The problem with gravity

When weak overcomes strong

Suppose you are holding two magnets that have snapped together. Grab one of them and twist it around. Whichever way you hold it, the other magnet stays firmly attached, even upside down. This is remarkable for a simple reason: the entire Earth, all six trillion trillion kilograms of it, is pulling on that magnet with everything it has, and the magnetic force between two tiny magnets is enough to resist it. It is a simple experiment that reveals the immense gap between the electromagnetic force and gravity.

These are two of the four fundamental forces of nature. The other two are the strong and weak nuclear forces. But there is a big problem. The electromagnetic, strong and weak forces are all described by a single framework, the Standard Model of particle physics, while gravity is described by general relativity alone. On top of that, gravity is vastly weaker than any of the others. What makes it so special? We still do not know, although there is no shortage of proposals.

There is a second puzzle. If the other three forces are so much stronger, why is gravity the one we see everywhere in the universe? Why does gravity keep planets in orbit rather than electromagnetism? Why are black holes gravitational and not a product of the weak force? Why is the weakest force the one that holds planets, solar systems and entire galaxies together?

The answers are simpler than you might expect. The electromagnetic force only acts on things that carry electric charge, and thankfully almost everything is electrically neutral, containing nearly equal amounts of positive and negative charge. This is obvious when you think about it, or magnets would stick to everything. The weak force is responsible for radioactive decay, and we do not see the world around us spontaneously decaying into other elements, so it matters only on the scale of atoms. The strong force holds atomic nuclei together, but its reach is so short that it does not even affect the electrons orbiting the nucleus.

So when all is said and done, gravity is the only force left standing on the scales of daily life, and it stays in charge all the way up to the scale of the universe. Gravity holds planets together, keeps them in orbit, gathers stars into galaxies, creates black holes, and governs the birth, life and death of the cosmos. When it comes to the motion of things in the universe, it dominates everything else.

We are wrong about gravity

General relativity has passed every crucial test we have thrown at it, again and again, and it stands as our theory of gravity. So how can we know it is wrong, despite it being right time after time? Quantum mechanics.

Quantum mechanics, or more precisely quantum field theory, lies at the heart of the electromagnetic, weak and strong forces. It is a strange theory whose predictions seem to defy logic. Yet the quantum theory of electromagnetism is one of the most accurate theories ever written down. Its predictions have been tested with ever more precise experiments, and every one has confirmed it. Quantum field theory rules the microscopic world.

How do we reconcile gravity, which describes the very large, with quantum field theory, which describes the very small? The natural thing to try is to use gravity to describe the very small. But when you do, things go horribly wrong. The mathematics falls apart. You get singularities, divergences and nonsensical answers. General relativity cannot be pushed to these scales; it cannot be quantised (in the jargon, it is non-renormalisable). So general relativity cannot be a complete theory, or it would describe both the very large and the very small. It must, in some way, be wrong.

Is there hope?

What can we do? The holy grail of physics is a unified theory of all the fundamental forces: a theory that works at every scale and every time, the large and the small, the ancient and the future. There must be an answer, but we have not found it yet. There has been no shortage of proposals.

Many theorists, most of them extraordinarily clever, have put forward their own candidates. Most fall under the banner of quantum gravity, the search for a theory that replaces general relativity and describes both the large and the small. Examples include

string theory,
loop quantum gravity,
and causal dynamical triangulations,

among many others, each taking an entirely different approach. How do we decide which is right? By scrutinising each theory in exhaustive detail, extracting every prediction it makes, and running experiments to try to prove it wrong. The one whose predictions survive is the winner.

But all of these theories make their predictions about the very, very small, far beyond the reach of any experiment we can currently build. It will likely be many years before we know what gravity really looks like on subatomic scales. Technology has to catch up with the theorists.

Until then, the universe remains split in two: the ultra-small and the overwhelmingly large.