Ask Ethan: Which films correctly show time travel?

Original author: Ethan Siegel
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The train "Jules Verne" from the third part of "Back to the Future." Perhaps Einstein imagined it was not at all when he formulated his thought experiments related to the theory of relativity, but the degree of scientific character can still be assessed.

The way we travel in time, at a speed of one second per second, is so boring that we take it for granted. However, according to Einstein's theory of relativity, we can travel in time not only at different speeds (if we increase the speed to values ​​close to the speed of light), but also in different directions, forward or backward, building a bridge between two unrelated places of space -time. Time travel, forward or backward, has long been one of the leitmotifs for our imagination and stories; Who would not want to explore an unknown future or go back in time to correct past mistakes? However, to compose a scientifically correct story is a completely different task. What movies have done it best? This is what our reader wants to know:

I am a big fan of time travel movies (no matter how they are explained). Which films best and most accurately use this plot tool?
Let's think about what should be in a good time travel movie, and how your favorite movies are doing so.

Relativistic journey to the constellation of Orion. The closer you get to the speed of light, the more distorted the visible space is and the visible distance to the stars is reduced, and the less time passes during your journey. For illustrations, the StarStrider program , a relativistic planetarium from FMJ-Software, was used.

If you strive for scientific relevance, you need to understand how time travel looks. One of the most revolutionary ideas of Einstein's theory of relativity was that space and time are inseparable from each other. The universe consists of four-dimensional tissue of space-time, and all objects, particles and radiation exist on its background. This leads to a strange and non-intuitive phenomenon: your movement through time is influenced by your movement through space, and vice versa.

Light clocks will go differently for observers moving at different speeds, due to the constancy of the speed of light. Einstein's HUNDRED talks about how to transform these times and distances.

Any object that exists in space-time will immediately notice the following three things:

  1. For other objects moving relative to it, the distances will be reduced, and the time will be lengthened.
  2. Relative to them, light always moves at the same speed, c, the speed of light in a vacuum.
  3. Their movement through space-time is determined by the curvature of space-time, which depends on the surrounding matter and energy.

If you are in a certain frame of reference (for example, you stand motionless on the surface of the Earth), then everyone who moves relative to you will travel a greater distance in space, which means a shorter time.

Movement at a speed close to the speed of light will cause time to flow differently for a moving person and for a person who remains in a fixed frame of reference.

Therefore, the twin paradox works just like this: the one who left the Earth and moves at a speed close to the speed of light will age lessthan his identical twin remaining on Earth. Anyone who moves through space with greater speed will experience slow motion through time. If we start to take into account the General Theory of Relativity, and the effects of gravity, then staying in a strong gravitational field will have a similar effect on you: you will feel the passage of time as normal, but those who are far away from you will age much faster. This effect is most pronounced near the singularity of the black hole, after you have passed through the event horizon.

Mathematical diagram of the Lorentz wormhole. If one of its ends is created from positive mass-energy, and the other from negative, then the wormhole can become passable.

But in GR there is another interesting possibility: the appearance of wormholes. Wormholes are often perceived as a short path in space, but there is no reason for them to go only through space; space-time will do the same here! You can use it if you succeed in creating, stabilizing and passing (or sending information) through it, and going forward or backward for an arbitrary amount of time. You can even create loops, or closed timelike curves , valid mathematical solutions that exist under certain conditions.

For example, in the context of GR, you can go back in time to a certain place; you only need to make a certain adjustment.

Warp enginepictured for NASA. If you create a wormhole between two points of space, in which one input will move at a relativistic speed relative to the other, observers from both ends will age at very different speeds.

If you create a massive black hole from matter, and then another BH from negative mass (which theoretically should exist), then you can create a wormhole between them. Spread them to any distance and disperse one of the ends of the wormhole to a speed close to the light. And while you are moving along with the accelerated end, you can at any moment pass through it, and end up on the other side of the wormhole unharmed. And most importantly, since you moved at a speed close to light speed, for you the time went differently. When you move in the opposite direction, it turns out that almost nothing has changed at home. You can travel for hundreds of years, and then return to the point of departure a few seconds after you left it. In this sense, the journey back in time is quite possible physically.

Is time travel possible? With a fairly large wormhole, for example, created with the help of a supermassive black hole, connected to its counterpart, consisting of negative mass-energy, completely.

There are many possibilities, as well as many films that take advantage of the combination of a time machine and creative narration. Of course, there are many films, in this sense, saving on scientific credibility.

