Physics has an ox with Avengers Endgame. Spoilers!

When you watch Avengers: Endgame, you can sit back and enjoy the ride while our heroes are trying to save the universe. But if you are one of those who hope that the key points of the Marvel film plot are based on sound science, we have bad news.

First, if you have not watched the movie, STOP READING NOW. This is your spoiler warning.

Because we are discussing here the journey back in time and how Iron Man and the rest of the Avengers are coming out of their post-Thanos sadness. The characters strive to distinguish their approach to time travel from Back to the Future and everything in between, but it's not Hollywood, but Heisenberg.

"In simple terms, I do not think that everything we've learned about gravitation and quantum mechanics suggests that it's possible to travel back in time in the past," said Leonard Susskind, a professor. Stanford's influential, who claims the glory of physics theory, a fundamental revision of the laws of the universe.

Even the most plausible approach of time travel – wormholes that can connect different points in the temporal and spatial structure of the universe – requires some seriously strange and potentially non-existent material forms. You will not be able to travel in the past to murder your grandfather, let alone resurrect the Spider-Man franchise of Marvel Cinematic Universe.

But the good news for time travel enthusiasts is that physicists have taken trips back in time more seriously over the last few decades. Look.

Thank you, Albert Einstein and your crazy maths

The main reason for optimism: the mathematics of Einstein's theory of general relativity open up very interesting possibilities.

"General relativity can allow a trip back in time," said physicist Stephen Hawking. "Science fiction fans must not lose heart."

General relativity describes the universe as four dimensions – the three familiar dimensions of space, related to a temporal dimension. This explains gravity as a bending in this space-time. Relativity is strange and has awakened the minds of many people, but you probably know the idea visualized in a two-dimensional space with suns and planets stretching a layer of rubber. Einstein's results are verified by phenomena such as the slightly slower frequency of the GPS satellite clock than that of the Earth and the gravitational lens phenomenon in which large galaxies curve the path of light coming from more distant galaxies.

Walking a loop in a three-dimensional space is not mind-blowing. But the math of relativity also opens another option to follow a path that includes the temporal dimension – what is called a closed time curve. In fact, following such a curve could bring you back to where you started at the same time you left.

Real time travel research often relies on structures that support closed time curves. These include black holes, infinitely long cylinders – and the most popular time travel idea, the wormholes.

Wormhole wandering

Wormholes are an integral part of sci-fi movies for a good reason. They offer a way to connect different places and times in the universe, with two different mouths connected by a tube to move from one side to the other.

Well, at least they exist.

The key to traveling in time in a worm is to accelerate one of its two mouths to a speed close to the light so that a clock turns relatively slowly, then put the mouth in the still mouth where the time has passed more slowly. "By crossing the wormhole from the right mouth to the left, you can travel back in time," said Caltech Nobel laureate physicist Kip Thorne, who helped legitimize scientific investigations into the journey into the world. time, and his colleagues in a 1998 article on wormholes and time travel.

Here is a problem: you must prevent the vortex from collapsing.

"The connection between one part of the space-time and the other does not last long enough for anything," said Richard Muller, a physics professor at the University of California at Berkeley and author of Now, The Physics of Time. Each end of the wormhole would collapse into a black hole.

With a form of so-called exotic matter – something with a negative energy density – you can leave the wormholes open, but no one knows if this is even possible, if it's not at the smallest scales of quantum mechanics.

"You need something very special," said Muller. "Even black energy does not do that," he said, citing the relatively new phenomenon discovered that is causing the accelerating expansion of the universe.

Another problem: to make a wormhole, one would have to back space-time. So yes, do something as huge as the sun does, but in a profoundly different way.

Travel faster than light

Einstein's equations show that the faster you travel, the slower is the time spent in the region where you started to accelerate. They also indicate that time goes back if you exceed the speed of light.

Too bad you can not go that fast. "It would take an infinite amount of power to accelerate beyond the speed of light," Hawking said.

Relativity actually means that you could travel in the future. Accelerate in a spaceship for a few decades to travel at a speed close to that of light, then turn and slow down and you will have aged during these few decades, as centuries passed for the planet.

We have nothing close to a propulsion system that could power this type of spacecraft. If we could stay very close to a rapidly rotating black hole, the time would pass quite slowly – as in the movie Interstellar, where an hour on a planet meant that seven years had passed on Earth. (Thorne, a film advisor, has endorsed this idea.)

S & # 39; pack around Tipler cylinders

Here is another place where you could find a closed time curve: a very large cylinder that rotates very fast.

"General relativity suggests that if we build a sufficiently large rotating cylinder, we create a time machine," concluded physicist Frank Tipler in a 1974 article. Again, there are practical concerns, such as 39 requirement that the cylinder must be unnecessarily long – to infinity, or perhaps very long if you are close to the middle – and made of materials strong enough not to shatter when shot quickly.

This book prompted science fiction author Larry Niven to write a short story with exactly the same title as Tipler's article: "Rotating Cylinders and the Possibility of a Global Causation Breach". (Although Tipler cylinders are common in discussions of closed time curves, note that Tipler's latest work linking physics and Christianity alienated scientists who expressed skepticism, to put it politely.)

Goodbye, free will

Hollywood could have another big problem with physics: Time travel takes away any real control that each of us has on its own future, Muller said.

"If the wormholes exist and we can go back in time, it is a consequence in physics that we would not be able to exercise our free will.We could not change d & # 39; opinion, "he said. He exposes his case in a paradox that he calls tachyon killing involving a gun capable of firing particles faster than light.

You could argue that most Avengers: Endgame follow a fairly predictable path. But characters who have no real ability to influence their future? Let's hope this never gets in the movies.