Ask Lemmy

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Time travel does exist, but you can only go forward. You just need to approach the speed of light relative to a frame of reference, and you will travel a shorter time span compared to it.

From a narrative sense the "nexus" theory is certainly the most amusing, which is probably why Terry Pratchett posited it works exactly that way on numerous occasions. It turns out that history really is kings and battles and speeches and dates, and in order for history to have actually happened someone has to observe those critical events. The things in between really don't matter. History as a whole further finds a way of happening whether people are involved in it or not, and regardless of -- or possibly despite -- anyone attempting to hinder, help, or change it. The key events will always happen eventually. All anyone can do is slightly influence how long it takes for them to do so, which is why there are so many boring spans in history where it seemed like nothing really happened; That's because it didn't. Possibly until some history monk noticed, and came along to pull out whatever spanner was holding up the works.

Probably the branch off one.

Though, speaking of time travel, I really don't understand/like the whole Harry Potter dementor (however it's spelt) lake scene in the movie where future Harry saves past Harry. How does that work? Wouldn't in an initial timeline Harry have to somehow save himself before he could travel back in time to save his past self? The way I see it, it just looks like an infinite cycle of Harry saving his past self with no origin point.

My belief is if you went to the past your actions would fully effect the future, no branches or anything else of course this will create paradoxes but if your a time traveller you will still exist even if you prevent your birth, if then you go back to the future there will be no record of your existence.

Hope that makes sense.

In either scenario, I'm more interested in where the matter you're made of will come from:

• Either you go to the past and suddenly add additional atoms to the universe.
• All the atoms you're made of will suddenly be taken from their origin to form you.
• You'll be made from entirely new atoms created from pure energy meaning your arrival will cost ~6.75 Quintillion as in 10^18 Joules (I eyeballed the speed of light here so don't @ me).

Branch off probably

The current scientific theory is that time exists across space in cones that would require one to move faster than the speed of light to alter. Going to go with that one for now since I have no idea personally.

I like the idea that the timeline you exist in is that and can still be determined but everything that happened in the pasr is set in stone. Future time travel is not possible.

Whatever it is, I don't believe paradoxes are possible (other than language ones that basically just confuse any attempts to resolve a statement or set of statements to true or false without breaking any physical laws or causality).

That said, I don't think an unstable time loop would necessarily be impossible. Eg, you go back in time and kill your grandfather before your father is conceived, which results in you never existing in the timeline, which then means no one is there to go back in time and kill your grandfather, which means the loop disappears and the timeline snaps back to the version where you do go back, and it continually alternates from there.

Not sure if any future outside of the unstable loop would exist, I think that would depend on if there's a higher dimension of time that these loops could play out over.

Or, if everything experiences the same present at the same time, it's also possible that after the first loop, it wouldn't go back to resolve the whole "killer pops out of literally nowhere" because it was in the past and no time traveler is bringing the timeline back to there, so it's all in the past. Though I think in that case, you wouldn't disappear after killing your grandfather. You'd just be an enigma that would require going outside of time to understand the origin of.

Tbh though I'm 99% sure time travel just isn't possible (paradoxes or not), just a fun thing to think about. And no, I don't consider quantum effects being symmetrical in time to be time travel, they are just cases where you can reverse cause and effect and still have a valid cause and effect sequence.

Wormholes. Travel some place faster than light and see light from the past from your source of travel when you arrive, travel again back to your original spot and theoretically you travel backwards in time to before the light from the past that you just saw was even produced yet. Might work the same for just seeing the future if you glimpse through a wormhole that leads to someplace in the future by doing an Allie oop to further into the future someplace far away, then back to someplace in your future but your destinations past. Speed and gravity both impact time. A wormhole fits that description to a T.

You’ve already taken said actions but just didn’t know about it so nothing changes

12 Monkeys did this one perfectly.

You can't change things because if you undid the thing, then there wouldn't be a reason to undo the thing. If you go back in time, you are just going to do what you already did because that is in the past.

I like the one where the motion of the universe is not accounted for, so the travelers drop into empty space. But someone figures out how to use that to travel through space.

Time Wars are fun though. Each prime timeline moving others toward them.

Whichever one is objectively correct based on empirical evidence.

Fun fact: time travel does exist, and I am myself a time traveler. The fact that I'm travelling at one second per second along with everyone else is just a minor detail.

I have always been a fan of stable time loops so I guess option 2 is the best one for me.

One trope I'd like to see more of is loops which are not stable themselves, but are stable as a group. Eg a 2-loop has loop A in which someone goes back in time and changes history leading to a new timeline loop B. Someone in loop B later goes back in time and changes history in a way that turns the timeline back into loop A.

My headcanon is that your option 3 is basically an n-loop that we only see the first few loops of.

Its a one way trip. You can never go back to the original universe.

I really like the nothing is changeable and travel is possible and anything you do while traveling has already happened / was already going to happen concept

If it actually existed, then obviously I would subscribe to whatever theory most accurately described how it worked. That's science.

If you're asking which theory I would predict is most likely, knowing only that time travel was possible as a starting point, then there are only two that I'm aware of that are logically consistent. Either:

• Single fixed timeline, whereby if you go back in time then whatever you do there was already a part of history from the start. You won't be able to "change" anything because you were always there. This is the approach described by the Novikov self-consistency principle.

• Multiple worlds, in which if you go back in time you just end up following a different "branch" of history forward from there.

Any of the models that let you "change your own history" are logically inconsistent and therefore utterly impossible. They just can't exist, like a square triangle or 1=2. They may be fine for entertaining movie plots but don't take them seriously.

I like the persistent present. We simply live with the paradoxes.

"Remember when Hitler was assassinated in 1919, 1933, 1936, and 1939, then off'd himself in his bunker in '45?"

Primer because Primer. (Video warning and some spoilers for a bunch of different films.)

I don't know if I would subscribe to it, but it is one of the more interesting ideas for time travel.

From a quantum perspective the Deutschian and similar models are honestly pretty compelling. They essentially require matching up the past and present in a consistent way that can remove paradoxes.

These make the most sense because it's entirely possible to write down spacetimes that contain "closed time like curves" (CTCs) ie. paths connecting past and present and you can then just let physics play out on these models (or more commonly using black box quantum circuits). The only consistent way to do it is to make sure the past and future side of such curves agree. It's not my area at all, just something colleagues of mine did, but from memory there are nice approaches using the path integral formalism that work really nicely in these scenarios.

All that's to say that I don't think time travel leads to anything changing, the past will have always agreed with whatever time travel happens in the future.

Having worked very briefly with the spacetimes that produce CTCs, I don't expect we'll be able to time travel because they usually violate the weak energy condition.

https://en.m.wikipedia.org/wiki/Quantum_mechanics_of_time_travel