The C is the speed of photon ( light ) ?

[dailydc]

Hello everyone .
In Einstein's famous formula E=mc^2 , the C is speed of photon in Vacuum or is different ?

 

[DaveC426913]

Hello everyone .
In Einstein's famous formula E=mc^2 , the C is speed of photon in Vacuum ?

A photon does move at c in a vacuum, yes. But c is a fundamental constant of the universe; it is not specific to the speed of photons.

 

[Pengwuino]

A photon does move at c in a vacuum, yes. But c is a fundamental constant of the universe; it is not specific to the speed of photons.

Just to clarify something in case the OP gets tripped up here, 'c' is specifically the speed that a massless particle must travel at - it is indeed something fundamental to the universe. The photon has been measured to be massless to some very high precision though so people say 'c is the speed of light' with the pretty much universal implication that photons are massless.

 

[dailydc]

A photon does move at c in a vacuum, yes. But c is a fundamental constant of the universe; it is not specific to the speed of photons.

Sorry for my English .Yes , i know "C" could be attributed to the speed of gravity and other energy in vacuum .

But Someone said to me "C" is not exactly equal to the speed of photons in vacuum .
Rather "C" is limited a speed of photon in a vacuum
That Means "C" is higher that speed of photon in vacuum because a photon in vacuum arrest in bending of space .
I got confused

 

[HallsofIvy]

Sorry for my English .


Yes , i know "C" could be attributed to the speed of gravity and other energy in vacuum .

But Someone said to me "C" is not exactly equal to the speed of photons in vacuum .

Well, that someone was wrong. "c" is the speed of photons in vacuum. (Strictly speaking you should use "c", not "C".)

Rather "C" is limited a speed of photon in a vacuum
That Means "C" is higher that speed of photon in vacuum because a photon in vacuum arrest in bending of space .
I got confused

So am I. I have not idea what "a photon in vacuum arrest in bending of space".

 

[GrayGhost]

Sorry for my English .

Yes , i know "C" could be attributed to the speed of gravity and other energy in vacuum .

But Someone said to me "C" is not exactly equal to the speed of photons in vacuum .
Rather "C" is limited a speed of photon in a vacuum
That Means "C" is higher that speed of photon in vacuum because a photon in vacuum arrest in bending of space .
I got confused

My best guess, is that your "someone" was pointing out that light's speed can be measured as "slower than c" thru a gravity well, per an observer far removed fromm the well. Since gravity exists everywhere, then he may have been thinking that light should technically appear to travel just under c wrt any distant POV when considered over a very very large expanse. However no matter where one is in vacu, whether inside a gravity well or not, when light is measured in one's local area it is always precisely c.

GrayGhost

 

[WannabeNewton]

My best guess, is that your "someone" was pointing out that light's speed can be measured as "slower than c" thru a gravity well, per an observer far removed fromm the well.

How is that possible: one cannot even define velocity globally in arbitrary space - times.

 

[GrayGhost]

How is that possible: one cannot even define velocity globally in arbitrary space - times.

Well, spacetime is warped by rest mass. The photon travels along the curvature within a gravity well. Although we know the curvature is there, we don't see the dent in space. In fact, we don't even see space, as it's transparent. Instead, we see the POE appear to reside in a place that it really does not ... as a brick at the bottom of a swimming pool. Ie, lensing effects. The curved pathlength light traverses thru a gravity well is longer than the straight euclidean path it appears to take. Since it always travels at c, and since the longer curved path appears "as a shorter euclidean straight path", the photon must appear to slow down as it traverses the well, and thus it "only appears" to travel slower than c, while in reality it always exists at c everywhere, all the time, at any local spot measured.

GrayGhost

 

[WannabeNewton]

I'm saying your point doesn't make sense because you can't unambiguously define the velocity of a photon if you are making global measurements.

 

[GrayGhost]

I'm saying your point doesn't make sense because you can't unambiguously define the velocity of a photon if you are making global measurements.

