Time travel may be possible, but only in parallel timelines
As is often seen in science fiction, nothing lasts forever with a time machine - you can always step back and change everything. But is time travel really possible in our universe or is it just science fiction?
Our modern understanding of time and causation is derived from general relativity. The theory of the theoretical physicist Albert Einstein combines space and time into a single whole - "space-time" - and provides a surprisingly complex explanation of their operation at a level that cannot be compared with any other established theory. This theory has been around for over 100 years and has been experimentally tested with extremely high accuracy, so physicists are confident that it provides an accurate description of the causal structure of our universe.
For decades, physicists have been trying to use general relativity to find out if time travel is possible. It turned out that time travel equations could be written, and they are completely consistent and compatible with the theory of relativity. But physics is not math, and equations don't matter if they don't correspond to what's in reality.
The arguments against time travel are serious. There are two main questions that make us think that these equations may not be true. The first question is practical: to create a time machine apparently requires exotic matter, that is, matter with negative energy. All matter that we see in everyday life has positive energy - matter with negative energy is not something to be found just around the corner. We know from quantum mechanics that such matter could theoretically arise, but in too small amounts and for too short a time.
However, there is no evidence that it is impossible to produce exotic matter in sufficient quantities. Moreover, you can discover other equations that allow you to travel through time without the use of exotic matter. So this problem may just be a limitation in our current technology or understanding of quantum mechanics.
The second major problem is less practical but more significant. The observation that time travel seems to contradict the logic of time travel paradoxes. There are several types of such paradoxes, but sequence paradoxes are the most problematic. A popular technique in science fiction, the sequence paradox comes up whenever a certain event occurs that causes a change in the past, but change itself prevents that event from happening in the first place.
For example, consider a scenario where you enter a time machine, use it to go back five minutes in time, and destroy the machine as soon as you enter the past. Now that you've destroyed the Time Machine, you won't be able to use it five minutes later. But if you can't use the Time Machine, then you can't go back in time and destroy it. Therefore, it is not destroyed, so you can go back in time and destroy it. In other words, a time machine is destroyed if and only if it is not destroyed. Since it cannot be destroyed and not destroyed at the same time, this scenario is inconsistent and paradoxical.
A common misconception in science fiction is that paradoxes can be "created". It is, for this reason, that time travelers are usually warned not to make significant changes in the past and to avoid encountering their past. Examples of this can be found in many time travel movies, such as the Back to the Future trilogy. But in physics, a paradox is not an event that can actually happen - it is a purely theoretical concept that points to the inconsistency of the theory itself. In other words, the sequence paradoxes not only imply that time travel is dangerous, but also that it is simply impossible.
This was one of the reasons why theoretical physicist Stephen Hawking formulated his Defense of Chronology Hypothesis, which states that time travel should be impossible. However, this hypothesis still remains unproven. Moreover, the universe would be a lot more interesting if, instead of eliminating time travel because of paradoxes, we could just eliminate paradoxes themselves.
But what's the point of going back to the past if you can't change the past? In fact, it can solve any paradox. The idea is very simple. When you exit the time machine, you enter a different timeline. In this timeline, you can do whatever you want, including destroying the time machine, without changing anything in the original timeline you came from, as you cannot destroy the time machine in the original timeline that I actually traveled back in time, and therefore there is no paradox.
Time travel is possible, but is it dangerous?
In science fiction novels, the time machine is most often used as a means of travel into the past or the future. But if such devices have been developed, their functionality has not yet been officially confirmed. Meanwhile, Sarah Bridle, professor of astronomy and cosmology at the University of Manchester's Department of Physics and Astronomy, recently told BBC Radio that time travel is possible. In any case, if we are talking about a "trip" into the future. But it is not so simple.
It has been noticed that aircraft passenger clocks always run a little forward.
"So if you get on a plane and travel around the world, you will be delayed by one-millionth of a second," says Professor Bridle.
Another thing is that it will be completely imperceptible to us because there will be no major changes in the world in such a short time. So there is no point in using an ordinary airplane as a time machine.
