The Big Questions: Physics (28 page)

Perhaps most important of all, though, this breakthrough suggested that the seemingly different forces might not be all that different at heart, even though the weak force acts over the shortest ranges, and on uncharged neutrons, whereas the electromagnetic force acts over enormous distances, and on charged particles. In fact, not only can we not say which is the stronger force, we suddenly find ourselves facing a shocking question.

If the electromagnetic and the weak forces were once the same force, who is to say that spontaneous symmetry breaking didn’t give rise to all of the forces of nature? Perhaps we can’t say one force is the strongest, simply because they are all manifestations of one ancient superforce. In order to explore that possibility, we have to consider the remaining element: the strong nuclear force.

Just as the weak force had to exist in order to explain beta-decay, the mutual repulsion between protons in a nucleus made the strong force a necessity, otherwise the nucleus could not hold together. Strong is an appropriate name for the force: typically, it appears to have a hundred times the strength of the electromagnetic force that would tear the nucleus apart. Measuring its strength was the easy part of taming the strong force, however; explaining its existence was much more difficult. It’s not enough to know that such a gargantuan force is the only way that a nucleus can hold together. What creates it?

The ideas behind this strong force were developed in the early 1970s. It was known that quarks make up the protons and neutrons in the nucleus. Each quark has a characteristic that physicists call its colour. For this reason, the theory tying the strong force to the quarks is called ‘quantum chromodynamics’, or QCD. According to QCD, the strong force binds quarks together using an interaction that, unlike the electromagnetic and gravitational forces, does not diminish with distance. The force grows stronger as quarks move further apart just as if they were connected by a spring.

This peculiar property, which emerges from the equations of QCD, gives the strong force the power to bind quarks together wherever they might be found. Its nature is borne out by the fact that, despite many searches, we have never found a free-roaming lone quark. QCD says that the strong force is created by a boson known as a gluon. Gluons were seen in experiments for the first time in 1979. The theory was already on a firm footing by that
time, however: when quarks, complete with their predicted characteristics, began to be spotted in particle accelerators in the late 1960s and early 1970s, QCD was considered a proven theory.

But what really excited physicists was the fact that QCD is built on the same symmetry-breaking idea as the electroweak force. It seemed entirely plausible that they could be closely related – and pulled together into one description of the behaviour of matter: the ‘grand unified theory’, or GUT. And here is where the quest hits the skids. After three decades of searching, we are still not sure if the strong force really is from the same stable as the electroweak force.

The problem is that unification is far from straightforward. What is required is another symmetry, like the strangers gathered in a room scenario – but this time there are even more of them. Somehow this bunch of indistinguishable strangers has to spontaneously break up in a way that describes five different types of particles – the three differently coloured quarks and the electron and its associated neutrino – and three forces.

The unification is almost impossible to recreate on Earth: reaching the energy for this symmetry breaking requires particle accelerators 100 billion times more powerful than the Large Hadron Collider (LHC), our most powerful atom-smasher. However, there are other ways to test the idea. According to any grand unified theory, quarks must be able to change into electrons and neutrinos, and physicists’ best-looking candidate for this grand theory (known as SU(5) because of the five particles that arise from it) has just such a process up its sleeve. It involves the proton in a kind of radioactive decay, and makes a prediction about how often that will happen.

It’s just a shame it gets it so very wrong. The theory says a proton will last approximately 10
³³
years before decay. About a quarter of a century ago, physicists built huge tanks of highly purified water surrounded by detectors that would register such
an event occurring. From the theory, and the number of protons in their tanks, they expected a few decays per year. So far, though, they have seen nothing. We still have another shot, though – and this one might be vindicated in the LHC. It is called ‘supersymmetry’.

Supersymmetry arises from the fact that physicists split particles into two camps: the fermions, such as the electron and the quarks, which make up matter; and the bosons, such as the photon and the gluon, which create the forces. These two different kinds of particles follow two different sets of rules. And supersymmetry, or SUSY, says each one has a ‘superpartner’ from the other camp that will behave the same in any experiment.

That is possible because the essential difference between fermions and bosons comes from the quantum property known as spin. Bosons have integer spin – 1, 2, 3 and so on – while fermions have spins that are half-integers: 1/2, 3/2 etc. SUSY involves applying a kind of perspective change, something akin to looking at a clock from the front or the back. This change of view alters quantum spin (just as the sense of rotation of the clock’s hands is different when viewed from the back), but not other qualities, such as electric charge or quark colour.

