Tag Info

Hot answers tagged

23

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 ...


9

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

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 ...


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

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

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

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

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

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 ...


5

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, ...


5

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 ...


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

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

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 ...


3

A particle is just an excitation in some quantum field. These fields permeate all space and are coupled to each other. As one of these excitations evolves in time, it can take any number of paths. The probability amplitude for a particle to be at some location after some amount of time is the sum of all the possible paths the particle could have taken to get ...


3

Decaying particles are described by complex energies, the imaginary part of which encodes life-time information. They are observable; in case of very short-lived particles such as the Higgs in the form of resonances, http://en.wikipedia.org/wiki/Resonance_(particle_physics) , i.e., a peak in the production rate of products of Higgs decays. The decay itself ...


3

The standard model predicts that the Higgs boson has a lifetime on the order of $10^{-22}$ seconds. That means that if the Higgs were moving close to the speed of light, it could move something like $34\gamma$ times the diameter of a proton (on average) before it decays. $\gamma$ is the time dilation factor from special relativity which is $$\gamma = ...


3

Yes, light can interact with "virtual particles". It can also interact with itself via virtual particle interactions (see Delbruck Scattering), although I believe direct observation of this effect is currently outside of our experimental capability. Edit: Just realised I didn't address the second part. When a photon propagates, the propagation receives ...


3

Nothing goes on; the vacuum is completely inert. Virtual particles don't exist in time, except in a (literally) figurative sense. They don't have associated states, hence no expectations, probabilities, uncertainties. See http://physics.stackexchange.com/a/22064/7924 and Chapter A8 ''Virtual particles and vacuum fluctuations'' of my theoretical physics FAQ ...


3

The other answers are quite complete, but magnetic (and electric) lines DO cross in some special cases. An example would be the magnetic quadrupole configuration: This doesn't contradict the other answers, but it means that at the centre there is no field and there are several directions of getting there (it would be an equilibrium point).


3

It is better to begin with amplitudes calculus in position space, because things become more clear (usual "Feynman diagrams" correspond to momentum space). Suppose, for instance, a massless scalar field theory, with a $\phi^4$ interaction. In this theory, each vertex has 4 "legs", which may be real particles or field perturbations/field correlations (also ...


3

OK, I watched the video. It consists of two parts. The first part talks about General relativity and the introduction of a cosmological constant, which from the argument should not exist in completely empty space. He then goes to the Quantum Field Theory vacuum which has the continuous creation and annihilation of all possible fields of virtual particles ...


3

First of all try to understand what vacuum fluctuations are. Virtual pairs exist everywhere appearing and disappearing below the threshold of our detection. Every photon, every charged particle is "dressed" by the vacuum fluctuations as it goes along. The following diagrams are a shorthand, they represent integrals that have to be calculated in order to get ...


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 ...


2

Your question has been addressed in two physics.stackexchange articles: are-elementary-particles-actually-more-elementary-than-quasiparticles and what-is-the-relationship-between-string-net-theory-and-string-m-theory In short, vacuum is not inert but a dynamical medium. Casimir effect has experiemntally demonstarted that vacuum is indeed a dynamical medium. ...


2

Virtual particles influence physics at every point of space, whether or not there is a nearby atomic nucleus or orbital. All electrons in an atom receive energy shifts analogous to the Lamb shift (from virtual photons), aside from other quantum corrections. In fact, the influence of the virtual particles only becomes truly measurable if there are some nearby ...


2

They can be real, no problem with that. However, all Z and W observed are virtual. And yes, they are off-shell, whatever that means to you. We actually measure their width, for instance, http://arxiv.org/pdf/0909.4814 (it's more or less 2 GeV around a mass of more or less 80-90 GeV). What is observed is a pole in the S matrix for some final states in ...



Only top voted, non community-wiki answers of a minimum length are eligible