Redshift and speed of light

killium · Sep 9, 2004

According to mainstream science, the speed of light is constant, even if the source is moving relative to the observer. The wavelenght of any wave, including light, is proportionnal to the speed of the wave. So, if the speed is constant, and the wavelenght is proportionnal to it, where does the Doppler effect (and the redshift) comes from ? <div class="Discussion_UserSignature"> </div>

Saiph · Sep 9, 2004

It arises precisely because the speed is constant. Or rather, because the speed is independent of the source velocity. Imagine the following scenario's: You're stationary, and playing catch with a friend. He's throwing a ball towards you every ten seconds. If neither of you move, you'd then catch a a ball every ten seconds. Now, you run towards him. Each time you catch a ball, you're a little closer. So each one traveled less distance, and thus took less time to reach you, than the previous ones. So even though your pal is chucking them at 1 every 10 seconds, you close the distance so that you catch one every 9. If you stop closing the distance (you stand still) the normal rate of one every 10 seconds will come back. Now, if you run away, each ball takes a little longer to reach you than the previous, so the rate drops, to 1 every 11 seconds. The same thing happens to the rate if he runs towards or away from you as well. If the speed of the ball varied, these trends could still occur, but only if you moved vaster than the average variation (otherwise it would be lost in the "noise" of the variation). Now, for light, each ball represents a repeating part of the light wave (say the crest). So if you approach, you encounter the crests faster (along with every other part of the wave) and slower if you recede. <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

unclefred · Sep 9, 2004

Your statement "The wavelenght of any wave, including light, is proportionnal to the speed of the wave" is correct but misleading and you are using it wrong. The way you are using it would imply that light of two different colors must have different speeds since speed and wavelength are inversely related. That is wrong. The correct statement is: Frequency x Wavelength = Velocity. As the wavelength changes the frequency changes and the velocity remains the same. The Dopler shift works the same for light as it does for sound or any other wave phenomena. The wavelength and frequency change and the speed remains unchanged.

5stone10 · Sep 9, 2004

I cringe whenever I see this qualification >> "According to mainstream science..."/safety_wrapper>

Saiph · Sep 9, 2004

your post triggered an elaboration: Wavelegth*frequency=velocity. For all waves but light, the following holds (i.e. it's not an analogy). As we move, relative to the source, the velocity of the wave is measured differently. Just as if I was to approach a moving car, I would measure the velocity different (the relative velocity is different). If I go 600 mph, the speed of sound as I measure it,will be different ahead and behind (cause I'm almost pacing the wave, it's almost stationary). As such the frequency and wavlength have to change. For light, you have to factor in relativistic effects to explain the doppler shift (which is observed...) in order to get the doppler effects to agree with the "everybody sees the same speed" observation. I.e. in making dopler and constant speed agree, the relativistic effects arise. <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

unclefred · Sep 9, 2004

I disagree, the formula Wavelegth*frequency=velocity is true for all waves including light. The velocity of light is measured no differently than any other wave. It is measured by the time it takes to transverse some known length in a given media. If you measure the speed of sound (or of light) ahead of you and behind you when you are going 600 miles and hour, you will find no difference. Why? Because you can only measure the velocity of the sound when it is inside your measurement device, and the conditions in your measurement device are identical for both measurements. If it is an open platform, then the air will be going through the measurement device at 600 mph in both cases and yield the same answer. If it is a closed platform (like inside an airplane) then the air will be stationary in both cases and again the same answer. The same is true if it were a submarine, except it would be water instead of air. I disagree with your statement that relativistic effects are needed to explain Doppler shift. Doppler happens any time there is relative motion, even at very slow speeds. Also, everybody does not see the same thing. An observer with a different speed will see something different. Doppler affects the wavelength observed, not the speed. They are independent for both light and sound. The same event observed by two different people will appear at different colors depending on their relative velocities.

Saiph · Sep 10, 2004

Viewing the speed as different is a valid approach to visualizing the problem. In the moving frame, the sound will appear to go from one location to another at a slower speed (assuming an "open platform"). True, you're moving (and so is the measuring device), but it's an apparent shift. Anyway, here's a nice link about Doppler shift, and the apparent shifts in wavelength (applies for light and sound): http://hyperphysics.phy-astr.gsu.edu/hbase/sound/dopp.html Here's another one that explains the reason for (and no real need for) the two different approaches to Doppler shift (that for sound, and another for light). http://www.mathpages.com/rr/s2-04/2-04.htm <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

