Einstein died an annoyed man. After he shot to international stardom with his theory of General Relativity, he had devoted his life to a “unified theory” that connected the two major forces in the universe — gravity and electromagnetism — but it only ended in frustration. But in the sixties, scientists came across a mathematical equation that promised to change everything we knew. String Theory says that everything — from quark to atom to human to planet — is fundamentally composed of little vibrating strings.
You’re probably asking, “So what?”
The reason string theory is such a big deal is that if true, it successfully describes all four natural forces — gravity, electromagnetism, and strong and weak nuclear forces — with a mathematical elegance that other theories only dream about. But before we understand string theory (and that’s a relative term), we must understand why it needs to exist.
The story of gravity
When Newton described gravity, it made perfect mathematical sense. More importantly, it could be verified experimentally — scientists love it when that happens. Even so, Newton and his fellows couldn’t figure out how gravity really worked. Do bodies just reach out and grab each other with invisible arms?
Enter Einstein. He saw the four dimensions — length, width, depth and time — as a fabric, capable of stretching and warping under influence. This influence, he said, came from the planets and stars. Imagine a large piece of cloth, stretched to firmness. If you were to toss a watermelon on it, it would create a sizable depression on the cloth. If you threw a tennis ball into the mix, it would not only create a smaller depression, it would start to roll towards the watermelon. This is essentially how gravity works — stars and planets create their own depressions — called gravity wells — in space-time, and small objects either fall into these wells, or swirl around them as satellites.
While the world was still busy praising this new theory, Einstein wanted to take it one step further. Only a few years earlier, Maxwell Planck had proved that electricity and magnetism were just manifestations of the same force — electromagnetism. To Einstein, it seemed logical that electromagnetism and gravity were manifestations of something bigger, and set out to find out what that was. The problem, however, was that electromagnetism and gravity are vastly different.
We tend to think of gravity as this strong, anchoring force that’s holding the universe together — because, after all, without gravity, everything will just float into nothingness. In reality, though, gravity is one of the weakest forces in the universe. It may be strong enough to keep you in your chair, but it’s too weak to pull you through the chair, towards the ground, to the center of the Earth. Or to prevent you from jumping, for that matter. Let’s understand this better.
You and your chair (and everything else) are made of atoms — protons and neutrons at the centre, with electrons orbiting around them. This outer shell of electrons is negatively charged, so when the atoms of your body come close to those of your chair, they repel each other. This repulsion — an electric force — is so strong that even if you turned the tallest, pointiest building in the world upside-down and made it cover only a square foot of ground, it’ll never go through it. In fact, at the atomic level, gravity is so weak that scientists often disregard it in their calculations.
If gravity and electromagnetism come from the same place, it doesn’t make sense that one is billions of billions of times weaker than the other. While Einstein puzzled over this, things got a bit worse…
As scientists delved deeper into the atom, they discovered two things: one, that the atom was made up of protons, neutrons and electrons and that the universe has two more major forces — the strong nuclear force, which holds protons and neutrons together in the atom, and the weak nuclear force, which makes atoms radioactive. Secondly, they discovered that at the atomic level, the universe changes completely.
At the subatomic — or quantum-level, the fabric of space-time isn’t really a fabric at all. It’s bumpy and frothy and it’s called quantum foam. It contradicts everything Einstein had believed in — the universe as structured and rational and orderly.
In this quantum foam, there’s no such thing as certainty — all events only have a probability. So if you’re a little electron and you want to be in Mumbai, there’s only a probability that you’ll end up there — just like there’s a probability you’ll end up in Delhi, or Bangalore, or at the top of Everest. In fact, you’ll really end up everywhere, just in different universes — so you could be in Mumbai in this universe, but you’re in Delhi in another. These crazy theories — among others form quantum mechanics. For all its bizarreness, quantum mechanics is surprisingly accurate. It explains the behavior of the universe at the subatomic level, and any experiment that set out to verify any of its predictions has ended in success.
