Until the beginning of the twentieth century, it was believed that time isabsolute value. But after Albert Einstein published general theory of relativity (GR), it became clear that time is a more subjective concept and has to do with the observer who measures it. And yet, many continued to interpret time as if it was a straight railway line, along which you can only move forward or backward. But what if this railway line branches out or even has circumferential tracks, moving along which the train returns to the station that it has already passed? In other words, is it possible to travel to the future or the past? Beginning with HG Wells' famous novel The Time Machine, science fiction writers are fantasizing about everything. But in real life, it is impossible to imagine something like that. After all, if someone in the future had invented a time machine, would he really not have warned us about the threat of the COVID-19 pandemic or about the dire consequences of global warming? But nobody came to us. Perhaps it is worth looking at time from a different angle?

Quantum mechanics is a branch of theoretical physics that describes physical phenomena in which the action is comparable in magnitude to Planck's constant.

Content

- 1 Progress of time
- 2 Can you go to the past?
- 3 Transition to the quantum level
- 4 The arrow of time
- 5 Conclusions

The passage of time

Our concept of time goes back to the painting,described by Isaac Newton: the arrow of time moves only forward, depriving us of any opportunity to return back to the past. At the same time, general relativity says that the course of time is different for observers in different gravitational fields.

This means that time passes near the surface of the Earth.slower, as the force of gravity on the planet is stronger than in orbit. And the stronger the gravitational field, the greater this effect. You can read more about why time at the top of the mountain and at the beach flows differently here.

It turns out that Newton's laws of motion put an endthe idea of the absolute position of time in space, and the theory of relativity put an end to this idea. Moreover, as physicists Stephen Hawking and Leonard Mlodinov write in their book A Brief History of Time, time travel is possible.

This is interesting: Guide to the theory of the Multiverse: are there other worlds?

The theory of relativity shows that the creationa time machine capable of moving us into the future is indeed possible. After its creation, all you have to do is go inside, wait a while, and then go out - and find that time on Earth went differently than for you. That is, much faster. Of course, no one on the planet possesses such technologies, but their appearance is a matter of time. After all, if you think carefully, what is needed to invent such a machine?

First, it must accelerate to near-lightspeeds (remember that the speed of light reaches 300,000 km / s), and secondly, we should recall the famous paradox of twins, with which physicists are trying to prove the contradiction of the special theory of relativity, which says that from the point of view of "stationary" observers, all processes moving objects slow down.

According to the special theory of relativity (SRT), all physical laws are the same for all freely moving observers, regardless of their speed.

Let's clarify a little - this method assumes,that the time machine you entered takes off, accelerates to near-light speed, travels that way for a while (depending on how far ahead in time you go) and then comes back. When the journey ends, leaving the time machine, you realize that much less time has passed for you than for all the inhabitants of the Earth - you have made a journey into the future. But if from now on we perceive time differently, maybe the laws of physics will tell you how to travel into the past?

Don't Miss: What is Einstein's General Theory of Relativity?

Can you travel back in time?

The first hint of what a person can dotime travel, appeared in 1949 when the Austrian mathematician Kurt Gödel found a new solution to Einstein's equations. Or a new structure of space-time, admissible from the point of view of general relativity.

In general, speaking about Einstein's equations, it is important to understand that they satisfy many different mathematical models of the Universe. These models differ, for example, in initial or boundary conditions.

And in order to understand whether they correspond to the universe in which we live, we must check their physical predictions.

Gödel, being a mathematician, became famous for the fact thatproved - not all true statements can be proved, even if it comes down to trying to prove all true statements, for example, using simple arithmetic. Thus, like the uncertainty principle, Gödel's incompleteness theorem can be a fundamental limitation of our ability to know and predict the universe.

**The uncertainty principle** - the principle formulated by Heisenberg and asserting that it is impossible to simultaneously accurately determine both the position and the speed of a particle; the more accurately we know one thing, the less certain the other.

Interestingly, Gödel's space-time hadan interesting feature: the universe in his view rotated as a whole. But Einstein was very upset that his equations allowed such a solution. General relativity in his understanding should not allow time travel. Gödel's equation, however, does not correspond to the universe in which we live, but his work allowed the world to look at time (and at the same time at the universe) differently.

So, space-time is known to be cramped.interconnected. This means that the issue of time travel is intertwined with the issue of travel at speeds in excess of 300,000 km / s, that is, the speed of light. And when it comes to photons, general relativity, alas, fades into the background, and quantum mechanics takes its place.

More about what quantum mechanics studies, and most importantly how, we talked about in this article, I recommend reading it!

Going to the quantum level

Not so long ago, a team of physicists from UniversitiesVienna, Bristol, the Balearic Islands and the Institute for Quantum Optics and Quantum Information (IQOQI-Vienna) have shown how quantum systems can simultaneously develop along two opposite time arrows (forward and backward in time). In other words, **quantum systems can move both forward and backward in time**.

