I am putting this here, but it is quasi-related to Science Fiction. In almost every story I have read/seen involving time travel the machine involved goes into the future or the past and winds up exactly in the same place on earth but at a different time. If the machine is or remains stationary, shouldn’t the machine wind up in space when it comes out at a different time? Since the earth and solar system are constantly moving through space at a few thousand miles an hour. Even a short trip to the past or future would have the earth not where it was when you left in your time machine. So, does a time machine have to account for the earth’s movement? And does any sci-fi actually address this?
The problem is that there is no such thing as being absolutely stationary. There’s no way to tell whether something is moving through space at a constant velocity, or standing still.
I’ve wondered this myself. I have read at least one story that dealt with this - the time travel device in question was in fact a sealed, underground laboratory, which ended up in space, but the inventor had planned for that. Otherwise, I suppose you could postulate that gravity doesn’t cease to act on the time machine, even in transit, which would keep it neatly anchored to the earth. Of course, then you have to worry about where the time machine is - does it just sit there, and you pop out a thousand years later? Can you go back in time from when it was invented? Can it cross its own path? Anyways, I think it’s complicated enough that most authors don’t even address it.
If it’s stationary relative to the Earth, then there’s no problem: starting on the Earth and staying on the Earth is being stationary relative to the Earth.
This is the reason why I think “non-continuous” time travel (where one is instantly in another time without moving through the “times” in between) is impossible.
If we could/can accurately predict our orbital flight path, couldn’t we just pick a time period and a point along it, and our machine would appear somewhere on Earth( the position in space our planet occupied 100 years ago, for example?), with a little fine tuning to prevent ending up in an ocean, or something?
Indistiguishable, perhaps I wasn’t clear. But when I meant stationary relative to earth, I meant something like this: You are on a train platform. Car number 19 is in front of you. The engine starts up and you stay where you are while the train moves forward. You are stationary relative to Car number 19, but clearly the distance between you increases. Regardless of whether it’s you who is really moving or the train that’s moving, the space between you increases.
Forget about relative to earth. Momentum of the time traveler would have to be preserved. If the process of time travel somehow eliminated the force of gravity during transit, the time traveler would be hurled off the earth in whatever direction he was going at the moment he pushed the button. If, however, time travel worked by small jumps with all the usual forces still applying, he’d get pulled along as if he never left, and there would be no issue.
But he has a bigger problem. No surface stays at the same level for all time. If he starts in a place that has been eroded, and winds up at the same level,. he’d be inside the ground which would probably ruin his entire day. If the ground erodes from under him he’d do a Coyote - materialize, see the ground, stick up a sign saying “whoops” and plummet to it, probably breaking the machine if not him.
If I ever write a time travel story, the travelers are going to do it in a spaceship where at the least the second of these problems won’t apply.
If you can bend space-time enough to travel centuries in the blink of an eye then I think a little teleportation is the least of your problems. Haven’t tried it myself though.
The notion of something staying absolutely still in the solar system while the Earth rolls away from under it, is completely arbitrary. That’s not staying still at all, because the solar system is in motion, the galaxy is in motion, etc.
There is no way to determine what it would actually mean to be not in motion at all, so (apart from all the other problems with time travel), it has to play out one of two ways:
-The time machine stays tied to some object, such as the Earth (and so reappears in the same place on Earth)
-The time machine enables us, for the first time ever, to determine such a thing as absolute position. I think that means having to rewrite a lot of our understanding of physics, with appropriate apologies to Einstein et al.
I guess I call it stationary relative to the car because it appears that you are standing still while the car is moving. That’s different from being in the car in which you’d be stationary and the car would be stationary even it were moving away from the station. But the distance between you and the station would increase, but the space between you and the car would not. So, in “relation” to the car in my original scenario, you’d be standing still while it moved. There is no absolute frame of reference, I know, but you can work out how things move or stay still in relation (relatively) to one another, right?
I think having students compute the motion, through force vectors, for a time machine under various assumptions would make an excellent freshman physics homework problem. Or test question, if you’re feeling sadistic.
Yes. Only in relation to one another. That’s the point. It’s easy to say that something is moving or not, relative to something else. It’s impossible to say that something has stopped moving, relative to everything.
Asimov used just this idea to kill a man in a story once. Billiard ball shot through small a “time machine” just far enough into the future to puncture his heart where he stood across the table from it.
I’m completely blanking on the name of the story, though…