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Why does light have invarient speed?

by mdeng
Tags: invarient, light, speed
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Mentz114
#55
Jan7-08, 05:45 PM
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Ming,
thus can’t be explained by the principle of relativity.
You've got completely the wrong way round. It is a postulate of relativity that everyone measures the same speed for light.

The postulate is supported by the fact that the laws of physics require it to avoid contradictions.

Also, is the Wiki really the best source you have ? I must say I find your arguments incomprehensible but I don't think you understand relativity at all.
DaleSpam
#56
Jan7-08, 06:11 PM
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Quote Quote by mdeng View Post
I hope/think you are not referring to the question of mine about why light has a constant speed c to anyone and everyone.
As you noted in other posts the more scientific question is "how". Science answers "how" questions much better than "why" questions. However, that was not what I was refering to in this case.

What I am talking about is this:
Quote Quote by mdeng View Post
Well, it's not a proof, is it? "Proof by example" is not a proof
If you refuse to allow experimental evidence in the answer then you are rejecting the scientific method and therefore you are not asking a scientific question. In fact, your question of this thread appears to be a philosophical or mathematical question.
DaleSpam
#57
Jan7-08, 06:20 PM
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Quote Quote by rbj View Post
i fail to see how mdeng can accept the broader postulate of relativity, that "any law of nature should be the same at all times; and scientific investigations generally assume that laws of nature are the same regardless of the person measuring them", yet insist that a quantitative parameter of some of those laws can vary and, for some reason, needs yet another postulate to tie it down to a fixed value (at least between inertial observers). i don't get it, and i doubt that mdeng will convince me that i'm the one that's missing something in the logic here.
I tend to agree with you on this point. I have often thought that the first postulate was sufficient and that the second postulate is simply a corolary to the first one. But I have been told by rather reliable sources that I was wrong, they were actually two separate postulates, and I didn't feel strongly enough about it to argue. I have a similar "wrong but not strong" opinion about Newton's first and second laws.
phyti
#58
Jan7-08, 08:06 PM
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If the theory is based on the first and second postulates, then the speed of light 'c' is guaranteed, but the abstract mathematical manipulations will not reveal the 'how' or 'why'. The purpose of the theory is to produce numbers that agree with measurements/perceptions by the observer. To explain 'why', you have to analyze the behaviour in terms of physical processes. There is an answer to this, just as there is for time dilation.
mdeng
#59
Jan7-08, 10:06 PM
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Quote Quote by DaleSpam View Post
As you noted in other posts the more scientific question is "how". Science answers "how" questions much better than "why" questions. However, that was not what I was refering to in this case.
:) And in the sense of the article I quoted, I agree with you about 'why' vs. 'how'.

Quote Quote by DaleSpam View Post
What I am talking about is this:If you refuse to allow experimental evidence in the answer then you are rejecting the scientific method and therefore you are not asking a scientific question. In fact, your question of this thread appears to be a philosophical or mathematical question.
My statement was actually meant to refer to my misconception of rbj's reasoning as proving one postulate by another or using SR as an absolute truth. I don't refuse experimental evidence at all. That's what physics and all empirical science are about when seeking truth (or "how" :-).

My original question is about the (physical) mechanism/process, not philosophy/abstract-math.
mdeng
#60
Jan7-08, 10:10 PM
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Quote Quote by DaleSpam View Post
I tend to agree with you on this point. I have often thought that the first postulate was sufficient and that the second postulate is simply a corolary to the first one. But I have been told by rather reliable sources that I was wrong, they were actually two separate postulates, and I didn't feel strongly enough about it to argue. I have a similar "wrong but not strong" opinion about Newton's first and second laws.
I don't know what your reliable sources are, but what they told you appears to be consistent with what I have read so far (except for some loose introductory articles).
mdeng
#61
Jan7-08, 10:17 PM
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Quote Quote by phyti View Post
If the theory is based on the first and second postulates, then the speed of light 'c' is guaranteed, but the abstract mathematical manipulations will not reveal the 'how' or 'why'. The purpose of the theory is to produce numbers that agree with measurements/perceptions by the observer. To explain 'why', you have to analyze the behaviour in terms of physical processes. There is an answer to this, just as there is for time dilation.
Right, I have no issues with the revealing math results or their accuracy, but I am curious about any insights on how nature does 'c' and what this insight may tell us over and above SR. BTW, did you mean "there will be an answer" or there is one already?
rbj
#62
Jan8-08, 12:57 AM
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Quote Quote by DaleSpam View Post
I tend to agree with you on this point. I have often thought that the first postulate was sufficient and that the second postulate is simply a corolary to the first one.
i'm glad to think it wasn't just i that was going crazy. like we're in Opposite World where we get to switch who is in a subset of what. are the quantitative parameters of a law part of the law?

