Time travel is self-suppressing | Hacker News Reader

Time travel is self-suppressing

arxiv.org

41 pointswarrenm71 comments

Maybe they are all mostly dead and ever-more-feral survivors ridden by the crippling radiation- and pollution-borne genetic sicknesses are birthing still-born and slowly dying out while picking through the debris left from the civilizational collapse caused by global warming, ai, and the resulting world wars.

And the last stronghold of civilization are genetically superior, warlike, numerous, but illiterate Tate descendants hidden in the mountains of Romania, unable to build anything more advanced than a cudgel used in the rituals to determine the alpha leader.

Most time travel theories ignore the fact that the earth is not fixed in space. It is moving relative to the sun in the solar system and the solar system is moving relative to the center of the galaxy, and the galaxy is… etc. A motion in each of these systems is not 100% accurately predictable forward or backward in time.

This fact alone means that any time traveler is most likely to arrive in the middle of empty space.

> Most time travel theories ignore the fact that the earth is not fixed in space

This is a misconception that bugs me. The problem isn't that the Earth isn't fixed in space, it's that there's no such thing as a fixed point in space. Position is only defined relative to other objects. If you're going to use time travel in a story or something then it has to use something like an anchor object to determine destination. I.e. the relative location of the traveler and the anchor is replicated from the future to the past.

Relativity just says "nothing about space seems to require a preferred reference frame", not "such a thing as a preferred reference frame can't possibly exist". If we're allowing for the discovery of time travel in the story, I'm willing to allow for such a discovery as well.

In reality I'd bet neither are realistic, but that's what makes the stories interesting.

We would assume that the time-space traveler would have to tell the machine both the time and space directions from their current position in time and space. Assuming the time-space traveler cannot stop to observe his or her “location” in time-space coordinate along the way in small increments, he would have to calculate the entire travel trajectory beforehand.

I am saying this trajectory calculation relative to current coordinates is impossible. Even modern satellites with super precise instruments still need regular ongoing “adjustments.” Time travel requires many order of magnitude more precision than satellite orbital maintenance.

Go backwards in time 15 minutes at a time. At this short distance your calculation error will be small, and you can land your hovercraft back on earth to correct for any drift. Then go backwards another 15 minutes, repeat ad infinitum. Even present day aircraft have autopilot, so surely this can be automated too.

I think you have a good premise for a science fiction story right here: Say some "magic" (i.e. invented) physics quirk allows you to travel both into the future and the past, but all you can do is essentially accelerate and rewind time drastically. You don't "jump" to a time, there's still a physical presence, and colliding would have catastrophic results.

The logistical impacts from that would yield plenty of storytelling material: If you want to travel back in time, you need some ancient cellar that has been undisturbed since the target timeframe. If you want to skip forward, you need to establish that cellar, and round trips are limited by the space available.

That is - essentially - how 2002's _The Time Machine_ showed travel: Alexander's machine was 'stationary' on the earth, but time passed around him in a massively accelerated manner

to take this one step further - go back one infitesmal back in time and adjust position one infitensmal, thusly, a fixed time machine

Even if you could magically arrive at the right point, how would you get the right momentum? If the Earth were standing completely still, it would still be spinning at a horrendous speed.

I'll give a half-baked counter to this: we know gravity impacts the flow of time through relativity. There is currently no evidence that time travel wouldn't be impacted by gravity in some way. Maybe the way time in time travel interacts with gravity protects you from this problem? Probably not, but it has just as much evidence to support it as your claim of time travel will dump you in empty space.

You’re positing some unknown influence will cause everything to work out well in the ends without any evidentiary basis. Occam’s Razor suggests that you’re more likely to be wrong than parent.

Of course the idea that your point of origin must be fixed from time A to time Z if you’re willing to allow for time travel is itself flawed. If you could somehow move an object to an arbitrary time you could move them to an arbitrary point in space, and your ability to calculate may be significantly greater on the grounds that you’d have more advanced technology than us. It’s all scifi woo though until someone actually time travels.

I disagree with this interpretation of what I said. We HAVE evidence that time and gravity interact. It's actually more of a violation of Occam's Razor to suggest that time travel is somehow exempt from that interaction than to claim that yes, time travel should in someway be subject to the influence of gravity.

It’s even crazier if you imagine that whole universe might be countless universe lengths away from its starting point every microsecond for all we know. Acceleration is the only thing we feel.

You’re very likely to travel into an undefined void even if you map out and calibrate the whole system.

That why it is important not to mess up coordinate system. With wrong calculations they fall to ground. Or they are buried under ground. And space is full of frozen bodies.

1. Genetically superior 2. Tate Descendents

Pick One.

