Legend has it that Albert Einstein spent his last hours on Earth tracing something on a piece of paper in a final attempt to formulate a theory of everything. Sixty years later, another legendary scientist in the field of theoretical physics, Stephen Hawking, will leave this world with similar thoughts. We know that Hawking believed that the so-called M-theory was our best chance to create a complete theory of the universe. But what is it?
Ever since Einstein's general theory of relativity was formulated in 1915, every theoretical physicist has dreamed of reconciling our understanding of the infinitely small world of atoms and particles with the infinitely large scale of space. While the latter is perfectly described by Einstein's equations, the former is predicted with extraordinary accuracy by the so-called Standard Model of Fundamental Interactions.
Our current understanding is that the interaction between physical objects is described by four fundamental forces. Two of them - gravity and electromagnetism - appear for us at the macroscopic level, we deal with them every day. The other two - weak and strong interactions - appear on a very small scale and only when we are dealing with subatomic processes.
The Standard Model of Fundamental Interactions provides a single structure for three of these forces, but gravity does not want to fit into this picture in any way. Despite the accurate description of large-scale phenomena such as the behavior of a planet in orbit or the dynamics of galaxies, general relativity fails at very short distances. According to the Standard Model, all forces are mediated by certain particles. In the case of gravity, the work is done by the graviton. But when we try to calculate the interactions of these gravitons, meaningless infinities appear in the equations.
A complete theory of gravity must work at any scale and take into account the quantum nature of fundamental particles. This would allow gravity to fit into a combined structure with three other fundamental interactions, thereby creating the notorious theory of everything. Of course, since Albert Einstein died in 1955, significant progress has been made in this area. Our best candidate today is called M-theory.
The string revolution
To understand the basic idea of M-theory, you need to go back to the 1970s, when scientists realized that instead of describing the universe based on point particles, it would be better to describe them as oscillating strings (energy tubes). A new way of understanding the fundamental constituents of nature has led to the solution of many theoretical problems. First of all, a single vibration of a string can be interpreted as a graviton. And unlike standard gravity, string theory can describe its interactions mathematically and not get weird infinities. This means that gravity can be included in the combined structure.
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After this exciting discovery, theoretical physicists have worked hard to understand its consequences. But, as is often the case with scientific research, the history of string theory is full of ups and downs. At first, people were perplexed that she predicted the existence of a particle that moves faster than light, the so-called "tachyon". This prediction contradicted all experimental observations and cast a serious shadow over string theory.
Nevertheless, this issue was resolved in the early 1980s with the introduction of so-called “supersymmetry” into string theory. She predicts that each particle has its own superpartner and, by an unusual coincidence, the same condition actually eliminates the tachyon. This first success is widely known as the “first string revolution”.
Another unusual feature is that string theory requires ten space-time dimensions. Currently, we only know four: depth, height, width, and time. While this appears to be a major obstacle, several solutions have been proposed so far, and it currently seems to be more of an unusual feature than a problem.
For example, we could exist in a four-dimensional world without any access to additional dimensions. Or the extra dimensions could be “compact” and fit into such small scales that we would not notice them. However, different compactifications would lead to different values of physical constants and different laws of physics. A possible solution is that our Universe is only one of many in an infinite “multiple universe” governed by different physical laws.
M-theory
There was one more problem that haunted the string theorists of the day. Careful classification revealed the existence of five distinct sequential string theories, and it was unclear why nature should choose one of the five.
This is where M-theory comes into play. During the second string revolution in 1995, physicists suggested that five successive string theories are in fact different faces of a unique theory that exists in eleven time-space dimensions called M-theory. It incorporates each string theory in a variety of physical contexts while remaining workable for everyone. This incredibly fascinating picture has led most theoretical physicists to the idea that M-theory will become a theory of everything - and it is also mathematically more consistent than any other proposed theory.
Be that as it may, so far M-theory has not been able to produce predictions that can be verified experimentally. Supersymmetry is currently being tested at the Large Hadron Collider. If scientists could find signs of the existence of super partners, this would finally strengthen the position of M-theory. But modern theoretical physics is not yet able to give verifiable predictions, and experimental physics cannot present experiments for this verification.
Most of the great physicists and cosmologists are obsessed with finding this beautiful and simple description of the world that could explain everything. And although we are still far from this, without brilliant and creative people like Hawking, this would be completely impossible.
Ilya Khel