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[Aqizzar]

And people wouldn't believe you anyways if you just walked around saying your from the future.

Well, they would if you showed up with like a laser gun or a hoverbelt or a robot butler or something.  But instead, self-confessed "time-travelers" typically come across as slightly more coherent hobos than spacemen.

Maybe the time travel mechanism relies on an addiction to Old Milwaukee and wearing smelly clothes?

[qwertyuiopas]

Maybe the mechanism cannot properly convey matter?
Or yhe planetary magnetic field prevents time travel?
Or you can only time travel at rates of -1 to +1, so you would die of old age on your way? And eventually your time machine willl cease to exist?

[Eagleon]

There are innumerable ways of convincing people you're from the future. One is to have such technology that you can kill anyone that disagrees with you

I personally prefer my idea that the "future" is erased upon travelling back in time. That or it just stops for the people in the time-traveller's future.

Warning: contains conjecture on higher-dimensional physics which I honestly have never seriously studied

Spoiler (click to show/hide)

If you imagine time as two dimensional, it helps - regular time is a straight line, but travel back in time and you jot upward a bit and then straight back. According to the Pauli exclusion principle, as I understand it, it's basically impossible for matter to exist occupying the same space without bad destructive weird things to happen. So paradoxes happen when the time-line (literally here) intersects itself. If I'm right, the only way you could revisit the same time vector (I love getting to use the word vector correctly ehehe) and thus revert any changes you made is if time were three dimensional.

Anyway. It avoids paradox by making the future irrelevant - the time traveller (say it was you) simply exists in the new time, and he'd have to wait or go into stasis of some sort in order to see the effects of his changes. This could have already happened, and no one could possibly know unless the traveller made it obvious. It could actually be happening all the time in minute intervals, as long as the subtraction were less than the pace of time... or even if it weren't. We think things are going ahead in time, but if we lose all memory of having been further ahead, we'll have no clue what's happening.

Read: Time itself could be being eaten away in any amount of interval at all O.O I just scared myself by thinking about this xD
And yes, I know this is probably all garbage physics. But it's fun to think about.

[Osmosis Jones]

Spoiler: Frustrated physicist rant (click to show/hide)

Pauli exclusion principle states that two fermions cannot occupy the same state in the same position (bosons can, and that leads to some funky effects, but nothing to do with time travel).

This means two electrons can't have the same energy state in an atom, and that two electrons that share identical wavefunctions will be subjected to a repulsive force as they are brought together. It doesn't mean going back in time causes an explosion, especially as this would require every single subatomic particle aligning exactly, while being at the exact same energy level.

Given that there are ~10^23 atoms per gram of material (for say carbon), think about the odds that would imply.


With regards to the idea that because we haven't seen time travellers, we must all be dead in the future, it contains waaaaaayyyy too many assumptions to be even remotely tenable.
Here's the easiest rebuttal; methods of time travel generally require either infinitely massive objects with relativistic speeds, exotic matter, or physics that has no experimental support whatsoever.
They few I have seen that seem remotely plausible either cannot travel before the creation of the machine, or do not allow you to exit the time machine (you literally cross an event horizon).

As such what little we know about time travel suggests that, IF IT POSSIBLE AT ALL, it is prohibitively difficult, quite possibly fatal, and unlikely to be able to travel to a point in time prior than it's own invention, which is where we currently reside.

[Sean Mirrsen]

There are always possibilities of breaches in the universal laws. Laws, as rules, can have exceptions. (That boson thing, for one)  It is possible that by setting very specific conditions (down to geptillionths of whatever measurement unit), one can bypass the universe. These exceptions become laws by themselves, of course, unless they are not made public. For example, before resonance was discovered, it was likely believed that waves or oscillations do not affect each other. But set the oscillations to a very specific frequency, and you get resonance. Same may apply to other laws. This is part of the background of my Multiworlds setting, btw.

And back to timetravel. Another part of the aforementioned background describes what I'd call "dynamic parallel universes". Parallel universes, in the 5th dimension and above (assuming the 4th dimension to be the entirety of a given universe's timeline, the 5th dimension represents variations of the timeline) actively try to merge themselves together. I remember some physical principle behind that, but can't recall what it is exactly. Something about lowest amount of energy in a system. But the universes can't merge unless they are at least nearly identical at a given point, so they branch out when the timelines are different, and join back when they are identical. What results from this is that when you travel back in time, the timeline branches off in the past. But if you leave no impact whatsoever and go back to the future, at your point of return the child timeline would "branch in" and merge with the original timeline. There would be a permanent "bulge" in the timeline at the point you appeared and until you left, but the rest of it would eventually absorb back into the main stream because it's very nearly identical. Matching quantum waveforms and all that technobabble.

