At the beginning of the 20th century it might have seemed that there was nothing new to discover in physics. Not anymore. Today it is becoming increasingly clear that there are problems that physics – at least as we currently know it – isn’t able to solve. Perhaps we just need more data, perhaps we need a new fundamental theory of reality.
Hosted by Miriam Frankel, science editor at The Conversation, and supported by FQxI, the Foundational Questions Institute, the six-part series explores the greatest mysteries facing physicists today – and discuss the radical proposals for solving them.
Part 1: Time
We cannot imagine reality without time flowing through it. But on the most fundamental level, physicists aren’t even sure whether time actually flows or even exists. In our first episode, we look at whether it could potentially move backwards as well as forwards.
But if that’s true, then why is our experience of time moving from the past to the future so strong? One answer is that entropy, a measure of disorder, is always increasing in the universe. When you run the numbers, explains Sean Carroll, a physicist at Johns Hopkins University in the US, it turns out that the early universe had very low entropy. And in a closed system, entropy must always rise, creating an arrow of time.
Emily Adlam, a philosopher of physics at the Rotman Institute of Philosophy at the University of Western Ontario in Canada, on the other hand, believes the mystery of why our universe started with low entropy is a problem that ultimately stems from the fact that physics is riddled with assumptions about the time.
Adlam argues the best way to understand time would be to remove it entirely from our theories of nature – to strip it out of the equations. Interestingly, when physicists try to unite general relativity with quantum mechanics into a “quantum gravity” theory of everything, time often disappears from the equations.
Part 2: Fundamental constants
The values of many forces and particles in the universe, represented by some 30 so-called fundamental constants, all seem to line up perfectly to enable the evolution of intelligent life. But there’s no theory explaining what values the constants should have – we just have to measure them and plug their numbers into our equations to accurately describe the cosmos.
So why do the fundamental constants take the values they do? This is a question that physicists have been battling over for decades.
Some physicists aren’t too bothered by the seemingly fine-tuned cosmos. Others have found comfort in the multiverse theory. If our universe is just one of many, some would, statistically speaking, end up looking just like ours. But many physicists, including Paul Davies, a theoretical physicist at Arizona State University, are holding out for a more fundamental theory of nature which can explain exactly what values the constants should have in the first place.
That said, in the absence of a deeper theory, it’s hard to estimate exactly how fine-tuned our universe is. Fred Adams, a physicist at the University of Michigan, has done a lot of research to try to find out. He’s discovered that the mass of a quark called the down quark (quarks are elementary particle which make up the atomic nucleus, for example) can only change by a factor of seven before rendering the universe, as we know it, lifeless.
This suggests the universe is less fine tuned than a radio. Intriguingly, his work has also shown it is possible to get universes that are more life-friendly than ours.
Part 3: The multiverse
The idea that we live in a universe among many others has been around for a long time. But does it have any basis in science? And if so, is it a concept we could ever test experimentally?
Katie Mack, Hawking chair in cosmology and science communication at the Perimeter Institute for Theoretical Physics in Canada, argues the idea that the universe is much bigger than we can observe is totally well accepted in cosmology. It is also supported by theories known as inflation, which has lots of experimental support, and string theory.
For Andrew Pontzen, a professor of cosmology at University College London in the UK, quantum mechanics is the best reason to believe in the multiverse. According to quantum mechanics, particles can be in a mix of different possible states, such as locations, which is known as a “superposition”. But when we measure them, the superposition breaks and each particle randomly “picks” one state.
So what happens to the other possible outcomes? Well, they may all play out in different universes.
Not everyone is so keen, however. Sabine Hossenfelder, a research fellow at the Frankfurt Institute of Advanced Studies, believes the theory is near impossible to test – something Pontzen and Mack disagree with.
Part 4: Quantum weirdness
Quantum mechanics, which rules the microworld of atoms and particles, is famously weird. It even seems to suggest there’s no objective reality.
According to quantum theory, each system, such as a particle, can be described by a wave function, which evolves over time. The wave function allows particles to hold multiple contradictory features, such as being in several different places at once – this is called a superposition. But oddly, this is only the case when nobody’s looking.
