Everything is not so simple
Imagine that you have a broken egg on your face, and this is not a figure of speech. An attempt at juggling with eggs resulted in one of them falling and breaking on your head, and now you need to go to the shower and change your clothes.
But wouldn't it be easier to turn the time back a minute? After all, the egg broke in just a couple of seconds - why can't you do the same thing, just the opposite? Just put the shell back together, throw in the white and yolk - and that's it. You would have a clean face, clean clothes and no yolk in your hair.
It sounds ridiculous - but why? Why can't I undo this action? In fact, nothing is impossible in this. There is no natural law that would forbid doing this.
Moreover, physicists report that any moment that occurs in everyday life can happen in the opposite order at any moment in time. So why can't you “break back” the eggs, “burn back” the match, or even “dislocate back” the leg?
Why don't these things happen every day? Why is the future different from the past at all? This question looks pretty simple, but to answer it, you need to go back to the birth of the Universe, turn to the atomic world and reach the boundaries of physics.
Like many stories in the world of physics, this one dates back to the great physicist Isaac Newton. The bubonic plague engulfed Britain in 1666, and it was she who forced Newton to leave Cambridge University and go home to his mother, who lived in rural Lincolnshire. There Newton got bored and, being isolated from the outside world, took up physics.
He discovered three laws of motion, including the famous maxim that every action has its own opposition. He also came up with an explanation for why gravity works.
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Newton's Laws are incredibly effective in describing the world around us. They can explain many phenomena, from why apples fall from trees to why the earth revolves around the sun.
But they have a strange property - they work the same way and vice versa. If an egg breaks, then Newton's laws say that it can return to its original state. Obviously this is wrong, but virtually every theory that has been developed by scientists since Newton has exactly the same problem.
The laws of physics simply do not take into account how time flows - forward or backward. They care about it as much as information about whether you write with your right hand or with your left. But you definitely care!
As far as you know, time has an arrow that indicates its direction, and it is always facing the future. You can mix up east and west, but you will never mix up yesterday and tomorrow. However, the fundamental laws of physics do not distinguish between past and future.
The first person who seriously faced this problem was the Austrian physicist Ludwig Boltzmann, who lived in the second half of the 19th century. In those days, all the ideas that are now accepted as an axiom were controversial.
In particular, physicists were not as convinced as they are today that everything in the world is made of particles called atoms. In the opinion of most physicists, the idea of atoms could not be proved, it could not be verified by practical methods.
Boltzmann was convinced that atoms actually exist, so he used this idea to explain all everyday things, such as the flame of fire, the work of the lungs, and also why tea cools when you blow on it. He thought he could understand all these things using the concept that was so close to him - the theory of atoms.
Some physicists were impressed by Boltzmann's work, but most rejected it. He was soon ostracized by the scientific community for his ideas.
However, it was he who showed how atoms are related to the nature of time. In those days, the theory of thermodynamics appeared, which describes how heat behaves. Boltzmann's opponents insisted that the nature of heat could not be described; they said that warmth is just warmth.
Boltzmann decided to prove that they were wrong, and the heat is caused by the chaotic movement of atoms. He was right, but he had to spend the rest of his life defending his point of view.
Boltzmann began by trying to explain something strange - "entropy." According to the laws of thermodynamics, everything in the world has a certain amount of entropy, and when something happens to this object, the entropy increases.
For example, if you put ice cubes in a glass of water, they will melt and the entropy in the glass will rise. And the growth of entropy differs from everything in physics - the process moves in one direction. Physicists have long wondered whether the way time flows is determined by an increase in entropy.
As you might guess, Boltzmann was the first to raise this issue, but then many other scientists began to study this issue. As a result, it became clear that time could potentially flow in the opposite direction - but only if the entropy decreases, which is simply impossible.
However, if time can flow in the opposite direction, it is possible to build a time machine. In 2009, British physicist S. Hawking hosted a party for time travelers - the trick was that he sent out invitations to the party a year later (none of the guests showed up).
So travel back in time is most likely impossible. Even if this possibility existed, Hawking and others argue that you can never get to a point in time until the moment your time machine was built.
