The World According to Physics

by Jim Al-Khalili

Physicists’ relentless drive to unify their theories—to bring together the laws of the universe and encapsulate them in a single neat mathematical equation—a ‘theory of everything’—often appears to be no more than an obsession with simplicity and compactness, an effort to package up the complexity of all natural phenomena using the minimum number of underlying principles. In fact, it’s subtler than that. Throughout the history of physics, the more we’ve discovered about the workings of nature, the more connections we’ve found between seemingly unconnected forces and particles, and the fewer rules and principles we’ve needed to explain an ever-wider range of phenomena.

Unification is not something we deliberately set out to achieve; it has emerged as a result of our deeper understanding of the physical world. But this success undeniably comes with a certain aesthetic appeal that drives us to keep going along the same lines. And we have been astonishingly successful at it.

But I’m afraid that to truly appreciate its importance and the role that different symmetries have played in theoretical physics over the past century is somewhat beyond the remit of this short book.

The hunt for a unified theory is sometimes described as an attempt to gather all the forces of nature into one framework, suggesting that there exists just one ‘superforce’ and that the different interactions we know of in nature—electromagnetism, gravitation and the two short-range forces within the confines of atomic nuclei—are all different aspects of this single force.

Physicists have so far had a good deal of success with this broad project of unification. I have already described how Newton understood that what causes the apple to fall from the tree is the same universal force (gravity) that controls the motion of the heavenly bodies across the sky. This was not at all obvious at the time, even though it might seem so to us today. Before Newton, it was believed that objects fell to the ground because everything had a ‘tendency’ to move to its ‘natural’ place—...

Newton’s law of universal gravitation brings these phenomena together by stating that all masses are attracted towards each other, with a force proportional to the product of their mass and inversely proportional to the square of the distance between them. It doesn’t matter whether it is an apple...

Another huge leap forward along the path to unification took place almost two centuries after Newton, when James Clerk Maxwell showed that electricity and magnetism are in fact different facets of the same electromagnetic force. So, the electrostatic attraction between a scrap of paper and a balloon that has been rubbed on your clothing has its origin in the same electromagnetic force that attracts a paper clip towards a magnet. Almost all phenomena we see in nature are due ultimately to one or other of these two forces...

We have already seen that, at a fundamental level, the gravitational field is nothing more than the shape of spacetime itself, a revelation that was also due to a unifying idea. By combining space with time, Einstein revealed a profound truth: that only in four-dimensional spacetime can all observers (however fast they are moving relative to each other) agree on the separation between two events. A decade later, his general theory o...

But that wasn’t enough for Einstein, who spent the most part of the next four decades of his life searching unsuccessfully for a unified theory that would combine his theory o...

We now know that there are, in addition to gravity and electromagnetism, two other forces—the strong and weak nuclear forces—that only act over very tiny distances, but which are just as important as far as the fundamental laws of nature are concerned. And it was the unification of the electromagnetic force with one of t...

But this important advance in our understanding of the nature of the fundamental forces only came about with the evolution of quantum mechanics from a theory describing the microcosm in terms of particles and waves to one involving fields. I touched very briefly on the meaning of fields in chapter 3 in the context of gra...

may have given you the impression that once quantum mechanics was completed almost a hundred years ago, most physicists busied themselves with applying it to real problems in physics and chemistry, leaving just a few of the more philosophically minded to carry on arguing about what it all meant. To a large extent, this version of history is true. But it is also true that quantum mechanics continued to develop in sophistication throughout the first half of the last century.

special theory of relativity.

quantum mechanics

electromagnetic fie...

quantum field...

precise way of describing the electromagnetic interaction of matter with ligh...

exchange of photons.

physics of particles

physics of...

particle-like manif...

localised particles of matter,

quantum fields.

Pauli exclusion principle,

This means we do not perceive their quantum fields so easily.

By the late 1940s, mathematical problems with the description of quantum fields were finally resolved, and the theory known as quantum electrodynamics (QED) was completed. To this day, it is regarded as the most accurate theory in all of science. It is also the physical theory that explains at a fundamental level almost everything in the world around us, since it underpins all of chemistry and the nature of matter—from the way the electronic circuitry and microchips in my laptop work to the neurons firing in my brain, commanding my fingers to move across the keyboard. This is because QED is at the heart of all interactions between atoms.

