Do Time Machines Account for the Movement of Planets and Galaxies?

[Mr Whippy]

Hello,

This might be an old question by now, though I have never seen any mention of it yet.

I was considering all the fictional time machines and even theoretical the laser time tunnel idea. When it occurred to me that all have a certain flaw, in that they are all based on a stationary destination.

For example if I was going to send a particle down Ronald Mallett’s laser tunnel or use a Delorean like machine. Rather than appear in the past or future in the same location as they originally were, they would most likely end up in empty space.

Why? Because of course the planet is moving along with everything else in the universe. Go back a half a year and not only is the planet on the other side of its orbit, but the whole solar system and the indeed the galaxy it’s in has moved millions of miles for when you left, even light years if you traveled far enough in time.

To be able to send a particle down a time tunnel you would have to keep all the other time frame tunnels stationary in space. Or if you were in a Delorean, go rather faster than 88mph.

Just my thoughts on the matter.

Cheers.

 

[JesseM]

The time machines proposed in general relativity don't involve instantaneously disappearing from one time and reappearing in another, they involve a continuous path through a sort of twisted region of spacetime, so where you end up depends on the details of the path. For example, with a wormhole time machine, the wormhole has two "mouths", and if you enter mouth A in 2010 and travel through the wormhole's "throat" to exit from mouth B, you might exit mouth B in 2005--where you would find yourself would just depend on where mouth B happened to be located in 2005 (it's a physical object so it could be carried along close to the Earth if you wished). I imagine that if Mallett's idea could work then similarly you would exit wherever the laser tunnel happened to be located at the time you were exiting, although, numerous objections have been made to the theoretical validity of his idea.

As for the notion that the Earth has "moved", this doesn't really make sense in relativity because it presupposes some sort of absolute stationary reference frame, whereas in relativity the question of whether an object is moving or stationary depends on an arbitrary choice of reference frame, with all frames equally valid. This is discussed in the wikipedia time travel article:

Time travel, or space-time travel?

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 fact 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[57] (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".[58]