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What is a photon if not electromagnetic field? I have always seen photons explained as waves of changing electric and magnetic amplitude. I speculate that the electric and magnetic fields that comprise photons always go to saturation at two single points. This saturation property gives rise to quantum phenomena.

If a photon slows down does it become heavier? E=mc2. If m goes up then c comes down to keep E constant.

As to what it really is, the clues are scattered throughout history and in the evidence. You and Robert Kemp have found some I know, but for the life of me I don't understand why it's speculation and why none of it is in the curriculum: the photon conveys energy, and the dimensionality of energy is stress x volume.

Or maybe the photon will have less energy when it slows down and it's mass stays constant []

Mass is interaction of particles with the Higgs field.

If a photon slows down does it become heavier? E=mc^{2}. If m goes up then c comes down to keep E constant.

So you are the Farsight I know from other places [] Good to see you here! I think we mostly agree about the photon construct. Too bad we haven't come up with some great principal that will make this view of nature useful for predicting experimental outcomes.

I think the consensus now is that the photon never really slows down. It simply follows a longer path to traverse certain materials.

But broadly speaking, you're right. See "the Weight of Light" concerning Robert Pound and Glen Rebka: ...sorry, you cannot view external links. To see them, please REGISTER or LOGIN

Yea, if you use E=mc² on a photon with mass 0, you'll get E=0...

My hunch is that the "mass" of the photon increases thus causing the space in the glass to bend. Thus the light wave bends in the glass and goes back to its original path when it is out of glass.

I'm sorry, what you say is wrong. The frequency increases because the energy increases. c stays constant.

Quote from: lightarrow on 21/08/2009 20:28:45I'm sorry, what you say is wrong. The frequency increases because the energy increases. c stays constant.It isn't wrong, lightarrow. It's unfamiliar to you and it isn't what you've been taught, but it isn't wrong. We measure the speed of light using clocks calibrated by the motion of light, hence we always measure the same value locally. But Einstein talked about the variable speed of light. See his 1911 paper ...sorry, you cannot view external links. To see them, please REGISTER or LOGINyou didn't quote all it's written:QuoteGeneral RelativityEinstein went on to discover a more general theory of relativity which explained gravity in terms of curved spacetime, and he talked about the speed of light changing in this new theory. In the 1920 book "Relativity: the special and general theory" he wrote: . . . according to the general theory of relativity, the law of the constancy of the velocity of light in vacuo, which constitutes one of the two fundamental assumptions in the special theory of relativity [. . .] cannot claim any unlimited validity. A curvature of rays of light can only take place when the velocity of propagation of light varies with position. Since Einstein talks of velocity (a vector quantity: speed with direction) rather than speed alone, it is not clear that he meant the speed will change, but the reference to special relativity suggests that he did mean so. This interpretation is perfectly valid and makes good physical sense, but a more modern interpretation is that the speed of light is constant in general relativity.The problem here comes from the fact that speed is a coordinate-dependent quantity, and is therefore somewhat ambiguous. To determine speed (distance moved/time taken) you must first choose some standards of distance and time, and different choices can give different answers. This is already true in special relativity: if you measure the speed of light in an accelerating reference frame, the answer will, in general, differ from c.In special relativity, the speed of light is constant when measured in any inertial frame. In general relativity, the appropriate generalisation is that the speed of light is constant in any freely falling reference frame (in a region small enough that tidal effects can be neglected). In this passage, Einstein is not talking about a freely falling frame, but rather about a frame at rest relative to a source of gravity. In such a frame, the speed of light can differ from c, basically because of the effect of gravity (spacetime curvature) on clocks and rulers.If general relativity is correct, then the constancy of the speed of light in inertial frames is a tautology from the geometry of spacetime. The causal structure of the universe is determined by the geometry of "null vectors". Travelling at the speed c means following world-lines tangent to these null vectors. The use of c as a conversion between units of metres and seconds, as in the SI definition of the metre, is fully justified on theoretical grounds as well as practical terms, because c is not merely the speed of light, it is a fundamental feature of the geometry of spacetime.Like special relativity, some of the predictions of general relativity have been confirmed in many different observations. The book listed below by Clifford Will is an excellent reference for further details.Finally, we come to the conclusion that the speed of light is not only observed to be constant; in the light of well tested theories of physics, it does not even make any sense to say that it varies.

General RelativityEinstein went on to discover a more general theory of relativity which explained gravity in terms of curved spacetime, and he talked about the speed of light changing in this new theory. In the 1920 book "Relativity: the special and general theory" he wrote: . . . according to the general theory of relativity, the law of the constancy of the velocity of light in vacuo, which constitutes one of the two fundamental assumptions in the special theory of relativity [. . .] cannot claim any unlimited validity. A curvature of rays of light can only take place when the velocity of propagation of light varies with position. Since Einstein talks of velocity (a vector quantity: speed with direction) rather than speed alone, it is not clear that he meant the speed will change, but the reference to special relativity suggests that he did mean so. This interpretation is perfectly valid and makes good physical sense, but a more modern interpretation is that the speed of light is constant in general relativity.The problem here comes from the fact that speed is a coordinate-dependent quantity, and is therefore somewhat ambiguous. To determine speed (distance moved/time taken) you must first choose some standards of distance and time, and different choices can give different answers. This is already true in special relativity: if you measure the speed of light in an accelerating reference frame, the answer will, in general, differ from c.In special relativity, the speed of light is constant when measured in any inertial frame. In general relativity, the appropriate generalisation is that the speed of light is constant in any freely falling reference frame (in a region small enough that tidal effects can be neglected). In this passage, Einstein is not talking about a freely falling frame, but rather about a frame at rest relative to a source of gravity. In such a frame, the speed of light can differ from c, basically because of the effect of gravity (spacetime curvature) on clocks and rulers.If general relativity is correct, then the constancy of the speed of light in inertial frames is a tautology from the geometry of spacetime. The causal structure of the universe is determined by the geometry of "null vectors". Travelling at the speed c means following world-lines tangent to these null vectors. The use of c as a conversion between units of metres and seconds, as in the SI definition of the metre, is fully justified on theoretical grounds as well as practical terms, because c is not merely the speed of light, it is a fundamental feature of the geometry of spacetime.Like special relativity, some of the predictions of general relativity have been confirmed in many different observations. The book listed below by Clifford Will is an excellent reference for further details.Finally, we come to the conclusion that the speed of light is not only observed to be constant; in the light of well tested theories of physics, it does not even make any sense to say that it varies.

