According to QFT. The quantum vacuum is a particular state of quantum fields; it's not a "place" where quantum fields "exist in.

No problem with that. But does the word "vacuum" without the quantum word before it has the same meaning? I'm asking because in other usages like written in the Scientific American May 2014 edition:

"The Higgs may hold other clues. The discovery of the Higgs boson shows that there is a Higgs energy field turned on everywhere in the universe that gives mass to elementary particles. This means that the vacuum of “empty” space is a busy place, with both Higgs energy and virtual particles producing complicated dynamics. One might then wonder if the vacuum is really stable or if some unlucky quantum event could one day trigger a catastrophic transition from our universe to a clean slate."

Here they describe "vacuum" as a place...

in QFT.. we know quantum vacuum is a state, and not a place.

Is it because the article is a popularization or does "vacuum" has realy another meaning from "quantum vacuum"? What do you think?


1 Answer 1


They are simply being cavalier in their terminology. The vacuum as you said is the ground state of the QFT in question. Roughly speaking, "space" (more properly spacetime) is the "place" where the fields "live". "The vacuum" is the groundstate of all the interacting fields in space. The article is talking about an instability wherein there could be a state with a slightly lower energy density to which the "apparent vacuum" could transition.

I am not an expert in this area at all, but there are many open questions which I think are the points that you are confused about. For example, the universe itself is not in a vacuum state, if anything it is probably in some thermal state. Furthermore, it's not clear that the universe as a whole can be said to be in a particular quantum state. Could some small region of the universe be said to be approximately in a ground state? Presumably such a region is one far from any source of excitations, in some sense "empty" (which is where the word "vacuum" to describe the ground state probably comes from). Going back to the idea that space is a "place" where fields "live", a more proper description would require a theory of quantum gravity, since generally it is believed that gravity itself must also be described by a quantum theory, which presumably interacts with the other fields, and therefore also is involved in the "vacuum state".

So generally speaking, when we refer to the word vacuum in this context, we are talking about the ground state of some QFT. As to the questions I raised above, hopefully someone more knowledgeable than I could clarify some of this.


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