Should a virtual photon be described as a "short-lived" photon?

[.Scott]

I am reading this Quanta Magazine article.

And I ran into this paragraph that surprised me:

If the muon sat isolated from other particles, its g-factor would be exactly 2. But quantum theory requires that all other particles influence the g-factor. As the muon wobbles, it releases particles such as photons, which are too short-lived to show up in detectors. These can release other particles, which can release still more particles. The muon quickly reabsorbs all these fleeting particles, and the only trace they leave behind is that the muon wobbles a little bit more. Through these intricate chains of emission and reabsorption, every particle in existence has some small effect on the movement of the muon.

Of course, photons would not normally described as "short-lived".
Shouldn't the author have tossed in the word "virtual" someplace?

Actually, I see the word "virtual" is included in the diagram that follows.

 

[mfb]

The author is writing for a broader audience, not for physicists. He is a particle physicist, so he knows it's not a very accurate description.

 

[ohwilleke]

The author is writing for a broader audience, not for physicists. He is a particle physicist, so he knows it's not a very accurate description.

And, while it isn't a very accurate description, in this "lies told to children" example, the way in which it is inaccurate doesn't seriously hinder an approximately correct intuition and understanding of what is going on. It is heuristically more helpful than harmful. Virtual photons are "short-lived" in the sense that they have a lifetime of zero, which is the limit that short-lived approaches.

 

[PeroK]

I am reading this Quanta Magazine article.

And I ran into this paragraph that surprised me:

If the muon sat isolated from other particles, its g-factor would be exactly 2. But quantum theory requires that all other particles influence the g-factor. As the muon wobbles, it releases particles such as photons, which are too short-lived to show up in detectors. These can release other particles, which can release still more particles. The muon quickly reabsorbs all these fleeting particles, and the only trace they leave behind is that the muon wobbles a little bit more. Through these intricate chains of emission and reabsorption, every particle in existence has some small effect on the movement of the muon.

Of course, photons would not normally described as "short-lived".
Shouldn't the author have tossed in the word "virtual" someplace?

Actually, I see the word "virtual" is included in the diagram that follows.

There is an Insight article about what virtual photons are. Essentially, they are a calculation device. It's not that a muon emits and reabsorbs a given number of vitual photons. If you want an approximation to the muon's g-factor, you calculate it directly and get 2. You get a better approximation if you consider, with the appropriate weighting, that a muon releases and reabsorbs a single virtual-photon. Note that in this calculation, the photon "lives" for zero time, in the sense that there is no specific time associated with the calculation. It's an abstract calculation of a "virtual" event. It's not an event that actually happens - that's why you can't detect it. Note also, that you sum over all the photon four-momenta (including those physically impossible "off-shell" four-momenta).

You get an even better approximation if you consider, with the appropriate weighting, that the muon ALSO emits and reabsorbs two photons. Again, these are virtual events.

The muon's g-factor is ultimately the infinite sum of the calculation involving an ever greater number of photons. Although, even this is not quite the whole mathematical story.

As has been mentioned, one of the golden rules of popular science is to latch onto virtual particles and insist that they are real events. Glossing over their abstract, virtual nature and the physical impossibility of all these events happening continuously in zero time.

Why they can't say it's only a calculation device defeats me. Perhaps you wouldn't be allowed to publish if you said that!

 

[ohwilleke]

Why they can't say it's only a calculation device defeats me. Perhaps you wouldn't be allowed to publish if you said that!

In popular non-fiction articles, word count is king. The number of words you'd need to explain the concept of a virtual particle properly would probably use a good share of the words available for the story.

Book length popular non-fiction, where the word count imperative is less intense and the reader is expected to have more patience, tend to do a better job at explaining these kinds of things more precisely,