Talk:On shell and off shell

On the topic of off mass shell, I'm not convinced energy violation is an issue. For instance, in Feynman diagrams, energy and momentum conservation are explicitly included in the calculation. Virtual particles just don't have the usual correspondence between the two. 74.131.48.12 07:51, 29 August 2007 (UTC)

Minus sign

I was about to add the following line:

In literature, one may also encounter $p^{\mu }p_{\mu }\equiv p^{2}=-m^{2}$, where the extra minus sign is due to sign convention in the metric.

although I'm not sure it is appropriate, relevant and sufficiently general. --CompuChip (talk) 10:26, 2 October 2009 (UTC)

Conceptually, I'm okay with the idea of on-shell and off-shell, but this article really confuses me. The symbol usage looks dubious. And, If $E=mc^{2}$ then doesn't the exampled equation reduce to $-|{\vec {p}}\,|^{2}c^{2}=0$? But more importantly, that equation doesn't clarify anything for me at all. I'm mystified. -- 99.233.186.4 (talk) 13:02, 20 March 2010 (UTC)

If $E=mc^{2}$ then the particle is at rest and so $-|{\vec {p}}\,|^{2}c^{2}=0$ is true. —Preceding unsigned comment added by 94.15.20.39 (talk) 23:19, 4 January 2011 (UTC)

I'd say in literature a minus sign is encountered pretty often as the standard convention is metric of signature $(-,+,+,+)$. Also it should be noted in the article that $m$ is rest mass (if I'm not mistaken...). — Preceding unsigned comment added by 77.70.120.85 (talk) 08:35, 7 May 2013 (UTC)

I think it's encountered often enough that it's worth mentioning. I'll go ahead and add it as well as some examples. --Anagogist (talk) 12:15, 7 May 2014 (UTC)

As a student of Quantum Field Theory I can say this article is spot on and shouldn't be changed. I myself cannot cite any sources because I don't know any. The information on this page could be said to be a composite of multiple sources on QFT and Relativity in a very peicemeal fashion but, is a very good description as is. I have never seen the scalar context given here before and found it very interesting which prompted me to write this. Also, to address an issue cited above, anyone studying at this level will know the energy term is TOTAL energy where as mc squared is rest energy only. In fact, set p to zero in the equation cited above on this talk page and take the square root of both sides and you will see where the famous equation comes from. But, this subject and concept are quite advanced to the point that inclusion of such information would be out of place here. I realize Wikipedia is considered for everyone but to fully explain Mass Shell, its context and usage in Quantum Field Theory to a layperson would be quit an involved article in itself indeed. — Preceding unsigned comment added by 67.65.250.125 (talk) 20:58, 12 July 2014 (UTC)

Re: "negative and positive on-shell E then simply represent opposing flows of positive energy"

I'd like to see consideration mentioned for a case where the opposing flows can be (if not always) destructive in nature, and cancelling as the creation of an 'off mass shell' vector, even if temporary (We see this all the time in water wave propagation, acoustic sound (dead zones) and photon wave destructive interference) . This would be equivalent to the same process as creating an eigenstate from complex vectors only from our fram the translational frame of reference is our coordinate system to complex. In other words, the process appears one sided mathematically. So far, I don't know if this is mathematically founded or not (matrix translation), at this time it appears (to me) the reverse of this is unfounded mathematically; however, if that's the case then it opens a whole new consideration for mathematics in quantum mechanics. I think it's simpler to think of conjugates as dimensional rotations, even though I can't get away from 'classical' understanding of the concept of conjugates as a one way street mathematically. (Perhaps in matrices, the coordinate reference can be any frame including a 'classical' complex frame and I forgot or didn't realize this. I even see this in the Pauli exclusion principle where from another frame, the collapse is from on shell, to off shell. Providing, of course, I'm understanding this 'shell' concept properly in that 'virtual' means imaginary complex abstract or am I misunderstanding that by squaring everything the process is intended to use 'virtual' as special case of local realism, which I believe is under question right now due to entanglement.)

In our current frame, a complex sum of squares results in eigenvalues which are real. If we reverse the process we create either 4 complex vectors, or if we reverse the concept mathematically we create the same as complex to real, a single vector, then real sums of squares might create a single complex vector (though not mathematically based on my understanding). Perhaps my understanding of Linear Algebra is weak, and I need to realize that the math has a polar independence from any 'real' frame of coordinates. I'll admit to that, I'm weak in this; however I still find the case of quantum decoherence existing in 'real' conjugates moving the process from 'on shell' to 'off shell', and visa versa. (Since I consider myself a scientist, naturally I consider opposing viewpoints and would love to hear such an argument in support or contrast).Cyberchip (talk) 12:23, 5 January 2015 (UTC)

References

Wow

For a mathematician , this kind of thing is at the fringe of sense, yet it's stimulating and I think re QFT it is much of the point. Can we put a delimiting fence about it, however large and loose? 198.129.67.88 (talk) 22:45, 29 March 2016 (UTC)

Mass shell

I am far too polite to ask what kind of idiot would write:"This is because the q 2 {\displaystyle q^{2}} q^{2}-dependence of the propagator is determined by the four-momenta of the incoming and outgoing particles." when the key variable q is left undefined? but I must admit the sentiment is close to how I feel. Note to editors: if you use a symbol/variable, DEFINE THE SYMBOL/VARIABLE (with the obvious exception of the common mathematical symbols) q is commonly used in physics for location, charge, and several other things; it is NOT obvious to me what is meant here (it's not even obvious whether q is a vector or a scalar, although q² suggests the latter).174.130.48.127 (talk) 15:23, 22 July 2017 (UTC)

Answer: These idiots are called Wikipedia authors and they, quite unanimously, show this flaw. I have seen such a problems numerous times and there is an easy explanation for this: In Wikipedia it is not about understanding but about reputation masturbation in front of your peers. Therefore, unfortunately, many theoretical physics articles are only understandable by the specific peers and a catastrophy from an encyclopedic or didactic point of view. Here I assume that q is the position variable (where q is a common choice in quantum mechanics for this). It is a vector and q² is supposed to be its norm. The article tried to describe inverse square of distance laws here, which drop out when taking the integral of the propagator. But unless you knew this beforehand you will not learn it from this article. Shame on you, Wikipedia. — Preceding unsigned comment added by 217.95.172.17 (talk) 12:19, 29 November 2017 (UTC)

Thomson, pg 118: $q$ is "the four-momentum of the exchanged virtual particle X ", $q=p_{a}-p_{c}$, and the term ${\frac {1}{q^{2}-m_{X}^{2}}}$ is the propagator. $p_{a}$, $p_{c}$ and $q$ are 4-vectors (4-momenta) and $q^{2}$ is a four-vector scalar product.

Improving the article as far as defining the variable, but also don't understand particle physics. And it should have some more accessible references for those without textbooks access. CyreJ (talk) 19:22, 25 June 2019 (UTC)