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This is my first question on this site, which I recently found and can’t get enough of.

In many time travel scenarios, the machine is static in space. My question is how to explain this. For example, in HG Wells' works, it's always in the same location. I’m referring the earth rotating around the sun, as well as the sun moving across the cosmos.

Even if you made a timeship, moving a couple hundred years could mean light years, without FTL you are stuck in deep space.

I have also contemplated using a time machine as a sort of FTL calculating some time in the past or present when another star system will be in the same location we currently are.

Is there a realistic answer to why we stay grounded in the same space while moving through time? It might be easily answered by the theory of relativity, and I just don’t understand.

Note: I’m not thinking about using near light speed time travel, because obviously you would be moving anyway. Thinking more of the getting inside and flipping some switches.

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    $\begingroup$ Welcome to Worldbuilding.SE Travis. This is an interesting question but I'm finding the way you laid it out a bit confusing. I see that you're wondering about location changes relative to the movement of the cosmos, but some of the details you gave make it unclear exactly what you're asking. I did an edit on your question for grammar etc (the shift key is your friend!) and hopefully did not change the meaning of anything. I also wonder if you really want the tag science-based since it's about something we don't really have the science for; if you're looking for theory, it might work. $\endgroup$ – Cyn says make Monica whole Mar 15 at 16:36
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    $\begingroup$ There is no universal rest frame of reference - internalize this truth and your question solves itself ;) I wwill elaborate on this in an answer when I have some time. $\endgroup$ – Renan Mar 15 at 16:37
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    $\begingroup$ The edit is great, thank you. I just used science based thinking, the answers would better explain the logic. I know the easy answer it’s a space-time machine, which I consider more of a wormhole device. I look forward to your explanation Renan. I may need to rework the whole question, I will think on it. $\endgroup$ – Travis Mar 15 at 16:48
  • $\begingroup$ I like the neat idea of using Time Travel for FTL. There are three problems, how to get back as you will have some new velocity vector added to your old one (good space propulsion required). You will have to put up with the fact that you cannot travel in present time. And each jump will have to be done via open space as you will never have two points pass the same point in space at the same velocity. $\endgroup$ – KalleMP Mar 16 at 9:12
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The concept of space and time being combined into spacetime may be the relevant factor here. It is possible that while you are traveling through time, that you will still be under the influence of Earth's gravity. Earth's gravity well might function as sort of an anchor or tether, that will stop you from flying off into the cosmos. Often times in fiction you have to feed a time machine a "time" and "place". It is possible that these fictional devices have a sort of GPS built in that will calculate where a certain spot on Earth will be located in the cosmos at any given time. So long as you don't travel back to before the formation on the Earth you should be just fine.

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    $\begingroup$ This. Since there is no universal rest frame of reference, you can't just "pop" from one point to another. You have to follow a path in spacetime. $\endgroup$ – Renan Mar 15 at 17:01
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    $\begingroup$ You answered faster than me about gravity. But +1 for using the words anchor and tether, as a non-native speaker, I couldn't think of them. $\endgroup$ – kikirex Mar 15 at 17:12
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Use gravitational mass as reference.

(I wrote a time-travel story years ago and I stumbled accross this problem too. In my novel, I explained it in the simplest possible way and discarded any question that may have resulted from it, because plot.)

The device takes the mass of the most dense object nearby as a dot of reference in time, this object being Earth itself. When time-travelling, taking a place as a point of reference makes no sense, since everything moves, either within the cosmos, or on Earth itself: continents may rise and sink while you are time-travelling, and the only reason you do not end miserably into molten rock is plot armor.

But if your time-machine has a gravity densimeter (I just made up that name, feel free to make up a new one by yourself), it can estimate the distance from the most important gravitational mass nearby, and adjust itself to always stay tangent to this mass, while keeping its orientation (the differences in the mass of the Earth being negligible on a grand scale).