It might sound like a convenient fiction, but it is a highly respected mode of thinking that stands amongst the best ideas in physics. The burning question, of course, is whether it is true. Besides spin, one other property of the superpartner particles is changed: their mass. They are much, much heavier than the set of particles that we are familiar with. That means that, thanks to
E = mc
²
, they will only exist at high energies. Thankfully, however, the LHC’s collision energy of 14 TeV should be high enough to see the lightest of them, which are thought to come into play at around 1 TeV.

Though it sounds promising, these particles are still difficult to detect. They hardly interact with normal matter, and will fly out
of the machine almost without trace. That means the only hint of supersymmetry might be some energy missing from the LHC’s detectors. Since other theories suggest that some normal particles might be disappearing into other, ‘hidden’ dimensions of reality, it’s a recipe for false positives and missed sightings.

If we do see unquestionable sightings of supersymmetric particles, though, we can feel sure that the grand unified theory of the forces of nature is on solid ground. It will be entirely reasonable to assume that the strong, weak, and electromagnetic forces arise from a common source, a ‘prime mover’, as the Greeks would say. There is just one fly in the ointment, though. What about gravity? Is that also part of the unification, or is it a separate entity? If we can’t say there is a strongest force, can we at least say that gravity is the weakest?

Gravity certainly is weak. When we draw the likely unification diagram for the forces, showing the energy where they (might) unite, it’s hard to put gravity on there, unless your graph is bigger than the known universe. While the other forces converge from a factor of 100 or so apart, gravity is simply off the scale. But
there is a get-out. It is enormously technical, but, boiling it down, it says that the gravitational interaction depends on mass, which is proportional to the energy involved. In the SUSY picture, when considering the high-energy conditions of unification, gravity drops into the picture at an alluring scale – almost, but not quite, where the other forces unite.

It’s not a fully convincing answer, but it does suggest that gravity and all the other forces of nature might spring from one ultimate force. This superforce only existed in the first moments after the universe was formed. In that situation, asking which of the forces is stronger is like asking which of the particles is more particle-like. Though different, they are all aspects of one characteristic. Gravity vs electromagnetism just won’t work; they appear to be fighting from the same corner.

Beyond the quantum world lies the realm of information

This will be the last question physics answers – if, that is, it is even possible to do so. Constructing a grand unified theory of forces is all very well, and the hunt for quantum gravity, the theory that unites the science of the very small with the science of the very large, is exciting and useful. But neither of these will answer the fundamental question: what is reality made from?

Some may argue that this quest lies beyond the reaches of science. But it is in the very nature of physics to find answers to seemingly impossible questions. The history of physics is littered with ‘impossible’ tasks that have turned out to be very possible indeed. It is easy to forget that Archimedes stunned the ancient world with his innovative thinking. Whether or not he really worked out a way to tell if the king’s crown was made from pure gold, he achieved enough of a scientific reputation to have his life protected by Roman mandate. Similarly, Newton’s description of how gravity works seems obvious now, but its formulation was truly a
tour de force
at the time.

St Augustine described magnetism as not unlike a miracle, but now we know the microscopic processes behind the whole gamut of electromagnetic phenomena. The physics done in the past seems a little prosaic now; indeed, the concepts are so straightforward to us that we learn many of them as children. So, will schoolchildren of the future be bored in lessons on the fundamental nature of reality?

The nature of reality is an avenue that humans have tried to explore since at least the time of the ancient Greeks, and it is to then that we can trace our own quest for the nature of reality. The Greeks had several schools of thought on this question. Perhaps most influential was Plato, who believed in a realm of perfect abstractions of physical entities. Everything in the material world drew its existence from these ‘ideal forms’, and everything was but a shadow of the ideal object.

‘To Earth, then, let us assign the cubic form, for Earth is the most immovable of the four and the most plastic of all bodies, and that which has the most stable bases must of necessity be of such a nature.’

PLATO

This realm, accessible only through the training of the mind, was not just about physical objects, such as trees or mountains. It also applied to mathematical ideas: Plato envisioned an ideal mathematical reality populated by the ideal solids. These five geometrical shapes created a connection between the mathematical and the physical. In his dialogue
Timaeus
, for instance, Plato linked the cube to the Earth: ‘To Earth, then, let us assign the cubic form, for Earth is the most immovable of the four and the most plastic of all bodies, and that which has the most stable bases must of necessity be of such a nature.’ A similar logic linked the tetrahedron to fire, and the icosahedron to water, the octahedron to air and the dodecahedron to a mysterious ‘fifth element’, or quintessence.