How does a photon carry force?

[FireBones]

I've looked through about 40-50 of the threads dealing with photons and have not found one that describes how the photon transmits the force of the EM field.

In Feynman's QED, he talks about how a nucleus keeps an electron in orbit by exchange of photons, but I don't see how a photon can provide a push, much less a pull.

If I think of photon as a light quanta and consider light as a perturbation in the EM field, I'm stymied by the fact that the ripples in the EM field are transverse to the direction of propagation. A particle at (1,0,0) transmitting a light quanta to a particle at (0,0,0) creates an EM wave that oscillates in the y- and z-directions, so this does not seem availing to a force in the x-direction.

 

[tiny-tim]

… I'm stymied by the fact that the ripples in the EM field are transverse to the direction of propagation. A particle at (1,0,0) transmitting a light quanta to a particle at (0,0,0) creates an EM wave that oscillates in the y- and z-directions, so this does not seem availing to a force in the x-direction.

Hi FireBones!

(leaving aside the question of whether photons do transmit EM force …)

The ripples look longitudinal to me, just like the ripples when you throw a stone into water.

And the energy flow of an EM field with (transverse) electric and magnetic components E and B is in the direction of the Poynting vector, E x B.

 

[jeblack3]

Consider an electron some distance, much more than the size of an atom, from a proton. (Let's not worry for the moment about what happens at the atomic scale.)

The reason the electron accelerates is because of interference between the case where a photon is exchanged, and the case where it isn't.

To start, do you understand why the electron doesn't accelerate if the proton isn't there? Why it approximately obeys Newton's first law?

 

[FireBones]

Hi FireBones!

(leaving aside the question of whether photons do transmit EM force …)

The ripples look longitudinal to me, just like the ripples when you throw a stone into water.

And the energy flow of an EM field with (transverse) electric and magnetic components E and B is in the direction of the Poynting vector, E x B.

I'm not concerned with energy flow here. I'm interested in force.

Ripples caused by a stone thrown into water are transverse.

 

[FireBones]

Consider an electron some distance, much more than the size of an atom, from a proton. (Let's not worry for the moment about what happens at the atomic scale.)

The reason the electron accelerates is because of interference between the case where a photon is exchanged, and the case where it isn't.

To start, do you understand why the electron doesn't accelerate if the proton isn't there? Why it approximately obeys Newton's first law?

Jeblack, I don't really know what to make of your question... I guess I don't know why an electron wouldn't obey Newton's first law...

But I am interested in understanding how "interference between the case where a photon is exchanged and the case where it isn't" causes the acceleration.

 

[LostConjugate]

An electron pushes on all other electrons around it.

If electron1 is pushing on electron2 and e1 accelerates in a perpendicular direction the pushing force of e1 on e2 will now have an additional component in this perpendicular direction.

This component is what we measure as the force of the electromagnetic wave on another electron. The original electric force is not commonly included in equations, though it comes up every once in awhile.

The act of applying a force in a way is like making a connection between you and the object. If you are pushing on a wall and you simultaneously start pushing up with your legs you will find that you are now pushing the wall both away from you and up. Not that it will go anywhere.

For a complete understanding see http://web.mit.edu/smcs/8.02/

 

[LostConjugate]

In Feynman's QED, he talks about how a nucleus keeps an electron in orbit by exchange of photons, but I don't see how a photon can provide a push, much less a pull.

.

The photon provides kinetic energy, the direction does not matter. The reason why is because there is no physical state that exists where the electron crashes into the nucleus. This means that if an electron is given kinetic energy it will always be beneficial to keeping the electron in orbit.

 

[FireBones]

An electron pushes on all other electrons around it.

If electron1 is pushing on electron2 and e1 accelerates in a perpendicular direction the pushing force of e1 on e2 will now have an additional component in this perpendicular direction.
http://web.mit.edu/smcs/8.02/

I don't see how this relates to my original question.

Yes, if one electron moves perpendicular with respect to the displacement vector between them, the new force will have a component perpendicular to the old displacement vector...but what does that have to do with anything?

My question is "how does a proton (at rest) exert force on an electron by means of a photon." How does a photon "carry force" in the x-direction if its EM perturbation is in the y/z plane?

