Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Murchie85 on 26/07/2010 23:24:43

Title: How does travelling faster than light speed constitute time travel?
Post by: Murchie85 on 26/07/2010 23:24:43

Hi there,

I am just linking this on from rami's question about the top speed of light.So I would like to know why is it considered that travelling FTL is analogous to travelling back in time, for example the theoretical particles Tachyons.

I know roughly speaking that an object with no relative velocity ages at the speed of light, and the faster you go the slower time for your clock travels and so forth. Although what is the exact reasoning, (I am sure it is relativity and unfortunately it has not been covered in my course as of yet ... well fully). I guess what I am looking for is a more rigorous explanation.

All thoughts appreciated

Adam

Title: How does travelling faster than light speed constitute time travel?
Post by: yor_on on 27/07/2010 03:14:18

I won't swear to it but I think it's because of some of the math involved points to it be analogous too traveling backwards in a temporal direction when you pass the speed of light. It's not provable other that in theory as I know, and I'm not sure my memory of it is correct. Looked and found this.

FTL (http://en.wikipedia.org/wiki/Faster-than-light)
Time travel (http://en.wikipedia.org/wiki/Time_travel#Via_faster-than-light_travel)

Title: How does travelling faster than light speed constitute time travel?
Post by: LeeE on 27/07/2010 18:38:25

The relationship between the rates of movement through space and movement through time is described by the Lorentz equation.  However, there is no scope for FTL movement in the Lorentz solution and FTL movement would require departure from it.

The Lorentz solution doesn't preclude -ve movement through time but you've got to cross the time axis to do so.

Have a look at the diagram below:

We exist in the 'normal' grey shaded +ve/+ve region, where all movement through space and time can be considered to be +ve.  The Green plot shows that while our spatial speed is low our temporal speed is high, whilst the purple plot shows how our temporal speed approaches zero as our spatial speed increases.

However, to reverse our direction in time we don't need to exceed 'c', and as you can see from the diagram there simply isn't the scope to do so, but instead we just need to cross the axis into the yellow shaded +ve/-ve region, as shown by the Red coloured plot.

The trouble is that there just doesn't seem to be any way of crossing that axis, to start travelling backwards in time i.e. having a -ve temporal speed, because just reaching the axis, where the temporal speed drops to zero, requires infinite energy, even though we never exceed 'c'.

Title: How does travelling faster than light speed constitute time travel?
Post by: Murchie85 on 29/07/2010 13:16:35

Thanks LeeE, that does make sense for sure, although there do exist theories that allow for ftl and associate negative temporal displacement with it http://en.wikipedia.org/wiki/Tachyons. I guess we can only hypothesis until we actually observe it...

Title: How does travelling faster than light speed constitute time travel?
Post by: Eric A. Taylor on 30/07/2010 09:07:51

I had a very hard time picturing time dilation until I read a book that described it in this way:

Imagine an American football running back (the person who carries the ball down field as apposed to a receiver who catches passes). Lets say our player can run at 7 yards per second. He gets the ball and runs at top speed (7 yards per second.) No one touches him and he is able to run at top speed for the entire play. He's knocked down 2.1 seconds after he got the ball. He should have gone 15 yards. First down and more GO TEAM!!! But wait the reff spots the ball only 6 yards down field. Is the team being cheated? What happened? Because our player had to deal with defensive linemen he was unable to run strait down the field, he had to run a little towards the sidelines. Some of his forward speed (7 yards per second) was used to evade other players.

If two people are sitting in a room together and are not moving relative to each other they are both moving through time at 186,000 miles per second. (In the same way our running back would be moving down field at 7 yards per second if he runs strait down field) If one guy wants to walk into another room he needs to use some of his velocity through time to move through space. Because of this effect, just by the act of walking away he'll age just a little bit slower  than his friend. Lets say he walks at 3 meters per second for one minute. His friend will see him moving through time not at 300000000 meters per second like himself but rather he'll see his friend moving though time (aging) at 299999997 meters per second. So he'll see his friend's watch take 1.000...0000001 seconds to tick.

If his friend says "Hey you know where the beer is REALLY cold? Pluto! I'll be right back." and sets off to get some really cold beer at 299999997 meters per second his friend's watch will take quite a lot longer to tick 1 second. To the the guy fetching the Pluto beer just a few minutes have gone by, but He'll get back to the house to discover the game ended hours ago and his friend is at work the next day (and he's late)

At least that's how I understood it. I could be wrong.

Here's something really odd. Because photons are using all their velocity trough time to move through space they don't age at all. From the photons point of view it's created and destroyed (absorbed by an atom) at exactly the same time. A photon from the cosmic background radiation, that has been zipping through space for billions of years is exactly the same age as the photon that entered your eye from the computer monitor .0000....000001 seconds ago.

Title: How does travelling faster than light speed constitute time travel?
Post by: flr on 30/07/2010 16:05:30

Quote from: Eric A. Taylor on 30/07/2010 09:07:51

Here's something really odd. Because photons are using all their velocity trough time to move through space they don't age at all. From the photons point of view it's created and destroyed (absorbed by an atom) at exactly the same time. A photon from the cosmic background radiation, that has been zipping through space for billions of years is exactly the same age as the photon that entered your eye from the computer monitor .0000....000001 seconds ago.

From our point of view photons travel in space and time.
From photon's point of view .... I am not sure.

One way to look at it is indeed as you formulated (photons 'don't age').

Or maybe space and time is something apparent, kind of relativistic effect due to the fact we have speeds lower than c.

And finally, we can always question whether Lorentz equation can say anything about space and time from the point of view of particles at precisely c.

Title: How does travelling faster than light speed constitute time travel?
Post by: Eric A. Taylor on 30/07/2010 21:16:13

Some of the particles studied in accelerators that last just picoseconds will last much longer (microseconds) when moving close to C. It's not possible to make any particle with mass move at 100% C. Conversely any particle that has no mass MUST move at C in whatever medium it goes through. Light moves more slowly when not moving through a vacuum.

Title: How does travelling faster than light speed constitute time travel?
Post by: yor_on on 06/09/2010 15:53:57

Nice explanation Eric, and I think you're perfectly correct in your description, and it actually helped me remember yours LeeE :)

And yes Flr, I'm wondering too about that.

==
Heh, good to be back.