tag:theconversation.com,2011:/ca/topics/atomic-clocks-18330/articlesAtomic clocks – The Conversation2022-11-21T02:42:13Ztag:theconversation.com,2011:article/1949222022-11-21T02:42:13Z2022-11-21T02:42:13ZIt’s time-out for leap seconds: an expert explains why the tiny clock adjustments will be paused from 2035<figure><img src="https://images.theconversation.com/files/496357/original/file-20221121-18-vwota1.jpeg?ixlib=rb-1.1.0&rect=54%2C99%2C5952%2C3908&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><span class="source">Shutterstock</span></span></figcaption></figure><p>Meeting in Versailles, France, on Friday, the Bureau International des Poids et Mesures (BIPM) has <a href="https://www.nytimes.com/2022/11/19/science/time-leap-second-bipm.html">called time-out</a> on “leap seconds” – the little jumps occasionally added to clocks running on Coordinated Universal Time (UTC), to keep them in sync with Earth’s rotation. </p>
<p>From 2035, leap seconds will be abandoned for 100 years or so and will probably never return. It’s time to work out exactly what to do with a problem that has become increasingly urgent, and severe, with the rise of the digital world.</p>
<h2>Why do we have leap seconds?</h2>
<p>Roll back to 1972, when the arrival of highly accurate atomic clocks laid bare the fact that days are not exactly 86,400 standard seconds long (that being 24 hours, with each hour comprising 3,600 seconds).</p>
<p>The difference is only in milliseconds, but accumulates inexorably. Ultimately, the Sun would appear overhead at “midnight” – an indignity metrologists (people who study the science of measurement) were determined to prevent. Complicating matters further, Earth’s rotation, and thus the length of a day, actually varies erratically and can’t be predicted far in advance.</p>
<p>The solution arrived at was leap seconds: one-second corrections applied at the end of December and/or June on an ad hoc basis. Leaps were scheduled to ensure the timekeeping system we all use, Coordinated Universal Time (UTC), is never more than 0.9 seconds away from the Earth-tracking alternative, Universal Time (UT1).</p>
<p>But all this was before computers ruled the Earth. Leap seconds were an elegant solution when first proposed, but are diabolical when it comes to software implementations.</p>
<p>This is because a leap second is an abrupt change that badly breaks key assumptions used in software to represent time. Base concepts such as time never repeating, standing still, or going backward are all at risk – as well as other quaint notions like each minute lasting exactly 60 seconds. </p>
<h2>Leaping into danger</h2>
<p>Question: what’s worse than mixing computers and leap seconds? Answer: mixing billions of interconnected networked computers, all trying to execute a leap second jump at (theoretically) the same time, with a great many failing in a wide variety of ways. </p>
<p>It gets better: most of those computers are learning about the impending leap second from the network itself. Better still, almost all are constantly synchronising their internal clocks by communicating over the internet to other computers called time servers, and believing the timing information these supply.</p>
<p>Imagine this scene then: during leap-second madness, some time-server computers can be wrong, but client computers relying on them don’t know it. Or they can be right, but client computer software disbelieves them. Or both client and server computers leap, but at slightly different times, and as a result software gets confused. Or perhaps a computer never receives word that a leap is happening, does nothing, and ends up a second ahead of the rest of the world. </p>
<p>All of this and more was seen in the analysis of timing data from the last <a href="https://data.research.uts.edu.au/publication/2e573e44ae983b08f3e03f50540d059e/ro-crate-preview.html">leap-second event</a> in 2016. </p>
<p>The ways in which computer confusion over time can impact networked systems are too numerous to describe. Already there are documented cases of significant outages and impacts arising from the most recent leap second events. </p>
<p>More broadly though, consider the networked critical infrastructure our world runs on, including electricity grids, telecommunications systems, financial systems, and services such as collision avoidance in shipping and aviation. Many of these rely on accurate timing at millisecond scales, or even down to nanoseconds. An error of one second could have huge and even deadly impacts. </p>
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<a href="https://images.theconversation.com/files/496344/original/file-20221121-24-7tg7jv.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Satellites GLONASS and Geo-IK displayed overhead at an exhibit in Moscow." src="https://images.theconversation.com/files/496344/original/file-20221121-24-7tg7jv.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/496344/original/file-20221121-24-7tg7jv.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/496344/original/file-20221121-24-7tg7jv.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/496344/original/file-20221121-24-7tg7jv.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/496344/original/file-20221121-24-7tg7jv.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/496344/original/file-20221121-24-7tg7jv.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/496344/original/file-20221121-24-7tg7jv.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=566&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">Russia voted against the decision to abandon leap seconds, in part because this will require a major update to its global navigation satellite system, GLONASS, which incorporates leap seconds.</span>
<span class="attribution"><span class="source">Shutterstock</span></span>
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<h2>Time’s up!</h2>
<p>In recognition of the growing costs to our computer-based world, the idea of doing away with leap seconds has been on the table since 2015.</p>
<p>The International Telecommunications Union, the standards body that governs leap seconds, pushed back a decision several times. But pressure continued to grow on multiple fronts, including from major tech players such as Google and Meta (formerly Facebook). </p>
<p>The majority of international participants in the vote, including the US, France and Australia, supported the recent decision to drop the leap second.</p>
<p>The Versailles decision is not to abandon the idea of keeping everyday timekeeping (UTC) aligned with Earth. It’s more a recognition that the disadvantages of the current leap second system are too high, and getting worse. We need to stop it before something really bad happens!</p>
<p>The good news is we can afford to wait the suggested 100 years or so. During this time, the discrepancy may grow to as much as a minute, but that’s not very significant if you consider what we endure with daylight savings time each year. The logic is that by dropping the leap second right now, we can avoid its dangers and allow plenty of time to work out less disruptive ways to keep time aligned.</p>
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Read more:
<a href="https://theconversation.com/a-brief-history-of-telling-time-55408">A brief history of telling time</a>
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<h2>How could we deal with this down the track?</h2>
<p>An extreme approach would be to fully adopt an abstract definition of time, abandoning the long-held <a href="https://www.nytimes.com/2022/11/14/science/time-leap-second.html">association between time</a> and Earth’s movements. Another is to make larger adjustments than a second, but far less frequently and with far better preparation to limit the dangers – perhaps in an age where software has evolved beyond bugs.</p>
<p>The decision of how far we’re willing to let things drift before a new approach is decided upon has its own deadline: the next meeting of the Bureau International des Poids et Mesures is set for 2026. In the meantime, we’ll be stuck with leap seconds until 2035. </p>
<p>Since the Earth has surprisingly begun to <a href="https://theconversation.com/the-length-of-earths-days-has-been-mysteriously-increasing-and-scientists-dont-know-why-188147">spin faster</a> in recent decades, the next leap second may, for the first time, involve removing a second to speed up UTC, rather than adding a second to slow it down. </p>
<p>Software for this case is largely already in place, but has never been tested in the wild – so be prepared to leap into the unknown.</p>
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Read more:
<a href="https://theconversation.com/scientists-are-hoping-to-redefine-the-second-heres-why-157645">Scientists are hoping to redefine the second – here's why</a>
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<img src="https://counter.theconversation.com/content/194922/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Darryl Veitch has received funding from the ARC for network timing research. </span></em></p>The majority of nations voted to scrap leap seconds – the little jumps added to UTC time to keep it aligned with Earth’s rotation. What can we expect moving forward?Darryl Veitch, Professor of Computer Networking, University of Technology SydneyLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1611202021-05-19T16:13:43Z2021-05-19T16:13:43ZClocks that tell time more accurately use more energy – new research<figure><img src="https://images.theconversation.com/files/401267/original/file-20210518-13-1a2rmfe.jpg?ixlib=rb-1.1.0&rect=192%2C100%2C4107%2C1993&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/collection-vintage-clock-hanging-on-old-561021889">Shutterstock/Alexey Wraith</a></span></figcaption></figure><p>Clocks pervade our lives, from the cellular clocks <a href="https://theconversation.com/uk/topics/body-clock-2947">inside our bodies</a> to the <a href="https://theconversation.com/scientists-are-hoping-to-redefine-the-second-heres-why-157645">atomic clocks</a> that underlie satellite navigation. </p>
<p>These atomic clocks can measure time accurately to within one second in billions of years. But there could be a price to pay for this accuracy, in the form of energy.</p>
<p><a href="https://journals.aps.org/prx/abstract/10.1103/PhysRevX.11.021029">Our new experiment</a> found clocks that measure time more accurately consume more energy than their less accurate counterparts. This suggests nature imposes a fundamental energy cost for keeping time, and it may mean there’s a limit to how accurate we can make clocks.</p>
<p>The branch of science that studies the energy required for different physical processes is called thermodynamics. Its laws are inescapable, and all our machines are constrained by them, including power stations, computers and engines.</p>
<p>A key principle of thermodynamics is that energy always eventually flows from hot objects to cold ones. If we reverse the flow in one place, such as a refrigerator, we must pay for it elsewhere, such as in a power station. </p>
<p>A consequence of this is that everything in the universe will ultimately reach the same temperature. At this point life, which relies on energy flow, will become impossible. <a href="https://theconversation.com/the-fate-of-the-universe-heat-death-big-rip-or-cosmic-consciousness-46157">This grim scenario</a> – which lies in the far distant future, if the universe lasts that long – is known as heat death.</p>
<p>The one-way evolution driven by the laws of thermodynamics, often called the arrow of time, profoundly constrains what technology can and can’t do. For example, there’s a maximum useful energy that can be extracted by burning a given amount of fuel at a given temperature. No engine will ever be more efficient than this. Thermodynamics also imposes a price for rewriting information, and constrains the efficiency of any possible computer memory.</p>
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<img alt="The dust of interstellar space." src="https://images.theconversation.com/files/401270/original/file-20210518-15-be37xc.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/401270/original/file-20210518-15-be37xc.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=293&fit=crop&dpr=1 600w, https://images.theconversation.com/files/401270/original/file-20210518-15-be37xc.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=293&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/401270/original/file-20210518-15-be37xc.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=293&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/401270/original/file-20210518-15-be37xc.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=368&fit=crop&dpr=1 754w, https://images.theconversation.com/files/401270/original/file-20210518-15-be37xc.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=368&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/401270/original/file-20210518-15-be37xc.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=368&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<span class="caption">Interstellar space.</span>
<span class="attribution"><a class="source" href="https://www.shutterstock.com/image-illustration/360-degree-interstellar-cloud-dust-gas-1778340206">Shutterstock/Jurik Peter</a></span>
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<h2>Studying time</h2>
<p>There may be other thermodynamic machines constrained in this way. Some intriguing hints suggest that clocks are a third example. </p>
<p>Simulations of <a href="https://journals.aps.org/prx/abstract/10.1103/PhysRevX.6.041053">clocks inside bacteria</a> and the latest <a href="https://physics.aps.org/articles/v10/88">“quantum” clocks</a> show that, even though their innards are completely different, both of them must be supplied with energy to create the same flow from hot to cold. This is the cost they must pay to keep time, and the thermodynamic theory of clocks predicts that it must increase when the accuracy of the clock improves.</p>
<p>To find out whether such a constraint applies to real clocks, we and our colleagues, including PhD candidate Anna Pearson, built a <a href="https://journals.aps.org/prx/abstract/10.1103/PhysRevX.11.021029">particularly simple clock</a> based on a pendulum clock, in which the flow of energy could be measured and controlled. </p>
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Read more:
