Everyone would like to return to the past at least once in their life in order to correct a mistake or even save someone's life. Ethan Siegel in Forbes answers the question of how dangerous time travel is in theory. Will temporary paradoxes destroy our universe? In any case, the time traveler will never return to the world he left.
We all dream of time travel. We want to go back in time to fix something wrong, or undo a mistake we made, or save someone's life, or prevent something terrible - the amount of good we could do seems immeasurable. We have no idea how to do this, but, probably, there is something essential, because of which travel to the past is impossible.
Was Doc Brown from the Back to the Future trilogy right? This is exactly what Alex Knapp wants to know by asking his question:
“My 8-year-old has seen Back to the Future films for the first time and would like to know if, as Doc Brown suggests, creating a time paradox could disrupt the space-time continuum and destroy the universe?”
It may be a little hyperbolic, but temporary paradoxes evoke feelings deeper than just a little uneasiness. Let's think about why.
When it comes to what the universe really is today, one should understand how unlikely that particular outcome was. Over the course of 13.8 billion years, all the particles that exist in only one visible part of our universe (there are about 1090 of them) interact and collide with each other countless times.
And only thanks to these interactions, stars and galaxies were formed in large quantities, as well as heavy elements, organic molecules and planets (for example, Earth), which provide the conditions for the emergence of life.
The path that our planet traveled before the advent of humans was a string of highly unlikely outcomes due to the sheer number of possibilities that the universe could allow. A tiny change that may have been unnoticed ten years ago, like whether a buzzing fly caught your attention, may have determined whether you noticed the future love of your life or not. The way your foot touches the hiking trail can affect the likelihood of a young, venomous snake (which likely bit you). The movement of water molecules along the ocean shore can make the difference between a pleasant day at the beach and a day when a child tragically drowned.
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In physics, we call this phenomenon "chaos". Small differences in the initial states of the system can lead to very different results over time. If we somehow traced the entire history of each particle in the universe, we would find that each of them has traveled a certain path, and in the history of each there was a certain set of interactions. This path was by no means constant; if it were possible to go back in time and repeat the origin of the universe with the help of simulation, then the same result would be extremely unlikely.
Many processes in our universe are random, including at the fundamental, quantum mechanical level. Their outcomes are inherently uncertain and inherently unpredictable. In quantum physics, we can only calculate the probability of an outcome, not any particular outcome.
The point of all this is to emphasize that the universe that exists today, exactly as it is, was the result of an overwhelming number of quantum decisions that have occurred over the entire history of its existence. The probability of getting exactly the same result again - even under the same conditions and the same laws - is practically zero.
Therefore, if you are talking about going back into the past and changing this past, you will definitely not get into the same universe that exists today. Everything will necessarily be different, and this can lead to dire consequences.
Let's give an example. What if you went back in time - like Marty McFly - and intervened in a situation critical to your own existence? What if you stopped your parents from meeting and loving each other? Of course, in this case, you could not have been born. However, you clearly exist. So how could you travel back in time and go back in time to do something like stop your parents from conceiving you?
In thought experiments like this, many have assumed (philosophically) that time travel to the past is impossible. Associated with this is a paradox known as the “grandfather paradox,” according to which the universe should not allow a person to travel back in time to kill their own grandfather, thereby excluding the possibility of their own existence.
But could such time travel disrupt the space-time continuum? Would it lead to the destruction of the entire universe?
Probably not. Paradoxes tell us that our traditional way of thinking about an object does not give us a complete picture of it at best. But this does not mean that the consequences will be as dire as the destruction of the space-time continuum or the destruction of the universe. It could simply mean that there is something else going on in the universe beyond what our thoughts initially tell us.
Is this a catastrophic paradox? I can say for sure no. Given the laws of physics as we understand them today, there are two possible ways out of this paradox.
First, we can say that the past will forever remain unchanged and no matter what we do, we cannot change it. But it is not at all a fact that this is incompatible with the idea of time travel to the past! Imagine that you find out that your best friend is about to be in trouble, and you need to go back in time to prevent it. You get into your time machine, go back in time, warn your friend and come back.
Tempting, right?
Well, in this scenario, this has already happened! Your friend would remember meeting what you incarnated into during time travel, remember receiving a warning, and preparing well for the inevitable. You would take a trip back in time, do what it takes, and come back. All this has already happened - you did not even have time to notice it, because the events that are happening to you now have already happened in the past of the universe.
According to this first scenario, the way out of the paradox is to note that everything that you would like to do while traveling back in time and that you would like to influence has already happened. How the universe ended up in its current state is due in part to the actions you took at every stage of your existence - including the stages when you traveled back in time.
True, in this case you give up the ability to change the universe by your own actions. You know very well that what you do today is incredibly important for tomorrow's results. Whether you go to work, beat your boss, or drive your car into the sea - all this will lead to very serious consequences. But if you went back in time and acted there, these consequences would already be determined. The thought is confusing and disturbing.
The second option is to assume that the past can be changed and your actions matter to it. The universe as it exists today will not be tied to its present state if you return to the past. In a sense, your every action creates a new, alternative history of the universe. You can kill your own grandfather before your parents were conceived, you can prevent your parents from meeting and falling in love with each other. You can kill Hitler before World War II or kill Brutus, Cassius and Mark Antony before they kill Caesar.
In short, you can change history.
In this case, you must give up only the immutability of the universe in which you live - from the fact that it remains the same after traveling back to the past. Your actions can change the future, but you have to pay for it: everything that happens after your adventures in the past is rewritten and becomes a new alternative history. The universe as you knew it in the "present future" can change. Or it cannot change - in a universe in which you can no longer live.
In the place where you are now, all future events will unfold in a completely different way than if you had not returned. And it is quite possible that in this new future you will not even be born. Perhaps you were just born the moment you arrived at your destination on your time travel. You would literally be a guest from another universe.
We do not yet fully understand the true nature of our reality. We do not know if our observable universe with the rules of quantum mechanics is the only one of this kind, or if there are alternative parallel universes somewhere (of course, there are noteworthy theories about this). We don't know if the universe is deterministic or not. And while quantum physics seems to be pretty clear about the lack of determinism, there can be logical flaws. And we don't know if the past is unchanging or can be changed.
Time travel into the past is certainly mathematically possible, but whether it is physically possible is a moot point. Although, if they are possible, they will not help you or your friends to avoid what is destined in this universe. Today, the only real result can be considered the result of past events, already recorded as a fait accompli. What is really important for the future is what we do in the present.
Ethan Siegel