# I Diffraction Redshift and Emission Theory

1. Dec 3, 2015

### greswd

In the emission theory of light, light waves can move at any speed.

We can still apply the Doppler effect, but to the best of my knowledge, only the frequency changes, not the wavelength.

The pattern for a diffraction grating only depends on the wavelength right? And we have observed redshifts and blueshifts using a diffraction grating.

Although emission theory has long been discredited, is the existence of diffraction redshift a valid reason to discard emission theory?

2. Dec 3, 2015

### Ken G

When I google "diffraction redshift", all I get is your question, so I'm not sure what you mean by that phrase. A diffraction grating doesn't redshift anything, it just sends the different colors off into different directions, like a rainbow (which is actually caused by refraction, but the effect is similar). We wouldn't say the red band in a rainbow is light that has been "redshifted", we would say the red light came off in that direction. It sounds like you are asking if emission theory, which explains Doppler shifts and the Michelson-Morley null result, can explain diffraction. But it certainly can, because there's no movement of anything but the light itself to consider in diffraction, so emission theory looks like a normal wave theory of light in that context.

Where emission theory has its problems is time-of-flight effects. I'm not sure if emission theory could even be made consistent with observations of the timing of the eclipses of the moons of Jupiter, as per Roemer's 1676 measurement. Most likely, emission theory claims that the differences in when those eclipses would be seen is too small to distinguish relativity from emission theory, but more modern types of experiments should have no such difficulty.

3. Dec 3, 2015

### greswd

What I mean is redshift observed and measured by means of a diffraction grating.

4. Dec 3, 2015

### Staff: Mentor

I assume you mean this:

https://en.wikipedia.org/wiki/Emission_theory

If so, please note the multiple reasons for discarding emission theory given on that page.

I don't see how this question can be answered without an actual derivation of how emission theory makes a different prediction for diffraction experiments from our best current theory. (I agree with Ken G that "diffraction redshift/blueshift" is not the best way to describe the prediction of our best current theory, but it is really the prediction vs. experiment that matters, not what we call it.) Do you have such a derivation? Or can you give a reference that has one? If you can, please PM me. Until then, this thread is closed.