 

[LostConjugate]

I don't see how this relates to my original question.

Yes, if one electron moves perpendicular with respect to the displacement vector between them, the new force will have a component perpendicular to the old displacement vector...but what does that have to do with anything?

My question is "how does a proton (at rest) exert force on an electron by means of a photon." How does a photon "carry force" in the x-direction if its EM perturbation is in the y/z plane?

The same way the force from an electron still maintains a force in the x-direction when the EM is in the y/z. In an EM wave there is always a force in every direction, because of the pure electric and magenetic forces.

 

[FireBones]

The photon provides kinetic energy, the direction does not matter. The reason why is because there is no physical state that exists where the electron crashes into the nucleus. This means that if an electron is given kinetic energy it will always be beneficial to keeping the electron in orbit.

I'm not interested in "why doesn't the electron crash into the nucleus" questions...I'm interested in how the photon allows the proton to exert force on the electron. And I fail to see how force [a vector quantity] can be articulated upon an object purely by its receipt of energy. Clearly direction does matter since force is not a scalar.

 

[FireBones]

The same way the force from an electron still maintains a force in the x-direction when the EM is in the y/z. In an EM wave there is always a force in every direction, because of the pure electric and magenetic forces.

Nothing you have said has explained how a photon allows one charged particle to exert forces on another.

 

[Frame Dragger]

Nothing you have said has explained how a photon allows one charged particle to exert forces on another.

Doesn't, "By adding energy and possibly causing emission" answer that question? It seems fundamental to the issue of emitters and absorbers, and I'm not clear where your confusion is coming from.

 

[LostConjugate]

Nothing you have said has explained how a photon allows one charged particle to exert forces on another.

How charged particles exert forces on each other I do not think we have an explanation for.

What you are asking is how does the electric force exist.

There may be a QM explanation for it, but its way beyond me if there is. From what I understand we only know the properties of the electric force.

 

[Antiphon]

The energetic (non virtual) photon is the one you describe and it would transmit a transvers force. These are waves.

The force you describe is longitudinal but the photons have no energy. They are virtual with infinite wavelength (or perhaps an undefined wavelength?).

A virtual photon is not a free quantum like the light emitted by an atom. It has zero energy.

 

[FireBones]

Doesn't, "By adding energy and possibly causing emission" answer that question? It seems fundamental to the issue of emitters and absorbers, and I'm not clear where your confusion is coming from.

Because simply adding energy does not explain the exertion of a force.

Forget the proton/electron example for now. Just consider two electrons in free space. One electron [A] at (0,0,0), the other at (1,0,0).

Now, how does a photon traveling from A to B pull B toward A? Simply saying that "B absorbs the photon, increasing its kinetic energy" doesn't give any indication of why B should be accelerated in the direction of A.

energy is a scalar, so you cannot explain a force exerted by an object as purely due to the energy... To use a rather silly personification: an electron that gains energy might know it should be speeding up, but it won't know in what direction.

Furthermore, if electron B happens to be moving with respect to electron A, this force might not be increasing its kinetic energy.

The issue here is not ionization energy and atomic physics. I was just using Feynman's book as an example of someone talking about force exchange through photons. I'm talking about the very general question of "how does a photon exert force in a direction parallel to its transmission?"

 

[Frame Dragger]

Because simply adding energy does not explain the exertion of a force.

Forget the proton/electron example for now. Just consider two electrons in free space. One electron [A] at (0,0,0), the other at (1,0,0).

Now, how does a photon traveling from A to B pull B toward A? Simply saying that "B absorbs the photon, increasing its kinetic energy" doesn't give any indication of why B should be accelerated in the direction of A.

energy is a scalar, so you cannot explain a force exerted by an object as purely due to the energy... To use a rather silly personification: an electron that gains energy might know it should be speeding up, but it won't know in what direction.

Furthermore, if electron B happens to be moving with respect to electron A, this force might not be increasing its kinetic energy.

The issue here is not ionization energy and atomic physics. I was just using Feynman's book as an example of someone talking about force exchange through photons. I'm talking about the very general question of "how does a photon exert force in a direction parallel to its transmission?"

