# Speed light from distant stars

1. Aug 6, 2013

### cliffes

Light from distant stars is doppler shifted. Is the measured speed of light from these stars lower than the speed of light from the sun? Would one expect the permativitity of free space to vary over the universe?

2. Aug 6, 2013

### Staff: Mentor

The speed of light (measured speed of the light as it arrives here) is the speed of light (the fundamental constant).
No. In addition, this would require a spherical symmetry, and there is no reason why earth should be in the center of such a variation.

3. Aug 6, 2013

### phinds

The speed of a photon in a beam of light emitted by a very distant star that is moving away from Earth at a very high speed and heavily red-shifted, will be measured as c

The speed of a photon in a beam of light emitted by a very close star that is moving towards Earth at a very high speed and heavily blue-shifted, will be measured as c

The speed of light in a vacuum is c. Period. The speed of the emitting source is irrelevant.

4. Aug 6, 2013

### Naty1

It helps when starting out in relativity to recognize some rules which simplify 'new ways of thinking' relative to our everyday experiences. A guide which I found helpful was to remember when considering relativistic situations is that in the flat spacetime [no gravity] of special relativity the speed of light is always observed at 'c'...both locally and distant. Time and space may vary, but light is constant at speed 'c'.

But in general relativty, the curved characteristic of spacetime [gravity] can cause measures of distant phenomena to appear as if light moves at other than 'c'...for example it may move along a curved trajectory. But locally, where curvature can be ignored, light is always the same old 'c'. In other words, even if an observer measures some distant phenomena as appearing to have a lightspeed other than c, right there at the location of the phenomena, a local observer will record good old 'c'.

One would expect it to not vary...on the other hand, we have been fooled by Mother Nature on other occasions!

What we observe, each day, as older light than yesterday reaches us from the most distant parts of the universe, it is redshifted according to the same pattern as the prior day's light. If that pattern suddenly changed, we'd be tipped off there is something 'different' way out there where light originated some 13.8 billions years ago. So far it appears that as the distant particle horizon expands away from us, local characteristics there are the same as they are closer to us...redshift retains it's expected character.

all of these ideas are consistent with earlier posts here.

5. Aug 6, 2013

### cliffes

Many thanks for the replies.
I am puzzled by frequency shift which I think relates to change in energy of a photon -however the doppler effect is normally explained as a frequency change in the light wave(not the energy of a particle). Intuitively I would expect photons to loose energy as they travel the huge distances in space. Is there evidence for this? Does it show up as a frequency shift if c remains constant?

6. Aug 6, 2013

### phinds

They DO lose energy due to the redshift / traveling though an expanding universe. If they did not, the CMB would swamp everything and we would see blazing light in all directions.

Note that this is NOT the "tired light" hypothesis, which attempts to disproved expansion, and which has been thoroughly discredited.

7. Aug 6, 2013

### Naty1

E =hf Energy and frequency change are two sides of the same coin. they go together.

Not in static spacetime. Light does lose energy in a fiber optic cable however.

Two things cause an observed frequency shift. One is relative motion between emitter and absorber[observer]...Doppler shift:

The other is a change in relative clock rates....as for example different gravitational potentials.

Local observations trump distant ones.

8. Aug 7, 2013

### cliffes

Many thanks again,
Am I right in thinking the tired light theory failed in part due to red and blue doppler shifts from the rotating arms of spiral galaxies?
I understand that the measured speed of light in a vacuum is constant regardless of the motion of source or observer. However a medium such as glass "slows down" the photons - without changing the frequency. Any measured speeds for c lower than the vacuum value are presumably put down to the medium- typically our atmosphere. I wondered if the experiment had actually been done (rather than the result assumed) to compare the speed of light from different stars rather than to look at frequency shifts. I am not sure how sensible the question is -it maybe that the result is clear from all the astronomical data that has been collected over the years.

9. Aug 7, 2013

### Staff: Mentor

Abberation of light is a direct result of the speed of light, and it has to be taken into account for all precision measurements of stellar positions. So yes, it has been measured - and it has to be taken into account to do the actual measurements properly.

10. Aug 7, 2013

### Naty1

An interesting aspect of this interpretation is that in a contracting universe, lights regains 'lost'
energy.