C in a variable density enviroment.

[Earl]

As Space is clearly not a consistent vacum, as we can easily observe nebula etc and there are many none visual clouds and also we have the flux, how does this interfere with the speed a photon travels at as it crosses a galaxy? Also gravitational manipulation on its route?

[michael.h.f.wilkinson]

C varies with the index of refraction. Whilst space is not a perfect vacuum, it is so close to a perfect vacuum the index of refraction will be 1.0000000000 or so. After all, air at normal atmospheric pressure has an index of refraction of 1.000293. Gravity warps space-time, so whist the speed of light does not change, the actual distance travelled may

[andrew s]

Interestingly the neutrino pulse detected from SN 1987A  got to earth before the photons as they interact less with the ISM than the photons.

Regards Andrew

[Earl]

C varies with the index of refraction. Whilst space is not a perfect vacuum, it is so close to a perfect vacuum the index of refraction will be 1.0000000000 or so. After all, air at normal atmospheric pressure has an index of refraction of 1.000293. Gravity warps space-time, so whist the speed of light does not change, the actual distance travelled may

Do we have a way of measuring that across differeing areas of trhe galaxy ie, horse head vs pillars of creation?

[michael.h.f.wilkinson]

Not that I am aware of

[saac]

Do we have a way of measuring that across differeing areas of trhe galaxy ie, horse head vs pillars of creation?

Should be a fairly straightforward proposition really after all you are simply measuring the speed of light in Hydrogen, (predominantly).  As said earlier the refractive index (n) of a gas is directly related to its density (r) -  as an empirical rule the following holds   -    n = 1 + k(r/r_ref)  ; r_ref is the density of the gas at a reference condition.  I would be surprised if you could not readily find reliable references for the speed of light across a range of gases and densities from say the NPL, this type of thing is their bread and butter.

Jim

[robin_astro]

Yep you can observe this effect at radio wavelengths with pulsars, where the time of arrival time of the pulse depends on the wavelength at which you make the measurement. (known as dispersion) This is due to the refractive index/speed of light in the interstellar medium being wavelength dependent. It can be used to estimate the distance to pulsars.

Robin

[robin_astro]

Also on a larger scale, differences in the flickering of  multiple gravitationally lensed quasar images can potentially  be used to determine distances to intervening galaxies eg

http://spaceflightnow.com/news/n0011/12quasar/