The only current answer I know of is that involving the exchange of virtual photons, which to me is the same as "We can make it work on paper, but this isn't real life." As for assigning an electron a direction... that seem to fly in the face of how an electron is defined in QM.

 

[FireBones]

The energetic (non virtual) photon is the one you describe and it would transmit a transvers force. These are waves.

The force you describe is longitudinal but the photons have no energy. They are virtual with infinite wavelength (or perhaps an undefined wavelength?).

A virtual photon is not a free quantum like the light emitted by an atom. It has zero energy.

That helps a little...at least it basically says "Anything you know about photon as a light quanta has nothing at all to do with how photon act as force carriers."

But it also seems to me that you are saying we have no idea how or why photons exert force...but rather that they simply do.

Do these virtual photons have momentum? It seems like they must since they are essentially transferring momentum from one particle to the other. Except it would also mean that, in some cases, the photons have momenta in the direction opposite to their motion

Can anyone elaborate/confirm/correct this?

it seems pretty strange for photons to be conceived of in these two completely different ways...

 

[Frame Dragger]

I think we need a mentor or advisor on this one. Tiny Tim... Don't go tiptoeing through the tulips yet!

 

[Antiphon]

The way photons exert forces on charges is by modifying the phase term in thei QM wave function. The presence of an electric field will shift the position and momentum of the electrons wave packet. One might say there's no force, just a readjustment to the dynamic variables of the electron. In doing this the photon is absorbed (including it's energy). This is how it works for virtual photons too except the QM phase change is pointed longitudinally and the energy gained doesn't come from the virtual quantum but from the radiation fields of the accelerating charges.

In other words no energy is expended by pulling on something. It has to move as a result of your pulling on it.

 

[Frame Dragger]

The way photons exert forces on charges is by modifying the phase term in thei QM wave function. The presence of an electric field will shift the position and momentum of the electrons wave packet. One might say there's no force, just a readjustment to the dynamic variables of the electron. In doing this the photon is absorbed (including it's energy). This is how it works for virtual photons too except the QM phase change is pointed longitudinally and the energy gained doesn't come from the virtual quantum but from the radiation fields of the accelerating charges.

In other words no energy is expended by pulling on something. It has to move as a result of your pulling on it.

... So this is just another aspect of Perturbation in a QFT?

 

[FireBones]

Consider an electron some distance, much more than the size of an atom, from a proton. (Let's not worry for the moment about what happens at the atomic scale.)

The reason the electron accelerates is because of interference between the case where a photon is exchanged, and the case where it isn't.

To start, do you understand why the electron doesn't accelerate if the proton isn't there? Why it approximately obeys Newton's first law?

Jeblack,
I found a discussion that appears to explain what is going on by passing to momentum space, and I think it is saying the same thing you are.

http://math.ucr.edu/home/baez/physics/Quantum/virtual_particles.html"

I need to reread this discussion, but it is along the lines of what I was looking for. An honest effort to explain how photons actually can transmit force.

 

[FireBones]

The way photons exert forces on charges is by modifying the phase term in thei QM wave function. The presence of an electric field will shift the position and momentum of the electrons wave packet. One might say there's no force, just a readjustment to the dynamic variables of the electron. In doing this the photon is absorbed (including it's energy). This is how it works for virtual photons too except the QM phase change is pointed longitudinally and the energy gained doesn't come from the virtual quantum but from the radiation fields of the accelerating charges.

In other words no energy is expended by pulling on something. It has to move as a result of your pulling on it.

Thanks, Antiphon...can you take a look at the link I just posted in my response to jeblack and see if it is what you are getting at? It is the sort of argument I can follow, so if that is what you are discussing, then I think I'm good here. If not, feel free to critique his version or modify it, etc.

I found that article well written in the sense that it was very much focused on answering just the question involved [and cutting through a bunch of other considerations that are important when doing calculations but not so important when trying to drill down on this one specific conceptual issue.)

 

[Vanadium 50]

And I fail to see how force [a vector quantity] can be articulated upon an object purely by its receipt of energy. Clearly direction does matter since force is not a scalar.

OK, I think we are getting to the kernel of the problem. This has nothing to do with photons or quantum mechanics: force is the gradient of potential energy. Are you OK with this or do we have to address it?