<a href="https://theconversation.com/scientists-are-hoping-to-redefine-the-second-heres-why-157645">Scientists are hoping to redefine the second – here's why</a>
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<p>Our “pendulum” – perhaps more accurately described as a drum – was a suspended membrane, just 50 nanometres (billionths of a metre) thick, which vibrated at a set frequency. Each vibration corresponded to one tick of the clock. We could increase the strength of these vibrations by supplying energy to the membrane in a controlled way. Determining the accuracy of the clock became a matter of measuring how regularly the ticks occurred, which we did using an electrical circuit.</p>
<p>Just like any other engine, the clock had to release part of the energy supplied to it as heat. In our design, this heat contributed to the signal from the electrical circuit. We could measure both the accuracy of the clock and the price in terms of heat released.</p>
<p>The thermodynamic theory of clocks made two predictions about our experiment. First, the more energy we supplied, the more accurately the clock should run. Second, the amount of heat released by the clock should increase in proportion to its accuracy. </p>
<p>Both these predictions came true. What’s more, the ratio between the accuracy and the heat released was close to the value the theory predicts, once the electrical noise in the experiment was taken into account.</p>
<h2>The cost of measuring time</h2>
<p>Our results show there is indeed a price for measuring time accurately, at least for this simple clock. Interestingly, our theory predicts quite accurately the energy consumption of more complex clocks in everyday life. For example, it says that a wristwatch should consume at least one microwatt (millionth of a watt) of power – which is indeed slightly less than the actual consumption.</p>
<p>So do humans’ efforts to measure time inescapably accelerate the universe’s journey towards heat death? We don’t need to worry, for two reasons.</p>
<p>First, some clocks, particularly the most accurate atomic clocks, are much more efficient than our theory predicts. This shows the thermodynamic constraint we have found does not apply in the same way to all clocks, meaning we still lack an all-encompassing understanding of timekeeping. </p>
<p>More importantly, the energy dissipated by clocks is minuscule on a universal scale. The heat death of the universe may eventually happen – but the cause will lie not in ourselves, but in the stars.</p><img src="https://counter.theconversation.com/content/161120/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Edward Laird receives funding from the UK Connected Places Catapult. He has served as a consultant on timekeeping for LocatorX.</span></em></p><p class="fine-print"><em><span>Natalia Ares receives funding from the Royal Society, EPSRC Platform Grant (EP/R029229/1), Grant No. FQXi-IAF19-01 from the
Foundational Questions Institute Fund, a donor advised fund of Silicon Valley Community Foundation, from Templeton World Charity Foundation, and from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (Grant Agreement No. 948932).</span></em></p>This is the first time a measurement has been made of the entropy generated by telling time.Edward Laird, Lecturer in Experimental Condensed Matter Physics, Lancaster UniversityNatalia Ares, Royal Society University Research Fellow, University of OxfordLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1576452021-03-24T16:04:36Z2021-03-24T16:04:36ZScientists are hoping to redefine the second – here’s why<figure><img src="https://images.theconversation.com/files/391132/original/file-20210323-18-o7x7oa.jpg?ixlib=rb-1.1.0&rect=97%2C52%2C4745%2C2649&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Measuring time.</span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/image-illustration/composition-space-time-flight-spiral-roman-1221181900">Shutterstock/FlashMovie</a></span></figcaption></figure><p>Everyone needs to know the time. Ever since the 17th century Dutch inventor Christiaan Huygens made the first pendulum clock, people have been thinking of good reasons to measure time more precisely. </p>
<p>Getting the time right is important in so many ways, from running a railway to doing millisecond trades on the stock market. Now, for most of us, our clocks are checking themselves against a signal from atomic clocks, like those on board the global positioning system (GPS) satellites. </p>
<p>But <a href="https://www.nature.com/articles/s41586-021-03253-4">a recent study</a> by two teams of scientists in Boulder, Colorado might mean those signals will get much more accurate, by paving the way to effectively allow us to redefine the second more precisely. Atomic clocks could become so accurate, in fact, that we could begin to measure previously imperceptible gravity waves.</p>
<h2>Brief history of time</h2>
<p>Modern clocks still use Huygens’ basic idea of an oscillator with a resonance – like a pendulum of a fixed length that will always move back and forth with the same frequency, or a bell that rings with a specific tone. This idea was greatly improved in the 18th century by John Harrison who realised that smaller, higher frequency oscillators have more stable and pure resonances, making clocks more reliable. </p>
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<img alt="An old pendulum clock with a wooden wall behind." src="https://images.theconversation.com/files/391131/original/file-20210323-20-eowbly.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/391131/original/file-20210323-20-eowbly.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/391131/original/file-20210323-20-eowbly.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/391131/original/file-20210323-20-eowbly.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/391131/original/file-20210323-20-eowbly.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/391131/original/file-20210323-20-eowbly.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/391131/original/file-20210323-20-eowbly.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<span class="caption">An old pendulum clock.</span>
<span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/old-pendulum-clock-on-background-wooden-262861325">Shutterstock/Tillottama</a></span>
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<p>Nowadays, most everyday clocks use a tiny piece of quartz crystal in the shape of a miniature musical tuning fork, with very high frequency and stability. Not much has changed with this clock design in the past hundred years, although we’ve got better at making them cheaper more reproducible. </p>
<p>The massive difference these days is the way that we check – or “discipline” – quartz clocks. Up until 1955, you needed to keep correcting your clock by checking it against a very regular astronomical phenomenon, like the Sun or the moons of Jupiter. Now we discipline clocks against natural oscillations inside atoms. </p>
<p>The atomic clock was first built by Louis Essen. It was used to redefine the second in 1967, a definition that has remained the same since. </p>
<p>It works by counting the flipping frequency of a quantum property called spin in the electrons in caesium atoms. This natural atomic resonance is so sharp that you can tell if your quartz crystal clock signal wanders off in frequency by less than <a href="https://doi.org/10.1088/1681-7575/aae008">one part in 10¹⁵</a>, that’s a millionth of a billionth. One second is officially defined as 9,192,631,770 caesium electron spin flips.</p>
<p>The fact we can make such accurately disciplined oscillators makes frequency and time the most precisely measured of all physical quantities. We send out signals from atomic clocks all over the world, and up into space via GPS. Anyone with a GPS receiver in their mobile phone has access to an astonishingly accurate time measurement device. </p>
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Read more:
<a href="https://theconversation.com/why-we-will-probably-never-have-a-perfect-clock-105444">Why we will probably never have a perfect clock</a>
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<p>If you can measure time and frequency accurately, then there are all kinds of other things you can accurately measure too. For example, measuring the spin flip frequency of certain atoms and molecules can tell you the strength of the magnetic field they experience, so if you can find the frequency precisely then you’ve also found the field strength precisely. The smallest possible <a href="https://doi.org/10.1063/1.5128716">magnetic field sensors</a> work this way. </p>
<p>But can we make better clocks that allow us to measure frequency or time even more precisely? The answer might still be just as John Harrison found, to go higher in frequency. </p>
<p>The caesium spin flip resonance has a frequency corresponding to microwaves, but some atoms have nice sharp resonances for optical light, a million times higher in frequency. Optical atomic clocks have shown extremely stable comparisons with one another, at least when a pair of them is placed only a few metres apart. </p>
<p>Scientists are thinking about whether the international definition of the second could be redefined to make it more precise. But to achieve this, the different optical clocks that we would use to keep time precisely need to be trusted to read the same time even if they are <a href="https://doi.org/10.1038/nphoton.2016.235">in different labs</a> thousands of miles apart. So far, such long distance tests have been <a href="https://doi.org/10.1038/s41567-020-01038-6">not much better</a> than for microwave clocks. </p>
<h2>Better clocks</h2>
<p>Now, using a new way of linking the clocks with <a href="https://journals.aps.org/prresearch/abstract/10.1103/PhysRevResearch.2.033395">ultra-fast lasers</a>, researchers have shown that different kinds of optical atomic clocks can be placed a few kilometres apart and still agree within 1 part in 10¹⁸. This is just as good as previous measurements with <a href="https://doi.org/10.1038/s41566-020-0619-8">pairs of identical clocks</a> a few hundred metres apart, but about a hundred times more precise than achieved before with <a href="https://doi.org/10.1038/s41567-017-0042-3">different clocks or large distances</a>.</p>
<p>The authors of the new study compared multiple clocks based on different types of atoms – ytterbium, aluminium and strontium in their case. The strontium clock was situated in the University of Colorado and the other two were in the US National Institute of Standards and Technology, down the road. </p>
<figure class="align-center ">
<img alt="A diagram showing three atomic clocks being compared at a distance to each other." src="https://images.theconversation.com/files/391130/original/file-20210323-14-1hn2o4b.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/391130/original/file-20210323-14-1hn2o4b.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=410&fit=crop&dpr=1 600w, https://images.theconversation.com/files/391130/original/file-20210323-14-1hn2o4b.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=410&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/391130/original/file-20210323-14-1hn2o4b.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=410&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/391130/original/file-20210323-14-1hn2o4b.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=515&fit=crop&dpr=1 754w, https://images.theconversation.com/files/391130/original/file-20210323-14-1hn2o4b.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=515&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/391130/original/file-20210323-14-1hn2o4b.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=515&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">The experiment.</span>
<span class="attribution"><a class="source" href="https://www.eurekalert.org/multimedia/emb/259564.php">Hanacek/NIST</a>, <span class="license">Author provided</span></span>
</figcaption>
</figure>
<p>The study connected the clocks with a laser beam through the air over 1.5km from building to building, and this link was shown to be just as good as an optical fibre under the road, in spite of air turbulence.</p>
<p>But why do we need such accurate clocks? Although the atoms in the clock are supposed to be exactly the same wherever the clock sits and whoever looks at it, tiny useful differences can appear when the measurements of time are so precise. </p>
<p>According to Einstein’s theory of general relativity, gravity <a href="https://theconversation.com/explainer-what-are-gravitational-waves-53239">distorts space-time</a>, and we can measure this distortion. Optical clocks have already been used to detect the difference in the Earth’s gravitational field by <a href="https://doi.org/10.1038/s41566-020-0619-8">moving just a centimetre</a> in height. </p>
<p>With more accurate clocks, maybe you could sense the creep in stress of the Earth’s crust and <a href="https://doi.org/10.1093/gji/ggv246">predict volcanic eruptions</a>. Gravitational waves produced by distant <a href="https://doi.org/10.1103/PhysRevLett.116.061102">black hole mergers</a> have been seen – maybe we will now be able to detect much weaker waves from less cataclysmic events using a pair of <a href="https://doi.org/10.1103/PhysRevD.94.124043">satellites with optical clocks</a>.</p><img src="https://counter.theconversation.com/content/157645/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Ben Murdin does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>New research has tested the latest generation of atomic clocks.Ben Murdin, Professor of Photonics and Quantum Sciences, University of SurreyLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1390812020-05-22T13:07:17Z2020-05-22T13:07:17ZPairing lasers with microwaves makes mind-bogglingly accurate electronic clocks – a potential boon for GPS, cell phones and radar<figure><img src="https://images.theconversation.com/files/336835/original/file-20200521-102632-z9s0ez.jpeg?ixlib=rb-1.1.0&rect=3%2C0%2C2392%2C1595&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Mating laser-driven atomic clocks like the one shown here with microwaves promises more accurate electronic devices.</span> <span class="attribution"><a class="source" href="https://www.nist.gov/image/16pml015ytterbiumclocknphillipshrjpeg">N. Phillips/NIST</a></span></figcaption></figure><p>Time and frequency standards are a key part of technologies we have come to rely on in our daily lives, from GPS navigation and cellphone networks to the electrical power grid. The importance of these systems and the constant drive to improve their performance has led to the development of atomic clocks that keep time and measure frequency with incredible accuracy. </p>
