Question

If you are moving at high speed toward a light source and you observe a shift in the frequency that seems to imply that there is an additiion of velocity between observer and light speed. But, there is supposedly no such addition.

Answer 1

so.. whats the question?

Answer 2

it is the doppler effect

Answer 3

Good question . I think what you mean according to Einstiens second statement of special relativity velocity of light is invariant to all inertial observers.
But this does not mean that there cannot be any change in frequency. Even with the velocity remaining constant there can be change in frequency for electro magnetic radiatons because \[c =\upsilon \lambda\]
here if either frequency or wavelenght changes it is only in accordance with the equation and change in either of them is compensated by subsequent change in the other

Answer 4

Vicky007 understood what I was questioning. I was wondering how could an observer be moving toward an incoming light wave and observe a higher frequency without it being the result of the addition of velocities of the observer and the source. It really seems to go against common sense and logic if you think about it. To move toward an incoming wave with a velocity you would think that you would have to be running across the wave fronts faster and this would be observed as a higher perceived frequency of the incoming source. But, even if the incoming source frequency does get higher, and it's not due to velocity additions (of light source and observer velocity) then what would be the cause of the frequency change? Something just doesn't make sense when you try to imagine the scenario.

Answer 5

I'm aware that what I'm describing is called the Doppler blue shift. But, there is something missing in all the descriptions about the cause of this phenomenon. They are telling us (in textbooks) that the velocity of an observer moving in toward a light source will not be described by adding the velocities of the observer and the source within an inertial frame; instead , it is described as a constant velocity of light between a moving observer and stationary source. Yet, they somehow expect us to make the leap of imagination that this constant velocity of light will be seen by the moving observer to shift in frequency and wavelength c = h * wavelength such that c remains constant. So, my question is that if it is not the addition of velocity of the observer's velocity added to the speed of the light which causes the change in observed wavelength and frequency of the light source (blue shift) then what is the immediate cause? It makes me want to join the people who say that the speed of light is not constant.

Answer 6

here addition of velocity does not make sense for the increase in observed frequency
Its actually due to relative motion between the source and the observer .No matter what the relative velocity of an electromagnetic wave be if you are moving toward a source then you will encounter more wave fronts in unit time than what is supposed to be observed by a stationary observer . thus resultant frequency increases . Its actauly change in distance not change in velocity that is causing doppler effect

Answer 7

I appreciate the replies, but something still is missing in all these (yours) and textbook descriptions. I mean that a change of distance is really the result of the observer's relative velocity (d/t). It seems to be just playing with words, unless you mean there is no formal "velocity change" as a vector but there is a "relative motion" as cause or something. Then it's just an issue of semantics and the relaitve motions between observer and source ARE the cause of the blue shift using different terminology. I can see that the speed of light is a constant and it is not violated if you raise the frequency while lowering the wavelength. There no laws are technically violated. But, the cause of the frequency shift is not described in PARTICULAR and specific language. It is only described in particular language that it is NOT an addition of velocities.

Answer 8

I guess part of the problem I have with your answer Vicky is that you are saying relative motion is the cause of the blue shift, but velocity addition is not ... and, I'm wondering what is the difference between these terminologies? Am I missing some formal definitions in the difference between velocity and relative motion, or is everyone vague on the actual answer and just playing with words to preserve the magical constant status of the speed of light (as some have suggested)?

Answer 9

http://www.relativityoflight.com/Chapter21.html

Answer 10

While one would be tempted to think of it as the addition of velocities, a more intrinsic explanation is the addition of energies. It is true that regardless of ones inertial frame, you will measure the speed of light to be the same. However, photons have a specific energy, which according to quantum theory depends solely upon their frequency i.e. \(E=fh\) where \(h\) is Planck's constant.

If you collide head on with a photon, your detector will be moving with a specific kinetic energy. The photon will also have a "kinetic energy" which is identical to the Energy given above. Since our detector can claim to be at rest (since it is in an inertial frame), then we see the total energy of the collision as being an increase in the energy of photon, and hence a blue-shift in its frequency or wavelength. If we are moving away from the photon, then the impact energy will be less, but by the argument of relativity, from our rest frame we will see the photons red-shifted.

Answer 11

I just looked through my respected physics textbook "Fundamentals In Physics" by Halliday, Resnick, Walker 7th edition. It describes the head on blue shift Doppler equation as:
f(new) = f(original) times the square root of [1 plus v/c divided by 1 minus v/c]. Since the "1" represents the speed of light then it seems that the equation itself is not directly adding vector velocities, but it is seeming to add and subtract a ratio of velocities from the speed of light and using that to create a factor to multiply by the original frequency. So, adding and subtracting the speeds of light and source is in there in an indirect way.

It also states: " The Doppler effect for light waves depends on only one velocity, the relative velocity "v" (vector) between source and detector, as measured from the reference frame of either." So, it seems to be saying that vector addition of velocities is the answer, but not admitting a paradox that the light speed stays constant with reference to a moving observer. Does that shed any light on this to anyone?

Thanks for the above reply, but I'm not satisfied with it yet.