That may not have been the best link, better link below.
I'm drawing up a spacetime diagram and I think you're right. We would see mars' future and mars would see ours but a signal would not return before it left. It couldn't get to its own past lightcone. Ok, I'm doing an awful job with this.
There are paradoxes associated with FTL. This link shows a better example than my rover mishap:
In essence, a person shoots a tachyon bullet that kills someone else. He is however killed by a tachyon bullet himself before he can fire the shot that killed the other person. Neither person fires a shot, yet they both die from the bullet the other fired. It's a paradox - an FTL paradox. Difficult to describe, but the link does a good job - I'd really suggest you read it.
According to relativity it isn't just light that travels at c but simultaneity. In essence, "now" travels at the speed of light. The constant c is fundamental to the universe. It shows up in a lot of physics equations - it's certainly much more than the speed of light.
I wouldn't say we know the universe forbids it. We do know that no particle we have ever observed (either with mass or massless) can exceed the speed of light.
-modest
OK this link explains it more completely but what about the possibility of the Abercrombie warp drive? In this scenario a space ship travels at any arbitrarily high speed by taking it's local space with it. This is mathematically possible but would it violate causality?
Sorry I missed it. I just discovered Hypography a few days ago and I love it!
So, what is the thinking about regions of the universe where the the light cones will NEVER intersect with ours?
Actually I have read about this, given the speed of the expanding universe is greater the further away you are then at some point the recession would be greater than c . At this point these galaxies would disappear from our part of the light cone of the universe but still exist in the light cone of other galaxies they were closer to. This would indicate that at some point all galaxies not bound to each other gravitationally would simply disappear from each other's part of the universe never to be seen again.
I went to the site you recommended and it specifically said that if faster than light travel was possible then communication faster than light would be possible so that would indicate the the problem is with the speed limit not causality. If you assume that getting a signal before you sent it is nonsensical then if you found a way to transmit information faster than light it wouldn't nesesarrily result in time travel of the signal. It also said the time travel of the signal could occur in "some" reference frames, this indicates to me that some would not.
The devil is in the details.
We really need some math on this. Unfortunately, I can't think of the proper formulas of the top off my head, but I know a few people here that would.
I seem to remember CraigD posting some mathematical formulas showing the conundrum of using a value higher than . Basically, iirc, it made the equation "something"/zero. This is a violation of mathematics, you cannot divide by zero. I believe this is what the wiki on Tachyons was talking about when it says that Tachyons have imaginary proper time.
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Actually I have read about this, given the speed of the expanding universe is greater the further away you are then at some point the recession would be greater than c . At this point these galaxies would disappear from our part of the light cone of the universe but still exist in the light cone of other galaxies they were closer to. This would indicate that at some point all galaxies not bound to each other gravitationally would simply disappear from each other's part of the universe never to be seen again.
It seems to me that this says something about the geometry of the universe. I'm just not sure what. I've always wondered about the hubble constant, and the observation that everything appears to be receding in all directions, and that the farther away you look the faster things are receding. It would be ironic if the Hubble telescope ultimately proved Hubble wrong.
Granted the limitations on the observable universe imposed by the speed of light, is it reasonable/possible that the curvature of the goemetry of the universe itself (space/time) can account for the red shift we see? I've always wondered about this.
Uh...yep...that's the one.
Now if I could only find where Craig posted those equations.
*freeztar goes looking
__________________ Hypography Science Forums Moderator
--- "There are no passengers on Spaceship Earth. We are all crew." - Marshall McLuhan
"We must not forget that when radium was discovered no one knew that it would prove useful in hospitals. The work was one of pure science. And this is a proof that scientific work must not be considered from the point of view of the direct usefulness of it." - Marie Curie
OK this link explains it more completely but what about the possibility of the Abercrombie warp drive?
Very much like a wormhole, warp drive has possible mathematical solutions. So, people do propose methods of FTL that are based on (or in the very least don't violate) relativity.
Quote:
Originally Posted by Moontanman
In this scenario a space ship travels at any arbitrarily high speed by taking it's local space with it.
Except for watching star trek, I'm not too versed in warp theory
General relativity doesn't preclude spacetime moving faster than light. In fact, the concordance model is based on it. So the possibility is at least there. I believe the practical aspects are, however, very pessimistic.
Quote:
Originally Posted by Moontanman
This is mathematically possible but would it violate causality?
Yeah, that's a thorn in my side. If ever you arrive somewhere before your light - if ever you travel outside your lightcone - you travel back in time. That isn't ease to deal with (at least for me). There are again, proposed solutions such as having the universe you arrive in after your FTL journey not be the same universe your left from. Seems a bit drastic - but never say never in physics I guess