<p>Conventional atomic clocks use the billions-of-times-a-second vibrations of atoms like cesium to calibrate microwave signals, which are read by other devices such as GPS satellites, to keep time. The most accurate atomic clocks, however, calibrate optical signals from laser beams rather than microwaves, and they use atoms like ytterbium that oscillate even faster than cesium – hundreds of trillions of times per second. </p>
<p>Optical clock frequencies are so stable that it would take more than 14 billion years – the age of the universe – for one of these clocks to be off by a second. But researchers haven’t been able to feed these ultrafast optical signals at their full performance into electronic devices.</p>
<p>Our team of physicists and engineers, with members from the University of Colorado, University of Virginia and the National Institute of Standards and Technology (NIST), has found a way <a href="https://science.sciencemag.org/cgi/doi/10.1126/science.abb2473">to link optical atomic clocks with microwave signals</a> without compromising the amazing performance of the optical clock signals. The resulting microwave tracked the optical clock with a precision of under a quadrillionth of a second. A quadrillion is a thousand trillion. This yields a 100-fold improvement over the cesium fountain clock, the gold standard for microwave atomic clocks. </p>
<h2>Keeping time</h2>
<p>The very best microwave clock today is the cesium fountain clock, which <a href="https://doi.org/10.1088/1681-7575/aae008">oscillates near 10 GHz</a> or about 10 billion cycles per second. Carefully tracking the clock cycles makes it possible to deliver a clock frequency with high stability. The best cesium fountain clocks can provide about 13 digits of precision after tracking one second’s worth of oscillations. Averaging over longer times increases the precision, and if you’re willing to wait for days or weeks you can improve the precision of the clock frequency to about 16 digits. With 16 digits of precision, it would take 300 million years for a clock to be off by a second. Microwave atomic clocks, housed in metrology institutes worldwide, are used to define the international standard for the second. </p>
<p>Microwave atomic clocks underlie much of today’s technology. For example, GPS measures the relative delay of timing signals from overhead satellites to determine your position. Without the nanosecond-level stability of the clocks onboard the GPS satellites, the relative timing delay among satellites would vary randomly, making it impossible to find your position accurately. </p>
<p>High-performance clocks are also extremely important for science. One example is very long baseline interferometry (VLBI) where microwave and millimeter wave signals are detected at observatories spread across the globe, and are combined to form images of cosmic objects. High stability clocks are needed to effectively time stamp the received signals so they can be combined in a meaningful way to form an image. A recent example of this technique at work was the <a href="https://doi.org/10.3847/2041-8213/ab0ec7">first-ever images of a black hole</a>. </p>
<p>Over the past decade, a number of optical clocks have surpassed the performance of their microwave counterparts. Optical clocks operate at 100s of terahertz – more than 100 trillion cycles per second – and can now provide 16 digits of precision in one second or better. In just a few hours, they can offer a whopping <a href="https://doi.org/10.1038/s41586-018-0738-2">18 digits of precision</a> or more. This has opened up exciting new avenues in scientific research with atomic clocks, including <a href="https://doi.org/10.1126/sciadv.aau4869">the search for dark matter</a>, testing <a href="https://doi.org/10.1103/RevModPhys.90.025008">whether fundamental constants of nature are truly constant</a> and chronometric leveling where gravity’s effect on an atomic clock rate can be used to <a href="https://doi.org/10.1038/s41566-020-0619-8">measure Earth’s gravitational potential</a>. With the extraordinary performance of optical atomic clocks, a redefinition of the second now seems inevitable.</p>
<p>New applications become available by bringing optical-clock-level stability to microwaves. </p>
<p>GPS could be more accurate, positioning you to within a few centimeters rather than a few meters. Better GPS would improve the performance of aircraft auto pilots and self-driving cars. With more precise timekeeping, electronic communications like cellphone signals can transmit more information.</p>
<p>Radar is dependent on the frequency stability of the transmitted microwaves. With higher precision microwaves, radar sensitivity could see sizeable improvements, particularly for detecting slow-moving targets. <a href="https://doi.org/10.1016/j.asr.2019.05.042">Moving VLBI to space</a> and outfitting it with improved timestamping could greatly increase resolution and observation time, making it possible to image more objects in the universe.</p>
<h2>Combing frequencies</h2>
<p>Bringing optical atomic clock precision to microwave signals was achieved with a tool known as an <a href="https://doi.org/10.1364/JOSAB.27.000B51">optical frequency comb</a>. The frequency comb, named for its array of discrete, evenly spaced laser frequency tones, emits a train of sub-picosecond light pulses. A picosecond is a trillionth of a second.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/336809/original/file-20200521-102682-xc4f6i.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/336809/original/file-20200521-102682-xc4f6i.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/336809/original/file-20200521-102682-xc4f6i.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/336809/original/file-20200521-102682-xc4f6i.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/336809/original/file-20200521-102682-xc4f6i.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/336809/original/file-20200521-102682-xc4f6i.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/336809/original/file-20200521-102682-xc4f6i.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=566&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">The black rectangle (center) is a high-speed photodiode that converts laser pulses to high, super-stable microwave frequencies, bringing the incredible accuracy of optical atomic clocks to everyday electronics.</span>
<span class="attribution"><span class="source">Franklyn Quinlan/NIST</span></span>
</figcaption>
</figure>
<p>When the frequency comb is connected to an optical clock, the rate at which these pulses are emitted is a well-defined fraction of the optical clock frequency. Shining these pulses onto a high-speed optical-to-electrical converter makes it possible to generate a microwave signal that oscillates at a well-defined fraction of the optical clock frequency, and whose stability and accuracy matches that of the optical clock. </p>
<p>Armed with this level of performance, a new generation of microwave timekeeping opens the door for many scientific and technological advances.</p>
<p>[<em>Deep knowledge, daily.</em> <a href="https://theconversation.com/us/newsletters?utm_source=TCUS&utm_medium=inline-link&utm_campaign=newsletter-text&utm_content=deepknowledge">Sign up for The Conversation’s newsletter</a>.]</p><img src="https://counter.theconversation.com/content/139081/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Andrew Ludlow receives funding from the NIST, DARPA, and NASA. </span></em></p><p class="fine-print"><em><span>Franklyn Quinlan receives funding from The National Institute of Standards and Technology and DARPA. </span></em></p>Researchers have made some of the most accurate clocks imaginable in recent years, but the trick is harnessing those clocks to electronics. Using lasers to tune microwaves bridges the gap.Andrew Ludlow, Lecturer of Physics, University of Colorado BoulderFranklyn Quinlan, Physicist, National Institute of Standards and TechnologyLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1151832019-04-11T08:54:09Z2019-04-11T08:54:09ZMarclay’s Clock: 24-hour installation highlights a modern obsession with time<figure><img src="https://images.theconversation.com/files/268394/original/file-20190409-2909-1964qt0.jpg?ixlib=rb-1.1.0&rect=2%2C5%2C995%2C410&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><span class="source">Mikhail Leonov via Shutterstock</span></span></figcaption></figure><p>There is something unnerving about The Clock, Christian Marclay’s acclaimed installation, which was <a href="https://www.tate.org.uk/whats-on/tate-modern/exhibition/christian-marclay-clock">recently on display</a> at the Tate Modern in London before moving to Australia where it has appeared in Melbourne to sell-out crowds. This remarkable piece of contemporary art, which has travelled the world gathering awards and critical acclaim, is a montage of scenes drawn from thousands of films, from Orson Welles’ The Stranger to James Bond in Live and Let Die, which feature a clock or timepiece of some kind. </p>
<p>The installation itself also functions as a 24-hour clock, the time displayed in each segment corresponding to that in the “real world” – at 12pm, for instance, viewers are treated to the ringing of bells in High Noon. Rarely does the action in one film clip lead logically to the next —- the narrative arc of The Clock is simply the passage of time itself. And yet, throughout, viewers remain transfixed, eagerly anticipating the next scene, the next moment in time.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/xp4EUryS6ac?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
</figure>
<p>Our incapacity to live in the present is hardly a modern phenomenon. As St Augustine <a href="http://www.leaderu.com/cyber/books/augconfessions/bk11.html">wrote in around 400 AD</a>: “No time is wholly present … all time past is forced on by the future.” What Marclay’s installation foregrounds, however, is the degree to which our experience of time has become associated with the means to measure and quantify it – in a word, with clocks. Few public or private spaces remain today which have not been infiltrated by some kind of time-reckoning device, whether a clock, watch, computer screen or smart phone.</p>
<p>Throughout history and across cultures, of course, humans have developed diverse means of dividing the natural cycles of day into segments to help track the passage of time. Water clocks, sundials, sand-glasses and bells have all been used in various ways to identify, measure and announce the time. And “knowing” the time has obvious advantages – it allows the members of a given community to coordinate their activities, to pursue their own individual goals while congregating or interacting at particular moments. </p>
<p>Much <a href="https://doi.org/10.1111/j.1475-682X.1980.tb00383.x">emphasis has been placed</a>, for example, on the importance Benedictine monks attributed to the strict regulation of time, the division of the day into “hours” of prayer, sleep and work. During the Middle Ages in Europe, the repeated, at times cacophonous, ringing of church and town bells served to structure the daily life of local inhabitants.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/268386/original/file-20190409-2931-1brwpbx.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/268386/original/file-20190409-2931-1brwpbx.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=380&fit=crop&dpr=1 600w, https://images.theconversation.com/files/268386/original/file-20190409-2931-1brwpbx.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=380&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/268386/original/file-20190409-2931-1brwpbx.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=380&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/268386/original/file-20190409-2931-1brwpbx.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=477&fit=crop&dpr=1 754w, https://images.theconversation.com/files/268386/original/file-20190409-2931-1brwpbx.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=477&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/268386/original/file-20190409-2931-1brwpbx.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=477&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">A Flemish Book of Houses, dating back to between 1470 and 1500.</span>
<span class="attribution"><span class="source">Unknown artist, Bruges</span></span>
</figcaption>
</figure>
<h2>Clock-watching</h2>
<p>With the <a href="https://www.scientificamerican.com/article/a-chronicle-of-timekeeping-2006-02/">emergence of mechanical clocks in the 1300s</a>, the time was increasingly displayed in public spaces, providing a focal point for the organisation of social life. But the 18th century witnessed an explosion in the manufacture of a different, more personalised device – the watch. By the late 1700s, it <a href="http://www.hup.harvard.edu/catalog.php?isbn=9780674002821">has been estimated</a>, the annual world production of watches stood at between 300,000 and 400,000. Henceforth, time was portable and wealthy individuals could <a href="https://doi.org/10.1086/ahr.112.3.685">adjust their own personal watches</a> to public clocks, bringing home a more accurate knowledge of the time. The habit of clock-watching had emerged.</p>
<p>During the 19th century, however, this practice turned into a veritable obsession. A number of factors stimulated this phenomenon, among which were the development of industry and new means of transport and communication. Railway timetables, time-stamped telegrams and factory discipline all called for stricter conformity to the time of the clock. And it was during the 19th century that time was standardised across countries and, with the <a href="http://www.thegreenwichmeridian.org/tgm/articles.php?article=10">International Meridian Conference of 1884</a>, the globe. </p>
<p>Time was no longer determined locally, in each town or village, according to the position of the sun, but “transmitted” by electric wires, from a specified location – in Britain, of course, this was Greenwich, which then became the reference point for the world’s time zones. </p>
