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 Jul 22nd 2017, 11:29 AM #1 Member   Join Date: May 2017 Location: tampa bay florida Posts: 31 Photons and Force Under Newtonian Physics we are taught that 'things' gain motion by the imparting of a force on such 'things.' What is the force that gives motion to a photon? __________________ XML
 Jul 22nd 2017, 12:31 PM #2 Senior Member   Join Date: Nov 2013 Location: New Zealand Posts: 534 I will take a guess and say the electromagnetic field of an oscillating charge "generates" an electromagnetic wave. Edit: I am yet to figure out if a photon is a "thing" or just energy tied up in a changing electromagnetic field. I await further education on this. Last edited by kiwiheretic; Jul 22nd 2017 at 02:14 PM.
 Jul 22nd 2017, 01:06 PM #3 Member   Join Date: May 2017 Location: tampa bay florida Posts: 31 Does the charge dissipate or change in any way in 'generating' the electromagnetic wave? __________________ XML
Jul 22nd 2017, 01:08 PM   #4
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 Originally Posted by wad Under Newtonian Physics we are taught that 'things' gain motion by the imparting of a force on such 'things.' What is the force that gives motion to a photon?
That only applies to objects which have what's called a "non-zero proper mass" aka "non-zero rest mass" not to photons. In inertial frames (i.e. in the absence of a gravitational field" the velocity if a photon is both constant and invariant. In non-inertial frames it can change. The gravitational force on a photon is non-zero. But that only changes its velocity. A photon never starts out from being at rest.

Jul 22nd 2017, 02:10 PM   #5
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 Originally Posted by wad Does the charge dissipate or change in any way in 'generating' the electromagnetic wave?
I believe charge is always conserved like energy and momentum so it never "dissipates".

 Jul 22nd 2017, 02:38 PM #6 Member   Join Date: May 2017 Location: tampa bay florida Posts: 31 And what then is lost in exchange for the gain of a photon? __________________ XML
 Jul 22nd 2017, 02:47 PM #7 Senior Member   Join Date: Nov 2013 Location: New Zealand Posts: 534 Energy must be added to the system for a photon to be produced. An accelerating charged particle will lose energy in the form of electromagnetic radiation (which I picture as oscillating electric and magnetic field lines). A particle will only accelerate if a force is applied. It almost seems to be a bit of a "chicken and egg" problem. Which came first, the electromagnetic fields that cause the electric charges to move or the moving electric charges which cause changing electromagnetic fields. My personal view is that electric charges are a more "basic" constituent of matter than photons (which I believe, without any real evidence, are derived from properties of the changing electromagnetic fields) but someone on this forum may be able to prove me wrong. Should be an interesting discussion either way. Last edited by kiwiheretic; Jul 22nd 2017 at 02:49 PM.
Jul 22nd 2017, 02:57 PM   #8
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 Originally Posted by kiwiheretic Energy must be added to the system for a photon to be produced. An accelerating charged particle will lose energy in the form of electromagnetic radiation (which I picture as oscillating electric and magnetic field lines). A particle will only accelerate if a force is applied. It almost seems to be a bit of a "chicken and egg" problem. Which came first, the electromagnetic fields that cause the electric charges to move or the moving electric charges which cause changing electromagnetic fields. My personal view is that electric charges are a more "basic" constituent of matter than photons (which I believe, without any real evidence, are derived from properties of the changing electromagnetic fields) but someone on this forum may be able to prove me wrong. Should be an interesting discussion either way.
A photon moving through a gravitational field has potential energy. That has to be taken into account when considering conservation of energy.

Jul 22nd 2017, 06:08 PM   #9
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 Originally Posted by Pmb A photon moving through a gravitational field has potential energy. That has to be taken into account when considering conservation of energy.
What then is the gravitational potential energy of a photon?

Jul 22nd 2017, 06:30 PM   #10
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 Originally Posted by kiwiheretic What then is the gravitational potential energy of a photon?
That depends on the gravitational field you have in mind. For things like the earth, sun, black holes, etc. it's U = -GMm/r[sup]2[/sup]

As to why, this is best explained by deriving it. I don't know what your math skills are but if you are able to follow along then I can explain. For that reason I created a webpage to describe it. It's at: Gravitational Red Shift

If you went to some websites you'll run into mistakes since they assume that mc^2 is the energy of a photon in such cases. It's not. m is proportional to P^0 = time component of 4-momentum whereas E = P_0 which is the time component of the 1-form associated with the 4-momentum and in general they are not the same. Here's why: Conserved Quantities

Last edited by Pmb; Jul 22nd 2017 at 06:33 PM.

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