### One Big Problem with Time Travel

There is one aspect of time travel that nobody considers, whether in science or science-fiction (at least, i am not aware of it), something that has the potential of rendering time travel, even if it is possible, totally futile. So, what is it? In time-travel, we are only moving through time, and not through space. So, if we are at a given set of co-ordinates X, Y, Z at the present time A, and we go back in the past to some moment B, we would still, essentially, be at the same co-ordinates XYZ. Now, here's the point. Earth is moving through space; moving with velocities we might not even have a clear idea of. For definite, we know the earth is moving around the sun, the sun is moving around the centre of the galaxy, and the galaxy itself is moving. That makes up a lot of motion. So that at any given two instants earth would be at two different places. So, if i travel in time, whether in past or future, while retaining my location in space, i would end up in empty space, cause earth would be somewhere far far away...

[Edit: Since JesseM resolved this issue and clarified the problem in her comment, i am posting it under the post for the convenience of future readers:

JesseM: None of the time travel schemes proposed in general relativity actually involve the sci-fi idea of dematerializing in one time and rematerializing in another, instead they involve travel through a warped region of spacetime like a wormhole--and with a wormhole, where you end up in the past would just depend on where the "mouth" of the wormhole happened to be located at the time you exit. For example, if the mouth was in orbit around the Earth then you'd end up near Earth no matter what time you travelled to.

That said, the idea that there is some objective truth about which point in space in the past is the "same position" that the Earth is in today is in total contradiction with relativity. See this section of the wikipedia time travel article:

An objection that is sometimes raised against the concept of time machines in science fiction is that they ignore the motion of the Earth between the date the time machine departs and the date it returns. The idea that a traveler can go into a machine that sends him or her to 1865 and step out into the exact same spot on Earth might be said to ignore the issue that Earth is moving through space around the Sun, which is moving in the galaxy, and so on, so that advocates of this argument imagine that "realistically" the time machine should actually reappear in space far away from the Earth's position at that date. However, the theory of relativity rejects the idea of absolute time and space; in relativity there can be no universal truth about the spatial distance between events which occurred at different times[52] (such as an event on Earth today and an event on Earth in 1865), and thus no objective truth about which point in space at one time is at the "same position" that the Earth was at another time. In the theory of special relativity, which deals with situations where gravity is negligible, the laws of physics work the same way in every inertial frame of reference and therefore no frame's perspective is physically better than any other frame's, and different frames disagree about whether two events at different times happened at the "same position" or "different positions". In the theory of general relativity, which incorporates the effects of gravity, all coordinate systems are on equal footing because of a feature known as "diffeomorphism invariance"[53]. ]

M. Umer Toor said…
YOU really need to stretch this short article into a standard one, for elaboration that is necessary. You're not explaining how are you moving? In which direction are you moving? And how, or whether it is in agreement with thermodynamic laws and with the direction of the the cosmic motion of time, or not?
Abdullah Shahid said…
I got everything you said except the last statement. :-D What exactly are you implying?
Umer said…
You won't be in empty space. Time and space are NOT independent entities (Einstein's relativity) and by altering yourself in time your alter your position in space and vice versa.

Plus, you can't trate space co-ordinates of space separately from time co-ordinate t, of course, due to the same reason.

According to Physics, time travel is possible in one dimension, i.e you can go to future but can't go to the past. (that too would require too large travelling speeds).
Awais said…
@ Abdullah

Ok, let me put it this way.
At present time A, you are at coordinates X, Y, Z, and earth is also at coordinates X, Y, Z cause you are standing on it.

Now when you go back to past at moment B, you would still be at coordinates X, Y, Z but earth would not be at coordinates X, Y, Z... it would be at some other position, cause at that instant in the past earth's position in space would be different from the position it was in when you were in the present. Got it?
Awais said…
@ Umer

Yes, i am aware that time and space are not independent entities. But even if we assume that if we go back in time, and our position in space would also change, that doesn't mean that we would still land on earth in the past. To do so, the change in the position would have to be precisely coordinated with that of change in earth's position between those two instants. And given the variety of velocities with which earth is moving, such a calculation is not going to be successful.

And how is travelling into the future possible according to physics, since travelling at lower than light speeds is not going to send you in the future?
misspecs said…
Okay, I'm not a physics person here so sorry for butting in (I'm prolly going to sound an A class idiot but I HAVE to know more!), but I don't get this: keeping Einstein's relativity Theory in view, time and space are not independent. So, I'm assuming time travel breaks the time/space continuum. So, if the space continuum breaks, isn't it possible to land at the co-ordinates the Earth was at some specific point in time? You know, to put it differently: why must one land at the same point in SPACE only in a different TIME?

