(More or less conceptual) questions about the photon

[trand]

Hello!

I have essentially no knowledge of quantum physics, these are just questions that popped up in my mind today. I'm new to these forums, so I posted here as the quantum physics forum might be a platform for more advanced problems.

First of all, what happens to the photons, when an electromagnetic bond between molecules is broken? For example, when ice melts, what happens to the photons that carried the force to keep the molecules close together before? Are they simply released when the molecules fly away and we should experience marginal fluctuations in light levels, or is their energy (partially, at least) absorbed by the surrounding water molecules?

Could photons give their energy away completely, ceasing to exist? And when a new chemical bond (I guess it's simplest to think of only ionic and hydrogen bonds here) forms with the release of energy, where do the photons come from, to act as the bond? Are all electrically charged particles simply surrounded by a probability field of a photon forming/disappearing and in cases of counteraction with another charged particle the fields' individual probabilities of a photon forming differ and they start to level each other which would cause photons to appear? In that case (or any case a photon would appear), where do they get their energy (if photons appear/disappear constantly there wouldn't be a change in the particle system's average energy though)?

Secondly, a bit unrelated perhaps, but, speaking of a non-equilibrium thermodynamic process, when ice melts at 0C, would the average kinetic energy of the molecules also increase by the amount of energy the now free photons could give off?

 

[Bloodthunder]

1. There are different kinds of bonds between atoms/molecules, but I'll just list the ones I think you are referring to.
There is no electromagnetic bond. I think you are referring to the electrostatic bond, which is between a positive and negative ion.
Between water molecules in ice, there is hydrogen bonding, the attractive interaction of a hydrogen atom with another molecule's oxygen.
You also have van der Waals' forces, which is the sum of the attractive or repulsive forces between molecules.
Then you have covalent bonds, which is the bonds due to "sharing" of electrons between the hydrogen and oxygen atom.

When ice melts, energy is required. So the ice absorbs energy.

When water freezes, heat is removed from the water. So the closest of what I think you are talking about is where the phonon goes to. (though not quite...)

Anyway, photons can be created and destroyed.

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The simplest answer is that when a photon is absorbed by an electron, it is completely destroyed. All its energy is imparted to the electron, which instantly jumps to a new energy level. The photon itself ceases to be. In the equations which govern this interaction, one side of the equation (for the initial state) has terms for both the electron and the photon, while the other side (representing the final state) has only one term: for the electron.

The opposite happens when an electron emits a photon. The photon is not selected from a "well" of photons living in the atom; it is created instantaneously out of the vacuum. The electron in the high energy level is instantly converted into a lower energy-level electron and a photon. There is no in-between state where the photon is being constructed. It instantly pops into existence.

So the question is: where does the photon come from?

Strangely, it doesn't seem to come from anywhere. The universe must put the extra energy somewhere, and because electrons in atoms are electromagnetic phenomena, a photon is born with the required energy. In a weak-force interaction, say the decay of a neutron, that energy goes into a neutrino particle which is also instantaneously created. Each force has its own carrier particles, and knows how to make them.

From: http://curious.astro.cornell.edu/question.php?number=85
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2. If that happens, we'll have an infinite amount of energy! =) The ice absorbs heat (so draws in phonons, in a sense) here to break the bonding between the molecules. So the vibrational energy of the water would increase.And anticipating your question...

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A Photon is an elementary particle, the quantum of the electromagnetic field and the basic "unit" of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force. The effects of this force are easily observable at both the microscopic and macroscopic level, because the photon has no rest mass; this allows for interactions at long distances. Like all elementary particles, photons are governed by quantum mechanics and will exhibit wave-particle duality – they exhibit properties of both waves and particles. For example, a single photon may be refracted by a lens or exhibit wave interference, but also act as a particle giving a definite result when quantitative mass is measured.

Whereas a Phonon is a quantized mode of vibration occurring in a rigid crystal lattice, such as the atomic lattice of a solid. The study of phonons is an important part of solid state physics, because phonons play a major role in many of the physical properties of solids, including a material's thermal and electrical conductivities. In particular, the properties of long-wavelength phonons give rise to sound in solids—hence the name phonon from the Greek φωνή (phonē) . In insulating solids, phonons are also the primary mechanism by which heat conduction takes place.

From: http://answers.yahoo.com/question/index?qid=20091022053831AAVamRg
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[m.e.t.a.]

I think trand was referring to the virtual photons which mediate the electromagnetic force—not the real photons which are emitted and absorbed during electronic transitions.

I would reinterpret the question loosely as, "Please briefly explain the theory of virtual photons as the mediators of the electromagnetic force".

If this is the question, I too would be interested in a (simplified) answer.

 

[Bloodthunder]

For m.e.t.a.'s question...

The virtual photon is just the middle man of sorts in between the two charged particles.

This was my teacher's explanation, or what I can remember anyway.
We suppose 2 electrons in close proximity as 2 ice skaters, with 1 holding a ball. The act of throwing the ball from 1 to the other pushes the 2 ice skaters apart. It is this act of ball passing that repels the 2 ice skaters from each other.
Bringing back to the electrons, the ball is the virtual photon.

 

[BruceW]

Virtual photons pass between two charged particles, causing the electromagnetic force.
By definition, the virtual photon passes between two charged particles near instantaneously. This appearance of a virtual photon does violate energy conservation, but because it only exists for a very short time, the energy violation is allowed by Heisenberg's uncertainty principle.
A real photon is what we classically interpret as light. It carries its own energy and can exist independently (unlike a virtual photon). A real photon can be absorbed by an atom/molecule, which then gains an amount of energy proportional to the frequency of the absorbed photon. Conversely, if an atom gives off a real photon, it must change its state to a lower energy level.
Ionic bonds are due to the electromagnetic force (exchange of virtual photons). Covalent bonds are due to two (or more) molecules sharing electrons, which is ultimately also motivated by the electromagnetic force.

 

[pallidin]

Virtual photons pass between two charged particles, causing the electromagnetic force.
By definition, the virtual photon passes between two charged particles near instantaneously.

What is meant by "near instantaneously" in this context?
Greater than c?

 

[BruceW]

yes, they can move faster than c. Virtual gravitons (mediators of the gravitational force) can also travel much faster than c. Einstein said that no message can travel faster than the speed of light. These virtual particles don't carry information, which is why they are allowed to travel faster than c.

 

[pallidin]

Uh... virtual particles are mathematical constructs only, at this time.
There is ZERO proof of their existence.

 

[BruceW]

I agree that they don't exist in the intuitive sense of the word

 

[SpectraCat]

yes, they can move faster than c. Virtual gravitons (mediators of the gravitational force) can also travel much faster than c. Einstein said that no message can travel faster than the speed of light. These virtual particles don't carry information, which is why they are allowed to travel faster than c.

Umm .. I don't think that is correct. If two charged particles are interacting (e.g. attracted or repelled by EM force), then they must be sharing information. If the system is in equilibrium, and then particle A moves, then particle B will respond. In order for that to happen, the information about particle A's motion must somehow be transmitted between the particles. As I understand it, that information is transmitted by exchange of virtual photons, and cannot happen faster than light (although I agree that the theory allows for superluminal virtual photons). I say this with the caveat that I am not an expert in QFT or relativtistic QM, so I may have misunderstood something.

 

[BruceW]

If the (charged) particle A moves, then I'd have thought it would communicate with B using a real photon. (i.e. an accelerated charged particle gives off light).
I always assumed a virtual photon traveling at c is simply a real photon.
But I don't know much about this stuff either...