# Speed of Light

by Kannan Kailas
Tags: light, speed
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 Originally posted by russ_watters [B]I've heard this arguement before and it boils down to "the laws of the universe are conspiring against us to make it APPEAR that the speed of light is constant." [b]
The “laws of the universe” don’t “conspire”. They do what they’re supposed to do, and if we can’t figure out exactly what they do, then we are just plain dumb.

I suppose you think that the color “red” is where you see it? Look around your room for something that is red. See it? Ok, where is the “red”?

It’s in your brain. It’s not at the place where you see it. Electromagnetic waves have no color, just like compression and vacuum waves in air have no “sound” until that wave phenomena reaches deep inside your brain in the form of electrical impulses.

 Originally posted by russ_watters [B]It appears you are ok with the implication that time isn't constant, but are not ok with the implication that DISTANCE is not constant. Thats unusual because the distance is the easier one to picture as it is also not a constant in Newtonian physics. That makes it easier for most people to accept.[b]

“Time” depends on how fast things move, vibrate, or oscillate, when compared to the motion, vibration, or oscillation rate of something else. Time is a kinetic energy/motion phenomenon, and different kinds of “clocks” speed up and slow down at different rates in the same places and under the same conditions. Anything that moves, vibrates, oscillates, or hums is a “clock”. An atomic clock doesn't represent all of "time". It only represents internally vibrating atom rates.
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 Originally posted by russ_watters The speed of light is constant. It is always measured by all observers, everywhere, to be C.
This is not true. How can you make such a bold erroneous statement? Einstein disagrees with you. He said in 1911 that the speed of light slows down when it passes near the sun. It speeds up again when the photons move away from the sun. That’s what causes a light ray to bend when it passes near the sun. You need to study his actual papers and not popular media reports about them. The media reporters just don't understand much of what he said, and they're always getting things mixed up.
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 Originally posted by David This is not true. How can you make such a bold erroneous statement? Einstein disagrees with you. He said in 1911 that the speed of light slows down when it passes near the sun. It speeds up again when the photons move away from the sun. That’s what causes a light ray to bend when it passes near the sun. You need to study his actual papers and not popular media reports about them. The media reporters just don't understand much of what he said, and they're always getting things mixed up.
No, sorry, russ is correct. Perhaps you need to take your own advice? russ actually knows quite a bit about relativity!

Every observer who builds an apparatus at rest with respect to him, and allows light to go through it, will always measure the speed of light as c.

If you built an apparatus near the sun, the apparatus would indicate that the light is travelling at c. Light always travels at c locally. When you say "light slows down near the Sun," you mean that an Earth-bound observer would say it took longer than it would've on Earth. A Sun-bound observer, however, would say it took just as long as it should've.

- Warren
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 Originally posted by chroot No, sorry, russ is correct.
Well, now, let’s see. Russ said:

“The speed of light is constant. It is always measured by all observers, everywhere, to be C.”

That’s not quite the same as what you said.

You said:

”Every observer who builds an apparatus at rest with respect to him, and allows light to go through it, will always measure the speed of light as c.”

And that’s pretty much what I said earlier:

”So, when local photon speed is measured in a certain gravitational potential by an atomic clock in that same gravitational potential, the speed is measured as being “c”.”

See? You are referring to a “local” observer observing the photons’ speed “locally”, and that is also what I was referring to. I added a specific kind of clock which we can use to time the local speed of the local photons. But Russ’ less specific statement could include “local” observers observing the motion of distant photons moving from one distant place to another.

 Originally posted by chroot If you built an apparatus near the sun, the apparatus would indicate that the light is travelling at c. Light always travels at c locally.
That's what I said, but you've got to specify what type of clock you are using for the measurement, and to get "c", an atomic clock is requried.

 Originally posted by chroot When you say "light slows down near the Sun," you mean that an Earth-bound observer would say it took longer than it would've on Earth. A Sun-bound observer, however, would say it took just as long as it should've.
Right, if both observers are using resting atomic clocks where they are located.

So, Russ’ statement is not quite correct, and it's misleading:

“The speed of light is constant. It is always measured by all observers, everywhere, to be C.”