Hardly anyone remembered the films “Time Patrol” (Timecop), “Hot Tub Time Machine”, or “Bill & Ted's Excellent Adventure” incredible adventures of Bill & Ted's Excellent Adventure with time machines. In the film Idiocracy, time travel is used in the only sense that time passes, and objects and people remain unchanged. Superman rewinds time back to save Lois Lane’s life in the original Superman movie, but it’s not about science, but about superpowers. The same can be said about the recent film about Dr. Strange or the cult "Warlock", or about the film "Harry Potter and the Prisoner of Azkaban"; using magic to move scientific glasses in time will not add you. In a variety of films, time travel is primarily used, as a plot move, without regard to scientific accuracy. Even in the film “Army of Darkness”, although it is funny, there is no reliable mechanism for the movement of time that takes place in the story.

Reading a spell from the Necronomicon and transfer to the past - such a plot is suitable for an entertaining film, but it does not pass a scientific test.

But some films, although they do not tell, and do not directly show the mechanism for moving in time, describe surprisingly exactly what time travel might look like. Moving forward is quite simple: you need to get closer to the speed of light, return to the starting point, and you find yourself in the distant future. That is how the Planet of the Apes on the planet sent a man to a distant future on a dystopian Earth, and that is why Star Wars is so annoying with his “hyperdrive”. Rapid movement has a significant impact on the speed of time, and takes you into the future no matter what you do.

The hyperdrive from Star Wars seems to depict relativistic motion in space, close to the speed of light. But no one is aging at a speed other than normal, and this violates the principles of relativity.

Returning to the past, especially at a certain point in time, is the basis of the plots in time-travel films. There are two theories about the operation of this mechanism:

  1. The passage of time is unchanged; everything that happened is already recorded, and when you move backwards, you cannot change the course of events. Your move is already built into the course of events.
  2. The timeline can be changed; the changes you make when going backwards lead to a different future, perhaps even rejecting your existence.

Two excellent examples of the first theory are the Twelve Monkeys and The Time Loop, where the future is already recorded. Moving through time allows you to live and interact with the past, but does not change the course of history. Events unfolding so that you go back in time, have already occurred. You just live your life, knowing the fate of the world.

The idea of ​​moving into the past long occupied people - for example, with the help of Delorean DMC-12 from Back to the Future. After decades of research, we may have stumbled upon a physically feasible solution, but Delorean will most likely not need it.

On the other hand, there is the possibility that your future is not recorded, even if you yourself came from the future. In the series “Back to the Future” and in the films “Terminator” 1 and 2, much has been built on this. Although they describe exactly how time travel works physically, omitting some key ingredients, travelers' actions can change the future. Kyle Reese and Sarah Connor can prevent or postpone the day of judgment by fighting the terminator sent back in time to kill or prevent the very existence of the boy who will fight with the machines. Marty McFly travels in time to save his friend's life, but he needs to be careful not to erase from the story himself in the process. These are two of the best examples of films with a variable future. Also films from the Star Trek series do well with this - a 2009 film,

Best of all, the black hole was depicted in the film Interstellar, and it rather accurately depicts the event horizon for a completely specific class of rotating black holes. In the depths of the gravitational well, time for the observer goes differently than outside.

Two films stand out for their scientific credibility and level of detail: Interstellar and Contact. Interestingly, the same scientist, Kip Thorn, worked as a consultant for both films, and both of them use the idea of ​​black holes and wormholes. In the depths of the black hole's gravitational well in Intereslar, the time for the observer goes at a different speed, which leads to a relativistic turn of the plot. In the film “Contact”, an instant on Earth corresponded to a galaxy tour that lasted almost a day, and, potentially, throughout the Universe. The physics of black holes, wormholes and GTR in these films appear in all their glory, and they look quite picturesque.

Bill Murray is drinking a coffee pot in front of Andy McDowell in one of the scenes of the film "Groundhog Day"

Finally, there is another, probably the most realistic and interesting film using time travel in the form of a “time loop”: “Groundhog Day”. Any decision of GR that allows the existence of closed time-like curves is usually rejected because of philosophical concepts like the “ grandfather paradox ”, but these mathematical solutions are consistent and can describe reality, especially if we accept that the beginning of the loop “nulls” the state of the system. In the Groundhog Day, this moment is perfectly used, and the time loop is destroyed only when a sufficient number of changes accumulate in this ridiculous, moral tale of good and self-determination. Although it lacks scientific knowledge, the image of the time loop is well done here (although I have not yet seen The Edge of the Future).

Here are films, from those that I saw, correctly describing time travel from a scientific point of view.

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