The bartender says, “We don’t serve your kind in here”
A faster-than-light neutrino walks into a bar …
The media is champing at the bit to proclaim a discovery of faster-than-light travel by a subatomic particle, with some going as far as claiming “Einstein was wrong: relativity theory busted”.
The scientists responsible for the experiment and analysis let slip they have some preliminary data that suggests the particles travelled faster than light, but they seem to be the only ones not jumping to conclusions just yet.
The team at the Oscillation Project with Emulsion-tRacking Apparatus (OPERA) in Italy regularly measures the detection of neutrinos emitted from another experiment at the Large Hadron Collider (LHC) in Switzerland, 730 kilometres away.
Neutrinos (electrically neutral subatomic particles) are rather indifferent to the presence of trivial things such as Earth, and zip through without so much as a passing interest (their cross-section, the probability for interaction, is extremely small). Owing to their small mass, they should do so at approximately the speed of light, c (see video below) – the speed light travels in a vacuum, known quite well to be 299,792,458 metres per second.
Using GPS timing and position data, the OPERA team claim to know the distance between the point at which neutrinos are emitted from the LHC and the point at which they are detected in Italy to a precision that allows them to predict the time the neutrinos should arrive to within ten nano-seconds (a nanosecond being a billionth of a second).
What they claim to have found, though, is neutrinos arriving 60 nano-seconds (0.00000006 seconds) early. If accurate, this would be a six standard-deviation result – enough to convince physicists that something is genuinely awry.
The scientists concerned have released the findings to the scientific community in the hope that, if something has been overlooked, it will be picked up by their peers. The peer-review process is usually quite efficient at eliminating likely sources of error, and in this case there are plenty of possibilities. But on the face of it, it seems the OPERA team has been very careful.
There’s the issue of knowing the exact positions of the source and detector to within the quoted uncertainty – keeping in mind that in the extra 60 nano-seconds the neutrinos are supposedly travelling they will cover a total of 18 metres. This means knowing those two positions – and the geodesic distance between them – to within three metres out of 730,000 metres.
Traditional civilian grade GPS has an accuracy of about 15 metres. More sophisticated methods are used for proper surveying, such as differential GPS (10-centimetres accuracy). At the very top range is “carrier phase tracking”, which can beat one-centimetre accuracy.
This does require the GPS antenna to be above ground, though, so one also needs to take into account the timing for signals to travel along wires to the underground experiments.
The OPERA scientists made use of the more precise GPS system and a cesium atomic clock to ensure their timing and positions were as accurate as possible.
Presuming for now all the possible sources of error are accounted for, what would this result mean? Time-travel seems to be the go-to topic when faster-than-light particles are mentioned, but don’t hold out hope for a TARDIS just yet.
If a particle is able to travel faster than c, a few odd things happen. Critically, it breaks (special) relativity, which states there’s an absolute speed-limit – the speed at which massless particles travel – that doesn’t depend on relative motion.
One practical aspect of relativity is that the concept of simultaneity is frame-dependent. If two events occur at different locations (say, flashing a torch) then, depending on how you are travelling relative to each of those events, you may see them occurring at different times (for instance, if you are accelerating relative to one then you will see it occur later, as if time is slowed). The order in which you observe them to occur depends on the relative motion.
Now, if one of those events was flashing a torch (photons) and the other was flashing super-luminous particles (travelling faster than light) your interpretation would not just be that they occurred at different times, but that one must have travelled back in time.
So, the particles can appear to travel back in time, but there’s still no method of accelerating a cyborg killing-machine to super-luminal speeds.
The peer-review process is an important step to deciding whether or not to believe a particular result, but is the latest potential finding an isolated incident? Apparently not. In 2007 the MINOS experiment observed the same thing, albeit with a smaller significance (1.8 standard deviations – not enough to get excited about).
Measurements of arrival times of photons and neutrinos from supernova SN1987a in 1987 provided a much better agreement with the speed of light, but those neutrinos were of a much lower energy. The possibility remains that velocity depends on energy.
Somewhat less rigid explanations include neutrinos taking “shortcuts” through extra dimensions. Undoubtedly, many more possible explanations will arise if all conventional sources of error are excluded.
The much more likely scenario is that the analysis has overlooked some seemingly insignificant but critical aspect, and that re-analysis will led to a very good agreement with the speed of light.
Should that be the case, the follow-up press-release will no doubt refer to the “Phantom of the OPERA”.