 

[tiny-tim]

I think we need a mentor or advisor on this one. Tiny Tim... Don't go tiptoeing through the tulips yet!

Sock it to me? ​

Actually, I've been hiding in the tulips , because this whole discussion is getting totally weird.

i] Virtual photons don't exist (except in the maths) … they aren't real … the clue's in the name!

ii] virtual photons (in the maths) do have energy, and do have momentum, but they aren't connected to each other by the on-shell equation

For a very full discussion on virtual photons, see the thread https://www.physicsforums.com/showthread.php?t=372021" ​

(btw, Vanadium 50 knows a lot more about this than i do!)

 

[FireBones]

OK, I think we are getting to the kernel of the problem. This has nothing to do with photons or quantum mechanics: force is the gradient of potential energy. Are you OK with this or do we have to address it?

My question is "how do photons carry force," so I think it pertains to both photons and [if Jeblack and a Antiphon are to be believed] quantum mechanics.

I can except how an energy field gives rise to force [because its gradient obviously is a vector], but that was not my objection. A previous post-er was saying that "force" was due to the particle receiving energy. I don't think he had a potential field in mind, as he was referring to a photon transmitting that energy.

 

[Frame Dragger]

Sock it to me? ​

Actually, I've been hiding in the tulips , because this whole discussion is getting totally weird.

i] Virtual photons don't exist (except in the maths) … they aren't real … the clue's in the name!

ii] virtual photons (in the maths) do have energy, and do have momentum, but they aren't connected to each other by the on-shell equation

For a very full discussion on virtual photons, see the thread https://www.physicsforums.com/showthread.php?t=372021" ​

(btw, Vanadium 50 knows a lot more about this than i do!)

I don't blame you at all, but to be fair I think the "non-existent" (beyond the math and diagrams) nature of virtual photons was established. Really, someday there has to be a better way of describing ths interaction without such confusing language; "virtual photons"... that is begging for misunderstandings.

As for the last, the only example I can think of would be Hawking Radiation, but then, those photons are not virtual... (head... hurts).

 

[FireBones]

Sock it to me? ​

Actually, I've been hiding in the tulips , because this whole discussion is getting totally weird.

I don't see what is weird about it...other than that several people appeared to ignore the question I was asking...which seems like a perfectly reasonable question: If photons are the mediator of the EM force, then how do they do it?

I think the link I posted explains this in the way that Jeblack and Antiphon were hinting at.

i] Virtual photons don't exist (except in the maths) … they aren't real … the clue's in the name!

What is "real"? Just because something is called "virtual" doesn't mean it doesn't exist. The algebraic extension of the Real numbers to make it algebraically closed is called the "imaginary" unit but imaginary numbers have real manifestations. One could make an argument that negative numbers are no more "real" than imaginary ones...they both are algebraically motivated elements that have manifestations in the real world.

But getting back to virtual photons. You say they "aren't real." If you simply mean that they cannot be detected, then the question becomes "what does it mean for something to be detected?" Quarks were originally simply seen (at least by some physicists) as mere mathematical abstractions, or "ways to make the math work out."

If we have as our model the standard model, which says that the EM force is due to these beasts, then every time we observe the EM force in the lab, it is an (indirect) observation of them. We have confidence in saying that a "real" photon exists because we see, for example, an excitation of an atom's electrons. We observe a change and say it counts as an observation of an energetic photon. If a charged particle causes another charged particle to change its momentum, how is that not the same situation. We see a change in momentum and attribute it to a "virtual" photon just like we see a change in (for example) an electron or atom's energy state and attribute that to a "real" photon. We didn't observe the "real" photon prior to seeing the change it made.

 

[LostConjugate]

A photon is a quanta of the EM wave, and the EM wave carries force proven by experiment. So by definition a photon carries force.

 

[FireBones]

A photon is a quanta of the EM wave, and the EM wave carries force proven by experiment. So by definition a photon carries force.

Uhm...I think I've been about as clear as I can be that my question is not whether but how, which has been satisfactorily answered (I think) by the link I found...which corresponds to what Jeblack and Antiphon [I think] were saying.

 

[Frame Dragger]

@FireBones: No... virtual photons are a mathematical trick, they have and never HAVE had anything to do with a physical reality. They are useful in describing and calculating... that's it. We're not talking about pions here.

