Time travel

July 28, 2006 6:22 AM

Does the lack of time travellers, or artifacts from the future, indicate that time travel into the past will never be invented/possible/feasible at any point in the future? Is anyone looking for time-travel artifacts?

I'm interested in the philosophical angle on this, can we prove the non-existence of time travel by the absence of its artifacts?

posted by Mwongozi to science & nature (79 comments total)

Or is it all being covered up? Or can anyone/thing travelling back in time not interact?

There's been lots of cool sf stories, but I've never heard of anyone dedicating themselves to searching out futuristic ooparts - only ever historic items that suggest aliens or advanced civilisations...
posted by Chunder at 6:25 AM on July 28, 2006

Perhaps it's impossible to use a time travel device to travel to a point in time before the device was built.
posted by Prospero at 6:25 AM on July 28, 2006 [1 favorite]

Perhaps, they came back to get whatever it was they left behind and we will never find anything.
posted by JohnnyGunn at 6:29 AM on July 28, 2006

Waddaya mean "no artifacts"? John Titor left behind the freaking manual for his time machine.
posted by Thorzdad at 6:31 AM on July 28, 2006

Maybe they can only view the past? Ooooooooooo...
posted by shepd at 6:31 AM on July 28, 2006

Perhaps we're just the first run-through of the timeline.

I think you would like the Temporal Anomalies in Time Travel Movies site.
posted by BackwardsCity at 6:33 AM on July 28, 2006

I'm convinced that in the future, I am a celebrity, and the guy I see reading the paper every day on the corner by my office is actually a fan from the future. Every time I throw away a paper or gum wrapper, I'm sure that he collects it and sells it on the future's version of E-bay.

So no, lack of evidence is not evidence against. They're just very discreet.
posted by Terminal Verbosity at 6:33 AM on July 28, 2006 [1 favorite]

[reworded the post slightly w/ the OP's OK to be a little less chatty]
posted by jessamyn at 6:46 AM on July 28, 2006

I emailed this to the OP when the question was temporarily deleted:

Perhaps we are inaccurate in our notion of how time travel would work. Maybe there are time travelers among us, but outside our perception. Along the lines of what shepd said, perhaps time travel is a read-only format (intentionally engineered that way to minimize the chances of altering history or having some catastrophic interaction with yourself) and we are blind to the presence of these travelers (similar to when the various 'Ghosts of Christmas' took Scrooge through time).
posted by daveleck at 7:07 AM on July 28, 2006

Every time I throw away a paper or gum wrapper, I'm sure that he collects it and sells it on the future's version of E-bay

This auctioning activity will have become so widespread that no manufacturing jobs will exist in the future: our children's children will have picked up our detritus in our timeline and resold it online. Time travel will have destroyed the future's economy, so much so where it will have become too expensive to ever again operate a time machine to leave artifacts behind for us to find.
posted by Blazecock Pileon at 7:22 AM on July 28, 2006 [1 favorite]

The most likely explanation is that if time travel has been invented, either that scientific result or its aftermath have caused such problems that the future is desperately trying to prevent itself from happening by hiding all clues as to its own possibility.
posted by felix at 7:36 AM on July 28, 2006

How do you know there aren't quantum artifacts and we just don't appreciate what they really mean in the grand scheme of time? Crystal skulls, woodhenge, heck the Sphinx was ancient by the time the Pyramids were built- the Egyptians probably retrofitted the existing lion-headed statue with the face of the pharaoh Khafre. How did those Thracians master gold and silverwork in the Bronze Age?

Of course, one has to wonder why only artists have been allowed to meddle around in history. Perhaps because they would make the least and most difference at the same time, heading off paradox by being paradox to start with? Hmmm...
posted by headspace at 7:36 AM on July 28, 2006

A lack of evidence is never proof of anything. It could be that the hypothetical time travel device will only work on living things. Even guys with fillings would be excluded. That would make it hard to leave anything behind that would indicate time travel if the time travellers were careful (IE didn't allow engineered organisms to TT).
posted by Mitheral at 7:44 AM on July 28, 2006

They arrive naked. And stay here.
Or they don't want to mess with the prime directive.
posted by slimepuppy at 7:49 AM on July 28, 2006 [1 favorite]

I subscribe to both lesser production values and the idea that Jean Clause Van Damme protects us from the future.
posted by biffa at 8:01 AM on July 28, 2006

I've concluded that either (1) there is some system of multiple (and presumably uncountably infinite) parallel universes among which time travelers would effectively navigate when jumping through time, or (2) time travel will never be practical to execute.

