Ask Ethan: How close are we to the Theory of Everything?

The idea that all interactions and particles observed today are manifestations of a single, comprehensive theory is attractive, but it requires additional dimensions and heaps of new types of particles and interactions

Long before Einstein, people who studied the Universe had a dream to find a single equation covering as many phenomena as possible. Instead of having a law for each physical property of the Universe, it would be possible to combine them into a single, comprehensive platform. All the laws of electric charges, magnetism, electric currents, induction and other things were combined into one platform by James Clerk Maxwell in the middle of the XIX century. Since then, physicists have dreamed of a Theory of Everything : a single equation governing all the laws of the Universe. What progress have we made? This is the question of our reader who wants to know:

Has science made progress in relation to the Theory of Great Unification (TVO) and the Theory of Everything? Could you explain what would mean for us if we could find the unifying equation?

Yes, progress has been made, but we have not yet reached the goal. In addition, there is not even confidence that the Theory of Everything exists at all.


Electromagnetic, weak, strong and gravitational interactions are four known fundamental interactions of the Universe.

The laws of nature, as far as we have discovered them, can be divided into four fundamental interactions: the force of gravity, controlled by the General Theory of Relativity, and the three quantum forces controlling the particles and their interaction - strong nuclear interaction, weak nuclear interaction and electromagnetic interaction. The earliest attempts to create a unified theory of everything began shortly after the publication of GR, even before we learned the fundamental laws governing nuclear interactions. These ideas, known as the Kaluza-Klein theory , attempted to combine gravity and electromagnetism.


The idea of ​​combining gravity and electromagnetism dates back to the early 1920s and to the works of Theodore Kaluza and Oscar Klein.

Adding an additional spatial dimension to Einstein GR, the fifth one (except for the standard three spatial and one temporal) resulted in the appearance of Einstein gravity, Maxwell electromagnetism and a new, additional scalar field. The additional dimension should be small enough so as not to interfere with the laws of gravity, and the additional scalar field should not have a discernible effect on the Universe. Since it was impossible to formulate the quantum theory of gravity with such an approach, the discovery of quantum physics and nuclear forces - which this unification attempt was unable to take into account - deprived the approach of popularity.


Quarks, antiquarks and gluons of the Standard Model have a colored charge, except for all other properties, such as mass and electric charge. The standard model can be written as a single equation, but within it interactions will not be merged.

However, strong and weak nuclear interactions led to the formulation of the Standard Model in 1968, which brought together strong, weak and electromagnetic interactions under one all-encompassing umbrella. All particles and their interactions were taken into account, several new predictions were made, including a large prediction about combining. At high energies of the order of 100 GeV (the energy required to accelerate one electron to a potential of 100 billion volts), the symmetry uniting the electromagnetic and weak interactions should be restored. The existence of new massive bosons was predicted, and with the discovery of the bosons W and Z in 1983, this prediction was confirmed. Four fundamental interactions have been reduced to three.


The idea of ​​unification suggests that all three interactions of the Standard Model, and possibly even gravity, at high energies are combined into a single platform.

The union was already an interesting idea, but models began to develop it. People have suggested that at even higher energies strong interaction will unite with the electroweak; it was from here that the idea of ​​Great Unification Theories arose. Some have suggested that at even higher energies, perhaps around the Planck scale, gravity will also join the rest; This was one of the main motivations of string theory. An interesting feature of these ideas is that if you need a union, you need to restore symmetries at high energies. And if the Universe at high energies has symmetries broken at the moment, this can be translated into something observable: new particles and new interactions.


Particles of the Standard Model and their supersymmetric twins. This spectrum of particles is an inevitable consequence of the unification of the four fundamental interactions in the context of string theory.

So what new particles and interactions are predicted? It depends on which of the variants of theories of association to choose. These include:

• Heavy, neutral particles, similar to dark matter.
• Supersymmetric particle partners.
• Magnetic monopoles.
• Heavy, charged scalar bosons [with zero spin / approx. trans.].
• Many particles similar to the Higgs particle.
• Intermediary particles in proton decay.

Although we can confidently assert the existence of dark matter from indirect observations, none of these particles or the predicted decays were observed in experiments.


In 1982, in an experiment led by Blas Cabrera , with eight loops of wire, a change in eight magnetons was recorded: a sign of a magnetic monopole. Unfortunately, there was no one at the time of detection in the laboratory, and no one has since been able to reproduce this experiment or find a second monopole.

