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Old Apr 6th 2014, 07:18 AM   #1
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Lightbulb What is a photon?

Can someone please explain what a photon is? Is it a particle? Thanks
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Old Apr 6th 2014, 09:40 AM   #2
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Yes, it's an elementary particle with zero mass and zero charge, but it has momentum and spin. It is the quantum of electronmagnetic radiation (light), and the carrier particle for electromagnetic force.
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Old Apr 6th 2014, 12:24 PM   #3
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Isn't momentum a composite property based on the mass, and velocity? How can it have momentum if it doesn't got any mass? Thanks
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Old Apr 6th 2014, 02:56 PM   #4
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That's true in classical mechanics. But consider that light photons have energy, and can impart that energy into physical particles, such as when an electron is knocked out of an atom by the photoelectric effect. In order for momentum to be conserved this means that the photon must have had momentum. In a related way, the pressure that light exerts is equal to its momentum times the speed of light.
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Old Apr 7th 2014, 04:31 AM   #5
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What do you mean by energy? Thanks
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Old Apr 7th 2014, 08:05 AM   #6
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Originally Posted by Anonymous View Post
What do you mean by energy? Thanks
See: http://en.wikipedia.org/wiki/Energy
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Old Apr 8th 2014, 11:00 AM   #7
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Nobody Knows

On the wikipedia page suggested by ChipB there is a suggesed "extra reading" section.
I particularly liked:
Feynman_energy.pdf

If Richard Feynman indicates that no-one knows what energy is,
I doubt you will find out on an internet forum...
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Old Apr 8th 2014, 11:33 AM   #8
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I agree ... Energy is a concept that is intuitive to understand but can't really be defined except in terms of other concepts. It's like mass, or inertia, which is defined as the property of an object that relates its acceleration to force, but then force is defined in terms of how it operates on an object's mass to cause acceleration.
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Old Jul 23rd 2017, 07:53 AM   #9
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Originally Posted by Anonymous View Post
Can someone please explain what a photon is? Is it a particle? Thanks
A photon is a quantum of electromagnetic field, which comprises an electric field and a magnetic field. The electric field is potential (i.e. irrotational) and has a negative energy, which implies a negative inertia (a negative resistance to acceleration). That is why the electric field self-accelerates to an infinite speed and acts from the future. Thus, the magnetic or actual (i.e. rotational) component of a photon is being attracted from the future by the photons electric component, which acts at an infinite speed and thus is omnipresent.
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