[Neruz]

All the lack of time travellers proves is that we don't understand time, GUESS WAT GIES? WE ALREADY KNEW THAT!

[Jreengus]

Well if I invented/was part of a team that invented a time travel machine I'd make sure that the machine can't take you any further back than the day it was first released into the public. Aside from anything else the alternative is that the first person to buy it travels back 10 years before you even made it and gives it to their past self/parents to pretend they invented it.

[Neruz]

As ways to fuck the world over with time travel go, that one is pretty laid back.

[Osmosis Jones]

Spoiler: For Sean (click to show/hide)

There are always possibilities of breaches in the universal laws. Laws, as rules, can have exceptions. (That boson thing, for one)  It is possible that by setting very specific conditions (down to geptillionths of whatever measurement unit), one can bypass the universe.

It's a commonly accepted facet of quantum mechanics that, on the Planck scale (verrrryyyy smalll, like, so small atoms are 10^25 times larger), space time is a wild froth with wormholes and other garbage. That's fine. So, yes, at a 1 geptillionth (that's a fun word, normally we're stuck with things like atto) of a metre, time travel could very happily occur. The problem is scaling it up. I'm not saying that time travel is impossible, per se. I'm saying it is impossible for any civilisation due to the difficulty in manufacturing the situation.

For wormholes (which mind you, I consider to be the most reasonable by a long shot of the possible methods), you need a form of exotic matter with, from memory, negative gravitional attraction. That is something that has never been observed. That either means it is impossible, or extremely unstable.

When I say it's unstable, bear in mind we have observed things that decay in 10^-25 seconds, so we are talking a short timescale here.

If it is extremely unstable, that means any wormhole we open will most likely decay near instantly as the stuff we prop it open with decays in a shower of gamma radiation (or neutrinos, or the energy/particle combination of your choice). On top of that, we have to get the matter we generate to coincide with exact time and position of the opening wormhole, which has odds like finding a needle in a haystack larger than the size of the known universe.

On top of that, we have to ask about the energy requirements required to create that matter. Maybe the reason we haven't seen it so far is because we haven't had the accelerating energy required. If so, how much energy will we need? Look at what is required for the LHC. Bear in mind that's only a fraction of the energies contained by cosmic rays. If travelling backwards in time requires supernovae, how likely is it to be performed to visit a time period of probably not that great importance?

...and so on.

Look, put simply, time travel is, by the admittance of some very great luminaries in the fields of theoretical physics, not explicitly impossible by the laws of physics (though there are also some that believe the universe will actively thwart any attempt to create time machines... I'm honestly not so sure how I feel about that, it sounds a bit too hand wavy, and the maths they try to invoke is waaaaaayyyyyyy beyond my ken). I'm just saying it's very likely to be impossible by the limits on what is actually achievable by any real being.

Tl;dr? I'm trying to say that basically, economics will thwart us all.

[Neruz]

There doesn't appear to be any theoretical reason why one could not engage in time travel; the problems are pretty much entirely practical ones.

[Osmosis Jones]

There doesn't appear to be any theoretical reason why one could not engage in time travel; the problems are pretty much entirely practical ones.

Well, not yet at least, but yah that's my understanding 

[Neruz]

In theory time should be three dimensions just like space, a 6 dimensional object should be able to quite happily jaunt around what we percieve as time without any worries, and we should be able to jaunt through what we percieve as time, again without any worries.

The trick is the practical side; how one actually achieves this. Due to a distinct lack of 6 dimensional objects to observe this has proven rather difficult.

[Jackrabbit]

Whenever I think about what humans would do with time machines, I remember that we humans are as much masters of trolling as we are science.  I can absolutely guarantee that if time travel ever became possible, given the unlimited time and possibility of the future, somehow, someway, some cheesedick would go back to the beginning of humanity or the planet or the universe and unmake everything just for kicks.

We're humans, we do that.

I believe the fact that we've never seen proof of a real time traveler from the future is proof that time travel is either impossible or discrete, because otherwise we wouldn't exist at all.

That is possibly the most hilarious thing I have read in the last six months.

[Neruz]

Pfft, you can be sure within a year of Time Travel being invented somone will go back in time and shoot their great grandfather just to see what happens.

[Twiggie]

what? but i am from the future. everyone lives in mud huts except me. so now everyone does live in mud huts.

i had a palace though. with a time machine. thats how im here. anyways, must be getting back to my empire, the serfs will be rebelling again.