Physics Help Forum What is the frequency of a single photon?

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 Jan 9th 2015, 03:39 PM #1 Junior Member   Join Date: Jan 2013 Posts: 24 What is the frequency of a single photon? What frequency means for a single photon?
 Jan 9th 2015, 04:27 PM #2 Senior Member     Join Date: Apr 2008 Location: Bedford, England Posts: 668 A photon is a packet of energy, The frequency is a measure of the energy content of the photon. __________________ You have GOT to Laugh !
 Jan 10th 2015, 07:53 AM #3 Junior Member   Join Date: Jan 2013 Posts: 24 Then when it is redshifted where its energy goes to? When it is blueshifted whence it gains energy? Last edited by harve; Jan 10th 2015 at 09:21 AM.
 Jan 11th 2015, 11:21 AM #4 Physics Team     Join Date: Jun 2010 Location: Morristown, NJ USA Posts: 2,344 Red shifting or blue shifting is perceived by the observer and is caused by the motion of the observer relative to the source. Asking where the energy goes is like asking where kinetic energy of an object goes if the observer is moving with it, so that the object's relative speed is 0. Imagine a fire a bullet from a gun - it has a huge amount of kinetic energy, based on its motion relative to me. But for an observer in a jet plane traveling at the sane speed as the bullet the bullet has no kinetic energy. Who's right? We both are.
Jan 17th 2015, 08:03 AM   #5
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 Originally Posted by ChipB Red shifting or blue shifting is perceived by the observer...
Even when we're talking about gravitational redshift/blueshift. Not many people know this, but it's pretty obvious if you think about it. Consider sending a 511keV photon into a black hole. The black hole mass increases by 511keV/c². The photon didn't gain any energy. Conservation of energy applies, as you'd expect, because gravity is not a force in the Newtonian sense.

Jan 17th 2015, 11:32 AM   #6

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 Originally Posted by Farsight Conservation of energy applies, as you'd expect, because gravity is not a force in the Newtonian sense. [/SIZE][/FONT]
I'm unclear about the reasoning in your comment. Usually it is the geometric interpretation of GR that people argue against, not it's validity as a force. My web search gave me nothing...Are you saying that GR does not predict a conservative force?

-Dan
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 Jan 18th 2015, 01:43 PM #7 Senior Member   Join Date: May 2014 Location: Poole, UK Posts: 132 No. What I'm saying is that most people will say that a downward-moving photon is blueshifted, and that it acquires a higher frequency and gains energy. It doesn't. Its frequency doesn't change, nor does its E=hf energy. The variation of energy for the particle is zero. However you measure a higher frequency because you and your clocks go slower when you go lower. The photon doesn't change, you do. You lose energy, so the photon appears to have gained it.
Jan 18th 2015, 02:46 PM   #8

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 Originally Posted by Farsight No. What I'm saying is that most people will say that a downward-moving photon is blueshifted, and that it acquires a higher frequency and gains energy. It doesn't. Its frequency doesn't change, nor does its E=hf energy. The variation of energy for the particle is zero. However you measure a higher frequency because you and your clocks go slower when you go lower. The photon doesn't change, you do. You lose energy, so the photon appears to have gained it.
Oh it gets better because if you are viewing the photon from a large distance from the Earth the photon is red shifted.

I'm going to have to ponder this one. If this didn't involve light then I'd suggest talking about the photon in its own reference frame but we can't do that since it's moving at c.

-Dan
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Jan 19th 2015, 02:28 AM   #9
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 Originally Posted by topsquark Oh it gets better because if you are viewing the photon from a large distance from the Earth the photon is red shifted.
An ascending photon doesn't get redshifted! Its frequency doesn't change one jot! When I lift you up to space I do work on you. You gain energy. So the photon appears to have lost energy. But it hasn't.

There's some really interesting things lurking in general relativity. You tend not to bump into them, and they don't seem to get taught. It's only when you "do your own research" that you find out about them. A related matter is invariant mass. And as you may have guessed already: it varies! How's that for shiny?

 Jan 19th 2015, 11:35 AM #10 Junior Member   Join Date: Oct 2014 Posts: 13 I'm guessing that Farsight is suggesting that there is one absolute system of coordinates in which to consider the physical events, such that photons never, ever receive a redshift or blueshift because the photons are always the same, just produced by a physical system that is dime dilated relative to the receiver. This contradicts standard GR, where one can use any system of coordinates. In many systems of coordinates, the redshift or blueshift is introduced to the photons.

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