Hmmm. Well, velocity is defined wrt your own coordinate system as stationary. It seems to me that spacetime is flat, with wells here and there due to mass. All the wells are in the same direction wrt flat spacetime. All wells must create a longer pathlength (non-euclidean) than what is apparent (since we perceive them as euclidean). This concept should apply globally.

Here's a couple supporting links ...

http://www.speed-light.info/speed_of_light_variable.htm" ...

In 1915 (10 years after Special Relativity) Einstein developed another theory called General Relativity that deals with gravitational fields and according to this latest theory the velocity of light appears to vary with the intensity of the gravitational field. For example, an observer outside gravitational fields measures the speed of light locally (in his location) at 299792.458 km/s but when he looks towards a black hole he sees the speed of light there to be as slow as a few meters/sec.​

http://www.physlink.com/Education/AskExperts/ae13.cfm" ...

A non-mathematical discussion of this can be found in:

'The Riddle of Gravitation,' Peter G. Bergmann, Charles Scribner's Sons, NY (1987).

See, in particular, pages 65-66 and, especially, the first full paragraph on page 66. Here, Bergmann takes the deflection of light by the gravitational field of a star as evidence of the decreased speed of light in a gravitational field.

The speed of light is _not_ constant in a gravitational field, but depends upon the reference frame of the observer. An observer anywhere in free fall will measure (locally) the traditional value of c. An observer sufficiently far away from the source of the field will conclude likewise that the speed of light is c (locally). But, the observer far away from the source will likewise conclude that the speed of light closer into the source decreases as the source is approached.​

Also ...

http://www.speed-light.info/speed_of_light_variable.htm" ... Suppose that you have a clock and a ruler (which is not rotating with respect to stars) and that you are not accelerating (inertial). Locally (where you are) you will always measure the speed of light at 299792.458 km/sec. However in the presence of gravity if I am at a different location than yours then I could measure the speed of light at your location to be any value smaller than or greater than 299792.458 km/sec. It depends on where I am and where you are (it depends on locations). So in the presence of gravity the speed of light becomes relative (variable depending on the reference frame of the observer). This does not mean that photons accelerate or decelerate. This is just gravity causing clocks to run slower and rulers to shrink.

GrayGhost

 

[harrylin]

Yes, c is a vacuum constant which can be slightly different at different places as determined with a non-local measurement. In particular:

The average speed of light (= distance / propagation time ) is measured to decrease in the presence of the Sun,
- http://en.wikipedia.org/wiki/Shapiro_delay
(Note that this does mean that light decelerates and accelerates according to the definitions of such a standard measurement system.)

Einstein predicted the bending of light near the Sun due to the gradient in c (this made him famous),
- http://www.Alberteinstein.info/gallery/gtext3.html

Harald

 

[dailydc]

Well, that someone was wrong. "c" is the speed of photons in vacuum. (Strictly speaking you should use "c", not "C".)So am I. I have not idea what "a photon in vacuum arrest in bending of space".

hello

In other words,
He said if space was flat A photon does move at "c" in a vacuum
like left of picture

But we know space is not flat and is flexible , bending , warped or something like that In an overview .
like right of pictue

So the feature of space causes Photons can not easily move at "c" in vacuum and decrease their speed . is that true ?

 

[HallsofIvy]

I believe Harrylin's post #11 addresses that.

 

[WannabeNewton]

Hmmm. Well, velocity is defined wrt your own coordinate system as stationary. It seems to me that spacetime is flat, with wells here and there due to mass. All the wells are in the same direction wrt flat spacetime. All wells must create a longer pathlength (non-euclidean) than what is apparent (since we perceive them as euclidean). This concept should apply globally.

Parallel transport of vectors is path dependent in curved space - time (\bigtriangledown_{\mu } \bigtriangledown_{\nu }U^{\alpha }\neq \bigtriangledown_{\nu } \bigtriangledown_{\mu }U^{\alpha }) so you can't unambiguously define the velocity of something globally.

