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⚛️ Quantum Gravity: The Hundred-Year Puzzle ⚛️

How the Universe keeps us guessing about its innermost secret

A brief bit of history, because there is nothing much actually current …

Relativity: The Grandfather of Modern Physics

Einstein published his Special Theory of Relativity during his Annus Mirabilis, 1905; the year he also proved that atoms exist (with his calculations of Brownian motion), and advanced the notion of quantum theory (with his paper on the photoelectric effect). It was another ten years, though, before Einstein cemented Relativity’s place in physics text books, when he published his General Theory of Relativity. That paper was published on November 25th, 1915, which also happened to be Thanksgiving in America. And it is indeed something for us to be thankful for. Relativity is the grandfather of modern physics.

Quantum Mechanics: The Mad Hatter of Science

Fast-forward another ten years. Even before Einstein, Planck had kicked off the idea of quanta in 1900, but it was really the likes of Heisenberg and Schrödinger that got things going in the mid-1920s. Quantum mechanics introduced an alien world of probability that underpins our reality where things just don’t make logical sense. It’s a bit like Alice in Wonderland meets sub-atomic particles. It’s a carnival House of Horrors ride run by gremlins. Particles with bizarre properties pop in and out of existence, an electron exists like Tweedledum and Tweedledee in two places at once, and the Mad Hatter doesn’t know if the Dormouse is dead or alive until he looks in the teapot. You’re often excluded from knowing two facts at once, and there’s a lot of philosophical twaddle about an ‘observer.’ (Quantum events were quite happily occurring autonomously long before we came along to observe like some sort of Cheshire Cat up a tree, grinning and making waveforms collapse). Figuring out the meaning of quantum mechanics is like trying to give Schrödinger’s cat a bath.

String Theory: Cosmic Vibes

But what about more up-to-date physics? String theory comes to mind, right? And yet string theory is already more than half-a-century old! The idea was born from work in 1968 on the strong nuclear interaction and a 200-year-old equation, but it wasn’t until 1970 that the equation describing tiny quanta made of vibrating strings was proposed. Superstrings: tiny vibrating filaments that jiggle in more dimensions than can be comprehended. It’s as if our world sounds like somebody twanging a rubber band, but these other dimensions are like an orchestra playing Adagio for Strings. The theory may be elegant and bold, but it has produced exactly no experimental confirmation. That doesn’t mean there isn’t truth to it, though.

Loop Quantum Gravity: Minecraft for Physicists

After Einstein’s Special relativity was reformulated, the term Loop Quantum Gravity was coined in 1987 to describe another modern idea suggesting that the universe is pixelated. It seems to be a logical conclusion of the notion of quanta. If there are discrete jumps in everything in the quantum world, then it stands to reason that, when you look close enough, space and time must be made of teeny-tiny chunks. Loop quantum gravity says spacetime isn’t a smooth fabric after all, and when you get small enough it’s made of minuscule Lego blocks of existence. Like Minecraft, but with much smaller cubes. And hideously complicated mathematics. How do we reconcile this idea with superstrings? What if superstrings can only vibrate in certain harmonics, like the notes played on a violin? Could that create the kind of pixelated space that loop quantum gravity calls for?

M-Theory: the New Kid on the Block

In 1995, M-theory was proposed describing an 11-dimensional universe in which higher dimensions contain membranes, or “branes,” that govern the strings in string theory. It’s ambitious and impressive. And probably not the whole story, but it has to be a harbinger of a new way to think about the problem. We can’t readily comprehend a fifth dimension, let alone how powerful dimensions might be beyond that—folded up and inaccessible in our world, but quietly governing the quantum realm and the cosmos in the background. If we could unfold more dimensions of time, would that mean it’s possible to zoom around time like we do in our three dimensions of space? Either way, these higher-dimensional ‘branes give us a framework to begin to see how individual superstrings are connected. Could it help to explain how quanta behave across time? Like the half-life of a radioactive decay, or quantum entanglement?

Where Do We Go From Here?

Physicists have a problem. On the one hand, we have Einstein’s general relativity: serene, elegant, and as firmly established as the fact that gravity only works downwards. On the other hand, we have quantum mechanics: hyperactive, unintuitive, and constantly throwing the dice that Einstein didn’t want to believe in. Both are brilliant. Both work astoundingly well. But, like a Stradivarius and a kazoo, they refuse to play nicely together.

