Fnord wrote:
Light travels away from a source as an electromagnetic wave. When it encounters the slits of an inteferometer, it passes through and divides into two wave fronts. These wave fronts overlap and approach a screen. At the moment of impact, however, the entire wave field disappears and a photon appears. Quantum physicists often describe this by saying the spread-out wave "collapses" into a small point - a particle.
Light waves travel at the speed of light (c = 299,792,458 meters per second in a perfect vacuum). Changing the energy content of a wavefront will not alter its velocity, only its amplitude (brightness) or frequency (hue).
I used to explain the doppler effect used by police radar to people by likening it to throwing a "perfect" tennis ball at a moving object and measuring the speed of its return.
It went like this:
If the tennis ball comes back faster, the object is moving toward you. The faster it is moving toward you, the faster the ball comes back. If the tennis ball comes back slower, than it is moving away from you. The slower it is when it comes back, the faster the object is moving away from you. If the tennis ball returns with the same velocity as when it returns, the object is stationary. In all cases, when you are measuring the velocity of the tennis ball, you are measuring the kinetic energy of the ball.
With electromagnetic waves, the speed doesn't change. Rather, the energy is reflected in the wavelength/frequency of the wave. When you aim the radar at the vehicle, the returned beam has the same wavelength and frequency if the vehicle is stationary -- that is, the energy of the beam didn't change. If you aim the radar at a vehicle traveling away from you, the returned electromagnetic wave has a longer wavelength and lower frequency and thus a lower energy. The faster the vehicle is traveling away from you, the longer the wavelength and lower the frequency. If you aim the radar at a vehicle traveling toward you, the returned electromagnetic wave has a shorter wavelength and higher frequency and thus a higher energy. The faster the vehicle is traveling toward you, the shorter the wavelength and higher the frequency you will measure.
Whether or not you are throwing tennis balls at the car or electromagnetic waves, it is the change in energy of the reflected wave that indicates its speed. The difference is in how the energy manifests itself and the way it is measured.