#### vanhees71

Gold Member
Well, in my QFT lecture nobody taught "vacuum fluctuations", nor do I in my lectures ;-). There are radiative corrections, symbolized by Feynman diagrams containing loops, and these Feynman diagrams are just a very clever method to write down complicated formulae in a very efficient way. The "real" thing, i.e., what's measurable calculated in this way are transition-probability matrix elements $|S_{fi}|^2$, which usually are expressed in terms of cross sections and which are measured by the experimentalists with there detectors. There's not a single observation of "vacuum fluctuations", and there never can be, because to measure something you have to use a detector, and introducing a detector makes the situation not to be vacuum anymore.

The Casimir effect is indeed, as discussed at length in this thread and elsewhere here on the forums, indeed pretty analogous to the van der Waals force. It's not due to fluctuations of the vacuum but due to quantum fluctuations of real (not virtual!) charges and fields.

#### Karolus

Nothing is a hoax. It is a different way to describe things. You can use virtual particles - just keep in mind that they are mainly a mathematical tool in perturbation theory. Their existence is about as real as the existence of an integral sign, another mathematical tool used in perturbation theory.
So even an electron is as real as the psi schrodinger, as real as a Greek letter or mass is just as real a Latin letter ...
in short, all of physics is as real as a series of mathematical signs...

#### Karolus

Well, in my QFT lecture nobody taught "vacuum fluctuations", nor do I in my lectures ;-). There are radiative corrections, symbolized by Feynman diagrams containing loops, and these Feynman diagrams are just a very clever method to write down complicated formulae in a very efficient way. The "real" thing, i.e., what's measurable calculated in this way are transition-probability matrix elements $|S_{fi}|^2$, which usually are expressed in terms of cross sections and which are measured by the experimentalists with there detectors. There's not a single observation of "vacuum fluctuations", and there never can be, because to measure something you have to use a detector, and introducing a detector makes the situation not to be vacuum anymore.

The Casimir effect is indeed, as discussed at length in this thread and elsewhere here on the forums, indeed pretty analogous to the van der Waals force. It's not due to fluctuations of the vacuum but due to quantum fluctuations of real (not virtual!) charges and fields.
So Feynman lost a lot of time to identify arrows as fermions, wavy lines as photons and so on .. a picturesque symbolism just to make a tattoo ... who knows, maybe his car, though as I read, had drawn them on his car ...

#### mfb

Mentor
So even an electron is as real as the psi schrodinger, as real as a Greek letter or mass is just as real a Latin letter ...
in short, all of physics is as real as a series of mathematical signs...
You can detect electrons in a detector. You cannot detect virtual particles in a detector.
So Feynman lost a lot of time to identify arrows as fermions, wavy lines as photons and so on .. a picturesque symbolism just to make a tattoo ... who knows, maybe his car, though as I read, had drawn them on his car ...
Your sarcasm doesn't help. Feynman invented calculation tools. Those tools are useful. He did not discover new particles, he just found a much better way to describe existing particles and do the calculations.

#### vanhees71

Gold Member
So Feynman lost a lot of time to identify arrows as fermions, wavy lines as photons and so on .. a picturesque symbolism just to make a tattoo ... who knows, maybe his car, though as I read, had drawn them on his car ...
How do you come to this conclusion? Among all the ingenious ideas by Feynman his diagrams are among his most important. Without Feynman diagrams, I'm pretty sure we'd not have been able to use QFT to describe all known elementary particles in such a detailed way as is now represented by the Standard Model, let alone the solution of the difficult problem of renormalization. Last but not least the diagram technique is nowadays used not only in particle physics but also in any other field using QFT methods as in condensed-matter physics etc.

#### weirdoguy

So Feynman lost a lot of time to identify arrows as fermions, wavy lines as photons and so on
Your attitude is like "I know better then working physicists do!" but most of the things that you say are just common misconceptions and it's really quite easy to check their fallacy. How many academic courses on QM and QFT have you taken? How many advanced book have you read? It seems to me that you base your knowledge only on pop-sci books and stuff, and that is not a good basis for your sarcastic attitude. Show us some math that justifies it. Or tell us in which textbook we'll find information that virtual particles are as real as electrons. If you really did take any serious course on QM/QFT you'd already know what virtual particles are (Feynman certainly did), or that particle-wave duality is not part of modern QT.

#### ftr

I think you all misunderstood what Karolus was saying. He is certainly not against Feynman since his picture(playing Samba, whatever) is his Avatar.

English can be tricky when it comes to indirect talk. He was trying to say ( I think) that the term is and has been widely used and there is probably good reason, although the math is the same for QED, but Casimir controversy and multiple definitions of vacuum do make things confusing.

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