I'm gonna guess that... umm... the posts are deleted when we talk about multiple things... so, one example at a time from me.

How do you guys explain the doppler effect. When a star is traveling away from us, the frequency of light from that star changes. Two things determin frequency of a wave: speed and wavelength. The star's chemical composition doesn't change when it's speed changes, so the wavelength should stay the same... the only thing that changes when the star changes speed.... is speed.

So... Because we observe light traveling at different speeds relative to us based on the speed of it's source... how can it be said that speed travels the same speed relative to all frames of reference?
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Originally posted by babooshka4761

How do you guys explain the doppler effect. When a star is traveling away from us, the frequency of light from that star changes. Two things determin frequency of a wave: speed and wavelength. The star's chemical composition doesn't change when it's speed changes, so the wavelength should stay the same... the only thing that changes when the star changes speed.... is speed.

So... Because we observe light traveling at different speeds relative to us based on the speed of it's source... how can it be said that speed travels the same speed relative to all frames of reference?

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And btw Babooshka, the doppler effect has a very nice everyday example. Stand on a street corner and listen to car engine sounds. As they come toward you the pitch is higher than when they pass and recede. The speed of sound is more or less a constant in the area, so the only effect of speed is a wavelength (frequency) change. Simple as that, the universe does the same thing with light only on a grander scale.

Copernicus said it well, speed affects _all_ wavelengths, so all that happens to the spectrum of a star is it gets shifted up or down by an amount proportional with relative speed.

Daniel

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Careful, dciobota. The doppler effect for sound is different, because sound travels through a medium.

copernicus2:
If an object (like a meteor, say) falls into a gravitational well, it gains energy i.e. it speeds up - kinetic energy. A photon falling into a gravitational well gains energy also. Because the energy of a photon is simply given by E = hf, this corresponds to an increase in frequency i.e. a blueshift. The opposite happens to a photon climbing out of a gravitational well.
Also, the momentum of light is given by p = E/c. For individual photons, this becomes p = h/lambda.
p = mv is only correct for low speeds. So, p = mc does not exist!! The correct relativistic formula is p = mc(gamma), where (gamma) is the gamma factor given by 1/(1 - (v/c)^2)^1/2.
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Yes, you're right, but the analogy still holds, as we're talking about an travelling object emitting a wave where the speed of the wave is a constant.

I knew I'd probably catch flak on that one, just trying to show Babooshka an easy to visualize scenario.

Clear, medium-less skies,

Daniel


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Coper: "because the light is exiting a greater gravity, so that it redshifts, or it enters greater gravity so that (I am told but do not know this for sure, or why) it then blueshifts."

BLOODY MARVELOUS! an actual logical explanation. I do agree, shifting light frequencies could be due to the planet's increase in gravity instead of motion relative to us. That's possible.

However, that would mean all that we know about the motion of the universe based on the doppler effect studies would be falsified. This would mean that assuming the source is moving due to the shifts would be false, instead of moving, they're increasing in density (thereby changing chemical composition, and possibly altering wavelength)

However... I have to disagree on your first statement, saying we've always tested light to be the same speed. I don't think those tests were done to properly conclude that. It would be like going to one McDonalds, ordering one chicken nugget and saying "all chicken nuggets from all resteraunts are exactly this shape."

We simply havn't tested it in a wide enough range of circumstances.

Unfortunately, I don't think we CAN test it properly because the limits we put on ourself based on the assumption of what we'll find when we test those limits is sort of a self fulfilling prophesy (temporarily forgot how to spell that one. I just spent 5 hours in a VERY small car... leave it alone). Basically, if we assume all chicken nuggets come from McDonalds, we will only look at the chicken nuggets from McDonalds... therefore, our findings will PROVE that all chicken nuggets are like the ones from McDonalds, when, in fact, they are not.


dcio: "And btw Babooshka, the doppler effect has a very nice everyday example. Stand on a street corner and listen to car engine sounds. As they come toward you the pitch is higher than when they pass and recede. The speed of sound is more or less a constant in the area, so the only effect of speed is a wavelength (frequency) change. Simple as that, the universe does the same thing with light only on a grander scale.