This is because, if time travel becomes a practical feat, a crazy person will go back and fuck things up irrevocably. And if there is only the one universe (paradoxes bedamned!), and someone has gone back and fucked everything up, then we wouldn't be having this discussion.
posted by cortex at 8:05 AM on July 28, 2006

Technically you would only need one time traveler convention.
posted by equipoise at 8:07 AM on July 28, 2006 [1 favorite]

You should read "Pastwatch" by Orson Scott Card.
posted by vkxmai at 8:09 AM on July 28, 2006 [1 favorite]

Jean Clause van Damme is too busy with his uncle's reindeer and sleigh. I have the definitive collection of Terminal Verbosity artifacts. That guy on the corner is only one of my sources.

Possibly anything from the future is instantly vaporized in a paradox-generated burst of energy. If that's the case, then time travel is probably not feasable.
posted by Kirth Gerson at 8:11 AM on July 28, 2006

Time travel is so last year.
posted by popcassady at 8:18 AM on July 28, 2006 [4 favorites]

This is because, if time travel becomes a practical feat, a crazy person will go back and fuck things up irrevocably. And if there is only the one universe (paradoxes bedamned!), and someone has gone back and fucked everything up, then we wouldn't be having this discussion.

How do you know someone hasn't gone back and fucked everything up?
posted by Orange Goblin at 8:21 AM on July 28, 2006 [1 favorite]

Technically you would only need one time traveler convention.

Here is the original Cat and Girl cartoon.

And here is the website for the actual one and only Time Travelers' Convention, held at MIT on May 7, 2005. Don't worry if you missed it--if you have a time machine and a strong tolerance for paradox, you can still crash it any time.
posted by Iridic at 8:22 AM on July 28, 2006

Of course it's possible. They've just never worked out the whole "earth moving through space" issue, and appear in a cold, hard, vacuum.
And die.
Unless they've got space suits, then they float around for a while.
posted by signal at 8:23 AM on July 28, 2006

You could google around for ooparts for fun. Be aware, of course, that such searches will lead you not only to well-mannered Forteans, but to creation scientists, ancient astronauts, general gadflies and the usual assortment of kooks.
posted by gimonca at 8:24 AM on July 28, 2006

Does the lack of time travellers, or artifacts from the future, indicate that time travel into the past will never be invented/possible/feasible at any point in the future?

It doesn't seem that anything follows from the lack of such evidence. Time travel could "have been" feasible, possible and invented in the future - but the absence of evidence in July 2006 explained by them never having used the technology, never having come back as far as July 2006, or having been so careful with any visits they did make as to have never left any realistically detectable trace.
posted by ed\26h at 8:27 AM on July 28, 2006

8,500 Delorian's were built. There has to be at least one time traveller out there.
posted by NotMyselfRightNow at 8:27 AM on July 28, 2006

There's always Larry Niven's objection to time travel: in any Universe where time travel is possible, it will never be invented.
posted by Johnny Assay at 8:33 AM on July 28, 2006

How do you know someone hasn't gone back and fucked everything up?

You're much more optimistic about the relative severity of the fuckup than I am. I'm talking about extinction-level events here. I'm talking about humans erasing humanity at or before the ball got rolling.

Granted, some prior civilization might have wiped themselves out in a past fuckup and we're the clever cavemen who have come in their wake, but I don't buy that, either—it depends on the presumption that time travel was discovered by some unreproducible accident. If it's a discoverable scientific process, we'll either discover it or wipe ourselves out before we get around to discovering it; if we discover it and it becomes practical to travel, someone will eventually go back and fuck things up beyond measurable repair.
posted by cortex at 8:44 AM on July 28, 2006

If time travel is possible, why would any time travellers come back to 2006?

Maybe time travel will be invented way in the future to revisit an event so totally awesome that it will actually necessitate time travel because people must witness it again.
posted by Third at 8:49 AM on July 28, 2006

OUGHTN'T WE DIG UP GALILEO AND BURN THE CORPSE?
posted by hortense at 8:51 AM on July 28, 2006

Perhaps we couldn't figure out how to go back in time, only forward. In that case, check back tomorrow.
posted by cklennon at 8:51 AM on July 28, 2006

Surely someone would have prevented 9/11.
posted by surferboy at 8:51 AM on July 28, 2006

Prospero has it. The only remotely scientifically plausible designs of time machines (which are barely plausible, but they outrank DeLoreans and police boxes) imply that they are not vehicles you get in and move through time in, but locations at which you can, by moving in the right way move through time as well as space.

