This Is Why Physicists Think String Theory Might Be Our 'Theory Of Everything'

It's one of the most brilliant, controversial and unproven ideas in all of physics: string theory. At the heart of string theory is the thread of an idea that's run through physics for centuries, that at some fundamental level, all the different forces, particles, interactions and manifestations of reality are tied together as part of the same framework. Instead of four independent fundamental forces — strong, electromagnetic, weak and gravitational — there's one unified theory that encompasses all of them.

In many regards, string theory is the best contender for a quantum theory of gravitation, which just happens to unify at the highest-energy scales. Even though there's no experimental evidence for it, there are compelling theoretical reasons to think it might be true. Back in 2015, the top living string theorist, Ed Witten, wrote a piece on what every physicist should know about string theory. Here's what that means, even if you're not a physicist.

When it comes to the laws of nature, it's remarkable how many similarities there are between seemingly unrelated phenomena. The mathematical structure underlying them is often analogous, and occasionally even identical. The way that two massive bodies gravitate, according to Newton's laws, is almost identical to the way that electrically charged particles attract-or-repel. The way a pendulum oscillates is completely analogous to the way a mass on a spring moves back-and-forth, or the way a planet orbits a star. Gravitational waves, water waves, and light waves all share remarkably similar features, despite arising from fundamentally different physical origins. And in the same vein, although most don't realize it, the quantum theory of a single particle and how you'd approach a quantum theory of gravity are similarly analogous.

The way quantum field theory works is that you take a particle and you perform a mathematical "sum over histories." You can't just calculate where the particle was and where it is and how it got to be there, since there's an inherent, fundamental quantum uncertainty to nature. Instead, you add up all the possible ways it could have arrived at its present state (the "past history" part), appropriately weighted probabilistically, and then you can calculate the quantum state of a single particle.

If you want to work with gravitation instead of quantum particles, you have to change the story a little bit. Because Einstein's General Relativity isn't concerned with particles, but rather the curvature of spacetime, you don't average over all possible histories of a particle. In lieu of that, you average instead over all possible spacetime geometries.

Working in three spatial dimensions is very difficult, and when a physics problem is challenging, we often try and solve a simpler version first. If we go down to one dimension, things become very simple. The only possible one-dimensional surfaces are an open string, where there are two separate, unattached ends, or a closed string, where the two ends are attached to form a loop. In addition, the spatial curvature — so complicated in three dimensions — becomes trivial. So what we're left with, if we want to add in matter, is a set of scalar fields (just like certain types of particles) and the cosmological constant (which acts just like a mass term): a beautiful analogy.

The extra degrees of freedom a particle gains from being in multiple dimensions don't play much of a role; so long as you can define a momentum vector, that's the main dimension that matters. In one dimension, therefore, quantum gravity looks just like a free quantum particle in any arbitrary number of dimensions..

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