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Thread: A question regarding the doppler effect

  1. #1 A question regarding the doppler effect 
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    For starters, I am not a scientist. I am a 16 year old boy doing upper secondary school. Besides from that Im am very intrested in Cosmology and Physics. And now to my question:

    I have read some about the doppler effect and understood to some level, I have also managed to get some brief understanding of the theory of relativity, and one day a question arose. The theory of relativity states that nothing can move faster than light, thereby nothing can speed up light. So if I would mount a flashlight on a F16 the light would still move at the same speed relative to the plane.

    This is the point where I need a little help, because the way I see it, if the light is moving at the same speed why should I percive its waves to be compressed or elongated?

    Also another question that ive been thinking alot about, does sound have similar properties to light in the way that it has a constant speed?


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    The speed of sound is dependent on the medium in which it is propagating. So the speed of sound varies with air density and through liquids and solids at different temperatures (higher density, higher speed), while the speed of light is constant in a vacuum and is only perceived to slow down through mediums as a result of the time taken of the absorption/emition of photons by the medium’s constituent molecules’ electron clouds.

    The red and blue shift of light emitted from sources at various relative accelerations change the frequency, i.e. the number of complete oscillations per unit time. The Doppler shift works the same way for sound and light in this instance. The position of the source of the wave has changed between each pulse. So the distance between the two pulses (areas of higher density with sound) are less as per a function of the time between each emition and the speed of the source. So each pulse still travels at the speed of sound through that medium, but they are closer together (higher frequency) than if the source had been stationary. That is also where the sonic boom comes from, i.e. the speed of the source equals the speed at which the sound is emitted, so the air cannot move away from nose of the plane.

    Did that answer your question?


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  4. #3 Re: A question regarding the doppler effect 
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    Quote Originally Posted by Bubble
    For starters, I am not a scientist. I am a 16 year old boy doing upper secondary school. Besides from that Im am very intrested in Cosmology and Physics. And now to my question:

    I have read some about the doppler effect and understood to some level, I have also managed to get some brief understanding of the theory of relativity, and one day a question arose. The theory of relativity states that nothing can move faster than light, thereby nothing can speed up light. So if I would mount a flashlight on a F16 the light would still move at the same speed relative to the plane.

    This is the point where I need a little help, because the way I see it, if the light is moving at the same speed why should I percive its waves to be compressed or elongated?

    Also another question that ive been thinking alot about, does sound have similar properties to light in the way that it has a constant speed?
    I can add this to what Kalster says regarding light.

    The velocity of light is fixed relative to the emitting source and not to space.

    Relative to space, you need to add or subtract the 'local space velocity ' to fix the speed to space.
    My opinion is that when an emitting object is approaching an observer, you need to add that velocity to the VoL relative the space. With a receding object, you subtract its velocity.
    When an object is moving laterally to an observer, you do nothing.

    The light is transmitted through the electric fields that surround the emitting objects and not through space as it is cirrently taught since these fields are also moving with the source.

    Caomo
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    Take a look at this animation. It shows a light source with two "observers" (the red and blue dots). in this animation the light source has no relative motion to the observers. Each part of the light wave propagates out as a spherical wavefront at c and reaches each observer equally spaced apart.



    Now look at this animation. Same light source, only with a relative motion towards the blue dot.



    Each wavefront is still emitted and propagates at c as a spherical wave front from the point of emission. But each succesive point of emission is a little closer to the blue dot and a little further from the red dot. Thus each successive wave front takes a little less time at c to travel to the red dot, and a little longer to reach the red dot. IOW, the wave fronts traveling in the direction of the blue dot follow each other a little more closely and the wave fronts traveling in the red dot's direction are more spread out, and you get a Doppler shift.
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


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  6. #5  
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    Quote Originally Posted by Janus
    Take a look at this animation. It shows a light source with two "observers" (the red and blue dots). in this animation the light source has no relative motion to the observers. Each part of the light wave propagates out as a spherical wavefront at c and reaches each observer equally spaced apart.



    Now look at this animation. Same light source, only with a relative motion towards the blue dot.



    Each wavefront is still emitted and propagates at c as a spherical wave front from the point of emission. But each succesive point of emission is a little closer to the blue dot and a little further from the red dot. Thus each successive wave front
    takes a little less time at c to travel to the red dot, and a little longer to reach the red dot. IOW, the wave fronts traveling in the direction of the blue dot follow each other a little more closely and the wave fronts traveling in the red dot's direction are more spread out, and you get a Doppler shift.
    That second illustration of yours is not realistic.
    It shows the electric field as 'frozen' in space.
    This is not true because these electric fields are moving with the light source.
    However, the observer on the right would see these waves as 'blue shifted' because of the higher ve;ocity of c relative to that observer.
    So these waves should not show any compression as is shown.
    However, the observer on the right would see shorter wavelengths because of the higher velocity of c relative to him.