This means you cannot travel back in time before the machine was made, or after the point at which it is destroyed.

Imagine for instance that the Statue of Liberty was a time machine, which works by running round it anticlockwise. This obviously keeps working until you get back so far that the Statue hasn't been built yet, because then you don't have a time machine to run around.

Now we'd actually be talking about jumping through wormholes or playing in the vicinity of black holes or whatever rather than tourist attractions, but you get the idea. I hope.
posted by edd at 8:52 AM on July 28, 2006 [4 favorites]

Jesus Christ has played a hilarious practical joke on all of us.
posted by one_bean at 8:54 AM on July 28, 2006

Wouldn't that imply that the minute you found such a wormhole you'd be utterly swamped in time travelers? It'd be like a clown car.
posted by Justinian at 8:54 AM on July 28, 2006 [2 favorites]

< wetblanket>Time is a benchmark of change, not a dimension that can be traversed. Time travel is, and will always be, impossible.< /wetblanket>

Look at the bright side: No time travel = no time travel-related paradoxes!

While it's handy to treat time as a dimension for engineering purposes (makes the math SO much easier..), when you get into the realm of astro-physics, treating time as a dimension is grossly misleading.
posted by LordSludge at 9:06 AM on July 28, 2006

When I was ten and read this thread in my Metafilter Textbook, I never dreamed that I would be able to go back and post an answer myself.

The answer is that there is no lack of time traveller evidence. You see it everywhere.
posted by nomad at 9:10 AM on July 28, 2006 [3 favorites]

Surely someone would have prevented 9/11.

Why? In the scheme of things, there is a lot more tragedy out there than two towers falling down killing a couple of thousand people.

That's a bit like the old God argument (but if God exists why does ... still happen)

i just reckon there will never be time travellers in the sense you are thinking. Maybe a visualiser (like a timeTV or something. With all the reruns we get these days it can feel like time travel.)

Imagine if you will Google Earth, live and recordable. Now that would be cool!
posted by twistedonion at 9:11 AM on July 28, 2006

Surely someone would have prevented 9/11.

Perhaps the time travelers allowed 9/11 as it was the only way to prevent something even worse.
posted by DevilsAdvocate at 9:11 AM on July 28, 2006

Maybe this and all points in time in the past are so utterly devoid of interest that no one
posted by clearlynuts at 9:16 AM on July 28, 2006

er, no one ever comes back this far
posted by clearlynuts at 9:16 AM on July 28, 2006

Time travel is, and will always be, impossible.

Scientist in 1580: Yes, your holiness; someday man will fly through the sky from city to city, in giant metal birds! Some of these birds will be able to pierce the heavens and visit other worlds!

The Pope: Kill him.
posted by weirdoactor at 9:20 AM on July 28, 2006

lack of evidence might not be proof against time travel per se. it could simply mean that humanity will be destroyed before the technology for time travel is available.

mostly, i agree with signal's post about the earth moving through space. if you travelled even one second forward or backward in time, the earth will have moved dramatically, and perhaps fatally (imagine suddenly being hundreds of feet in the air, or embedded in solid rock).

any successful time travel device would also have to compensate for earth's movement over the period of the quantum leap. and for extremely long leaps, that calculation may become too complex or too error-prone to manage.

so time travel might be possible, but only over relatively short time spans, none of which are long enough to reach our present time. in that case, we may eventually start to see travel artifacts as we draw closer to the point at which time travel is "invented".
posted by el-gregorio at 9:28 AM on July 28, 2006

A couple problems with the 'they come naked' scenario:

1. They would introduce all sorts of germs into the environment. That could lead to detection or even some kind of catastrophe.

2. Considering how everyone in the future is made up of matter from the past, you'll have a sitation where somehow two copies of the exact same matter and energy will exist at the same time. Which sounds pretty impossible and if possible would probably be detectable.

