## The Maths of Time Travel [Physics/Maths]

For the discussion of the sciences. Physics problems, chemistry equations, biology weirdness, it all goes here.

Moderators: gmalivuk, Moderators General, Prelates

### The Maths of Time Travel [Physics/Maths]

I'm pretty sure that there are solutions to the general relativity equations that imply that time travel is possible. (wikipedia says so, and I've read about it a few times)

However, I cannot for the life of me remember what these conditions are, or where I might find them, or even better, explain them here.

<bakemaster> Only German Sausage can prevent forest fires
<felstaff> Hype is like a giant disappointment ray aimed squarely at the finished article.
<watson> Treat me like a criminal, Holmes!
TMT4L

German Sausage
3 of 5

Posts: 2933
Joined: Mon Jan 01, 2007 9:45 am UTC

Can I get back to you in about six months when I've finished my astro units?

Hawknc
Oompa Loompa of SCIENCE!

Posts: 6963
Joined: Mon Oct 02, 2006 5:14 am UTC
Location: Melbourne, Australia

theory is that objects with a velocity higher than c have a negative time component right?
ave_matthew wrote:in a perfect system a gallon of body fat is worth one third of the US GDP
zealo

Posts: 321
Joined: Sun Dec 17, 2006 11:36 am UTC
Location: perth, australia

zealo wrote:theory is that objects with a velocity higher than c have a negative time component right?

Imaginary, actually, so perhaps not too helpful. (The time dilation factor is the square root of 1-(v/c)^2, so the square root of a negative number if v>c.)

Also, there aren't solutions to GR that allow anything to accelerate through c in its local frame, which means that even if ftl travel was also time travel, that way of doing it wouldn't be possible in a system consistent with GR.

I have heard of two specific solutions. One is if the entire universe were rotating. I believe this is the GÃ¶del metric. The other involves rotating cosmic strings (i.e. long, stringy black holes) around each other, creating closed timelike curves in the vicinity of where they pass.

These are obviously not really practical solutions, especially the one where the whole universe has to rotate. I think there are others that are more feasible (in that they only need a few galaxies' mass of negative energy to work, instead of the whole universe, for instance), but I don't know what they are.
In the future, there will be a global network of billions of adding machines.... One of the primary uses of this network will be to transport moving pictures of lesbian sex by pretending they are made out of numbers.
Spoiler:
gmss1 gmss2

gmalivuk
Archduke Vendredi of Skellington the Third, Esquire

Posts: 19271
Joined: Wed Feb 28, 2007 6:02 pm UTC
Location: Here, There, Everywhere (near Boston, anyway)

I've read that you can time travel with a garden-variety rotating black hole, by using frame dragging to accelerate to a speed greater than c with respect to a faraway observer, while still traveling at less than c in the rotating frame. This allows you to travel back in time. Is that right?
Herman

Posts: 559
Joined: Wed May 02, 2007 2:46 am UTC

Yes, you can travel forward in time.

Motivation is when your dreams put on work clothes. -- Ben Franklin

Avatar from My Little Pony: Friendship is Magic, owned by Hasbro.

Vaniver

Posts: 9356
Joined: Fri Oct 13, 2006 2:12 am UTC

Herman wrote:I've read that you can time travel with a garden-variety rotating black hole, by using frame dragging to accelerate to a speed greater than c with respect to a faraway observer, while still traveling at less than c in the rotating frame. This allows you to travel back in time. Is that right?

I never was able to make much headway with the mathematics of the Kerr metric, but according to Wikipedia, you're right. The Kerr vacuum (which is what describes a rotating black hole) does indeed have some closed timelike curves associated with it. This means a massive particle can start at some point in space-time and return to that same point in its own future, which is basically a fancy way of saying it can travel back in time.
In the future, there will be a global network of billions of adding machines.... One of the primary uses of this network will be to transport moving pictures of lesbian sex by pretending they are made out of numbers.
Spoiler:
gmss1 gmss2

gmalivuk
Archduke Vendredi of Skellington the Third, Esquire

Posts: 19271
Joined: Wed Feb 28, 2007 6:02 pm UTC
Location: Here, There, Everywhere (near Boston, anyway)

gmalivuk wrote:I have heard of two specific solutions. One is if the entire universe were rotating. I believe this is the GÃ¶del metric. The other involves rotating cosmic strings (i.e. long, stringy black holes) around each other, creating closed timelike curves in the vicinity of where they pass.
I've also heard of the Alcubierre metric, Krasnikov tubes, the Tipler cylinder, and, of course, wormholes.
This message composed of 100% post-consumer electrons.
My blag

NathanielK

Posts: 108
Joined: Wed May 02, 2007 5:47 am UTC
Location: Maryland, USA