In that link:...sorry, you cannot view external links. To see them, please REGISTER or LOGINyou didn't quote all it's written:

The use of c as a conversion between units of metres and seconds, as in the SI definition of the metre, is fully justified on theoretical grounds as well as practical terms, because c is not merely the speed of light, it is a fundamental feature of the geometry of spacetime.

In that link: ...sorry, you cannot view external links. To see them, please REGISTER or LOGIN which says:"The Golden Age of General Relativity is the period roughly from 1960 to 1975 during which the study of general relativity, which had previously been regarded as something of a curiosity, entered the mainstream of theoretical physics". What people are generally taught about relativity, is not quite how it is. Please note that I'm a strong supporter of relativity and Einstein, along with empirical evidence and experiment. Some people are not.

I find all this very interesting. Could someone explain how c is a fundamental feature of the geometry of spacetime please?

On the one hand the article says the interpretation is perfectly valid and makes good physical sense, and later on it says it does not even make sense. It not only contradicts Einstein, it contradicts itself.

I'm not pulling the wool over your eyes with this. People like PhysBang defend the modern interpretation by directing ad-hominems at people like me without offering any evidence or logical argument, attempting to distract you from the very simple unassailable logic supported by the evidence that's there in the history and in the observation.

The latter is "hidden in plain view". For example, imagine that I'm in a region of high gravitational potential holding a light clock in an orientation that avoids any issues of radial length contraction. PhysBang is holding an identical light clock in free space, and you're observing us both via telescopes and a TV monitor that puts both light clocks up on a split screen. What you see, is that my light goes slower than his.

What people are generally taught about relativity, is not quite how it is.

Please note that I'm a strong supporter of relativity and Einstein, along with empirical evidence and experiment. Some people are not.

Why must you provide such an obvious academic falsehood? And so poorly? You even quoted the sentence that shows that you're wrong: "Finally, we come to the conclusion that the speed of light is not only observed to be constant; in the light of well tested theories of physics, it does not even make any sense to say that it varies".

It is not an ad hominem to point out that you have a documented history of using the same deceptive practices again and again.

If you begin your logic with false premises, then it doesn't matter how good your logic is. If you were able to, on your own, derive a usable theory with a variable speed of light, then you might have a case. However, the best that you have is two mistakes made by Einstein. Indeed, you quote a single passage of text from Einstein but never address a single formula.

How could this be possible? This violates every test of the speed of light that we have ever performed. What one might see is that the light clock that you were holding is distorted by gravity. If we could see and measure the speed of light for your clock, we would see the light moving at c. If you can demonstrate otherwise, let us see your calculations.

Well, let's see how everyone has got the equations incorrect.

Some people are not supporters of people who have mistaken ideas about science with a book to sell.

What's obvious is that Einstein talked about the variable speed of light. It's there in 1911, and in 1916. And yet here you are saying it's an "obvious academic falsehood". It just won't wash, Physbang, and phlogiston just doesn't cut it.

I'm pointing out the clear evidence, there's no deception on my part. You're the one with the problem concerning the examination and discussion of this evidence.

It's Einstein's premise, and it isn't false. He started with a postulate in Special Relativity saying the speed of light is constant, and later explained that it didn't apply to General Relativity. What we read in the 1920 translation is this:"In the second place our result shows that, according to the general theory of relativity, the law of the constancy of the velocity of light in vacuo, which constitutes one of the two fundamental assumptions in the special theory of relativity and to which we have already frequently referred, cannot claim any unlimited validity. A curvature of rays of light can only take place when the velocity of propagation of light varies with position. Now we might think that as a consequence of this, the special theory of relativity would be laid in the dust. But in reality this is not the case. We can only conclude that the special theory of relativity cannot claim an unlimited domain of validity; its results hold only so long as we are able to disregard the influences of gravitational fields on the phenomena (eg of light). But Einstein was German, he didn't speak English in 1916 when he wrote this book. The word he used wasn't velocity, it was geschwindigkeit, which means both velocity and speed. This and the context tells us that when he approved the translation, he was talking about speed, like the Baez article says. He was talking about speed because even something as simple as a mirror changes the velocity of a beam of light. This was a popular science book. The word velocity here is the common usage, as in "high velocity bullet". Claiming that he was talking about a vector-quantity velocity reduces this paragraph to a ridiculous tautology. He would have been saying light curves because it changes direction. His meaning is obvious, it’s clear as a bell.

It's possible, and it's what we see.

Lightarrow will see my light moving slower than yours on his split screen. And it doesn't violate every test of the speed of light we have ever performed. What the heck do you think the ...sorry, you cannot view external links. To see them, please REGISTER or LOGIN

I stand for scientific progress, nothing more. And it isn't me with mistaken ideas, or Einstein. It's you.