Why physics isn't dead
Relativity and quantum theory help us make sense of the universe – but physics still has huge mysteries left to unravel.
This article is a preview from the Spring 2017 edition of New Humanist. You can find out more and subscribe here.
You have climbed a mountain. Getting to the summit has taken every last ounce of energy, and you are exhausted but euphoric. As you pause to catch your breath, you look up at the next mountain in the chain – and gasp. It is a million billion times as high as the one you’ve just climbed.
This is the position that physicists find themselves in. They have used all their knowhow and ingenuity to build the Large Hadron Collider near Geneva. It has bagged them the Higgs boson, the particle that gives all other particles mass, and they are euphoric at their success. But their next great challenge involves working with objects far smaller, and a million billion times more energetic than anything the LHC can reach. The Planck energy is enough to make a grown physicist weep, but if we can scale these heights, we can find the point at which space and time and gravity emerge from something more fundamental; the ultimate secret of the origin of the universe.
The daunting challenge has led some to claim that fundamental physics is over. It has morphed, they say, into science fantasy, with theorists now free to publish any mad theories they can dream up, safe in the knowledge that no conceivable experiment will ever prove them wrong.
Nothing, however, could be further from the truth. There are two mathematical physics principles that we know are true which predict exactly what our current observations and experiments tell us about the world. The first is special relativity and the second is quantum theory. Physicists are not, it turns out, free to invent any theory they like. Their theory must be consistent with both special relativity and quantum theory. And this constraint is so ridiculously tight that the majority of theories that physicists come up with are instantly ruled out.
Relativity describes the world at speeds approaching that of light, a place where space shrinks, time slows and the two become mere facets of the same entity: space-time. Quantum theory describes atoms and their constituents, an Alice-in-Wonderland world where things happen for no reason, and subatomic particles can be in two places at once and influence each other instantaneously even if on opposite sides of the universe.
To emphasise how extraordinarily restrictive is the straitjacket of special relativity and quantum theory, imagine a very competent physicist who knows nothing about the world and who is locked in a windowless room with two blackboards. On Blackboard One are written the principles of special relativity and quantum theory. Blackboard Two is empty apart from a message: “Use this blackboard to deduce the consequences of the other blackboard.”
The physicist picks up a piece of chalk and begins scribbling. The first thing he thinks about is quantum spin. This is one of those weird quantum properties that have no analogue in the everyday world. Particles that have it behave as if they are spinning – only they are not. The property, like everything else in the microscopic world, is measured in discrete chunks, or “quanta”.
It might appear that a subatomic particle can possess any multiple of these chunks – for instance 9.5 or 27 or 801. However, our physicist quickly discovers that nature is far more restricted and must choose its spins from a much reduced palette. Out of an infinity of conceivable spins, only five are compatible with the twin constraints of special relativity and quantum theory: 0, 1/2, 1, 3/2 and 2.
The spin of a particle determines how it interacts with other particles and the phenomena for which it is responsible. Our physicist decides to consider particles of each spin, one at a time, and to write down on the empty blackboard everything he can deduce about them.
First, he discovers that quantum theory requires that particles with spin 1/2 (also known as “half-integer spin”) obey the so-called Pauli Exclusion Principle. This endows them with a strong tendency to avoid each other. The need for each such particle to have a lot of elbow room means that, when large numbers of them come together, they form spread-out, extended objects.
Particles with spin 1/2 – known as “quarks” and “leptons” – are the fundamental building blocks of matter. The electron, for instance, is a commonly occuring lepton. As the American physicist Richard Feynman said, “it is the fact that electrons cannot get on top of each other that makes tables and everything else solid.”
Next, our physicist considers particles of spin 1. His workings tell him that they can be exchanged between the building blocks of matter and that this exchange gives rise to forces. There are three possibilities which lead to three distinct fundamental forces of nature.
He’s right. The three “interactions” have been given the names electromagnetic force, weak nuclear force and strong nuclear force. The strong force binds triplets of quarks into protons and neutrons, and confines them in a “nucleus”. Electrons are then bound to a nucleus by the electromagnetic force to create an atom.
Our incarcerated physicist deduces not only the existence of 92 types of naturally occurring atom – from hydrogen, the lightest, to uranium, the heaviest – but a dizzying variety of chemical compounds that arise from all the ways in which the basic atomic building blocks may be combined.
So much for particles with spin 1/2 and spin 1; what about spin 0? Our physicist deduces that a particle with spin 0 creates a “field” which permeates all of space and resists the passage of other particles. By doing this, it gives them inertia, or mass. And yes, such a particle exists in the guise of the Higgs boson. Its discovery was triumphantly announced to the world by physicists at the LHC in July 2012.
Next, our physicist considers spin 2. He finds that a particle with spin 2 has the property that it interacts with every other particle, giving rise to a “universal force”. It takes a bit of calculation but he is able to show that an inevitable consequence of the existence of a spin 2 particle is the general theory of relativity, Einstein’s theory of gravity.