<p>By 1900, Switzerland alone was exporting <a href="http://www.hup.harvard.edu/catalog.php?isbn=9780674002821*">more than 7m watches</a> and watch movements every year, suggesting the extent to which individuals were adjusting their personal lives to the structures of public time.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/268403/original/file-20190409-2914-1tj3ve9.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/268403/original/file-20190409-2914-1tj3ve9.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/268403/original/file-20190409-2914-1tj3ve9.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/268403/original/file-20190409-2914-1tj3ve9.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/268403/original/file-20190409-2914-1tj3ve9.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/268403/original/file-20190409-2914-1tj3ve9.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/268403/original/file-20190409-2914-1tj3ve9.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Centre of the universe: Greenwich Meridian.</span>
<span class="attribution"><span class="source">By Giacomo Baudazzi via Shutterstock</span></span>
</figcaption>
</figure>
<p>By the turn of the 20th century, punctuality had become the hallmark of modern society. Resistance to the imposition of standard clock time, whether in rural communities or Western colonies, was considered a sign of backwardness, and “keeping up” with time had become a new source of anxiety. In 1881, the American neurologist George Beard listed “clocks and watches” among the causes of what he described as an <a href="https://archive.org/details/americannervousn00bearuoft/page/102">epidemic of “nervousness”</a>: “They compel us to be on time”, he wrote, “and excite the habit of looking to see the exact moment, so as not to be late for trains or appointments.”</p>
<h2>Looking to the future</h2>
<p>In the early 20th century, the German historian Karl Lamprecht <a href="https://archive.org/details/zurjngstendeuts02lampgoog/page/n187">similarly detected</a> a link between the contemporary concern for punctuality and a widespread, pathological “excitability”. Modern society, it seemed, as it does today, had come to depend on a universal effort to be on time. “If all the watches in Berlin suddenly went wrong in different ways,” the sociologist <a href="https://gutenberg.spiegel.de/buch/die-grossstadte-und-das-geistesleben-7738/1">Georg Simmel wrote in 1903</a>, “… its entire economic and commercial life would be derailed for some time.”</p>
<p>Beard, Lamprecht and Simmel had recognised the Janus-faced quality of the clock as a cornerstone of social life. A well-adjusted clock only ever indicates the present moment in time. But it also situates us in a continuum of events and interactions, inviting us to look ahead, whether with excitement, apathy or anxiety, to our future engagements. </p>
<p>It is this modern paradox that Christian Marclay’s installation illustrates so well. Incessantly reminded of the time, the viewer sits in anticipation of the future, experiencing what the French writer <a href="http://www.gallimard.fr/Catalogue/GALLIMARD/Blanche/Guide-des-egares">Jean D’Ormesson described</a> as a “static precipitation [which] transits as briefly as possible through this paradoxical state which is its aim and its heart, and which we call the present”. </p>
<p>For a moment, groups of visitors are invited to experience together the temporally structured present which they share, a present which dissolves when they, as individuals, decide it is time to leave.</p><img src="https://counter.theconversation.com/content/115183/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Jean-Michel Johnston is a researcher on the 'Diseases of Modern Life: Nineteenth Century Perspectives' project at the University of Oxford, supported by the European Research Council under the European Union’s Seventh Framework Programme (FP/2007-2013) under Grant Agreement Number 340121.</span></em></p>Critically acclaimed art installation highlights the way that the ubiquity of clocks and watches has transformed our relationship to time and the present.Jean-Michel Johnston, Postdoctoral Research Fellow, University of OxfordLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/923832018-03-01T10:56:57Z2018-03-01T10:56:57ZWhy a 10,000-year clock is being built under a mountain – and why 10,000 years is too long<figure><img src="https://images.theconversation.com/files/208465/original/file-20180301-152552-kownkq.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/closeup-old-clock-mechanism-gears-137397236?src=UUr8-_wUdNRFAes4pIsjMQ-1-27">Didecs/Shutterstock.com</a></span></figcaption></figure><p>Construction is underway on a “<a href="http://www.10000yearclock.net/learnmore.html">10,000 year clock</a>”. This is a clock that will have a year hand, a century hand, and a cuckoo that comes out every 1,000 years. The American inventor Danny Hillis <a href="https://www.wired.com/1995/12/the-millennium-clock/">wrote about</a> the idea in 1995, and, thanks to a US$42m investment from Amazon CEO Jeff Bezos, the clock is now being installed inside a Texas mountain. The aim of building a clock designed to work for 10 millennia, Bezos <a href="http://www.10000yearclock.net/learnmore.html">says</a>, is to encourage long-term thinking.</p>
<p>The striking thing about a 10,000 year clock is that it’s measuring a very long time, rather than a very short time. We’re getting better at measuring short bits of time – London has been home to a “caesium fountain” atomic clock since 1955, which set new standards for precision timekeeping. In 2013 “optical lattice clocks” were <a href="https://www.nist.gov/news-events/news/2013/08/nist-ytterbium-atomic-clocks-set-record-stability">100 times more accurate</a> than caesium fountains, and experiments continue to improve on this. </p>
<p><div data-react-class="Tweet" data-react-props="{"tweetId":"965957024109379602"}"></div></p>
<p>The 10,000 year clock, meanwhile, will potentially be the world’s most inaccurate clock. To ensure accuracy, the clock will recalibrate at noon each day, so, if all goes well it can be at most 12 hours out. But, as Lewis Carroll pointed out, a stopped clock is right twice a day. If this clock stops it will be only be right once every 1,000 years. Is such monumental imprecision a bad thing?</p>
<h2>What is a clock for, anyway?</h2>
<p>How useful clocks are depends on what they are used for. Clocks have become more precise, for example, because activity has needed to be coordinated more precisely. </p>
<p>Think of the marine chronometer, a portable timepiece invented in 1761 by John Harrison, which allowed sailors to calculate their location at sea more accurately. Think of Greenwich Mean Time, which was adopted across Great Britain in 1847, to help people catch trains. Think of the wristwatch, which was used in World War I to coordinate infantry advances with artillery barrages. More recently, GPS uses the time kept by caesium fountains to allow your sat nav to help you navigate your way across the country and have the voice of Brian Blessed congratulate you on successfully reaching your destination. </p>
<p>In a little over 10,000 years, the ghostly voice of Jeff Bezos may well congratulate an intrigued audience on witnessing the 10th cuckoo appear out of his clock. But there doesn’t seem to be a practical purpose served by a 500ft tall mechanical clock dug into the Sierra Diablo mountain range. It is roughly as useful stopped as it is ticking.</p>
<p>But this misses the point. Clocks aren’t just for the coordination of activity, they are often symbolic. Clocks are often used as retirement presents, jewellery and public art. John Taylor’s <a href="http://www.johnctaylor.com/the-chronophage/">Chronophage</a> – or “time eater” – is an artwork in Cambridge that’s been reminding the public of their time being ravenously eaten since 2008. Similarly, the 10,000 year clock is primarily a symbol, rather than a timepiece. Its aim is to get us to think about things beyond what’s currently dominating the news agenda.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/208466/original/file-20180301-152584-vluor6.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/208466/original/file-20180301-152584-vluor6.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/208466/original/file-20180301-152584-vluor6.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/208466/original/file-20180301-152584-vluor6.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/208466/original/file-20180301-152584-vluor6.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/208466/original/file-20180301-152584-vluor6.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/208466/original/file-20180301-152584-vluor6.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Tourists looking at John Taylor’s Corpus Clock, which features an insect named Chronophage – ‘Time Eater’.</span>
<span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/cambridgeukseptember-32017tourists-looking-john-taylors-corpus-719930587?src=2-3FX_M1Am9iJL5GvA0FWw-1-1">Pajor Pawel / Shutterstock.com</a></span>
</figcaption>
</figure>
<p>The 10,000 year clock is a symbol of the <a href="http://longnow.org">Long Now foundation</a> – both aim to foster long-term thinking and responsibility. This is a laudable goal. It is good to ensure that companies don’t prioritise quarterly profits over the sustainability of their businesses. It’s good when governments don’t prioritise re-election over public health issues or the environment. It’s good to ensure that one’s pension provides security in one’s retirement, even if it is many decades away.</p>
<h2>In the long run we’re all dead</h2>
<p>I’m sceptical, however, that “long-term” thinking is what we really want. As the late John Maynard Keynes <a href="https://books.google.co.uk/books?id=8HsuAAAAYAAJ">pointed out</a> in 1923: “In the long run we are all dead.” Extending our thinking from weeks or months to decades or centuries would be a good thing, but once we start thinking in millennia, we stretch ourselves too far. Corpus Christi college, Cambridge, home of the Chronophage, does a perfectly decent job of reminding its students to think about decades and centuries, rather than weeks and months, having been there since 1352. But this is all “medium-term”; the college isn’t old enough to have witnessed its first cuckoo from a 10,000 year clock. Neither is the British monarchy, let alone the United Kingdom.</p>
<p>Percy Shelley’s poem <a href="https://www.poetryfoundation.org/poems/46565/ozymandias">Ozymandias</a> describes a traveller coming across the broken remains of a statue of Ozymandias (aka the Egyptian pharaoh Ramses II, 1303-1213BC), inscribed with the words “LOOK ON MY WORKS, YE MIGHTY, AND DESPAIR!”. Discussing this poem, the philosopher Christopher Bennett <a href="https://books.google.co.uk/books?id=dj2hBgAAQBAJ&pg=PT55">asks</a>: </p>
<blockquote>
<p>No doubt at the time we strive and strive to do things that we can feel proud of; but in the scheme of things, how important can anything we do really be, when even the achievements of Ozymandias, ‘king of kings’, will one day be reduced to dust? </p>
</blockquote>
<p>This is my worry with “long-term thinking”: such thinking risks removing us from the contexts (our life plan, our society, our civilisation) in which things matter to us. Indeed, it removes us from contexts in which talk of “mattering” makes sense at all. Another philosopher, Thomas Nagel, <a href="https://books.google.co.uk/books?id=5cryOCGb2nEC">cautioned</a> against losing touch with the human scales on which our actions, plans and lives make sense: “To see myself objectively as a small, contingent, and exceedingly temporary organic bubble in the universal soup produces an attitude approaching indifference.” </p>
<p>If we think on a 10,000 year scale, what do railways, or pension schemes, or countries matter? The result is paralysis, rather than responsibility. Much better, then, to stick to the medium-term.</p><img src="https://counter.theconversation.com/content/92383/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Dr Graeme A Forbes is a Lecturer in Philosophy at the University of Kent, and a Senior Associate of the Centre for Philosophy of Time, Milan. </span></em></p>A clock designed to work for 10 millennia is being built – but what is the point of it?Graeme A Forbes, Lecturer in Philosophy, University of KentLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/759932017-04-11T08:06:28Z2017-04-11T08:06:28ZHere’s how Doctor Who’s time machine measures up with real instruments of space and time<figure><img src="https://images.theconversation.com/files/164574/original/image-20170409-29403-paxiug.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The TARDIS.</span> <span class="attribution"><a class="source" href="https://en.wikipedia.org/wiki/File:Tardis_BBC_Television_Center.jpg">Babbel1996/wikipedia</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span></figcaption></figure><p>There’s no denying that we’ve seen some absolutely staggering accomplishments in physics in the past year or so, particularly in our ability to measure space and time with unprecedented levels of detail. But being a lifelong “Whovian” excited about Doctor Who returning to our screens once again, I wondered how these accomplishments stacked up to those of the fictional Time Lords.</p>
<p>The crowning achievement of the Doctor has to be the TARDIS, the blue box from the show that’s bigger on the inside and allows the Doctor and his companions to travel “all of time and space, everything that ever happened or ever will” as Matt Smith’s eleventh Doctor once put it. But throughout the history of the show, the Doctor’s TARDIS has shown itself to be rather unreliable, regularly turning up at the wrong place or time. Given these faults we might think that the TARDIS <a href="http://io9.gizmodo.com/5720207/when-did-doctor-whos-time-traveler-get-so-good-at-controlling-his-tardis">isn’t quite what it’s cracked up to be</a>.</p>
<p>While the show has featured many, often conflicting, descriptions of how the TARDIS works, the key to the Time Lords’ time travelling ability seems to be the “Eye of Harmony”, essentially a star in an eternal state of collapsing into a black hole. In terms of real science though, the same theory that predicted black holes – Einstein’s general relativity – has solutions which permit time travel (in fact <a href="https://arxiv.org/abs/1310.7985">one possible way</a> of doing this has been given the name TARDIS).</p>
