Usually, we take it for granted that our past is unchangeable. The story always goes the way we remember it. But how can we know that things are this way? If time travel is possible, that the travelers at other times can always jump back in time and change things on your own. As we will see whether it really is or not?


We asked several experts to travel in time - although none of them, of course, did not make such trips personally. (And if they did, it keeps its secrets with you.

First of all, the fact of changing the past is really difficult to establish, because "it is assumed that when the time traveler change our past, this immediately leads to the change of our memories of the past, to keep them relevant to this "new" past", says Professor of philosophy Christian Wuthrich. "If the change of the past instantly and completely consistent, that is, include changing all of our memories and records of the past, then we really cannot with certainty be noted that this happened."

But according to experts, there are ways by which you can ensure that someone still does not change constantly our past.

Time travel is impossible

We often hear this from many experts. As told ion Thaler, a physics Professor from the University of Illinois: "the Problem is that we don't know how to build a theory which allows time travel. And without theory is quite difficult to say what the phenomena will be the signs of such a journey". Thaler wrote the section travel in time for the Usenet Physics FAQ, in which he explains that the theory of relativity can assume the existence of "temporary closed curves", which allow time travel - but the famous "grandfather paradox" (in which you return back in time and kill your grandfather while he is still a child) proves that it is impossible. Thaler says: "in short, "temporary closed curves" (slang name for physical assumptions, which allows time travel), apparently, incompatible with quantum mechanics".

Even if time travel is possible, in any case, you cannot change the past

This opinion is also very common. Wutrich says that many philosophers of science agree that the past must be consistent in order to avoid multiple temporal paradoxes. This restriction "consistency" means that there is only one past, and it is fixed. However, an expert on quantum physics Todd Brun says: "Even if you go back into the past with the intention of changing history, events rotate so that will force you to take some action, instead of to follow your will (and the story will now include the existence of time travellers)".

This is the reason why many philosophers time like stories like "All you zombies" by Robert Heinlein, which is strictly consistent, says Nick of Haggett, author of the book: "Always and everywhere: travelers in physics and philosophy". Haggett, Professor of philosophy at the University of Illinois, adds:

Today we have no understanding of what it means to change the past. To say that something has changed, it means that at one time it was that way, and in another moment like that, right? And how you can change the past? This should mean that yesterday was first like that, and then another - but first of all, you did not come with time travel in this day, and then once was in it, right? But in our history, you arrived at this day, so the first version, it never happened, and the past has not changed since your arrival - again, because it was not anything different.

An alternative to this view is the hypothesis that every time you go back in time and change something, you create a new universe, according to the theory of "multiple worlds" Hugh Everett. This new reality will continue to have a consistent past that everyone will remember exactly - but it will be past, which appeared as a result of changes made by the traveler.

Brun says:

Perhaps there are many universes, and change our past makes the universe to share. As strange and scientific-fantastic as it may sound, we should take seriously this feature from the point of view of quantum mechanics, which is one of the leading interpretation is the theory of so-called "multiple worlds, where each quantum event causes the universe to share. But even in this case no one can go back and change history own universe.

Perhaps you can travel back in time only until the moment when he invented the first time machine

If only someone already did not invent such a machine, and we just don't know that we are safe. Explains Professor of physics at Vanderbilt University Thomas weyler:

Time travel, if available at all by itself, can send you only the moment of creation of the first machine of time. As in the past, our civilization there is no time machine, it's immutable, while the past is more advanced civilizations can be changed.

No signs

The only accurate way to know that the time travellers are in our past, and something in him change is if they start to brag about. That, being human, they will begin to do. Says Nick Of Haggett:

Imagine that tomorrow you back in time to two days (i.e. during yesterday), and makes on TV sensational statement about today. Thus, today we will know that tomorrow you have influenced the past.

And even then we cannot be sure that these travelers in time to change things, because we will remember only one version of the past.

Would be physical traces

Depending on the applied method of time travel, you may expect to have any physical traces, says Haggett. In the novel by Carl Sagan Contact, time travel is possible due to the way in space-time, which leads to the past. According to Haggett, this type of wormholes "requires exotic forms of matter, to keep it open, which may be spotted". Also in this case is the question of the conservation of energy - when you appear in the past, you are formed of matter and energy, which it was not, and that you are actually imported from the future. This can create tracks that can be spotted - unless of course someone will actually go back to our age. That is, briefly, the results of time travel will be invisible to those of us who are stuck in linear time, but the methods themselves travelling, possibly, will leave some traces.

Games with cause and effect will change the laws of probability

If someone can really go back in time and change it, it can lead to the fact that the investigation ahead of reasons, says Todd Brun. And this in turn may mean that everything goes topsy-turvy, leading to logical contradictions that we will be able to notice. He explains:

The confusion of cause and effect may make it impossible in other cases, the event is much more likely. So if we suddenly discover that the events violate our healthy of mind assessment of probabilities, it may mean that there is a place where time travel. Essentially, this means that people can notice the existence of the machine of time - perhaps even before it is created. But it's hard to tell what you should pay attention.

The history testifies that they do not

Let's give the final word to Professor of physics at Harvard University Gary Feldman: "If the time travellers constantly change our past, they seem to do this is not very successfully. Why don't they have prevented two horrific and senseless war for the past century?"