Although each potential location in a superposition has a certain probability of appearing, the second you observe it, the particle randomly picks one – breaking the superposition. Physicists often refer to this as the wave function collapsing. But why should nature behave differently depending on whether we are looking or not? And why should it be random?
Not everyone is worried. “If you want to explain everything we can observe in our experiments without randomness, you have to go through some really weird and long-winded explanations that I am much more uncomfortable with,” argues Marcus Huber, a professor of quantum information at the Technical University of Vienna.
Chris Timpson, a philosopher of physics at the University of Oxford, talks us through the different interpretations of quantum mechanics. Chiara Marletto, a quantum physicist at the University of Oxford, talks about her meta-theory called constructor theory which aims to encompass all of physics based solely on simple principles about which physical transformations in the universe are ultimately possible, which are impossible, and why.
Part 5: Life
What’s the difference between a living collection of matter, such as a tortoise, and an inanimate lump of it, such as a rock? They are, after all, both just made up of non-living atoms. The truth is, we don’t really know yet. Life seems to just somehow emerge from non-living parts.
The physics of the living world ultimately seems to contradict the second law of thermodynamics: that a closed system gets more disordered over time, increasing in what physicists call entropy. Living systems have low entropy. A messy lump of tissue in the womb, for example, can grow into a highly ordered state, such as a foot with five toes.
In this episode we hear about two main approaches to a new physics of life. Jim Al-Khalili, a broadcaster and distinguished professor of physics at the University of Surrey in the UK, explains how quantum biology might help. It’s based on the strange world of quantum mechanics, which governs the microworld of particles and atoms. The idea is that living systems may use quantum mechanics to their advantage – promoting or halting quantum processes.
“Evolution has had long enough to fine-tune things or to stop quantum mechanics from doing something that life doesn’t want it to do,” explains Al-Khalili, who carries out research in the area.
Sara Walker, an astrobiologist and theoretical physicist working as a professor at Arizona State University in the US, favours another approach, however. She is trying to create a new physical theory of life based on information theory – which takes information to be real and physical.
Information seems to be crucial to life. Living organisms have an inbuilt set of instructions, DNA, which non-living things simply don’t have. Similarly, when living beings invent things, such as rockets, they rely on information, such as knowledge of the laws of physics, stored in their memory.
Part 6: Theory of everything
Finding a theory of everything – explaining all the forces and particles in the universe – is arguably the holy grail of physics. While each of its main theories works extraordinarily well, they clash also with each other – leaving physicists to search for a deeper, more fundamental theory.
Our two best theories of nature are quantum mechanics and general relativity, describing the smallest and biggest scales of the universe, respectively. Each is tremendously successful and has been experimentally tested over and over. The trouble is, they are incompatible with one another in many ways – including mathematically.
Physicists have already managed to unite quantum theory with Einstein’s other big theory: special relativity (explaining how speed affects mass, time and space). Together, these form a framework called “quantum field theory”, which is the basis for the Standard Model of Particle Physics – our best framework for describing the most basic building blocks of the universe.
But the standard model only describes three out of the four fundamental forces in the universe – electromagnetism, and the “strong” and “weak” forces which govern the atomic nucleus – excluding gravity. While the standard model explains most of what we see in particle physics experiments, there are a few gaps. And one way to bridge them could be to introduce a whole new force of nature – something recent experiments have seen hints of – explains Vlatko Vedral, a professor of physics at Oxford University.
But what should a theory of everything include? Would it be enough to unite gravity and quantum mechanics? And what about other mysterious properties such as dark energy, which causes the universe to expand at an accelerated rate, or dark matter, an invisible substance making up most of the matter in the universe?
As Chanda Prescod-Weinstein, an assistant professor in physics and astronomy at the University of New Hampshire, explains, physicists prefer to use the term “theory of quantum gravity” over “theory of everything”. She describes the two main attempts. One is string theory, which suggests the universe is ultimately made up of tiny, vibrating strings. Another is loop quantum gravity, which suggests Einstein’s space-time arises from quantum effects.
But is either of them correct? And do we actually need them?
Great Mysteries of Physics is created and presented by Miriam Frankel and produced by Hannah Fisher. Executive producers are Gemma Ware and Jo Adetunji. Social media and platform production by Alice Mason, sound design by Eloise Stevens and music by Neeta Sarl.