A TENACIOUS DREAMER, propped up against a pillar by the pool table, reminds us that it is also possible that what we think of as fundamental forces just aren’t. The quarks, leptons and bosons of the Standard Model may contain smaller constituents, just as the atoms of the Periodic Table turned out to be made of other things. The solutions to the outstanding problems of the Standard Model might then be found in the interactions of these new, even smaller, pieces. Again, this was partly motivated as a way to avoid the need for a Higgs boson. However, there are still versions around in which the Higgs too is a composite particle, made up of even smaller things.
Such a scenario would involve new fundamental forces, and probably mean that the forces we currently think of as fundamental are not, but emerge from some higher-energy, smaller-distance theory which lives out in the far east.
There are also ideas in which the current forces remain, but we add a new one, or change an existing one. Since gravity is a common thread in many of the problems with physics – Dark Matter, Dark Energy, the lack of quantum gravity in the first place – it is very reasonable to expect that General Relativity needs to be modified in some way. That’s a thought that has occurred to many physicists. However, General Relativity is so subtle, and so, well, general, that replacing it, or even successfully tweaking it, is a very hard thing to do.
Still, physicists are persistent, and there are new ideas coming forward all the time. One possible tweak is to postulate a new particle which carries a ‘fifth force’ on top of electromagnetism, the weak and strong interactions, and gravity. Maybe out in the far east there is some other form of transport, beyond our road, rail and airways, and distinct from gravity?
To explain Dark Energy, this force has to affect all matter – as gravity itself does – and operate over large distances. Such forces have been looked for already, and if they affect the motion of the planets in the Solar System, for example, they have to be enormously more feeble than gravity, otherwise we would have seen them already. But if they are enormously more feeble than the gravitational force between stars and galaxies, they won’t make any difference to the Dark Energy or Dark Matter problems, so that’s a waste of time.
One possible way around this conundrum is a process called ‘screening’, in which the strength of a force depends upon the environment it is in. It is even possible that such a force is screened by matter itself. It is possible to build theories whereby in dense regions of the universe (like the Earth, for instance) the force can be hidden, while in empty space the force can operate.
In the case of the Dark Energy problem, which is what some such theories are aiming to solve, this can provide exactly what the data need. The force can make the universe accelerate at large distances, while having no measurable effect on the orbit of the planets. As a bonus, this new force can also have a significant impact on the way galaxies rotate, which might at least partially address the Dark Matter issue as well.
The way this new force works is reminiscent of the way the theory of Brout, Englert and Higgs gives mass to fundamental particles. It involves a scalar boson – a particle like the Higgs boson, which has no spin – and it involves the idea of symmetry breaking. If that isn’t familiar enough to you to help, consider a pointillist painting, in which an image, and even the colour mix within it, is composed of many tiny dots.
When averaged over dense regions of space, a symmetry hides the fifth force. This is like looking at the picture from a metre or so away. The dots are hidden in the colours and landscape of the painting.
Close to the painting, the dots are visible. In the same way, for things the size of atoms or smaller, the averaging doesn’t happen, so the fifth force may show up.
And on very long distance scales, space is very empty, so the density is low and again the force reappears. Similarly, a very long way from the painting, it is just a single dot again.
By the standards of wild yarns, this idea seems at least to be testable in an excitingly wide range of experiments. Upcoming observatories will characterise gravity and Dark Energy on astrophysical scales. Precise atomic physics experiments could measure the effect of the fifth force on atoms, and the LHC may also produce, or rule out, some varieties of these weird, so-called chameleon particles.
A theoretical framework, such as is provided by supersymmetry, or the Standard Model of particle physics, plays the role of the picture on the box of a jigsaw puzzle.fn1 When looking at a jigsaw piece, the picture gives you an idea of where it might fit, and how it might connect to the others. Trying to do a jigsaw puzzle without looking at the picture on the box is enormously more difficult that looking at the big picture.
Of course, once you see where a piece might fit in, you still have to try it to see if it really does so. And in the science version of this puzzle, we also have to bear in mind that our picture is almost certainly incomplete (for example in the case of the Standard Model, which definitely describes lots of data) and possibly completely wrong (for example, supersymmetry or extra dimensions, where there is currently no data). But to some extent any picture is better than none, and anyway we don’t have much choice. At some point we might fit enough pieces together to realise it was wrong, throw it out and try a new picture. A sort of paradigm shift that would see a jigsaw puzzle manufacturer go out of business under the weight of returned Christmas presents.
The framework provided by a good theory, or collection of theories, gives focus to research and makes it harder for a new theory to gain acceptance. A maverick new theory – and there are many around – must either fit with the existing picture, or replace it completely. In the latter case, it has to accommodate all the jigsaw pieces that are already snugly interlocked. This, not a conspiracy of lizards or illuminati (or even hide-bound conservatives), is why a dramatic ‘Einstein was wrong’ type of idea is unlikely to be taken very seriously without a lot of supporting evidence, and the ability to accommodate previous evidence that he was pretty much right. It will generally not just postulate monsters out east, it will probably contradict things we already know about lands we have already explored in detail.
In the sense that it emphasises the importance of the whole, and the interdependencies of the parts, the approach described above is a holistic view of science. In an essay entitled ‘When Scientists Go Astray’, the great theoretical physicist Philip W. Anderson describes this as a ‘seamless web’:
a body of firmly established theory, now extending from physics through molecular biology, which in many situations, traps dubious observations. Already known laws, like conservation of energy, quantum mechanics, relativity, and the laws of genetics, constrain the explanation of any given result in a fashion which can be unique, or nearly so, and makes errors easy to spot. Much of science is ‘overdetermined’ in this sense.
It is by no means infallible but it is the best we have.