Quantum vacuum

1. Nov 14, 2013

kye

The quantum vacuum is said to be the ground state of the combination of all virtual particles contributions from all particles and forces. How about in deep space where there is only CMB... so the virtual particles present is only from contribution from the CMB or complete particles meaning the lab on earth with all matter present and deep space with only CMB present has the same vacuum ground state? Is it like the Mach concept where all the particles in the universe is said to contribute to Inertia... Likewise, all the particles in the universe contribute to the quantum vacuum even if they are not present in a section of space?

2. Nov 15, 2013

kye

To rephrase my question, is the quantum vacuum just due to the particles and forces (mathematically, perturbations of them).. or do they have more primary roles that can affect the particles and forces? What is your thought about them? And what's the consensus?

Second. In QFT, the quantum vacuum is the ground states of the particles. But in cosmology where spacetime and the vacuum expands in inflation, the vacuum seems to have other meaning. Can someone confirm whether this is the case that there is two meanings of the word "vacuum"?

3. Nov 15, 2013

Staff: Mentor

If you cant get an answer here, I'd recommend the Quantum Theory forum.

4. Nov 15, 2013

Chronos

The dark energy concept is consistent with the false vacuum idea, where the energy content of the vacuum is non-zero. This is typically ascribed to virtual particle pairs which pop in and out of existence almost instantly. Here is a brief paper about the quantum vacuum you may find of interest: http://arxiv.org/abs/0909.2989. It is co-authored by Pisin Chen, who is himself an interesting character.

5. Nov 15, 2013

kye

I just read it, quite interesting. Question, there is this passage

"The vacuum fluctuations of the known matter fields cannot gravitate"

My question is whether the matter fields include all fields (electron field, quarks field, etc.) in location in space where there is no matter.. only CMB... so are vacuum fluctuations in all of space the same even if there is matter present or not in a particular locality?

6. Nov 15, 2013

bapowell

Quantum fields are defined throughout all space, whether or not there happens to be particulate matter present.

7. Nov 15, 2013

kye

Is it because the state vector contains all the state of the fields even not present on the interaction domain or locality? or do you see the vacuum as possessing and expressing the latent characters of all matter fields even when they are not there? what is the mathematical reason and justification?

8. Nov 15, 2013

bapowell

But the matter fields are there -- that's the idea. When you define the electron field, for example, you assign it a value throughout space, $\psi(x)$. In other words, an electron can be created anywhere the field is defined. Since we expect electrons to be able to exist across the universe, even in the remotest of places, the electron field should be defined across the universe.

9. Nov 17, 2013

kye

Is this right... you define a field and it extends throughout the universe and not just locally? But note that in the paper Chronos shared, "The vacuum fluctuations of the known matter fields cannot gravitate" giving rise to the 120 magnitude puzzle.. what's the proof the quark field is there in deep space where there is no matter. What experiments have been done on earth to detect fields of particles that are there. Maybe the reason the vacuum fluctuations don't gravitate is because they are not there (where the matter of the field is missing.. and the field not there...)?

10. Nov 17, 2013

bapowell

Anywhere there could be particles, there must be a quantum field, as particles are localized excitations of the field. You are proposing that there could be places in the universe where particles cannot exist. If this is what you mean to say, then how would one go about testing it?

Testing for the positive presence of quantum fields essentially amounts to verifying that particles physics works. The field itself is a mathematical construct; the physical manifestations of the field include particles and vacuum fluctuations. We know that each of these things exist here on Earth. In the same way that we generally assume the extension of the laws of physics to apply elsewhere in the universe, we do so for the quantum field.

Now, the statement that the quantum vacuum does not gravitate is central to understanding the dark energy problem. It is true that a naive summation of vacuum fluctuations up to the Planck scale leads to an enormous disparity with the observed expansion rate. However, it is not particularly obvious that we should be simply summing the vacuum contributions of each field to compute the total gravitational vacuum energy density. In flat spacetime, we subtract off the infinity on the grounds that it is unphysical. There are, however, instances where the vacuum energy does physically manifest itself, e.g. the Lamb shift and the Casimir effect. Both of these involve restricted dimensions: the quantum vacuum only shows itself when it is in some sense confined (specifically, when the vacuum modes get discretized). We might expect something similar to happen in GR.

