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I'd like to ask a question that is not in my field but it's bothering me. Based on that, I'm sorry if I make dumb mistake in my assumptions.

The idea of travelling in time means "going" at some place, but not in the same time. If I decide to go back to a few days ago, this does NOT only means I would have to do the complex process of reverting time, but (and this in my point/question), I would also have to be physically moved.

Here's my theory : Today, the earth is at a certain position around the sun. In three days, this position will be different. Suppose we can successfully revert the time. If I go three days ago, I will be in space, because the earth was not at the same place as today.

Now this is even more complex because the earth goes around the sun but that's not our only movement : our galaxy is moving, our universe is moving. From what I know, the planets are not even only rotating, but also falling.

So in order to achieve time travel, wouldn't we also need to know the exact position of where whe want to be, based on the calculation of every elements we rely on, from our earth rotation to our universe expanding, moving and falling?

Isn't that one of the reason we can not achieve travelling in time?

Thank you for your enlightment. And I'm really sorry if I said something stupid.

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    $\begingroup$ I'm pretty sure there was a Dinosaur Comic strip on exactly this issue. Can't find it though. But you're right, you need to make sure that the spatial coordinates of your time machine are comoving with Earth. Also, you need to find a mechanism that solves the issue that wherever you end up, there will be an atmosphere or at least some particles that occupy space, which will end up inside your body. $\endgroup$ – pela Jun 2 '15 at 8:59
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    $\begingroup$ I like this question. Seriously, it's something that time travelers need to consider. Or sci-fi writers. Or something. Anyway, +1. $\endgroup$ – pela Jun 2 '15 at 9:13
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    $\begingroup$ @CuriousOne "Science is a method to explain natural phenomena that have been observed. It can't say anything about phenomena that have not been observed". This is so wrong I'd like to slap your face in person for spouting such nonsense. Did Dirac observe the anti-electron when he wrote down his equations? Did Englert, Brout, Higgs, and others observe the Higgs boson before introducing the mexican hat potential? Did Galle just decide to point his telescope at a random spot in the sky and then send a letter to Le Verrier asking him to explain what he saw? $\endgroup$ – Raskolnikov Jun 2 '15 at 9:44
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    $\begingroup$ @Raskolnikov but all those theoreticians worked based on experiment (set aside that Dirac was a strong believer in mathematical beauty as indicator of truth). They explained observed phenomena (e.g. the spin of electrons, the mass of the W and Z bosons, the irregularities in the planetary movement) by introducing hypothetical objects that consistently explained observation (and were later actually observed). But they did not believe the objects to be scientific truth until they were experimentally verified. $\endgroup$ – Sebastian Riese Jun 2 '15 at 11:57
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    $\begingroup$ We all travel in time. Just not willfully. And fortunately the earth travels at the same rate so that we are not left gasping for air ... and a place to stand. $\endgroup$ – docscience Jun 2 '15 at 12:41
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Isn't that one of the reason we can not achieve travelling in time?

I'm afraid not. You can't travel in time because time is a dimension in the sense of measure, not a dimension that offers freedom of motion. Take a close look at what a clock really does. It isn't some kind of cosmic gas-meter gizmo with time flowing through it. It doesn't literally measure the flow of time. Instead it features some kind of mechanism that converts some kind of regular cyclical motion, such as the swings of a pendulum or the vibrations of a crystal, into the gradual movement of the big hand and the little hand around the clock face. The interior mechanism of a clock is called a "movement", because clocks "clock up" motion. A clock is more like a gearbox than a gas meter.

I think it's neat to draw a comparison with heat. Heat is an emergent property, wherein the temperature of a hot gas is something like a measure of the average motion or average kinetic energy of the gas molecules. The faster they're moving, the hotter the gas. But you know full well that you can't literally climb to a higher temperature. In similar vein you can't literally travel to a different time. You could take a fast out-and-back trip through space and suffer time dilation, but time dilation isn't time travel. I could watch you every inch of the way through my gedanken telescope, at no point do you disappear from the present and reappear in the middle of next week. You just suffer a reduced rate of local motion that's all, because of your macroscopic motion through space. Because the total rate of motion is limited to the motion of light.

The bottom line is that time travel is nothing to do with where planets were, or where they're going to be. The bottom line is that I can hop forward a metre, but you can't hop forward a second. And I can hop backwards a metre but you can't hop backwards a second. Because you can't move through what is merely a cumulative measure of motion. So I'm afraid time travel is science fiction. Sorry.

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  • $\begingroup$ this is nicely presented, but it feels slightly specious to me e.g. time is "merely cumulative measure of motion". $\endgroup$ – innisfree Jun 2 '15 at 14:16
  • $\begingroup$ @innisfree : it's a question of what do clocks do? Einstein was cagey about it with his operational definition, and then there's Palle Yourgrau's A World without Time: The Forgotten Legacy of Godel and Einstein. IMHO the blurb for this is a bit misleading, in that time exists like heat exists. A hundred years will kill you just as surely as 100 degrees C. But it isn't something you can move through. $\endgroup$ – John Duffield Jun 2 '15 at 17:00

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