"If you want to go much farther in time, you have to go faster," says Bridle. "You have to go near the speed of light."
According to the scientist, if we were on a spacecraft capable of flying around the Earth seven times a second, it would be a week, and on Earth a hundred years. We would go back to the future. However, such a spaceship would use a tremendous amount of energy, admits Sarah Bridle. Therefore, hardly anyone would undertake such an experiment.
In addition, a black hole can be used for time travel. It is believed that time does not flow in a black hole like it does in our reality, and if we had reached it, we would have a good chance of being transported many years forward on the Earth scale. However, little is known about the properties of such objects. What if we just break into atoms when we are near the hole?
Meanwhile, theoretically, there are points in the Universe that can serve as portals to other dimensions, both spatial and temporal. But on the other hand, if in fantastic works the heroes immediately and without any damage teleport to where they need and where they do not need, and then calmly return, then in our case no one guarantees survival and safety. It takes years, perhaps centuries of experimentation to "practice" such journeys. And even if we go into the future, we are unlikely to go back to our time. In a word, this is an option for kamikaze.
Sarah Bridle believes, however, that humanity does not currently have the technology that can really take people into the future. But even if it did exist, remember that with such a "jump" we will simply lose a lot of time for us. Are you ready to adapt to new realities? What if we don't like the future at all? I will give the scientist a return to the past, there will be no more.
Does time travel allow you to kill your own grandfather?
Contrary to popular belief, some scientists believe that time travel is possible. Simulations of the behavior of photons, carried out some time ago by physicists at the University of Queensland, seem to support this theory.
The so-called grandfather paradox is a claim that is meant to prove that time travel is impossible. The most common example is that if someone in time travel kills his grandfather before conceiving his father, that person will not be born. If he's not born, he won't travel back in time and kill his own grandfather, so he'll be born and kill his own grandfather. So we have a paradoxical theorem. However, some believe that the contradiction is only apparent.
Performing simulations, physicists investigated two possible outcomes for a time-traveling photon. In the first case, the first photon would travel back in time through a wormhole and interact with its previous version. In the latter case, the second photon would traverse normal space-time but would interact with a photon stuck in a closed time loop. By analyzing the second photon, it was also possible to study the behavior of the first photon.
The results of the study made scientists understand that time travel is possible, at least on a quantum level. However, it is uncertain whether larger particles or even whole groups of molecules could make such a journey, while human time travel remains a great mystery. Therefore, further research in this direction can be expected.
The grandfather's paradox does not rule out time travel
It is widely believed that a trip to the past is impossible, if only because of the "grandfather paradox" - a hypothetical situation in which a time traveler travels back in time and kills his grandfather without the traveler being born. However, researcher Tim Maudlin believes that the "grandfather paradox" only restricts temporary travelers, but does not in itself rule out the possibility of such a trip.
The classic plot of science fiction films often refers to the fact that someone travels into the past and does something there that has disastrous consequences for the present. An example of such a plot is the grandfather's paradox. Tim Maudlin, the philosopher of science, and researcher of the metaphysical foundations of physics and logic tried to understand the "grandfather paradox" and understand whether this idea interfered with time travel into the past.
The argument goes like this: if you could 'go back in time, you could go back to the time when your grandfather had no children and kill him, and then you would cease to exist and you could not start such a journey. Just if that happened, one of your parents would not be born, which means you would not be born, so you wouldn't be there to go back to the past.
The grandfather paradox is usually presented as a reductio ad absurdum or a refutation of the claim that time travel is possible. Thus, the hypothesis must be impossible because of the grandfather paradox, and thus time travel is impossible or even contrary to the fundamental idea of causality if viewed only in the context of the "grandfather paradox." However, Maudlin believes that the paradox itself still does not prevent travel into the past.
The grandfather paradox does not prove that you cannot go back in time, only that you cannot go back and kill your grandfather. According to the scholar, there is nothing wrong with going back in time and saying "Hi" to your grandfather, for example. "
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