[wizofaus]

I do. If it were possible we'd surely see them everywhere. Even if time travel was a one-way trip there's enough future billions of us that there'd be massive numbers with the sort of incurable fascination seth the past that they'd be motivated to travel back and see what it was like. Doesn't really seem any more or less likely than alien intelligence at any rate.

[tzs]

That reminds me of an amusing story I read in Analog several years ago. I don't remember the name or author.

It was about the first time travel trip. The team that developed the first time machine decided to send the first traveler to visit Shakespeare, figuring that Shakespeare had a flexible enough mind to not be freaked out by the visit.

When the traveler got to Shakespeare they were right that he did not freak out. In fact he took it entirely in stride. The time traveler was a little confused that Shakespeare was taking it so well. Shakespeare even asked what gift the time traveler had brought, saying that "all the early ones brought gifts".

The time traveler had in fact brought a gift--a nicely bound volume of Shakespeare's collected works. Shakespeare looked at it, said something about maybe he could sell the binding, then said probably not, and tossed it on a pile of books, which the traveler realized was a pile of similar books.

Shakespeare noticed that the traveler was now throughly confused and realized that the traveler was in fact one of the very earliest, and explained that most of early travelers brought books.

The traveler was still confused over the idea that Shakespeare had met other time travelers, saying "but I'm the first time traveler!". Shakespeare told him that he may have been the first to leave, but he certainly wasn't the first to arrive, and said at some stages in his life he was being visiting frequently by time travelers, which was actually annoying--although not as annoying as it was for Jesus, who Shakespeare says another time traveler decided to introduce them once.

At that point numerous other time travelers started arriving. They were reporters from throughout the timeline popping in to try to get an interview with the first time traveler. The first time traveller is now close to completely losing it, and Shakespeare says he can handle it and steps in to act as a press agent for the first time traveler.

If backwards time travel turns out to possible my guess is that there will be some limitation that prevents scenarios like the one in that story from happening. My guess is either (1) the time machine will only be able to go back to when it was created (think of it like going back to a save point in a game), or (2) when a time machine goes back to some point in spacetime it creates some sort of exclusion zone in a region around that point that precludes any other time machine from arrive at a point in that exclusion zone.

[wizofaus]

I suspect time-travelling and Shakespeare is a whole sub-genre, the one I know is one where Shakespeare travelled forward in time and enrolled in a university Shakespeare course - which he then failed... There must be at least one where all his works were actually sent back in time for him to copy from, leading to all sorts of questions as to who originally wrote them.

[tzs]

The story you mention is probably "The Immortal Bard" [1] by Isaac Asimov.

[1] https://en.wikipedia.org/wiki/The_Immortal_Bard

[wizofaus]

That's the one, I read it pretty recently actually!

[wizofaus]

BTW...I actually asked ChatGPT (just 3.5, don't have 4 access) what story it might be based on a description. Couldn't do it, gave lots of nonsense answers along the way, but when I prompted it with the name of the author and the word "immortal" it finally got it. Was kinda surprised actually, I'd think that's the sort of thing an LLM should be able to do quite well.

In fact, on further experimentation, my only conclusion is god help anyone who tries to use ChatGPT to help with studying literature.

[Grimm665]

If you haven't watched the movie Primer yet, you may enjoy it.

[detrites]

Maybe every instance of time travel the universe splits in two, to prevent all the causal loop paradoxes etc?

Ie, time is always a tree branching, and traveling back in time doesn't change that?

[wizofaus]

Well, sure, it's possible my consciousness is one that's travelled along every single branch where the backwards-time-travel didn't happen, but that strikes me as extraordinarily unlikely if there have been even only a fifty such attempts in all human (future) history.

[detrites]

What I mean is when a particle travels back in time, the universe branches forward in parallel, from that particle, at the instant it arrives. This resolves all paradoxes as the independently instantiated time streams can't interact.

I believe there are present theories of time/space that rely on this kind of idea.

It also might mean any time traveller could never get back to the exact "when" they came from. Though if there was a way to traverse parallel time streams, there'd be no paradox as the moment they arrived "back" would also branch.

[petilon]

Time traveling humans is more likely for the following reason: It requires only one thing: worm hole or some other yet-to-be-invented mechanism for traveling to the past. For this to be alien intelligence, two things are required: First alien intelligence has to exist, and second, they too need a mechanism for speedy travel, to travel to another galaxy such that they can reach the destination within an individual alien's lifetime.

[TechBro8615]

Aliens could exist with or without speedy travel. We can assume slow-traveling aliens must be from long-lived civilizations, but we can't assume fast-traveling aliens are from short-lived civilizations.

Slow-traveling aliens likely come from a long-lived origin civilization (although it's possible that origin civilization went "extinct" millions of years ago, but its descendants continue to reproduce of spaceships traveling slowly outward in different directions, these descendants would arguably be from the same origin civilization, which must definitionally be long-lived).

Fast-traveling aliens might be from a civilization doomed to be short-lived, but they achieved FTL travel so we just happen to meet them. They could have popped up a million years ago, and be on schedule for extinction in another million years. But since they can travel quickly, they don't need to be a long-lived civilization in order for it to be likely that we might encounter them. They could be one of many short-lived civilizations.

In a universe without FTL travel, the probability that we encounter an alien civilization is dependent on the expected duration of an alien civilization; the more long-lived civilizations that exist, the more likely we'll encounter one, because they've had more time to slow-travel. In such a no-FTL universe, there could be a high probability of civilization, but with a low expected civilizational lifetime. So we'd be unlikely to encounter any civilization, despite the high number of them in the universe.

So what I find ironic is that even with a bunch of aliens crashing (regardless of how slowly) onto our planet, we can't actually infer much new information about the Fermi paradox, or whether we've made it past the Great Filter. Either we're encountering civilizations that must be long-lived because they're slow-traveling, in which case there may not be a filter because the paradox was resolved by the lack of FTL travel; or they can be both long-lived and unknowably short-lived, in which case we don't know where the filter is because any civilization we meet could go extinct next (perhaps achieving FTL travel even achieves some prerequisite for a specific class of extinction event?).

So either there might not be a filter, or we don't know where it is. The most informative scenario would be for us to meet a long-lived, fast-traveling civilization.

[wizofaus]

I'm probably more positive alien intelligence exists than I am that humanity will last long enough to discover such a mechanism. To be clear, I'd say both are quite likely - I just very much doubt the mechanism actually exists.

[spywaregorilla]

describing something as yet-to-be-invented to contextually imply that it exists and will be invented is a strange proposition

[kalkaran]

And they have to want to