Copernicus said it well, speed affects _all_ wavelengths, so all that happens to the spectrum of a star is it gets shifted up or down by an amount proportional with relative speed."

Here's the problem with that.... compression and rarefaction of sound waves in the atmosphere happens because sound's speed is limited by the medium through which it travels. For that to be applicable to light's doppler effect, light speed would have to be limited by the medium through which it travels... thus light wouldn't be a constant in all frames of reference, proving my point.

This is why I like the doppler effect... whether light works the same as sound or not, it still proves my point.

... for the other side of the coin... assume light isn't limited by the medium through which it travels... and think of why a jet makes a sonic boom. The jet emits sound with the same frequency as always, but, because the jet is keeping up with the sound wave, the sound wave can't escape the nose of the jet... this eventually builds into a shock wave. If light behaved this way... the light wouldn't be traveling the correct speed relative to the source (it would be "stuck on the nose of the jet"). Were it not for the source keeping up with the wave, the frequency at which it's emited would stay the same. If the speed (relative to the source) is fixed, and the frequency at which it's emited is fixed, the wavelength would be fixed... the only alterable would be the speed at which the wave passes the objects.

Basically... if light acts like sound: the doppler effect would show that light isn't traveling "C" relative to it's source because it's limited by the medium. If light travels the same speed regardless of the medium, then the doppler effect would show light to travel at a speed other than "C" relative to outside observers. Either way, the law is broken.


VERY good responces on this one... actual logic instead of (term left out)... very nice people. A responce to the above would be good.

__________________

~Sir Gregor
"Before you go and discredit it, learn more physics. Don't discredit something you know nothing about - it makes you look like an idiot."

quote:

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(By Shambolic) copernicus2:
If an object (like a meteor, say) falls into a gravitational well, it gains energy i.e. it speeds up - kinetic energy. A photon falling into a gravitational well gains energy also. Because the energy of a photon is simply given by E = hf, this corresponds to an increase in frequency i.e. a blueshift. The opposite happens to a photon climbing out of a gravitational well.
Also, the momentum of light is given by p = E/c. For individual photons, this becomes p = h/lambda.
p = mv is only correct for low speeds. So, p = mc does not exist!! The correct relativistic formula is p = mc(gamma), where (gamma) is the gamma factor given by 1/(1 - (v/c)^2)^1/2.

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dciobota, Babooshka,

I think that light can also behave as a soundwave traveling through a medium, but the medium of space through which light travels does not have to be stationary, same as a wind is not stationary air and will alter the effect of Doppler shifts. Having said that, it would mean that we have not yet found any evidence of space having any kind of 'etherlike' wind to it, and so basically discarded with the ether concept all together, where it was more as a mathematical convenience then a necessary explanation of how the spacevacuum works. There is new work being done now on understanding this spacevacuum better, that it is not devoid of energy, and may in fact be energy filled. Just a possibility, thinking aloud, it may be that spacevacuum energy is so dense, so inertia rich, that in fact it is essentially stationary for all practical purposes... not here near the Earth, but way out there in the far reaches of space far from the galaxy. Conceptually, this makes sense to me, since if it is inertia rich, then it is also gravity rich, taken that the two are related, and that light coming out of deep space into the gravity light heliosphere should be redshifting naturally. But this is speculation only, since we still do not now that gravity, or inertia, is different in deep space, though there are theories of so-called 'dark matter' which makes this possibility somewhat more possible.

About your illustration:

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This is why I like the doppler effect... whether light works the same as sound or not, it still proves my point.

... for the other side of the coin... assume light isn't limited by the medium through which it travels... and think of why a jet makes a sonic boom. The jet emits sound with the same frequency as always, but, because the jet is keeping up with the sound wave, the sound wave can't escape the nose of the jet... this eventually builds into a shock wave. If light behaved this way... the light wouldn't be traveling the correct speed relative to the source (it would be "stuck on the nose of the jet"). Were it not for the source keeping up with the wave, the frequency at which it's emited would stay the same. If the speed (relative to the source) is fixed, and the frequency at which it's emited is fixed, the wavelength would be fixed... the only alterable would be the speed at which the wave passes the objects.

