Time Travel is impossible

[Prozac the Robert]

If you can travel through time, space is probably quite easy.

[McC]

Could you elaborate? For example, if you went back to kill gramps, had the revolvor in your hands, and had him in your sights, would a bird suddenly drop its load on you and screw you up, causing the missed bullet to zing past, alerting the cops to your presence, and forever denying you the chance to kill him?

More or less. It's not to say that the universe is trying to make sure time is preserved, it's simply saying that since it never happened in the first place, but you went back in time, that you had always gone back in time and always failed already.

[Uraniun235]

the other is the concept that time travellers always were part of the past.

Doesn't that suggest predestination, though? If a time traveller was always part of the past, then that means he must go back at some point.

[jcow79]

Time travel has always been difficult for me to wrap my brain around. On it's face I've always dismissed it as impossible.
However when you read about the experiments of taking synched atomic clocks and placing one on an airplane and leaving one on the ground, fly the plane in the same direction of the earths rotation for a few hours and find out that the clock on the plane has counted slower than that of the one on the ground, albeit only a fraction of a measurement. I can't seem to make sense of it. Did the high altitude alter the reliability of the clock on the plane? Or did the plane actually "time travel" due to it's increased velocity?

[Sharpshooter]

I just had another thought: could time travel succeed for as long as it doesn't impact the past (in any signifigant way)?

We've already explored the idea that a person can't affect the past because of the self-refuting nature of their actions. However, let's say a person creates a portal in time that would deposit an article that enters in a place only they would know and be able to reach - say, ten meters under the surface of their backyard. They find a replica of some relic left by whatever former residents their regions might have, chuck it into the potal, then close it down and start digging in their backyard. Lo and behold, ten meters and the time span's worth of shifting downward, they find the relic, now considerably aged, and sell it as an authentic item for a good mint.

I don't think it would self-refute, given that it doesn't prevent the subject from being able to project said relic into the past - since they only benifit after they throw it in, in the past, they stll had the motivation to do so, thus not mucking things up.

Of course, this might also depend upon the degree of sensitivity time has - as the Simpsons showed, Homer sitting on a fish in the past caused his family to be rich and for donuts to rain from the sky, so throwing that relic in might cause the path of history to derail completily, thus refuting the events and setting up some giant paradox of sorts. On the other hand, history might be quite stodgy and stuck in its place, so unlss something on a massive scale intervenes in the norma course of events, things will continue on as normal.

What are the theories regarding that, if any?

[Mad]

...*slap*

Large displacements in time, Robert, large displacements. Affecting the rate at which we perceive time to pass isn't the same thing as instantaneously relocating to different points in time.

Why would time travel be instantanous? You're moving through a dimension (the 4th dimension). Can you just instantly jump from one location to another in 3 dimensions? So why would it be any different for the 4th dimension?

If time travel were possible, then one would think we would see a bunch of time travelers here. Especially around big events. Because they will be there, and will always have been there. So whenever something big happens that time travelers might want to investigate, all the time travelers that ever wanted to see it for the rest of time will all be there at the same time. It'd get pretty crowded.

Yup. Its something everyone overlooks. I think the general excuse is that somehow the mass most time travel ships are on or orbiting keeps a 'hold' pn them somehow. Hell its all I've got.

The show Seven Days actually mentioned compensating for the difference in Earth's position on the pilot episode.

[General Zod]

If time travel were possible, then one would think we would see a bunch of time travelers here. Especially around big events. Because they will be there, and will always have been there. So whenever something big happens that time travelers might want to investigate, all the time travelers that ever wanted to see it for the rest of time will all be there at the same time. It'd get pretty crowded.

that assumes that any joe average that wants to can simply hop into a timeship/pod/whatever and jump about through time on a whim. it'd be far more likely time travel would be highly regulated and only a handful of individuals with proper clearance and such would be permitted to do so.

[Chmee]

...*slap*

Large displacements in time, Robert, large displacements. Affecting the rate at which we perceive time to pass isn't the same thing as instantaneously relocating to different points in time.

Why would time travel be instantanous? You're moving through a dimension (the 4th dimension). Can you just instantly jump from one location to another in 3 dimensions? So why would it be any different for the 4th dimension?

If time travel were possible, then one would think we would see a bunch of time travelers here. Especially around big events. Because they will be there, and will always have been there. So whenever something big happens that time travelers might want to investigate, all the time travelers that ever wanted to see it for the rest of time will all be there at the same time. It'd get pretty crowded.

That guy with the rainbow wig and the 'John 3:16' sign that keeps popping up at sporting events ..... 25th century ....

[GrandMasterTerwynn]

Well, yes, time travel violates causality. Unless you subscribe to the spurious theory that there are infinite timelines which branch out from every potential decision point. (Of which, just sitting still will generate a number of timelines equal to all the potential combinations of quantum states that all the particles in your body could exist in at any given moment.)

The only way that time travel would not violate causality is if it became impossible to travel back past the point your time machine first became operational. Of course, you could still have the grandfather paradox (except this time it would be you going back in time and smashing the time machine, preventing it from being around when you decided to go back in time to destroy it.) This means that, somehow, the universe would step in to prevent you from going back in time to smash the time machine the first second it became operational.