And it's a pity - for many reasons, since we were very actively looking for it all. In 1982, one of the experiments that looked for magnetic monopoles registered a single positive result, thanks to which he had many followers trying to discover monopoles. Unfortunately, that positive result was an anomaly, and it was never reproduced by anyone. Also in the 1980s, people began to build giant cisterns with water and other atomic nuclei in search of evidence of proton decay. As a result, these tanks were altered under neutrino sensors, and not a single proton decay was recorded. The limitation of the proton lifetime is now more than 10 ³⁵ years - about 25 orders of magnitude more than the age of the Universe.


The tank of the Super-Kamiokande experiment filled with water, which established the most stringent restrictions on the proton lifetime. Later, such detectors became excellent neutrino observatories, but did not register a single proton decay.

This is also bad, since the Grand Unification offers a neat and elegant way to create an asymmetry between matter and anti-matter in the Universe. In the early days, the Universe was hot enough to produce matter / antimatter vapors and all the particles that could be. In most TVOs, two such existing particles are superheavy bosons X and Y, which have charges and contain pairs of quarks and leptons. It is expected that the asymmetry is manifested in how their versions for matter and antimatter break up, which can lead to a predominance of remnants of matter over antimatter, even if initially there was no dominance. Unfortunately, again, we still have to find evidence of the presence of such particles and their interactions.


A symmetric collection of bosons of matter and antimatter (X, Y, anti-X, anti-Y) can, with the necessary properties of TVO, generate the asymmetry of matter / antimatter observed in the universe today

Some physicists stand on the fact that there must be such symmetries in the Universe, and that their evidence is simply outside the energies available at LHC. Others come to a less convenient opportunity: nature is probably not seeking to unite. Perhaps there is no TVO describing our physical reality; perhaps the quantum theory of gravity does not combine with other interactions; it is possible that problems of baryogenesis and dark matter have other solutions that are not following from these ideas. After all, the final arbiter of what the Universe looks like is not our ideas about this, but the results of experiments and observations. We can only ask the universe what it is; we have to listen to the answer and act on it.


The Lagrangian of the Standard Model is a single equation that includes particles and the interactions of the Standard Model. It has five independent parts: gluons (1), weak bosons (2), interaction of matter with weak interaction and the Higgs field (3), spirits-particles, eliminating the redundancy of the Higgs field (4) and Faddeev-Popov spirits , affecting redundancy weak interaction (5). Neutrino masses are not included here.

Although we can write the Standard Model in the form of a single equation, this is not a unification theory in the sense that different, separate, independent members control the different components of the Universe. Different parts of the Standard Model do not interact with each other - the color charge does not affect the electromagnetic or weak interactions. There is also no answer to the questions about why interactions such as CP-invariance violation do not occur in the strong interaction that should occur.


When restoring symmetries (at the highest value of the potential), a combination occurs. However, the violation of symmetries at the base of the hill corresponds to the Universe that we have today, where there are new types of massive particles.

Many hope that the union contains the answer to these questions, and that it will solve many open problems and mysteries of modern physics. However, any kinds of additional symmetries — symmetries that are restored at high energies, and now broken — lead to the emergence of new particles, new interactions, and new physical rules that the Universe should play. We tried to carry out reverse engineering of some predictions, using those rules that are necessary for everything to work - however, the particles and associations that we hoped to see did not appear. Combining will not help you to bring out such manifest properties as chemistry, biology, geology, consciousness - but it will help us better understand where it came from and how it all came from.


The cosmic history of the entire known universe shows that we owe the origin of all matter and the whole world to the end of inflation and the beginning of the hot Big Bang.

Of course, there is another possibility: that the Universe may not unite. The fact that many different laws and rules exist for a reason: the symmetries invented by us are just mathematical features, and not descriptions of the physical Universe. For every elegant, beautiful, convincing physical theory, there is another, equally elegant, beautiful and persuasive physical theory - but false. In these, as in all scientific questions, the task of mankind is to ask the right questions. Well, the task of the universe is to provide us with the answers. Whatever they are, we have such a universe as it is. And we need to understand what these answers mean.

Ethan Siegel - astrophysicist, popularizer of science, blog Starts With A Bang! He wrote the books Beyond The Galaxy , and Treknologiya: Star Trek Science [ Treknology ].

FAQ: if the universe is expanding, why aren't we expanding ; why the age of the universe does not coincide with the radius of the observed part of it .