what is the meaning of a "force carrier"?

[gib65]

So I understand that the photon is the carrier of the electromagnetic force, but what does this mean? Does it mean that photons have a positive or negative charge? And when an electron emits a photon, why doesn't its charge change? I mean, if the emitted photon is carrying an electromagnetic force, I'm assuming it got it from the electron, and so the electron should have less charge after the photon leaves it. But I know electrons always have a charge of -1. So what does it mean for a photon, or any forcing carrying particle, to "carry a force"?

[swansont]

Photons are neutral; they interact via the electromagnetic force because they are oscillations of electric and magnetic fields. The electromagnetic interaction is modeled as being an exchange of virtual photons, which carry momentum and energy, hence they are carriers of the force in the interaction. All of quantum theory has a Boson as a force carrier for each of the different kinds of force. The properties of the particle are tied in with the characteristics of the force, and vice-versa. That the photon is massless means that the EM force will have a 1/r² form and an infinite range. Severian gave a short derivation of this in (IIRC) a recent thread about whether the photon has mass.

[gib65]

Well I *sort of* understand this. But does this mean that if electrons have no photons to emit (i.e. they are very cold), neither do they have any electromagnetic force? The way I understand it, electron emit photons when they drop to a lower energy level. So if all the electrons in an object are at their lowest energy level (before they fall into the nucleus), they shouldn't have any energy that can be lost in the form of photons. But then this means that they can't exchange photons with each other, which means that they attract and repel each other, which means that there's no elecromagnetic force.

 

As you can see, I'm having a tough time wrapping my head around this. I've got to be holding on to a misconception somewhere, but where?

[5614]

Photons are made when an atom wants to lose some energy.

 

The are called "force carriers" because they carry a force with them. A photon carries energy and momentum. If you consider energy and momentum as a force then photons are a force carrier, because they carry the energy/momentum between the atom which wants to lose energy and another atom which the photon happens to bump into.

[swansont]

Well I *sort of* understand this. But does this mean that if electrons have no photons to emit (i.e. they are very cold)' date=' neither do they have any electromagnetic force? The way I understand it, electron emit photons when they drop to a lower energy level. So if all the electrons in an object are at their lowest energy level (before they fall into the nucleus), they shouldn't have any energy that can be lost in the form of photons. But then this means that they can't exchange photons with each other, which means that they attract and repel each other, which means that there's no elecromagnetic force.

 

As you can see, I'm having a tough time wrapping my head around this. I've got to be holding on to a misconception somewhere, but where?[/quote']

 

 

The photons that are exchanged in exerting the EM force are virtual. They "borrow" energy in accordance with the Heisenberg uncertainty principle- as long as this happens over a short enough time, it's allowed. These photons are not detected except as a manifestation of the force.

 

Electrons changing energy levels in atoms involve real photons that can interact with anything that photons interact with.

[LucidDreamer]

These photons are not detected except as a manifestation of the force.

How do we know they are photons then? Do they display some of the same characteristics of interaction as real photons? If so, then what are these characteristics? I'm assuming that the lack of mass is one of them. Thank you for your patience.

[swansont]

How do we know they are photons then? Do they display some of the same characteristics of interaction as real photons? If so, then what are these characteristics? I'm assuming that the lack of mass is one of them. Thank you for your patience.

 

Because of the infinite range of the force it must be a massless particle, and it has to interact via the electromagnetic force. So if it isn't a photon, it still has the properties of a photon. So the model has it as a photon.

[gib65]

Electrons changing energy levels in atoms involve real photons that can interact with anything that photons interact with.

 

So let me get this straight. Virtual photons are emitted from electrons without necessarily causing the electron to drop in energy level(s), and are only used to repel other particles with negative charge or attract particles with positive charge. Whereas, actual photons are emitted from electrons when they do drop in energy level(s) and does not necessarily constitute an exchange. Instead they simple propogate outwards as EM radiation and can be obsorbed by other electrons which gives them a boost in their enery level(s) (and is gradually converted into heat).

 

Does this dual functionality apply to all force particles?

[swansont]

So let me get this straight. Virtual photons are emitted from electrons without necessarily causing the electron to drop in energy level(s)' date=' and are only used to repel other particles with negative charge or attract particles with positive charge. Whereas, actual photons are emitted from electrons when they do drop in energy level(s) and does not necessarily constitute an exchange. Instead they simple propogate outwards as EM radiation and [i']can[/i] be obsorbed by other electrons which gives them a boost in their enery level(s) (and is gradually converted into heat).

 

Does this dual functionality apply to all force particles?

 

The exchange particles for the strong and weak nuclear forces are the gluon, and the W and Z, respectively. As far as I know, there are not found on their own. However, the strong force is also described as using meson exchange - the gluons are present in the interaction between individual quarks, which do not appear on their own, but can be traded between nucleons in quark-antiquark pairs, which are mesons. Mesons exist as real particles, but are unstable and decay fairly quickly.

 

Gravitons, the proposed exchange particles for gravity, are supposed to exist as real particles as well, and would be emitted from accelerating masses.

 

The key may be that the massive exchange particles, were they to exist in a real state, would decay rapidly to other particles. But I'm beyond my depth here, so this is partly informed speculation and partly dredging up my one semester of nuclear/particle physics.