FTL Communication

[Overdog]

One consequence of the light-cones I've always been fascinated with is the idea that from the moment of the big bang (if that's what happened) our view of of the universe has been constrained to just the parts of the universe where the light cones from those parts have intersected with our light cone. In otherwords, there may be parts of the universe that we can never detect, or have knowledge of...parts of the universe who's light-cone never intersects with ours.

[Moontanman]

No, because to get there in one second it would have to be travelling *way* FTL.

 

Here's some more info on that subject:

 

Speed of light - Wikipedia, the free encyclopedia

 

 

 

I found this a very helpful read on the subject. (same link as above, just further down)

 

Speed of light - Wikipedia, the free encyclopedia

 

It seems as if the universe forbids information traveling FTL. :)

 

 

Tachyon - Wikipedia, the free encyclopedia

 

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.

[freeztar]

One consequence of the light-cones I've always been fascinated with is the idea that from the moment of the big bang (if that's what happened) our view of of the universe has been constrained to just the parts of the universe where the light cones from those parts have intersected with our light cone. In otherwords, there may be parts of the universe that we can never detect, or have knowledge of...parts of the universe who's light-cone never intersects with ours.

 

Indeed.

 

We had a discussion about this a little while back, but I can't remember which thread. :)

 

The basic premise was that in this scenario:

 

G_A...................G_B.....................G_C

 

Galaxy B was within the light cones of both Galaxy A and C, but Galaxy A could not see Galaxy C and vice versa.

[Moontanman]

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:

 

Why FTL implies time travel (tachyon pistols)

 

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?

[Overdog]

Indeed.

 

We had a discussion about this a little while back, but I can't remember which thread. :)

 

The basic premise was that in this scenario:

 

G_A...................G_B.....................G_C

 

Galaxy B was within the light cones of both Galaxy A and C, but Galaxy A could not see Galaxy C and vice versa.

 

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?

[Moontanman]

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.

[freeztar]

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 [math]c[/math]. 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.

[Overdog]

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.

[modest]

We had a discussion about this a little while back, but I can't remember which thread.

 

Probably:

 

http://hypography.com/forums/space/14743-deep-space-deep.html

 

Mostly toward the end it looks like.

 

-modest

[freeztar]

Probably:

 

http://hypography.com/forums/space/14743-deep-space-deep.html

 

Mostly toward the end it looks like.

 

-modest

 

Uh...yep...that's the one. :naughty:

Now if I could only find where Craig posted those equations.

 

*freeztar goes looking

[modest]

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.

 

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.

 

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 :naughty:

 

-modest

[sanctus]

Probably:

 

http://hypography.com/forums/space/14743-deep-space-deep.html

 

Mostly toward the end it looks like.

 

-modest

That thread post 19 and then 41 to end

[Overdog]

That thread post 19 and then 41 to end

 

Ok, I read the thread. Great thread...I may revive it later. Thanks.

 

So the answer to my question is no, the part of the universe we can see appears to be very close to flat.

[sanctus]

Ok, I read the thread. Great thread...I may revive it later. Thanks.

 

So the answer to my question is no, the part of the universe we can see appears to be very close to flat.

Well the almost flat or flat models are the ones that fit best the data atm if i remeber right, that doesn't mean that other models that are not flat aare necessarily wrong.

[Overdog]

Are there any theoretical ways to communicate faster than light in our universe?

 

A highly theoretical idea I heard once was this:

 

Imagine a pair of scissors that is light years in length. You close the scissors, and cut a piece of string that is 4 light years away from where you are standing.

 

Hmmmm...you could send morse code with it if you wanted...

 

No laws are violated. Unfortunately it requires a physical, mechanical connection across the entire distance...

[modest]

Are there any theoretical ways to communicate faster than light in our universe?

 

A highly theoretical idea I heard once was this:

 

Imagine a pair of scissors that is light years in length. You close the scissors, and cut a piece of string that is 4 light years away from where you are standing.

 

Hmmmm...you could send morse code with it if you wanted...

 

No laws are violated. Unfortunately it requires a physical, mechanical connection across the entire distance...

 

No, compression in a rod or tension in a string will not travel faster than light. Even when pushing a "rigid" rod the compression wave will propagate through the rod at less than light speed.

 

Probably the easiest way to think of this is the string or rod being made of atoms which must push on one then another down the line sequentially. It is not then exactly rigid and will not send information simultaneously.

 

-modest

[sanctus]

That wouldn't work because, assuming you have enough strength to close the scissors, the speed of propagation of the force you applied to close the scissors also propagates no faster than the speed of light...that means the end of the scissors at 4 lightyears away would at the earliest start to move 4 years after you applied the force...

 

 

EDIT:lol modest you beat me by one minute :)