Studying general relativity, our physicist recognises the existence of a long-range law of attraction, which causes large bodies to orbit other large bodies. We of course know of planets that orbit stars and galaxies that orbit other galaxies. Our physicist locked in a windowless room knew of none of these. Remarkably, however, he has been able to deduce the existence of the large-scale universe. No one has yet found a particle of spin 2. And there is good reason to believe that, if it exists, it will be extremely difficult to detect. However, such a particle fits the bill for the “graviton”, the hypothetical carrier of the force of gravity.
Next, our physicist considers the one remaining spin: 3/2. Spin 3/2 particles permit “supersymmetry”, in which all the half-integer spin particles are merely the obverse face of integer-spin particles. As yet, we have no experimental evidence that nature uses particles of spin 3/2. But given that all the other spins in its palette exist, there is a strong suspicion that this does too.
The electron, for example, is hypothesised to have a supersymmetric twin, dubbed the “selectron”. The super-partners of known particles are considered to be good candidates for the Universe’s “dark matter”, which is known to outweigh the visible stars and galaxies by a factor of about six. The reason we have not yet detected supersymmetric particles, physicists suggest, is that they are very massive and that creating them requires more energy than is currently available in collisions at the Large Hadron Collider.
Although our physicist has now considered particles of every permissible spin and deduced their behaviour, there is one more thing that he can deduce from special relativity and quantum theory. According to the two principles, each subatomic particle must have a partner with opposite electric charge or spin.
Whenever a particle is created as a “quantum fluctuation” of the universe’s vacuum, it must always be accompanied by its “antiparticle”. For instance, a negatively charged electron is always conjured into existence alongside a positively charged “positron”.
So the full inventory of the world turns out to be the following: 12 basic building blocks (six kinds of quarks and six kinds of leptons, which carry force); 12 force-carriers (the “photon” of electromagnetic force, three “vector bosons” of weak nuclear force and eight “gluons” of strong nuclear force); plus the Higgs boson and all the anti-particles.
Collectively, these constitute the “Standard Model” of particle physics, the culmination of 350 years of toil by scientists. It is no exaggeration to say that the Standard Model + the general theory of relativity = the World.
Our physicist, locked in a windowless room with nothing more than a blackboard and chalk, has been able to deduce the main features of the world. As the American physicist Nima Arkani-Hamed says, “physics is shockingly constrained by quantum theory and relativity. They almost make the universe inevitable.”
Almost. The twin constraints do not determine the masses of the fundamental particles nor the total number of quarks and leptons. All normal matter is assembled from just four particles – the up-quark, down-quark, electron and electron-neutrino. (A proton in an atomic nucleus, for instance, is made of two up-quarks and a down-quark, and a neutron two down-quarks and an up-quark.)
However, nature has not stopped here. It has created heavier versions of the basic four particles – the strange quark, charmed quark, muon and muon-neutrino – and heavier still versions: the bottom quark and top quark, tau and tau-neutrino. Such particles play essentially no role in the Universe today, since the energy to create them existed only in the first split second of the Big Bang.
The Standard Model does not reveal why nature has triplicated its basic building blocks – or why the fundamental particles have the masses they do. It is a strong indication that the Standard Model is not the final word on nature but merely an approximation of a deeper theory, yet to be found. These shortcomings, however, should not detract from the fact that the principles of special relativity and quantum theory are an unbelievably tight straitjacket.
Physicists are not, after all, free to idly speculate. The idea that we cannot tell that a theory is correct until we do an experiment is false. It’s extremely difficult to find theories that fit the twin principles of relativity and quantum theory. Almost everything you try fails. “What really interests me is whether God had any choice in the creation of the World,” said Einstein. Once special relativity and quantum theory existed, it seems he did not.
At present there is only one candidate for a deeper “theory of everything” that satisfies the constraints of both special relativity and quantum theory: “string theory”. String theory views the fundamental building blocks of the universe not as point-like particles but as “strings” of mass-energy. They vibrate like violin strings, and each oscillation corresponds to a subatomic particle such as an electron or an up-quark. What excites physicists is that one particular vibrating string corresponds to a graviton, the particle that our current experiments have so far been unable to detect. This means string theory automatically incorporates Einstein’s theory of gravity, which until now has stayed beyond the reach of quantum physics.
Unfortunately, the strings are postulated to be mind-bogglingly tiny – equivalent to the Planck length, which is 10 to the power of -35 metres, or a million billion times smaller than a hydrogen atom – and mimicking all the known particles and forces requires them to vibrate not in 3 dimensions of space but 9. (The extra dimensions are postulated to be unnoticeable because they are rolled up far smaller than an atom.)
Physicists have discovered five different string theories, which they now know are aspects of a deeper theory, dubbed M-Theory. Living in 10 dimensions of space, the theory contains not only one-dimensional objects (strings) but 2D objects, 3D objects and so on. In fact, strings may not even be the most important thing in string theory.
M-Theory is like the tail of an elephant stumbled on by a blind man in the dark. Finding the creature to which the tail is attached is going to require a revolution in our thinking as big as, if not bigger than, the revolutions that gave birth to relativity and quantum theory.