Insights The Physics of Virtual Particles - Comments

vanhees71

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I don't know what you mean. You cannot observe off-shell particles. Tell me where you think virtual particles are observed.
 

naima

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@VanHees,
You say "Tell me where you think virtual particles are observed." I say the opposite

you say that asymptotic free states are observable.
@Demystifier, you give me free particles as an example for a not observed thing.

I think that
measured <=> not virtual
not measured <=> virtual
and as we have partial measurements that
partially measured <=> partially not virtual.
 

naima

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I recommend Neumaier insights post on virtual particles.
I would like to add that they only appear in processes where one need to add the amplitudes of the different possibilities and to square them to get the probability.
This is the case with Feynman diagrams, the path integral or with Young slits.
If in the Young experiment (and in all cases) the possibilities are measured, we do not need virtual particles with amplitudes (the result would be false) it is enough to add the probabilities.
One advantage is that intermediate situations (partial measurements) can be considered using a fringe visibility parameter.
 
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A. Neumaier

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A. Neumaier

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I updated my Insight article by adding at the end a lot of factual information on vacuum fluctuations and related topics, based on the fairly precise definition of vacuum fluctuations on p.119 in the quantum field theory book by Itzykson and Zuber 1980.
 

vanhees71

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To be pedantic: You write

According to the Born rule, the distribution of a quantum observable gives the probabilities for measuring values for the observable in independent, identical preparations of the system in identical states. Thus the presence of a Gaussian distribution means that the attempt to measure the electromagnetic field in the vacuum state cannot be done with arbitrary precision but has an inherent uncertainty.
I would write

According to the Born rule, the distribution of a quantum observable gives the probabilities for measuring values for the observable in independent, identical preparations of the system in identical states. Thus the presence of a Gaussian distribution means that the value of the electromagnetic field in the vacuum state is not determined with arbitrary precision but has an inherent uncertainty.

The fluctuations of observables are not due to the limitations of measurement accuracy but due to the state the system is prepared in. This is also often discussed in a misleading way in context of the usual uncertainty relation. Also in this case the uncertainty/fluctuations of observables are due to the impossibility to prepare the system in such a way that both incompatible observables have a determined value; it's not a limitation to the accuracy you can measure the one or the other observable.
 

A. Neumaier

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According to the Born rule, the distribution of a quantum observable gives the probabilities for measuring values for the observable in independent, identical preparations of the system in identical states. Thus the presence of a Gaussian distribution means that the value of the electromagnetic field in the vacuum state is not determined with arbitrary precision but has an inherent uncertainty.
Yes, that's an improvement. I updated the page.
 
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