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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