Doppler effect and time dilation?

Question

Just to clarify in general the principle of doppler effect and time dilation in Relativity:

1) As far as I understand, the doppler effect only affects the wavelengths of the light we perceive, not the speed of light itself. This would result to blue-shift/redshift as we see the object (weather moving away/towards us).

2) At velocities close to speed of light relative to an outside observer, the moving object (say a clock) would show slower time progression.

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Could you please explain why such time dilation as observed by the outside observer occurs?

According to relativity, speed of light travels at constant velocity at all reference frames. If the light in the high velocity frame leaves at "X" rate, it should approach the observer at the same "X" rate, so time dilation should not exist (or should not be observable to the observer), and only the blue/red shifts should occur.

PS: Im a mechanical engineer but never had the chance to study relativity in-depth so simple explanations will be much appreciated! Hopefully I will be able to catch up with the terminology you'll be using later.

Answer 1

1) yes, more or less.

2) no. it's not that simple.

Answer 2

"Doppler effect and time dilation?"



"1) As far as I understand, the doppler effect only affects the wavelengths of the light we perceive, not the speed of light itself."



Think of each "peak" being emitted from closer or further away.



"This would result to blue-shift/redshift as we see the object (weather moving away/towards us)."



Sure. Classical Doppler shift. What we'd see at less than, say, 1% of the speed of light.



"2) At velocities close to speed of light relative to an outside observer, the moving object (say a clock) would show slower time progression."



And the two combine:

http://en.wikipedia.org/wiki/Relativistic_Doppler_effect



"Could you please explain why such time dilation as observed by the outside observer occurs?"



There are no answers to "why" questions in Science. The best we can get to is *how* we think it works. If you believe that no one moving by you at any speed, can change your "laws of physics", then a million people flying by you cannot change your "laws of physics", then you should always get a constant speed of light, right?

http://en.wikipedia.org/wiki/Time_dilation#Simple_inference_of_time_dilation_due_to_relative_velocity

... How else can that light clock work?



"According to relativity, speed of light travels at constant velocity at all reference frames."



*Inertial* reference frames. Ones that are not accelerating...



"If the light in the high velocity frame leaves at "X" rate, it should approach the observer at the same "X" rate, so time dilation should not exist (or should not be observable to the observer), and only the blue/red shifts should occur."



Right, red and blue form two causes... time dilation and classical Doppler effect.



"PS: Im a mechanical engineer but never had the chance to study relativity in-depth so simple explanations will be much appreciated! Hopefully I will be able to catch up with the terminology you'll be using later."



Get "Spacetime Physics" by Taylor and Wheeler, or one of the other choices from this list:

http://www.physics.adelaide.edu.au/~dkoks/Faq/Administrivia/booklist.html#special-relativity

Answer 3

1) Yeah I think that's pretty much it.



2)

If I understand your question correctly, I think that you might be thinking that time dilation is something observed by the person observing the light ray moving towards them. Time dilation is a "subjective" phenomena, and what I mean by that it only happens from the reference frame of the light ray, it wouldn't be obvious to the resting observer.



If you imagine observer A at rest, and observer B moving at a constant velocity, then observer B's clock is going to tick slower than observer A's clock. But observer A still sees observer B moving at the same velocity, and from observer B's reference frame, observer A is moving with minus that velocity. So observer A doesn't notice the time dilation, but hypothetically he can measure the time dilation by checking the difference between an interval of time measured by observer A and a time interval measured by observer B (assuming the two clocks were synchronized before they did this experiment).



As for "why" this time dilation occurs, time dilation basically comes out of the Lorentz transformation, which is a transformation used to switch between inertial reference frames in special relativity. You can transform an interval of time from observer A's reference frame into observer B's reference frame and you'll find that the interval of time is longer for B than it is for A. You could also say that since the speed of light is constant in every reference frame, this naturally implies that time (and other quantities) are not the same in every reference frame.



I hope that helped.