What is time? Augustine the Blessed said: "I know what time is, until I think about it." According to the standard model of physics, time is the fourth dimension, in addition to the three spatial dimensions. So you can pass through it. For years, science fiction writers have relished the possibilities of time travel in a variety of ways. Every century we master more and more new technologies, discover new aspects of science. What is left for us to learn about time travel before we start making it a reality?
You may have noticed that we are constantly moving in time. We move through it. At the basic level of the concept, time is the rate of change of the universe, and regardless of whether we like it or not, we are subject to constant changes. We get old, the planets move around the Sun, things are destroyed.
We measure the passage of time in seconds, minutes, hours and years, but this does not mean at all that time flows at a constant speed. Like water in a river, time passes in different ways in different places. In short, time is relative.
But what causes temporary fluctuations on the way from cradle to grave? It all comes down to the relationship between time and space. Man is able to perceive in three dimensions - length, width and depth. Time complements this party as the most important fourth dimension. Time does not exist without space, space does not exist outside of time. And this couple is connected in a space-time continuum. Any event that takes place in the Universe must involve space and time.
In this article, we will consider the most real and everyday possibilities of travel through time in our Universe, as well as less accessible, but no less possible paths through the fourth dimension.
Temporary travel to the future
If you want to live a couple of years a little faster than someone else, you have to deal with space-time. Global positioning satellites do this every day, three billionths of a second ahead of the natural course of time. In orbit, time flows faster because the satellites are far from Earth's mass. And on the surface, the mass of the planet carries with it time and slows it down on a relatively small scale.
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This effect is called gravitational time dilation. According to Einstein's theory of general relativity, gravity bends spacetime, and astronomers use this corollary when studying light passing near massive objects.
But what does this have to do with timing? Remember - any event that occurs in the universe involves both space and time. Gravity not only pulls together space, but also time.
Being in the flow of time, you will hardly notice a change in its course. But objects that are massive enough - like the supermassive black hole alpha Sagittarius, located in the center of our galaxy - will seriously warp the fabric of time. The mass of its singularity point is 4 million suns. This mass slows down time in half. Five years in orbit of a black hole (without falling into it) is ten years on Earth.
The speed of movement also plays an important role in the speed of the current time. The closer you get to the maximum speed of movement - the speed of light - the slower time flows. By the end of the journey, the clock on a fast-moving train will begin to be "late" by one billionth of a second. If a train reaches 99.999% light speed, in one year in a train carriage, you can travel two hundred and twenty-three years into the future.
In fact, hypothetical future journeys in the future are built on this idea, pardon the tautology. But what about the past? Can you turn back time?
Temporary travel in the past
We found that the journey into the future happens all the time. Scientists have proven this experimentally, and the idea is at the heart of Einstein's theory of relativity, which turns 100 this year. It is quite possible to move into the future, the only question is "how quickly"? When it comes to travel back in time, the answer is to look at the night sky.
The Milky Way Galaxy is about 100,000 light-years wide, which means that light from distant stars must travel thousands and thousands of years before it reaches Earth. Catch this light, and, in essence, you will simply look into the past. When astronomers measure cosmic microwave radiation, they are looking into the cosmos as it was 10 billion years ago. But that's not all.
There is nothing in Einstein's theory of relativity that precludes the possibility of traveling to the past, but the very possible existence of a button that could bring you back to yesterday violates the law of causality, or cause and effect. When something happens in the Universe, the event generates a new endless chain of events. The cause is always born before the effect. Just imagine a world where the victim would die before the bullet hits her head. This is a violation of reality, but despite this, many scientists do not exclude the possibility of travel to the past.
For example, it is believed that moving faster than the speed of light can send back into the past. If time slows down as an object approaches the speed of light, can breaking this barrier turn back time? Of course, when approaching the speed of light, the relativistic mass of the object also grows, that is, it approaches infinity. It seems impossible to accelerate an infinite mass. In theory, warp speed, that is, the deformation of speed as such, can deceive the universal law, but even this will require colossal energy costs.
What if time travel to the future and the past depends not so much on our basic knowledge of space, but more on existing cosmic phenomena? Let's take a look at a black hole.
Black holes and Kerr rings
Orbit around the black hole long enough, and gravitational time dilation will throw you into the future. But what if you fall right into the jaws of this cosmic monster? We have already written about what will happen when plunging into a black hole, but we did not mention such an exotic variety of black holes as the Kerr ring. Or Kerr's black hole.
In 1963, New Zealand mathematician Roy Kerr proposed the first realistic theory of a rotating black hole. The concept includes neutron stars - massive collapsing stars the size of St. Petersburg, for example, but with the mass of the Earth's Sun. We have included neutron holes in the list of the most mysterious objects in the Universe, calling them magnetars. Kerr theorized that if a dying star collapsed into a rotating ring of neutron stars, their centrifugal force would prevent them from becoming a singularity. And since the black hole will not have a singularity point, Kerr figured that it would be possible to get inside, without fear of being torn apart by gravity in the center.
If Kerr black holes exist, we could pass through them and exit into the white hole. It's like the tailpipe of a black hole. Instead of sucking in everything it can, the white hole will, on the contrary, throw out everything it can. Perhaps even in another time or another universe.