 

[LostConjugate]

If you found an answer to how a photon carries force in the direction it is traveling that would be very interesting as it would explain why charged particles attract/repel. The link you found is about virtual photons however.

 

[Antiphon]

@FireBones: No... virtual photons are a mathematical trick, they have and never HAVE had anything to do with a physical reality. They are useful in describing and calculating... that's it. We're not talking about pions here.

They may be a mathematical trick but the forces are quite real. The OP is wondering what (in the theory) transmits longitudinal force. I'm pretty sure it's the virtual photon.

Can anyone address the obvious questio: if virtual photons have no physical reality then isn't QED missing some major physical mechanism that it "fakes in" with the virtual photon?

 

[lightarrow]

I don't see what is weird about it...other than that several people appeared to ignore the question I was asking...which seems like a perfectly reasonable question: If photons are the mediator of the EM force, then how do they do it?

I imagine the fact photons have momentum and that a force is a variation in momentum doesn't help, is it? You want to know the mechanism. But you should have a model for the photon, and we don't have...

 

[FireBones]

If you found an answer to how a photon carries force in the direction it is traveling that would be very interesting as it would explain why charged particles attract/repel. The link you found is about virtual photons however.

...which do transmit the force!

http://hyperphysics.phy-astr.gsu.edu/hbase/forces/exchg.html#c1"

"[URL
http://en.wikipedia.org/wiki/Force_carrier

@FireBones: No... virtual photons are a mathematical trick, they have and never HAVE had anything to do with a physical reality. They are useful in describing and calculating... that's it. We're not talking about pions here.

They transmit force. Are you suggesting force has nothing to do with physical reality?

 

[lightarrow]

...which do transmit the force!

http://hyperphysics.phy-astr.gsu.edu/hbase/forces/exchg.html#c1"

"[URL
http://en.wikipedia.org/wiki/Force_carrier
They transmit force. Are you suggesting force has nothing to do with physical reality?

But which is (I am just asking) the physical meaning of a particle (I'm talking of virtual particles of course) which energy E and momentum p don't obey E² = (cp)² + (mc²)² ?
How much or to which extent can we believe in the physical existence of a matematical tool which is very useful in physics but cannot be directly measured? (Difficult question, I know)

 

[Frame Dragger]

I'm suggesting that "virtual photons" describe an intermediate period which current theory cannot. Actually, I'm suggesting no such thing, did anyone even BOTHER to pay attention to what Tiny Tim posted?

 

[SpectraCat]

Hmmm ... I have heard it said a lot that virtual particles were added just to "make the math work out", but that does not gibe with my understanding. My understanding was that they were added to QED to make the *physics* work out. Specifically, unless there is some stream of particles that are subject to special relativity being exchanged by the electron and proton, then they are undergoing "spooky action at a distance", right? Isn't this what is illustrated by the thought experiment of what would happen if the proton suddenly blinked out of existence? According to SR, there must be a delay before the electron "knows" that the proton is gone. This can only happen if the proton and electron are somehow exchanging "information" about each other's presence, and unless I misunderstand, that is the function of the virtual photons.

 

[FireBones]

But which is (I am just asking) the physical meaning of a particle (I'm talking of virtual particles of course) which energy E and momentum p don't obey E² = (cp)² + (mc²)² ?

Can you explain why virtual photons do not obey this law? I'm not challenging that you are correct, but I hadn't seen before that virtual photons fail this equation. Or am I misreading you?

How much or to which extent can we believe in the physical existence of a matematical tool which is very useful in physics but cannot be directly measured? (Difficult question, I know)

But this gets back to what I had asked earlier. What does it mean to measure something "directly." When we say we "detect" a real photon what we mean is that we see an excitation occur and we say this is a detection of energy being transferred from some other source to the atom/electron/phonon/whatever that absorbed the photon. We did not "see" the photon midflight, we saw its effect and we call that a "detection" of the photon.

But isn't the same thing happening with, say, an electron being accelerated to a proton? We see a change in momentum of the electron and deduce that momentum has been transferred..but by what agent? Can't the measurement of a change in momentum count as just as much a "detection" of a "virtual" photon as the the change in energy of an electron/atom/phonon/whatever be a 'detection" of a "real" photon?