<p>Whether nature actually allows such solutions to exist is still an <a href="https://theconversation.com/how-to-build-a-time-machine-54873">open debate among theoretical physicists</a>, and even if time travel could happen we certainly don’t know how to build a time machine. So we’ll just have to compare the Doctor’s TARDIS with our best instruments of simply measuring time and space.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/QgHyDuFrWpM?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">How good is the TARDIS?</span></figcaption>
</figure>
<p>What we really need to compare here are these instruments’ relative precision. A simple way of thinking of this is as the ratio of the smallest thing you can measure with an instrument to the largest. In the case of a metre ruler that would be 1 millimetre compared to 1,000 millimetres (a metre), or simply one in 1,000.</p>
<h2>Measuring space</h2>
<p>In terms of measuring space our best ruler by far is advanced <a href="https://www.advancedligo.mit.edu/">Laser Interferometer Gravitational-Wave Observatory</a> (LIGO). Gravitational waves are mysterious ripples in the fabric of space and time that travel across our universe at the speed of light – stretching space in one direction and shrinking it in the direction that is at right angles. LIGO was the experiment that last year directly detected the <a href="https://theconversation.com/explainer-gravitational-waves-and-why-their-discovery-is-such-a-big-deal-53239">minute changes in distances travelled by light beams</a>, caused by gravitational waves. </p>
<p>These changes in distance are some 1,000-10,000 times smaller than the size of the nucleus of an atom, and they’re detected over a four-kilometre distance. That’s a level of sensitivity that’s up to one part in 10<sup>23</sup> – a huge number consisting of a one with 23 zeros after it: 100,000,000,000,000,000,000,000.</p>
<p>Now, considering the TARDIS’s playing field is “all of space”, it’s staggering that even when it turns up at the wrong place it simply manages to land on the right planet (usually Earth). The observable universe is some 10<sup>27</sup> metres in diameter while the Earth’s is a comparatively tiny 1.3m metres. So simply being able to find our planet within only the observable universe is a feat requiring some one in 10<sup>70</sup> relative precision. And that number only gets bigger when we consider how big the universe might extend beyond what’s actually visible.</p>
<h2>Measuring time</h2>
<p>When it comes to time, scientists have been developing new atomic clocks which are much better than the old Caesium ones that have been used to define what a second is. All these new clocks <a href="https://arxiv.org/abs/1602.03908">essentially count the number of waves</a> of specific colours of visible light emitted by atoms – a unique property of each element. Our current best clock uses Ytterbium atoms and is stable enough to yield relative precision a little less than one in 10<sup>18.</sup></p>
<p>But how do you compare this to the TARDIS? As it covers everything that ever happened or ever will happen, we need to essentially find out when the universe will die to be able to make a comparison. It’s currently 13.8 billion-years-old, but that’s still a very long way ahead. Given our current understanding of the amount of matter and energy in the universe, it won’t be until some 10<sup>100</sup> years that all of the stars, planets and galaxies will have died, all protons and neutrons will have decayed and even all the supermassive black holes will have evaporated. This is what is known as the <a href="https://theconversation.com/the-fate-of-the-universe-heat-death-big-rip-or-cosmic-consciousness-46157">heat death of the universe</a>.</p>
<p>Given that in the show, the TARDIS tends to turn up only a few years or a decade or so off the intended target, a ballpark figure for the TARDIS’s precision in time is around one in 10<sup>100.</sup> So despite it seemingly looking a bit rubbish in the show from time to time, we’ve still got a long way to go before we can match it. This is certainly something I’ll be keeping in mind when watching the show.</p><img src="https://counter.theconversation.com/content/75993/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Martin Archer does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Disappointed about Doctor Who’s TARDIS ending up at the wrong place at the wrong time? Don’t be – it’s incredibly precise.Martin Archer, Space Plasma Physicist, Queen Mary University of LondonLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/705832016-12-29T20:58:33Z2016-12-29T20:58:33ZWait a moment: 2016 goes a little longer thanks to a leap second<figure><img src="https://images.theconversation.com/files/150726/original/image-20161219-24276-l7zapx.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">That time is it on Earth?</span> <span class="attribution"><a class="source" href="http://www.measurement.gov.au/Pages/default.aspx">National Measurement Institute</a></span></figcaption></figure><p>To the time-poor of the world: take heart, for 2016 is a generous year. Not only were you granted a leap day on 29 February, you will soon score a New Year’s Eve countdown bonus, a <em>leap second</em>, to hold off 2017 for a final sip or regret.</p>
<p>Whereas leap years add a day to align the calendar with the seasons, <a href="https://www.nist.gov/pml/time-and-frequency-division/leap-seconds-faqs#often">leap seconds</a> align our everyday clocks with the Sun’s position in the sky, that is, with the Earth’s rotation.</p>
<p>Currently our planet takes roughly 86,400.00183 seconds (on average) to turn, instead of the expected 86,400 seconds you get by multiplying 24 hours by 60 minutes by 60 seconds. This may not sound like a great difference, but it amounts to a full second every 18 months. If left unchecked, it would become noticeable over time, and ultimately become problematic.</p>
<p>How did we get into this awkward situation? Why not just define a second so that there are exactly the right number? This sensible idea was tried in 1874, but hit a snag: the Earth keeps changing. </p>
<p>In terms of today’s standard <a href="http://www.bipm.org/en/measurement-units/">SI</a> second (defined via atomic physics), the above discrepancy is due to the fact that the day is losing about 0.0015 seconds per century, due largely to tidal friction. </p>
<p>Not only that, it also changes quite erratically due to mass redistribution.
For example, it is slowed by oceanic thermal expansion due to global warming, just as a playground spinning seat slows, via the conservation of angular momentum, when you place your body farther from the centre.</p>
<p>Leap seconds are used to make sure our usual timekeeping system, Coordinated Universal Time (<a href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">UTC</a>), never gets more than 0.9 seconds away from the Earth-tracking alternative, Universal Time (<a href="https://en.wikipedia.org/wiki/Universal_Time">UT1</a>).</p>
<p>But unlike leap years, leap seconds cannot be calculated centuries in advance. Because the Earth moves erratically, it must be observed closely, and leap seconds scheduled on an as-needed basis. </p>
<p>In UT1, seconds actually vary in duration, being stretched and compressed to match the Earth’s variations. In UTC, all seconds are standard SI seconds, which is much simpler, but it means that if you want to slow down or speed up UTC, there is no alternative but to jump. </p>
<p>All the leap seconds so far have been “positive”, meaning that an extra second is inserted, corresponding to jumping the clock back, and so slowing it down. </p>
<h2>Time’s up for the leap second?</h2>
<p>The leap second system has been with us since 1972. It represents an important chapter in the entangled history of civilian timekeeping, and of the definition of the second itself. Its days, however, may well be numbered. </p>
<p>For a number of years, support has been growing within the <a href="http://www.itu.int/en/Pages/default.aspx">International Telecommunications Union</a>, the standards body governing leap seconds, to abolish it. </p>
<p>The chief reason is complexity. Simply put, hardware and software can and do get things wrong. And the potential impacts are serious, from failures in navigation leading to collisions, to erroneous financial transactions, computer crashes and the inability to specify UTC times reliably into the future, because the leap second times are not yet known! </p>
<p>Because UTC jumps back at a leap second, effectively the second before the leap is repeated. Managing such “time travel” is inherently complex and error prone, so much so that in many cases the recommended action is to simply shutdown the system and restart it after the leap.</p>
<p>A dramatic illustration of the problem can be found in the internet. All computers have software clocks that generally rely on communication with time servers over the network to synchronise to UTC. Network timekeeping is a core internet service, and at its heart are the <a href="https://ntpserver.wordpress.com/2008/09/10/ntp-server-stratum-levels-explained/">Stratum-1 servers</a>, which have direct access to reference hardware such as atomic clocks. </p>
<p><a href="http://crin.eng.uts.edu.au/%7Edarryl/Publications/LeapSecond_camera.pdf">We collected data</a> from around 180 such servers around the world during the June 2015 leap second event, and assessed them from two points of view. </p>
<p>First, the clocks themselves: did they jump cleanly and sharply exactly as required?</p>
<p>Second, at the protocol level, that is with respect to the messages the servers send to the computers that rely on them: did they inform them properly of the upcoming leap?</p>
<p>Overall, we found that, at most, 61% of the servers were performing correctly. Many of the servers are well known and highly utilised, potentially impacting thousands of clients, possibly resulting in security vulnerabilities. </p>
<p>An expanded experiment is currently underway for the 2016 event, involving almost 500 servers, including from the widely used <a href="http://www.pool.ntp.org/en/">ntppool</a> project. </p>
<p>This is part of a broader network timing project at UTS led by myself together with Dr Yi Cao, which aims to refashion the global system, and in particular to make it scale in a trusted way to the <a href="https://theconversation.com/au/topics/internet-of-things-1724">Internet of Things</a>.</p>
<p>Finally, we must point out that leap seconds occur simultaneously across the globe, and it can’t be midnight everywhere.</p>
<p>Thus, as I confirmed with Dr Michael Wouters, responsible for Australia’s reference time at the <a href="http://www.measurement.gov.au/Pages/default.aspx">National Measurement Institute</a>, for us it will occur at 11am AEDT on January 1, 2017. Save the last sip till then.</p><img src="https://counter.theconversation.com/content/70583/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>This work was supported by the Australian Research Council and Symmetricom. </span></em></p>2016 has been a long year, but it’ll be made slightly longer care of a leap second. But why do we need such things?Darryl Veitch, Professor of Computer Networking, University of Technology SydneyLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/532652016-06-08T03:54:47Z2016-06-08T03:54:47ZWhy the Deep Space Atomic Clock is key for future space exploration<figure><img src="https://images.theconversation.com/files/118610/original/image-20160413-22045-1x9tufq.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">DSAC is prepping for a yearlong experiment to characterize and test its suitability for use in future deep space exploration.</span> <span class="attribution"><span class="source">Jet Propulsion Laboratory</span>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span></figcaption></figure><p>We all intuitively understand the basics of time. Every day we count its passage and use it to schedule our lives.</p>
<p>We also <a href="https://timeandnavigation.si.edu/">use time to navigate our way</a> to the destinations that matter to us. In school we learned that speed and time will tell us how far we went in traveling from point A to point B; with a map we can pick the most efficient route – simple.</p>
<p>But what if point A is the Earth, and point B is Mars – is it still that simple? Conceptually, yes. But to actually do it we need better tools – much better tools.</p>
<p>At NASA’s Jet Propulsion Laboratory, I’m working to develop one of these tools: the Deep Space Atomic Clock, or <a href="http://www.nasa.gov/mission_pages/tdm/clock/index.html">DSAC</a> for short. DSAC is a small atomic clock that could be used as part of a spacecraft navigation system. It will improve accuracy and enable new modes of navigation, such as unattended or autonomous.</p>
<p>In its final form, the Deep Space Atomic Clock will be suitable for operations in the solar system well beyond Earth orbit. Our goal is to develop an advanced prototype of DSAC and operate it in space for one year, demonstrating its use for future deep space exploration.</p>
<h2>Speed and time tell us distance</h2>
<p>To navigate in deep space, we measure the transit time of a radio signal traveling back and forth between a spacecraft and one of our transmitting antennae on Earth (usually one of NASA’s Deep Space Network complexes located in Goldstone, California; Madrid, Spain; or Canberra, Australia).</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/125453/original/image-20160606-31942-k1xnh1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/125453/original/image-20160606-31942-k1xnh1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/125453/original/image-20160606-31942-k1xnh1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=514&fit=crop&dpr=1 600w, https://images.theconversation.com/files/125453/original/image-20160606-31942-k1xnh1.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=514&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/125453/original/image-20160606-31942-k1xnh1.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=514&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/125453/original/image-20160606-31942-k1xnh1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=646&fit=crop&dpr=1 754w, https://images.theconversation.com/files/125453/original/image-20160606-31942-k1xnh1.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=646&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/125453/original/image-20160606-31942-k1xnh1.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=646&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The Canberra Deep Space Communications Complex in Australia is part of NASA’s Deep Space Network, receiving and sending radio signals to and from spacecraft.</span>
