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Old Nov 22nd 2013, 10:12 AM   #1
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Faster than the speed of light?

Ok I have a question regarding black holes and light. If a photon is travelling directly towards the centre of a black hole, when it comes within its gravitational pull, theoretically it is being given GPE therefore it must go faster than the speed of light because a black hole has enough energy to overcome a photon travelling in the opposite direction. IF this is true then surly it breaks some of the laws of physics? IF not why not?
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Old Nov 22nd 2013, 02:56 PM   #2
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Originally Posted by Jamesbolt View Post
Ok I have a question regarding black holes and light. If a photon is travelling directly towards the centre of a black hole, when it comes within its gravitational pull, theoretically it is being given GPE therefore it must go faster than the speed of light because a black hole has enough energy to overcome a photon travelling in the opposite direction. IF this is true then surly it breaks some of the laws of physics? IF not why not?
The speed of a photon is constant. It is always c under any circumstances. But you are right about your GPE arguement...something has to change as the photon heads into the black hole. It is not the speed of the photon that changes, but its wavelength. A photon's energy is measured by its wavelength and when energy is added it shifts the frequency of the radiation to a higher level. ("Blue shift.")

-Dan
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Old Nov 25th 2013, 06:51 AM   #3
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Originally Posted by topsquark View Post
The speed of a photon is constant. It is always c under any circumstances. But you are right about your GPE arguement...something has to change as the photon heads into the black hole. It is not the speed of the photon that changes, but its wavelength. A photon's energy is measured by its wavelength and when energy is added it shifts the frequency of the radiation to a higher level. ("Blue shift.")

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Wow! Bingo! What would happen if a photon traveled out from a gravitational field?
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Old Nov 25th 2013, 06:59 AM   #4
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It would lose energy. E = h f where h is the Planck constant and f the freq of the photon. So the freq is lowered or wavelength increased ie, a red shift should take place i guess
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Old Nov 25th 2013, 07:09 AM   #5
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Originally Posted by physicsquest View Post
It would lose energy. E = h f where h is the Planck constant and f the freq of the photon. So the freq is lowered or wavelength increased ie, a red shift should take place i guess
So observation of a distant massive object would provide a red shift?

If so the red shift would not change unless the mass/density of the object changed or it were receding?
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Old Nov 25th 2013, 07:31 AM   #6
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Originally Posted by Troll View Post
So observation of a distant massive object would provide a red shift?

If so the red shift would not change unless the mass/density of the object changed or it were receding?
As stated the light would be redshifted. If you are talking about an object you have an additional effect: the deeper the object is in a gravitational potential well, it's clock moves slower according to an outside observer. The frequency shift here is called a "gravitational red shift."

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