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How slowly can light travel?

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James Thomas Rook Jr. -

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A common misconception is that light travels at different speeds through different mediums. People say that the speed of light in a vacuum is cc, and then say that the speed of light through glass is slower than cc. This simply is not true.

The speed of light is always cc. Then why does it take longer for it to travel through glass? Experiments show that light "moves about  200000 km/s through glass" (as opposed to 300,000 km/s through a vacuum), so how could the speed of light be constant?

Because of this little thing called interaction.

Imagine you are receiving a prize, maybe for writing a great paper about physics. As you walk from point A to point B, you take 2 steps every second. The walkway, being 60 steps long, takes you 30 seconds to get from A to B. Your pathway looks like this.

Pretty simple right? I would think so. 

But let's say you didn't win the prize just by yourself. Some people also helped. In this case, you might want to thank your contributors as you walk to the prize. If you walk at a speed of 2 steps a second, it takes you 120 seconds now to get from the same point A, to the same point B. How is this possible? Both times you were walking at 2 steps a second, yet it took twice as much time as before? Well, let's take a step back and look at the picture.

Well that explains it. You were moving both times at 2 steps a second, but you interacted with other external things.

Now, let's substitute you for light, the walkway for glass, and the contributors for glass molecules.

No matter what, light is always moving at cc. The speed of light does not change in a vacuum, but the time from point A to point B does change, depending on what interacts with the light.

So in short, no, light does not change speeds depending on the medium it travels through. The speed of light is always cc.

Ignacio Cabero

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That is a semantics problem, and I disagree with that simplistic answer.  Velocity is measured by the distance traveled in a particular unit of time.  If you change the definitions, you can justify ANYTHING, as was done above.

For example, laser light through a particular Bose-Einstein condensate can be slowed down to about 38 miles per hour ... and whether it is passed along through quantum effects, or by Keebler Elves who got the day off from making cookies, it is STILL 38 miles per hour.

Time ON a photon is always ZERO, because at the speed of light, time stops.

That is why Quantum entangled particles can communicate instantly across ANY distance ... not limited by the speed of light.

Understanding WHY Cerenkov radiation makes that beautiful blue light in the water at the bottom of a swimming pool atomic reactor is the key to figuring it out.



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