# Thread: Speed of Light, Speed of Source Question

1. ## Speed of Light, Speed of Source Question

Does someone have a good analogy of why the speed of the source only effects the frequency and not the velocity of light? Does it have to do with that massless quality photons have?

2. I cannot imagine a good analogy with any familiar phenomenon. Such phenomena involve classical physics, and one needs to forget everything we know about classical physics to understand Einstein's theory of relativity. We now know that classical physics is an oversimplification that was not measurably off before the Michelson-Morley experiment with a super-duper interferometer.

I will not attempt a rigorous explanation because I have forgotten most of the details I learned in college 40 years ago. A rough description under Einstein's theory is that the relative motions cause a space/time warp that causes the velocity of any photon to be invariant as measured by the observer in his or her frame of reference, regardless of the motion. Space/time is the way it is, even if it defies our old fallible ideas of common sense.

3. I think you can still make an analogy to classical waves in a medium, like waves on water or sound in air. Note that both of those exhibit speeds that are independent of the source speed, so we are onto something. What's tricky about light is that it doesn't have a medium, but you can just say that the medium is in effect your own inertial frame of reference, and come up with basically the same picture of why source speed doesn't affect wave speed as you have for water or sound. Now, why your own reference frame works like a medium for light is another matter-- but there really isn't an explanation for that, it just is. You are right that it has a lot to do with photons being massless particles, so that the speed of the wave is completely disconnected from the energy of the particle.

4. Perhaps a good way to think of it is:

Any massless object has to travel at a constant speed across the fabric of space. I know the fabric of space doesn't actually exist as we know it, for it is an analogy.

It doesn't matter what the speed of the emitter is, the massless object that was emitted will always travel at a constant speed once it is travelling through space.

If you put a ball on a slope it starts to accelerate down the slope due to gravity until it reaches its terminal speed for that gravity. However fast you are moving up the slope when you put the ball down, it will eventually end up rolling down at the same speed. The same applies to light (but gravity isn't involved in the same way!).

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## fabric of space

Originally Posted by speedfreek
Perhaps a good way to think of it is:

Any massless object has to travel at a constant speed across the fabric of space. I know the fabric of space doesn't actually exist as we know it, for it is an analogy.

It doesn't matter what the speed of the emitter is, the massless object that was emitted will always travel at a constant speed once it is travelling through space.

If you put a ball on a slope it starts to accelerate down the slope due to gravity until it reaches its terminal speed for that gravity. However fast you are moving up the slope when you put the ball down, it will eventually end up rolling down at the same speed. The same applies to light (but gravity isn't involved in the same way!).

speedfreek. Actually there is a fabric of space. There were four separate physically valid interpretations of the negative result of the Michelson-Morley experiment (Isaac Asimov,"On Physics" circa 1970)...one of them, and the most widely accepted is that there is no luminiferous ether. But there is the omnipresent neutrino sea. It is relatively isotropic far removed from masses of any sort....though it's isotropy relative to DM is unknown, and it changes its isotropy with inertial and gravitational accelerations.
So, you could say the speed of light is constant relative to the neutrino sea. Pete.

6. Originally Posted by trinitree88
So, you could say the speed of light is constant relative to the neutrino sea. Pete.
So you keep saying, but isn't it true that the Michelson-Morely experiment was swamped in solar neutrinos coming from a particular direction? So does this not show the opposite, that the speed of light is the same regardless of the center of mass frame of the neutrino sea?

7. Originally Posted by BigDon
Does someone have a good analogy of why the speed of the source only effects the frequency and not the velocity of light? Does it have to do with that massless quality photons have?
I think it has to do with the properties of space-time.

I have thought of something of an analogy; but remember analogies have limits.

You are sitting stationary (you don't move as this would muck up the analogy) Some one on a motorbike approaches you at half the speed of sound. How quickly does the sound reach you? The rider increases speed to 80% of the speed of sound; at what speed does the bike sound reach you?

You see the sound still travels at the speed of sound irrespective of the bike's speed; all that changes is the frequency. Classical doppler effect. The analogy fails if you move towards the bike. As this would increase the speed of sound from your perspective whereas this doesn't happen with light.

The reason the sound doesnt come to you quicker from the bike is because as soon as the sound is emmitted it becomes dependent on the air's abillity to transmit sound. The reason light can't come to us quicker due to a moving source is because of the properties of space-time and it's abillity to transmit/allow light.

the speed of sound is derived from the Bulk modulus (B)and the equilibrium density (ρ). These are analogically similiar (with big limitations) to the permitivity and permeabillity (correct my terms) of vacuum that limits or sets the speed of light.

8. Sirius,

That is a spot-on excellent description. The way (flat, Minkowski) space-time "works", every observer thinks he is stationary with respect to the "medium" of EM propagation, with constants mu_0 and epsilon_0.

When your metric is not flat, due to acceleration or gravity, by doing the (complex!) math of Maxwell against a "curved" metric, you can express things in a form that looks exactly a variable mu and epsilon. This "index of refraction" depends on g_00 primarily. But, you still the medium as stationary everywhere.

However, the way it works is your own coordinates, your own ruler and clock are such that you always think mu and epsilon are mu_0 and epsilon_0 and your origin, locally that is.

Now, frame dragging gets even weirder and you can interpret that as the medium actually moving! This "moving" is actually where the term "frame dragging" comes from -- inertial frames are being "dragged" along.

-Richard

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