# Speed of light

sreekant
guys, i got a simple question i'll make it in parts after stating what i think is well known...

a) speed of light decreases in a medium...that is it decreases when it movers through matter

b) there is dark matter present throughout the entire galaxy and so is present on earth as well

c) we measure the speed of light experimentally and find its value as 299 792 458 m / s

does this not mean that due to presence of dark matter..the speed of light as we measure it here is not the absolute constant?..that there is a slight error in what we consider as a universal constant? ...please answer me..its eating my head out

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JaredJames
Firstly, please use proper English. Text speak is unacceptable.

a) Ok.

b) I'm not sure if that is actually the case. Someone else can ok that one.

c) Ok.

So I read your question as "does dark matter affect the speed of light and therefore is our measured maximum speed actually slightly off because it's actually travelling through the medium of dark matter?".

I'm under the impression that dark matter doesn't interact with light any different to 'regular matter'. As per another thread here, gravity doesn't affect the speed of light but can change it's direction. Someone else can probably explain it far better than I ever could.

Staff Emeritus
Gold Member
guys, i got a simple question ill make it in pts after stating wat i think is well known...

a) speed of light decreases in a medium...that is it decreases when it movers through matter
The apparent speed of light changes, but not the actual speed of light
b) there is dark matter present throughout the entire galaxy nd so is present on earth as well
Yes, but only in extremely small amounts. The total estimated amount of DM in the entire volume of the Solar system is about that of a Dwarf planet, so the percentage of that which would coincide with the Earth would be miniscule.
c) we measure the speed of light experimentally nd find its value as 299 792 458 m / s

does this not mean that due to presence of dark matter..the speed of light as we measure it here is not the absolute constant?..that there is a slight error in wat we consider as a universal constant? ...plz answer me..its eating my head out

One of the properties of (non-baryonic)dark matter is that it would not interact with light at all( other than gravitationally), so even if there was a significant amount, it would not slow light.

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