# Tag Info

11

There is only one kind of photon. Indeed, when we describe elementary interactions between two electrons for example, we call the photon "virtual" as opposed to a physical photon that might exist outside of this process. Still, these are the same particles, i.e. excitations of the same fundamental field, as the photons that make up light for example. ...

8

This is a perceptive question. Consider the following from the Wikipedia article "Virtual Particle": As a consequence of quantum mechanical uncertainty, any object or process that exists for a limited time or in a limited volume cannot have a precisely defined energy or momentum. This is the reason that virtual particles — which exist only ...

7

Virtual photon clouds are responsible for potentials, not electric and magnetic fields, and this is what makes the explanation of forces in terms of photon exchange somewhat difficult for a newcomer. The photon propagation is not gauge invariant, and the Feynman gauge is the usual one for getting the forces to come out from particle exchange. In another ...

7

The space between atoms depends very much on the medium you are talking about. In solids the typical distance between atoms is about the same as the size of the atoms themselves. In everyday gases at room temperature and pressure the distance between molecules is many times their size, and in deep space you can get densities as low as one proton per cubic ...

6

You have to realize that when we are speaking of photons, we are speaking of elementary particles and their interactions are dominated by quantum mechanics, not classical mechanics, and in addition special relativity is necessary to calculate anything about them. In general, we know about elementary particles because we observe their traces in detectors for ...

6

Update: I went over this answer and clarified some parts. Most importantly I expanded the Forces section to connect better with the question. I like your reasoning and you actually come to the right conclusions, so congratulations on that! But understanding the relation between forces and particles isn't that simple and in my opinion the best one can do ...

5

the wave function of a single photon has several components - much like the components of the Dirac field (or Dirac wave function) - and this wave function is pretty much isomorphic to the electromagnetic field, remembering the complexified values of $E$ and $B$ vectors at each point. The probability density that a photon is found at a particular point is ...

5

Imagine an elastic collision of two protons - something that they also see at the LHC but it's not the most interesting kind of interaction. The two protons will repel because of the electromagnetic interaction. Assume that the distance between them is never too small, relatively to the proton radius. But you may calculate the cross section of this process ...

5

The field lines in your drawing are not the trajectories of photons. The field lines show the direction of the force on a test magnetic dipole. The force, and therefore its direction, is mediated by virtual photons (or can be described that way) but those photons will travel in straight lines just like ordinary photons.

5

A major difference between real and virtual photons is that virtual particles are not required to have energy and momentum on the "mass shell". That is, virtual photons may have $E^2-p^2 \neq m^2$, while real photons must obey $E^2-p^2=m^2=0$. My memory disagrees with Neuneck (v1): I think that a coherent superposition of real photons is a laser, while ...

4

Virtual particles are not real It's in the name. You may draw Feynman diagrams where there are internal lines, and we call these internal lines virtual particles. They are not real. You will never detect a virtual particle. They are not really exchanged between the real charged particles. Virtual particles are a just-so stories designed to explain Feynman ...

4

These are just my thoughts as someone who studied the subject for a while: The concept of virtual photons that mediate interaction should not be seen as "what really happens". A virtual photon is not a real object (hence the name "virtual"), but an artifact of perturbation theory. If we knew an effective way (or even "a" way) to do the calculations without ...

4

I don't know where you get your intuition about how strong a virtual particle can push or pull. Why would virtual particles push any less than real ones? Virtual particle exchange is mathematically equivalent to other formulations of the theory. I don't know how to answer your question, because it is meaningless in logical positivism. If two ways of ...

4

The same way you determine if the interval is space-like or time-like. In fact you do it by computing the square of the four-momentum and examining the sign. Which sign is space-like and which time-like is a matter of convention, and varies from source to source. I like to compute the squared-interval as  (\Delta s)^2 = (\Delta t)^2 - (\Delta \vec{x})^2 ...

