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Why I Don't Believe in Time Travel (Nerd Stuff)  
09:33am 18/08/2010
Mister Nihil
(Fair warning: There's no real, hard physics in this. It's just a thing I thought of, thinking about time travel, because, you know, that's a thing I do sometimes.)

Time Travel sounds like a good idea, right? You step into a box, that box shifts to a time that is different from this time, presumably at a rate other than the usual one second per second rate of time travel we have so far enjoyed. Right? And then, you step out of the box, and what?

Well, think of this: Let's suppose you have magic technobox that will transport you one minute into the past. OK, not hard to imagine. Sounds good, maybe even easy. So, you step into the box, you set the controls, and the box moves only through time, sixty seconds ago. You step out of the box, and into the airless void of space, right? Because the Earth is sixty seconds ago. You hang, suspended outside the box, and, if you took a pretty deep breath and have good sunscreen, you could then watch as the Earth slams into you at about 67,000 MPH.

Oh, wait, you say. That's not nearly fast enough. That's just the Earth's velocity relative to the sun. By moving back in time, you get to worry about being slammed at the Earth's velocity relative to Everything, maybe?. Well, sure, but that number is much harder to find, what with it having no meaning from my point of view, on Earth, and what with it not actually being possible, so far as I know, to stop entirely, relative to the Universe, and things all having different velocities, and so stopping relative to them meaning different things, but the point is, you'd get sixty seconds of triumph followed by a very short career as ashy paste, wouldn't you?

So, does moving backward in time mess with your velocity relative to the sun (or to any other given point of reference)? It seems like there would be two answers: yes and no. If yes, you'd be slammed more softly, so yay for that, although the out in space thing is still a problem. If no, then the above scenario, paste, etc.

So, if no, and you maintain your original velocity relative to Earth, what if you travelled forward in time just a little? Maybe two seconds, instead of the usual one? Wouldn't that put the Earth, moving at just under 30 kM/S, about 30 kilometers further along than you? So, you'd be thirty kilometers back on the orbit of the Earth, which has a very small chance of being OK, and a huge chance of being very uncomfortable, and occupied variously by rock, magma or nothing at all. You would, though, not be paste. You'd just be, potentially, a little bit dead.

That's my thing. Time travel sounds like a blast, but unless we simultaneously develop non-spacial matter transportation and very, very smart computers, it doesn't seem practical, if only because we are, more or less, standing on a ship, moving at ridiculous speed. It's a big ship, but it's still going really fast.
    Yrs - PS 3 - - Link

(no subject)
06:45pm 18/08/2010 (UTC)
Yeah, time travel stuff is often pretty silly in that way.

On the other hand, it's entirely conceivable that we could invent some kind of appropriate navigational beacon at the same time as time travel.

Time travel, but *only to a functioning, existing, time-machine-type end point* in some ways makes a lot more sense than other, more general time travel. Among other things, you solve the problem you mention here without having to give mankind an absolute read on the zero-point of, like, the whole universe, which would be existentially kind of wacky (plus, like, who would congregate there? That'd be an obvious point to go visit if we were going to meet aliens, but I'm not sure that's a good thing).
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(no subject)
07:44pm 22/09/2010 (UTC)
But! In order to 'hold still' relative to the moving Earth through time, you would have to be moving in synch with a third reference point elsewhere in space. 'Holding still' in space while a planet 'moves away' is equivalent to, and requires the same force as, accelerating away from the planet.

Unless your time machine somehow finds a fixed point in the universe and exerts a force to keep itself motionless relative to that, it makes sense for the Earth's gravity well to serve as reference point for position in space.

It could be that when it moves through time, the device jumps in space by precisely the motion vectors it possessed at the moment it leaped, as though the Earth had disappeared entirely while the machine kept going the direction it was headed.

Since gravity is a distortion in space and time, though, there's no cause to assume that it has no effect on an object moving through time. Projected through time, the planet's gravity well carves a spiraling tunnel through space. Unless the time machine either becomes massless or has a mass of planetary scale, it's not unreasonable to expect that this tunnel's walls (which would require escape-velocity energies to breach) would keep the machine aligned with the planet as it curves through time, for much the same reason that we don't feel any torque when we rotate against the Earth's movement -- it's just that much heavier than we are.

There could still be some considerable jolts and jumps in the process. Gravity isn't always kind and gentle about how it controls our motions through space, either.
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(no subject)
07:45pm 22/09/2010 (UTC)
Igobue U. Gowique
Um, that was me just now with the long reply.
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