Speed of Light Relative?

I don't want to make any more of this than absolutely necessary, Brown, but since you were being a stickler for accuracy earlier in the thread, I feel it necessary to point out that you are incorrect here.

In fact, your introductory paragraph contained a major mistake. You said..."This argument is simply a matter of the definition of the Doppler effect."

It isn't -- or at least it wasn't at the point when I commented. The "thing" being discussed was not the "doppler effect" at all. The thing being discussed -- and the thing that caused me to comment -- was "doppler shift."

Go back and read the relevent passages and you'll see that I am correct.

"Doppler shift" deals with light waves.

My comments were right on the button.

The mass of a photon, the debate

http://focus.aps.org/story/v10/st9

http://silver.neep.wisc.edu/~lakes/mu.pdf

http://www.lns.cornell.edu/spr/2000-03/msg0023183.html

http://arxiv.org/abs/hep-ph/0306245

I was being a "stickler" for acuracy of concepts. To me there is a big difference between arguing concepts, and arguing the meaning of words or terms.

Using the wrong term to describe a concept you clearly understand is not that important. But describing an idea that is wrong is a different thing altogether.



Of course they were.

Two relatively non technical articles that discuss the issue

http://newton.ex.ac.uk/aip/physnews.625.html

http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/photon_mass.html

Wonderful links Acquiunk, But I do not wish to put my dog into a fight today. Nor was I really expecting one, Best M

Just my two cents.

Reading a bood by Brian Greene, very interesting and related to general and special relativity.

The title is "The Elegant Universe", ISBN 0-375-70811-1, Vintage / Random House.

elysia, welcome to A2K

Hello,

Any ideas on how does the light in a substance (glass etc.)
behave when the substance is moving with relativistic speeds?

Light in vacuum will travel with c, and all observers will measure
the same speed c. What about different observers measuring
speed of light in a lens, for example?

Let's say this speed is c1, and so we have c/c1 = sin(phi1)/sin(phi2).
This is refraction of light, which brings rays of light to the focal point.
When the lans is travelling at a great speed along the optical axis,
it's radius of curvature will be greater (because of contraction in direction
of movement). This would mean longer focal length, if c1 stays the same.
And this would also mean that a digital camera moving with a great
speed would seem to produce blurry pictures..
Unless of course c1 changes, to compensate for the bigger radius,
which would mean observed c1' must be less than c1.
This would indicate that speed of light in a medium (glass) slows down when observed at a relativistic speed. Any comments?

There is one other point, though - the light entering 'speeding camera' suffers a red shift (camera sees the light 'blue shifted') and this also influences c1' and c1 because of dispersion.

Cheers, Relative.

Anyone care to translate that?

Relative.

You post contains a fundamental error. (It may be that I just didn't understand your point).

The problem is the phrase "moving a relative speeds". In relativity you can't say that anything is moving *unless* you compare it to something else.

Let's say we have two people watching this situation.

Person A is sitting on top of the "substance". This person says that the "substance" is not moving. (If you are sitting on top of something this is often the case). In every way imaginable and with every test person 'A' will discover that the "substance" is motionless. This includes the refraction test with light you are suggesting.

Now person B sees the substance (and person A) moving near the speed of light (i.e. "relativistic speeds"). Person B will notice that the substance is shorter than Person A says it is. Person B will also see that Person A is going slower (i.e. time for Person A is advancing at a slower rate).

I should point out that Person A sees the who situation differently. Person A sees Person B moving at near the speed of light with all of the effect we have been discussing.

Thus the dillema. Person A thinks that she is motionless and that Person B is moving really fast. Person B thinks that *he* is motionless and it is Person A that is moving very fast.

There is no way for anyone to distinguish between the two of them. It is not correct to say that either is right.

If Person A and Person B are on "substances" and see light refractions. Neither one of them will see any evidence they are moving -- even when they do experiments with light. Person A will only see evidence of motion when she looks at Person B (and vise versa).

The Michelson-Morely experiment I linked to above was the first experiment that tried to detect a "motion" by studying light. It is very close to the expeiment that I think Relative is proposing.

But, it won't work. There is no way to determine if you are moving or not without comparing your position with another object. So you can not say "I am moving.". You can only say "I am moving *relative* to that thing over there.

I rather prefer the analogy that involves several observers

One person is on a boat anchored in a river. He cannot see shore
One person is on a raft floating in a river. He cannot see shore either.
One person is ashore. He can see both boats and a bridge
One person is on a motorboat travelling upriver.
One person is on a motorboat travelling downriver.
One person is in an airplane.
One person is another galaxy.

A man on a bridge drops a rock into the river.

What is the wavelength (frequency) of the ripples as seen by an observer
Did the rock fall straight down

Who's right

ebrown_p , thank you for your post.

Yes, I was being a little sluggish in presenting my thoughts. I will try to restate this in different words.
(Since the previous post I also noticed some details..)

Let's say we have this observer A sitting on top of a big lens. Behind him at some distance f from the lens is a white screen, where the image comes into focus , from the light passing through the lens.
The light is coming in from 'infinity' - the rays are parallel - and at the speed of light c.
What happens to the light when it enters the lens? Well, it slows down a bit to some speed c1, and this causes it to change direction. Since the surface of lens is spherical, with radius R, all parallel rays converge to a single point on the screen behind (positioned at focal length f).
Now the angles of rays relative to the lens are phi1 on the incoming side, and phi2 on the outgoing side. Because of the equation
c/c1 = sin(phi1)/sin(phi2) the rays converge to a focal point.

Observer B sees observer A , together with the lens and screen, passing by at a great speed v, which is say 98% of speed of light.
Because of this great speed, the lens and the distance between the lens and the screen, are contracted in the direction of movement. The distance now becomes f' < f and the radius of the lens is now R' > R.
Observer B sees rays of light entering the lens, going through the lens, and converging towards the screen.
Now imagine the frozen frame of the scene that B sees: the geometrical equation c'/c1' = sin(phi1')/sin(phi2') should still hold. The speed c does not change (c'=c), since the speed of light in vacuum is the same for all observers, whereas phi1', phi2' and c1' can be different than phi1, phi2 and c1.
We know that a lens with longer focal length has greater R, so since R'>R it should be alsof'>f, if the lens still focuses the image on screen.
But we also know that f'<f (because of contraction of f), so the only possible answer to this is that the refractive index of the lens is now greater, thus shortening the effective focal length.
And refractive index is c/c1 for the lens as seen by A, and c/c1' for the lens as seen by B. c/c1 < c/c1', therefore c1'<c1. Thus the speed of light going through the lens, as seen by B, will be less than the speed of light going through the lens as seen by A.
Since the contraction of f' and extension of R' is not limited, refractive index c/c1' is also not limited and we can get c1' as small as we want if we pick large enough v.

Now I am not saying this is absolutely correct or that this is in any contradiction with Relativity nor anything like that.
It just seems interesting to me that the speed of light in glass would slow down when the glass is moving with a high speed .

I wonder if my thinking is correct, though

Relative

It was clear when I was in high school, it was clear when I got two degrees in Physics in the 70s, and it's clear now.

I like you, ebrown_p, but, geez, I assure you that I understand.

Sorry Brandon, I got confused in a maze of twisty little embedded quotes.

The comment you quote above should have been addressed to curious George who asked about the speed of light.

Sorry. I think you and I agree here.

K