mithomps at indiana.edu wrote:
> That's because as red light is red shifted into the infrared ultraviolet light
> is red shifted to violet and blue (for motion away from the viewer, and
> similarly for motion towards the viewer).
Basically, light emitted (or reflected) from a moving source will have a higher
energy in the direction it is moving and a lower energy in the reverse
This energy difference is not reflected in a difference in speed since the light
travels at a constant velocity, but in the wavelength of the light.
> The color wouldn't change
> distinctively for non-relativistic speeds.
It depends on how you mean "distinctively". I've personally seen red shifts
at speeds of well under ten miles per hour (on an oscilloscope using a laser
to detect and measure differences in speeds). With the current state of
technology, I wouldn't be surprised if it could be detected and measured at
under one mile per hour.
(When using laser radar, if a target is traveling at highway speeds, it is very
difficult to accurately measure the red shift. This is not due to the speed
being too slow to accurately measure, but due to a quite large frequency shift.
On the other hand, using the same laser radar, you could quite easily measure
the speed of a turtle. That is why the laser radar systems for traffic speed
law enforcement use time of flight measurements rather than doppler shift
measurements to determine the speed of traffic. Time of flight is much
easier to measure at speeds much above 5 or 10 mph.)
> You'd only expect the hue to change
> when the velocity is so great that the very low intensity of, e.g., X-rays and
> gamma rays (in the case of motion away from the observer) or microwaves and
> radio waves (in the case of motion toward the observer) to make itself felt.
If you were talking about ordinary light, I would assume that you mean a
visual change in hue. But that does not appear to be the case because the
frequencies you mention above are not in the visible spectrum. Thus,
detectors are required and detectors can measure relatively slow speeds.
For example, when you're driving through a 15 mph school zone at 20 mph,
the police officer can easily and accurately calculate your speed from
the measurement of the red shift in the microwaves.
> This has nothing to do with the use of red shift in astronomy, which involves
> measuring the shifting of absorption lines in the spectrum.
If you and the star are traveling at the same speed in relation to each other,
there will be no shifting of the spectrum. If it is traveling toward you, you
will see a red shift and if it is traveling away from you, you will see a blue
shift in the absorption lines. How come you don't think that has nothing to
do with the doppler effect?