Where do photons come from?

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Humberto

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When you switch on a light, photons are created! They weren't there before. How does this work? Can they be destroyed again? Or did they exist all along, but they were released from inside another particle?
 
These little elementary particles are very intriguing. Photons are carriers for electomagnetic force and are responsible for holding matter together. But strangely, they seem to come from everywhere, and cannot be destroyed in the normal sense, rather absorbed by other particles.

The 1st law of thermodynamics tells us that energy can be transformed from one form to another, but it cannot be created or destroyed.
 
Photons are units of light energy. Energy cannot be created or destroyed, it is merely converted from one form into another. As far as I understand it photons are units of light with energy but no mass. Atoms will release photons when their electrons are excited. Electrons of course are the negatively charged particles that orbit the atomic nucleus.

Perhaps also not as well known is that electrons have a number of different energy levels and the general rule of thumb is that electrons with higher energy levels have orbits further away from the atomic nucleus (if you did science in high school they may have skirted this concept but not addressed it directly in great detail). Now when something causes energy to be invested into the a substance that change in energy is reflected in the electrons. An electron will become excited and temporarily rise to a higher energy level, it will move to an orbital state further away from the nucleus. However this state usually only persists for a tiny amount of time (a fraction of a fraction of a second) and so the electron is inevitably pulled back in towards the nucleus to resume its previous orbital level and its previous energy level. This reduction in energy requires that the electron release the surplus energy. This energy is released in the form of photons.

Regardless of the light source this underlying mechanism is what drives it. What changes is the way in which the atoms, or more specifically the electrons, are excited.

In an incandescent lightbulb the excitement comes from the movement of free electrons through the filament (an electric current - which is the movement of loosely associated electrons through a conductor). When electrons whizz through the filament they collide with the atoms that make up that filament. Those constant impacts cause the atoms being struck to wriggle and that wriggling raises the temperature of the atoms in the filament. Temperature is thermal energy, just another form of energy. The build up of thermal energy in the filament causes the electrons in the atoms making up that filament to begin temporarily rising up to higher energy levels. As mentioned previously, when they drop their orbit, their energy level, they release their surplus energy as photons... we perceive the photons falling within the visible spectrum as light.

Hope that helps.
 
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How does the physical structure of the photon form when the electron releases excess energy and is there an explanation for how the photon takes on "everywhere at once" behaviour?
 
Are photons themselves energy, or are they simply the messenger particle of the electromagnetic force?

Radio waves and gamma waves consist of photons, however the energy difference is due to the frequency of the waves, not the amount of photons.

And if photons have no mass, then why are they affected by gravity?
 
Humberto said:
Are photons themselves energy, or are they simply the messenger particle of the electromagnetic force?

Radio waves and gamma waves consist of photons, however the energy difference is due to the frequency of the waves, not the amount of photons.

And if photons have no mass, then why are they affected by gravity?
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By definition, photons are carriers that give and take when needed.

The effect of gravity is explained if you view them as particles. Photons have momentum, strong gravitational fields bend space-time and photons respond to that bend, not directly to gravity.

Perhaps porchrat is able to provide more insight about the effect of gravity on a photon in a wave-like state.
 
photons are the particles that are responsible for carrying electromagnetism. A type of force. Generally they are associated with light which is not the full story.

There are 4 forces each with its own carrier:

1) electromagnetism
2) gravity
3) weak nuclear force (like radiation)
4) strong nuclear force (bonds that hold atoms together)

so a photon is a particle that is responsible for carrying the visable light which we see and light we cant see such as gamma rays x rays infrared microwaves and so on. It is also responsible for transmitting the force magnetism.

A photon is a particle that cannot be charged or magnetized BUT it can induce both properties. When a photon has enough energy he can even displace an electron which is seen in solar panels and with cosmic rays.

So its not light (as commonly understood) and its not pure energy its a particle that carries a force
 
RiaX said:
photons are the particles that are responsible for carrying electromagnetism. A type of force. Generally they are associated with light which is not the full story.

There are 4 forces each with its own carrier:

1) electromagnetism
2) gravity
3) weak nuclear force (like radiation)
4) strong nuclear force (bonds that hold atoms together)

so a photon is a particle that is responsible for carrying the visable light which we see and light we cant see such as gamma rays x rays infrared microwaves and so on. It is also responsible for transmitting the force magnetism.

A photon is a particle that cannot be charged or magnetized BUT it can induce both properties. When a photon has enough energy he can even displace an electron which is seen in solar panels and with cosmic rays.

So its not light (as commonly understood) and its not pure energy its a particle that carries a force
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Do you have a link confirming the strong force holds the atoms together. All the material I've read states the nucleus is bound by the strong force and weak force binds the atom.

http://quantumrelativity.calsci.com/Physics/EandM1.html
 
Yes the strong force binds the nucleus together.

Photons don't have momentum though. p=mv, m=0
 
http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/photon_mass.html

What is the mass of a photon?

This question falls into two parts:

Does the photon have mass? After all, it has energy and energy is equivalent to mass.

Photons are traditionally said to be massless. This is a figure of speech that physicists use to describe something about how a photon's particle-like properties are described by the language of special relativity.

The logic can be constructed in many ways, and the following is one such. Take an isolated system (called a "particle") and accelerate it to some velocity v (a vector). Newton defined the "momentum" p of this particle (also a vector), such that p behaves in a simple way when the particle is accelerated, or when it's involved in a collision. For this simple behaviour to hold, it turns out that p must be proportional to v. The proportionality constant is called the particle's "mass" m, so that p = mv.

