The Physics of Avengers: Endgame
The Marvel Cinematic Universe makes ample references to physics to justify their plots and superheroes’ powers. Since the release of Avengers: Endgame, several people have asked me about it, since I am a physics professor and quantum scientist. So let us sift through what is science fact and what is science fiction.
Spoiler alert! We’re about to discuss plot elements of Avengers: Endgame.
Time travel is central to Endgame, as the Avengers’ plan is to go back in time, retrieve the infinity stones, bring them back to the present, and then revive everyone with a snap. Scientifically, time travel to the future is commonplace, but there is no evidence that time travel to the past is possible.
First, time travel into the future is constantly occurring, since in one second, you will be one second into the future. If everyone else has also aged one second, this may not be very interesting. But an amazing fact from Albert Enstein’s famous framework of general relativity is that clocks tick faster or slower for different people, depending on the strength of gravity or how fast they are moving. Experiment after experiment has validated relativity, including the first image of a black hole last month. In fact, GPS would not work without accounting for the different speeds of clocks in GPS satellites.
To time travel, imagine two individuals, where one person stays on Earth, while the other travels near a black hole. Since earth’s gravity is less than the black hole’s, time ticks more quickly for the people on Earth, whereas time ticks more slowly for the astronaut. The astronaut returns to earth. Twenty years may have passed for people on Earth, while only a week may have passed for the astronaut. To the astronaut, it is like she has traveled twenty years into the future. This idea was depicted quite well in the 2015 Oscar-winning movie Interstellar.
Notice that both people age in this example, just more quickly or slowly. Neither is getting younger, which would constitute going backward in time. In fact, the laws of thermodynamics prohibit time travel to the past by insisting that natural processes increase the amount of “disorder” (entropy) of a system. This is why we see eggs break, but broken eggs do not reassemble. To go backward in time would be to reassemble broken eggs, violating thermodynamics.
There are other reasons why time-travel into the past is implausible. If it were possible, then future advanced civilizations could go back in time and visit us, but we have no (credible) evidence of visits from people from the future. Also if time travel were possible, computers (both classical and quantum computers) would be absurdly powerful, unreasonably so that it is evidence against time travel.
So while we are constantly traveling in time into the future, even at different rates thanks to relativity, we cannot travel back in time without violating thermodynamics and other arguments.
We could end the discussion here, since a complete quantum theory of the universe must reproduce these classical results. But let us entertain time travel further. If time travel into the past were hypothetically possible, it would also need to resolve self-consistency. For example, it would need to solve the famous grandfather paradox: If you go back in time and kill your grandfather, then you would not have been born, then you would not be able to kill your grandfather, which means you would have been born.
(Along similar lines, in Endgame, it is inconsistent for Nebula to kill her younger self.)
There is no evidence that time travel exists in quantum mechanics, but if it did, it would also need to be self-consistent. In 1991, David Deutsch (who was name-dropped by Tony Stark — more on this later), showed that self-consistency is possible. In quantum mechanics, events have probabilities. To resolve the grandfather paradox, if you are born with probability one-half, then you kill your grandfather with probability one-half, then you would have been born with probability one-half, which is consistent.
When quantum systems are observed, however, they have definite values, not probabilistic mixtures. That is, when we look at you, we clearly see that you were born, and you are not a mixture of half born and half never born.
How a quantum mixture of half-half becomes definite is still a topic of debate and research. The most common view among physicists is the Copenhagen interpretation, which states that quantum mixtures become definite when measured.
Another view is the many-worlds interpretation. It proposes that when a measurement is taken, both outcomes occur, but each in parallel universes. So if you are quantumly half born and half never born, then upon measurement, one universe is created where you are definitely born, and another universe is created where you are definitely never born.
The many-worlds interpretation is likely the inspiration for the Sorcerer Supreme’s explanation in Endgame, that if one of the infinity stones is taken, another universe will be created.
Scientifically, however, the universes are parallel, meaning they cannot interact with each other. You are in your universe, and you cannot access the other(s).
Although there is no experimental evidence for the many-worlds interpretation, the writers of Endgame should receive credit for taking their inspiration from a scientific idea, even though science does not support time travel as depicted in the movie.
One way movies sound convincing is to name-drop actual scientific terms. A perfect example from Endgame is when Captain America, Natasha Romanoff, and Ant Man visit Tony Stark to propose using the quantum realm to go back in time. Tony dismisses it, saying, “Quantum fluctuation messes with the Planck scale, which then triggers the Deutsch Proposition.”
These three terms have some meaning, but together, the sentence is meaningless. Quantum fluctuations refer to temporary fluctuations in energy due to Heisenberg’s uncertainty principle. The name “fluctuations” may suggest an intimidating or worrisome instability, but they are normal, everywhere, and constantly occurring, even playing a role in the interactions between fundamental particles.
The Planck scale refers to incredibly small lengths, times, and energies for which a quantum theory of gravity is necessary. Quantum fluctuations and the Planck scale do not “mess” with each other, as Stark claims.
Finally, there is no such thing as the Deutsch Proposition, although David Deutsch is a real scientist, as previously referenced for his resolution of the grandfather paradox. Altogether, Stark’s sentence is scientific gobbledygook, but at least they are each related to quantum mechanics.
Later, Stark decides to take up the idea of quantum time travel. He asks his computer to draw a “Möbius strip” and asks for “eigenvalues” and a “singular value decomp(osition).” Again, these are legitimate scientific terms, but together, there is no meaning. Metaphorically and artistically, however, there is some merit to these choices.
A Möbius strip is just a strip of paper, turned and taped together. It it only has one side, so an ant walking along the strip eventually returns to where he started. If we metaphorically interpret the ant, not as returning to a point in space, but a point in time, then it alludes to time travel.
“Eigenvalues” appear throughout quantum mechanics. They are simply the values that you can get when you measure something. For example, if you measure the energy of an atom, you can only get certain possible outcomes, and these numbers are the eigenvalues.
As previously discussed, after a measurement, the quantum mixture (half born and half never born) becomes a definite state (born or never born). Finding the “spectral decomposition” is to find all the possible energies (eigenvalues) and states. Using these, one can determine how a quantum object evolves with time.
Combining this with the metaphoric interpretation of the Möbius strip, it could be that Stark found how to make quantum objects evolve such that they revisit a point in time, hence time travel.
Again, this is not scientifically accurate, but there are certainly instances where movies have taken greater creative liberty.
Despite these scientific inaccuracies (and many others, like Captain Marvel shooting photons from her hands, the creation of infinity stones from singularities, basically all of Ant-Man, etc.), Endgame has driven more interest in quantum mechanics and physics than I have ever seen. While it marks the end of the Infinity Saga, it may be the beginning of, the inspiration for, future scientists, like Star Trek and Star Wars were for previous generations.