Tall, powerful, beautifully bald, multitudinous and decease resistant!

Why are you giving Tate free advertising on a completely unrelated post? Just saying that asshole's name risks exposing more people to him.

Streisand Effect, people. If you hate someone and want them to go away you have to completely stop mentioning them online.

Seems to me your comment has more of a Streisand Effect quality than the one you're replying to.

There was an amusing story in the July 1979 issue of Isaac Asimov's Science Fiction Magazine called "The Merchant of Stratford" about what it would be like for people who were visited by time travelers from the future. The Internet Archive has that issue [1].

Here's a summary from memory (which probably has some errors since I read the story in 1979).

It's about the first time traveler. They decided the first trip will be to visit Shakespeare.

Shakespeare has no problems accepting that he's being visited by a time traveler, and asks what gifts the traveler brings.

The traveler is a bit confused, so Shakespeare explains that the early ones all bring gifts. The traveler has brought some gifts, including a nicely bound volume of Shakespeare's plays. Shakespeare looks at it, comments on the nice binding, says something about maybe he can sell it, then decides probably not, and tosses it on a pile of other such volumes brought by other time travelers.

The first time traveler is now getting pretty confused, and says something like "but I'm the first time traveler!", to which Shakespeare answer "but not the first to arrive". This is something often overlooked in time travel stories--just because you are the first to leave for a given destination doesn't mean you are the first to arrive.

Shakespeare mentions that he's frequently bothered by time travelers, but at least doesn't have it as bad as Jesus--that guy can barely do anything without a time traveler showing up. Shakespeare explains he knows because a time traveler thought it would be interesting to take Shakespeare to meet Jesus once. All the great figures of history are frequently visited.

Somewhere in there Shakespeare provides some drink and tries to calm down the inexperienced time traveler, who is freaking out over all this. Shakespeare is an old hand at dealing with newbie time traveler freak outs.

Then a bunch of other time travelers arrive, but not to see Shakespeare. They are reporters from the first time traveler's future, there to interview him about his historic visit to Shakespeare.

[1] https://archive.org/details/Asimovs_v03n07_1979-07/page/n123...

The conclusion is basically (Larry) Niven's Law of Time Travel. From "The Theory and Practice of Time Travel" (1971):

> If the universe of discourse permits the possibility of time travel and of changing the past, then no time machine will be invented in that universe.

The entire essay is worth a read, of course. Meanwhile the paper in the OP goes for a more mathematical approach.

Makes me think of this Asimov classic where you really can change the past

https://archive.org/details/dli.ernet.526852/mode/2up

where it makes sense because people established an orthogonal dimension of time and this movie

https://en.wikipedia.org/wiki/Predestination_(film)

based on another Heinlein classic

https://en.wikipedia.org/wiki/%27%E2%80%94All_You_Zombies%E2...

where you can't because it's all just a knot.

Of course you can "change the past" ... except you already did

Larry Niven wrote an essay about different theories of time travel in science fiction

https://www.scribd.com/document/503128860/Larry-Niven-THE-TH...

In some of them (like that Heinlein story) there is just one timeline and you at best get causal knots, in others (like Asimov’s End of Eternity) a time traveller experiences different timelines and whatever other experience is ‘undefined behavior’

I don’t think Nicen wrote stories about traveling into the past but plenty about traveling into the future with cold sleep, stasis boxes, and relativistic travel with Bussard Ramjets, I guess you could have done it even more stylishly if you could afford to buy the secret of the Outsider reactionless drive.

Future people will have a Holodeck to simulate the past. No reason to actually go there.

is it real? of course not. expensive? very.

Or Hollywood!

Maybe you can only create rifts, and you need to create one before travelling back and forth. So only time travelling after the first time travel rift has been opened. Otherwise it'd break continuity.

EDIT: If you created such rift and nobody would come out, then you'd have to start worrying.

Wouldn't such a gate imply that an unlimited amount of entities would arrive at the time of the gate?

Doesn't any scenario of time travel imply an unlimited amount of entities?

If you go back in time to observe (but not interfere with) your younger self, your younger self will get old and go back to that exact time too. So there will be an infinite number of your old selves observing your younger self.

Not to mention that travelling back also means adding matter to the universe.

> but not interfere with

...and how would you actually do that if we assume that your travel has added matter to the universe, rather than completed an iteration of a time loop that was happening there already?

Yeah, "interfering with" is open to interpretation. One might argue that you don't have to say "hi" to yourself to interfere with, and it is sufficient to be within the same light cone.

more or less how time travel in _Looper_ worked, iirc

Exactly. If nobody comes, that means the world ended before anybody could use it.

Or it takes a lot of energy or resources and you can send one person every few hundred years, severely limiting the flow of people.