In special relativity, it turns out that we are still able to define a particle's momentum p such that it behaves in well-defined ways that are an extension of the newtonian case. Although p and v still point in the same direction, it turns out that they are no longer proportional; the best we can do is relate them via the particle's "relativistic mass" mrel. Thus

p = mrelv .
When the particle is at rest, its relativistic mass has a minimum value called the "rest mass" mrest. The rest mass is always the same for the same type of particle. For example, all protons, electrons, and neutrons have the same rest mass; it's something that can be looked up in a table. As the particle is accelerated to ever higher speeds, its relativistic mass increases without limit.

It also turns out that in special relativity, we are able to define the concept of "energy" E, such that E has simple and well-defined properties just like those it has in newtonian mechanics. When a particle has been accelerated so that it has some momentum p (the length of the vector p) and relativistic mass mrel, then its energy E turns out to be given by

E = mrelc2 , and also E2 = p2c2 + m2restc4 . (1)
There are two interesting cases of this last equation:

If the particle is at rest, then p = 0, and E = mrestc2.
If we set the rest mass equal to zero (regardless of whether or not that's a reasonable thing to do), then E = pc.
In classical electromagnetic theory, light turns out to have energy E and momentum p, and these happen to be related by E = pc. Quantum mechanics introduces the idea that light can be viewed as a collection of "particles": photons. Even though these photons cannot be brought to rest, and so the idea of rest mass doesn't really apply to them, we can certainly bring these "particles" of light into the fold of equation (1) by just considering them to have no rest mass. That way, equation (1) gives the correct expression for light, E = pc, and no harm has been done. Equation (1) is now able to be applied to particles of matter and "particles" of light. It can now be used as a fully general equation, and that makes it very useful.

Is there any experimental evidence that the photon has zero rest mass?

Alternative theories of the photon include a term that behaves like a mass, and this gives rise to the very advanced idea of a "massive photon". If the rest mass of the photon were non-zero, the theory of quantum electrodynamics would be "in trouble" primarily through loss of gauge invariance, which would make it non-renormalisable; also, charge conservation would no longer be absolutely guaranteed, as it is if photons have zero rest mass. But regardless of what any theory might predict, it is still necessary to check this prediction by doing an experiment.

It is almost certainly impossible to do any experiment that would establish the photon rest mass to be exactly zero. The best we can hope to do is place limits on it. A non-zero rest mass would introduce a small damping factor in the inverse square Coulomb law of electrostatic forces. That means the electrostatic force would be weaker over very large distances.

Likewise, the behavior of static magnetic fields would be modified. An upper limit to the photon mass can be inferred through satellite measurements of planetary magnetic fields. The Charge Composition Explorer spacecraft was used to derive an upper limit of 6 × 10−16 eV with high certainty. This was slightly improved in 1998 by Roderic Lakes in a laboratory experiment that looked for anomalous forces on a Cavendish balance. The new limit is 7 × 10−17 eV. Studies of galactic magnetic fields suggest a much better limit of less than 3 × 10−27 eV, but there is some doubt about the validity of this method.
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[video=youtube;R9FVFh3HYaY]http://www.youtube.com/watch?v=R9FVFh3HYaY[/video]
 
Sinbad said:
Yes the strong force binds the nucleus together.

Photons don't have momentum though. p=mv, m=0
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You are attempting to use classical or Newtonian mechanics to describe something described by quantum mechanics.

The system involves things travelling at the speed of light. I don't quite understand why but the rules are different at that point. (my knowledge of quantum mechanics is next to worthless :p)
 
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porchrat said:
Photons are units of light energy. Energy cannot be created or destroyed, it is merely converted from one form into another. As far as I understand it photons are units of light with energy but no mass. Atoms will release photons when their electrons are excited. Electrons of course are the negatively charged particles that orbit the atomic nucleus.

Perhaps also not as well known is that electrons have a number of different energy levels and the general rule of thumb is that electrons with higher energy levels have orbits further away from the atomic nucleus (if you did science in high school they may have skirted this concept but not addressed it directly in great detail). Now when something causes energy to be invested into the a substance that change in energy is reflected in the electrons. An electron will become excited and temporarily rise to a higher energy level, it will move to an orbital state further away from the nucleus. However this state usually only persists for a tiny amount of time (a fraction of a fraction of a second) and so the electron is inevitably pulled back in towards the nucleus to resume its previous orbital level and its previous energy level. This reduction in energy requires that the electron release the surplus energy. This energy is released in the form of photons.

Regardless of the light source this underlying mechanism is what drives it. What changes is the way in which the atoms, or more specifically the electrons, are excited.

In an incandescent lightbulb the excitement comes from the movement of free electrons through the filament (an electric current - which is the movement of loosely associated electrons through a conductor). When electrons whizz through the filament they collide with the atoms that make up that filament. Those constant impacts cause the atoms being struck to wriggle and that wriggling raises the temperature of the atoms in the filament. Temperature is thermal energy, just another form of energy. The build up of thermal energy in the filament causes the electrons in the atoms making up that filament to begin temporarily rising up to higher energy levels. As mentioned previously, when they drop their orbit, their energy level, they release their surplus energy as photons... we perceive the photons falling within the visible spectrum as light.

Hope that helps.
Click to expand...

If I was a girl looking for an awesome science geek, I think you'd be my man. :love:
 
copacetic said:
If I was a girl looking for an awesome science geek, I think you'd be my man. :love:
Click to expand...
LOL thanks... I think :p

It wasn't an awfully good explanation though. I just told him where photons fit in when you turn the light on. Looking back I realise that isn't actually what he was asking. He was asking about the nature of photons as described by quantum mechanics and unfortunately I'm largely ignorant of that. I have found the contributions others have made so far very interesting though, especially that quick explanation of the momentum of photons.
 
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