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nothing can travel faster than the speed of light...

M

makken

Golden Member
Something I've always wondered about

If our sun were to suddenly disappear, would the earth feel its gravitation effects (ie. flung outta orbit) before we see the change? or would we retain our orbit around now-empty space until we actually see the change?
 
No mass less than the speed of light can travel faster than light. As it approaches lightspeed, it becomes infinitely more massive, which in turn requires more energy to accelerate it. That doesn't apply to gravity.

Also, there is a theoretical particle called a tachyon that is faster than light, but has the same problem in reverse, it can't be slowed down under the speed of light.
 
Originally posted by: c627627
Scientists observe sound traveling faster than the speed of light

http://www.physorg.com/news88249076.html
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These articles are so misleading...

Although such results may at first appear to violate special relativity (Einstein?s law that no material object can exceed the speed of light), the actual significance of these experiments is a little different. These types of superluminal phenomena, Robertson et al. explain, violate neither causality nor special relativity, nor do they enable information to travel faster than c. In fact, theoretical work had predicted that the superluminal speed of the group velocity of sound waves should exist.

?The key to understanding this seeming paradox is that no wave energy exceeded the speed of light,? said Robertson.
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No, gravity waves travel at the speed of light so it would NOT be instantaneous.
That is why it is hopefullyposssible to detect gravity waves; one end of the interferometer would be affected before the other.

Sagalore: A better way to think about would be to say that INFORMATION can not travel at a speed faster than c; if gravity waves were indeed instantaneous it would be possible to set up an information system that used gravity detectors and big masses; by moving mass you could transfer information faster than c to the dectors and this would be a violation of SR.
 
Originally posted by: SagaLore
No mass less than the speed of light can travel faster than light. As it approaches lightspeed, it becomes infinitely more massive, which in turn requires more energy to accelerate it. That doesn't apply to gravity.

Also, there is a theoretical particle called a tachyon that is faster than light, but has the same problem in reverse, it can't be slowed down under the speed of light.
Click to expand...

It does apply to gravity.
 
Originally posted by: TuxDave
Originally posted by: SagaLore
No mass less than the speed of light can travel faster than light. As it approaches lightspeed, it becomes infinitely more massive, which in turn requires more energy to accelerate it. That doesn't apply to gravity.

Also, there is a theoretical particle called a tachyon that is faster than light, but has the same problem in reverse, it can't be slowed down under the speed of light.
Click to expand...

It does apply to gravity.
Click to expand...

Tachyons are supposed to have negative mass or something odd...
 
Originally posted by: bobsmith1492
Originally posted by: TuxDave
Originally posted by: SagaLore
No mass less than the speed of light can travel faster than light. As it approaches lightspeed, it becomes infinitely more massive, which in turn requires more energy to accelerate it. That doesn't apply to gravity.

Also, there is a theoretical particle called a tachyon that is faster than light, but has the same problem in reverse, it can't be slowed down under the speed of light.
Click to expand...

It does apply to gravity.
Click to expand...

Tachyons are supposed to have negative mass or something odd...
Click to expand...

Was it that the square-root of their mass is negative? Something like that...
 
Originally posted by: futuristicmonkey
Originally posted by: bobsmith1492
Originally posted by: TuxDave
Originally posted by: SagaLore
No mass less than the speed of light can travel faster than light. As it approaches lightspeed, it becomes infinitely more massive, which in turn requires more energy to accelerate it. That doesn't apply to gravity.

Also, there is a theoretical particle called a tachyon that is faster than light, but has the same problem in reverse, it can't be slowed down under the speed of light.
Click to expand...

It does apply to gravity.
Click to expand...

Tachyons are supposed to have negative mass or something odd...
Click to expand...

Was it that the square-root of their mass is negative? Something like that...
Click to expand...

So, their mass is imaginary? According to Wikipedia and this article:
http://www1.shore.net/~ewall/ it may be on the imaginary plane. Huh.
 