Because a local observer at the earth will measure a slowdown in the speed of light as it passes near the sun, so we can’t say “It is always measured by all observers, everywhere, to be C”, since the earth observer would tell the sun observer, “The light slowed down when it passed near to you,” while the sun observer would say to the earth observer, “It traveled at ‘c’ when it passed me, but now it’s on the way to you traveling faster than ‘c’, relative to me.”
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Here is what Einstein said about the changing speed of light in his 1911 paper:

 Einstein said in 1911 “If we call the velocity of light at the origin of co-ordinates Co, then the velocity of light C at a place with the gravitational potential Φ will be given by the relation C = Co (1 + (Φ/C^2))”
It’s surprising how many people who talk about “relativity” have never read that paper and have no idea what that equation means.
 PF Patron Sci Advisor Emeritus P: 10,400 Yes, I think it's just a matter of semantics then. You can't really measure a photon's speed unless that photon goes through an apparatus at rest w.r.t. you -- and if it goes through an apparatus at rest w.r.t. you, you'll always measure it going at c. In other words, for every photon you actually measure with your own apparatus, you'll always get the answer c. When you're talking about observing a photon at another place (near the Sun, for example), you're not really measuring the speed of that photon directly, since it never comes through a local apparatus. You're inferring speed via other events, but not measuring it directly. If or when you do measure it directly, however, you can bet it'll be going c. Seems we're all on the same page, it's just a matter of wording. - Warren
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 Originally posted by chroot Seems we're all on the same page, it's just a matter of wording
The way you state it is good, but I think you need to add the atomic clock to your statement, and I think you need to be specific about the photons being at the clock at the time of measurement. We need to get rid of the old misconception that “the speed of light is always constant everywhere”, since that is not accurate and it is very misleading. Even if we change it to, “the speed of light is constant and is always measured by all observers, everywhere, to be c,” this is misleading too. I know it’s misleading because I see all different kinds of versions of that statement on the internet, and when I tell people that light photons slow down when they pass near a massive body, they argue with me and say that photons never slow down under any circumstances, which is incorrect.

When I try to explain to people the new Davis-Lineweaver way of thinking about photons reaching us from a distant high-z galaxy by gradually speeding up, relative to the earth, as they travel through deep space, a lot of people deny that, because they’ve been erroneously taught in high school and at universities that “the speed of light is constant everywhere”. When I ask them what they think about the Davis-Lineweaver paper, they go, “doh?” I asked a young guy who has a new PhD in astronomy what he thought of the Davis-Lineweaver paper and he went, “doh?” too. It’s a shame that our universities are so far behind in teaching this stuff correctly.
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 Originally posted by chroot Seems we're all on the same page, it's just a matter of wording

Ok, let me ask you this. What do you think of this statement:

“According to General Relativity, the wavelength of light (or any other form of electromagnetic radiation) passing through a gravitational field will be shifted towards redder regions of the spectrum. To understand this gravitational redshift, think of a baseball hit high into the air, slowing as it climbs. Einstein's theory says that as a photon fights its way out of a gravitational field, it loses energy and its color reddens. Gravitational redshifts have been observed in diverse settings.”

