There are few theories in physics with the potential to rewrite reality quite like string
theory. A theory so monumental in scope and ambition, it aims to accomplish the seemingly
impossible task of providing a singular model to explain the laws of the universe at all scales.
But, regardless of its scope and its growing fame, the scientific community has never agreed
upon its validity. And, for the watching world, it can still feel a little difficult to fathom.
So, this is Unveiled, and today we’re answering the extraordinary question;
what is string theory, and how exactly does it work?
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To over simplify it somewhat - string theory attempts to describe all particles in the universe
as infinitesimally small and vibrating strings. These strings are then one-dimensional objects,
and are hypothesized to be the most fundamental objects in the universe. If it’s correct,
then everything you are, know, see, even think, is the product of these strings.
Over a century ago, in 1915, Albert Einstein presented his newly formulated Field Equations to
the Prussian Academy of Science. These equations provided a beautiful description of how matter
interacts with each other via the force of gravity. They went above and beyond the work
carried out by Isaac Newton a few centuries prior, and explained gravity at the largest scales in
the universe, alongside predicting other phenomena such as time dilation and black holes. Since then,
the model has been proven in multiple ways, and is considered one of the most important
theories in physics. Unfortunately, however, as great as it is, it isn’t complete. General
Relativity - easily one of the most influential contributions to science - still doesn’t provide
an explanation for how gravity interacts on the scale of particles, in the quantum limit.
Surprisingly, the force of gravity is the weakest of the four fundamental forces,
which is perhaps not what you’d expect given that gravity literally moves planets. But,
this means that on the smallest scales of all (atomic and subatomic) gravitational interactions
are incredibly difficult to detect. All of the four fundamental forces have (or should have) a
respective particle that’s responsible for their effect… for example the photon, which mediates
the electromagnetic force. Gravity is then predicted to have its own mediating particle,
called the graviton. But, and again due to that comparative weakness of the gravitational force,
actually detecting the graviton is very, very hard to do. So hard, in fact,
that currently the graviton remains theoretical, only… and is thought to be impossible to observe.
And here’s where we get to string theory. In the 1960s, string theory began formulation
as an attempt to explain how what’s known as the strong nuclear force behaves, which is the
force responsible for holding hadronic particles like protons and neutrons together. The theory
proceeded to develop further in the 1970s, though, eventually attempting to describe all known
particles in the Standard Model. It was then in 1974 that the Japanese physicist, Tamiaki Yoneya,
realized that all forms of string theory predicted a massless particle, too. What was
doubly exciting, however, was that this massless particle behaved exactly as was predicted for the
hypothetical graviton. This was monumental, as no theory had naturally provided an explanation
for the graviton before… and still no other theory has since. It was soon after this that
the separate theory of Quantum Chromodynamics took over from string theory as the leading explanation
of hadrons, specifically. However, by now string theory seemingly had bigger fish to fry in
physics… its main purpose shifted, and it became considered as the major contender for explaining
gravity on a quantum scale. As, potentially, the major contender toward a theory of everything.
So that’s the story behind it but, still, how does string theory actually work? Again,
it predicts every particle in the Standard Model, at their most fundamental,
to be vibrating strings - which are assumed to be a Planck length in size;
i.e., the shortest measurable distance. These strings are one-dimensional objects,
and the frequency of their quantum vibrations is what determines what type of particle
the string represents. From a mathematical perspective, the theory is pretty incredible,
providing many advancements which have now been applied to numerous other fields of science. On
the other hand, from a physical perspective, string theory has historically proven to be
a bit of a nightmare. During the 1980s, there were five separate string theories formulated,
with one particular version of those requiring a mind blowing 26 dimensions of reality to
work. But, eventually, in 1995, Professor Edward Witten proposed a new version called M-Theory,
which unified all five of the previous theories into one, and predicted only 11 dimensions in
total - ten spatial and one time. Meanwhile, there is another string theory - superstring
theory - that manages to work with only 9 spatial dimensions predicted, and one time; ten in total.
Broadly, though, what’s clear is that all types of string theory require extra,
unobservable dimensions. So what (or where) are these other dimensions? They’re generally
predicted to be compact; incredibly small and curled up. But, unfortunately,
as yet no one has been able to consistently explain the geometry of any of them,
and so there are essentially infinite ways that these extra dimensions could be described. And,
somewhat despairingly, it seems that the more research that’s been put into figuring them out,
the further string theory has strayed from working. On top of this, there’s no clear
way to physically test for the presence of string theory’s extra dimensions, either. So,
without any mathematical validity or physical observability, the quest has reached a bit of
a dead end… arguably (and perhaps substantially) damaging string theory’s reputation and momentum.
Today, many scientists then consider string theory to be a premature effort to solve the
problem of quantising gravity. General relativity was considered to be an almost fully formulated
theory within just years of its proposal. On the other hand, Quantum Field Theory, the theory
that explains how the fundamental particles of the universe behave, was nowhere near as fully
developed and formulated until the latter half of the century. String theory might’ve been like
quantum field, then… but, the difference is that in the decades since they first turned up, quantum
field theory has made significant progress, while string theory has floundered and become dormant.
There are direct links to be made, too. The main principle of quantum field theory is to describe
all particles as fields, as opposed to them being concentrated point charges. A simplification of
this concept would be to imagine all particles as clouds, as opposed to smooth balls. Probably
the most important offshoot here, however, is something called an Effective Field Theory.
Among other things, this is an intuitive method of making particle calculations, without knowing
the complete specifics of the physical phenomena they’re describing. Effective fields are then free
from mathematical restrictions, which in the days of Einstein made quantising gravity impossible. In
the past decade, there has been significant progress on formalizing general relativity
as an effective field theory. To finally explain how gravity works at a quantum scale.
So, what does this mean for string theory? Its past is complex and its future is a
little muddled. String theory hasn’t ever really been one singular theory,
and there has never been general agreement on which one is best. What’s more,
many of the researchers who pioneered string models now currently work in adjacent fields,
and are trying to look at quantum gravity from other perspectives. Tools that work within the
framework of string theory are still powerful, however. For example, calculations on how
gravitons scatter off each other have been done using entirely string-based methods. Regardless
of string theory’s seeming failure at producing a singular formal theory of everything, then,
its achievements and advances in mathematics should not go unnoticed or unpraised. To this
day it still remains the only theory to naturally predict a graviton, after all… as opposed to the
plethora of rival theories, such as Loop Quantum Gravity, which have never achieved the same feat.
In modern day physics, string theory doesn’t have the same momentum as it used to,
but its impact has still been significant. Beauty can still be seen within it, as it predicts even
the highest powers of the universe to be, at their base, simplistic, fundamental strings.
As for the future, there’s a chance we may someday find a new version of the theory,
free from the problems of prior attempts. Research today in the field is minimal, but there are still
dedicated teams devoted to making progress. It could yet be that we simply don’t have
the required mathematical tools at the moment… but, with time, we might develop them. For some,
there’s also the chance that string theory will one day solve (or contribute to solving)
some of the other big questions in physics, like uncovering the mystery of dark matter.
What do you think will happen? Do you think that, at its vibrating,
stringy core, this model could still prove to be the answer to everything? Or is there another,
entirely new theory just around the corner, one we are yet to even conceive?
What do you think? Is there anything we missed? Let us know in the comments, check out these other
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