 

[GrayGhost]

I'm saying your point doesn't make sense because you can't unambiguously define the velocity of a photon if you are making global measurements.

Parallel transport of vectors is path dependent in curved space - time (\bigtriangledown_{\mu } \bigtriangledown_{\nu }U^{\alpha }\neq \bigtriangledown_{\nu } \bigtriangledown_{\mu }U^{\alpha }) so you can't unambiguously define the velocity of something globally.

Well, I would agree in that you will likely never get the result totally accurate. You should be able to approximate it well enough though. You do the best you can.

When we consider the path of a photon, it travels the shortest energy-free'est path thru spacetime. IOWs, stretch a string across the surface of a globe between London and Los Angeles, pull the string taut. It will be the shortest possible path between 2 points on a curved space. We don't care about the other possible curved paths between the 2 cities. The photon always takes that energy free'est path, a straight "and shortest possible" path thru curved space between emission and detection.

The avg speed of the photon is based upon a number of assumptions, which we assume are correct, or at least most correct compared to any other competing theory. There are many issues here. First, the assumed distance the photon appears to traverse. Then the assumption regarding the precise curvature of space on the grand scale. Add, the assumption as to the extent of gravitation the photon passed thru from there to here. The photon was emitted from a point in spacetime, and thus "from an earlier epoch" ... and thus travels thru space and time "as spacetime itself expands" during its trek. All such things need taken into account.

GrayGhost

 

[WannabeNewton]

Well, I would agree in that you will likely never get the result totally accurate. You should be able to approximate it well enough though. You do the best you can.

I agree with you that global measurements of a photon's speed need not return c. All I'm saying is that due to that path dependence different observers making those global measurements will all get different values from one another so there won't be one single deviated velocity for the photon as per the global measurements (all made at the same time).

 

[bcrowell]

WannabeNewton is right. Sources like those in GrayGhost's #10 are referring to coordinate velocities, which are fundamentally meaningless. Those sources are making true statements about quantities that are of no physical interest.

 

[GrayGhost]

I agree with you that global measurements of a photon's speed need not return c. All I'm saying is that ...

due to that path dependence, different observers making those global measurements will all get different values from one another, so there won't be one single deviated velocity for the photon as per the global measurements (all made at the same time).

Well, I suppose what you say here is true. The result of observers traveling different inertial paths thru curved spacetime is (generally speaking) relative motion. 2 different observers moving relatively measure space and time differently, no matter where they are. I'm not proficient in GR, so I'll defer to another here who knows more than I do. Harrylin seems to have a handle on this, maybe he can weigh in on your response here.

GrayGhost

 

[GrayGhost]

WannabeNewton is right. Sources like those in GrayGhost's #10 are referring to coordinate velocities, which are fundamentally meaningless. Those sources are making true statements about quantities that are of no physical interest.

While I agree they are coordinate vs proper, I'm not so sure I agree that coordinate values are "fundamentally meaningless". They exist, and they have their role in physics, which cannot be ignored. To say they are meaningless is to say that the difference between 2 observer's sense of NOW is meaningless. The transmission of light signals prove that coordinate values are something more than unreal or meaningless. It's merely a matter of understanding the difference between proper vs coordinate, IMO.

GrayGhost

 

[WannabeNewton]

Physically meaningful quantities are those that remain invariant under a change of coordinates. Simple as that.

 

[GrayGhost]

Physically meaningful quantities are those that remain invariant under a change of coordinates. Simple as that.

Well, I of course realize the importance of invariants. However, I would disagree that a contraction of a moving length has no physical meaning or physical interest ... even though no material body ever changes in and of itself due to anothers POV.

GrayGhost

 

[WannabeNewton]

You can lorentz boost to the frame of the then contracted observer and get rid of any apparent contraction just like you can transform away the singularities at r = 2M in the schwarzschild metric by going to Eddington coordinates.

