Is Time Travel Possible According to General Relativity?

[benhsmith]

I don't have an extensive physics background, so I apologize if there's a well known or obvious answer to this question.

I'm familiar with the Newtonian version of time in which it's treated as a kind of fourth dimension. So there's x,y,z and t. Sitting here typing this, time is passing for me and I'm moving along some t axis.

I've also read about how things like closed time-like curves would mean that time travel is, theoretically, possible.

Intuitively, it seems to me that if the I and the Earth are moving along a t axis, then even if I could travel back to some previous point on this t axis, when I got there the Earth wouldn't be there anymore. Not just because it would have moved to some new point in three-dimensional space, but because it would have move to a new point in time. So, for example, if t0 is my current moment in time and t1 is, say, some moment in 2009, the physical matter that makes up me and the planet I'm standing on can only exist at t0 or t1, not both. Or, does all matter and energy exist at all points in time that it will ever pass through simultaneously? If it's the latter, how do we know that?

Can anyone recommend a book for non-physicists with a good explanation of how this works?

 

[ghwellsjr]

It doesn't work. Look in the sci-fi section of your local book store.

 

[JesseM]

Intuitively, it seems to me that if the I and the Earth are moving along a t axis, then even if I could travel back to some previous point on this t axis, when I got there the Earth wouldn't be there anymore.

How would that make sense unless you had a second time dimension? What you're suggesting is that if we had a God's eye view of all of spacetime (i.e. we could see every point on the t-axis at once), spacetime as a whole could change, with Earth only being at one point on the t-axis and this point "changing", but it can't be changing with regular time because each point on the t-axis is supposed to show what was happening at that time, so if the Earth was at position x=1 at time t=1 and at position x=3 at time t=2, then our God's-eye-view would show a version of the Earth at both of these positions and times. The usual way of thinking of spacetime is thinking that each object has a world line in 4D spacetime (or a 'world-tube' for a spatially extended object instead of a point particle, sort of a 4D worm with your conception on one end and your death and decay on the other), and if you take a 3D cross-section of 4D spacetime showing a snapshot of space at one particular moment, then the cross-section through your 4D world-tube would just be a snapshot of your 3D body at one moment.

 

[JesseM]

It doesn't work. Look in the sci-fi section of your local book store.

Closed timelike curves are certainly a theoretical possibility in general relativity, whether they are possible in reality can only be settled by a theory of quantum gravity, but it's an active area of study. For a physics perspective on time travel you might look at something like https://www.amazon.com/dp/0393312763/?tag=pfamazon01-20 by physicist Kip Thorne. But if the poster is more interested in the general question of how to make sense of the notion of "spacetime", a good book is https://www.amazon.com/dp/0226288641/?tag=pfamazon01-20.

 

[John232]

There is no such thing as an absolute time in physics. In modern physics time for each object is independent of the time for another object. There is no sheet of time needed that carriers everything along with it at the same rate. I find it hard to imagine how one object could experience more or less time than another when there would only be one time that everything expereince's at the same time.

Say if you fell in a black hole your time would almost stop but you could still exist even though the rest of the universe progressed forward to the end of time in that instant. Nothing gets cut off from existence because it didn't experience enough time to move forward with everything else in time.

 

[benhsmith]

Closed timelike curves are certainly a theoretical possibility in general relativity, whether they are possible in reality can only be settled by a theory of quantum gravity, but it's an active area of study. For a physics perspective on time travel you might look at something like Black Holes and Time Warps by physicist Kip Thorne. But if the poster is more interested in the general question of how to make sense of the notion of "spacetime", a good book is https://www.amazon.com/dp/0226288641/?tag=pfamazon01-20.

Thanks, General Relativity from A to B sounds like the book I've been looking for. There are lots of books and articles about the strange things, like time travel, that relativity suggests are possible, but they never explain basic ideas like spacetime and worldlines in much detail.