Deep into the night of September 14, 2015, researchers in Livingston, Louisiana, noticed their world had suddenly gotten smaller. The nanoscopically measured distance between the two mirrors they were observing abruptly, and very briefly, shortened—by less than ten-thousandth of the diameter of a single proton. Something had just warped the vacuum inside the four-kilometre-long L-shaped Laser Interferometer Gravitational-wave Observatory (LIGO) interferometer, disturbing two precisely aligned laser beams. The device gave a short warning chirp, the somewhat soft-pedalled echo of a cataclysm that happened a billion years ago. It might have made Albert Einstein smile, because it proved something he had predicted a century before.

In his theory of general relativity, Einstein calculated that the movement of massive objects through the cosmos would convulse its very fabric, compressing it here, stretching it there, creating a kind of peristalsis in space and time, driven by gravity.

The LIGO researchers had heard the sound of that warp passing through their lasers—the wake left by the collision of two black holes that released more energy than a billion trillion suns.

But even Einstein couldn’t explain everything. By definition, universal laws should work everywhere, but when tiny particles start smashing into one another with colossal energy they throw out Einstein’s rulebook. We now know that neutrons and protons, once considered the fundamental blocks of matter, are simply home to an entire microverse of ever-smaller, more bewildering entities. Deep inside each atom is a fleeting quantum swarm physicists call the “particle zoo”, and it stubbornly refuses to obey Einstein, or anyone else. Particle physicists came up with a separate, subatomic explanation—it’s called quantum theory, because it’s characterised not by order or predictability, but by randomness and uncertainty.

Just the same, quantum theory does a fair job of explaining a lot of odd behaviour, including the strong and weak nuclear forces, which act deep inside the nuclei of atoms. The strong force is the glue that holds the nucleus together. The weak force only manifests in very specific conditions, overpowering the strong force and blowing the nucleus apart in a fury we now recognise as radioactivity. But quantum theory is powerless to explain gravity—that is, without invoking a notional particle, the graviton, to wield it. The only way a graviton can make quantum theory embrace the known universe is to have no mass, spin in a very specific fashion, and travel at the speed of light. It’s the unicorn of particles. No one’s ever seen one.

So while there’s only one world, we still need two separate theories to explain how it works—one within the atom, one beyond it. What’s so bad about that?

Well, they contradict each other, for starters. General relativity dictates precisely the behaviour of matter, while quantum theory insists there is only a probability it will act in a certain way, so they can’t both be right. Most of the time, it probably doesn’t matter, but it drives physicists crazy when they try to understand things such as black holes, because these are both super-massive (general relativity) and very small (quantum theory).

Einstein wouldn’t brook the imperfection: he devoted much of his career to the pursuit of a theory that would deftly marry gravity to the rest of physics—the strong and weak nuclear forces, gravity, and the electromagnetic force. A theory of everything.

The fly in the ointment is electromagnetism—gravity’s enigmatic twin. Any schoolkid knows that any two objects will either be attracted to, or repelled by, each other, according to their electromagnetic orientation. But how does that tie into gravity, the other powerful attractive force? Maybe it doesn’t: to bring them both under a single “unified field theory”, Einstein had to invent a fifth dimension—tiny, convoluted and invisible.

How would that work? Well, when you arrange to meet a friend, you agree on two things: the place, which is a point in three-dimensional space, and the time—four dimensions in all. The fifth, argued Einstein and other adherents, represented a space from which those other dimensions were visible, but imperceptible from our own viewpoint. Imagine a fish beneath the surface of a pond disturbed by rain drops. The fish can see the disturbance, but not its source. Thus was quantum eccentricity explained: preserving fifth-dimensional symmetry meant dismantling some of the beautiful elegance of general relativity, and Einstein wouldn’t go there.

He eventually gave up, but others persisted.

The late 1960s produced string theory, which shed new particles rather than seeking them. In fact, runs string theory, there are no particles at all; just tiny, one-dimensional strings or loops of matter. There are no gravitons, no muons, no bosons, no neutrinos. They’re all the same stuff; they appear different to us only because they happen to be vibrating at different frequencies, much like the strings of a guitar. Vibrating one way, a particle is an electron. Another way, it’s a gluon. And here’s the zinger: vibrating yet another way, spinning just so, and travelling at the speed of light, it may just be a graviton, carrying gravity.  At a stroke, string theory seemed to make sense of the quantum cosmos, while leaving the macro world of the massive undisturbed. It accounted for all four forces of Nature.

Unfortunately, while string theory greatly civilised the particle zoo, it created even more dimensions—26 in its earliest iterations. Even today, “superstring” theory needs 10 dimensions to work (just like Einstein’s fifth dimension, they are, with the exceptions of the three co-ordinates we know well, very, very small). Tougher still, superstring predicts a nearly infinite array of other possible universes, a boggling concept some physicists are unwilling to entertain. Instead, they’ve weighed in behind loop quantum gravity, not so much a quest for unification as an attempt to make quantum theory stick to gravity by presenting space–time as clusters of matter linked by strings or loops. These interlinked fibres of particles, it proposes, act in such a way as to ‘create’ gravity, but without experimental evidence, it’s all guesswork. As with superstrings, there are still too many pieces missing, and the maths hasn’t been devised even to test the few pieces we do hold.

Most recently, cosmologists and particle physicists have begun to doubt the wisdom of trying to describe the known universe at all because, some argue, there are probably millions, maybe billions, of others out there, each abiding by its own natural laws. Enter M, or multiverse theory, which isn’t about a single, unified theory, but 10 to the power of 500 theories—each explaining its own universe, yet logically consistent with all the others. Rather than one law to rule them all, think of your multiverse as governed by bylaws—millions and millions of them. Just like your home town.