Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: John Chapman on 13/02/2009 21:18:35
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The speed of light is always constant, regardless of the position and relative speed of the observer.
For example, apparently if you were in a spaceship travelling incredibly fast and you were passed by a ship travelling at a similar speed in the opposite direction and at the same time you both passed a third stationary ship then the light from your headlights would be travelling at exactly the same speed relative to all three ships. Because speed is equal to distance divided by time then, to make this impossible thing happen, time and space will both become distorted to accommodate it. So, the speed of light is ALWAYS constant.
In that case how come light is subject to the Doppler effect and gives us Red Shift?
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According to Einstein's theory space and time are both distorted for objects moving relative to each other. There is another way to look at it that I like better. The Lorentz Aether Theory contended that all things were distorted when they moved. It did not warp space and time. All the distortion was in the material things that moved. Space and time remained constant. Things that moved experienced dimensional and time distortion. This allowed H. Ziegler to assign a cause (http://photontheory.com/Einstein/Einstein06.html#Ziegler) for relativity phenomena.
The Lorentz transformations are still used to calculate the distortions even in Einstein's theory. But the Lorentz theory is largely forgotten as is H. Ziegler.
Doppler shifting happens because motion tends to shrink and stretch waves whether it is sound or light. The constant speed of light doesn't effect the Doppler shift.
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Hi Vern
Thanks for that interesting reply. I've not come across Lorentz and H Ziegler before, although I am not a physicist. I shall have a go at reading the information you linked to - though I may not understand it.
I also didn't understand your last sentence
Doppler shifting happens because motion tends to shrink and stretch waves whether it is sound or light. The constant speed of light doesn't effect the Doppler shift.
That seems counterintuative. But then so does most of quantum physics to me! How can the waves be stretched by motion without the speed being effected?
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That isn't a QM thing. It's what happens with sound waves as well. If you are approaching the source of some waves then you bump into the peaks of the waves at a higher rate than the source was emitting them - giving you a higher frequency. The same sort of argument applies when either source or detector moving towards or away. Once launched, the waves travel at their normal wave speed.
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That seems counterintuative. But then so does most of quantum physics to me! How can the waves be stretched by motion without the speed being effected?
Think about if you were walking along a path and dropping a cookie every second so you could follow the return path. If you walk slow, the cookies will be closer together than if you walk fast. If you run along, still dropping a cookie every second, the cookies will be further apart still; they will in effect be red shifted.
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Thanks for entering the discussion, sophiecentaur
Don't the waves only travel at their normal speed relative to their source? Surely the sound of a police siren is approaching your ears faster as the car travels towards you than it does when the car has passed and is travelling away?
Isn't this the same as saying that if you shoot a tree from a train that's travelling at 200 mph it doesn't matter if you shoot it as the train approaches the tree or as the tree disappears into the distance? The bullet will hit it at the same speed!
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Hi Vern
If you run along dropping the cookies doesn't that mean that the leading edge of the line of biscuits is approaching you at a faster speed?
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It isn't the leading edge that makes the distortion; you need the whole distance between biscuits [:)] The wave length of light determines its colour; the wavelength is the distance from crest to crest, or as in the analogy, from cookie to cookie.
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Hold on. Let me think about this. This is beginning to make my head hurt!
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But the thing is that light is supposed to travel at the one speed.
Didn't Einstein say that two light beams traveling toward one would only appear to an observer to be approaching one another at the speed of light.So here is were I get confused with red shift if I as the observer only observe light traveling at the one speed how can I observe a red shift.This would suggest to me that I am seeing light at a speed different to the speed of light and if I understand Einstein that is not possible.
Cheers
justaskin
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The speed of light is always constant, regardless of the position and relative speed of the observer.
For example, apparently if you were in a spaceship travelling incredibly fast and you were passed by a ship travelling at a similar speed in the opposite direction and at the same time you both passed a third stationary ship then the light from your headlights would be travelling at exactly the same speed relative to all three ships. Because speed is equal to distance divided by time then, to make this impossible thing happen, time and space will both become distorted to accommodate it. So, the speed of light is ALWAYS constant.
In that case how come light is subject to the Doppler effect and gives us Red Shift?
λ = c/ν
Since c is constant and invariant, if the frequency ν is lower, wavelenght λ is greater and you have red shift.
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λ = c/ν
Since c is constant and invariant, if the frequency ν is lower, wavelenght λ is greater and you have red shift.
Eh? Is it possible to convert that answer into everyday language?
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Hi Vern
If you run along dropping the cookies doesn't that mean that the leading edge of the line of biscuits is approaching you at a faster speed?
