Speed of Light

russ_watters

If I understand you correctly, you are saying that reltive to a clock in its reference frame a photon travels at C and relative to a clock in a different frame, that photon doesn't travel at C? Thats one of those paradoxes that isn't a paradox. Its covered by SR/GR and its quite simple: you can't mix reference frames in that way. 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." Scintists sometimes try to make convoluted explanations like that, but its generally because they don't like the implications of what their own data/theories tell them.

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.

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

David

No, not exactly. I’m saying that if you want to arrive at a local speed of “c” for light photons, their speed must be measured locally by an atomic clock resting at a certain place on the surface of an astronomical body, and the photons have to pass by that particular clock, at very close range to the clock. And then that clock measures “c” for the photons at that clock. But if you use that same clock to measure the speed of photons that are traveling someplace else, you will measure greater or less than “c” for the speed of the photons.

This is not a paradox. The tick rates of atomic clocks go down at the same places the speed of the photons go down, and apparently in the same amount. This is not covered by SR at all. It’s not even mentioned in SR, because gravitational potential and acceleration are not mentioned in SR.

The reason I refer to an atomic clock is because it’s local rate is tied to the local speed of photons. Not all clocks slow down when and where atomic clocks slow down, or speed up when and where atomic clocks speed up.

In a real world you can mix reference frames any way you want to. But you have to know where the photons are that you are measuring, and where the atomic clock that you are using for a time reference is located , because an atomic clock resting in one place (such as at the earth), will not measure “c” for the speed of photons that are moving some place else (such as past the sun).

David

Thanks for the info. I’ve never heard of the MLET Template, but I just looked it up, and of course that is correct. The “ether” that controls the speed of light locally is apparently the fields that are generated by astronomical bodies and that move through space with the bodies.

Lorentz’s “ether” was the incorrect 19th Century version of this, but with only one large “ether” filling all of space inside a “fixed” non-expanding universe filled with “fixed” stars.

The Michelson Morley experiment was designed to detect the 18.6 mps “ether wind” that 19th Century physicists thought was present at the surface of the earth. But, it turns out that since the earth carried its own local “ether” through space with it, their experiment was like raising a flag up a flagpole inside their laboratory and being surprised that the flag didn’t flap around in the breeze that was blowing outdoors, outside their laboratory.

The most ironic thing is that, although Einstein in his 1905 paper claimed there was no “ether”, he used two of them in his theory. Each one was fixed inside each of his two relatively moving “frames”. That’s why he measured “c” for the speed of light in each frame. He literally postulated the “local ether” concept into existence in that theory. Of course, one error he made was that he had “length contraction” being cause for no particular real physics reason. That’s why he had to print a retraction in one of his 1907 papers saying that the “geometrical” shape of a relatively moving body does not contract at all, not because of “relative motion”. Of course, Lorentz had his “length contraction” due to the motion of a mass “through” his “ether”, with the “ether” putting up a resistance to the motion of the mass, thus “squeezing” it somewhat in the direction of its motion.

David

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.


“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.

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.

chroot

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

David

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.


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.

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.”

David

Here is what Einstein said about the changing speed of light in his 1911 paper:

It’s surprising how many people who talk about “relativity” have never read that paper and have no idea what that equation means.

chroot

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

David

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.

David

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.”

link to source

StarThrower

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.

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 - 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."

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. 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.

David

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.

David

“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??

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.

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.

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.