 

[GrayGhost]

You can lorentz boost to the frame of the then contracted observer and get rid of any apparent contraction just like you can transform away the singularities at r = 2M in the schwarzschild metric by going to Eddington coordinates.

Yes indeed, but that was not the issue at hand ...

If one sets up a laser beam that radiates orthogonal to the direction of motion wrt a passing luminal starship, the laser beam will be broken for a precise amount of time while the passing shiphull obstructs the beam. Theoretically, with fast enough processing systems, one can determine the length of the passing vessel ... assuming a controlled test where the velocity of the starship is known and verified. The contracted length is thus real per the lasing apparatus' POV ... even though we know (by SR) that no material body ever changes in and of itself (ie in its own proper length, an invariant) simply because others look at it. This all suggests to me that the contraction of a moving body is meaningful and significant because it is measurable.

GrayGhost

 

[harrylin]

WannabeNewton is right. Sources like those in GrayGhost's #10 are referring to coordinate velocities, which are fundamentally meaningless. Those sources are making true statements about quantities that are of no physical interest.

Such velocities "merely" allowed to make physical predictions that can be verified with standard measurement systems - in particular Einstein's GR prediction of light bending.

 

[rede96]

Yes indeed, but that was not the issue at hand ...

If one sets up a laser beam that radiates orthogonal to the direction of motion wrt a passing luminal starship, the laser beam will be broken for a precise amount of time while the passing shiphull obstructs the beam. Theoretically, with fast enough processing systems, one can determine the length of the passing vessel ... assuming a controlled test where the velocity of the starship is known and verified. The contracted length is thus real per the lasing apparatus' POV ... even though we know (by SR) that no material body ever changes in and of itself (ie in its own proper length, an invariant) simply because others look at it. This all suggests to me that the contraction of a moving body is meaningful and significant because it is measurable.

GrayGhost

Sorry if this is a bit off-topic but I would be interested to know if length contraction had been tested in the way you describe above? (Or just info on anyway it had been tested please.)

Thanks.

 

[harrylin]

Physically meaningful quantities are those that remain invariant under a change of coordinates. Simple as that.
[..]
You can lorentz boost to the frame of the then contracted observer and get rid of any apparent contraction just like you can transform away the singularities at r = 2M in the schwarzschild metric by going to Eddington coordinates.

Even in classical mechanics you can Galileo boost to the rest frame of a moving object and thus transform away its speed. Do you think that this makes the concept "speed" useless, so that mechanics is useful without it?

 

[harrylin]

Sorry if this is a bit off-topic but I would be interested to know if length contraction had been tested in the way you describe above? (Or just info on anyway it had been tested please.)

Thanks.

As far as I know only indirect tests have been done, such as Kennedy-Thorndike combined with Ives-Stilwell. I think that direct Lorentz contraction tests with man-made objects are not feasible with current technology. However, I vaguely remember to have read something about a possible astronomical test...

 

[rede96]

As far as I know only indirect tests have been done, such as Kennedy-Thorndike combined with Ives-Stilwell. I think that direct Lorentz contraction tests with man-made objects are not feasible with current technology. However, I vaguely remember to have read something about a possible astronomical test...

Ok, I'll have a look for some of those. Thanks.

 

[bcrowell]

WannabeNewton is right. Sources like those in GrayGhost's #10 are referring to coordinate velocities, which are fundamentally meaningless. Those sources are making true statements about quantities that are of no physical interest.

Such velocities "merely" allowed to make physical predictions that can be verified with standard measurement systems - in particular Einstein's GR prediction of light bending.

I agree that coordinate velocities are needed as tools at intermediate steps in a calculation. However, they do not correspond in any way to observations. Similarly, when we solve the Schrodinger equation for an electron, we have to use the absolute phase of the wavefunction at intermediate steps, but it's not physically meaningful in and of itself.

