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A friend of mine is reading the book "Black Holes: The Reith lectures" from Stephen Hawking.

He is reading the French version, so I cannot give you the exact line, but on page 38, it is said that if someone was looking at me while I'm being absorbed by a black hole, he would see me slowing down, then floating, then my image would become paler, then redder before disappearing.

His question is: why red? Red is the least energetic wavelength, so why would he not see me violet instead?

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He would see red because the light wave coming from the person falling would get stretched due to the massive gravitational pull of the black hole. Since the wave length is increasing, it becomes red. This phenomenon is known as gravitational redshift. The light wave has to spend energy to overcome the gravitational force. Hence it loses energy by reducing it's frequency

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  • $\begingroup$ You on the other hand would see him getting violet due to the same phenomenon $\endgroup$ Dec 6 '17 at 9:51
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    $\begingroup$ "You on the other hand would see him getting violet due to the same phenomenon" - This is true for a person hovering a fixed distance outside the horizon but is it true for a person freely falling towards the horizon? $\endgroup$ Dec 6 '17 at 13:06

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