Or does one NEED to land at XYZ only in a different time.

misspecs said…
Oh okay. Saw your other comment just now. Uh huh...so what you're basically saying is, its impossible to calculate exactly where to land in order to land at the right place.

Uh huh. Gotcha.
GOTCHA!@
Awais said…
Space and Time form a Space-Time continuum together, but it is not necessary that a change in Time coordinates has to be tagged by a change in Space coordinates. For instance, the three space dimensions are not independent from one another either, and yet it is possible to change only one's Z coordinate while retaining the same X and Y coordinates. Hence, when people normally talk about time travel, they are assuming that only the Time coordinates are being changed and not the Space.

It is possible, yes, to change both simultaneously (as Hiro Nakamura does in Heroes tv series), but for us to go back in past or in future and still remain on earth, we would have to calculate the exact change in the position of the earth, and given the variety of rotational and linear velocities earth is being subjected to at the cosmic level, such a calculation is not likely to be possible.

In another words, time travel isn't just supposed to be time travel... to be successful, it would have to be space-and-time-travel :) and apparently nobody had paid much attention to the space-part, whether in fiction or in science.
JesseM said…
None of the time travel schemes proposed in general relativity actually involve the sci-fi idea of dematerializing in one time and rematerializing in another, instead they involve travel through a warped region of spacetime like a wormhole--and with a wormhole, where you end up in the past would just depend on where the "mouth" of the wormhole happened to be located at the time you exit. For example, if the mouth was in orbit around the Earth then you'd end up near Earth no matter what time you travelled to.

That said, the idea that there is some objective truth about which point in space in the past is the "same position" that the Earth is in today is in total contradiction with relativity. See this section of the wikipedia time travel article:

An objection that is sometimes raised against the concept of time machines in science fiction is that they ignore the motion of the Earth between the date the time machine departs and the date it returns. The idea that a traveler can go into a machine that sends him or her to 1865 and step out into the exact same spot on Earth might be said to ignore the issue that Earth is moving through space around the Sun, which is moving in the galaxy, and so on, so that advocates of this argument imagine that "realistically" the time machine should actually reappear in space far away from the Earth's position at that date. However, the theory of relativity rejects the idea of absolute time and space; in relativity there can be no universal truth about the spatial distance between events which occurred at different times[52] (such as an event on Earth today and an event on Earth in 1865), and thus no objective truth about which point in space at one time is at the "same position" that the Earth was at another time. In the theory of special relativity, which deals with situations where gravity is negligible, the laws of physics work the same way in every inertial frame of reference and therefore no frame's perspective is physically better than any other frame's, and different frames disagree about whether two events at different times happened at the "same position" or "different positions". In the theory of general relativity, which incorporates the effects of gravity, all coordinate systems are on equal footing because of a feature known as "diffeomorphism invariance"[53].
Abdul Sami said…
blooody hell !!!

do u kno i was thinkin about writin this down since the last few days and only the fact that i had to travel to slovenia and hav been away on business has been preventin me from doin so !!!

lol... another example of 2 remote people thinkin the exact same thing at the exact same time !!!
Abdul Sami said…
we could use the last version of the delorian or the tardis tho... both had location built into them as well :D !!
Awais said…
@ JesseM

Thank you for sharing this with us. I now realize the error in my thinking :)

One lesson to be learnt from this is: Always check up what's written on wikipedia before writing about it on your blog. ;)
Umer said…
Nicely explained JesseM.

A little comment about travel being possible towards future and not in past:

Travelling at lower than light speeds may not, and in fact won't, send us in future in a sense that most people visualize. But it would, in a sense, that travelling at high speeds (still lower than speed of light, though) would slow down time considerably for the traveller and thus while the aging process of the rest of the people would continue at normal pace, his aging process would slow down to quite some extent. Initially, with the increase of speed, this would be a negligible change, but with speeds approaching the speed of light, this difference would become enormous. A rough theoretical estimate through a simplest problem would tell us that for a person travelling at .99c (this speed has been attained in laboratory but only, of course, on subatomic particles) the passage of 2 hours would equate passage of around 240 hours for a person living in normal circumstances here on earth. With the speed of light or above it, this would, sort of, go to infinity. :D