I do have a further question though... photons are the mediator of the EM force. [There have been a few threads that start out with the question 'What is a photon?' and occasionally someone will answer "the force carrier of the EM force." as the only answer.]

But this means that all photons have to be force carriers, not just these virtual ones. While I alluded to virtual ones in the Feynman book, it seems that this is unnecessary. If "the carrier of the EM force" is a suitable definition for "photon," than any photon should be transmitting EM force...not just the virtual ones.

And so now I'm wondering what in the world that means for just everyday, ordinary photons. For example, we say something like "An electron drops from n=3 to n=1 orbital and a photon is released." What does this electron moving orbitals have to do with a conveyance of EM force?

Is it possible for this force to be attractive rather than repulsive? If only virtual photons can carry repulsive force, can someone tell me how the argument in http://math.ucr.edu/home/baez/physics/Quantum/virtual_particles.html" changes when the photon in question is 'real' rather than virtual?

[Perhaps the answer to that last question is that "real" photons are detected, hence collapsing their wave function, hence you no longer have the interference between the two momenta amplitudes?]

 

[Frame Dragger]

Hmmm ... I have heard it said a lot that virtual particles were added just to "make the math work out", but that does not gibe with my understanding. My understanding was that they were added to QED to make the *physics* work out. Specifically, unless there is some stream of particles that are subject to special relativity being exchanged by the electron and proton, then they are undergoing "spooky action at a distance", right? Isn't this what is illustrated by the thought experiment of what would happen if the proton suddenly blinked out of existence? According to SR, there must be a delay before the electron "knows" that the proton is gone. This can only happen if the proton and electron are somehow exchanging "information" about each other's presence, and unless I misunderstand, that is the function of the virtual photons.

All of that is beyond current physics however, which is where the mathematics comes in. I could be wrong, but the thread linked to by Tiny Tim was pretty definitive as I remember. Hans De Vries had some interesting points to make as well.

 

[jeblack3]

Jeblack,
I found a discussion that appears to explain what is going on by passing to momentum space, and I think it is saying the same thing you are.

http://math.ucr.edu/home/baez/physics/Quantum/virtual_particles.html"

I need to reread this discussion, but it is along the lines of what I was looking for. An honest effort to explain how photons actually can transmit force.

I was looking for that link, but couldn't find it.

You don't have to go into momentum space to understand this, though. You can do it in ordinary position space. Consider an electron some distance from a proton. The probability amplitude to absorb a photon is larger on the side of the electron's wavepacket closest to the proton. And that probability amplitude is imaginary. So when you add the contribution to the new wavefunction from the electron absorbing a photon to the contribution from the electron not absorbing a photon, the effect is to multiply the electron's wavefunction by a phase, which the phase depending on how close you are to the proton. This sets the electron in motion towards the proton.

 

[haael]

The simplest intuitive answer is: virtual photons that mediate attraction forces have negatie energy. Virtual particles violate conservation of energy, right? They can violate it both ways - too much or too little.

When you throw a ball forward, you are pushed backward. But when you throw a ball of negative weight forward, you will be pulled forward, too.

And now slightly more formally:
Virtual particles are physically real, but they are not particles. There are no esoteric photons floating here and there. It's the electromagnetic field that carries forces. Just as photons are excitations of electromagnetic field, the electromagnetic field is a logical anti-excitation of virtual photons.

But we can not think of virtual particles as actual particles. They are not.

 

[seniornegro]

Can you explain why virtual photons do not obey this law? I'm not challenging that you are correct, but I hadn't seen before that virtual photons fail this equation. Or am I misreading you?




But this gets back to what I had asked earlier. What does it mean to measure something "directly." When we say we "detect" a real photon what we mean is that we see an excitation occur and we say this is a detection of energy being transferred from some other source to the atom/electron/phonon/whatever that absorbed the photon. We did not "see" the photon midflight, we saw its effect and we call that a "detection" of the photon.

But isn't the same thing happening with, say, an electron being accelerated to a proton? We see a change in momentum of the electron and deduce that momentum has been transferred..but by what agent? Can't the measurement of a change in momentum count as just as much a "detection" of a "virtual" photon as the the change in energy of an electron/atom/phonon/whatever be a 'detection" of a "real" photon?


