Time travel will never have any effect on anything.

I have a unified theory that includes bits of everything.

• it’s possible to communicate in the same timeline between the future and past by using gravity (think something like Interstellar)
• By using gravity as an nondestructive line of communication, it’s possible to change events in your current existence without a causal shift or split.
• by sending actual matter into a timeline either ahead or behind where it’s supposed to exist it causes a causal shift or split in the timeline. this means you will never be able to see the future and can only contaminate the past enough to damage your present causing an inverted negative effect to your going back in time anyway.
• nexus events can only exist when it has a high gravitational marker attached to it. eg: a star will always go supernova, when doesn’t matter as much as the fact that it’s unstoppable because the gravitational function it applies on a universal scale across all timelines, known and unknown. think of it like flashing a flashlight into a room filled with mirrors. each mirror is the physical plane of existence for the timeline (the beam of light). the beam will hit the first plane and then bounce off all the others in the order in which the photons scatter. if you could slow down and witness the photons, they wouldn’t all hit at the same time nor at the same strength.
• It’s only possible to communicate across time using gravity and only if someone has picked up the “receiver” in the past. meaning if we’re not listening for the call “now”, we will never receive the call from “tomorrow”. I think this type of communication is completely within the reach of our technology today and quite possibly is being used today without world knowledge.

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?”

The one that exists.

I subscribe to multiverse theory. It’s probably the safest route and probably most likely.

The most interesting one to me, and the one that makes the most sense, is that changes propagate forward in time at the same speed as everything else, so 1 second per second. Why would causality suddenly decide to go any faster than that? This effectively means that all “alternate timelines” exist on the same timeline, and overwrite each other as they move forward.

You can visualize this by coloring the original timeline red. When you time travel backwards, you arrive at an earlier point on the timeline and it begin overwriting it orange, with the “head” of the orange section expanding into its future, which is previously red. If someone travels into the orange area again, it turns yellow, etc. If the instant where you time travelled backwards to make the orange region gets overwritten, the color of the timeline to the left of the orange region would begin expanding to overwrite it at the same speed as any other change.

This does lead to some interesting things, like two time travel loops that include the same point in time literally slowly corrupting the timeline. One loop, where you travel back, wait until when you left, then travel back again, would cause the future from your departure point to continually be overwritten by each new loop color, sending constant-width “bands” of colored time forward before they’re overwritten by the band from the next loop. Two loops’ bands would almost certainly not be commonly divisible, so you’d eventually end up with “bands” moving forward and within the loop that get smaller and smaller, fragmenting the timeline into colored noise. If you lived on the timeline, though, you wouldn’t notice-- even if you’re in a timeline band that’s only 1 second wide, you move with it, so nothing seems out of the ordinary. But if you travelled back to the same point in time repeatedly to check on it, or could freeze yourself in time and watch the bands pass through your point in time, things would be changing incredibly quickly. This also means that waiting time in the future before travelling backwards in time would let the past have time to be overwritten by a different band, so the same point in time would be different depending on when you left the future. All timeline damage would be repaired (at band-expansion speed) if you could remove all instances of time travel backwards to the offending loops, though.

IRL, the speed of causality depends on your speed, too, and in theory, timeline changes would expand outward at the speed of light. My brain is not big enough to think through all the potential consequences of relativistic weirdness and time travel at once, though. I suspect it would allow for “bands”/fragmentation not only in time but in space as well.

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I believe it’s impossible in the real universe.

Sure there are solutions of general relativity that contain time loops, but they require stuff like an infinitely long cylinder, or escaping a spinning black hole, or negative energy. I just don’t believe beings made of finite matter and with finite energy will ever be able to time travel (except into the future at various rates) and that’s the only kind of beings I think exist.

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.

I think time travel as a being who perceives one dimensional time linearly is not possible. And for any entity who doesn’t perceive time linearly it would be no different from traveling in a spacial dimension. It’s just travel. Anything that entity does in that point is a permanent fixture to the entities that perceive it linearly.

So yes, if someone could travel in time in the SciFi sense, they wouldn’t be able to change anything in their past experience (direct experience or prior to their perception, but in their event line) because that’s already part of that point in spacetime to anyone who experiences it linearly.

But also, it’s likely that time is not one-dimensional just like we know space is not only three-dimensional. So it is possible that you could end up in a separate “branch” of time that your past self from your perspective will never experience (directly or as past events), because it’s not the same point in spacetime as the event in your direct past timeline. But it’s not like there is a specific set of “branches”. They likely don’t branch off from a single trunk into the other dimension(s) or if they did “branch”, it was at the same time as all other “branches”, the beginning of the universe, not as specific events occur like in SciFi. And the changes you make in those branches were always part of those branches to people who will perceive the future of that timeline.

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.

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.

The first one. Specifically because of wave function collapse (ie. The cat is both alive and dead until the box interacts with the universe)

I either could have or could not have travelled back in time to the year 1927. In our present universe, the wave function collapsed and revealed that I didn’t. If I went back in time to 1927, I’d essentially be re-rolling the dice, causing the wave function to collapse again, this time revealing that I did in fact move back in time.

Re-rolling the dice doesn’t change the initial roll. It’s immutable in the fabric of reality. All I’m doing is creating a new universe in which I did travel back in time.

If I were to then move forward again, I’d be in the new timeline, not the original one. There’s no going home again. Which is why Sam Beckett was never able to return home. He spent four seasons creating different universes where one person’s life was better at the expense of a bunch of others.

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.

• You branch off into another timeline and your actions make no difference to the previous timeline

New actions, new consequences.