- #106
haushofer
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Perhaps people, including the popular literature, should stress more that when talking about QM or QFT, metaphores/analogies are used to translate the math into tangeble concepts.
TrickyDicky said:"QFT for the gifted amateur" by Lancaster, sidenote 2 in page 348
TrickyDicky said:You can think heuristically of an 'on shell particle slightly off shell' as a renormalized particle i.e.: a "dressed particle" in the sense that it is obtained from a "bare particle" (on shell) with the radiative corrections by virtual particles(off shell).
I wasn't implying that a bare particle has to be on shell, as a matter of fact that is why I referred to the renormalized dressed particle, because after renormalization one can no longer refer to on and of shell in the same way we do with the pre-renormalized Feynman diagram. See the definition of dressed particle in wikipedia:"dressed particle refers to a bare particle together with some excitations of other quantum fields that are physically inseparable from the bare particle. For example, a dressed electron include the chaotic dynamics of electron-positron pairs and photons surrounding the original electron." I figured it might aid as a graphical heuristic to get an idea of what "slightly off shell" means, if it doesn't work for you or find it misleading just disregard it.PeterDonis said:Except that a bare particle does not have to be on shell, since bare particles can correspond to internal lines, and a dressed particle that corresponds to an external line is on shell once you realize that the physically measured mass is the renormalized mass, not the bare mass.
bhobba said:No - because virtual particles don't actually exist. They are simply an artefact of the mathematical methods used called perturbation theory and something called a Dyson Series:
http://en.wikipedia.org/wiki/Dyson_series
Thanks
Bill
ddd123 said:Yes but there are no apparent forces in GR, at least that's the only way I can interpret the "appear". A thermal bath can't just appear, either it's there or it isn't. Otherwise you may be appearing to me this moment and fall into a Jabberwock if I accelerated in a certain way wrt you.
Never disagreed on the reality of virtual particles that I'm aware of; wrt on/off shell I think it is just an issue about words. QFT is admittedly hard to translate to english.JK423 said:TrickyDicky and PeterDonis are you still disagreeing on the reality of virtual particles or on the definions of off/on shell?
JK423 said:TrickyDicky and PeterDonis are you still disagreeing on the reality of virtual particles or on the definions of off/on shell?
PeterDonis said:For example, the question "are virtual particles real?" doesn't have an answer in the math
Gerinski said:So quantum fluctuations are real and they have observable properties similar to as if they were particles, it's just that they should not be called particles in the popular literature
PeterDonis said:My personal preference is to stop asking those kinds of questions, but not everybody wants to do that.
yoron said:A thermal bath can't just appear, either it's there or it isn't.
yoron said:Whether it is possible to change the order of cause and effect just by viewing two events from a different frame.
PeterDonis said:The key point is that the notion of "a particle being present" turns out to be observer-dependent when quantum effects are taken into account
Vanadium 50 said:the notion of "a particle is registered in a detector" turns out not to be observer-dependent.
A thermal bath consists of particles. These particles can be counted by a number operator as for the harmonic oscillator. However, this operator is only invariant with respect to inertial observers. Hence, accelerating observers will count a different amount of particles. See e.g. Nikolic's "myths and facts"-paper, which has an excellent treatment on this.yoron said:Actually I do think you have a excellent point in lifting up 'thermal baths' ddd, " A thermal bath can't just appear, either it's there or it isn't.
Vanadium 50 said:Imagine a box that counts particles and displays how many it detects in a bright LED. Observers, both accelerated and non-accelerated will agree on the number displayed on the LED. They may well disagree on the source or histories of the particles, but there is no dispute as to the number.
haushofer said:A thermal bath consists of particles. These particles can be counted by a number operator as for the harmonic oscillator. However, this operator is only invariant with respect to inertial observers. Hence, accelerating observers will count a different amount of particles. See e.g. Nikolic's "myths and facts"-paper, which has an excellent treatment on this.
Vanadium 50 said:The number of particles that the detector registers is invariant.
bhobba said:How you reach that conclusion from this thread beats me.
If they were real all ways of doing the QFT calculations would have them - but they don't.
Thanks
Bill
mattt said:Advise:
first: study it all in its mathematical rigorous (if possible) formulation (it will take many many long years of hard study).
second: only after that, read popularizations (pop-science books, famous quotes, whatever...) if you like, but now knowing perfectly well what actually lies beneath.If you ever want to (really) understand some of it, never start with pop-books.
Gerinski said:Well, there are real phenomena which get commonly attributed to virtual particle interactions (Casimir effect, vacuum polarization, the contribution to mass of the vacuum fluctuations etc).
Indeed that's just what I was asking. If there are real effects and you say that their cause are not virtual particles since they are nothing real, just kindly explain me what is the real cause of those effects and I will be happy. Again that's why I said: "there's something real some people call virtual particles even if they are not virtual particles. Just explain me what it is". If so the problem is just in the name given to the cause, the cause is still something real, isn't it?ddd123 said:Bhobba, since the unsettling part of all these posters seems to be the lack of real-ness of mathematical formalisms vs. the real effect they are associated with, why don't you answer them that the real cause of the real effect is simply the quantum field (modes)? Is it simply a coincidence or is there a reason why you don't want to answer in this way?
Jilang said:Oscillations of quantum fields that just don't have enough energy to make a real standalone particle that can be detected?
bhobba said:Haven't you been reading the thread? That they are caused by virtual particles is simply due to the pertubative formalism used. There are other ways of doing the calculations where virtual particles do not appear. Hence the cause can't be virtual particles.