 

[bcrowell]

Sorry if this is a bit off-topic but I would be interested to know if length contraction had been tested in the way you describe above? (Or just info on anyway it had been tested please.)

http://www.edu-observatory.org/physics-faq/Relativity/SR/experiments.html#Length_Contraction

 

[GrayGhost]

Sorry if this is a bit off-topic but I would be interested to know if length contraction had been tested in the way you describe above? (Or just info on anyway it had been tested please.) Thanks.

No, it has not.

Because we cannot get any macro body to accelerate to a luminal enough velocity, there has been no direct measurement of length contraction to date. We accelerate subatomic particles in the supercolliders, however being so tiny, there's no hope there. However, we have verified these relativitic effects by experiment just to name a few ... relativistic mass increase, time dilation, frame dragging, moving clocks tick slower, higher clocks tick slower, cosmological redshift, etc. Since all relativistic effects arise in unison with relative motion, it's rather unfathomable to imagine a body would not length contract when moving relatively ... all the relativistic effects are differing aspects of a single underlying mechanism.

GrayGhost

 

[WannabeNewton]

Even in classical mechanics you can Galileo boost to the rest frame of a moving object and thus transform away its speed. Do you think that this makes the concept "speed" useless, so that mechanics is useful without it?

But that is again coordinate speed. You can do the same thing in relativity. What is meaningful however, with regards to the object's 4 - velocity, is U^{\alpha }U_{\alpha } because it will not change when you define new coordinate functions and apply transformation laws to all respective quantities.

 

[dailydc]

A photon does move at c in a vacuum, yes. But c is a fundamental constant of the universe; it is not specific to the speed of photons.

Hello everyone again
I research hard but answer is no !
So what's your idea ?See my explanation and Argument
Refer to the link
https://secure.wikimedia.org/wikipedia/en/wiki/Speed_of_light
And this explanation

The speed of light (meaning speed of light in vacuum), usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second

focus on vacuum
Means that vacuum is perfect vacuum
But we know that there are no perfect vacuum because free-space have minimum of energy
Refer to the link
http://en.wikipedia.org/wiki/Vacuum_energy
So we have real vacuum or partial vacuum and we can not find any perfect vacuum

Physicists often discuss ideal test results that would occur in a perfect vacuum, which they simply call "vacuum" or "free space", and use the term partial vacuum to refer to real vacuum.

Conclusion :if photons travel or move at a perfect vacuum there are does move at C
But there are no perfect vacuum at all and photons move at partial vacuum so they lose a little of their speed .

So
photons does not move at "c" in our universe even if they are in free-space .

Hope undesrtand

 

[DrGreg]

Actually photons always travel at c (whether they are in a vacuum or not) even though light, in an imperfect vacuum, may travel slightly slower than c.

When a photon hits an atom, it may be absorbed by the atom, then, after a short delay, another photon is emitted. Delays like this mean that the average speed of light can be less than c even though each individual photon travels at exactly c.

The above explanation is over-simplified. For a more accurate explanation see the FAQ https://www.physicsforums.com/showthread.php?t=511177

 

[harrylin]

Hello everyone again
I research hard but answer is no ! So what's your idea ?
[..]
if photons travel or move at a perfect vacuum there are does move at C
But there are no perfect vacuum at all and photons move at partial vacuum so they lose a little of their speed .

So photons does not move at "c" in our universe even if they are in free-space . Hope undesrtand

Put like that, photons are never in perfectly free space. But I think that in high vacuum light (incl.photons) propagates at 299,792,458 m/s. That's perfect enough for me.

 

[harrylin]

[..] coordinate velocities[..] do not correspond in any way to observations. [..].

??!

But that is again coordinate speed. You can do the same thing in relativity. [..]

Exactly, and coordinate speed - which is what is actually measured - is just as useful there. Again: do you really think that coordinate speed in mechanics is useless, so that rocket science and GPS are useless too?!

 

[bcrowell]

??!

Exactly, and coordinate speed - which is what is actually measured - is just as useful there. Again: do you really think that coordinate speed in mechanics is useless, so that rocket science and GPS are useless too?!