unclefred · Sep 13, 2004

The speed is not different and viewing is as such is not a valid way to visualize the problem. The original post says "the speed of light is constant, even if the source is moving relative to the observer." and that is correct. The statement is also correct for sound. The speed of a sonar pulse shot from a moving submarine travels at a constant speed. The speed of the pulse shot forward is the same as the speed of the pulse shot backward. This is also true for the train horn. The Doppler affect describes the frequency or wavelength of the perceived wave, not its speed. The Doppler effect applies exactly the same to both sound and light. Your second reference states it very well and I quote "This sometimes gives the impressions that relativity requires us to apply a different set of rules to the propagation of sound than to the propagation of light, BUT OF COURSE THAT IS NOT THE CASE." Further into the article is states it again and I quote "In truth, relativity has just one formula for the Doppler shift, which applies equally to both sound and light." At high relative speeds one needs to add the time dilation factor to the Doppler factor since both are happening. Thus there is a red shift by time slowing down on the fast moving item and there is also red Doppler shift by the relative velocities. Sometimes these are combined into one equation. I refer you to sophomore physics (see last section called The Doppler Effect) at www.phys.virginia.edu/classes/252/srel_twins where this is described.

killium · Sep 13, 2004

Thank you all for the replys, they cleared some mis-conception i had about the subject. If i understand right now, we can conceive the thing like this: Suppose a sound device (a horn or wathever...) that is speeding away from you. Let see what happens at T=0. The horn put some compression force on the local air molecule. This compression will then start travelling from this point, in every direction, at the speed of sound. At T=0.00001 (!) the horn have moved to a new location in the direction of its movement, and places a new compression force to the "new local" air molecules. That compression will again start travelling (from that new position) in all directions at the speed of sound. The observer (the listener!) will receive those 2 compression events, at the speed of sound, with a delay in between, thus the reduced frequency noted. This delay is equal to 0.00001 + the time the second compression event took to cover the distance done by the horn during this 0.00001 interval (since this is happenning in a continuous way, we just have to integrate (or derive ?) the equation). Is that it ? <div class="Discussion_UserSignature"> </div>

unclefred · Sep 13, 2004

Ranur, you must take into account the medium for both light and air. The correction may not be as easy as just adding or subtracting. It depends on the situation. Also the speed of Light does not always measure to be C. Consider this experiment: measure the speed of light in an evacuated tank. You will measure the speed as C. Fill it with air and the speed slows. Fill it with water and the speed slows even more. Circulate the water with the light and the speed increases. Circulate the water against the light and the speed decreases. It does not mater if you are moving or if the water is moving, only relative speed in important. One will always measure the same speed for the same set of circumstances (same media moving at the same speed). Change the media and the measured speed changes. This is true for light as well as sound.

siarad · Sep 14, 2004

If you through the ball back it's received once every 10 seconds i.e. 0.1Hz. This is irrespective of your speed, so where did Doppler shift go. Knowing the speed for the media, the distance between the two can be calculated, ASDIC, RADAR. However the closing time, the time between throwing & receiving is reducing asymptotically i.e. accelerating. Surely integrating w.r.t. time would give the closing speed irrespective of the media

siarad · Sep 14, 2004

Thanks you've just proved air doesn't exist. Well the famous M & M experiment proved Aether doesn't exist by finding C being the same in all directions just as you've shown sound. I've never understood this assumption, I don't think it proof, provided by M & M & recently did a topic so saying.

Saiph · Sep 14, 2004

even though the ball is thrown at 0.1 Hz does not mean it is recieved at such an interval. As I said, if you move, each ball has to move a different distance (longer or shorter) than the one before. As such you'll catch them at a different rate (the doppler shift). Yes, if you know the speed of the medium, you can find a closing speed that doesn't relate to it. Or you can use an entirely different method that doesn't require the medium (like radar) to determine your speed. However, if you method involves the medium in question, and you do not know it's speed, you're stuck. <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

siarad · Sep 14, 2004

Sorry I seem to have not made my post clear or you missed the word back If the ball is thrown & bounces back it is received at 0.1 Hz, just as it is thrown initially, irrespective of any relative motion. I also pointed out the closing speed is known even though the transmission speed is not, assuming it's constant.

unclefred · Sep 14, 2004

The equation E= H(Nu) is totally off subject. This thread is about speed and Doppler effects. The equation is irrelevant here. As far as I can tell, my previous posts are correct.

larper · Sep 14, 2004

<blockquote>In reply to:<hr />If the ball is thrown & bounces back it is received at 0.1 Hz, just as it is thrown initially, irrespective of any relative motion. <hr /></blockquote> No, it is not. If red shift does not exist, then why do cops use radar guns? It really is not that hard. You are arguing just for the sake of arguing, it seems. <div class="Discussion_UserSignature"> Vote Libertarian </div>

unclefred · Sep 14, 2004

If one throws balls and the receiver throws them back, the Doppler effect is doubled, not cancelled. As pointed out in the previous posts, if the distance between the thrower and receiver is decreasing, each successive ball will have a shorter distance to cover and thus will take less time to arrive at the destination. Thus the receiver will get the balls closer together in time than the thrower sent them. Now if the receiver throws them back, it is exactly the same problem all over again. The distance is decreasing and the receiver will get the balls closer in time than the thrower sent them. Thus you have Doppler going both ways and the effect is doubled. This would be true for sound and for light

siarad · Sep 14, 2004

So where are the extra balls coming from for you to return more often than they're being thrown at you, go & get a ball & try. You're mixing up the return interval with the frequency methinks & I covered this in showing neither the medium or travel speed are required to be known to measure the closing speed.

