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Theoretical physicists like to ask big questions. How did the universe begin? What are its fundamental constituents? And what are the laws of nature that govern those constituents? If we look back over the 20th century, we can identify two pillars on which our current theories rest.

The first is quantum mechanics, which applies to the very small: atoms, subatomic particles and the forces between them. The second is Einstein’s general theory of relativity, which applies to the very large: stars, galaxies and gravity, the driving force of the cosmos.

The problem we face is that the two are mutually incompatible. On the subatomic scale, Einstein’s theory fails to comply with the quantum rules that govern the elementary particles. And on the cosmic scale, black holes are threatening the very foundations of quantum mechanics. Something has to give.

An all-embracing theory of physics that unifies quantum mechanics and general relativity would solve this problem, describing everything in the universe from the big bang to subatomic particles. We now have a leading candidate. Is it the much anticipated “theory of everything”?

Building blocks

At the end of the 19th century, atoms were believed to be the smallest building blocks of matter. Then it was discovered that they have a structure: a nucleus made of protons and neutrons, with electrons whizzing around it. In the 1960s, the atom was divided even further when it was theorised, then confirmed by experiments, that protons and neutrons are composed of yet smaller objects, known as quarks.

Do these layers of structure imply an infinite regression? All the theoretical and experimental evidence gathered so far suggests not: quarks really are the bottom line. We now believe that quarks are fundamental building blocks of matter along with a family of particles called the leptons, which includes the electron (see table).

More or less everything we see in the world around us is made from the lightest quarks and leptons. The proton consists of two up quarks and one down quark, while a neutron is made of two downs and one up. Then there is the electron along with the electron neutrino, an extremely light particle involved in radioactivity.

Nature is not content to stop there. There are two more “generations” of quarks and leptons which are like the first, but heavier. In addition, all these particles have antimatter partners which have the same mass but opposite charge.

Four fundamental forces

What holds the quarks and leptons, or building-block particles, together? As far as we can tell, there are four fundamental forces: gravity, which keeps Earth going round the sun; electromagnetism, responsible for phenomena such as light and electricity; the weak nuclear force, responsible for radioactivity; and the strong nuclear force, which binds neutrons and protons in the atomic nucleus.

But what exactly is a force? The modern view is based on a theoretical framework called quantum field theory. This says that the forces between building-block particles are carried or “mediated” by another set of particles. The most familiar of these is the photon, the mediator of the electromagnetic force. When two electrons repel one another, they do so by swapping a photon. This idea was backed up so well by experiments that theorists invented other force-carrying particles: the gluon for mediating the strong force, and the W and Z particles for the weak force.

Sure enough, the existence of the gluon and W and Z was confirmed in the 1970s and 80s. When you put all this together with the as yet undiscovered Higgs boson, whose job it is to give particles their mass, you get the standard model of particle physics.

The standard model is a remarkably robust mathematical framework which makes very definite predictions about particle physics that have so far withstood all experimental tests. For example, the fact that quarks and leptons come in three generations is not put in by hand but is required by mathematical consistency; the standard model would not work if one member of the family was missing. For this reason, theory demanded the existence of the top quark, which was duly discovered in 1995.

Many regard the standard model as one of the greatest intellectual achievements of the 20th century. Yet it cannot be the final word because vital questions remain unanswered.

Gravity

One glaring omission from the standard model is gravity; where does that fit in? According to Albert Einstein’s view of gravity, apples fall to the ground and the Earth orbits the sun because space-time is an active and malleable fabric. Massive bodies like the sun bend space-time. A planet that orbits a star is actually following a straight path through a curved space-time. This means we have to replace the Euclidean geometry we learned at school with the curved geometry developed by the 19th-century mathematician Bernhard Riemann.

Einstein’s description of gravity has been confirmed by watching light from a distant star being bent around the sun during a total solar eclipse. This is a very different picture of a force from that given by the standard model of particle physics, which says that forces are carried by particles. Extending this idea would suggest that gravity is mediated by a force-carrying particle known as the graviton.

Read next article: “Theory of everything: The road to unification“