I have a similar "wrong but not strong" opinion about Newton's first and second laws.
as if an acceleration rate of zero is a subset of the second law. why would you think such an heretical thing?
lightarrow
#63
Jan8-08, 10:05 AM
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Quote Quote by mdeng View Post
I meant v_1=-v_2 and |v_1| = c. I guess it still holds.
In that case R itself is not defined because artgh(1) is not defined.
Note that the case v1 = c cannot however studied in SR because v1 is the speed of the moving ref. frame S' with respect to the stationary ref. frame S (v2 is the speed of the object with respect to S') and we know that no ref. frame with that speed can exist.
Xeinstein
#64
Jan8-08, 09:45 PM
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Quote Quote by mdeng View Post
What is the physics answer to the question of why light has an invariant speed
to anyone and everyone, other than this is what light is? There must be a
reason why light behaves this way (or perhaps not necessarily this way
always). I'd think something must have happened external to the light to give
it this peculiar property. Put it in another way, what's wrong with the
classical physics where velocity would follow the law of vector arithmetics,
when applied to light?

Thanks,
- Ming
It's a matter of time.
The solution, Einstein explained, lay in a reconception of the idea of time.


Einstein lifted the idea that the speed of light is constant intact from electromagnetic theory, devised forty years earlier by the Scottish-born physicist James Clerk Maxwell. Part of Einstein's larger ambition was to reconcile electromagnetism with Galilean relativity. Then one night in May 1905, after discussing the problem with his longtime friend Michele Besso, Einstein saw how to do so.

Thank you!" Einstein greeted Besso the following morning. I have completely solved the problem."

The solution, he explained, lay in a reconception of the idea of time. Any velocity is simply distance divided by time. In the case of light, though, the velocity isn't just 186,282 miles per second; according to Einstein's postulate, it's always 186,282 miles per second. It's a constant. It's on one side of the equal sign, humming along at its imperturbable rate. On the other side of the equal sign are distance and time, which become, by default, variables. They can undergo as many changes in value as you can imagine, as long as they continue to divide in such a way that the result is 186,282 miles per second. Change the distance, and you have to change the time.
You can solve the problem too.
Xeinstein
#65
Jan20-08, 11:43 AM
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Quote Quote by belliott4488 View Post
I would say that historically we first discovered that the speed of light is invariant and then from that learned the properties of space and time (as described by Special Relativity). Now that we know those properties, however, I would venture to say that it is a property of space and time that massless particles always move at the maximum speed that any object can obtain, which is also invariant for different observers. Light happens to be an example but is otherwise not special.