unclefred · Sep 14, 2004

The formula you are talking about is valid but none of the terms in the equation E= h(Nu) are velocity, thus no matter how valid the equation is, it cannot add anything to this discussion. It does not relate frequency and velocity as you suggest. The equation calculates the energy of a photon in joules. h is Planck's constant 6.624 x 10-34 joule seconds. Nu is the frequency of the particular light wave. Strange, I don't see velocity anywhere in there. Do you? Please show me where the speed of light is in the equation. I do not understand your statement "There is NO such formula which works for light as it does for sound waves." There are several equations that work for both light and sound and previous posts (some made other people) provide a reference to those web sites. If you are referring specifically to the equation that you posted, then you are correct. That particular equation does not apply to sound. I don't understand you statement "that you will NOT engage on the facts". The discussion is entirely on the facts and supported by references to web sites. What is it that you think is not factual?

igorsboss · Sep 15, 2004

Although light propogates in a vaccuum, arguments to not. Here is the relativistic doppler effect equation. In frames S and S', with frame S' moving relative to S with velocity (v), at angle (theta), let there be a wavefront emitted from a point of emission at rest in frame S', with rest frequency (f_source). Further, let that wavefront be subsequently detected by an observer at rest in frame S, who observes the wavefront's frequency as (f_obs). Then f_obs = f_source * (sqrt(1-v*v/c*c)/(1+(v/c)cos(theta))) For a source directly receeding, theta=0. In this case, the equation simplifies to f_obs=f_source * sqrt((1-v/c)/(1+v/c)). For a source directly approaching, simply switch the sign of the velocity. Interesting: At theta=90 degrees, any nonzero velocity implies a diminished frequency (or "redshift"), unlike the classical theory. The key difference with the SR doppler effect is that the source frame is subject to time dilation. At faster relative velocities, time passes slower, so wavefront frequencies decrease. The classical doppler effect does not take this into account. (Gleaned from "Nonclassical Physics", Randy Harris, ISBN 0-20183436-7, section 1.6 "The Doppler Effect")

igorsboss · Sep 15, 2004

So, if the speed is constant, and the wavelenght is proportionnal to it, where does the Doppler effect (and the redshift) comes from ? The relativistic doppler shift comes from time dilation. "Two events occuring at the same location in one frame will be separated by a longer time interval in a frame moving relative to the first." (Harris, as cited earlier)

Saiph · Sep 15, 2004

Ranur: Redshift is used commonly in the Doppler aspect as well as the cosmological. But you are right, there are two different causes of redshift, and they are similar, but not synonomous, in how they are treated. Siarad: Simple, They're thrown at 1 Hz (though we really should be using s^-1 for conventions sake). You catch them at 0.9 Hz because the distance between you and the thrower is smaller. Now, you throw them when you catch them (perfect reflection), which is at: 0.9 Hz. However, you're still closing on the other guy. So he recieves them at: 0.8 Hz (the same change in frequency the first time). So, basically, you get them more often, so you can throw them more often. <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

siarad · Sep 15, 2004

You are kidding me I hope. You're now close enough to, in your parlance, catch them at 0.5Hz so you return two balls for the one thrown to you, seems like legerdemain to me.

Saiph · Sep 15, 2004

actually, I'm not. http://hyperphysics.phy-astr.gsu.edu/hbase/sound/radar.html Talks about police radar and how you have to consider the fact that the reflected radar pulse is shortened not only due to the cars motion towards you, but doubly so. It recieves a shortened version, and transmits a shortened version. The extra energy to pull this off, btw, comes from the throwers forward motion. Anyway, I can't believe I haven't mentioned this in previous threads: The reason doppler shift works for light, despite the constant speed of light, is because you percieve a shorter wavelength (as I've illustrated) AND the frequency is smaller. Both variables change in such a way as to keep the end result for c the same. Geesh, I'm kicking myself right now. <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

larper · Sep 15, 2004

<blockquote>In reply to:<hr /> You're now close enough to, in your parlance, catch them at 0.5Hz so you return two balls for the one thrown to you<hr /></blockquote>Nope. Still wrong. Distance has nothing to do with the frequency at which they are returned to you. Only ONLY ONLY the relative motion of the two players matter. It really is not that hard. <div class="Discussion_UserSignature"> Vote Libertarian </div>

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