<span class="attribution"><span class="source">Jet Propulsion Laboratory</span>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>We know the signal is traveling at the speed of light, a constant at approximately 300,000 km/sec (186,000 miles/sec). Then, from how long our “two-way” measurement takes to go there and back, we can compute distances and relative speeds for the spacecraft. </p>
<p>For instance, an orbiting satellite at Mars is an average of 250 million kilometers from Earth. The time the radio signal takes to travel there and back (called its two-way light time) is about 28 minutes. We can measure the travel time of the signal and then relate it to the total distance traversed between the Earth tracking antenna and the orbiter to better than a meter, and the orbiter’s relative speed with respect to the antenna to within 0.1 mm/sec.</p>
<p>We collect the distance and relative speed data over time, and when we have a sufficient amount (for a Mars orbiter this is typically two days) we can determine the satellite’s trajectory. </p>
<h2>Measuring time, way beyond Swiss precision</h2>
<p>Fundamental to these precise measurements are atomic clocks. By measuring very stable and precise frequencies of light emitted by certain atoms (examples include hydrogen, cesium, rubidium and, for DSAC, mercury), an atomic clock can regulate the time kept by a more traditional mechanical (quartz crystal) clock. It’s like a tuning fork for timekeeping. The result is a clock system that can be ultra stable over decades.</p>
<p>The precision of the Deep Space Atomic Clock relies on an inherent property of mercury ions – they transition between neighboring energy levels at a frequency of exactly 40.5073479968 GHz. DSAC uses this property to measure the error in a quartz clock’s “tick rate,” and, with this measurement, “steers” it towards a stable rate. DSAC’s resulting stability is on par with ground-based atomic clocks, gaining or losing less than a microsecond per decade. </p>
<p>Continuing with the Mars orbiter example, ground-based atomic clocks at the Deep Space Network <a href="http://dx.doi.org/10.2514/6.2014-1856">error contribution</a> to the orbiter’s two-way light time measurement is on the order of picoseconds, contributing only fractions of a meter to the overall distance error. Likewise, the clocks’ contribution to error in the orbiter’s speed measurement is a minuscule fraction of the overall error (1 micrometer/sec out of the 0.1 mm/sec total). </p>
<p>The distance and speed measurements are collected by the ground stations and sent to teams of navigators who process the data using sophisticated computer models of spacecraft motion. They compute a best-fit trajectory that, for a Mars orbiter, is typically accurate to within 10 meters (about the length of a school bus).</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/112618/original/image-20160223-16416-e9vv59.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/112618/original/image-20160223-16416-e9vv59.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/112618/original/image-20160223-16416-e9vv59.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=350&fit=crop&dpr=1 600w, https://images.theconversation.com/files/112618/original/image-20160223-16416-e9vv59.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=350&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/112618/original/image-20160223-16416-e9vv59.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=350&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/112618/original/image-20160223-16416-e9vv59.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=440&fit=crop&dpr=1 754w, https://images.theconversation.com/files/112618/original/image-20160223-16416-e9vv59.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=440&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/112618/original/image-20160223-16416-e9vv59.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=440&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The DSAC Demonstration Unit (shown mounted on a plate for easy transportation).</span>
<span class="attribution"><span class="source">Jet Propulsion Laboratory</span>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<h2>Sending an atomic clock to deep space</h2>
<p>The ground clocks used for these measurements are the size of a refrigerator and operate in carefully controlled environments – definitely not suitable for spaceflight. In comparison, DSAC, even in its current prototype form as seen above, is about the size of a four-slice toaster. By design, it’s able to operate well in the dynamic environment aboard a deep-space exploring craft.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/112619/original/image-20160223-16429-1kljchg.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/112619/original/image-20160223-16429-1kljchg.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/112619/original/image-20160223-16429-1kljchg.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=216&fit=crop&dpr=1 600w, https://images.theconversation.com/files/112619/original/image-20160223-16429-1kljchg.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=216&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/112619/original/image-20160223-16429-1kljchg.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=216&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/112619/original/image-20160223-16429-1kljchg.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=272&fit=crop&dpr=1 754w, https://images.theconversation.com/files/112619/original/image-20160223-16429-1kljchg.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=272&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/112619/original/image-20160223-16429-1kljchg.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=272&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">DSAC mercury ion trap housing with electric field trapping rods seen in the cutouts.</span>
<span class="attribution"><span class="source">Jet Propulsion Laboratory</span>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>One key to reducing DSAC’s overall size was miniaturizing the mercury ion trap. Shown in the figure above, it’s about 15 cm (6 inches) in length. The trap confines the plasma of mercury ions using electric fields. Then, by applying magnetic fields and external shielding, we provide a stable environment where the ions are minimally affected by temperature or magnetic variations. This stable environment enables measuring the ions’ transition between energy states very accurately.</p>
<p>The DSAC technology doesn’t really consume anything other than power. All these features together mean we can develop a clock that’s suitable for very long duration space missions.</p>
<p>Because DSAC is as stable as its ground counterparts, spacecraft carrying DSAC would not need to turn signals around to get two-way tracking. Instead, the spacecraft could send the tracking signal to the Earth station or it could receive the signal sent by the Earth station and make the tracking measurement on board. In other words, traditional two-way tracking can be replaced with one-way, measured either on the ground or on board the spacecraft.</p>
<p>So what does this mean for deep space navigation? Broadly speaking, one-way tracking is more flexible, scalable (since it could support more missions without building new antennas) and enables new ways to navigate.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/112620/original/image-20160223-16429-10eq5ad.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/112620/original/image-20160223-16429-10eq5ad.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/112620/original/image-20160223-16429-10eq5ad.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=341&fit=crop&dpr=1 600w, https://images.theconversation.com/files/112620/original/image-20160223-16429-10eq5ad.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=341&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/112620/original/image-20160223-16429-10eq5ad.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=341&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/112620/original/image-20160223-16429-10eq5ad.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=428&fit=crop&dpr=1 754w, https://images.theconversation.com/files/112620/original/image-20160223-16429-10eq5ad.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=428&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/112620/original/image-20160223-16429-10eq5ad.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=428&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">DSAC enables the next generation of deep space tracking.</span>
<span class="attribution"><span class="source">Jet Propulsion Laboratory</span>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<h2>DSAC advances us beyond what’s possible today</h2>
<p>The Deep Space Atomic Clock has the potential to solve a bunch of our current space navigation challenges.</p>
<ul>
<li><p>Places like <a href="http://mars.jpl.nasa.gov/">Mars</a> are “crowded” with many spacecraft: Right now, there are five orbiters competing for radio tracking. Two-way tracking requires spacecraft to “time-share” the resource. But with one-way tracking, the Deep Space Network could support many spacecraft simultaneously without expanding the network. All that’s needed are capable spacecraft radios coupled with DSAC.</p></li>
<li><p>With the existing Deep Space Network, one-way tracking can be conducted at a higher-frequency band than current two-way. Doing so improves the <a href="http://dx.doi.org/10.2514/6.2014-1856">precision of the tracking data by upwards of 10 times</a>, producing range rate measurements with only 0.01 mm/sec error.</p></li>
<li><p>One-way uplink transmissions from the Deep Space Network are very high-powered. They can be received by smaller spacecraft antennas with greater fields of view than the typical high-gain, focused antennas used today for two-way tracking. This change allows the mission to conduct science and exploration activities without interruption while still collecting high-precision data for navigation and science. As an example, use of one-way data with DSAC to determine the gravity field of Europa, an icy moon of Jupiter, can be achieved in a third of the time it would take using traditional two-way methods with the flyby mission <a href="http://www.jpl.nasa.gov/missions/europa-mission/">currently under development</a> by NASA.</p></li>
<li><p>Collecting high-precision one-way data on board a spacecraft means the data are available for real-time navigation. Unlike two-way tracking, there is no delay with ground-based data collection and processing. This type of navigation could be crucial for robotic exploration; it would improve accuracy and reliability during critical events – for example, when a spacecraft inserts into orbit around a planet. It’s also important for human exploration, when astronauts will need accurate real-time trajectory information to safely navigate to distant solar system destinations.</p></li>
</ul>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/125593/original/image-20160607-15049-1aoivp3.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/125593/original/image-20160607-15049-1aoivp3.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/125593/original/image-20160607-15049-1aoivp3.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=306&fit=crop&dpr=1 600w, https://images.theconversation.com/files/125593/original/image-20160607-15049-1aoivp3.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=306&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/125593/original/image-20160607-15049-1aoivp3.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=306&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/125593/original/image-20160607-15049-1aoivp3.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=385&fit=crop&dpr=1 754w, https://images.theconversation.com/files/125593/original/image-20160607-15049-1aoivp3.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=385&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/125593/original/image-20160607-15049-1aoivp3.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=385&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The Next Mars Orbiter (NeMO) currently in concept development by NASA is one mission that could potentially benefit from the one-way radio navigation and science that DSAC would enable.</span>
<span class="attribution"><span class="source">NASA</span>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<h2>Countdown to DSAC launch</h2>
<p>The DSAC mission is a hosted payload on the <a href="http://www.sst-us.com">Surrey Satellite Technology</a> <a href="http://www.sst-us.com/missions/otb/otb/otb-the-mission">Orbital Test Bed</a> spacecraft. Together with the DSAC Demonstration Unit, an ultra stable quartz oscillator and a GPS receiver with antenna will enter low altitude Earth orbit once launched via a SpaceX Falcon Heavy rocket in early 2017.</p>
<p>While it’s on orbit, DSAC’s space-based performance will be measured in a yearlong demonstration, during which Global Positioning System tracking data will be used to determine precise estimates of OTB’s orbit and DSAC’s stability. We’ll also be running a carefully designed experiment to confirm DSAC-based orbit estimates are as accurate or better than those determined from traditional two-way data. This is how we’ll validate DSAC’s utility for deep space one-way radio navigation.</p>
<p>In the late 1700s, navigating the high seas was forever changed by <a href="http://www.rmg.co.uk/discover/explore/longitude-found-john-harrison">John Harrison’s</a> development of the <a href="http://collections.rmg.co.uk/collections/objects/79142.html">H4</a> “sea watch.” H4’s stability enabled seafarers to accurately and reliably determine longitude, which until then had eluded mariners for thousands of years. Today, exploring deep space requires traveling distances that are orders of magnitude greater than the lengths of oceans, and demands tools with ever more precision for safe navigation. DSAC is at the ready to respond to this challenge.</p><img src="https://counter.theconversation.com/content/53265/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Todd Ely receives funding from NASA through Caltech for the DSAC mission at JPL.