 P: 220 David, a few questions if you don't mind. A photon of mass m is moving through a gravitational field, caused by a star of mass M. Let the force on m exerted by M be given by: F = GMm/r^2 Where r is the distance from the center of mass of the star, to the photon. Let us presume that the mass of the star is just right, to cause this photon to travel around the star in a perfect circle. Please calculate the tangential speed of the photon. Presume the star is at rest in an inertial coordinate system.
 P: 1,443 David - my person feeling is that the speed of light is affected by gravitational fields - as you know or course, the traditional relativity interpretations relate the gravitation red shift to a change in photon frequency due to the energy associated with the gravitational potential - rather than a change in velocity - I am also convinced that the velocity of light has varied temporally - starting out at what could be said to approach infinity as time is wound backward - some years ago I derived some equations that showed the velocity of light is determined by the rate of expansion rather than vice versa - these same derivations also suggested that the expansion rate was variable (approaching infinity as t approached zero). If the new observations survive scrutiny - it will be most gratifying personally. The operative word here is "IF."
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 Originally posted by chroot Seems we're all on the same page, it's just a matter of wording.
I'm not so sure, since he's using that as the basis of an arguement that C isn't constant.
 “Time” depends on how fast things move, vibrate, or oscillate, when compared to the motion, vibration, or oscillation rate of something else. Time is a kinetic energy/motion phenomenon, and different kinds of “clocks” speed up and slow down at different rates in the same places and under the same conditions. Anything that moves, vibrates, oscillates, or hums is a “clock”. An atomic clock doesn't represent all of "time". It only represents internally vibrating atom rates.
According to SR, time itslf is a varible. You are indeed correct that different types of clocks are affected differently by their environment. However, every clock is also affected in exactly the way Einstein predicts in SR/GR - because time itself is variable.
 Ok, let me ask you this. What do you think of this statement: ...baseball hit high into the air, slowing as it climbs.
Thats fine as long as you realize its not an exact analogy. A photon doesn't slow down as it leaves a gravitational field. Both the baseball and the photon lose energy, but that is manifest in different ways.
 Russ’ less specific statement could include “local” observers observing the motion of distant photons moving from one distant place to another.
No. This is sora a catch-22, since you can't "observe the motion of distant photons."
 Because a local observer at the earth will measure a slowdown in the speed of light as it passes near the sun...
How exactly would you do that? Again, catch-22.
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 Originally posted by yogi David - my person feeling is that the speed of light is affected by gravitational fields - as you know or course, the traditional relativity interpretations relate the gravitation red shift to a change in photon frequency due to the energy associated with the gravitational potential - rather than a change in velocity -
Seems to me that Einstein said the light is emitted at a lower frequency because the harmonic oscillation rate of an atom slows down in a stronger gravitational field, thus atomic clocks “tick” more slowly in a stronger gravitational field. That's basically what the atomic clock guys at Boulder told me.

When did this idea that “light “struggles” to “climb out” of a gravitational field and “loses” some of its frequencies” begin to enter physics?? Einstein specifically said the light does not change frequencies after it is emitted.
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 Originally posted by russ_watters I'm not so sure, since he's using that as the basis of an arguement that C isn't constant. According to SR, time itslf is a varible. You are indeed correct that different types of clocks are affected differently by their environment. However, every clock is also affected in exactly the way Einstein predicts in SR/GR - because time itself is variable.
“Time itself” is based on things oscillating, vibrating, and moving. There is no “time” without motion.

How are you defining “time itself”?

Have you ever found a pendulum clock to slow down in a higher gravitational potential just as atomic clocks do and at the same rate? Pendulum clocks actually speed up in a higher gravitational potential.

Why did he say in 1911 that we couldn’t use atomic clocks in part of his thought experiment, because we had to use different kinds of clocks that weren’t affected by gravitational potential??

 Originally posted by russ_watters Thats fine as long as you realize its not an exact analogy. A photon doesn't slow down as it leaves a gravitational field. Both the baseball and the photon lose energy, but that is manifest in different ways. No. This is sora a catch-22, since you can't "observe the motion of distant photons." How exactly would you do that? Again, catch-22.
A photon loses frequencies? But Einstein said they don’t. He didn’t say they slow down when the leave a gravitational field, he said they speed up. The “baseball analogy” isn’t anywhere close to the correct analogy. That analogy implies that black holes are filled with bright white light that “struggles” to get out and then just gives up and turn around and go back to the center of the black hole.