To assume that the universe can do that starts to smack of the introduction of some mysterious omnipotent universal intelligence term, which only acts to keep the timeline consistent. And, as rational folk, we don't like to think about spurious terms like that. So yes, time travel is entirely impossible.

[Queeb Salaron]

I just had another thought: could time travel succeed for as long as it doesn't impact the past (in any signifigant way)?

Jesus Parkinson's Christ, man, haven't you ever seen Back to the Future?

Really, these are not new ideas.

[Sharpshooter]

Jesus Parkinson's Christ, man, haven't you ever seen Back to the Future?

Really, these are not new ideas.

Sorry - last thing on my mind, to be honest. Enough mindfucking for one subject, anyway.

[wolveraptor]

Time travel has always been difficult for me to wrap my brain around. On it's face I've always dismissed it as impossible.
However when you read about the experiments of taking synched atomic clocks and placing one on an airplane and leaving one on the ground, fly the plane in the same direction of the earths rotation for a few hours and find out that the clock on the plane has counted slower than that of the one on the ground, albeit only a fraction of a measurement. I can't seem to make sense of it. Did the high altitude alter the reliability of the clock on the plane? Or did the plane actually "time travel" due to it's increased velocity?

that would be relativity. Anything that travels closer and closer to the speed of light will cause the measured time to fluctuate from the norm (the norm being earth sea-level for us). Essentially, time passes more slowly at high speeds.

[Kuroneko]

However when you read about the experiments of taking synched atomic clocks and placing one on an airplane and leaving one on the ground, fly the plane in the same direction of the earths rotation for a few hours and find out that the clock on the plane has counted slower than that of the one on the ground, albeit only a fraction of a measurement. I can't seem to make sense of it.

One can understand it inuitively as imagining that with respect to some reference frame, every object has some velocity through space, in the normal sense that its position changes by some amount Δx every Δt in the reference frame, and a velocity through time, in the sense that some amount of time Δτ passes on the object's clock for every Δt in the reference frame. The key step is that the sum of the squares of these 'velocities' is always constant--the faster the object travels in space, the more slowed down its clock is with respect to the reference frame, and vice versa. That's actually what the Minkowski metric of spacetime says: if the reference frame has coordinates (t,x) for time and space [let's work in one spatial dimension here], and τ is the time as measured by some other objects (its "proper time"), then the quantities are related by dτ² = dt² - dx², with units scaled so that c = 1. Now, velocity through space is naturally dx/dt, and "velocity through time" (in the sense described above) is dτ/dt, so we can simply divide by dt² and with simple algebra obtain (dτ/dt)² + (dx/dt)² = 1.

Did the high altitude alter the reliability of the clock on the plane?

Well, no, but general relativity requires this to be a factor. Both the Earth and the plane clocks make circular orbits around the Earth's centre (or, at least, pieces of them, if the experiment is over befor ea complete revolution is made). Circular orbits Schwarzschild metric are have time dilation (dτ/dt)² = (1-3m/r) [1], relative to a stationary observer at infinity, and the Earth's mass is M = 4.436e-3m (in units of G = c = 1). If the clock's altitudes are known, the rate of time between them can be calculated without difficulty, but note that since Earth clock has lower r-coordinate, it is slowed down more relative to the observer at infinity; correspondingly, it will measure the plane clock to be speeding up. So, in actuality there are two competing effects: the plane's velocity, which makes the plane clock slow down, and the plane's altitude, which makes the clock speed up. The former is larger, but the latter is still not insignificant. If their positions are Earth's equatorial radius and the plane flies 5.00km above it, then the plane clock should be speeding up at 0.817ns/s, which is well within an atomic clock's precision to measure.

Or did the plane actually "time travel" due to it's increased velocity?

That depends on how one thinks "time travel" should be defined. The above is really just another variation of the so-called twin paradox. This is why spacetime is such a popular picture--if one maps the traveling twin's path in spacetime (or, here, the plane), one will realize that even though their final destination is at the same point, they took completely different paths in spacetime. So, why shouldn't their clocks measure different times? To expect them to do otherwise would be like expecting a trip from New York to Florida at a predetermined speed to take the same amount of time regardless of whether one decides to go via the shortest path or stop in London first. The only caveat is that, unlike in normal space, relativistic spacetime maximizes distance (proper time) at geodesics instead of minimizing it, so the straightest path in spacetime will always be the longest.

[Tychu]

There are a few points I don't agree with. First, as addressed by dragon, you could get around the problem by theorising alternate timelines. That notwithstanding:

Your paradox only addresses time travel to the past, it doesn't say anything about going to the future. So you should really say "time travel to the past is impossible"

Why should the whole time-space continuum be destroyed, and a huge "implosion" occur just because of a little paradox? I think this is an bit of a leap in logic; the universe may be perfectly happy to carry on regardless of a paradox (so long as it doesn't notice ). And you haven't addressed the energy issues involved in instantly destroying the whole universe. Where would the implosion energy suddenly come from/go to? (Admittedly this energy thing could maybe in itself make an argument against the possibility of time travel to the past)

Lastly, I am not at all convinced that changing something in the past will instantly change the "present" or "future" - if I kill off my grandaddy back in the past, I don't see why I should fade away (a la Back to the Future) or instantly vanish in the "present".

time travel to the past and future is on your own perspective of time
if you travel to the future its someone elses past and then the same things can happen if you go in your own past