Kerr black holes remain a theory, but if they do exist, they are portals of sorts, offering one-way travel to the future or past. And while an extremely advanced civilization could evolve in this way and travel through time, no one knows when the "wild" Kerr black hole will disappear.
Wormholes (wormholes)
Theoretical Kerr rings are not the only way to possibly "shortcut" paths to the past or the future. Science fiction films - from Star Trek to Donnie Darko - often deal with the theoretical Einstein-Rosen bridge. To you these bridges are better known as wormholes.
Einstein's general theory of relativity allows the existence of wormholes, since the theory of the great physicist is based on the curvature of space-time under the influence of mass. To understand this curvature, imagine the fabric of spacetime as a white sheet and fold it in half. The area of the sheet will remain the same, it will not deform itself, but the distance between the two points of contact will clearly be less than when the sheet was lying on a flat surface.
In this simplified example, space is depicted as a two-dimensional plane, and not four-dimensional, which it actually is (remember the fourth dimension - time). Hypothetical wormholes work in a similar way.
Fast forward to space. The concentration of mass in two different parts of the universe could create a kind of tunnel in space-time. In theory, this tunnel would connect two different segments of the space-time continuum with each other. Of course, it is quite possible that some physical or quantum properties prevent such wormholes from emerging on their own. Well, or they are born and immediately perish, being unstable.
According to Stephen Hawking, wormholes can exist in quantum foam, the smallest medium in the universe. Tiny tunnels are constantly being born and burst, linking separate places and times for short moments.
Wormholes may be too small and short-lived for a person to move, but what if one day we can find, hold, stabilize and enlarge them? Provided, Hawking notes, that you will be ready for feedback. If we want to artificially stabilize the space-time tunnel, the radiation from our actions can destroy it, just as sound reversion can damage a speaker.
Cosmic strings
We're trying to squeeze through black holes and wormholes, but is there another way of time travel using a theoretical cosmic phenomenon? With these thoughts in mind, we turn to physicist J. Richard Gott, who outlined the idea of a cosmic string in 1991. As the name suggests, these are hypothetical objects that may have formed in the early stages of the development of the universe.
These strings permeate the entire universe, being thinner than an atom and under strong pressure. Naturally, it follows from this that they give gravitational thrust to everything that passes near them, which means that objects attached to the cosmic string can travel in time with incredible speed. If you pull two cosmic strings closer to each other or place one of them next to a black hole, you can create what is called a closed timelike curve.
Using the gravity produced by two cosmic strings (or a string and a black hole), the spacecraft could theoretically send itself back in time. To do this, you would need to make a loop around the cosmic strings.
By the way, quantum strings are hotly debated right now. Gott stated that to travel back in time, one would loop around a string containing half the mass-energy of an entire galaxy. In other words, half of the atoms in the galaxy would have to be used as fuel for your time machine. Well, as everyone knows, you cannot go back in time before the machine itself was created.
In addition, there are temporary paradoxes.
Time travel paradoxes
As we said, the idea of travel back in time is slightly clouded by the second part of the law of causality. Cause comes before effect, at least in our Universe, which means it can ruin even the most thought-out time travel plans.
To begin with, imagine if you travel 200 years back in time, you will appear long before you were born. Think about it for a second. For some time, the effect (you) will exist before the cause (your birth).
To better understand what we are dealing with, consider the famous grandfather paradox. You are a time travel assassin, your own grandfather is your target. You sneak through a nearby wormhole and walk up to a living 18-year-old version of your father's father. You raise your pistol, but what happens when you pull the trigger?
Think about it. You have not been born yet. Even your father hasn't been born yet. If you kill your grandfather, he will not have a son. This son will never give birth to you, and you cannot travel back in time with a bloody task. And your absence will not pull the trigger in any way, thereby denying the entire chain of events. We call this a loop of incompatible causes.
Alternatively, consider the idea of a sequential causal loop. Although it makes one think, it theoretically eliminates temporal paradoxes. According to physicist Paul Davis, such a loop looks like this: a mathematics professor goes into the future and steals a complex mathematical theorem. Then he gives it to the most brilliant student. After that, the promising student grows and learns in order to one day become the man from whom the professor once stole a theorem.
In addition, there is another model of time travel that involves a distortion of probability when approaching the possibility of a paradoxical event. What does this mean? Let's get back into the shoes of your grandfather's killer. This time travel model can virtually kill your grandfather. You can pull the trigger, but the gun will not fire. The bird will chirp at the right moment, or something else will happen: a quantum fluctuation will not allow a paradoxical situation to take place.
And finally, the most interesting thing. The future or the past that you go to can simply exist in a parallel universe. Let's imagine this as the paradox of separation. You can destroy anything you want, but this will not affect your home world in any way. You will kill your grandfather, but you will not disappear - perhaps another “you” will disappear in a parallel world, or the scenario will follow the paradox patterns we have already considered. However, it is possible that this time travel will be a one-time trip and you will never be able to return home.
Are you completely confused? Welcome to the world of time travel.
Ilya Khel