Is red shifted light travelling at a speed less than c?
Light emitted from a light source moving away from an observer at a speed v would intuitively be expected to be travelling at a speed c – v but is in fact still measured to be moving at a speed of c. The measurement of speed is based on the time interval between the light being emitted and the light being detected at the destination.
The difference between light detected from a stationary source and light detected from a receding source is that the latter is red shifted which means that its wavelength has increased and consequently that it is less energetic. But what does that really mean?
One can visualise it as follows:
A Quanta of light (Photon) is released from moving light source. The next quanta (Photon) is released at a distance d from the first. Thus a relatively stationary observer will observe a greater distance between each quanta than an observer in the same inertial frame of reference as the moving light source; this is manifested as an increase in wavelength or decrease in frequency.
If the wave from a stationary light source has a length L then the wave from a moving light source has a length L + n.
If we consider that the full energy of the photon only arrives at the crest of the wave then the amount of energy arriving per second from the moving light source is less than that from the stationary light source.
It takes longer for a FULL quanta of light to reach a point A where the light source is moving in a direction away from A than light from a relatively stationary source.
Although energy from each quanta of light will arrive in a continuous stream as its waveform unfolds it cannot accurately be said to have arrived until the whole packet of energy has been absorbed at the destination point. As an analogy a locomotive leaves station A and collects one mile of carriages in front of it on its way to station B. The first carriage being pushed by the locomotive may arrive at a station B at 09:00 but the locomotive doesn’t arrive until 09:03.
The speed of each quanta should be more accurately calculated as distance/time where time is the interval between the FULL quanta being discharged at source and the FULL quanta being fully absorbed at the destination. As its wavelength increases there can be a considerable interval between the arrival of the front of the wave and the back of the wave.
In conclusion red shifted light from a receding light source can be measured in terms of the quantity of energy transmitted and received per second as travelling at a speed less than c.
