Read more: “Crunch time for physics: What’s next?“
BLAME the ancient Greeks: they started it. The idea seemed sensible enough. If we dig down, we can uncover the building blocks of reality: what matter is ultimately made of, what rules govern its behaviour. Dig down far enough and we will hit the ultimate gold – a theory of how everything works.
In some senses, we have been doing quite well. Quantum theory’s weirdness might bamboozle us, but the standard model of particle physics built upon it successfully reduces things to a few fundamental particles and just three forces. General relativity’s mind-warping treatment of space and time delivers a cosmology that describes with stunning accuracy a universe dominated by a fourth force, gravity. Yes, there are wrinkles in both theories, but surely they will be ironed out as we move towards (see diagram) an ultimate theory of physics that unifies the two?
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Put that to many physicists, and you will get a grumpy response. “Elementary particle physics is pretty much in the state chemistry was in with Dmitri Mendeleev,” during the creation of the periodic table, says at the University of Oxford. “It has been classifying things, and has recognised there is an underlying structure, but it has no idea what that underlying structure is.”
Crucially, quantum theory and general relativity remain fundamentally incompatible. That is not a problem when we use relativity to describe the very large – stars, galaxies, the cosmos – and quantum theory for the very small – molecules, atoms, subatomic particles. But for a full understanding of the universe we have to know how the tiny newborn universe became so big: going back towards the big bang requires both theories to work together. So too, possibly, does the existence of black holes. As Stephen Hawking and Jacob Bekenstein showed in the 1970s, these general-relativistic monsters might destroy information, something forbidden by quantum theory.
Even something as basic as space and time highlights how badly these two theories get along. Relativity’s space-time is a smooth four-dimensional sheet; the quantum field theories that underlie the standard model suggest space is pixelated into units with sizes of about 10-35 metres, and do not even treat time as a real and observable thing.
Asked to choose between the two theories, most physicists’ money is on quantum theory being “right”, because its mathematics is such a successful prism through which to view the world. Others, from Einstein onwards, have taken issue with quantum theory’s seeming “irreality” and spooky, counter-intuitive correlations between apparently unrelated objects. If we cannot find a convincing physical reason why these correlations are just so, they argue, perhaps quantum theory is just an approximation to something better.
Attempts to go beyond this impasse have drawn on favoured mathematical ideas such as symmetry. One result is supersymmetry, a theory widely regarded as a way station on the road to string theory, a favoured candidate for an ultimate theory. String theory predicts that space has hidden extra dimensions, invoking symmetries embedded in these dimensions to “fold” energy into geometric shapes that look like certain fundamental particles, or mimic the way space curves in the presence of mass.
The theory has produced some credible depictions of particles, among them the long-sought graviton, a quantum particle that would carry the force of gravity. It thus takes steps towards a unified picture of all four forces of nature on the basis of quantum theory. But like other proposed frameworks for a theory of everything (see “Six routes to an ultimate theory“), it has a big flaw. “String theory does predict new things, but they’re almost certainly not testable in the foreseeable future,” says of Arizona State University in Tempe.
That failing means the idea of a theory of everything has quietly disappeared, says of Radboud University in Nijmegen in the Netherlands. “For a while you would see it in papers, in the heyday of string theory, but it has gone totally out of fashion.” of Imperial College London goes further. A theory of everything is “psychologically compelling”, he says, but there is no reason to think one exists – or that we can find it. That we have got so far with mathematics is a remarkable fact, but it does not mean we can go all the way.
One problem is that mathematics provides infinite ways in which numbers and abstract quantities can be processed – but no indication of what exists beyond it. “Mathematics only reveals truth about abstract objects,” says Deutsch. “Physics is not so much trying to study those objects as to find out which one of them corresponds to reality.” The fraction of the pure mathematics we know about that we have used so far to construct physical theories is relatively small.
For example, all the relationships between particles and fields and space and time can be represented by a subset of mathematical operations that are computable on a Turing machine, the basis of all our computers. “We don’t know why,” says Deutsch. “It just seems to be a brute fact.” Progress towards a theory of everything may require branching into areas of mathematics that today are not regarded as computable.
Deutsch’s own hunch is even more radical. He says we must abandon the idea that has served us so well over the past century: that if we start with the mathematics, reality will follow. Instead we must look first to problems in our understanding of the physical universe – the universe’s missing matter, say, or why gravity is so much weaker than the other forces – and try to work out what changes to our world view might solve it. “Then you have to formulate it mathematically, and test it. It has to be in that order,” says Deutsch. “Quite a lot of theoretical physics tries to do the mathematics first, and that’s never going to work.”
Is it worth the effort? Davies thinks so – but only if we are clear that even a theory of everything, at least as physicists define it, will not contain all the answers. “It wouldn’t help solve problems of the origin of life or the nature of consciousness,” he says. of Harvard University puts things even more bluntly: “Even if we knew the ultimate underlying theory, how are you going to explain the fact that we’re here?”
Perhaps the greatest reason to continue the search, though, lies not in the goal but in the routes we take to reach it. Our greatest, life-changing scientific insights have come from attempts to simplify physics and bring together disparate areas of understanding. James Clerk Maxwell united our understanding of electricity and magnetism, providing the theoretical bedrock on which most modern technology has been built. Einstein unified ideas about energy and mass to come up with E=mc2 – opening up the nuclear age, among other things. “Historically, these things have usually led somewhere,” says Davies.
For a purist, this might be an excessively utilitarian take. But anyone who searches for an ultimate theory purely for its own sake risks ultimate disappointment, says Davies, and also risks making the same mistake as those who believed at the end of the 19th century that physics was complete. “You could come up with some marvellous scheme, and you could put it up in a stained glass window and celebrate it as a wonderful achievement of the human intellect. But there would always be the possibility that someone comes up with a better one.”
Six routes to an ultimate theory
STRING THEORY
Particles and forces exist as tiny vibrating strings. Different string vibrations produce particles with different characteristics: electrons, quarks, the Higgs boson. The action takes place in 10 (mainly tiny) dimensions.
M-THEORY
A more general 11-dimensional theory that ties together variants of string theory.
LOOP QUANTUM GRAVITY
Breaks space up into small chunks and uses knot theory to tangle up links between them, giving these links the characteristics of elementary particles.
CAUSAL DYNAMICAL TRIANGULATION
Proceeds in a similar way to loop quantum gravity, but involves time as well as space.
AN EXCEPTIONALLY SIMPLE THEORY OF EVERYTHING
A 2007 paper by the independent researcher Garrett Lisi caused a rumpus by claiming a unified theory of all forces lies in the symmetries of a mathematical object known as the E8 Lie group. But even its author admits it is so far little more than a “speculative proposal”.
INFORMATION THEORY
Thermodynamics rules everything, including the flow of information. A better understanding of how might provide clues as to why things are as they are – and narrow down the parameters of any ultimate theory.
This article appeared in print under the headline “Are we nearly there yet?”
