Science

Time travel: the grandfather paradox and infinite energy

Part 2 of a series scientifically examining the idea in fiction.

February 16, 2024

In part 1 of this series, I broke down why the current state of physics doesn’t realistically allow for either jumping far into the future or traveling to the past.

But it’s so much fun to fantasize about the potential of time travel, even if it only works in science fiction. As a physicist, when I read a time travel story or watch a time travel movie, I’m always interested in understanding how the creators make it work. I’ve seen many good interpretations of how cause and effect while time traveling could operate. Most usually fall into three main categories.

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The first is the idea that a time traveler can change events in the past, with immediate effect on the future timeline – including her or his own existence. My favorite example of this is the original Back to the Future film. Michael J. Fox’s character, Marty McFly, accidentally travels to the past and inadvertently changes the event that brought his parents together. Since they may not fall in love anymore, Marty’s existence and that of his siblings is at risk. This change has immediate effects on the timeline, as his siblings start to disappear from a photo Marty carries. In the movie’s climax, Marty himself starts to fade out of existence, just as he’s playing rock ‘n’ roll guitar.

There’s a huge, well-known logical paradox within this type of time travel, which Back to the Future perfectly exemplifies. If Marty prevents his parents from getting together in the past, then he’ll never be born. But then he could never travel back in time to prevent his parents from getting together. So, they do get together and Marty is born and then he goes back in time, preventing them from meeting …

This logical inconsistency is most commonly known as the grandfather paradox. The YouTube channel minutephysics postulates a solution that involves quantum physics. They suggest that the two contradictory timelines could exist simultaneously as a kind of superposition of quantum states. As far as we know, though, quantum mechanics like this only applies to microscopic objects – definitely not to whole timelines. Furthermore, any observation of a quantum state would collapse the superposition, making one timeline reality.

So if changes made to the past can’t affect the future timeline the traveler came from, what would they alter? Most works of science fiction solve this problem through the use of parallel realities. If time travel alters something in the past, the original timeline doesn’t change. Instead, a new, parallel timeline splits off from the original and continues on its own, altered course. And since the very arrival of a time traveler in the past is a change, that’s exactly when the parallel timeline starts.

The parallel timelines solution is the most common method used in both the Marvel and DC comic book universes. We find the idea perfectly summarized in one panel all the way back in the classic Days of Future Past storyline, in Uncanny X-Men 142:

Both Marvel and DC’s multiverses have an untold number of parallel universes, accounting for any and all alterations to the timeline that could possibly be imagined. This idea has now also been adopted into both companies’ movie universes. The Marvel Cinematic Universe is currently in the midst of its overarching “Multiverse Saga,” and the most recent Flash film centered on the idea of time travel creating parallel universes.

The creation of alternate timelines does seem to eliminate logical contradictions when changing the past, but – despite how much I love a good multiverse story – there are a number of new issues with this interpretation of time travel. A universe has a whole lot of energy in it. So where does it all come from, when a new parallel universe is created through time travel? It seems to come out of nowhere, but that’s not allowed by the law of conservation of energy.

Furthermore, if time travel exists, it’s going to be used more than once, and each use creates an alternate timeline, right? And at some point, in each of those alternate timelines, time travel will also exist and be used multiple times, creating even more parallel universes. It’s easy to imagine the number of alternate universes growing at an exponential rate. Even more energy!

Time travel Marvel universes — Marvel Comics

Beyond the physical issues, let’s consider the philosophical implications. The existence of alternate timelines basically destroys any concept we have of individual identity. Who am I, really, if there are thousands of parallel “me’s” out there? Are they very similar or completely different than I am? Is there an original me, and then just variants? How would I know which one I am? How much does it matter?

Is there a universe in which a horrible act of evil was avoided, while people in other universes have to live with it? Why should some people in one reality have it so good, when another timeline could be a desolate wasteland? The existence of parallel realities would completely ruin any concept of universal justice.

I’ll admit those aren’t logical contradictions that eliminate the possibility of alternate, parallel timelines, but there may be a less morally distasteful way of imagining time travel, which I’ll look at in the third and final installment of this series.

Every February, to help celebrate Darwin Day, the Science section of AIPT cranks up the critical thinking for SKEPTICISM MONTH! Skepticism is an approach to evaluating claims that emphasizes evidence and applies the tools of science. All month we’ll be highlighting skepticism in pop culture, and skepticism *OF* pop culture.

AIPT Science is co-presented by AIPT and the New York City Skeptics.