The Big Question: Is Time Travel Possible?

Is Time Travel Possible?

Recent Hollywood blockbuster Avengers: Endgame is the latest in a long line of movies in which time travel features prominently in the plot. But is it plausible, considering the laws of physics?

By Tom Wong, PhD, Assistant Professor of Physics

Spoiler alert! We’re about to discuss plot elements of Avengers: Endgame.

Time travel is central to Endgame, as the Avengers attempt to go back in time, retrieve the all-important infinity stones and bring them back to the present. 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 Einstein’s famous framework of general relativity is that clocks tick faster or slower for people depending on the strength of gravity or how fast they are moving. Experiment after experiment has validated relativity.

Consider the case of black holes.

Imagine a hypothetical man (John) who stays on earth, while his friend (Mary) travels near a black hole. Since earth’s gravity is less than the black hole’s, time ticks more quickly for John on earth, and more slowly for Mary. When Mary returns, 20 years may have passed for John, and only a week for Mary. To Mary, it’s like she has traveled 20 years into the future. This idea was depicted quite well in the 2015 movie Interstellar.

Notice that both John and Mary age, just at different speeds. Neither is getting younger, which would constitute going backward in time. In fact, time travel to the past defies the laws of thermodynamics, which assert 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.

So we are constantly traveling in time into the future, even at different rates thanks to relativity. But unlike in Hollywood movies, we cannot travel back in time without violating thermodynamics and other arguments.

The physics of quantum mechanics also relates to another important element of Endgame: parallel universes.

In quantum mechanics, events occur with probabilities. For example, a quantum computer bit can be a combination of 0 and 1, not simply only 0 or 1. When we measure it, however, we get a definite value of 0 or 1. How this combination becomes a definite value is still a topic of debate and research.

The most common view among physicists is the Copenhagen interpretation, which states that a quantum combination simply becomes definite when measured, and there is nothing more.

Another view is the many-worlds interpretation. It proposes that when a measurement is taken, both outcomes occur, but each in a parallel universe. So the bit is 0 in one universe and 1 in another.

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).

Traveling back in time. Jumping universes. For now, it’s only possible with the Avengers. (Assemble!)  


Name-Dropping

One of the ways 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 each 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. And while Stark’s phrase is scientific gobbledygook, at least the ideas are each related to quantum mechanics, and it just might inspire a future scientist.

About the Author: Tom Wong’s research interests include quantum computing, and he leads the Quantum Computing Group at Creighton. Wong also serves on the editorial board of Quantum Information Processing, a quantum computing journal.