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No. 3216:

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Beamed directly into your brain by: aliyaist

The author writes:

Whenever time travel appears in a work of fiction, there are a half-dozen questions you need to ask about the rules.

How much can you change? From a purely physical standpoint, the two most likely scenarios are infinite ripple effects or the Novikov Self-Consistency Principle.

In the former scenario, merely bumping into a person in the street will change the genetics of all of the children they have for the rest of their lives. The tiny changes made by the birth of a boy instead of a girl, or vice versa, or them receiving this recessive gene from their father instead of that one - these will accumulate and spread until, barely a generation after the change, the two timelines will have literally not one person in common. No change to the timeline can be considered "minor", because minor effects propagate infinitely to spread across the globe.

In the latter scenario, literally nothing you can do will change the past because your changes are already part of the past. Your attempts to make a change are already part of the timeline that has already been established; they will either directly cause the event you're trying to prevent, or you will encounter insurmountable barriers on the way.

In practice, though, few science fiction stories lie at one of these two extremes. Authors find it much more fascinating to write "what if" scenarios than to write about how everything stayed the same, and contrasts are more notable the more the world around them resembles our own.

Are there multiple universe or just one? If there is only one universe, then any changes you make to it (if changes are possible) will erase the timeline that you came from. Each of these cases lead to followup questions:

When you time travel, are you entering/creating a different universe or remaining within your own? (for multiple universes only) If you create a new universe with each trip you take, you have to consider the awesome responsibility you have for the billions of people whose existence you bring into being merely by taking your trip, even if you change nothing. Then again, if the universe works by means of infinite parallel universes branching off of every decision point, then you're doing the same thing every time you decide whether or not to blink. So, eh, might as well go back in time and enjoy yourself, right?

If you're merely entering an already-existing parallel universe during your trips to the past, then you have another sub-question: Does your subsequent trip forward brings you into the future of the world you just changed, or back into your own future? If the former, then any changes you make won't benefit you unless you decide to live them out in real time; if the latter, you have to accept that you'll never see your original universe again.

If you're remaining within your own universe, then the existence of other universes doesn't really impact your time travel - unless you connect your time machine to a universe-hoping machine and operate both in tandem.

Are time travelers immune to changes made to the timeline? This question is valid in both single-universe stories and multiple-universe stories in which you travel into the past of the same universe. When you change the past, does your memory of that past get altered? If your leg got broken but you went back in time to prevent it, does your current leg magically heal? Or does the fact that you're not currently in your present mean that the alterations to the timeline pass you by? If your body is not immune to the changes but you still retain memories of your old timeline, does that prove the existence of the human soul? What if you remember both timelines?

Do changes affect the entire timeline simultaneously, or does it take time for the ripple effects to propagate into the future? This question is only valid if you are travel into the future of, or originally came from, the same universe whose past you just changed. Can you travel to the future faster than the wave of change, and watch the entire world suddenly shift around you? Will your altered memories take time to catch up to you, or will they be altered immediately? If you prevent your parents from meeting, will you instantly disappear yourself, or will it take time a la Back to the Future?

Can you go back in time and meet yourself? Many stories say you can't do this or it will cause a paradox. Other stories don't have a problem with this. Others say you can go back to the same time but not to a place where you might meet yourself or that you can go back and meet yourself as long as you don't actually make physical contact with yourself (which, too, makes little sense from a physics standpoint). Still others say that the universe will prevent you from even being able to travel into a time where you already exist because you can't have "two of the same thing at the same time" - which is curious, because your body exchanges chemicals with the outside world at a high enough rate that you're technically not made up of the same matter as you were only a few years ago. Or does that prove the existence of the human soul?

One interesting take on this question has you going back in time and actively replacing yourself - that is to say, you go back in time, and the moment you appear in the past, all the matter that makes up your body that is already in the past suddenly disappears. So if you're a known time traveler, people can know that a future you has arrived in the present if present you suddenly vanishes. (Oddly enough, stories that use this method never seem to mention those people seeing a pile of digestive system contents that are part of you in the present but aren't included in the matter coming back from the future - nor do they mention the consternation of the farmer who suddenly finds that one of his still-living chickens is missing the leg that future you happened to eat before his trip.)

Is the time travel physical or mental? This question is simple enough: Do you go back in time, or do you merely project your consciousness into the past? If the latter, can you project it into anybody or only into your own body? (This question is incompatible with many of the answers to previous questions, so answer them with care.)

How long does it take to time travel? In some stories, moving into the past is essentially instantaneous. In others, you feel or see yourself moving down a time stream at varying speeds. Rarely, you have to sit in the time machine for the same amount of time as your intended trip - which makes traveling back to earlier than a few weeks highly impractical, and traveling into the future completely pointless.

And, finally: what are the risks of time travel? These can vary wildly. Maybe there are Time Gremlins that can attack your vehicle. Maybe there are Time Police that can do unspeakable things to you if you mess with the timeline in an unauthorised fashion. Maybe if you step outside of the time machine while in transit you'll rapidly age until you disintegrate. The possibilities here are infinite.

These are the major questions you have to ask when establishing the rules behind a time travel plot, but there are many more. What other questions can you think of? Let's talk about them in the forums. (And the first person to guess the full set of answers as they apply to Marcus's current trip wins a prize.)