I think the idea of time travel is just silly once you sit down and think about it. We've known for 80 years that space and time are part of the same thing, but still assume they arent and then try to figure out ways to travel in time while ignoring space/spacetime.
posted by the ghost of Ken Lay at 9:30 AM on July 28, 2006

Considering how everyone in the future is made up of matter from the past, you'll have a sitation where somehow two copies of the exact same matter and energy will exist at the same time. Which sounds pretty impossible and if possible would probably be detectable.

What are the distinguishing characteristics of a hydrogen atom, that you would be able to recognize two copies of the same one?
posted by cortex at 9:40 AM on July 28, 2006

"Let's hope that UFOs are real and that they are time-travelling ships from friendly ETs, or time travellers from our future - because if they are not real, it looks pretty grim for your children and their children." - Jack Sarfatti
posted by General Zubon at 9:57 AM on July 28, 2006

Maybe some of the cutting edge inventions that are coming out these days are not actually being invented now. Maybe they're being invented in the future and after they become very popular everyday items someone may just be bringing the ideas back in time to make money on them before the real inventors actually invent them.
posted by gfrobe at 9:58 AM on July 28, 2006

The problem I see is that you cannot create matter or energy from nothing. The classic time travel scenario is also a magical matter and energy creation machine as well as somthing that can magically splice time from spacetime.
posted by the ghost of Ken Lay at 10:10 AM on July 28, 2006

Not only is the Earth moving around the Sun, but also the Sun around the Galaxy, and the Galaxy outward from the Big Bang, [My apologies to any Creationists lurking in this Time Travel thread].

I think simulated time travel may some day be feasible. You could visit the future by being in Suspended animation until it becomes the present.

Then to "visit" the past, I like twistedonion's idea: that reality can be acurately recorded on a grand scale and you can rewatch it in a virtual realization. Maybe even walk around like a "ghost" in a holographic reproduction of the event(s). I'm thinking of how on Star Trek when they record an event and upload the file to the holodeck to investigate -as observers- in 3D.
posted by yeti at 10:12 AM on July 28, 2006

Does the lack of time travellers, or artifacts from the future

if the future society was advanced enough, would we be able to identify the travellers or their artifacts?
posted by pyramid termite at 10:25 AM on July 28, 2006

el-gregorio: Couldn't they simply do a time jump in a short range space ship, outside of Earth's orbit, and then fly the rest of the way? That's what I did err I would have done.
posted by mutantdisco! at 10:31 AM on July 28, 2006

The problem I see is that you cannot create matter or energy from nothing. The classic time travel scenario is also a magical matter and energy creation machine as well as somthing that can magically splice time from spacetime.
posted by the ghost of Ken Lay at 10:10 AM PST on July 28

Thats actually a bit of a redundant statement. The conservation of Energy is a direct result of the fact that physical laws are invariant under time translations.

Noether's theorems are one of the coolest things in physics which most people do not understand, or have never even heard of. You also cant understand Heisenber's uncertainty relations without starting with Noether.
posted by vacapinta at 10:32 AM on July 28, 2006

If the time travel device is something we can create; a wormhole, for example, where one end is tied to the place and time of its invention, and the other end follows us here & now, then the spacial issues "go away". You can use the device to go back to the times and locations it has been present. You are still limited, however, to only going back as far as the creation of the device, hence, no going back to muck with us, and no artifacts.

Watch the movie Primer for what happens when a couple guys make just such a device.
posted by knave at 10:37 AM on July 28, 2006

He left his friggin' spectacles here. They were a gift! What, do you want the whales back too?
posted by blue_beetle at 10:38 AM on July 28, 2006

Nearly every time travel story we've heard, somebody goes back and screws with the whole of the world, history is changed, key people don't get born, and it all falls to bits.

So I suggest, in the future, people HAVE gone back -- they've gone back TONS of times. And they fucked with everything, and the whole world fell to hell.

But when the future clocked around again and time travel was invented "again", the appropriate powers recognized the dangers, made a permanent decree that time travel must never be used again. And then they travelled back to the furtherest possible time, and reset the entire world so it ran again from the start, with no time travelling interruptions.

Time travel cancelled itself out.
posted by Milkman Dan at 10:43 AM on July 28, 2006

The location argument (reverse time travel is not possible because of the movement of the earth on its orbit, universe expansion, etc.) does not hold a lot of water for me, because of the fact that time travel is already possible, in the form of time dilation. The only problem is, there is no obvious way to do the reverse of time dilation, in order to "travel back in time." If it were discovered to be theoretically possible, however, I would imagine it would be a process similar to time dilation, and would require acceleration or deceleration relative to the surroundings.

