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I have been reading this article about the quantum vacuum state, and in the section that I linked to, there is a video showing an experiment that shows visibly that quantum fluctuations are actually happening (see the video in the link). The quantum fluctuations are visible because they amplified them using Spontaneous parametric down-conversion

Now from what I learned here, quantum fluctuations (or virtual particles) do not exist (and links therein). They are just tools of perturbation theory in QFT when taking into account interactions to make the mathematics work. Another reason I learned is that the fluctuation-dissipation theorem states that when a system is in fluctuation, the energy dissipates as heat or other forms of energy. So then, the vacuum cannot be fluctuating.

I seem to be confused now. Is the video fake or is what the video/wiki claiming not true and is something else entirely?

This question is sort of a follow-up to my previous question.

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    $\begingroup$ I've never done an experiment that detected a thing called a square root. So every calculation I've ever done which used a square root used something that wasn't real. Should I therefore be confused that an experiment verified the Pythagorean theorem? $\endgroup$
    – Connor Behan
    Commented May 3, 2023 at 2:26

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You would be forgiven for thinking that the linked wiki article/video was somehow "an image of the real quantum vacuum and virtual particles" instead of the computer output of some physical modelling attempting to explain quantum processes with imagery.

Surprisingly, the claim that someone has sampled or directly measured vacuum fluctuations comes up from time to time, even in the professional physics community (though the language used to make these claims is often ambiguous or worded "carefully").

This example you cited, using SPDC, and other examples elsewhere in the physical literature, are attempts to create a visual image of something that originates strictly from the mathematics of QFT. It is very tempting to associate visual images of technical processes, and many have done the same, but ultimately, this physical picture is wrong.

The concept of virtual particles is well-defined in the mathematics of the standard model, i.e., QFT, and not in the physical world. Hence the term virtual particle.

The question about the "physical existence of quantum vacuum fluctuations" has been asked here before and probably will be again.

Hopefully not, because the answer is once again no. They are not processes that occur in physical space.

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