Originally posted by: SagaLore
No mass less than the speed of light can travel faster than light. As it approaches lightspeed, it becomes infinitely more massive, which in turn requires more energy to accelerate it. ....
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This is an archiac concept. The mass of any body is invariant. Put yourself on a spaceship accelerating away from earth, do you suppose that you, the occupant, would observe a mass change? Of course not, so how can you say the mass goes to infinity? The entire concept of relativistic mass is falling in to disfavor, all that matters is the rest mass.

The concept of `relativistic mass' is subject to misunderstanding. That's why we don't use it. First, it applies the name mass--belonging to the magnitude of a four-vector--to a very different concept, the time component of a four-vector. Second, it makes increase of energy of an object with velocity or momentum appear to be connected with some change in internal structure of the object. In reality, the increase of energy with velocity originates not in the object but in the geometric properties of space-time itself."
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From this site

 
An archaic concept yes, but an accurate description and a very sound theory. Particle accelerators routinely accelerate electrons to near light speed and observe a huge increase in mass. Its no lie, relativity is real, both general and special have been experimented to death and so far have not failed any test.(Neglecting of course quantum gravity)

The tachyon theory while mathematically correct has yet to predict anything or produce measurable results--the key to any great theory. From what I've heard the standard model can explain the same phenomenon as the tachyon theory and every thing else sub-atomic to boot.(again neglecting quantum gravity)
 
Originally posted by: Biftheunderstudy
Particle accelerators routinely accelerate electrons to near light speed and observe a huge increase in mass.
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Well, yes but generally they measure that "mass" i GeV, i.e. the "fundemental" property here is energy; not mass (in kg).
As RossGr has already pointed out it is much easier to stick to identifiying mass with rest mass; it makes the interpretation of experiments much easies to understand.


 
Honestly, gravity is not understood well enough to answer your question. There is a lot of theory but no one is sure how gravity propagates. The leading theory is the graviton, which has yet to be detected, but would travel at the speed of light. Thus, theoretically, the Earth would maintain its gravitational orbit until light stopped reaching it (assuming the sun just disappeared one day).
 
Originally posted by: makken
Something I've always wondered about

If our sun were to suddenly disappear, would the earth feel its gravitation effects (ie. flung outta orbit) before we see the change? or would we retain our orbit around now-empty space until we actually see the change?
Click to expand...

This type of speculation is pretty pointless. When doing a "geduncken" experiment you must make sound physical assumption this is basicly GIGO. There is no way that the mass of the sun can simply vanish. So your principle assumption is nonphysical. How can you expect to get meaningful physical results when you are making nonphysical assumptions.
 
Originally posted by: Biftheunderstudy
An archaic concept yes, but an accurate description and a very sound theory. Particle accelerators routinely accelerate electrons to near light speed and observe a huge increase in mass. Its no lie, relativity is real, both general and special have been experimented to death and so far have not failed any test.(Neglecting of course quantum gravity)

The tachyon theory while mathematically correct has yet to predict anything or produce measurable results--the key to any great theory. From what I've heard the standard model can explain the same phenomenon as the tachyon theory and every thing else sub-atomic to boot.(again neglecting quantum gravity)
Click to expand...

Did you read the FAQ I linked to? It seems that relativistic mass is found in pop sci books. It is the explanation given by real physicist to laymen. It is NOT the concept used by those who actually work in the field. So I guess if you are happy with a baby talk explanation you should hang on to relativistic mass.. other wise drop it and do some research on how it actually works.

I am not sure where you get this idea of a "tachyon" theory as something separate from Relativity. They are one and the same, it is relativity which allows for the possible existence of tachyons. They are in the math, but does that mean they exist physically? Not a question we can answer at this time.
 
Yes the mass that you measure when you are traveling at a certain velocity is invariant, it depends on your frame of reference. The particle physicist observing in increase in the electrons mass while its traveling near the speed of light is its relitavistic mass, the only mass that matters in that case since the rest mass is only seen from the electrons point of view.

I am aware that you hardly ever deal with mass or velocity only its energy, yet the argument about the mass of the spacecraft increasing as the velocity approaches the speed of light is still valid. Its an argument against accelerating anything with mass to the speed of light regardless of which mass you use.