No, coordinate speed is never measured. It can't be measured, because it has a different value if you describe the problem in different coordinates. For example, the redshift of a photon from a distant galaxy is what is measured. The coordinate velocity of the distant galaxy is not measured; it isn't even defined unless I first choose coordinates.

The distinction in Newtonian mechanics is unimportant, since Newtonian mechanics has an unambiguously defined way of talking about the velocity of object A relative to distant object B. GR doesn't.

I don't claim that coordinate-dependent quantities are completely useless. They are often useful, and I said so in #29, where I made the analogy with the phase of an electron's wavefunction, which is useful but not measurable. GPS uses coordinates called earth-centered inertial (ECI). They are useful. It's necessary for GPS to choose some coordinates. However, the choice is arbitrary, and therefore the coordinates don't correspond to measurable quantities.

 

[DrGreg]

Exactly, and coordinate speed - which is what is actually measured - is just as useful there. Again: do you really think that coordinate speed in mechanics is useless, so that rocket science and GPS are useless too?!

As a general rule, in curved spacetime coordinate values are not directly measured; you usually need to apply some conversion factor, except at those events where the components of the metric are equal to those of the Minkowski metric. That's what the metric is for, to convert coordinate values into "physically meaningful" quantities such as proper time and proper length.

I don't think anyone has claimed that coordinate speed is useless; the expressions used were "not physically meaningful" and "of no physical interest". My previous paragraph supports those views.

The only speed you can measure directly is between two particles whose worldlines intersect at a common event. When particles are separated you run into the old "parallel transport" problem and you have to agree on an artificial (i.e. coordinate-dependent) convention on how to parallel-transport one vector to the other. Having such a convention can be extremely useful but nevertheless the resulting coordinate velocity isn't "physically meaningful".

 

[bcrowell]

Not to beat a dead horse, but here's another point to help try to explain what DrGreg and I have been saying in the last couple of posts. Let's take a specific galaxy, UDFy-38135539, which is one of the most distant ever observed. You can find information about it by googling. Its redshift is 8.55. What is its coordinate speed relative to us? The answer is that its coordinate speed can be anything I like. I can choose coordinates in which a galaxy moving with the Hubble flow has all its coordinates constant except for t; in such coordinates, both UDFy-38135539 and our own galaxy have coordinate speeds that are nearly zero. I can choose other coordinates such that our own coordinate speed is still zero, but UDFy-38135539's is some fraction of c. I can also make it equal to c, or greater than c.

 

[dailydc]

Actually photons always travel at c (whether they are in a vacuum or not) even though light, in an imperfect vacuum, may travel slightly slower than c.

When a photon hits an atom, it may be absorbed by the atom, then, after a short delay, another photon is emitted. Delays like this mean that the average speed of light can be less than c even though each individual photon travels at exactly c.

The above explanation is over-simplified. For a more accurate explanation see the FAQ https://www.physicsforums.com/showthread.php?t=511177

Thank you for good replying .
Can i really trust to the Article of Sir ZapperZ ?
As a science it has to be invoked ?
It was a good information for me .

 

[bcrowell]

Can i really trust to the Article of Sir ZapperZ ?

That one was written before I was here, but anyway our current process for posting a FAQ entry is that it goes through a process of review by the mentors, and it doesn't get posted unless it reflects their consensus. Of course, the best thing is to be able to evaluate for yourself whether you trust a particular argument, rather than trying to figure out whether the source is authoritative.

 

[atyy]

So the feature of space causes Photons can not easily move at "c" in vacuum and decrease their speed . is that true ?

In some sense this is true. For example, the paper first describing http://en.wikipedia.org/wiki/Shapiro_delay" says "Because, according to the general theory, the speed of a light wave depends on the strength of the gravitational potential along its path, these time delays should thereby be increased ..."

However, the "speed of light over a large distance" and a "gravitational potential" are not always meaningful concepts, and are a somewhat looser way of speaking.

