2 years, 1 month ago
What is a virtual photon? What are the primary characteristics of a photon?
Massless
Electromagnetic Wave resonating at a specific frequency
Size?
Emitted from the electron orbit as a quanta
Electromagnetic Wave resonating at a specific frequency
Size?
Emitted from the electron orbit as a quanta
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M$1 Answer
Virtual photons are in a sense a construct intended to describe in a particle sense how the EM interaction is mediated. Virtual particles do not have to have the same mass as the real particle's (which for a photon is zero) but may have the same mass anyway.
There is an excellent description for the lay person at http://math.ucr.edu/home/baez/physics/Quantum/virtual_particles.html which explains this in some detail. The virtual photon has a specific momentum and energy which it transfers, which through the Heisenberg Principle implies the virtual photon is not well-localized (i.e. size is not specified). The virtual photon is emitted by a charged particle as a means of affecting another charged particle in its field.
- quote -
What are virtual particles?
One of the first steps in the development of quantum mechanics was Max Planck's idea that a harmonic oscillator (classically, anything that wiggles like a mass bobbing on the end of an ideal spring) cannot have just any energy. Its possible energies come in a discrete set of equally spaced levels. An electromagnetic field wiggles in the same way when it possesses waves. Applying quantum mechanics to this oscillator reveals that it must also have discrete, evenly spaced energy levels. These energy levels are what we usually identify as different numbers of photons. The higher the energy level of a vibrational mode, the more photons there are. In this way, an electromagnetic wave acts as if it were made of particles. The electromagnetic field is a quantum field.
Electromagnetic fields can do things other than vibration. For instance, the electric field produces an attractive or repulsive force between charged objects, which varies as the inverse square of distance. The force can change the momenta of the objects. Can this be understood in terms of photons as well? It turns out that, in a sense, it can. We can say that the particles exchange "virtual photons" which carry the transferred momentum. Here is a picture (a "Feynman diagram") of the exchange of one virtual photon.
(removed the diagram due to formatting limitations of Mahalo Answers, see the link above for the diagram)
The lines on the left and right represent two charged particles, and the wavy line (jagged because of the limitations of ASCII) is a virtual photon, which transfers momentum from one to the other. The particle that emits the virtual photon loses momentum p in the recoil, and the other particle gets the momentum.
This is a seemingly tidy explanation. Forces don't happen because of any sort of action at a distance, they happen because of virtual particles that spew out of things and hit other things, knocking them around. However, this is misleading. Virtual particles are really not just like classical bullets.
How can they be responsible for attractive forces?
The most obvious problem with a simple, classical picture of virtual particles is that this sort of behavior can't possibly result in attractive forces. If I throw a ball at you, the recoil pushes me back; when you catch the ball, you are pushed away from me. How can this attract us to each other? The answer lies in Heisenberg's uncertainty principle. Suppose that we are trying to calculate the probability (or, actually, the probability amplitude) that some amount of momentum, p, gets transferred between a couple of particles that are fairly well- localized. The uncertainty principle says that definite momentum is associated with a huge uncertainty in position. A virtual particle with momentum p corresponds to a plane wave filling all of space, with no definite position at all. It doesn't matter which way the momentum points; that just determines how the wavefronts are oriented. Since the wave is everywhere, the photon can be created by one particle and absorbed by the other, no matter where they are. If the momentum transferred by the wave points in the direction from the receiving particle to the emitting one, the effect is that of an attractive force.
The moral is that the lines in a Feynman diagram are not to be interpreted literally as the paths of classical particles. Usually, in fact, this interpretation applies to an even lesser extent than in my example, since in most Feynman diagrams the incoming and outgoing particles are not very well localized; they're supposed to be plane waves too. The uncertainty principle opens up the possibility that a virtual photon could impart a momentum that corresponds to an attractive force as well as to a repulsive one
- end quote -
There is an excellent description for the lay person at http://math.ucr.edu/home/baez/physics/Quantum/virtual_particles.html which explains this in some detail. The virtual photon has a specific momentum and energy which it transfers, which through the Heisenberg Principle implies the virtual photon is not well-localized (i.e. size is not specified). The virtual photon is emitted by a charged particle as a means of affecting another charged particle in its field.
- quote -
What are virtual particles?
