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What would happen if you went back in time and killed your grandmother?

If a person traveled back in time and killed their very own grandmother, what would happen to that person?

You ask a very good question! In a word, nothing.

We don't yet know how to travel back in time, though theorists claim that it is indeed possible. However, there is a very fundamental law of physics that says that the effect of an event can't influence its cause (or, in fancy words, one cannot violate causality). For example, you are an effect of your grandparents having children. Therefore your grandmother is a cause of your existence, and so physics says that you cannot do anything that might influence your grandmother in the past.

So we know that if you could go back into the past, you could not kill your very own grandmother; it just isn't allowed by physics. Strictly speaking, you can't even go back in time and talk to her, because that would be influencing her which is not allowed by causality. So, the question now becomes: how do you go back into the past without influencing your grandmother in any way?

This, indeed, is the part about travelling back in time that has us stumped. And, of course, why we don't know how to travel back in time yet.

There are a number of ways to get around causality, however; most of them are too complicated for me to understand! The main idea, though, is that when you go back in time, you don't go back to the Universe in which you presently exist, but into a mirror Universe that had the same past as you but a different present. For instance, suppose you wake up every morning and have either Corn Flakes or Raisin Bran for breakfast. Each time you choose Corn Flakes for breakfast, another Universe starts up in which you chose Raisin Bran. This happens for every choice that you make, every second of every minute of every day. That's an awful lot of Universes! But it means that when you visit the past, the choices that you make in the past (like killing your grandmother) don't affect your present life because they create another Universe separate from our own in which you don't exist. If this is the way that time travel into the past works, then nothing would happen to you if you killed your grandmother, because the effects of you being in the past are not felt in your present Universe. Sound like the plot of a Star Trek movie? Sometimes reality is truly stranger than fiction...

June 2002, Kristine Spekkens (more by Kristine Spekkens) (Like this Answer)

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