</span></em></p>Measuring time is a crucial part of navigation – particularly in space, where exacting precision is called for. The DSAC is poised to make a change that will aid future deep space missions.Todd Ely, Principal Investigator on Deep Space Atomic Clock Technology Demonstration Mission, Jet Propulsion Laboratory, NASALicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/554082016-05-16T02:04:10Z2016-05-16T02:04:10ZA brief history of telling time<p>We live in a world where time is all important. Nanoseconds mark the difference between success or failure to make an electronic transaction and where we are continuously reminded of “the time”: of being early or late, of having missed an appointment or arriving “before time”. In today’s world, time now governs our life.</p>
<p>In his bestseller, <a href="https://books.google.co.uk/books/about/A_Brief_History_Of_Time.html?id=9ysba1A1UF8C&source=kp_cover&redir_esc=y">A brief history of time</a>, physicist Stephen Hawking reminded us that: “The increase of disorder or entropy is what distinguishes the past from the future, giving a direction to time.” </p>
<p>There is no evidence that we can move backwards in time or that “time tourists” from the future are with us. But the arrow of time does carry us forward, and humans have measured this time through the ages in different ways.</p>
<h2>Sundials and water clocks</h2>
<p>We will never know who was the first man or woman to try to give structure to the measurement of time, although in the Bible, the book of Genesis exemplified change on a day-to-day basis, and with evening and morning. The Ancient Egyptians used simple sundials and divided days into smaller parts, and it has been suggested that <a href="http://curious.astro.cornell.edu/physics/161-our-solar-system/the-earth/day-night-cycle/761-why-is-a-day-divided-into-24-hours-intermediate">as early as 1,500BC</a>, they divided the interval between sunrise and sunset into 12 parts.</p>
<figure class="align-right ">
<img alt="" src="https://images.theconversation.com/files/113266/original/image-20160229-4105-pbtdph.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/113266/original/image-20160229-4105-pbtdph.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=656&fit=crop&dpr=1 600w, https://images.theconversation.com/files/113266/original/image-20160229-4105-pbtdph.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=656&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/113266/original/image-20160229-4105-pbtdph.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=656&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/113266/original/image-20160229-4105-pbtdph.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=825&fit=crop&dpr=1 754w, https://images.theconversation.com/files/113266/original/image-20160229-4105-pbtdph.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=825&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/113266/original/image-20160229-4105-pbtdph.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=825&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">An ancient Egyptian sundial.</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File%3AAncient-egyptian-sundial.jpg">University of Basel</a></span>
</figcaption>
</figure>
<p>Our familiar divisions of time are more recent and current terminology about time and time-keeping originated from the Babylonians and the Jews (the <a href="http://www.todayifoundout.com/index.php/2013/04/the-origin-of-the-7-day-week-and-the-names-of-the-days-of-the-week/">seven-day week</a> in Genesis). The Ancient Romans, during the republic, went with eight days – including a shopping day where people would buy and sell things. When the Roman emperor Constantine made Christianity the state religion early in the 4th century AD, the <a href="http://www.hermetic.ch/cal_stud/hlwc/why_seven.htm">seven‑day week was officially</a> adopted. </p>
<p>The sundial (of course an effective instrument only when the sun shines) was refined by the Greeks and taken further by the Romans a few centuries later. The Romans also used water clocks which they calibrated from a sundial and so they could measure time even when the sun was not shining, at night or on foggy days. Known as a <em><a href="http://www.britannica.com/technology/clepsydra">clepsydra</a></em>, it uses a flow of water to measure time. Typically a container is filled with water, and the water is drained slowly and evenly out of the container – markings are used to show the passage of time. </p>
<p>But the changing length of the day with the seasons in the Roman world made time measurement much more fluid than today: hours were originally calculated for daytime and based on a division of the day. The water clock made it possible to measure time in a simple and reasonably reliable way.</p>
<h2>Clocks come of age</h2>
<p>The better measurement of time has been a human fascination for centuries but in the 18th century the clock emerged as a scientific instrument in its own right, notwithstanding its conventional role to mark the passing of the hours.</p>
<p>The pendulum clock owes is <a href="http://www.cs.rhul.ac.uk/%7Eadrian/timekeeping/galileo/">refinement to Galileo noticing</a> the regularity of a suspended lamp swinging back and forth in the cathedral of Pisa, when he was still a student there.</p>
<p>The high water mark of an instrument of measuring time that was both perfectly fit for purpose and elegant was the marine chronometer <a href="https://theconversation.com/the-longitude-problem-how-we-figured-out-where-we-are-16151">invented by John Harrison in England</a>. It was a response to the need to measure time on board ship to a high level of precision, and so to be able to determine longitude (the pendulum clock was unsuitable for marine use due to the motion of the ship).</p>
<p>Harrison’s device drew on his brilliance in design and knowledge of the best materials. His clock enabled the measurement of time, and so a position at sea, to high accuracy. It gave the Royal Navy an unprecedented tool for navigation.</p>
<p>The work of 20th-century watch and clock makers continued that tradition – the skill of George Daniels in Britain in creating some of the best and most beautiful timepieces using traditional and hand-crafted methods can be seen in the permanent exhibition now at the <a href="http://www.salonqp.com/updates/watch-news/clockmakers-collection-reopens-at-londons-science-museum/">Science Museum in London</a>. </p>
<h2>Atoms and lasers</h2>
<p>Measuring time also changed in the 20th century changed through the <a href="http://www.npl.co.uk/60-years-of-the-atomic-clock/">development of the atomic clock in the 1950s</a> at the National Physical Laboratory. This allowed for new and better definition of time, and the second as its prime measure.</p>
<p>The invention of the laser in 1960 changed time measurement for ever. Lasers can produce pulses of a duration of a few attoseconds – 10⁻¹⁸ seconds – and the accuracy of international time measurement must reflect this. </p>
<p>Time today is defined not by a second that we may have expected to be a fraction – 1/86,400 – of the day. Instead, it is through an atomic frequency: formally done through something called the “caesium standard”. This <a href="http://www.livescience.com/32660-how-does-an-atomic-clock-work.html">measures the exact</a> number of “cycles” of radiation – 9,192 631,770 – that it takes for a caesium 133 atom to transition from one state of energy into another. </p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/OcDJX02PBPk?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
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<p>Time has moved away from terrestrial measurement to a measurement that could, in principle, be carried out on another planet or across the universe. The accuracy of this atomic time continues to be refined through research, and <a href="http://www.npl.co.uk/educate-explore/what-is-time/">work at National Physical Laboratory in the UK</a> is a world-leading presence. </p>
<p>And the future? To quote Hawking again: “Only time (whatever that may be) will tell.” We know it will involve the ongoing work of scientists to allow the accuracy with which we measure time to increase as we inevitably, it seems, find our lives becoming more ruled by time, its measurement and how it dictates what we do and when we do it. </p>
<p><em><a href="https://theconversation.com/about-time-episode-one-of-the-anthill-podcast-59355">The first episode of The Anthill</a>, a podcast by The Conversation, looks at different aspects of time: telling it, perceiving it, doing it and travelling through it.</em></p><img src="https://counter.theconversation.com/content/55408/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Kenneth Grattan does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>From sundials to atomic clocks, a journey through the way humans have measured time.Kenneth Grattan, George Daniels Professor of Scientific Instrumentation, City, University of LondonLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/593552016-05-16T02:04:05Z2016-05-16T02:04:05ZAnthill 1: About time<figure><img src="https://images.theconversation.com/files/130280/original/image-20160712-9281-1g34ia7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Tick tock.</span> <span class="attribution"><span class="source">shutterstock.com</span></span></figcaption></figure><p>Welcome to <a href="http://theconversation.com/the-anthill-a-new-podcast-from-the-conversation-59354">The Anthill</a>, The Conversation UK’s inaugural podcast. About time you might think, and so do we. So for our first episode we’ve taken that to heart and talked to a group of academics on the theme of time. </p>
<p>In a brief history of telling time, we ask physicist Kenneth Grattan about how humans perfected the measurement of time, from sundials to atomic clocks. Historian Richard Evans explains how time zones were created, and computer scientist Markus Kuhn tells us why the world would be in a spin without leap seconds. </p>
<p>To answer the age-old question of why exactly “time flies when we’re having fun”, Marc Buehner, a cognitive psychologist, explains why we perceive time the way we do. (Spoiler: he also reveals why it is we can’t tickle ourselves.)</p>
<p>Then theoretical physicist Marika Taylor answers the question: “Will time travel ever be possible?” </p>
<p>And, to take a slightly different angle on the theme of time, we hear from David Herd about the experiences of asylum seekers who live in a world of limbo. </p>
<p>We hope you enjoy listening. </p>
<hr>
<p><em>This episode of The Anthill was produced by Gemma Ware and Annabel Bligh, with interview help from Josephine Lethbridge and Michael Parker. Editing help from Emily Brown and Ally Kingston.</em> </p>
<p><em>The Anthill theme music is by <a href="http://www.melodyloops.com/search/How+to+Steal+a+Million+Dollars/">Alex Grey for Melody Loops</a>. Sound effects and background music by CorsicaS, eliasheuninck, bone666138 and digifishmusic via <a href="http://www.freesound.org/">freesound.org</a>.</em> </p>
<p><em>A big thanks to City University London’s Department of Journalism and to Dave Goodfellow.</em></p><img src="https://counter.theconversation.com/content/59355/count.gif" alt="The Conversation" width="1" height="1" />
A podcast on time: telling it, perceiving it, doing it and travelling through it.Annabel Bligh, Business & Economy Editor and Podcast Producer, The Conversation UKGemma Ware, Head of AudioLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/381092015-08-03T20:06:07Z2015-08-03T20:06:07ZSharper GPS needs even more accurate atomic clocks<figure><img src="https://images.theconversation.com/files/83221/original/image-20150528-11319-edvifk.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">How accurate is that GPS navigation? </span> <span class="attribution"><a class="source" href="https://www.flickr.com/photos/mwichary/2829184329/">Flickr/Marcin Wichary</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span></figcaption></figure><p>The <a href="https://theconversation.com/explainer-what-is-gps-12248">GPS network</a> might just be Earth’s greatest piece of infrastructure. It’s effectively a collection of clocks in space that serve up time information 24/7 free of charge to anyone on the planet who cares to listen.</p>
<p>These timing signals have all sorts of important applications, but most people will be using them to help get from A to B with the aid of GPS navigation tools. Well, actually to within about 10 metres of B. On the scale of humans and cars that’s a fairly substantial margin of error, as anyone who’s missed that left turn can attest to. </p>
<p>Up and coming technologies such as self-driving cars will probably rely upon a combination of local sensing and GPS signals to navigate independently without incident. So any improvement in GPS accuracy would be hugely advantageous in speeding up and rolling out the era of the autonomous car.</p>
<p>A GPS receiver listens and compares the different timing signals from the GPS satellites and then uses that information to calculate exactly where on Earth you are.</p>
<p>A variety of factors conspire against the accuracy of GPS navigation. Right now, for most civilians, the primary offender is the Earth’s ionosphere, which interferes with the timing signals as they commute from a satellite to your GPS receiver of choice.</p>
<p>But the second biggest contribution of error comes from the stability of the clocks onboard the GPS satellites.</p>
<h2>Timing is everything</h2>
<p>Every single GPS satellite is home to a family of atomic clocks (typically four) that derive their time from cesium (<a href="https://www.webelements.com/caesium/">Cs</a>) or rubidium (<a href="https://www.webelements.com/rubidium/">Rb</a>) atoms.</p>