One of the first steps in the development of quantum mechanics was Max Planck's idea that a harmonic oscillator (classically, anything that wiggles like a mass bobbing on the end of an ideal spring) cannot have just any energy. Its possible energies come in a discrete set of equally spaced levels. An electromagnetic field wiggles in the same way when it possesses waves. Applying quantum mechanics to this oscillator reveals that it must also have discrete, evenly spaced energy levels. These energy levels are what we usually identify as different numbers of photons. The higher the energy level of a vibrational mode, the more photons there are. In this way, an electromagnetic wave acts as if it were made of particles. The electromagnetic field is a quantum field.
Electromagnetic fields can do things other than vibration. For instance, the electric field produces an attractive or repulsive force between charged objects, which varies as the inverse square of distance. The force can change the momenta of the objects. Can this be understood in terms of photons as well? It turns out that, in a sense, it can. We can say that the particles exchange "virtual photons" which carry the transferred momentum. Here is a picture (a "Feynman diagram") of the exchange of one virtual photon.
(removed the diagram due to formatting limitations of Mahalo Answers, see the link above for the diagram)
The lines on the left and right represent two charged particles, and the wavy line (jagged because of the limitations of ASCII) is a virtual photon, which transfers momentum from one to the other. The particle that emits the virtual photon loses momentum p in the recoil, and the other particle gets the momentum.
This is a seemingly tidy explanation. Forces don't happen because of any sort of action at a distance, they happen because of virtual particles that spew out of things and hit other things, knocking them around. However, this is misleading. Virtual particles are really not just like classical bullets.
How can they be responsible for attractive forces?
The most obvious problem with a simple, classical picture of virtual particles is that this sort of behavior can't possibly result in attractive forces. If I throw a ball at you, the recoil pushes me back; when you catch the ball, you are pushed away from me. How can this attract us to each other? The answer lies in Heisenberg's uncertainty principle. Suppose that we are trying to calculate the probability (or, actually, the probability amplitude) that some amount of momentum, p, gets transferred between a couple of particles that are fairly well- localized. The uncertainty principle says that definite momentum is associated with a huge uncertainty in position. A virtual particle with momentum p corresponds to a plane wave filling all of space, with no definite position at all. It doesn't matter which way the momentum points; that just determines how the wavefronts are oriented. Since the wave is everywhere, the photon can be created by one particle and absorbed by the other, no matter where they are. If the momentum transferred by the wave points in the direction from the receiving particle to the emitting one, the effect is that of an attractive force.
The moral is that the lines in a Feynman diagram are not to be interpreted literally as the paths of classical particles. Usually, in fact, this interpretation applies to an even lesser extent than in my example, since in most Feynman diagrams the incoming and outgoing particles are not very well localized; they're supposed to be plane waves too. The uncertainty principle opens up the possibility that a virtual photon could impart a momentum that corresponds to an attractive force as well as to a repulsive one
- end quote -
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M$
I am reading the article now. He said, "high energy vibrational levels will produce more photons."
The harmonic oscillator implies that EM are discrete.
The virtual photon plane is everyone in space at once and disappears all at once. The virtual photon has no position, it is a plane wave that fills all of space. The transfers create attractive or repulsive forces.
The electromagnetic field wiggles like a wave.
The discrete Energy levels are identified as a certain number of photons.
The electric field can produce an attractive or repulsive force. (That is what I meant by positive and negative force on the electric field)
Virtual Photons carry transferred momentum. The virtual photon loses momentum and the other particle gets the momentum.
What is the momentum space wave function?
Will you display a diagram that shows how the EM interaction is mediated using virtual photons and photons?
Why does a virtual photon have a momentum and energy? It seems to be a mirror or symmetrical to the photon. Does this phenomena connect to the Heisenberg uncertainty principle?
Check the link at http://math.ucr.edu/home/baez/physics/Quantum/virtual_particles.html where there is such a diagram. The limitations of formatting here on MA make it impossible to type it in.
As for the rest of your comment, a virtual photon is not mirror or symmetric to a real photon. It is simply a quantum mechanical construct having the same quantum numbers as a real photon, and which exists only to transfer the momentum and energy you would derive from a field calculation of the effect of one charged particle's existence on another's. Since a virtual photon has the same quantum numbers as a real one, of course it must also have energy and momentum. The connection to the uncertainty principle is also described in the link above. It is simply that delta(momentum) x delta(position) > Plank's constant, and thus, to transfer a precise amount of momentum requires the virtual photon to have infinite extent in space, which works well for describing the EM field, as that field has infinite range (though the field strength drops as 1/R^2).
Did find the statement, "energies come in a discrete set of equally spaced levels", a revelation? Discrete (step like) set of energy levels. ZPE calls this discrete step the cavity and once the cavity grows to the size of a photon wave then vacuum energy flows in with a greater overall energy.