# Tag Info

25

Let me give a second, more technical answer. Observable particles. In QFT, observable (hence real) particles of mass $m$ are conventionally defined as being associated with poles of the S-matrix at energy $E=mc^2$ in the rest frame of the system (Peskin/Schroeder, An introduction to QFT, p.236). If the pole is at a real energy, the mass is real and the ...

14

The reason for many contradictory statements regarding the nature of virtual particles is that they are often invoked for heuristical explanations of phenomena that arise within the framework of quantum field theory. One then tries to justify those explanations by attributing certain properties to virtual particles they do not actually possess. What ...

12

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. ...

12

Photons do not exhibit the property of virtual particles, but it is not your reasoning that is faulty, you have simply fallen prey to an imprecise use of terminology. Let me start with my view of the wave/particle duality. Most of the images of "particles" and "waves" comes from a time when we really didn't understand the quantum world, and some ...

11

Here are from wikipedia drawings of the field lines of two magnets in two orientations, like-like, like-unlike . North pole to north pole North pole to south pole. The lines distort but do not intersect. These field lines are solutions of the formal Maxwell differential equations. Differential equations do not give discontinuous solutions, as ...

8

Yes there are "virtual" Higgs bosons. A virtual particle isn't really a particle but a ripple / disturbance in a field. So a virtual electron is a ripple in the electron field. A virtual higgs is a ripple in the higgs field. Virtual particles are just a convenient conceptual model for describing field disturbances in terms of particles. Matt Strassler ...

7

I don't think the particle-anti-particle picture is a very good one to grasp what's going on. Essentially, it's a consequence of zero-point energy. In classical physics, the lowest energy state of a system, it's ground state, is zero. In quantum mechanics, its a non-zero (but very small) value. The easiest way to see how this zero point energy arises is ...

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 ...

7

The terminology "virtual particle" comes from quantum field theory. Note the third word in QFT, theory. Theory means that it is a mathematical model for calculations which will, if the theory is valid, describe concrete measurements and behaviors of physical reality. The basic building block of QFT is the Feynman diagram: a mathematical prescription that ...

7

Short answer: A virtual particle is not the opposite of a classical particle. While the other answer captures some aspects correctly, there are still a few flaws and inaccuracies which in the following, I will try to set straight. Wave-particle duality Strictly speaking, quantum objects are neither waves or particles. They are entities behaving like ...

6

The idea that the universe is a vacuum flucuation has been around a long time. The first public mention of the idea I know of is from Edward Tryon in 1973, but I bet it had been discussed long before that. Do you have access to old copies of Nature? If so have a look at "Is the Universe a Vacuum Fluctuation?" by Edward Tryon, Nature 246, 396 - 397 (14 ...

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

In the normal usage, real and virtual are not properties of Feynman diagrams themselves, but of the particles depicted in them. The particles corresponding to external lines (attached to at most one vertex only) are real, the others (attached to two vertices) are virtual. A Feynman diagram may be considered as a repetitive part of a bigger diagram. This ...

6

If I understand your question correctly its just a matter of what you are calculating whether you put the external particles on shell or not. If you are, for example, calculating an amplitude to use for a cross section, you'll put the external particles on-shell and it will be what you call a 'real Feynman diagram'. If you are calculating an effective action ...

6

All observed particles are real particles in the sense that, unlike virtual particles, their properties are verifiable by experiment. In particular, W and Z bosons are real but unstable particles at energies above the energy equivalent of their rest mass. They also arise as unobservable virtual particles in scattering processing exchanging a W or Z boson, ...

6

Photons are force carrying bosons and come in both virtual and real varieties. There is nothing wrong with that. Virtual means off-shell, and real means on-shell. Even on-shell weak bosons decay very quickly, however, because there are plenty of modes with the right quantum numbers and much lower total mass (and thus lots of phase space). I want to ...

6

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 ...

5

First of all, virtual particles are indeed a consequence of the uncertainty principle – without any quotation marks. Virtual particles are those that don't satisfy the correct dispersion relation $$E = \sqrt{m^2 c^4 +p^2 c^2}$$ because they have a different value of energy by $\Delta E$. For such a "wrong" value of energy, they have to borrow (or lend) ...

4

Feynman diagrams are just that: diagrams. Real or virtual is what the particles depicted in them can be. A distinction should be made: In order to calculate an amplitude, one needs to integrate over all possible momenta of internal lines. Therefore, those propagators can be thought as virtual. Effectively, one sums over all virtuality levels of the internal ...

4

Ignore the gluon for the moment Regarding the momentum conservation law, how come we have a photon of spin 1 and at the end some meson with spin 0? First of all spins are angular momentum not momentum. Secondly the two quarks have a spin 1/2 which will add to either 0 or 1, and 1 conserves the angular momentum at the vertex. All intermediate lines ...

4

This question involves the concept of "virtual particle" which was discussed a few days ago here. In a nutshell, a particle is virtual when it is a connecting line in a Feynman diagram between two vertices. It has all the quantum numbers of its name ( photon, electron, etc.) but not the mass, which is the measure of the four vector describing it. In that ...

4

You can't violate the conservation of energy (at least not on long timescales) so you can't just create matter from nothing. If you're going to create matter from energy then that energy has to come from somewhere. One example of this is Hawking radiation. Although we often talk about a black hole event horizon as though it were an object, it is not. A ...

4

In curved space, and even more so in expanding curved space, the concept of a vacuum wherein there are no particles has significant ambiguity. This is all covered very thoroughly in chapter 3 of "Quantum Fields in Curved Space" by Birrell and Davies. In normal flat space, we can define a vacuum state with no particles that every inertial observer would agree ...

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 ...

3

The underlying level of nature is quantum mechanical and obeys special relativity laws, not Newtonian mechanics. Mass is a conserved quantity in Newtonian mechanics, but not in the underlying quantum mechanical framework. The classical framework emerges as the variables become large enough where h_bar, which characterizes the quantum mechanical level, can ...

3

it seems curious to me that a system can have a mass, and then accelerate to the speed of of a massless particle There is no paradox here. Special relativity doesn't preserve velocity - in fact the opposite - it allows one to move between frames of reference in which the velocity of objects is relative to the observer. [do the particles] accelerate ...

3

In Feynman diagrams all four vectors conform with special relativity algebra, BUT the internal lines, even though they have the name "photon" are virtual, which means the mass can be different than zero, only the quantum numbers identify them as a photon, quark, electron, etc.. Feynman diagrams are an iconic representation one to one with the integrals and ...

3

Both fields will distort each other. The topology will look like compressed field lines, but they will never intersect. If you flipped the magnets so that opposite poles are in proximity, then the field lines will combine together

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, ...

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