You can only use it once before it closes.

e.g. the machine is a big box. You can "start" the machine once and "end" it once. When you "start" the machine it instantaneously teleports everything inside it from when you "end" the machine.

If you "start" the machine and break it before "end"ing it, nothing gets sent through, or there's a giant explosion, or the universe collapses, etc.

Alternatively, you can "receive" and "send" any number of times on the machine. But every time you "receive" you get a unique ID, and you can only "send" to that ID once.

Could it be that time travel suffers from the dark forest problem?

A hostile and aggressive alien species with time travel capabilities would naturally use it to go back in time and eliminate any evolved species that similarly discovers time travel.

The energy required would definitely be enough to annihilate planets.

This is just the one model of time travel, isn't it? It's a bit weird how it uses the notion of rewriting to continue until there can be no rewriting. If there could never be any rewriting then you still permit the single universe model of time travel. Things can only ever happen the way they happen, if that was because of a time traveller, it always was. It permits the grandfather paradox, but I can't help but think that this papers argument could be reshod to say the the grandfather paradox is self-suppressing.

Submitted by Andrew Jackson, presumably the President of the United States of the same name.

Perhaps time doesn't objectively exist in the way it is understood as a premise of the paper?

I would argue time only 'exists' as a convenient way to describe our lives

(Incidentally, the Bible describes a singular Being Whom exists outside time.)

I'm partial to the idea that time travelers need a "gate" to arrive at, and until we have that, no traveling to the past. However the universe really likes avoiding being forced to compute a paradox, so it may well be many timelines.

I loved that about the film Tenet. It's very grounded compared to say Terminator (not that I don't like Terminator)

Same

Dark Matter (the TV show) also does this very well (plus multiverses)

"Primer" is arguably the best time travel film made/written.

this!!

Yeah, time travel probably isn't possible, but if it is, limiting the time traveler to only being able to go back to when the machine was invented at least solves the "why haven't we met a time traveler" problem.

It could also solve the "where will the extra atoms to instantiate a second copy of yourself come from?" problem if the machine uses some kind of filament to 3d-print the traveller.

sounds akin to how the transporter works in Star Trek

Ted Chiang's "The Merchant and the Alchemist's Gate" does this pretty well. It's a fun read (and short).

I like the approach taken by several authors from Asimov in "The End of Eternity" to Star Trek or Loki on TV: Time travel is not allowed except for entities that live outside of time in a way that is not meaningfully perceived by anyone else; When unsanctioned travel happens, it is easy to detect and retroactively suppress by these entities. Of course this can all be refuted or at least declared a transient state at most by the mess Time Cop is; Or how things end up in any of the other stories.

It’s also possible the time traveling civilization was killed off by boiling the oceans to plagiarize the entire creative output of humanity.

Looking at the actual link itself, is this one of those papers that takes a thought experiment and tries to evaluate it using abstract mathematics/statistics? That's what it looks like it's doing. How is it actually useful to apply Markov chains to such unknowable suppositions? Is this analytical philosophy

Time travel, like flight and invisibility, are a product humanity's imaginative nature: "What if I were not bound by the laws of the universe, what could I achieve?"

It's great for fiction, because of so many creative ways that you can structure your rules-violating-universe.

Didn't we have flying birds before even the dinosaurs, let alone the Homo Sapiens ? So we had physical evidence that flight was possible.

Flying things maybe, but birds are dinosaurs’ descendants.

Interesting:

> Therefore, I conclude that, assuming my model, time travel is self-suppressing: the timeline is continually rewritten until it inevitably reaches a timeline with no time machines ever being constructed. At this point, no further changes to the timeline are possible.

> inevitably

Fails right there. No strong argument was constructed to why this is inevitable.

I have always thought that time travel is a solution to the fermi paradox.

Title: Where Are All The Tourists From 3025?

wouldn't backwards time travel merely create an alternative timeline? for all we know, we simply live in the timeline that had to exist prior to the invention of time travel technology itself

they’ve all gone 4025, where the real party is.

I was about to say: This is not the best time to visit right now. Mabey later.

After 1000 years of recovering from the consequences of going back to 3024.

time travel backwards is impossible. but there are undoubtedly time travelers from the past going forward in time

I've already been doing this for a while now ;)

Hope you remembered to get extra plutonium. It isn't available at every corner drugstore yet.

All you need is to go fast to travel forward in time

be here now maxxi

I've always suspected the reason is this century is just incredibly boring and literally nothing notable or even remotely interesting happens.

Eh, plenty of things to steal.

Nah, everyone knows all the good stuff was before 1850 and after 2225. We're living in the 'fly-over country' of history.

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