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It would seem to me that the nose of the spacecraft blowing through lightspeed, if it were possible, would likewise create a light shockwave, a brilliant flash, probably at such high freqencies that it would be up there with gamma rays, and make a lot of spacevacuum noise.... except our ears would not hear it.

An afterthought: What would happen to the measured lightspeed after the imagined burst through the speed of light barrier? I suspect the answer is really 'nothing'. Light measured from that superlightspeed velocity reference frame would still show as v = c, but this would be from a new reference frame which is invisible to those on the original frame from which the spaceship took off. Intriguing point, though, is what happens at the exact moment when the ship is at v = c itself? ... Gamma-ray rainbows...?


[Edited by Coppernicus2 on 03/03/04 at 05:31]
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Disclaimer: Please note the ideas expressed here by me are cutting edge theory, very speculative in nature, and not physics as it is being currently taught. Caveat lector.

http://spiff.rit.edu/classes/phys314/lectures/doppler/doppler.html

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Originally posted by tmorten
http://spiff.rit.edu/classes/phys314/lectures/doppler/doppler.html

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Thanks for the great reference on The Relativistic Doppler Effect. The diagrams and equations are so easy to understand, what a delight! The either blue or red lightshift in classical Doppler shift are exactly the same, in either positive or negative direction, so they offset if moving tangentially (across the light of sight of observer). This is later adjusted for the 'relativistic Doppler effect' in the diagram showing the centrifuge experiment, 1962 "Measurement of the Transverse Doppler Effect..", where the light is sent from a central source towards the perimeter of the centrifuge, at 9.3 cm distance, and spun at up to 35,000 rpm (or 583.3 rps). A quick calculation yields that the velocity of the receptor on the perimeter, if set at 583 rps, is traveling at approx. v = 170 m/s, which then can be applied in the relativistic equation provided in the paper. The paper then goes on to say that the measured result bears out the relativistic equation.

The centrifuge experiment, however, is not shielded from operating in Earth's gravitational field, so the relativistic result may be tainted by this fact, that light traveling at the increased velocity, i.e., 170 m/s, as measured on the perimeter of the centrifuge, is also traveling at that velocity within the gravitational field. Would this not duplicate the expected Doppler shift of light through a gravity field? It would seem that the (1962) experiment is in essence doing just that. So yes, the results will show a Doppler shift as per Einstein, but no, it is not clear as to why this lateral light shift is taking place: Is the Doppler effect due to gravity or relativity?

Also, I found curious the title of the paper in the Journal of the Optical Society of America: "An Experimental Study of the Rate of a Moving Atomic Clock," (1938). Hmm... searching for Doppler light shift in a moving atomic clock? Therefore, can it be assumed that if light shifts, so will the oscillations in Cesium-133 in like manner? This necessitates the question: Does a Doppler lightshift of atomic oscillations in an atomic clock mean "time" is slowing, or is it merely a Doppler shift of atoms slowing, and nothing more?

If yes, there is a Doppler shift relativistically observed, where time slows; but if no, there is no definitive proof time slows down.. In like manner, nor does it mean that the Doppler shift of distant cosmic light is anything more than it traveling through gravitational fields, not the very weak field we know of here on Earth, but a very strong field (dark matter like) we still do not know about -- far out there. If it is the latter, then what does it mean for the thesis space is expanding, or the origin of this expansion in a Big Bang?



__________________
I have formally 'resigned' (tactical withdraw) from the Space-Talk boards; mine were many questions, ideas, but no real answers. Thanks. 04/10/04.

Disclaimer: Please note the ideas expressed here by me are cutting edge theory, very speculative in nature, and not physics as it is being currently taught. Caveat lector.