Instead, the more general idea is that the *local* speed of light is always the same. That idea completely determines the paths of light rays in curved spacetime. An equivalent way of stating this is "light rays follow null geodesics".

 

[harrylin]

In some sense this is true. For example, the paper first describing http://en.wikipedia.org/wiki/Shapiro_delay" says "Because, according to the general theory, the speed of a light wave depends on the strength of the gravitational potential along its path, these time delays should thereby be increased ..."
[..]
Instead, the more general idea is that the *local* speed of light is always the same. That idea completely determines the paths of light rays in curved spacetime. An equivalent way of stating this is "light rays follow null geodesics".

A precision here, as "local" can stand for different things: with "the local speed of light is always the same" you surely mean, not the local speed of light along its path as determined with a single reference standard for length and time (Shapiro, GPS, etc), but the local speed of light as measured with multiple *local reference systems* along its trajectory.


Harald

 

[harrylin]

[..] The only speed you can measure directly is between two particles whose worldlines intersect at a common event. [..]

No, coordinate speed is never measured. It can't be measured, because it has a different value if you describe the problem in different coordinates. For example, the redshift of a photon from a distant galaxy is what is measured. The coordinate velocity of the distant galaxy is not measured; it isn't even defined unless I first choose coordinates.[...]

As this is rather off-topic, here's my last comment. Measuring is based on a chosen measurement system with measurement standards to which one relates. Again: GPS definitely uses coordinate speed; and I guess that most people would agree that what GPS does, is "measuring".

I don't claim that coordinate-dependent quantities are completely useless. [..]

Good that that misunderstanding has been settled. Harald

 

[dailydc]

Actually photons always travel at c (whether they are in a vacuum or not) even though light, in an imperfect vacuum, may travel slightly slower than c.

When a photon hits an atom, it may be absorbed by the atom, then, after a short delay, another photon is emitted. Delays like this mean that the average speed of light can be less than c even though each individual photon travels at exactly c.

The above explanation is over-simplified. For a more accurate explanation see the FAQ https://www.physicsforums.com/showthread.php?t=511177

Hello again

Everyone can explain why speed of photon decrease in a gravitational field or Bose–Einstein condensate ?

We Know bosons aren't atom

 

[harrylin]

Hello again

Everyone can explain why speed of photon decrease in a gravitational field or Bose–Einstein condensate ?
We Know bosons aren't atom

"A Bose–Einstein condensate (BEC) is a state of matter of a dilute gas"
- http://en.wikipedia.org/wiki/Bose–Einstein_condensate


Harald

 

[dailydc]

"A Bose–Einstein condensate (BEC) is a state of matter of a dilute gas"
- http://en.wikipedia.org/wiki/Bose–Einstein_condensate


Harald

Yes , You're right .
I must see english wikipedia
Since english is not my first or seconds language , i always see our wikipedia language
And in that page was no reference to this case . sorry

But

Can you explain why speed of photon decrease in a gravitational field ?

 

[Dale]

Can you explain why speed of photon decrease in a gravitational field ?

It doesn't decrease in any inertial frame, but with tidal gravity present there are no global inertial frames.

 

[harrylin]

Yes , You're right .
I must see english wikipedia
[..]
But
Can you explain why speed of photon decrease in a gravitational field ?

Wikipedia isn't very reliable but usually it's a good place to start.

About the "why" we can only form hypotheses, and different people tend to give different answers to "why" questions - but see my post #11.

In other words, the speed of light (as well as many other things) is determined by the local state of the space through which it propagates. Thus Einstein came to admit: "the fact that "empty space" in its physical relation is neither homogeneous nor isotropic, [...] has, I think, finally disposed of the view that space is physically empty."
- http://en.wikisource.org/wiki/Ether_and_the_Theory_of_Relativity

 

[dailydc]

It doesn't decrease in any inertial frame, but with tidal gravity present there are no global inertial frames.

So what's shapiro effect ?