I’m sure this has been discussed before, but here it is again.
If time travel were or does become possible, wouldn’t it also have to incorporate some form of space travel as well?
As within the universe, the physical coordinates of where a certain place on the earth is 100 years ago, is probably not where it is today. If travel across time is possible, and you only considered that traveling back in time 100 years without considering the position of the earth within the universe, it would plop you in space where the earth currently is, but not where it was 100 years ago. Which would be quite deadly without the proper equipment.
If this is the case, then is safe time travel even remotely possible?
Time travel into the future does involve space travel - it involves you speeding away and back to the observer relative to which you want to travel in time to. This causes your subjective time to pass slower than the observer’s time, so you’ve experienced less time than them when you return - i.e., passed into their future.
Time travel into the past is impossible. The only way it can exist in any form with our current understanding is the existence of traversable wormholes, and even in that form, it is mostly believed that travel would only involve closed worldlines - i.e., the object observers the same series of events, forever and ever, returning to the start. Traversable wormholes have not been found, and it is not know how they might be stabilized and kept open.
If somebody finds a way around the issue previously mentioned, time travel should be perfectly possible so long as you include space travel into the mix. Note that very short travel times would add in the complication that, rather than appearing in space, you’d reappear inside the Earth, and that’s a lot more difficult to engineer for.
Some clever time travel stories think things through, and you always end up in the same spot on earth as you started from. (Wells’ original story,* The Time Machine*, included this detail.) Some more flippant time travel stories allow the machine to also go places “Bethlehem in year 0”, or “London, 1890”. The trouble is, this becomes teleportation if you just make the time difference half a second.
Larry Niven, in his teleportation stories, acknowledges the inertial frame of reference. If you teleport more than, say, 5 miles, the difference between your motion at point A and at point B is the difference in the rotation of the earth. You might be thrown toward the edge of the teleport booth. IIRC, Vernor Vinge in The Witling makes the same observation about teleportation.
Of course, most time travel stories (in fact all of them) gloss over this fact, taking the very Aristotle-like (not even Newtonian) point of view that the earth is the immobile center of the universe. Otherwise, you would need a device capable of plunking you down with a precision of inches over millions of miles. There are multiple components - you are spinning around the center of the earth at 25,000 miles every 24 hours, or about 1000 miles an hour (at the equator; zero at the poles); the direction of that travel is dependent on time of day. The earth goes around the sun about 300,000,000 miles/year or about 33,000mph. Plus, from the milky way inertial frame of reference, the sun itself and out entire solar system has a velocity too.
If your machine can’t compensate for momentum, you’ll be squashed against the wall or ceiling unless you return to the exact same time of day and time of year as you left.
So the answer is, it’s too complex. Because this is science fiction (or fantasy), you can make up the guiding principles however you want. most authors simply omit the excessive complexities.
Oh yeah, I forgot about this. But time travel means changing the rate of movement through time relative to an observer, so you’d still have to move up or down the well to reach a place with a different speed of time (compared to “proper” time)
*Primer *(probably the best though-out and most hard core time travel story out there) handles this very well. You have to stay inside the time machine for the same amount of time that you want to travel into the past, which means that you’re travelling with it, as it were, and the Earth, through space. If you want to go two hours into the past, you need to sit two hours, from your perspective, in the box.
One of my favorite Arthur C. Clarke stories, “Technical Error”, touched on this concept.
[spoiler]It basically involved a fellow who was caught in a massive electromagnetic spike inside an empty generator casing at a power station. He survived, but had been flipped through the fourth dimension and came back as a 3D mirror image. Long story short, he desperately needed to be flipped back, so they put him back in the generator and recreated the spike. And he simply vanished – nowhere to be found! With no other alternative, they put back the huge multi-hundred ton generator rotor and put the generator back online. Later the power station chief starts thinking that the fourth dimension is also the time dimension, and that maybe the fellow had also been displaced through time… meaning that he could materialize at any moment, inside a generator that is no longer empty!
And as he looks out his window toward the power station, a large mushroom cloud is rising into the sky.
I read a sci-fi story about a zero entropy envelope that lets to time travel into the future but not the past. Essentially, while you’re inside the Z.E.E. time stops for you while it keeps going for the world outside. When they switch it off, you’ve traveled forward in time. But you’re in the exact same place because you and your mass are being carried along with the Z.E.E. on the surface of the Earth.
However, if you have bi-directional time travel akin to teleportation, you should arrive at a point in space which is on your linear path based on the momentum you had at the time the device was activated. IMHO, your path through space would be unaltered by the mass of the Earth hence a 24-hour trip into the future would land you in outer space roughly 30,000 km away, at exactly the same spot you’d end up if the entire Earth except for you vanished for 24 hours and you coasted along ballistically.
That’ll work. Well, not really, but it sort of works. It may sound flippant, but time travel into the future is barely science fiction, it’s just a matter of practicality (it’s going to the past that is the hard part) . As mentioned upthread, positioning yourself at the right place in a gravity well does the trick. What you really want to do is go hang out near a black hole.
I guess I question the premise. Changing my motion in a spatial coordinate – say, stopping at a light and turning left – does not cause me to go flying off the face of the earth. Why would changing my motion in a temporal direction do so?
Eltro has given the only sensible answer so far. Most fictional depictions of time travel imagine the traveller somehow magically translocating from one location in space and time to another location in space and time, with no sensible method of getting from one location to another.
Wormholes are a completely different animal; they represent a region of space which is multiply connected, so that you can travel consistently between one location and another. To go to another location in space-time you would have to move the wormhole - good luck with that, since they would be about as difficult to play with as a black hole of comparable size.
So if your wormhole went to (say) Jupiter in the 18th century, that is where you would go and nowhere else. Of course, if you then got in a spaceship and came back to Earth from 18th century Jupiter you’d arrive on 18th century Earth, with hilarious and paradoxical results.
I suppose the other end of the wormhole could be linked to 18th century Earth instead of Jupiter - this would save on a space journey and make things easier all round. But as time passed at both ends of the wormhole, eventually the 18th century would change to the 19th century at one end, and the 21st century would turn into the 22nd at the other.
Eventually the early end of the wormhole would reach the 21st century, while the late end would reach the 24th; note that the early end of the wormhole would now be located in the same region of time and space as the original link between the 21st and the 18th. This means a person in the 18th century could now travel all the way to the 24th, simply by travelling through two wormholes (actually the same wormhole, just at different periods of its own history).
This, as Amy Pond once said, is where it gets complicated.
Assuming that when you reach your destination, you are still inside the “time machine”, which is a fixed, physical object like a refrigerator, anchored to the planet somehow, you will open the door and step out into the same physical geography that you started out in. The time machine will have been transported through space along with its environs.
So this would only become a problem as time machines become so advanced that hardware is no longer needed, and we can’t realistically think that far into the future of the technology.