    Cosmo
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  7. #6  
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    Thank you, both Kalster and Cosmo for the detailed explanation. I think I have a better understanding for this now.

    Another question tho: Would this effect be the same if the speed of the sound which was emitted was also in addition speeded up by the plane?
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  8. #7  
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    Quote Originally Posted by Cosmo
    Quote Originally Posted by Janus
    Take a look at this animation. It shows a light source with two "observers" (the red and blue dots). in this animation the light source has no relative motion to the observers. Each part of the light wave propagates out as a spherical wavefront at c and reaches each observer equally spaced apart.



    Now look at this animation. Same light source, only with a relative motion towards the blue dot.



    Each wavefront is still emitted and propagates at c as a spherical wave front from the point of emission. But each succesive point of emission is a little closer to the blue dot and a little further from the red dot. Thus each successive wave front
    takes a little less time at c to travel to the red dot, and a little longer to reach the red dot. IOW, the wave fronts traveling in the direction of the blue dot follow each other a little more closely and the wave fronts traveling in the red dot's direction are more spread out, and you get a Doppler shift.
    That second illustration of yours is not realistic.
    It shows the electric field as 'frozen' in space.
    This is not true because these electric fields are moving with the light source.
    However, the observer on the right would see these waves as 'blue shifted' because of the higher ve;ocity of c relative to that observer.
    So these waves should not show any compression as is shown.
    However, the observer on the right would see shorter wavelengths because of the higher velocity of c relative to him.

    Cosmo
    As usual, Absolutely wrong. The speed of light is c relative to all interial observers and is independent of either their velocity or the source's. That is one of the two postulates of Relativity. (But of course, you don't believe in Relativity, do you.)
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


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  9. #8  
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    Quote Originally Posted by Bubble
    Thank you, both Kalster and Cosmo for the detailed explanation. I think I have a better understanding for this now.

    Another question tho: Would this effect be the same if the speed of the sound which was emitted was also in addition speeded up by the plane?

    I would ignore Cosmo's explaination, as it does not adhere to the views of excepted science.
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


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  10. #9  
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    I see
    Well I gues I will Listen to you instead
    "Time only exist so everything won't happen at once" Albert Einstein
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  11. #10  
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    Yeah, Janus knows his stuff.
    Disclaimer: I do not declare myself to be an expert on ANY subject. If I state something as fact that is obviously wrong, please don't hesitate to correct me. I welcome such corrections in an attempt to be as truthful and accurate as possible.

    "Gullibility kills" - Carl Sagan
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  12. #11  
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    If you look at just one part of the wave, like maybe the highest point, you'd see that part was always still traveling at C. It's just that, since the object emitting it keeps getting further and further away, the total distance keeps growing, so it keeps taking slightly longer than expected for that part of the wave to get to you.



    Quote Originally Posted by KALSTER
    The speed of sound is dependent on the medium in which it is propagating. So the speed of sound varies with air density and through liquids and solids at different temperatures (higher density, higher speed), while the speed of light is constant in a vacuum and is only perceived to slow down through mediums as a result of the time taken of the absorption/emition of photons by the mediumís constituent moleculesí electron clouds.

    To be fair, what this means is that the speed of light also, just like the speed of sound, varies with the medium through which it travels.

    It's exactly like sound, except for one thing: sound can't travel through a vacuum, but light can travel through a vacuum.

    C is defined as the speed of light in a vacuum, because that's simplest for people to keep track of. (As opposed to keeping track of the speed of light in glass, or any number of other mediums)
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  13. #12  
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    Quote Originally Posted by Bobbo
    Another question tho: Would this effect be the same if the speed of the sound which was emitted was also in addition speeded up by the plane
    This is an intresting question.

    Here you have two different carriers of the waves.

    In the space that is a vacuum, the carrier is the electric field that moves with the emitter of the light.

    However, in the sound question, the carrier of the sound is the atmosphere (molecules) that transmit the sound and are at a standstill.
    So in this situation, the above illustrations would apply since in this case, the carrier is standing still. So the sound waves would be compressed as shown in the above 2nd illustration.

    Cosmo
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  14. #13  
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    Ja nus

    Yes, you are right. I do not believe in relativity, just as I do not believe in the BBT.

    Cosmo
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