What I'm imagining is one of those test rocket ships from the cartoons, accelerating to relativistic speeds. Suddenly the surroundings would start to age rapidly. If this were accomplished in the reverse, the machine would only go back to the time it was created, as many others have said.
posted by anomie at 10:46 AM on July 28, 2006 [1 favorite]

yeti: momentary pedantry going off topic but 'the Galaxy outward from the Big Bang' - no, the Big Bang happened everywhere, and everything is moving away from everything else.

However, you can use the remnant radiation of the Big Bang to figure out how fast you're moving and in which direction compared to the vast majority of the universe. It is, as you basically say, pretty impressively quick thataway.
posted by edd at 11:11 AM on July 28, 2006

Perhaps it's like in Ray Bradbury's short story A Sound of Thunder (read the story, skip the movie), where time travellers are forbidden from altering the past in any way, lest they change history from that point forward.
posted by geeky at 11:33 AM on July 28, 2006

edd: That is true but yeti may have been mistakenly correct. The galaxy, and all points in the universe, are rotating much like "the Earth moving around the Sun, but also the Sun around the Galaxy" if I'm not mistaken...(not an astrophysicist, just a {hopefully} educated hobbyist)
posted by cklennon at 11:38 AM on July 28, 2006

Perhaps because we're living in a simulation?
posted by stranger loop at 11:52 AM on July 28, 2006

I recently watch a show called "If The Earth Had No Moon" that sort of blew my mind. I learned that the moon is receding from the earth at something like an inch a year; which causes our days to get longer by a fraction. Meaning that since we've had a moon; days have been slowly getting longer. So a day in 2006 AD would be longer than a day in BC 2006.

Also, since the big bang; all matter in the universe is slowly expanding. This might (or might not) affect your destination.

So if you wanted to travel back in time, you'd have to not only figure out where the earth was at that time (in space, in the galaxy, etc.), but if you wanted to get close to a particular date, you'd need to figure for shorter days, wouldn't you?

Disclaimer: I am not a scientist. I am also not building a time travel device to go back and change the results of the 1986 National League Championship Series between the Mets and the Astros, and in dire need of calibration advice. Thank you.
posted by weirdoactor at 11:59 AM on July 28, 2006

This means you cannot travel back in time before the machine was made, or after the point at which it is destroyed.

So... when the final coat of paint dries on the hull, then *poof* it can't go back before that point? My point being, how does the universe know when the time travel machine is "created" (vs. partially or mostly created)? I don't see how this makes any sense. (Or do you mean that one could only time-travel to any point during which device was operational? Eh, okay.)

Maybe some of the cutting edge inventions that are coming out these days are not actually being invented now. Maybe they're being invented in the future and after they become very popular everyday items someone may just be bringing the ideas back in time to make money on them before the real inventors actually invent them.

This would be fairly simple to disprove by following the progress of inventors and/or interviewing them after the fact. (Of course, tee hee, maybe they're all part of the grand temporal consipiracy. And maybe we're all part of teh M4trix. sigh...)

This response is marked "best" -- no snark intended, but is the submitter just going for the funniest or most clever response? The original question implied a more serious tone.)

Scientist in 1580: Yes, your holiness; someday man will fly through the sky from city to city, in giant metal birds! Some of these birds will be able to pierce the heavens and visit other worlds!

Scientist in 2006: Energy cannot be created from nothing.

MeFite: But I *want* to believe!! (And I've seen all these movies that says it's possible...)
posted by LordSludge at 12:12 PM on July 28, 2006

Conservation of energy is much, much older than 2006.
posted by the ghost of Ken Lay at 12:26 PM on July 28, 2006

Scientist in 2006: Energy cannot be created from nothing.

You mean: Scientist in 2006: Energy is constantly being created from nothing, but there's no way to make any use of it.
posted by kindall at 12:27 PM on July 28, 2006

(What's the difference?)
posted by LordSludge at 12:30 PM on July 28, 2006

man will fly through the sky from city to city, in giant metal birds!

FWIW, heavier-than-air flight skeptics were always debunked by the existance of birds, bees, and bullets. It was never a convicing position to take and really doesnt come close to the well-tried theory of the conservation of energy.