Is there a place for both these theories? Are they at odds? Or two ways of looking at the same reality? Are we even asking the right questions anymore? Every time we try to squish relativity and quantum mechanics together, the equations turn into gibberish—like trying to explain infinity plus one while juggling Jello with your underpants on your head.

Quantum Gravity

So, what’s next? Aren’t we due for another new breakthrough? We are a quarter of the way through a new century with nothing much to show for it, apart from the Higgs boson after half a century of searching—under the sofa cushions, in the kitchen junk drawer, down the rabbit-hole—and finally finding it hiding in a corner of the Large Hadron Collider. How do we go about reconciling relativity and quantum mechanics? Enter the dream of quantum gravity. Like Sauron’s One Ring: one theory to bind them. Quantum gravity would supposedly unify everything; Life, the Universe and Everything. It could smooth the differences between all these theories, and maybe stop them from fighting like Schrödinger’s cat and Einstein’s dog. M-theory is the leading candidate with its higher dimensions, D-branes, and F-theory spin-off. Is it about time for quantum gravity to give up its secrets?

A New Theory—and It’s About Time

We don’t see much about theories to describe how time works. Chronons were theorized as the tiny units of quantum time back in 1980, with the idea kicking around for half-a-century before that. If ‘now’ (this exact moment) is a tiny quantum slice of time, what connection is there to events past or future in the quantum realm? What governs when things happen in the quantum world? Take a radioisotope like uranium, with a certain half-life. Something, somewhere, in a higher dimension must know how long it’s been since one atom decayed and determines the probability of when the next one can. One atom decays and then, as time passes, the probability of the next decay increases until … pop, and the brane clock starts again. Maybe time is a force that acts through probability. If the ‘branes in higher dimensions govern the behavior of the superstrings in our dimension, maybe there’s a lot M-theory can begin to explain.

Strings of Time

What if this chronon, the particle of time, is a string too? What if the frequency at which those strings of time vibrate determines how fast time passes? And I don’t mean when you’re bored versus when you’re having fun. What if interactions exist between strings of matter and strings of time?

Could that explain why time slows down when matter is present? The denser the matter, Einstein tells us, the slower time gets (it’s true, we can measure it).

If time stops at the surface of a black hole, is that why light can’t propagate and escape from the black hole?

Can that explain why, when matter gets close to the speed of light and (according to Einstein) weighs so much more, time slows down until it can’t slow down any more? Is that why we can’t exceed the speed of light?

How does gravity bend light? If light doesn’t have mass, gravity can’t pull on it, right? Maybe it’s time that bends space-time and not matter after all.

Maybe we’re looking in the wrong place for our answers to quantum gravity. Maybe we need to try looking under a different sofa cushion.

Imagine That!

What if we had a new theory about time and it answered our questions about quantum gravity? What if it incorporated the idea of branes in higher dimensions of time? What if it also explained quantum probability that governs the weak force and determines when radioisotopes decay? That would be a major breakthrough.

But Why Bother In Real Life?

Without quantum gravity, we can’t really explain black holes, the ultimate fate of the universe, or what happened at the Big Bang besides God saying “lights, matter, action!” But who actually gives a crap? The Big Bang is over; it’s so last season. And it’s not like black holes are much of a problem, I mean compared to, say, world hunger. And the end of the universe isn’t any time soon. So, in the end, who cares? Like in Monty Python’s Life of Brian: What did the physicists ever do for us? Apart from the aqueduct, lasers, nuclear energy, atomic clocks, solar panels, and wine—we’d miss the wine. But apart from all that, what have physicists ever done for us?

But curiosity prevails and the physicists persist. They scribble on chalkboards long into the night, while the rest of us smile and nod, and pour another glass of wine. We can’t predict what the benefit of the next big breakthrough will be, but we can be sure it will be something pretty profound.

Maybe the Universe Likes Its Secrets

Perhaps the truth is that quantum gravity doesn’t want to be explained. Maybe the universe, in all its infinite weirdness, has boundaries it won’t let us cross. Like in Hitchhiker’s Guide, maybe the question and the answer can’t both be known in the same universe. What’s a good mystery novel without a few intertwined storylines? But, like a good book, in the end, we want a resolution to the plot. It’s a constant battle for humankind to reveal the secrets of the universe, and the curious human mind needs to keep on trying to work out the ending of this cosmic whodunit.

Until then, relativity and quantum mechanics will continue their awkward dance, like the ten-year-old and his drunk aunt. And the rest of us? We’ll watch, like wallflowers, from the sidelines, grateful that somehow someone else can manage all the ridiculous mathematics.