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Source: https://writers.stackexchange.com/a/42539
License name: CC BY-SA 3.0
License URL: https://creativecommons.org/licenses/by-sa/3.0/

Source: https://writers.stackexchange.com/a/42539
License name: CC BY-SA 3.0
License URL: https://creativecommons.org/licenses/by-sa/3.0/

The easiest way to time-travel without paradoxes is the rewind universe.

Think of the time-machine as a bubble that preserves your body and mind. The entire universe around this bubble reverts to 1963. When you get out, the future is gone, it hasn't happened, and from now on, the universe will evolve again, and may evolve differently **even if you do absolutely nothing to affect it.**

The reason for this is (putting on a physicist's hat) due to quantum wavefunction collapse. When the wavefunction collapses (say, spontaneously for some reason), there are several super-positions of possible particle configurations, and the "collapse" means exactly ONE of these is selected **entirely at random** as "reality", all the others are discarded.

But if by some means your time-machine finds the magic switch that reverts the universe back to as it was in 1963, all the wavefunctions of that moment are restored, and as they collapse, they again select one eigenstate (the technical name for the super-positions) **entirely at random** as "reality".

So, although most things happen exactly as they did before, not everything will, this "fundamental randomness" will not be repeated the **same** eigenstate selections from the first nanosecond after the rewind.

Then you have the interference of yourself. You were preserved inside the bubble, all your memories and flesh were preserved. So in a sense you know the most-likely future, the bigger things are the more likely they are to repeat. If you already exist in 1963, then two of you exist; you are just another human 56 years old (from 2019). You could go kill your younger self, your parents, if you go far back all your ancestors.

It doesn't make a difference in this new world that you wouldn't be born or wouldn't survive, because you don't have to invent or use a time machine to get back there.

Only your brain contains the memories of the future that evolved before, with your parents or siblings. But memories aren't **real** , they are entirely encoded in the configuration of neurons in your brain, and you brought that with you.

As far as the universe is concerned, you just randomly materialized in 1963 and the universe continues from there. **There is no future** after the rewind, so there is no communication or cause-and-effect with the future.

If you travel **forward** in time, faster than one-second-per-second like we all do, then you will just see the slightly different evolution of this rewinded universe, and many things at the macro level will be nearly the same, and many things at the micro-level may be different.

Particularly, perhaps, the effects of spontaneous decisions that had little preamble to push them in a particular direction. Impulse buying, or impulse sex that resulted in a pregnancy, or impulse violence that changes the course of a life.

Other things you might count on, and ultimately change the course of the future. You could stop the JFK assassination, perhaps. You could be there (as your future self) to save a childhood friend of yours from drowning, or dying in a car accident.

Things like that do not change the future **you** lived through, but change the future of the reset world. You might find your 56-years younger self, because your friend lived, did not experience the shock or grief you did, and as a result chooses a slightly different path in life, and never invents a time machine (e.g. if you jump forward a year at a time to check).

There are no paradoxes, once you travel back, or rewind the universe, nothing of the future exists anymore, except for what was in the protected bubble: You, your brain, your memories, your notes, computers or whatever tech or anything else you brought with you. The people, the history, everything after the point in 1963 when you arrive has been obliterated, just as if it never existed. And the world starts again, with you in it. But even that is not a paradox, you and your machine are just energy (E=MC^2) in a particular configuration of particles. Nothing in quantum mechanics prevents that particular configuration from spontaneously appearing. (It is only astronomically unlikely, but not impossible.)

Original score: 5