Does Dark Matter Affect the Speed of Light?

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Discussion Overview

The discussion centers on the question of whether dark matter affects the speed of light, particularly in the context of its measurement and the implications for the constancy of this speed. Participants explore the theoretical and conceptual aspects of light's behavior in different media, including the presence of dark matter.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Exploratory

Main Points Raised

  • One participant states that the speed of light decreases when it moves through matter, suggesting that this could imply a variation in the speed of light due to dark matter's presence.
  • Another participant expresses uncertainty about the claim that dark matter is present throughout the galaxy and on Earth, indicating that this point needs further verification.
  • A different participant argues that while dark matter exists in small amounts, its presence would not significantly affect the speed of light, as it does not interact with light except gravitationally.
  • One participant clarifies that the apparent speed of light changes in a medium, but the actual speed remains constant, emphasizing that dark matter would not slow light down due to its non-electromagnetic interactions.
  • Another participant notes that normal matter slows light due to electromagnetic interactions, which dark matter does not possess, reinforcing the idea that dark matter cannot affect the speed of light.

Areas of Agreement / Disagreement

Participants generally disagree on the implications of dark matter's presence on the speed of light, with some asserting it has no effect while others question the constancy of the speed of light in the presence of dark matter. The discussion remains unresolved.

Contextual Notes

Participants mention the small quantities of dark matter in the Solar system and its non-interaction with light, but these points are not universally accepted or elaborated upon, leaving some assumptions unexamined.

sreekant
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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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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 traveling 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.
 
sreekant said:
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? ...please 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.
 
Last edited:
The reason why normal matter slows light waves is because of the electromagnetic interaction. Dark matter cannot interact electromagnetically, because otherwise it wouldn't be dark.
 

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