I do have a further question though... photons are the mediator of the EM force. [There have been a few threads that start out with the question 'What is a photon?' and occasionally someone will answer "the force carrier of the EM force." as the only answer.]

But this means that all photons have to be force carriers, not just these virtual ones. While I alluded to virtual ones in the Feynman book, it seems that this is unnecessary. If "the carrier of the EM force" is a suitable definition for "photon," than any photon should be transmitting EM force...not just the virtual ones.

And so now I'm wondering what in the world that means for just everyday, ordinary photons. For example, we say something like "An electron drops from n=3 to n=1 orbital and a photon is released." What does this electron moving orbitals have to do with a conveyance of EM force?

Is it possible for this force to be attractive rather than repulsive? If only virtual photons can carry repulsive force, can someone tell me how the argument in http://math.ucr.edu/home/baez/physics/Quantum/virtual_particles.html" changes when the photon in question is 'real' rather than virtual?

[Perhaps the answer to that last question is that "real" photons are detected, hence collapsing their wave function, hence you no longer have the interference between the two momenta amplitudes?]

A photon isn't real. Its a quanta of EMR. Its like a gallon or a pint...

A photon being a quanta of energy and energy being the amount of work that can be performed by a force, the photon carries the gas needed / transferred or expended by a force.

 

[Frame Dragger]

A photon isn't real. Its a quanta of EMR. Its like a gallon or a pint...

A photon being a quanta of energy and energy being the amount of work that can be performed by a force, the photon carries the gas needed / transferred or expended by a force.

Wow, those photon detectors have something coming to them! Yes, photons are real, detected, and have appreciable physical properties.

 

[acentauri]

For what it's worth, classical http://iopscience.iop.org/0295-5075/76/2/189?ejredirect=migration".

http://arxiv.org/abs/0907.1611" a related paper.

 

[SpectraCat]

A photon isn't real. Its a quanta of EMR. Its like a gallon or a pint...

A photon being a quanta of energy and energy being the amount of work that can be performed by a force, the photon carries the gas needed / transferred or expended by a force.

For what it's worth, classical http://iopscience.iop.org/0295-5075/76/2/189?ejredirect=migration".

http://arxiv.org/abs/0907.1611" a related paper.

Wow! Now that is pretty cool. I am not clear on the details yet (I can only access the arXiv paper at the moment), but it seems this would definitely support the "reality" of virtual phonons. One thing that is a bit worrisome is that the arXiv paper seems to say that the evanescent waves cannot be measured or interact with things ... this would contradict my own experimental experience. I have used total internal reflection techniques in spectroscopy, and we can observe coupling of the evanescent waves to sample material, and thus record an infrared spectrum. A colleague down the hall uses near-field techniques to precisely measure the displacement of beads in a magnetic tweezers experiment. In both cases we rely on the interaction of the evanescent waves with our samples. I guess I'll have to read that other paper at work tomorrow.

 

[Cthugha]

For what it's worth, classical http://iopscience.iop.org/0295-5075/76/2/189?ejredirect=migration".

http://arxiv.org/abs/0907.1611" a related paper.

Günter Nimtz is the same guy who claimed to have observed violations of special relativity in superluminal tunneling, where it turned out that all he has is some clever form of pulse shaping mixed with a bad understanding of what group and phase velocity mean.

One should be pretty careful about the conclusions drawn in his papers.

 

[Frame Dragger]

Very well 'said' Cat!

 

[JDługosz]

In Feynman's QED, he talks about how a nucleus keeps an electron in orbit by exchange of photons, but I don't see how a photon can provide a push, much less a pull.

Have you see http://math.ucr.edu/home/baez/physics/Quantum/virtual_particles.html" ?

 

[tiny-tim]

Have you see http://math.ucr.edu/home/baez/physics/Quantum/virtual_particles.html" ?

Just a detail …

that's not "the" Physics FAQ, it's John Baez's Physics FAQ.

 

[Frame Dragger]

Just a detail …
that's not "the" Physics FAQ, it's John Baez's Physics FAQ.

Yes... where the devil lies.