The split between the two forces (the symmetry breaking) is due to another field, called the Higgs field, which gives the W and Z particles mass while leaving the photon massless. This unification means that, at a fundamental level, the four forces of nature are reduced to just three: the electroweak force, the strong nuclear force, and gravity (which in any case is not actually a force at all, according to general relativity). You may disagree with me as to whether this has helped simplify matters.

‘colour charge’,

Note that the word ‘colour’ here is not to be taken in any way literally. The reason three types of colour charge were needed, rather than just two (as with electric charge), was to explain why protons and neutrons must each contain three quarks; and the reason the analogy with colour was chosen was because of the connection with the way the three different colours of light (red, blue and green) combine to produce white light. Thus, the three quarks in a proton or a neutron each carry a different colour charge: red, blue or green, which combine to produce a particle that has to be ‘colourless’.

The rule was that quarks could not exist by themselves because they carried colour; they could only exist in nature by sticking together to make up colourless combinations.2 For this reason, the field theory of the strong nuclear force that binds quarks together became known as quantum chromodynamics, or QCD.

The exchange particles between quarks are the gluons, a rather more evocative and appropriate name, I think you’ll agree, than that of those weak for...

Let us then take stock. Of the four known forces of nature, three are described by quantum field theories. The electromagnetic and the weak nuclear force are linked together by the electroweak theory, while ...

A yet-to-be fully developed theory that connects these three forces together is known as a grand unified theory (or GUT). But until we find one, we must make do with a loose alliance of the electroweak theory ...

Even its most ardent defender will admit that the Standard Model is probably not the last word on the matter. It has survived this long in part because we have nothing better to replace it with and in part because its predictions have so far been validated by experiments,...

And yet despite this being the best description we have of three of the four forces of nature, physicists would like nothing better than make some new discovery that conflicts with the Standard Model, in the hope of dis...

But as long as the predictions of the Standard Model continue to be confirmed by experiments, it...

We have discovered that the description of our everyday world at the length, time, and energy scales appropriate for Newtonian physics is only an approximation, and that beneath it are more-fundamental physical theories that come into their own at the extreme scales.

At one end, we have quantum field theory, which has led us to the Standard Model of particle physics and which accounts for three of the four known forces in the universe. At the other extreme, we have the general theory of relativity, which gives us the Standard Model of cosmology that encompasses the other force, gravity. This standard model of the very large is called a variety of different names, such as the concordance model, or the Lambda–cold dark matter model, or the Big Bang cosmology model. I will discuss it more fully in the next chapter.

Therefore, a question that physicists are often asked is why we feel it is so important, indeed whether it should even be possible, to keep going with our obsession with unification, to try to combine these two models describing enti...

Surely each works well in its own domain, and that should be enough for us. But again, I must stress that the purpose of physics is not simply to account for what we observe or to find some useful application based on it; physics is about...

So, this is where we are at the moment: stuck with two successful frameworks—quantum field theory and general relativity—which jus...

Indeed, they appear to have very little in common: their mathematical structures are incompatible. And yet this cannot be the whole story. We know that spacetime reacts to the matter within it. We also know that matter at the subatomic scale behaves according to the rules of quant...

If an unobserved electron is in a quantum superposition of being in two or more states at once, as we know electrons can be—for example, if their quantum state is spread out over some volume of space or in a superposition of different energies at once—then sure...

The problem is that general relativity just isn’t ‘quantum-y’, and it is far from straightforward how we can make it so. One of the problems with this is that subatomic particles have such tiny masses that ...

Still, the issue remains: How do we quantise the gravitational field? What do we need to do to bring quantum field theory and general relativity together? And if they are truly as incompatible as they seem to be, then which one of these two incredibly s...

In the mid-1980s, a candidate theory of quantum gravity was developed. It was based on a mathematical idea called supersymmetry, which I mentioned briefly in chapter 2. This candidate theory became known as superstring theory, and it captured the imagination of many mathematical physicists of my generation.

matter particles, or fermions

force carrier particles, or bosons

String theory had originally been proposed in the late 1960s as a theory of the strong nuclear force, but when quantum chromodynamics was developed in the ’70s and found to be so successful, string theory ...

But it was soon realised that by incorporating into string theory the idea of supersymmetry, it could be reborn as a candidate for a much grander undertaking than a theo...