 Originally posted by russ_watters This is sora a catch-22, since you can't "observe the motion of distant photons." How exactly would you do that? Again, catch-22.
Sure we can. When we see photons that have passed near the sun, we “observe” that the photons slowed down and changed directions when they went past the sun at close range.
 P: 1,443 David - As to Gravitational red shift - there are two common approaches to the red shift - one is that the photon frequency is lowered as it climbs out of the gravitational field - there are many books that take this approach - but there is also the view that the wavelenght is modified at the out set by the gravitational field - and that the photon frequency does not change - so I would imagine all that is left is the photon velocity that can slow - However, an example of the apparent frequency of the photon changing can be found in the cosmological red shift due to spatial expansion - as space stretches, we observe the Hubble red shift ... an effective change in the photon wavelength due to spatial distension during the period from emission to reception.
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 Originally posted by yogi David - As to Gravitational red shift - there are two common approaches to the red shift - one is that the photon frequency is lowered as it climbs out of the gravitational field - there are many books that take this approach - but there is also the view that the wavelenght is modified at the out set by the gravitational field - and that the photon frequency does not change - so I would imagine all that is left is the photon velocity that can slow - However, an example of the apparent frequency of the photon changing can be found in the cosmological red shift due to spatial expansion - as space stretches, we observe the Hubble red shift ... an effective change in the photon wavelength due to spatial distension during the period from emission to reception.

Thanks for the info. I think some of this stuff is pretty silly. I don’t know who invented the “space is expanding” idea. Einstein attributed it to Friedman. How can space “expand”? That’s ridiculous. Space is three-dimensional “nothing”, filled with moving bodies and their fields. Distance is expanding because the galaxies are moving through space away from one another. When you drive away from New York, you don’t say that “space is expanding” between you and New York. I think it’s nonsense to say that space is expanding. Funny how it expands only between the galaxies but not within them. Funny how M-31 is coming toward us but they don’t say that the space in between us and M-31 is “shrinking”.

One persistent problem they’ve had for years is trying to hold on to Einstein’s “constancy of the speed of light postulate” when it really doesn’t apply to nature. The authors of all the latest physics papers feel that they have to always agree with Einstein and pretend he never made any mistakes in the SR theory. They also have to pretend there is no “light propagating medium”, because “Einstein said so.” Modern cosmology is warping and distorting its theories while trying to explain observational results but without ever admitting Einstein made errors in his SR theory.
 PF Patron Sci Advisor Emeritus P: 10,400 Looks like this thread is heading to theory development... - Warren
 P: n/a Yogi, Look at the Harvard Tower experiment. Radiation emitted at ground level was of a lower frequency than radiation emitted at the top of the tower. Just what Einstein said in 1911, and I believe that theory was correct. But people try to explain it as light being emitted at a normal rate, then the light “struggles” to climb out of the gravity well. While it struggles, it somehow loses frequencies. The atoms are supposed to emit more frequencies at the bottom of the tower than were received at the top of the tower. So where do those lost frequencies go? If we consider them to be waves, then where do the lost waves go? No, the better explanation is that the atoms emit light of a lower frequency from the beginning. And also, the light starts out slightly slower at the bottom and speeds up a little as it leaves the stronger area of the gravitational field. But that speed change wouldn't cause a redshift at the receiver. It would only cause a slight delay in the initial light reception time at the top of the tower. The speed change is why light slows down when it passes the sun. It doesn’t “lose” frequencies at the sun and then “gain them back” when it gets away from the sun. The light just slows down as it passes the sun. We seen only bending of the light at the earth. We don’t see a redshift of the light. What a lot of physicists don’t seem to want to admit that it is not “empty expanding space” that regulates the speed of light, but evidently it’s the fields in the space that do it, and they, in effect, are the local “ethers” of space... the fields. In my opinion.
 P: 1,443 David - Roberson was the first to suggest spatial expansion as the cause of the red shift (Robertson of the Robertson/Walker metric). Almost every cosmologist takes the view that the galaxies are not traveling with respect to space - but with it. There are some profound reasons for that view. There are many so called accepted truths that are copied and endorsed in the physics community that need to be overhauled in the light of new discoveries. - - But in many cases there will be conflicting data and inconsistent interpretations - For example, I do not believe the issue of light velocity can be settled for or against SR based upon our present knowledge - just as in Einstein's 1911 paper to which you have referred - there were errors (Einstein had incorrectly determined the deflection angle of light by the Sun) - It is great to advance new ideas, but beware of rashness - its been my experience that when someone is absolutely sure they are right - they are usually wrong.

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