It's not the right analogy, though.
The waves travel through the air (let's talk about sound because it's more familiar) at he same speed, however fast you are going - why? Because, once you have 'let go of them' you can have no more influence on them.
Treat it like a very long train, going slowly through a station. Stand still and they will go past you at, say, on every second. If you run along beside the train, in the same direction, they will pass you at a slower rate - which would be zero if you ran at the speed of the train.
Running against the direction of the train will speed up the rate that you pass coaches.
The sound waves are the coaches and the frequency is how often they pass by you.
The formula
λ = c/f
can be rewritten as
c = λf
c is the speed of the waves
f is the frequency ('real' Physicists tend to use the symbol ν - the greek letter "nu" but that's just likely to scare you off even more. f stands for frequency for most mortals)
λ is the wavelength.
So speed = wavelength times frequency
Why?
Back to the train: If the coaches are going past you (frequency) at 2 per second and they are 20m long (wavelength) then the speed of the train could be calculated by multiplying the number of coaches going pas per second by the length of a coach (=40m/s). So the wave speed is, similarly, frequency times wavelength.
To appreciate the red shift effect, it is not necessary to get relativistic. Just treat the light waves as traveling through space at speed c and you still have your train going through the station and the 'doppler shift'.
You will observe the speed of sound (and the train) to be different, according to which direction and how fast you, as observer go. As it happens, relativity tells us that, however fast you are going, you will always measure the speed of light as the same value - but this is a bit of a red (shifted?) herring as far as getting an understanding of the basic red-shift mechanism. The sums come out a bit different but get used to the classical world first - then get relativistic.
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I’ve just had a eureka moment about the Doppler effect. I think I’ve got it.
Instead of a police siren consider an approaching speaker. As the speaker produces sound the diaphragm vibrates. The faster the vibration, the higher the pitch. As the diaphragm vibrates, it’s surface is moving towards you creating a pressure wave, albeit a small movement. If the whole speaker is moving towards you then, when it vibrates, the diaphragm is moving towards you faster than when it is stationary. Therefore the pitch is higher. The actual sound isn’t traveling any faster as it’s speed is determined by the medium through which it is traveling, in this case air. Sound must always travel at the same speed in air or it wouldn’t be possible to overtake your own sound if you were traveling supersonically.
Therefore, as the police car travels towards you the siren has a higher pitch even through the sound is not actually reaching your ears any faster. I think.
Maybe.
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I think you're associating wave length with the speed of the wave and there is no association there. The wave length is independent of the propagation of the sound; or light. The speed of propagation is constant, the wave length is variable and depends upon the relative speeds of source and observer. If you're going away from the wave, it will seem stretched out. If you're going toward the wave, it will seem squeezed together.
In the case of sound, you could measure a different speed of propagation if you were moving through the medium. In the case of light; its speed is constant no matter your movement through space.
According to Einstein, this constant speed is the result of a distortion of space and time.
According to Lorentz, this constant speed is the result of distortion of measuring sticks and clocks due to their material composition consisting only of a most fundamental substance that must always move at the speed of light.
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Well, I only just sort of had a bit of a grasp of what I possibly thought might be happening, maybe. Now that's trashed.
I'll give it some more thought and try to get my head round it.
Thanks for your continued attempts to try and put me right on this.
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Not trashed, I think you had it. Whether you're talking about the pitch of sound or the colour of light, it's solely to do with frequency, not speed.
And the reason frequency changes is because the source of the waves moves, like you say with your siren example.
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When I said
I only just sort of had a bit of a grasp of what I possibly thought might be happening, maybe.
what I meant was
I'm completely confident I've got this speed of light and Dopplar thing nailed! The only thing that's separating me from Einstein is that he has neater hair.
Actually, I think I was 90% there before Vern's last comment but now I've got my head round it more completely. But just to make sure, can one of you tell me this:
If the source of the sound is travelling towards me then the wavelength is shortened but the speed the sound travels at is unchanged. But if I am travelling towards the source of the sound then it's the other way round. The relative speed of the sound is increased (relative to my ears, that is) and the wavelength is unchanged but the peaks of the wave reach your ears in closer succession which has the same end result as the shorter wavelength scenario.
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Yes; I think you've got it right.
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Correct
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Thanks Verm.
Thanks Madidus_Scientia.
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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.
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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?
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Thank goodness no-one has mentioned shear waves.
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And black light?
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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.
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Why are you surprised? Beavers have very good eyesight.
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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.
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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.
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Thanks Vern. I feel confident I've got this wrapped up now.
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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.
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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/Pound–Rebka_experiment
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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.
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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/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?
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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.