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paolog t1_je9n938 wrote

  1. The speed of light is constant, so light can't slow down.
  2. Black holes (and other massive objects) bend the space around them, so the path of a passing object (or light) bends as the object goes through that space. That doesn't slow the object or light down - it only changes its trajectory.
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lmjoe t1_je9ni55 wrote

If the speed of light can't slow down, why is it always referred to as "the speed of light in a vacuum" ?

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Dillinger0000 t1_je9o35f wrote

So light is actually slowed down through different mediums ie) to about 75% of it's speed through water, and 2/3 of it's speed through for instance glass. Gravity itself is not a medium, so does not impact the speed of light.

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somirion t1_je9s48h wrote

It is not changed, we preceive it as changed.

​

Photon thropugh water or glass is moving with the same speed as in the vacuum.

But that photon is striking into every molecule/electron it finds on its way. So we preceive speed of ray of light as 2/3 c for example.

​

Just like photon emitted in nuclear fussion in a core of a Sun - it needs about 100 000 years to be emitted to the Solar System, because there are so many molecules that its bumping into on its way.

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dirschau t1_jea303h wrote

That is incorrect. The light does actually slow down without getting absorbed or bouncing.

What is important to understand is that a photon is the oscillation of the EM field, not just a particle. Charges (like an electron) interact with the the EM field both eays, they are acted upon and act on it. Oscillations cause charges to move, but moving charges cause oscillations in the field. These effects interact with eachother in such a way that it causes the oscillation (a photon) to propagate slower in that medium than the maximum speed (speed of light) even without being absorbed.

The way you're describing it, transparency would be impossible, everything would be translucent or opaque. Like, say, the core of the sun that you mentioned.

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tangential_quip t1_je9o8ge wrote

The speed of light does change depending on the medium it is passing through. For example, it is slower in water than through a vacuum and slower through glass than it is in water.

That, however, isn't relevant to the question being asked by OP.

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ben_db t1_je9r3tz wrote

I'm not certain of this, but my understanding is that light in a vacuum is unhindered. When it's passing through a medium, it interacts with the medium and either doesn't travel straight, or is absorbed and emitted. Someone correct me if this is wrong.

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paolog t1_je9rfco wrote

The speed of light through any one medium (air, glass, a vacuum) is constant, so that is what is meant even we say "the speed of light is constant".

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