Unfortunately, quantum mechanics and general relativity were horribly incompatible. Quantum mechanics makes perfect sense at the subatomic scale, but doesn’t apply to anything bigger. General Relativity, on the other hand, explains all the big stuff, but falls apart at the atomic scale. For the most part, though, this incompatibility didn’t seem to cause a major impasse.
But then came the proposition of the black hole — the entire mass of a star, concentrated into a tiny, subatomic point; its gravitational field so strong that even light can’t escape it. How do you treat such a formation? Do you apply the laws of quantum mechanics to it because it’s so small, or do you use General Relativity because it’s so heavy? The same problem applies to the Big Bang — general relativity can talk about what happened just after the Big Bang, but there’s no way to talk about the instant of the Bang, when the entire universe was just a single point. So far, any attempt to integrate quantum mechanics with general relativity has resulted in nonsense.
But if quantum mechanics applies everywhere, and General Relativity applies everywhere, why don’t they play well together? There had to be some way to unify the universe’s forces...
As scientists started smashing atoms together, they discovered that matter didn’t end at protons and neutrons — these subatomic particles were made of even smaller particles, called quarks. Naturally, the scientists kept smashing, and unearthed a whole mess of particles — enough to develop the Standard Model of particle physics.
According to the Standard Model, the forces of the universe are all caused by an exchange of fundamental particles, called messenger particles — magnets are attracted to iron, for examples, because both are throwing photons at each other. Similarly, protons and neutrons exchange gluons to stick together, and bosons are the reason that atoms are radioactive. This model of the universe ties electromagnetism, the strong nuclear force, and the weak nuclear force together with a neat little bow. Even better, there’s plenty of experimental evidence to suggest that this is, indeed, how the universe works.
But what of gravity? While the Standard Model explains so much, it still falls short of providing a plausible explanation for gravity. That’s where string theory comes in.
The crazy yarn
String Theory, as we mentioned before, says that even the most fundamental particle is made up of billions of little strings that can stretch, compress, and vibrate. Just like different vibrating guitar strings produce different sounds, the way these strings vibrate determines the nature of the particle they form.
Most theoretical physicists took an instant dislike to this new theory. There wasn’t — and isn’t now — any way to prove these strings actually exist — in the world of science, that means that string theory is still more philosophy than science. Worse, it made some really bizarre predictions. Firstly, it suggested that mass less particles could exist — and since even photons supposedly have mass, that’s a disturbing thought. Secondly, it proposes that our universe has not four, but eleven distinct dimensions — a brain-melting thought on its own — which opens up the door for multiple universes.
In fact, these multiple dimensions and universes are essential for string theory to truly explain gravity.
Strings in the Quantum World
At the subatomic scale, space-time is foamy — so much so, that it’s possible that in all that frothy chaos, the fabric of space-time may tear and repair itself on a regular basis.
Einstein’s general relativity forbids this; it makes room for the possibility of tears (called wormholes) that exist as a feature of the universe itself — just like your t-shirt has three holes when you buy it — but space-time isn’t supposed to rip at will. If it does rip, it may well be the end of the universe.
According to string theorists, however, these rips could exist, and it’s the strings that are keeping all hell from breaking loose. As these strings travel through the universe, they’re tracing out tubes, which form “bubbles” around these tears in space-time, effectively preventing them from doing any damage.
If space-time can rip at will, we might eventually be able to use these rips to teleport to and from wherever we want. It’s a tiny possibility, but it’s there, and that’s cool enough.
Inside the bubble chamber of an atom smasher, the fundamental particles of the universe make their existence known. Atoms break into smaller particles, which then follow the different paths you see here
Big Banging Branes
If we are to accept that we’re trapped in a four-dimensional brane in a larger universe with more dimensions, we might also have an explanation for the Big Bang. With all these branes floating about, it might be that another brane collided with ours, spewing all of its contents into ours.
