# Thread: light and sound question

1. im having a hard time visualizing a photon and why it sets the limit for speed in the universe.

A photon is like a wave ok and so is sound. We know that sound is the result of vibrations of materials on a macro scale. Now does this mean that the macro light that we see is a result of micro vibrations of the fabric of space-time, since waves behave similarly?

2.

3. Originally Posted by ttcfraser
im having a hard time visualizing a photon and why it sets the limit for speed in the universe.
It doesn't. The universe sets the speed limit of the photon (and everything else), not the other way around.

A photon is like a wave ok and so is sound. We know that sound is the result of vibrations of materials on a macro scale. Now does this mean that the macro light that we see is a result of micro vibrations of the fabric of space-time, since waves behave similarly?
Ripples is space-time are more of a description of gravity waves (gravitons) than light (photons). Light is self-propagating waves of electric and magnetic fields.

One difference between sound and light is that sound is a longitudinal wave (a compression wave) while light is a transverse wave (like a water wave).

4. Originally Posted by ttcfraser
im having a hard time visualizing a photon and why it sets the limit for speed in the universe.

A photon is like a wave ok and so is sound. We know that sound is the result of vibrations of materials on a macro scale. Now does this mean that the macro light that we see is a result of micro vibrations of the fabric of space-time, since waves behave similarly?
I you look carefully at the development of special relativity and the derivation of the Lorentz transformation you will find that special relativity is based on only two assumptions; 1) the laws of physics are the same in all inertial reference frames (implicit here is that there exists an inertial reference frame) and 2) the speed of light is constant in all inertial reference frames. In fact the derivation of the Lorentz transformation only requires item 1) and the assumption that there is some phenomena that propagates at a constant speed X in all inertial reference frames. One can then derive the Lorentz transformation which turns out in the usueal form with X in the role of c, the speed of light. From this it follows that ANY phenomena that propagates at a constant speed in all inertial reference frames must in fact propagate at speed X. Then one notes the experimental fact that light propagates at a constant speed in all inertial reference frames and therefore X = c.

We know that light is simply a bunch of photons. It is corpuscular. If light were a wave propagating in some medium in the manner of sound, then the speed would be constant with respect to that medium. In that case you would expect that the speed would change for an observer in motion relative to that medium, which would be inconsistent with respect to observation -- unless you simply postulate that mechanics obey the Lorentz transformation which is a a really complicated way of modeling a relatively simple situation.

5. Originally Posted by Janus
Originally Posted by ttcfraser
im having a hard time visualizing a photon and why it sets the limit for speed in the universe.
It doesn't. The universe sets the speed limit of the photon (and everything else), not the other way around.

A photon is like a wave ok and so is sound. We know that sound is the result of vibrations of materials on a macro scale. Now does this mean that the macro light that we see is a result of micro vibrations of the fabric of space-time, since waves behave similarly?
Ripples is space-time are more of a description of gravity waves (gravitons) than light (photons). Light is self-propagating waves of electric and magnetic fields.

One difference between sound and light is that sound is a longitudinal wave (a compression wave) while light is a transverse wave (like a water wave).
Ya, i realized that right before I went to bed, that is, because photons have no mass they therefore move unimpeded by the fabric of space time, and the mass objects upon it, at a space time maximum velocity of 186000m/s.

6. Originally Posted by DrRocket
We know that light is simply a bunch of photons. It is corpuscular. If light were a wave propagating in some medium in the manner of sound, then the speed would be constant with respect to that medium. In that case you would expect that the speed would change for an observer in motion relative to that medium, which would be inconsistent with respect to observation -- unless you simply postulate that mechanics obey the Lorentz transformation which is a a really complicated way of modeling a relatively simple situation.
Thats interesting but I from what I've read, a Medium dose effect the speed at which Light travels. This is apperent when you submerge a Nuclear Reactor in water, when the Reactor is running it gives off a Blue Glow called Čerenkov radiation, which is created when a Particle travels faster then the speed of light in that medium (for istance Electrons from the reactor).

7. Originally Posted by Tharghana
Originally Posted by DrRocket
We know that light is simply a bunch of photons. It is corpuscular. If light were a wave propagating in some medium in the manner of sound, then the speed would be constant with respect to that medium. In that case you would expect that the speed would change for an observer in motion relative to that medium, which would be inconsistent with respect to observation -- unless you simply postulate that mechanics obey the Lorentz transformation which is a a really complicated way of modeling a relatively simple situation.
Thats interesting but I from what I've read, a Medium dose effect the speed at which Light travels. This is apperent when you submerge a Nuclear Reactor in water, when the Reactor is running it gives off a Blue Glow called Čerenkov radiation, which is created when a Particle travels faster then the speed of light in that medium (for istance Electrons from the reactor).
Indeed, but the context is different. Dr Rocket's point was that light does not require a medium in which to propagate, unlike sound waves or water waves, or waves in a skipping rope. If it did require a medium, which in the 19th century they suspected it did, and which they therefore called the ether (or aether), that medium must be all pervasive but yet undetectable to us, because we cannot perceive any physical difference made by it. The famous Michelson-Morley experiments were geared not just to estimating the speed of light, but to finding out something about this supposed ether.

The simple solution is that light does not need a medium in which to propagate although, should it pass through a refracting medium, as you point out, different effects are noticed.

As Dr Rocket points out, the mathematics/logic of invariance actually leads us to the conclusion that light does not require a meium to propagate.

8. Originally Posted by Tharghana
Originally Posted by DrRocket
We know that light is simply a bunch of photons. It is corpuscular. If light were a wave propagating in some medium in the manner of sound, then the speed would be constant with respect to that medium. In that case you would expect that the speed would change for an observer in motion relative to that medium, which would be inconsistent with respect to observation -- unless you simply postulate that mechanics obey the Lorentz transformation which is a a really complicated way of modeling a relatively simple situation.
Thats interesting but I from what I've read, a Medium dose effect the speed at which Light travels. This is apperent when you submerge a Nuclear Reactor in water, when the Reactor is running it gives off a Blue Glow called Čerenkov radiation, which is created when a Particle travels faster then the speed of light in that medium (for istance Electrons from the reactor).
Yes a medium does affect the apparent speed of light. When I speak of the speed of light I am speaking of the speed of a photon, which is the speed of light in a vacuum. And Cerenkov radiation is the result of particles moving faster than the apparent speed of light in a medium -- quite common around nuclear reactors.

However, the apparent speed of light in a medium is the result of photons moving at the speed of light in a vacuum, interacting with electrons, being absorbed and re-emitted. The absorption and re-emission is what causes the apparent speed to be less than that in a vacuum. The photons still travel at c (the vacuum speed) in the inter-atomic vacuum.

It is the speed of light in a vacuum, c, the speed of a photon, that is the limiting speed, and the speed that is the same in all inertial reference frames.

Note that in this discussion, by a medium, we mean a physical medium, a material. We are not referring to "the fabric of space-time" or some such notion. The ratio of the speed of light in a vacuuml to the speed of light in the material is the index of refraction of the material.

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