Most scientists agree that the possibility of time travel is directly dictated by the equations that are fundamental to the physics of our world. But even the very thought of the possibility of such an adventure raises many questions. Let's look for answers in our material.
And there are many questions. Will the traveler travel to one of the parallel universes of the multiverse or stay in his own? Will the guest in time use the same opportunities as the "hosts" of their time frame?
Our columnist, physicist Daria Zaremba, talks about the paradoxes that arise on the path of a time traveler. Her past material, made in the form of a dictionary, is devoted to the theoretical possibilities of such travels, in this - "obstacles" will be considered.
I - Everett's Interpretation
Everett's interpretation is an interpretation of quantum mechanics, according to which we live in a multiverse - a many-world world, where new parallel universes are constantly being born with the same laws, but different states.
According to Everett's interpretation, every time you make a choice - for example, to cook borscht, not soup with noodles, to wear a black shirt instead of a white one - a parallel universe appears in the world, in which you made the opposite choice and cook soup in a white shirt.
That is, each tossing of a coin will “divide” the universe into two worlds with corresponding outcomes or, in other words, states. They will be identical to each other, but in each of them, due to a different initial state (heads / tails), the course of history will be different.
Promotional video:
Everett's many-world interpretation.
This model of the many-world universe is often used by scientists to avoid the paradoxes of time travel. Some theoretical physicists believe that travel to the past is nothing more than travel into such a parallel universe, and not at all travel back along the traveler's own timeline.
P - Paradoxes
The time travel paradox is a situation in which, due to the existence of a time machine, a system is in a state incompatible with the laws governing the evolution of this system.
Despite the many ways justified by the well-established laws of physics and mathematics, the attitude towards time travel in scientific circles is skeptical. The main reason is the paradoxes that arise from such travel.
It should be noted here that paradoxes are characteristic exclusively of travel to the past. In general terms, a paradox occurs when the use of a time machine violates the laws of physics and / or logic.
Imagine, for example, that you have a powerful laser beam that you direct into a time machine portal. As you exit it, you set up a system of mirrors so that the outgoing beam is reflected and returned to the time machine.
Thus, at the output you will get a beam with twice the intensity (twice as powerful). When such a beam is bounced back and gets back, a laser with an intensity four times greater than the initial one will come out of the portal.
Through such manipulations, it turns out that it is possible to instantly create a source of generating infinite energy. The violation of the fundamental laws of physics is evident.
There are a lot of such paradoxes, but the most famous and discussed is the so-called “grandfather's paradox”. It consists mainly in the fact that the traveler, changing his past, violates the principle of causality.
Imagine that for a long time you lived with a feeling of hatred towards your grandfather, whose tyranny almost reached the level of Hitler. And here you are given a unique opportunity: using the time machine, return to the past and kill this monstrous person.
Now imagine that you end up with your relative at the moment of his birth. What happens in this case? Your parents will not be born, as well as yourself, and therefore you … cannot kill your grandfather at birth. This is how the paradox of causation arises.
C - Superposition
Quantum superposition is a phenomenon in quantum mechanics in which each particle can be simultaneously in alternative (mutually exclusive) states.
For example, an electron can simultaneously rotate to the right and left, a quantum bit ("qubit" is a unit of information in a quantum computer) can simultaneously have values of 0 and 1.
In 1991, British theoretical physicist David Deutsch discovered that by transferring the quantum principle of superposition to the macrocosm, it is possible to avoid the paradoxes of time travel.
Superposition in multiverse.
First, the scientist found that when a traveler returns to the past, he actually does not move along his own timeline, but moves to an alternative timeline or, in other words, to a parallel universe.
Imagine, for example, that you were walking down the street and suddenly noticed your favorite actor, peacefully standing by the side of the road. The unexpectedness of the moment knocks you off the track, and you freeze like an idol.
At this time, a car of a prestigious brand drives up to the side of the road and takes away the actor you adore in an unknown direction. Wanting to change this situation, you move back in time for 20 minutes, and this time you still gain courage and decide to speak to the star.
But the actor behaves quite differently from the way, you remember, he behaved. You cannot predict his actions, the conversation goes on by chance, and he may not want to get into the car, but decides to give this evening to his inveterate fan.
That is, it will no longer be the past that is imprinted in your memory - it is not your past. Your past remained in your home universe (it could not disappear anywhere, because the law of conservation of energy and information operates in the world).
This theory is consistent with the many-worlds interpretation of quantum mechanics by Hugh Everett, which assumes the existence of a multiverse with constantly branching parallel universes.
Secondly, according to Deutsch, when a traveler goes into the past, he is in a state of superposition, since he simultaneously moves into two parallel universes with alternative states: in one of them he kills his grandfather and, accordingly, loses the opportunity to be born, and in the second, he remains unharmed.
Alternate timelines.
The probability of each of these events is. In this case, the traveler will remain conscious only in the universe where he did not create a paradox.
