# Is red-shifted light traveling at a speed less than c?

## Summary:

The wavelength of light from a moving source is red shifted which means that the wavelength has increased and the quantity of energy arriving per second at a relatively static destination is less than the quantity of energy emitted per second at the source.

If so then the original quantity of energy emitted (the message) takes longer to arrive in full than it would have done if emitted from a stationary source implying a true velocity of c - n.

## Main Question or Discussion Point

The wavelength of light from a moving source is red shifted which means that the wavelength has increased and the quantity of energy arriving per second at a relatively static destination is less than the quantity of energy emitted per second at the source.

If so then the original quantity of energy emitted (the message) takes longer to arrive in full than it would have done if emitted from a stationary source implying a true velocity of c - n.

In other words the true speed of a beam of light should be considered in terms of the time it takes for a quantum of light at the wavelength emitted to be received at the destination at the same wavelength or (if the wavelength has changed) the equivalent amount of energy emitted.

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PeroK
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Is there a question here?

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Is red shifted light travelling at a speed less than c?
No. And red-shifted light is light too.

• russ_watters
Yes, but the point I am making is that red shifted light is lower energy so that one would expect it to take longer for an equivalent amount of the emitted energy to be received at the destination.

Is there a question here?
yes, "
Can red shifted light be considered to be travelling at a speed less than c?

PeroK
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yes, "
Can red shifted light be considered to be travelling at a speed less than c?
No is the answer. Speed is distance/time.

Ibix
Can red shifted light be considered to be travelling at a speed less than c?
No. The tail end of a pulse has further to go than the front end, that's all.

Add a strobe light that is stationary with respect to you and happens to be next to the moving source when it starts transmitting. Then you would have two pulses emitted at the same time in the same place and arriving at the same time at the same place but, according to you, travelling at different speeds.

Nugatory
Mentor
one would expect it to take longer for an equivalent amount of the emitted energy to be received at the destination.
It does take longer, but that doesn’t imply that the light waves are moving more slowly. Consider a burst of light carrying some amount of energy. Redshift happens because the source and destination are moving apart, so the tail end of the burst will travel a longer distance than the leading end. With a constant speed of light, the longer distance implies a greater travel time so the time between arrival of the leading edge of the burst and arrival of the the trailing edge increases.

Staff Emeritus
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Can red shifted light be considered to be travelling at a speed less than c?

(PS I made it a little less loud in the quote)

• • HallsofIvy and phinds
Mister T
Gold Member
The speed $c$ of a beam of light in a vacuum is independent of the speed of the source. This is a postulate of Einstein's 1905 special theory of relativity. Thus, any consequence derived from the postulate cannot be a violation of that postulate.

Suppose a baseball pitcher is throwing a baseball from a moving platform towards a stationary catcher. The platform is moving away from the catcher. The speed of the ball as measured by the pitcher will always be greater than the speed of the ball as measured by the catcher. But the difference is not equal to the speed of the platform! Moreover, as the speed of the ball approaches $c$ the difference approaches zero. (As the speed of the ball approaches zero the difference approaches the speed of the platform.)

For a beam of light traveling at speed $c$ the difference is zero.

jbriggs444
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Thus, any consequence derived from the postulate cannot be a violation of that postulate.
True in the case at hand, certainly. Special relativity is self-consistent.

However, it is possible to have a formal system where a particular postulate leads to an inconsistency. Obviously, such a system is not consistent.

Last edited:
russ_watters
Mentor
Yes, but the point I am making is that red shifted light is lower energy so that one would expect it to take longer for an equivalent amount of the emitted energy to be received at the destination.
I think that wording is misleading because it is only true for a a certain interval of measuring a continuous power source. If the power source only lasts a finite time (a pulse), and you measure the entire energy output, you will find it to be lower than what was emitted. There is no "take longer" available to make the received energy equal the emitted energy.