Also, CoE might be found wrong in the future but that sure doesnt equal "hollywood style time travel is real."
posted by the ghost of Ken Lay at 12:32 PM on July 28, 2006

Time machines are fairly common in the future, but nobody wants to travel back to before the entire universe was united in a self-perpetuating orgiastic communal mind cloud.

It's like a guy in the middle of the south pole thinking cities haven't been invented yet.
posted by Hildago at 12:33 PM on July 28, 2006

MeFite: But I *want* to believe!! (And I've seen all these movies that says it's possible...)

Gosh. If I were going to base what I hoped life would be on movies I've seen; I'd probably choose good porn over sci-fi, and get a job as a pizza boy/plumber/cable guy. But thanks for the lovely snark.

The difference, LordSludge? There have been deposits of oil underground for eons. Only in the last century or so have we use that as a power source (combustion engines, I mean). Do you think that people five hundred years ago would have envisioned nuclear power? Or even basic things, like gel insoles for your shoes. If Magellan had indeed been "gellin'"; he would have been burned at the stake.
posted by weirdoactor at 1:18 PM on July 28, 2006 [1 favorite]

We're a 250ish-millennia-old species for whom agriculture was still a pretty neat idea 12 millennia ago and who started formulating the modern scientific method under 4 centuries ago.

I think we're pretty cocky monkeys to pretend to be able to state absolutely the limits of hypothetical technologies hundreds of millions of years beyond our "we've known about the neutron for over seventy years!" state.

Maybe time travel's impossible. Maybe it's possible but unfeasibly expensive. Maybe it's feasible but forbidden by an entity of such vast power that it can prevent it. Maybe Niven's right and time travel self-destructs.

Maybe Earthlings never amounts to much of anything in the universe and no entity who can time travel gives a rat's ass about visiting.

I don't think we can claim to prove anything about the non-existence of time-travel based on the absence of evidence without making a lot of wholly speculative assumptions about how time travel must work, what its effects must be, and what limitations time travellers would have.

(None of which means I think time travel is possible, let alone that time travellers are among us.)
posted by Zed_Lopez at 2:13 PM on July 28, 2006

What if time travel is already happening, we just don't know where?

I don't think it's possible to change things that have already happened. If there are time travellers visiting our space in time now, they're here now.
posted by divabat at 2:27 PM on July 28, 2006

Because people in the future got fed up with shouting "Your kids, Marty, your kids!".
posted by greycap at 3:53 PM on July 28, 2006 [1 favorite]

ckiennon and wierdoactor: we're not moving as a result of the Big Bang. We're staying right where we are and everything is moving away from us. Equally, over in a distant galaxy where they're having a similar discussion they're staying right where they're are and we're moving away from them. Confusing but true. Anyway, the result is that the cosmological expansion isn't a problem even if time machines did work in this manner. Other motions, the sort that result in you actually going past stuff, which cosmological expansion never does, those are going to be a problem for this hypothetical time machine.

LordSludge: Time machines are generally considered to be present by physicists when you've got what's called a Closed Timelike Loop. This a path you can follow that gets you back to where and when you started. This will happen at a particular point in your bending of spacetime or whatever trick you are doing. It might be that earlier on you've got something that is going to let you slow down or speed up time, but it's not going to get you actually back in time. You also have to put in the back to the same place condition, otherwise the usual tricks that relativity gets up to will let you confuse yourself and think you've gone back in time at a distant point, when from someone else's point of view you haven't.

It's simply when completing this loop back to your starting point is possible that you've got yourself a time machine.
posted by edd at 4:04 PM on July 28, 2006 [1 favorite]

From the philosophical and not scientific angle:

By what criteria would you recognize an artifact from the future? What characteristics would it need to have in order for you to be able to say, with reasonable confidence, "oh, this object is from the future"?

I can't think of any property or set of properties that would clearly indicate an object as an artifact from the future. Even if you discovered something that employed an unknown and advanced technology, what's to say some genius but unknown inventor didn't make it in his/her garage?

Well, how about a list of predications that proves 100% accurate? It would still be impossible to completely rule-out other explanations -- e.g., extremely remarkable coincidence.
posted by treepour at 4:49 PM on July 28, 2006

edd: Time machines are generally considered to be present by physicists when you've got what's called a Closed Timelike Loop. This a path you can follow that gets you back to where and when you started.