Thus, not only you can time-travel on Earth without moving (or so little that it will not affect anything), but also if you have to time-travel on a bigger scale, you can take any other astral object as reference, given it has a massive gravitational mass: be it a planet, a star, a black hole, etc.

Of course, the higher the density, the higher the time-distorsion factor will be.

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    $\begingroup$ Or always time travel in a balloon to avoid the uncertain solid surface contour. Sourcing Helium in some time periods may be a big challenge and getting shod down is almost certain in others. $\endgroup$ – KalleMP Mar 16 at 9:03
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The easy way to resolve this is to just have the machine "exist" throughout any time it travels through. Any forces applied to it from the outside can move it like normal. That's why it stays where it is (and also why it doesn't fall through the planet).

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  • $\begingroup$ "Any forces can move it"... "That's why it stays where it is" Did you mean to say that external forces CANNOT move it? I'm confused $\endgroup$ – DSKekaha Mar 15 at 21:12
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    $\begingroup$ @DSKekaha I think the idea is that the machine is sitting on the floor of your workshop while it is travelling so it travels with the floor that is attached to the earth. This does pose some problems if the surface is altered by rapid forces like a bulldozer or a lava flow. $\endgroup$ – KalleMP Mar 16 at 9:06
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    $\begingroup$ @KalleMP - It would be interesting to see what would happen if the surface is greatly altered...though a slower change might be worse. "traveling through time" would be, well, moving very, very quickly... I dunno how it'd interact with the bulldozer (maybe moving fast enough to be insubstantial to touch?) but I do know fire won't burn unless you're close enough for long enough to transfer the heat, a fire or lava flow might not be able to transfer much before moving "out of range" but an extended heatwave might bake the person inside the time bubble if more heat can absorb in over time. $\endgroup$ – Megha Mar 17 at 4:24
  • $\begingroup$ Perhaps it makes it incumbent on the time machine operator to situate his laboratory/machine on top of a exposed bit of granite that has been so exposed for the whole duration of his time travel adventure. Might add a neat plot twist if he has to relocate to retreat from the advance of urbanisation before the rock is removed for a new interdimensional bypass. $\endgroup$ – KalleMP Mar 22 at 8:58
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Speed can only be determined relative to something else. Why wouldnt time be different? When traveling the time machine has to have a referencepoint or it might lack the ability to travel at all. You might be able to travel as well but with the time machine already traveling through time it might just be too difficult to travel. And being off with your aim could also mean that you now occupy a rock, or any distance above ground from 1mm to 100.000km. Its not worth the risk.

Additionally we know that gravitational effects also affect time. It might simply be that the time machine function only when close enough to a large enough mass and use the gravitational effects to travel. When traveling through time you have to keep the time machine close enough to said mass or you get ejected. Traveling to another location through the gravitational effect of the planet changes the effects the time machine uses, making it uncertain where you end up... And when. Better stick to the same location!

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It's easy to make a time machine -- just get yourself a big cardboard box, climb in, and you will move forward in time at the rate of one second per (subjective) second. When you get out, it's still in the same place, relative to the building it's in.

And if that's true, then it should be true for any other time machine that isn't also a car or a plane or something like that. Only if it somehow "teleports" through time would there be a problem keeping it in the same place -- or steering it to some other place of your choice, TARDIS or Mr. Peabody style.

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Time Travel from Stationary Position?

We might attempt look at relative motion versus absolute motion.

If a traveller were to come to an absolute stop, to become absolutely stationary, he might find that his universe (capital U?) has instantaneously flown away from him, and that in becoming absolutely stationary he has somehow effectively detached himself from said universe (and therefore created another one, depending on the initial definition of 'universe').

But (according to whom?) he may be said to have motion relative to the universe he has just detached from, and in instantly coming to an absolute stop, he has done exactly the opposite. Zero motion may be absolute motion - to the traveller at least, he has for all intents and purposes superpositioned himself upon the universe, where previously all straight lines are now curves. With some further quackery it is around this point there might be room for time travel without appearing to move.

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