Earlier, to understand why, scientists found that time knows only one direction - forward. So you and I will have to remember **second law of thermodynamics**... It says that in a closed system **system entropy** (that is, the measure of disorder and randomness within the system) remains constant or increases.

See also: Is there time travel without paradoxes?

Если наша Вселенная представляет собой замкнутый a cycle rolled into a ball, its entropy can never decrease, which means that the universe will never return to an earlier point. But what if the arrow of time "looked" at phenomena where the changes in entropy are small?

The second law of thermodynamics is statisticala law that is, on average, true for a macroscopic system. In a microscopic system, we can see how the system naturally evolves towards situations with lower entropy, the authors of the scientific work write.

Here's what Giulia Rubino, the scientifica researcher at the University of Bristol and lead author of a new article: “Let's assume that at the beginning, the gas in the vessel only takes up half of it. Then imagine that we remove the valve that kept it within half of the vessel so that the gas can now expand freely throughout the vessel. "

As a result, we will see that the particles will startmove freely throughout the volume of the vessel. Over time, the gas will occupy the entire vessel. “In principle, there is a nonzero probability that at some point the gas will naturally return to fill half of the vessel, only this probability becomes smaller as the number of particles that make up the gas becomes larger,” explains Rubino.

Don't Miss: Is Time Travel Possible?

If there were only three particles of gasinstead of a huge amount of gas (consisting of billions of particles), these few particles could be back in the same part of the vessel from which they originally started. This is the physics.

Next, as you might have guessed, follows the secondthe law of thermodynamics is the so-called statistical law, which is true on average for a macroscopic system. “In a microscopic system, we can see how the system naturally evolves towards lower entropy situations,” the researchers note.

Arrow of time

To understand even more, we note that in the course of a new study, physicists wondered about the consequences of applying the paradigm described above in the quantum domain. According to **principle of quantum superposition**, individual units (for example, light) canexist simultaneously in two states, both in the form of waves and in the form of particles, manifesting itself in one form or another depending on what exactly you are testing.

You might be wondering: Does quantum mechanics imply a plurality of worlds, or what is Everett's interpretation?

Rubino's team examined quantum superpositionwith a state that develops both backward and forward in time. Measurements have shown that more often than not, the system ends up moving forward in time. If not for small changes in entropy, the system could indeed continue to evolve both forward and backward in time.

So how do these complex physical concepts relate to real human experience? Is it finally time to start packing for your time travel? Alas.

“We humans are macroscopic systems.We cannot perceive these quantum superpositions of temporal evolutions, ”says Rubino. For us, time is really moving forward. Perhaps this is the case when the world is a little undecided.

And indeed - at the most fundamental levelthe world is made up of quantum systems (which can move back and forth). A deeper understanding of how to describe the passage of time at the level of these elementary components could allow physicists to formulate more accurate theories to describe them and, ultimately, gain a deeper understanding of the physical phenomena of the world in which we live.

Read even more interesting articles about everything in the world, as well as about time travel and the Multiverse on our channel in Yandex.Zen. There are regularly published articles that are not on the site!

findings

However, not everyone agrees that the difference betweenmacroscopic and microscopic is clear. According to Popular Mechanics, Ramakrishna Podila, assistant professor of physics and astronomy at Clemson University in South Carolina, says that many-particle statistics are a more accurate way of describing things than single-particle statistics.

Even one particle has its own unique microstates. Podila believes that in our quest to understand time, we put equations above physical reality - and miss the point.

Linking the arrow of time to entropy, orcollapse of a quantum mechanical system (as indicated in the article) are not formal statements, but popular methods that are easy to use. Even the fact that time moves forward is not in itself an axiom, but a theory that astrophysicist Arthur Eddington invented and popularized in 1927.

More on the topic: Can quantum mechanics explain the existence of spacetime?

So maybe the idea that space andtime merge into one intertwined continuum, has the right to life. Since Albert Einstein formulated the theory of relativity, we stopped perceiving space as a three-dimensional figure, and time as one-dimensional.

Time has become the fourth element of the four-dimensional vector that describes space and time, says Rubino. This is a single, dynamic entity over which we are still racking our brains.

In conclusion, I want not only to thankthe reader for their attention, but also to quote scientists again: “Although time is often viewed as a continuously increasing parameter, our research shows that the laws governing its flow in quantum mechanical contexts are much more complex. This could mean that we need to rethink how we represent this quantity in all those contexts where quantum laws play a critical role. "

Fully read the text of the scientific workcan be in the journal Nature. By the way, do you think it is possible to travel in time and what new research tells us about the universe? We will wait for the answer here, as well as in the comments to this article!