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Author Topic: If the speed of light is constant, how does Red Shift happen?  (Read 20799 times)

Offline Vern

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If the speed of light is constant, how does Red Shift happen?
« Reply #25 on: 09/03/2009 12:28:32 »
Vern - I've looked at the page you linked to in your first reply and something is puzzling me:

Quote
This profound fact was recognized by H. Ziegler in 1906, two hundred years before it would finally be realized by the funded Physicists. We now know that this is the correct cause of the phenomena of relativity.

Two hundred years? I know about time dilation but are we realy in 2106 already?
Yes; back in 1995 I guessed it would be the year 2195 at least before physicists finally accepted the obvious fact that we've known since Lorentz wrote it down in 1900, before Einstein, or Michelson Morley. :)

I'm surprised that you noticed.
 

Offline DoctorBeaver

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If the speed of light is constant, how does Red Shift happen?
« Reply #26 on: 09/03/2009 12:59:40 »
Why are you surprised? Beavers have very good eyesight.
« Last Edit: 09/03/2009 13:03:09 by DoctorBeaver »
 

Offline John Chapman

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If the speed of light is constant, how does Red Shift happen?
« Reply #27 on: 15/03/2009 19:02:22 »
Hang on. I think Iíve got another Eureka moment coming. Itís coming........  Itís coming.......

......Iíve just had an idea about the speed of sound. I previously said I accepted that the speed of sound is constant - but I didnít really understand why. But it came to me in a moment of brilliance while I cooked my porridge (similar to when Archimedes devised a way of exposing himself to the women in his street without getting arrested).

What Iíve had trouble getting my head around is this: If two objects throw off a pressure wave and the second is travelling towards you, why is the second pressure wave not traveling faster than the first?

I reckon the answer is that the medium through which the pressure wave is travelling is not moving itself. Itís like the ripples on the surface of water. Ripples travel out in concentric circles but the molecules of water donít actually move outwards. They just bob up and down a little as the ripple passes across them. It doesnít matter how fast a pebble hits the water, the ripples still travel at a similar speed. Or like whipping a length of rope so that undulations travel down itís length but the rope doesnít actually travel outwards at all.

Sound is ripples in air. Is that correct? Please tell me it is Vern, Madidus Scientia, Dr B, et al.
 

Offline Vern

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If the speed of light is constant, how does Red Shift happen?
« Reply #28 on: 15/03/2009 19:45:14 »
Sound is compression waves in what ever medium it is in. It could be water or steel etc.

A sound source moving through the medium does not cause the the waves to move faster, they are simply bunched up in the direction of motion.
 

Offline John Chapman

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If the speed of light is constant, how does Red Shift happen?
« Reply #29 on: 15/03/2009 20:01:29 »
Thanks Vern. I feel confident I've got this wrapped up now.
 

Offline Matt Jakubowski

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I am a nut but If the speed of light is a constant why would the wave shift at all?  I have seen multiple explinations that make no clear sense. I don't accept any doppler wave answer due to the fact you absolutely can outrun sound.  I just want a clear example of light moving at a relative fixed speed giving different wavelengths.
 

Offline alancalverd

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Ignore sound - wholly different stuff, and its speed is not constant.

And Doppler is not the same thing as gravitational redshift.

The wavelength of light is given by L = hc/e where h is a constant, c is the speed of light (also a constant) and e  is the kinetic energy of a photon.

In moving from a low to a high gravitational potential the photon loses kinetic energy as it gains potential energy, so e decreases and L increases.

People are often confused by gravitational potential, which is zero in "deep space" and negative close to a massive object (an "attractor"). V = -GM/r where G is a constant, M is the mass of the attractor, and r the distance from the attractor.

Now consider Doppler shift. If I send out a light pulse every second, you will receive a pulse every second if I'm not moving. If I move away from you, and the speed of light is constant, the pulses will arrive at slightly longer intervals because each pulse has further to travel. So the perceived frequency of a receding source is lower, and of an approaching source is higher, than the frequency received when it is stationary.

Wavelength L = c/frequency, so L increases for a receding source and vice versa. 

The Pound-Rebka experiment was a neat proof of all this. https://en.wikipedia.org/wiki/PoundĖRebka_experiment
« Last Edit: 02/12/2016 14:20:07 by alancalverd »
 

Offline Colin2B

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I don't accept any doppler wave answer due to the fact you absolutely can outrun sound. 
At some stage you need to sort this out and understand doppler for sound, but as Alan says, it is irrelevant to this thread.
 

Offline Nilak

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Now consider Doppler shift. If I send out a light pulse every second, you will receive a pulse every second if I'm not moving. If I move away from you, and the speed of light is constant, the pulses will arrive at slightly longer intervals because each pulse has further to travel. So the perceived frequency of a receding source is lower, and of an approaching source is higher, than the frequency received when it is stationary.

Wavelength L = c/frequency, so L increases for a receding source and vice versa. 

The Pound-Rebka experiment was a neat proof of all this. https://en.wikipedia.org/wiki/PoundĖRebka_experiment

It is the proof there is a speed difference between  EM wave speed and the source speed. The difference is not measurable by the observer due to relative time dilation.

You are saying the opposite (in fact the SR theory says).


Lets imagine light is a wave that is not constant. What will happen to the frequency ? The same thing?
« Last Edit: 02/12/2016 19:26:41 by Nilak »
 

Offline puppypower

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Light can move at constant speed and also show a red shift, because light has two distinct, but connected aspects or legs. One leg moves at the speed of light. According to Special Relativity, and relative reference, something moving at C should appear to contract to a point. Therefore, since light is moving at the speed of light, all wavelengths should look the same to any inertial reference. The fact that there is a wide variety of wavelengths, based on relative reference, implies that light has a secondary leg that is inertial. Light is the bridge that spans the gap between inertial reference and the speed of light reference, displaying both properties.

Say we had a universe where space-time has nothing but energy in it. In this hypothetical scenario, there is no matter. Under these conditions there would be primarily, the speed of light reference. Energy doesn't clump, like matter, without matter (black hole), therefore space-time remains nearly totally expanded. There is some small local space-time contraction, due to the inertial legs of energy, but the movement of the energy makes these references unstable and fleeting. The stable reference is C. 

To form matter we would need at least some of the energy to gain potential, all the way into the gamma range. This would allow matter and anti-matter pairs to form, allowing stable inertial reference to appear. Notice that inertial reference; stable, only appears at extreme energy potential.

The speed of light reference reflects the ground state of the universe. The inertial leg of energy is at higher potential, while matter is at even higher potential.

Our universe net converts matter to energy, reflecting lowering of potential back to the ground state. The red shift reflect the inertial legs of energy lower potential also back to the ground state.
« Last Edit: 03/12/2016 11:46:37 by puppypower »
 
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