U - Narrow possibilities
Narrow possibilities - according to one of the theories, are the only possibilities that a traveler in the past will be able to dispose of.
There is a theory that while traveling in the past, the traveler will not have full free will. At his disposal there will be only opportunities, for example, to determine the placement of parts of his own body in space in the near future.
So, he can scratch his nose, walk along the street or wave to a friend. However, he will not be able to realize broader opportunities - for example, to sell an apartment, commit murder, or cancel his parents' wedding.
What does it mean? When you wave your hand to greet a friend is a manifestation of a narrow opportunity, but if you wave your hand at an auction, placing a bet on a Van Gogh painting, then there will already be a wide ability.
At the same time, the division into narrow and broad opportunities occurs depending on whether their implementation needs the "support" of external circumstances, "response" actions from the "outside world".
This is written, in particular, by professor of philosophy Kadri Vihwellin - a researcher in the field of ethics, metaphysics and free will, as well as theoretical physicist Bradford Skow - in the context of solving the paradox of the murdered grandfather.
X - Hawking's party
Hawking's Party is an experiment by the famous physicist and cosmologist Stephen Hawking to prove the possibility or impossibility of time travel.
In 2009, Stephen Hawking conducted an amazing experiment. He organized a real party for the time travelers! Everything looked traditional: music, balloons, snacks. Except for one detail: no one but Hawking himself was at the party.
And all because the physicist sent the invitation to attend this party after the "celebration" itself. According to Stephen's logic, if time travel is indeed possible, then someday someone arriving from the future will find this invitation and come back at the time of the party.
Hawking's party invitation.
However, other scientists such as David Deutsch conclude that this will never happen. Because when traveling to other times, the traveler actually travels to other universes, which are connected in a special way.
Those in which the traveler has a time machine are interconnected, while those in which he has not yet created it remain isolated.
The physicist described his conclusions in the fundamental work "The Structure of the Universe". This means that if Deutsch is right, you and I will never be lucky enough to see tourists from the future until we build our time machine.
C - Censorship violation of causality
Censorship of the violation of causality is the principle of the prohibition of violation of causality in travel to the past assumed by various theories.
The author of one of the theories that “defends” the principle of causality when traveling to the past is the master of quantum computers, American physicist Seth Lloyd.
According to Lloyd's theory, when a traveler travels back in time, some actions planned by him (like killing his grandfather or a copy of himself) will not be completed by him for reasons independent of his will.
In this case, the principle of postselection or subsequent choice, which operates in quantum mechanics, became the “working principle” of physics.
Its essence lies in the fact that in the quantum world, elementary particles are able to instantly choose the "correct" solution from several possible ones (this, by the way, is the "secret of success" of quantum computers).
Here's an example that Seth Lloyd and his team experimentally displayed. Researchers have teleportated a photon (in the quantum world, the phenomenon of teleportation is used in full swing).
Moreover, according to the scientist, the rematerialization (appearance) of the photon occurs in time earlier than its dematerialization (disappearance). In other words, rematerialization takes place in the past.
Thus, we get a time interval when both copies of the photon coexist together. This can reasonably be considered a simulation of the time travel of a photon.
Now we provoke a paradox - we push both copies of the photon together. What will happen? Absolutely nothing. As Seth pointed out in an interview with The New Mexican, no matter how many times you bring photons closer to each other, at the last moment one of them will always miss, change course due to sudden quantum fluctuations.
This is because a collision with its copy will give rise to a paradox and such a "decision" will be "wrong" for a photon.
According to the scientist, this can also happen with the actions of a traveler in the past: the chain of cause-and-effect relationships will surely break off somewhere, and the insidious plan to kill the grandfather will not be completed. Perhaps the same quantum fluctuations, for example, will cause the bullet to miss.
Scientists have long noted that the universe works on the principle of a quantum computer, and itself "calculates" its state at the quantum level. Perhaps, according to the same principle, it will be able to "interfere" with the traveler's actions in the past in order to avoid paradoxes.
The paradox of violation of causality was solved in a similar way by the Russian theoretical physicist Igor Novikov. He chose the Polchinsky paradox as a basis - another formulation of the grandfather's paradox.
The paradox of violation of cause-effect relationships on the example of the Polchinsky paradox.
In this paradox, the ball rolls and hits the portal of the time machine, is transported for a few seconds into the past and hits its copy so that it no longer falls into the time machine.
According to Novikov's decision, the ball will always roll out of the time machine at such an angle that it will definitely push a copy of itself in the direction of the portal.
This is how Novikov formulated his principle of self-consistency, which says: when moving into the past, the probability of an action that changes an event that has already happened to the traveler tends to zero.
However, we will probably not be able to talk in more detail about the solution to the "protection of chronology" when traveling to the past until we get a much better understanding of the nature of the relationship between gravity and quantum mechanics.
Daria Zaremba