<p>What is actually being measured in these clocks is the energy difference between two specific atomic states. When an atom changes from the high-energy state to the lower energy state, the energy difference is emitted in the form of light. The frequency, or ticking rate, of this light is what we count and how we define time. The crucial part is that, fundamentally, this energy difference is always the same.</p>
<p>All clocks – be they wrist, grandfather, atomic or otherwise – have some level of intrinsic error that causes them to lose seconds or drift away. Left to their own devices, the atomic clocks on board the GPS satellites would drift, meander or dawdle about by 10 nanoseconds a day.</p>
<p>That may not sound like much, but if you go ahead and multiply by the speed of light, you arrive at a GPS position error of three metres.</p>
<p>Thankfully, the GPS network is well monitored and corrections are applied to keep the clocks in-line, so that they are only responsible for about one or two metres of position error.</p>
<p>There is lots of work underway to improve the accuracy of GPS navigation, including different broadcast methods that can effectively eliminate the influence of the ionosphere.</p>
<p>But, ultimately, the performance of the clocks is fundamental to GPS. Clock technology is advancing all the time and with it comes lots of new opportunities for discovery and applications.</p>
<h2>Improved accuracy</h2>
<p>Here at the University of Western Australia our <a href="http://www.physics.uwa.edu.au/research/frequency-quantum-metrology">research group</a> is building an optical lattice clock based on ytterbium (<a href="https://www.webelements.com/ytterbium/">Yb</a>) atoms. </p>
<p>In addition to the ytterbium atoms, we also have an ultra-stable laser and a frequency comb: all the necessary components to produce an incredibly accurate optical atomic clock. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/84807/original/image-20150612-1471-7mftto.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/84807/original/image-20150612-1471-7mftto.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/84807/original/image-20150612-1471-7mftto.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=197&fit=crop&dpr=1 600w, https://images.theconversation.com/files/84807/original/image-20150612-1471-7mftto.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=197&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/84807/original/image-20150612-1471-7mftto.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=197&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/84807/original/image-20150612-1471-7mftto.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=247&fit=crop&dpr=1 754w, https://images.theconversation.com/files/84807/original/image-20150612-1471-7mftto.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=247&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/84807/original/image-20150612-1471-7mftto.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=247&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The UWA ytterbium lattice clock.</span>
<span class="attribution"><span class="source"> Romain Bara-Maillet and John McFerran</span>, <span class="license">Author provided</span></span>
</figcaption>
</figure>
<p>This type of clock is currently one of the best that you can make, with similar designs elsewhere achieving accuracies 100,000 times better than what you would find on a GPS satellite.</p>
<p>They are purported to be accurate at the level of 10<sup>-18</sup> of a second. If two such clocks started running when the universe began 13.8 billion years ago, to this day they would agree to within 1 second.</p>
<p>When complete, the lattice clock will be the only one of its kind in the southern hemisphere. The clock, along with our other cutting-edge time and frequency technologies, will form a ground-station at UWA for participating in space-clock comparison experiments.</p>
<p>In fact, we need this impressive clock in Australia to play a crucial role in an upcoming European Space Agency experiment.</p>
<h2>Missions in space</h2>
<p>The Atomic Clock Ensemble in Space (<a href="http://www.esa.int/Our_Activities/Space_Engineering_Technology/Challenging_Einstein_on_the_ISS_ACES_takes_a_step_ahead">ACES</a>) mission will place a different type of <a href="http://missions-scientifiques.cnes.fr/PHARAO/index.htm">cold-atom clock</a> on board the International Space Station (ISS), one that is about 1,000 times more stable than your typical run-of-the-mill GPS atomic clock.</p>
<p>The ACES mission is on track for a 2017 launch. Successfully getting this thing up in to space and operating on the ISS without incident will be a pretty significant achievement by itself. It is an important stepping stone on the path towards setting up future space-clock networks.</p>
<p>Over the course of a few years the timing signal produced by the ACES clock will be compared against different types of clocks all over the world, including the ytterbium clock under development at UWA.</p>
<p>This will allow us to undertake some important tests of fundamental physics, such as testing gravitational redshift and searching for subtle changes in the fundamental constants of nature.</p>
<p>Outside of space missions there are still a whole bunch of things you can do with an extremely stable and accurate optical lattice clock. For example, as the ticking rate of these clocks is strongly dependent upon the strength of the local gravitational field, two clocks separated by 30cm height already run at noticeably different rates.</p>
<p>This could eventually lead to a network of such clocks being used to accurately map out the Earth’s gravitational field, which could be useful for minerals exploration.</p>
<p>Even though scientific progress can sometimes feel a bit slow, it’s only a matter of time before advanced clocks are going to be incorporated into upgraded GPS satellites and helping to accurately drive your (presumably autonomous) car here, there and everywhere.</p><img src="https://counter.theconversation.com/content/38109/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Work discussed in this article is partially funded by the Australian Research Council.</span></em></p>They are the most accurate clocks on the planet and a few more in space. So why are researchers trying to make atomic clocks even more accurate?Stephen Parker, Research Associate, The University of Western AustraliaLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/440772015-06-30T20:10:27Z2015-06-30T20:10:27ZTemporal flux: why we need to keep adding leap seconds<figure><img src="https://images.theconversation.com/files/86821/original/image-20150630-5846-zpfab3.jpg?ixlib=rb-1.1.0&rect=82%2C13%2C1426%2C1237&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Time can feel like that sometimes, even if the Earth's rotation isn't slowing down.</span> <span class="attribution"><span class="source">JD/Flickr</span>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span></figcaption></figure><p>Today at precisely 10am Australian Eastern Standard time, something chronologically peculiar will take place: there’ll be an extra second between 09:59:59 and 10:00:00. </p>
<p>This will make 1st July 2015 (or 30th June 2015 in many other parts of the world) one second longer than the length of a standard day. The culprit is a “leap second”, although it’s far from unique. In fact, it’ll be the 26th one we’ve had since they were first introduced in 1972. </p>
<h2>How long is a piece of day?</h2>
<p>Why do we need to add an extra second to the day? Historically the second had been defined as a fraction of the day: one 86,400th of the total time for the sun to return to the same position in the sky. </p>
<p>That was precise enough for most purposes, but by the early twentieth century, astronomers had determined that the Earth’s rotation was not constant. It was actually slowing down. This meant that a second defined in this fashion would slowly lengthen over time. </p>
<p>The development of <a href="http://www.npl.co.uk/60-years-of-the-atomic-clock/">atomic clocks</a> in the 1950s allowed the second to be defined with incredible accuracy, with a variance of only one part in 10<sup>14</sup>. </p>
<p>Thus was the second redefined in 1967 by the <a href="http://www.bipm.org/en/committees/cipm/">International Committee for Weights and Measures</a> in 1967. It was no longer pegged to the Earth’s rotation. Instead it was defined in terms of a very particular physical property of a caesium-133 atom. </p>
<p>This mechanical definition has disconnected the second from the length of the solar day. In fact, the tables turned and the day was subsequently redefined in terms of this newly established atomic second: 86,400 seconds make up a standard day. </p>
<p>The length of the solar day – or the time it actually takes the Earth to complete a rotation – is no longer precisely as long as a standard day, and it has not been for a century. This is because the Earth’s rotation continues to slow. </p>
<p>The main reason it’s lagging is tidal friction from the Moon, which by itself would increase the length of the day by <a href="http://eclipse.gsfc.nasa.gov/LEcat5/secular.html">2.3 milliseconds each century</a>. </p>
<p>However, other geological process on Earth that shift mass around will also have an effect on the rotation rate, since the system mus conserve its total <a href="http://hyperphysics.phy-astr.gsu.edu/hbase/amom.html">angular momentum</a>. This can end up increasing the Earth’s rotation rate as well as decreasing it. </p>
<p>For example, the 2005 earthquake in Indonesia that caused the tsunami also decreased the <a href="http://www.nasa.gov/home/hqnews/2005/jan/HQ_05011_earthquake.html">length of the day by 2.68 microseconds</a>. </p>
<p>So we have to keep adding leap seconds to keep the time of noon at Greenwich (<a href="http://wwp.greenwichmeantime.com/">Greenwich Mean Time</a>) in line with noon as measured by the atomic clock (<a href="http://www.bipm.org/en/bipm/tai/tai.html">International Atomic Time</a>). This guarantees that the solar time (the rise and fall of the sun) doesn’t fall too far out of sync with our clocks.</p>
<h2>Taking time</h2>
<p>The task of adding these seconds was initially taken on by the <a href="http://adsabs.harvard.edu/full/2000ASPC..208..175G">Bureau International de l'Heure</a>, the executive body of the <a href="https://en.wikipedia.org/wiki/International_Time_Bureau">International Commission of Time</a>, which itself was part of the International Astronomical Union (<a href="http://www.iau.org/">IAU</a>). </p>
<p>In 1987 the IAU created a new organisation, the International Earth Rotation and Reference Systems Service (<a href="http://www.iers.org/IERS/EN/Home/home_node.html">IERS</a>). And from 1st January 1988, it became responsible for the leap second. </p>
<p>The leap second itself is an irregular occurrence. Between 1990 and 1999 there were seven leap seconds added. Yet between 2000 and 2009, only two extra seconds were added. In fact, it is so irregular that leap seconds are only announced by the IERS six months in advance. </p>
<p>This is a headache for computer systems. Software that doesn’t know about the leap seconds may see time going backwards and crash. When the previous leap second was added in 2012 the computerised reservation system for the <a href="http://www.news.com.au/travel/travel-updates/flights-back-to-normal-after-system-crash/story-e6frfq80-1226413756216">airline Qantas collapsed</a> and up to 50 flights were delayed. Similar problems affected sites such as <a href="http://www.wired.com/2012/07/leap-second-bug-wreaks-havoc-with-java-linux">Reddit, Foursquare and LinkedIn</a>.</p>
<h2>Temporal flux</h2>
<p>The future of the leap second is currently being debated. The chaos that it can cause on the world’s computer systems may not be worth the continued consistency with the heavens. </p>
<p>The New York Stock Exchange plans to <a href="http://gpsworld.com/june-30-leap-second-worries-markets-internet/">close its after hours trading 30 minutes early</a>. The web services arm of the online retailer Amazon plans to change their definition of the second for the day, such that they retain the 86,400 seconds in a standard day. This would mean that the clocks of Amazon Web Services would be slightly different to civil time.</p>
<p>Does the day to day time used by humans even need to remain linked to astronomical time? If the second retains the definition that it has, the need for leap-seconds will only increase. In 100 years there will need to be one at least one a year. And in a thousand year’s time we will need to add a new leap second every couple of months. </p>
<p>There are already timing schemes that do not follow the civil definition of time, such as the <a href="http://www.bipm.org/en/bipm/tai/tai.html">International Atomic Time</a> and the <a href="https://en.wikipedia.org/wiki/Global_Positioning_System">Global Positioning System time</a>. Computer systems could be linked to either of these. </p>
<p>Thus, this peculiarly long minute on 1st July 2015 serves as a useful reminder that time is no simple thing. We might want it to be pure and ordered, but as long as we live on a giant ball of shifting molten rock orbited by another huge ball of rock, all careening through space around an enormous ball of exploding gas, then it’s inevitable that we’ll need to adjust our clocks from time to time.</p><img src="https://counter.theconversation.com/content/44077/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>David Parkinson receives funding from the Australian Research Council and the University of Queensland, and is affiliated with the Australian Research Council's Centre of Excellence for All-sky Astrophysics.</span></em></p>Keeping time isn’t easy, particularly as the Earth’s rotation is slowing down, so we need to keep adding troublesome leap seconds.David Parkinson, Researcher in astrophysics, The University of QueenslandLicensed as Creative Commons – attribution, no derivatives.