In other words, I'd say the invariance of the speed of light is a by-product of the underlying properties of space-time, so the question becomes, why are space and time the way they are? I doubt there's a definitive answer for that yet.
I disagree.
I'd say the underlying properties of space-time is a by-product of the invariance of the speed of light which is a postulate of SR
Doc Al
#66
Jan20-08, 11:53 AM
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Quote Quote by Xeinstein View Post
I disagree.
I'd say the underlying properties of space-time is a by-product of the invariance of the speed of light which is a postulate of SR
Don't confuse how we deduce the consequences of relativity from the usual postulates with how we interpret "why" things are the way they are. I'd agree with belliott4488 that it's space and time itself that is structured in such a way as to make anything moving with speed c have an invariant speed with respect to any frame. It's interesting that light has such a property, but not fundamental.
Xeinstein
#67
Jan20-08, 12:47 PM
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Quote Quote by Doc Al View Post
Don't confuse how we deduce the consequences of relativity from the usual postulates with how we interpret "why" things are the way they are. I'd agree with belliott4488 that it's space and time itself that is structured in such a way as to make anything moving with speed c have an invariant speed with respect to any frame. It's interesting that light has such a property, but not fundamental.
I would say Einstein postulated the invariant speed of light in his 1905 paper first.
It was Minkowski who pointed out how important the geometry of spacetime was.
Einstein himself did not at first seem to think geometrically about spacetime.
Doc Al
#68
Jan20-08, 12:52 PM
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Einstein used the invariant speed of light to deduce how space and time behaved. (It's not just a "trick of light".) That's his huge contribution. True, the full modern view of the geometry of spacetime came later.
rbj
#69
Jan25-08, 12:05 PM
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Quote Quote by Doc Al View Post
Don't confuse how we deduce the consequences of relativity from the usual postulates with how we interpret "why" things are the way they are. I'd agree with belliott4488 that it's space and time itself that is structured in such a way as to make anything moving with speed c have an invariant speed with respect to any frame. It's interesting that light has such a property, but not fundamental.
Doc, i think that it is fundamental (perhaps not yet verified experimentally) that any of these fundamental interactions, EM, gravity, weak, strong, all ostensibly "instantaneous", are all believed to have a delayed effect on a distant object when viewed by an observer that is equi-distant from the source of the action and the object affected by the action. it's not that light just happens to propagate at a speed of c. it's that light is EM and EM is one of these fundamental interactions and all of these fundamental interactions have effect that propagate at the same finite speed.
Doc Al
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Jan25-08, 12:18 PM
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Quote Quote by rbj View Post
Doc, i think that it is fundamental (perhaps not yet verified experimentally) that any of these fundamental interactions, EM, gravity, weak, strong, all ostensibly "instantaneous", are all believed to have a delayed effect on a distant object when viewed by an observer that is equi-distant from the source of the action and the object affected by the action. it's not that light just happens to propagate at a speed of c. it's that light is EM and EM is one of these fundamental interactions and all of these fundamental interactions have effect that propagate at the same finite speed.
I agree with you. Light doesn't just "happen" to have a speed equal to the apparent "speed limit" of the universe. Something more fundamental is going on.

I don't think I expressed myself very well before. My point was that relativity itself is more fundamental than just a strange consequence of the behavior of light. (Some folks argue that relativistic effects are just illusions due to the strange nature of light. They are wrong.)
Xeinstein
#71
Jan25-08, 04:28 PM
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Quote Quote by mdeng View Post
Another question I had, as posted in the Quantum group, what happens to non-photon particles that are moving at a speed close to c and are moving against each other? Is the relative speed of the two particle beams (whose sum is > c) capped by c? Relativity theory says yes. But what would be the mechanics behind this phenomenon? And would this be called "invarance of upbound of relative speed"?
Yes, the relative speed of the two particle beams (whose sum is > c) capped by c.
This is a result of time dilation. The faster you travel in space, the slower you travel in time. Nevertheless, the length of 4-velocity of any inertial observer is always c
JesseM
#72
Jan25-08, 04:50 PM
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Quote Quote by Xeinstein View Post
Quote Quote by mdeng
Another question I had, as posted in the Quantum group, what happens to non-photon particles that are moving at a speed close to c and are moving against each other? Is the relative speed of the two particle beams (whose sum is > c) capped by c? Relativity theory says yes. But what would be the mechanics behind this phenomenon? And would this be called "invarance of upbound of relative speed"?
Two particles moving at .999c and -999c relative to one observer still see a relative speed less than c in their own frames.

Yes, the relative speed of the two particle beams (whose sum is > c) capped by c.
This is a result of time dilation. The faster you travel in space, the slower you travel in time. Nevertheless, the length of 4-velocity of any inertial observer is always c
It's worth distinguishing two types of "relative motion" here. In the frame where both particles are moving in opposite directions, each one is moving at less than c, but the distance between them can increase at a rate greater than 1 light-year per year. But in each particle's own rest frame, using rulers and clocks at rest in that frame to measure the distance covered by the other particle in a given time, the other particle's speed in this frame will be less than c.


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