Relativity Question *waves at TheEngineer*

I've heard it said before that photons don't break relativity because they are 'emitted' at light speed and don't accelerate to it.  If this is the case, why can't a different particle be 'emitted' at greater than light speeds?


Let the flaming begin!!

I've heard it said before that photons don't break relativity because they are 'emitted' at light speed and don't accelerate to it.  If this is the case, why can't a different particle be 'emitted' at greater than light speeds?


Let the flaming begin!!

There are particles, generalized as tachyons, that are emitted during radioactive decay that go faster than the speed of light. Interestingly, these particles take more energy to slow to the speed of light. So c is actually some sort of asymptotic energy barrier. It's really interesting stuff.

Additionally, c is a constant based off of early vacuum experiments. You can alter vacuums to speed up c, like in a casimir vacuum.

As long as I am hijacking your thread.

Photons are not traveling at c because they are emitted at that speed. This has been shown by cooling a photon, which slows it, then warming it, which speeds it back up to c. If it can only attain a velocity of c during its initial emition, these types of experiments could never work.

Typically, physicists will look at photons as waves for your thought argument. This allows them to ignore their mass. They will then look at them as particles when they need them to have mass. Basically we have the newtonian vs quantum laws that we pick and choose from. This is evidence that we have a flaw in our model at a more elementary level.

It's not that they are emitted at c so they aren't traveling at c, it's that they are emitted at c so they don't accelerate to c.

It's not that they are emitted at c so they aren't traveling at c, it's that they are emitted at c so they don't accelerate to c.

I understand. But then if you slow them down, how can they ever get back to c?

My point is they can because they are "massless."

Perhaps they don't get back up to c.  Perhaps they accelerate back up to near c.  Are the experiments you mention being done in vacuums?

In GR, I can't imagine anything being transmitted FTL. Although there is the group velocity aspect in which they travel FTL.

Other than GR though, superposition could be taken into account.

In GR, I can't imagine anything being transmitted FTL. Although there is the group velocity aspect in which they travel FTL.

Other than GR though, superposition could be taken into account.

GR specifically states that massy objects cannot reach c, however there is no restriction in any of its laws against massy objects moving faster than c. This supposes that they begin and always remain faster than c.

Perhaps they don't get back up to c.  Perhaps they accelerate back up to near c.  Are the experiments you mention being done in vacuums?

yes.

General Relativity does not prohibit the movement of particles at velocities greater than c.  It's just that objects with mass can't accelerate to c.  Now, it is theorized, that tachyons exist and that they travel at FTL speeds.  However, we can't see them until they have passed us by, otherwise, they would violate the no communication theorem.  They also don't violate causality as a person observing them would see two of them, one coming and one leaving, and would have no way of differentiating which one is the real tachyon, thus, losing the message in the background noise.

Thank you TheEngineer!  So communication at FTL speeds is impossible according to GR?  This is why a graviton that could be emitted at FTL speeds could not be the messenger particle for the force of 'gravity'?

Yes, information transfer at FTL speeds is strictly prohibited by GR.  Things can move at FTL speeds, but they can't tell us they are doing so.  This would cause all kinds of problems with causality, so there is usually a very neat mathematical solution as to why information can't be transfered at FTL speeds.  Such as a tachyon detector:  It would be able to detect the incoming tachyon, but it would also emit an exact copy of that tachyon, thus losing the information.

Yes, information transfer at FTL speeds is strictly prohibited by GR.  Things can move at FTL speeds, but they can't tell us they are doing so.  This would cause all kinds of problems with causality, so there is usually a very neat mathematical solution as to why information can't be transfered at FTL speeds.  Such as a tachyon detector:  It would be able to detect the incoming tachyon, but it would also emit an exact copy of that tachyon, thus losing the information.

That's not strictly correct. The "No-Communications" says we, not nature, cannot communicate at FTL speeds. Nature could, and probably does, communicate at FTL speeds.

From Wikipedia

In quantum information theory, a no-communication theorem is a result which gives conditions under which instantaneous transfer of information between two

observers is impossible.[/list]

Oh, good Lord, here we go again.

You must have a very narrow definition of an observer.

Oh, good Lord, here we go again.

You must have a very narrow definition of an observer.

Oh, it's much wider than your definition of local--That's for sure!

Way to back up your argument, TomG.

My definition of local in the context that it was used in that thread was less than global.  One satellite could not give global, 24/7 coverage as I was led to believe.