But a journey into the future? This is a different story. Of course, all of us time travelers are racing in the flow of time from the past to the future at a rate of one hour per hour. But like a river, the flow of time flows at different speeds in different places. Modern science offers several ways to bring the future closer. Here is a summary of their essence.
The easiest and most practical way to get to the distant future is to move very quickly. According to Einstein's theory of relativity, when you travel at a speed close to the speed of light, time slows down for you in relation to the outside world.
It's not just a hypothesis or thought experiment - it's a measurement result. With the help of two identical atomic clocks (some flew in a jet plane, others remained stationary on Earth), physicists proved that flying clocks tick slower due to speed.
In the case of an aircraft, the effect is minimal. But if you were aboard a spacecraft traveling at 90% the speed of light, time would pass 2.6 times slower for you than on Earth. And the closer your speed approaches the speed of light, the more extreme time travel becomes.
The highest speed achieved thanks to human technology can be called the speed with which protons sweep around the Large Hadron Collider - 99.9999991% of the speed of light. Using the theory of relativity, one can calculate that one second for a proton is equivalent to 27,777,778 seconds or, in practice, 11 months for us.
Surprisingly, particle physicists take deceleration into account when dealing with decaying particles. In the laboratory, muon particles typically decay in 2.2 microseconds. But fast-moving muons, which are produced when cosmic rays reach the upper atmosphere, decay 10 times longer.
The following method is also inspired by the work of Einstein. According to his theory of general relativity, the more you feel gravity, the slower time moves. For example, as you get closer to the center of the Earth, the force of gravity increases. Time passes more slowly for your legs than for your head.
Again, this effect has been measured. In 2010, physicists at the US National Institute of Standards and Technology placed two atomic clocks on shelves, one 33 cm taller than the other, and measured the difference in their ticking speed. The clock on the shelf below ticked more slowly because it was slightly more subject to gravity.
To be in the distant future, all we need is a place with extremely strong gravity, like a black hole. The closer you get to the border, the slower time moves - but this is risky, since crossing the line you can never return. In any case, the effect is not that strong, so the trip is probably not worth it.
Let's say you have the technology to travel long distances to get to a black hole (the closest is about 3000 light years away). During the travel itself, time will slow down much more than during travel through the black hole itself.
(The situation described in Interstellar, where one hour on a planet near a black hole is the equivalent of seven years on Earth, is too extreme and completely impossible for our universe, says Kip Thorne, the film's scientific advisor.)
Perhaps the most amazing thing is that GPS systems must take into account the effects of time dilation (both due to the speed of the satellites and the gravity that acts on them) in their work. Without these corrections, the GPS on the phone will not be able to determine your position on Earth, even within a radius of several kilometers.
Another option for traveling to the future is to slow down the perception of time by slowing down or stopping the life processes of your body, and then restarting them.
Bacterial spores can live for millions of years in suspended animation until the right temperature, humidity and food conditions start their metabolism again. Some mammals, such as bears and squirrels, can slow down their metabolism during hibernation, which greatly reduces their cells' need for oxygen and food. Will people ever be able to do the same?
Although the complete stop of the body's metabolism is not yet subject to modern science, some scientists are working towards achieving the effect of short-term "hibernation" lasting several hours. This may be enough time to help the person survive, for example, during cardiac arrest, before they can be taken to the hospital.
Another technique that puts the body into a hypothermic "hibernation" - replacing blood with cold saline - has worked in pigs and is currently undergoing clinical trials in humans in Pittsburgh.
General relativity also allows for the possibility of rapid travel through time-space tunnels, which could help cover distances of billions of light years or simply different times.
Many physicists, including S. Hawking, believe that the space-time tunnels, constantly appearing in different places of the quantum shell, are much smaller in size than atoms.
The trick is to grab one and enlarge it to human proportions - a feat that will require a tremendous amount of energy, but can only be possible in theory.
Attempts to prove such a method have failed, ultimately due to the incompatibility between general relativity and quantum mechanics.
Based on materials from the journal "Unknown"