4

Suppose a scattering process with a 3- particle vertex : $A \to B + \gamma$. Here we suppose that particles $A$ and $B$ are massive, with the same mass $m$, and $\gamma$ is the "virtual" photon. Let $a,b, q$ be the momenta of the particles $A,B,\gamma$. You have : $q^2 =(a-b)^2 \\= (a_0 - b_0)^2 - ( \vec a - \vec b)^2 \\=(a_0^2 - \vec a^2) + (b_0^2 - ... 4 Unboundedly many, because photon number is not conserved. Every time you push an electron with a classical field, you produce infinitely many soft-photons (if the universe is flat at infinity) and conversely, any long range field which pushes the electron has infinitely many soft-photons getting absorbed in a sense, although you can't tell photons apart, so ... 4 The short answer is that you just can't. Different ways of doing the accounting (e.g., different gauges) lead to different ways of counting up the virtual particles in any given situation. 4 When thinking about fundamental entities, it's quite easy to ask a question that, upon reflection, is contradictory. The questions of this kind take the form: What is [some fundamental thing] made of? The contradiction here is that there can only be an answer if the fundamental thing isn't fundamental! The electromagnetic field is one such fundamental ... 4 I will address the premise that the electron is in an orbit around the hydrogen atom. This is a classical picture overlayed on the basic quantum mechanical one. The electron around the hydrogen atom is in a "spherical" probability cloud about the proton. The above is the n=1,l=0,m=0 probability distribution, which is the lowest energy state. A single ... 3 Your seemingly unrealistic gedanken experiment is in fact a quite realistic. First, one can kick out the proton with help of a fast neutron. Next, to increase your "delay" time, you can consider a Rydberg atom with a high enough$n\$, so the electron velocity is rather small with respect to the light (and the maximum proton) velocity. What happens to the ...

3

I do not think that this question has an answer. A photon is a quantum mechanical object. a) there is no conservation of photons, virtual or not. b)there is no lower limit to the energy of the photons, so in principle they are infinite ( infrared problem) c)The energy of the virtual photons will depend on the motion of the charge and or the probe that ...

3

It's infinity. This is the soft photon problem, which requires infrared regularization.

3

Virtual particles appear when one wants to calculate cross sections and branching ratios for elementary particle interactions. This is done with the prescription of Feynman diagrams: Feynman Diagram of Electron-Positron Annihilation In the above diagram the external "legs" are real particles with the quantum numbers given in the standard model table, ...

3

I understand that one can measure a single photon being absorbed using a photomultiplier tube or CCD. Can one measure a single photon being emitted by monitoring the current through an LED or the recoil of an emitting ion? The photon is a particle. It will have particle interactions, i.e. scattering off electrons and/or the spill over electric and ...

3

You should not imagine a virtual photon as an individual object wandering from one charged particle to another. This picture is simply inapplicable. Unfortunately, Feynman diagrams mislead people to imagine such things. Actually, Feynman diagrams are good for calculation and bad for imagination. Feynman diagrams have been introduced to help physicists to ...

2

The question here really is: "When are photons not virtual?". As explained in this article: A virtual particle is one that has borrowed energy from the vacuum, briefly shimmering into existence literally from nothing. Virtual particles must pay back the borrowed energy quickly, popping out of existence on a time scale set by Werner Heisenberg's ...

2

Virtual particles, whether photons or electrons or... are, in the context of QFT, particles that are off-shell, i.e., their associated energy and momentum are not related by the relativistic energy-momentum relation. Please read this to get an idea of how virtual particle exchange can create attractive or repulsive forces. Photons are quanta of the modes ...

2

You are right, real photons always travel at the speed of light and would carry energy away from a magnet. From a field theory point of view, all static fields, whether electric, magnetic, the weak nuclear force or the strong nuclear force can be thought of as being mediated by virtual particles. So for either electric or magnetic fields, that would be ...

2

The short answer is the Heisenberg uncertainty principle allows for the attraction. Suppose you have two opposite charges, and the one on the left emits a virtual photon with momentum directed leftwards. The left charge begins to move towards the charge on the right. Now, where's the virtual photon? It's momentum is some exact value directed left, so ...

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