Agreed. I maintain, however, that if you get back to the same initial conditions, including not only your location, but the location and state of every freekin thing in the universe, that's identical to "going back in time". In fact, the "time" condition is irrelevant. Everything is going to happen the same again anyhow. (Note that this assumes a deterministic universe.) Of course, this is impossible, as it would take near-infinite energy to do this.

Z_L: I don't think we can claim to prove anything about the non-existence of time-travel based on the absence of evidence without making a lot of wholly speculative assumptions about how time travel must work, what its effects must be, and what limitations time travellers would have.

I *guess*... but one could make that claim about ANYthing, including pink unicorns.

w: Gosh. If I were going to base what I hoped life would be on movies I've seen; I'd probably choose good porn over sci-fi, and get a job as a pizza boy/plumber/cable guy. But thanks for the lovely snark.

Snark begets snark (if you'll read back). Case in point: If you want to know how to fuck like a porn star, you can open another AskMe thread and I'll hook you up. I, however, was offering a possible (social/cultural) explanation as to why people insist that time-travel must be possible. A bit snarky, in response to the snarky 1500s scientist comment, but at least relatively on-topic.

Actually, no, I am off-topic. I'm steering us towards whether time-travel is possible, as opposed to the original question, which specified using lack of artifacts as possible proof. To me, it's a bit like asking whether pink unicorns can fly or not. But it's still off-topic.
posted by LordSludge at 5:22 PM on July 28, 2006

it's a bit like asking whether pink unicorns can fly or not.

I agree completely. And given their wholly speculative nature, I wouldn't claim to know that they can't any more than I'd claim to know that they can.

MeFite: But I *want* to disbelieve!!
posted by Zed_Lopez at 6:11 PM on July 28, 2006

I can't think of any property or set of properties that would clearly indicate an object as an artifact from the future. Even if you discovered something that employed an unknown and advanced technology, what's to say some genius but unknown inventor didn't make it in his/her garage?

Being clearly mass-produced would be a dead giveaway, IMHO.
posted by kindall at 12:44 AM on July 30, 2006

If time is a measure of the propagation of information (thinking in terms of how time stops at the event horizon of a black hole, and no information can escape), the reversion of time may also be the erasure of information. To anthropomorphize this concept, a person travelling backwards in time may find themselves unable to remember how they got there. This is the "Hollywood" version, though; but I think it's been used in one or two movies which escape me at the moment.

Time may not be what you think it is. If time is the direction of entropy due to an expanding universe, it may be possible to reverse it locally, but that would result in being able to send small objects back in time such that they are younger than they were. Bad example, but if you sent a piece of ancient Greek pottery back in time via a localized Reverse Entropy field, it would be younger, but the outside universe would have aged normally in the few seconds/minutes it was inside the RE field. Most time travel stories operate on the opposite principle; reversing the flow of entropy for the entire universe except a small area; which would require a ridiculous expenditure of energy.
posted by Eideteker at 11:07 AM on July 30, 2006

I don't think anyone's mentioned one of the popular (?) quantum mechanical approaches to disproving time travel. I hope it's not too off-topic, as the original question asked about whether a lack of evident time travelers, not whether it was possible according to possible laws of the universe.

A paradox does not cause the universe to cease to exist. A paradox causes the universe to never have existed. Paradoxes literally can't exist, because they (and this is very loose) consist of an unstable waveform. If you think of it from the point of view of an electron 'orbiting' an atom, it can exist in something like a circular standing wave pattern with 1, 2, 3, etc. nodes. It can jump to a higher level if exposed to the right amount of energy to get there. Too much or too little and it won't go. Or, if you like, you can think of it as: it does go, finds that it's not in a stable configuration, and so the wave is never completed and does not collapse into being. So a universe (out of all the possible universes, not just the finitely probable ones (i.e., all the ones we can conceive of, not just the ones that can come into being by following physical laws)) which allows time travel would certainly have to do so in a way which doesn't allow paradox. Otherwise you're looking for a universe which does not obey its own physical laws; which, if such a thing can exist, is probably not too hospitable for life as we know it.

This is all out of my ass, so anyone who actually finished their physics degree feel free to pick nits or smash the whole erroneous mess with a hammer.
posted by Eideteker at 1:51 PM on July 30, 2006

I posted something about time travel once, but jessamyn went back in time and deleted it, like it was never there. In my head, she has a goatee like The Master.
posted by obiwanwasabi at 7:15 PM on July 30, 2006 [2 favorites]