While tachyon theory is related and dependant on relativity it is a separate entity much like string theory is related to both quantum mechanics and general relativity yet separate. Just because its in the math doesn't mean its physical, imaginary numbers are just a tool, they have no physical meaning in any branch of physics or otherwise. All of the phenomenon related to tachyons can be described by the standard model without FTL particles.

Does a 4rth year course on General Relativity count as research?
 
Originally posted by: Biftheunderstudy
Yes the mass that you measure when you are traveling at a certain velocity is invariant, it depends on your frame of reference. The particle physicist observing in increase in the electrons mass while its traveling near the speed of light is its relitavistic mass, the only mass that matters in that case since the rest mass is only seen from the electrons point of view.

I am aware that you hardly ever deal with mass or velocity only its energy, yet the argument about the mass of the spacecraft increasing as the velocity approaches the speed of light is still valid. Its an argument against accelerating anything with mass to the speed of light regardless of which mass you use.
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I guess the fact that it is a poor or even incorrect view doesn't bother you?
While tachyon theory is related and dependant on relativity it is a separate entity much like string theory is related to both quantum mechanics and general relativity yet separate. Just because its in the math doesn't mean its physical, imaginary numbers are just a tool, they have no physical meaning in any branch of physics or otherwise. All of the phenomenon related to tachyons can be described by the standard model without FTL particles.
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I disagree with the bolded statement. Tachyons are a facet of realtivity, they can only exist if you give physical meaning to imaginary numbers. I am reluctant to take that step.
At this point in time any theory of "tachyons" is pretty much pure speculation, only distanly related to physics.

Does a 4rth year course on General Relativity count as research?
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Evidently not. Since your opinions are not backed up by relaiable reference and you are contradicting what I consider reliable (and linked) sources, I can not give them much credance.

Fourth year as in Undergrad?
 
Originally posted by: RossGr
It seems that relativistic mass is found in pop sci books. It is the explanation given by real physicist to laymen. It is NOT the concept used by those who actually work in the field. So I guess if you are happy with a baby talk explanation you should hang on to relativistic mass.. other wise drop it and do some research on how it actually works.
Click to expand...

I guess my Masters in Physics is a waste of time as well then. I should have read a FAQ and Wiki instead.
cheers
 
Originally posted by: LazyGit
Originally posted by: RossGr
It seems that relativistic mass is found in pop sci books. It is the explanation given by real physicist to laymen. It is NOT the concept used by those who actually work in the field. So I guess if you are happy with a baby talk explanation you should hang on to relativistic mass.. other wise drop it and do some research on how it actually works.
Click to expand...

I guess my Masters in Physics is a waste of time as well then. I should have read a FAQ and Wiki instead.
cheers
Click to expand...

So good of you to make such a meaning contribution to the disscussion.

Come on over to PhysicsForums and throw your weight around. BTW I do not consider Wiki a good source for this type thing. Why do you mention it? I didn't.
 
I think you may have gotten the wrong ideea about my thoughts on tachyon theory, I'm agreeing with you.

As for SR, denying the lorentz transformation does not make the relativistic mass dissapear. Saying that the mass doesn't actually change is like saying there is no such thing as time dilation or length contraction. Its a coordinate change -- M->gammaM'. If you replace the space ship with an electron you can see that the relativistic mass increases making it harder and harder to accelerate. As the electron approaches c, its mass approaches infinity--from the scientists frame of reference. If this doesn't happen what explains the difficulty in reaching c?

To get back to the original question, in Newtonian gravity the force happened instantaneously much like Coulombs law in elecromagnetism. Einstein came up with special relativity and saw that gravity had to be included in the relativistic view of the universe since it violated it under newtons view. Years later he came up with General Relativity which was a way of incorporating special relativiy into gravity. So to answer your question, according to General Relativity gravity propogates at the speed of light.

Proof by contradiction works too, if gravity were instantaneous you could imagine builiding a machine that could send signals via gravity instantaneously and then all the paradox's that occur in FTL travel could happen.
 
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