Way to back up your argument, TomG.

Still waiting on your reference for that Einstein quote you claim. You sure seem to be taking your time. Maybe you're lying?

I guess I'm a little hesitant to agree with TomG...

Doesn't gravity work at FTL speeds? If I had an instrument that were unbelievable sensitive to changes in gravity... Couldn't we communicate, lightyears apart, by simply waving our arms around?

Ah, look at that!  TomG has made a statement he can't back up!  He now tries to change the subject in true TomG fashion.  It's getting harder and harder to tell the difference between the two Toms!

Doesn't gravity work at FTL speeds? If I had an instrument that were unbelievable sensitive to changes in gravity... Couldn't we communicate, lightyears apart, by simply waving our arms around?

No, 'gravity' is subluminal.

Ah, look at that!  TomG has made a statement he can't back up!  He now tries to change the subject in true TomG fashion.  It's getting harder and harder to tell the difference between the two Toms!

1) I challenged you to back up your quote before your post; and 2) Your post doesn't ask for anything or reference the claim you're looking me to support.

We're still waiting on that reference...

Quote from: TheEngineer on July 17, 2007, 01:21:56 PM

Ah, look at that!  TomG has made a statement he can't back up!  He now tries to change the subject in true TomG fashion.  It's getting harder and harder to tell the difference between the two Toms!

1) I challenged you to back up your quote before your post;

Try keeping the threads straight, TomG. 

2) Your post doesn't ask for anything or reference the claim you're looking me to support.

Those word games sure are cute.  Too bad you used to cry about others using them.  But I guess when you talk out your ass, you have to try to do something to divert our attention...

Quote from: Gulliver on July 17, 2007, 01:30:38 PM

Quote from: TheEngineer on July 17, 2007, 01:21:56 PM

Ah, look at that!  TomG has made a statement he can't back up!  He now tries to change the subject in true TomG fashion.  It's getting harder and harder to tell the difference between the two Toms!

1) I challenged you to back up your quote before your post;

Try keeping the threads straight, TomG. 

Quote

2) Your post doesn't ask for anything or reference the claim you're looking me to support.

Those word games sure are cute.  Too bad you used to cry about others using them.  But I guess when you talk out your ass, you have to try to do something to divert our attention...

1) Time is not that relative. You first, pal.
2) Since you're too lazy to ask for what you want and too pompous to provide references for your own quotes, don't be amazed when you don't get what you want as soon as you ask for it. All of life is not a government handout.

Quote from: narcberry on July 17, 2007, 01:19:52 PM

Doesn't gravity work at FTL speeds? If I had an instrument that were unbelievable sensitive to changes in gravity... Couldn't we communicate, lightyears apart, by simply waving our arms around?

No, 'gravity' is subluminal.

Ah, thanks.

Quote from: narcberry on July 17, 2007, 01:19:52 PM

Doesn't gravity work at FTL speeds? If I had an instrument that were unbelievable sensitive to changes in gravity... Couldn't we communicate, lightyears apart, by simply waving our arms around?

No, 'gravity' is subluminal.

Evidence? I thought 'gravity', in theory, traveled at, not below, the speed of light.

From Wikipedia

In mathematical physics, particularly in the context of classical theories of gravitation, the speed of gravity refers to the speed at which gravitational field propagates. This is the speed at which changes in the distribution of energy and momentum result in noticeable changes in the gravitational field which they produce.

Where no other theory is specified, discussion of the speed of gravity is normally in reference to general relativity, which predicts it to equal c.

I believe TheEngineer has already stated that gravitation travels in waves near c.  Anyway here's an article about that very subject for your enjoyment.

http://www.physorg.com/news103902795.html

I believe TheEngineer has already stated that gravitation travels in waves near c.  Anyway here's an article about that very subject for your enjoyment.

http://www.physorg.com/news103902795.html

He's stated it both ways, at c and "subluminal" (below c). I think he just erred with "subluminal" in this thread. I've already posted experimental evidence that gravity travels at c in another thread.

How do you know it travels AT the speed of light and not JUST UNDER the speed of light or something >_<

How do you know it travels AT the speed of light and not JUST UNDER the speed of light or something >_<

I don't know. I do believe that GR is clear in its prediction that gravity travels at exactly c. I accept GR as the best theory for determining such speeds. I also put confidence in the experimental evidence that puts a reasonable confidence level about the speed of gravity that allows only a small probability that it could be "subluminal".