[John Chapman (OP)]

Thanks Verm.

Thanks Madidus_Scientia.

[yor_on]

It's a difficult concept to encompass I think So I'll give you my idea of it too.
Hopefully I'm correct here, and if I'm not, point it out )

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Awh, should have pointed this out first, all that you read down here is based on one idea. The idea that lights speed is a constant.

What Einstein did here was to look at the work published by Albert Michelson and Edward Morley. It's called the Michelson-Morley experiment and was aimed to see if there was an aether, if there was then light traveling in for example a triangle should have different speeds depending on which direction it traveled as it was expected that our relative motion (Earths motion in space) then would give different results when lights speed was measured. As they couldn't find any implications of a aether but instead found light always traveling with the 'exact' same speed at all times, regardless of what motion the object from which the light emanates had when  measured. Einstein used this as a fact and postulated that the speed of light is always exactly the same in any inertial frame.

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The idea of red and blueshift, as I see it, goes back to Einsteins special relativity theory (1905). In it he states that the 'relative velocity' (speed and direction) is like a sliding scale coupled to 'time', 'distance', 'length' and 'mass'. As two objects accelerates or moves in a uniform motion relative each other, it won't natter if they are moving towards or from each other. The speed of light as measured from one frame of reference towards the other will always get the same speed 'c' (light's speed in a vacuum).

Both 'time' and 'distance' changes depending on what 'frames' you have chosen to compare with each other. Length will be seen to contract in the direction of the motion, as observed from another frame, and the mass measured will also change depending on your uniform motion relative some other object. Remember that our definitions don't have any 'gold standard' for what 'rest' is. Once we were satisfied with looking at phenomena on our earth, and then, what we called rest was very easy to define, and still is btw, on Earth that is, in our daily environment:) but when we started to look 'out' to the stars we found that there was no 'gold standard' way to define 'being at rest'.

So our definition of 'being at rest' today is when one 'frame of reference' relative another 'frame of reference' is seen to have the same 'velocity'. Then we say that they are 'at rest' relative each other.

Einstein came to those ideas from some thought experiments. One of them involve the concept of assuming that if you were in, let's say a elevator, without windows, moving at a uniform speed in a unchanging direction (-> velocity <-). Then, if making a physical experiment, Einstein claimed that you always would get the same result, no matter what 'uniformly moving elevator' you performed your experiment from.

The 'frame of reference' we are using here is also called a 'inertial frame' as 'it' won't change its possible 'velocity'. That example clearly state that the idea of 'speed' can only be seen as a 'relative' factor, as long as it is uniform and unchanging. If you had a laser pen for example and shone it at the wall the beam wouldn't bend, no matter what 'uniform velocity' you might find yourself at if comparing your 'frame' to another 'frame of reference'.

Let's say you are parked at some star at the 'Andromeda highway' seeing me pass in a extremly fast (uniformly moving) space-train near 'c'. As I pass you I shine a laser beam up at the mirror-ceiling in my carriage. From my perspective this beam goes straight up and bounces straight down again to my laser pointer. But as you see that beam, parked in your space-car, you will tell me that the beam went diagonally up and then down at the other tangent, forming a triangle.

What this seems to mean is, from your point of view, that this beam must have traveled a longer distance than it did from my point of view (aka frame of reference), would you agree to that? And if it did so, then it also must have taken more time, as it had a longer 'distance' to travel. So now you can also say that the 'time' on my space-train is 'slowed down' as compared to the 'time' you measured my laser beam in. Or if you like, I could say that the time you used when measuring it is 'moving faster'. And in both our 'frames of reference' we are having this 'uniform motion' (not accelerating).

Space trains laser beam (distance) seen from inside the train
-->

Space trains laser beam (distance) seen from your frame of reference.
--------->

Distance is a measure of time, and, as we are describing the exact same events (causality chain).
Then time on that space train must be slower from your frame of reference.


Now, what the heck has this to do with red and blue shift? Well let's assume that you are driving your space-car at 5000 mps (miles per second) meeting another space-car with me in at the opposite lane. Mine space-car also goes at 5000 mps so when you add those two velocities you will find that the total velocity is 10 000 mps.

But as it is dark in space we both use our headlights. If you now measured the light coming from mine space-cars headlight coming at you, the speed of my light still wouldn't come at you faster than 'c'. So what can light do to 'compensate' for those relative velocities when comparing our frames, well it 'blueshifts' and express itself as having a greater energy per time unit. When we passed each other we can see each others 'tail lights' and when you measure that lights speed you will find it to be 'red shifted'. It still comes at you at 'c' but it will relative you now have a lesser 'energy per time unit' so expressed as a wave it will be 'stretched out' in time and therefore seen as 'red' when hitting your retina. The idea 'Red' and 'Blue' here is just a measure of the 'energy' contained in this wave per time unit when observed between two 'frames of reference' as I understands it here.

I'm not sure I've explained it that good, but this is about how I see it

When I think of light I see it consisting of entities, all of an unchangeable amount of energy, so called light quanta. When light interacts with matter this idea seems to be true as shown by the 'photoelectric effect' where light is seen as a stream of energy quanta with a quantum energy 'E', proportional to the light frequency 'f' and as it then is 'particles' it also will have a momentum. But then again, this seems true under some circumstances and not under other as Lightarrow pointed out.

[DoctorBeaver]

Vern - I've looked at the page you linked to in your first reply and something is puzzling me:

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?

[DoctorBeaver]

Thank goodness no-one has mentioned shear waves.

[yor_on]

And black light?

[Vern]

Vern - I've looked at the page you linked to in your first reply and something is puzzling me:

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.

[DoctorBeaver]

Why are you surprised? Beavers have very good eyesight.

[John Chapman (OP)]

Hang on. I think I’ve got another Eureka moment coming. It’s coming........  It’s coming.......

......I’ve just had an idea about the speed of sound. I previously said I accepted that the speed of sound is constant - but I didn’t 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 I’ve 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. It’s like the ripples on the surface of water. Ripples travel out in concentric circles but the molecules of water don’t actually move outwards. They just bob up and down a little as the ripple passes across them. It doesn’t 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 it’s length but the rope doesn’t 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.

[Vern]

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.

[John Chapman (OP)]

Thanks Vern. I feel confident I've got this wrapped up now.

[Matt Jakubowski]

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.

[alancalverd]

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/Pound–Rebka_experiment

[Colin2B]

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.

[nilak]

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/Pound–Rebka_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?

[puppypower]

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.