Speed of Light

I'm sure there must be a simple explanation to this...I'm not an expert, but one of the concepts I understand is that the speed of light is constant and is independant of the motion of the observer. Why is it then, that red-shift occurs when observing distant galaxies which are moving away from us?Another concept I am given to understand is that the speed of light CAN vary according to the density of the medium in which it is travelling. My own simplistic logic therefore, would conclude that red-shift is the result of light travelling massive distances through sparsly occupied(but not empty) space.Can someone put me on the right track with my thinking please?Ian

Thread moved here from the Proposed New Topics forum.

Light doesn't shift red because its speed is changeing. The color of light is dependent on the wavelength (which depends on the frequency). As the source of light moves away from us, the frequency becomes slower which causes the wavelength to appear to be longer, which makes the light look more red. This is the Doppler Effect. You can also read about Red Shift on wikipedia.

I had to read your reply 3 times before I grasped it. But yes, that make perfect sense now. Thank you for your help.

It's the same reason a car horn sounds different when the car is moving away from you, compared to when it is stationary or moving toward you.Studying the doppler effect was one of my favorite parts of my high school physics class.

I was hoping you'd stick around for an answer to this other issue you raised: The question this raises is that if the speed of light is a fundamental constant, c, then how can it vary according to medium? Once this question is raised the answer is usually expressed more concisely, that c is actually the speed of light in a vacuum.But this isn't a very satisfactory answer, either. If one could shrink to atomic levels and climb between the molecules in a medium such as glass, would one really measure a slower speed of light in the space between molecules, which must also be a vacuum, just like the vacuum of space? Of course not.So why does a photon of light take longer to pass through a pane of glass than through the same distance in a vacuum? The answer is that it doesn't. The original photon entering the glass is not the same one that emerges. The molecules of a transparent or translucent medium absorb the photon, rise to a higher energy level, then retransmit a photon and drop back to their normal energy state. The precise molecular structure governs the direction of the new photon. In high quality optical glass, the direction is the same as the absorbed photon, except at the interfaces between mediums, such as that between glass and air. In translucent materials the direction of retransmission is somewhat more random. The extra time light takes to travel through a medium like glass is due to the time it takes for absorption and retransmission of photons. But in the space between molecules the photons of light travel at c, which is a fundamental constant.--Percy

The speed of light is constant in a vacuum. It was assumed that this speed was the fastest that a photon could travel and was used as a standard against photon propagation occurring in other environments.If our solar system is being drawn toward a black hole in the middle of our galaxy and far away other solar systems are collapsing toward the center of their galaxies, then indeed there would be a red shift even if there is no relative movements between the center of the galaxies.Our planet is moving through space around 67,000 mph but nowhere near the speed of other objects as they travel in ellipical orbits around the sun. Imagine the speeds that might be obtained around larger stars. Imagine the speed that particles could obtain falling toward a black holes.Your question is very valid. I await a measuring device other than the Dopler effect.

Could you explain that one? Wouldn't you also observe the wavelength contracting if they were going toward the black hole from a different frame of observation? Causing the light from the galaxies to change color?But what we observe is only that the wavelength is being pulled in one direction causing only a red-shift so it can ONLY indicate expantion.But if you have some reference where evidence has been found about the 'black hole/red-shift theory' you talked about I'd be really interested in reading about it.

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All great truths begin as blasphemiesI smoke pot. If this bothers anyone, I suggest you look around at the world in which we live and shut your fuckin' mouth.

The Doppler Effect is usually thought of as a source moving and its effect on the wavelength received at a frame of reference that is stationary. The same happens when the reference is moving and the source is stationary. The apparent wavelength elongates under both scenarios.Assuming that the galaxies are stationary, the stars in a distant galaxy revolving toward us would have their light shifted with shorter wavelengths. But any radial velocity of the stars caused by attraction toward the center of their galaxy (black hole or not) would diminish the effect. On the other hand, stars moving away from us would have the wavelength elongated further by the same radial velocity.When we measure the wavelength of light from a galaxy, you measure the summation of all the light we are receiving from the galaxy and we are not stationary. You receive more light from the near side of the galaxy than you do from the far side since the density of photons is decreasing the further they travel from a point source and the sources on the near side are closer. The further we are from a galaxy the more pronounced this effect would be. Thus the light from the near side would produce more waves and a more apparent elongated state at our stationary receiver. If the receiver is our planet and is moving toward the center of our galaxy, the effect would be intensified even if the galaxies were stationary.To answer your last question, I don’t have a reference to cite since I’ve been unable to find any reference that discusses the situation. So I simply question the use of Doppler and put forth a hypothesis that the results may be biased. I’d enjoy reading any reference that you know which discusses the situation as I described.

What radial velocity? Stars do not not have a radial velocity component in their motion around their galactic host (other than that caused by individual local proper motion that will be inward and outward) - there is no evidence to the contrary in any galaxy where we can monitor the individual motion of the constituent stars, and the Earth is certainly not falling towards the centre of the MMilky Way. Similarly, and rather fortunately, the Shuttle does not have a inward radial velocity component in its orbit around the Earth.

These little bobbles you speak of, those that actually exist, only amount to much with in the local group. Being random they tend to cancel each other out and are reflected by the fuzzing of the spectral line within a single galaxy and widening of the bell curve in a statistical measure of a pant load of galaxies. Expansion is cumulative and pushes the line farther and farther toward the red end of the spectrum.

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KindlyThere is a spider by the water pipe.

Yes. As the light moves toward us the wavelength is moving faster, as it is going away from us, it is going slower. If im not mistaken this process does change the color of the light though. If the galaxies were moving towards us they would not be red-shifting. I doubt you'll find any evidence for this. I'd need to see evidence for something like that. I'd figure astrophysicist and cosmologist would have already taken our rate of speed into account. I could be wrong though. Whats wrong with the Doppler? It's measurable, it makes predictions, it's falsifiable...What don't you like about it?Just because it goes against what you were able to postulate doesn't make IT wrong...I think it makes you wrong.

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All great truths begin as blasphemiesI smoke pot. If this bothers anyone, I suggest you look around at the world in which we live and shut your fuckin' mouth.

Gravity causes all revolving bodies to experience radial acceleration. Without it an object would simply fly off into space instead of orbiting. Orbiting requires maintaining enough tangential speed that a vertical component continuously offsets the gravitational attraction. If adequate tangential velocity is not obtained, the object falls back to earth.For galaxies its more complicated than orbiting earth since each object in the galaxy generates its own gravitational field. If a galaxy is collapsing, hypotheically from any point outside a galaxy that has a uniform distribution of luminous objects (except from above its axis), the objects nearest the observer will be traveling in an inward spiral - away from the observer.

Would I be the first that someone thought was wrong? A stellar group of folks I would say.I only suggest that Doppler should be confirmed by some other means before closing the book forever.

I hope cavediver missed ninth grade physics.

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KindlyThere is a spider by the water pipe.