But scientists aren’t comfortable with this idea. First, like the rest of string theory, there’s no way to prove this. Second, it implies that other universes existed before ours, and others before them. In essence, the universe doesn’t really have an origin. And a universe without an origin is a universe that doesn’t make sense at all.
’Branes and ’verses'
It’s hard enough to think of the universe as having four dimensions, much less eleven. String theorists suggest that we don’t see these dimensions because they’re either too small or too large to be observed.
To understand the first idea, imagine that you’re a super-massive being, looking down at the Milky Way. To you, Earth is just a point, if that; it’s one-dimensional. To us regular humans, though, it has a full three dimensions — we move back and forth, up and down, and left and right. It’s as if our three dimensions have been squished into a single one of your dimensions. In the same way, the extra dimensions that string theory predicts might just be squished into tiny, subatomic spaces.
The second, more popular idea, is that these extra dimensions are so large that we can’t notice them—just like we can’t notice that the world is round from where we’re standing. Imagine now that you’re playing a game on your shiny new LCD monitor. The game’s characters seem to be moving in three dimensions — they’re hiding behind objects, rushing at you, and running away from you. Of course, they’re not really moving in three dimensions — they’re trapped inside the two dimensions of your screen. What the game’s characters see as three-dimensional space, you see as an optical illusion. This might be what’s happening to our universe — our four dimensions might just be someone else’s optical illusion. With the possibility of extra dimensions, string theory says that it’s possible that one string can stretch to form a huge membrane — or brane, in short — that encompasses our entire universe. So while we think we’ve got it made with four whole dimensions, we’re still trapped inside a brane. And if one string can turn into a brane, so can others — each with its own universe. And just like the characters from your screen can’t jump out into the world, we can’t jump from brane to brane, even if they’re just a millimeter apart. And in all this weird wondrousness, we might have an explanation for gravity — or more specifically, why it’s so weak.
It might not be that gravity is a weak force, after all — it might be just as strong as electromagnetism, but something might be robbing it of its strength.
String theory talks of two types of strings. The first — the open-ended string — is tied to our brane by both its ends. If it wants to travel to another universe, too bad — it can’t. The second, however, is a closed loop, like a rubber-band. It isn’t confined to just one brane, it comes and goes as it pleases. These closed loops make up gravitons — the missing particle in the Standard Model. When bodies exchange gravitons, they are attracted to each other, producing gravity. But since gravitons are closed-loop strings, they’re escaping into other universes, weakening its influence in our universe.
And so, with the flighty graviton, string theory plugs a major hole in the Standard Model — it plausibly ties together all of nature’s forces, becoming the Theory of Everything. But is this too good to be true?
The great test
While string theory seems plausible, it’s only plausible in the way that the existence of God is plausible. There’s no way to prove either notion wrong — but that doesn’t mean they are false. On the other hand, there’s no way to prove them, either, so they aren’t really true, either. According to string theory’s critics, if it’s not testable, it’s not science. Case closed. It became even worse for string theory when it came out in five plausible forms. If there’s supposed to be a single theory that explains the universe, how can it come in five flavors, each of which makes sense? It was a while before these five theories were integrated into the string theory we know today — also called M-theory — but it’ll always have an embarrassingly fuzzy past.
A growing number of scientists, however, have faith in the theory — to them, it just makes too much sense to be wrong. Still, they need to figure out a way to test it, because that faith won’t last forever. And that test might just show up in an experiment we already have high expectations from.
If string theory is correct, there’s a small chance (0.5 percent, to be precise) that the Large Hadron Collider (LHC) will show us the proof. In its atom-smashing frenzy, the LHC might just manage to catch a glimpse of a graviton before it escapes to another dimension. It’s a small probability, but it’s better than nothing. Even if the LHC doesn’t spot a graviton, it’ll do nothing to harm the theory.
For now, however, if we are to believe in string theory, we must remain content with the hope that there might someday be a way to prove or disprove it, and that there really is a single way to describe all the forces in the universe