Grandfather Paradox

If you go back in time and accidentally kill one of your ancestors, what will happen to you?

When it comes to time travel hypotheses, it is one of the most popular and fascinating questions of all time. And it’s not just a plot device that science fiction writers and filmmakers use to propel a story along. The so-called grandfather paradox has been pondered by physicists and philosophers alike for nearly a century.

What is the grandfather paradox?

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Generally speaking, the grandfather paradox is a colloquial term for almost everyone paradox of causation. Causation, as you may remember from school, is the relationship between cause and effect. Although not quite a law, causation is certainly fundamental to our understanding of basic physics, which depends in part on the idea that cause must precede effect.

The a paradox part of the grandfather paradox occurs when a time traveler creates a self-contradictory scenario where the effect precedes the cause. In this example, if you kill your grandfather before he has any children, one of your parents was never born. Therefore, You are never born. But if you were never born, then you wouldn’t exist to go back and kill your grandfather, would you? A paradox!

Of course, the paradox doesn’t just apply to patricide cases, but to any hypothetical situation where a time traveler goes back in time to prevent something from happening. If they are successful and the event never happens, the time traveler has no reason to go back and change anything. So they may try again and … paradox! Whatever the traveler changes doesn’t have to be intentional either; accidental interference with a past event can also cause a paradox.

Who invented the grandfather paradox?

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Although the concept of going back in time and changing the past has been around almost since the dawn of science fiction, the credit for naming the grandfather paradox often goes to early fans and writers of science fiction magazines.

The specific time-travel trope of killing your grandfather (and the consequences that may follow) is mentioned several times in the 1920s pages of letters of pulp magazines such as the famous Amazing stories. In the late 1920s and early 1930s, now-unknown writers such as Charles Clokey, Nathaniel Schachner and Rene Barjavel delved into the paradox long before it was used in famous stories by Ray Bradbury or in popular movies and television shows ranging from Star Trek to Back to the Future to The Umbrella Academy.

Has anyone solved the grandfather paradox?

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As a purely logical problem, scientists and other thinkers have devoted a surprising amount of time to pondering the grandfather paradox over the past few decades. Physicists who study quantum mechanics, for example, have published some compelling but not easily explainable results documents which allow a temporary paradox to exist without necessarily violating the laws of quantum physics.

Meanwhile, others offered less scientific but more creative ideas to resolve the paradox. One of the more popular notions – a real “have your cake and eat it” theory – is that you won’t actually create a paradox or cancel your own existence by killing your grandfather.

A parallel universe

Instead, after your grandfather dies, you’ll find yourself still very much alive, but in an alternate timeline. The future (or your past, depending on how you look at it) will remain unchanged in your main timeline while the alternate timeline continues to unfold elsewhere in the multiverse. In fact, some theorists believe that this solution isn’t really talking about time travel so much as it is about traveling to parallel universes, in this case a place just like yours, right up until you leave it.

Read more: Scientists are trying to map the multiverse

Yet, in the end, both options—killing Grandpa and not erasing yourself from existence—can arise in this scenario. The paradox is resolved! (Though this decision raises a different thorny question: After killing your grandfather, can you go back to your original timeline, or will you remain in the alternate version?)

Principle of self-consistency

Other theories claim that the grandfather paradox simply shows you that time travel is not possible at all. The past cannot be changed; somehow the universe will correct its course before you have a chance to change the past. In short, you may find that your time machine simply won’t work, the best universal safety measure to prevent any paradox. A variation of this theory views cause and effect as an indestructible physical law, like gravity. In other words, just as you cannot simply decide to float off the earth, you would also be physically incapable of doing anything, either accidentally or on purpose, that would create a time paradox.

Paradox of Predestination

This brings us to the “what happened, happened” school of thought, which basically says that even if time machines were real, you’re kidding yourself if you think you can change things in the past. This theory relies less on cause and effect and more on the idea that everything is simple pre-defined. Your “going back in time to make a difference” – or whatever i think is change – it was always meant to happen.

Read more: A big advantage for time travel may be technically impossible

For example, let’s say that you do go back in time and kill your grandfather. Plot twist: It may turn out that he was never still your grandfather! In a cosmic case of mistaken identity, someone else could be revealed as your actual grandfather and you would still exist in the future as always intended. (But then Grandma might have some explaining to do.)

Read a wilder variation on this subject “resolves” the paradox by introducing another paradox. The idea here is that if you manage to kill your grandfather, you’ll eventually take his place and become, yes, your own grandfather. It’s true that scenarios like this tend to play out more in the minds of fiction writers than logicians and theorists.

Is time travel possible?

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Don’t laugh! This question is not as far-fetched as you might think. Are time machines real? No. Will they ever be? Probably not. Like the deceased Stephen Hawking and others note that if such devices were ever real, we’d probably have seen tourists from the future by now. But that doesn’t mean time travel itself is impossible, at least theoretically.

Theory of relativity

of Einstein theory of relativity establishes that time and space are connected, and that the faster you travel through space, the slower you experience time. This is known as time delay and you’ve probably heard the concept explained through the famous example of two twins – one who takes a trip in a fast rocket ship and one who stays on Earth. The closer the twin astronaut’s ship approaches the speed of light, the slower that twin will experience time. When this twin returns to Earth, they will be significantly younger than the twin that remained on the planet. Essentially, the astronaut twin will have traveled into the future. Of course, it would be a one-way trip, and humans still can’t (or maybe ever will) make a ship that approaches the speed of light, but it doesn’t matter – relativity still says that this form of time travel is possible.

Wormhole theory

After that, that’s it wormhole theory. Einstein’s theory of general relativity allows for the existence of wormholes. A sufficiently large source of mass and energy (such as a black hole or the merger of two black holes) can create these structures, making a tear in the very fabric of the universe, connecting two different points of space-time. Traveling through such a wormhole would, for our purposes, be equivalent to time travel. But then again, humanity has no way of creating such a phenomenon – even if we could find a naturally occurring wormhole stable enough to enter, it’s unlikely we’d ever be able to construct a craft powerful enough to survive the journey through her.

So, unfortunately for us – but fortunately for all our grandfathers – true time travel remains a practical impossibility. Instead, humans will just have to settle for the same form of time travel we’ve always used—not to the past, but to the future, one second at a time.

Read more: Is there a particle that can travel back in time?

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