While it's true that the energy content of the spacetime causes gravitation, it is generally assumed that the divergent parts of the stress tensor should still be subtracted. The complicating issue in GR is the lack of a unique vacuum: what vacuum do we use?? How do we isolate uniquely the divergent parts of the stress tensor? This has lead to the enormous field of research focused on renormalizing the stress energy tensor in curved spaces: it is complicated and as far as I know, no perfectly satisfactory (generally applicable) formulation has been found (see the excellent text by Birrell and Davies for details).

In analogy with the Casimir effect, Ford (http://prd.aps.org/abstract/PRD/v11/i12/p3370_1) sidestepped the above difficulties by looking at situations where the gravitational field was a small perturbation on Minkowski space. Here, the vacua are unique, the stress tensor is calculated for the perturbed spacetime, and then the perturbation is smoothly turned off and Minkowski space is obtained. If the difference between the perturbed stress tensor and that of Minkowski is finite and cut-off independent, then Ford argues this difference is the physical vacuum energy. This is sensible, but it only works in a very restricted set of circumstances.

So, it's an open and complicated problem. That a naive summation of vacuum fluctuations does not happen to agree with the observed expansion signals to me that this is not the right way to do the calculation. It does not necessarily imply to me that vacuum energy does not gravitate, or that, as per your suggestion, that quantum fields simply don't exist elsewhere in the universe.

11. Nov 17, 2013

kye

No. I meant let's say your computer you are reading this now, there are matter fields here because of the present of computer. In deep space (say between Neptune and pluto where there is no computer or matter), I wonder if the matter fields are still there..

Thanks.

12. Nov 17, 2013

bapowell

Yes, they are wherever you can potentially have particles, whether they happen to be there or not. Quantum fields can be thought of as providing the ability to have particles in a region of space, as the particles are localized excitations of the field.

13. Nov 17, 2013

kye

What experimental proofs have been done that proves that matter fields are there even if there is no matter and wherever you potentially have particles? Maybe you would cite particle accelerators as producing the resonances or particles by simply having enough energy in GeV or Tev to produce them. But here you are injecting energy into the vacuum. What is the evidence virtual particles of matter field are present in a local space or that doesn't have matter field? I just want to know experiment evidences, it's one thing to have them.. another to just have a theory of quantum fields, thank you.

14. Nov 17, 2013

bapowell

If you're asking whether we've flown out to Andromeda to see if there are electron fields there, no, we've not done this experiment. But, as I said above, we expect local physics to suitably extend across the observable universe. One would need a very good reason to suppose that there are regions of space in which no quantum fields exist, in which fundamental particle dynamics are not operative.

15. Nov 17, 2013

kye

My questions are really the virtual particles of the quantum vacuum that is said to be 120 magnitude bigger than observed and the worse prediction in all of physics.. so I'm not talking about Andromeda.. but say between Earth and the moon that doesn't contain matter. Do virtual particles still exist to justify the 120 magnitude difference anomaly? What's the experimental proof virtual particles of all matters are always present in the quantum fields even between earth and moon where there is no matter.

16. Nov 17, 2013

bapowell

I see. Virtual particles come along for the quantum field ride. If you postulate the existence of quantum fields, you get virtual particles. Yes, we expect there to be quantum fields between Earth and the Moon, and so we expect that there are quantum vacuum fluctuations occurring there as well. Keep in mind -- these are vacuum fluctuations -- i.e. they are associated with the quantum vacuum. No matter needs to be there for these to be present.

The 120 order of magnitude calculation pertains to the presumed energy density associated with these vacuum fluctuations. This energy density is assumed to exist uniformly throughout space, on account of the uniformity of the quantum vacuum. If you have space, and you've got particles that can potentially exist in that space, you've got quantum fields and you've got the quantum vacuum.

17. Nov 17, 2013

kye

Ok. Is the following distinctions correct:

quantum vacuum = ground state vacuum fluctuations of matter fields
vacuum = source of matter fields

in other words, the word "quantum vacuum" and "vacuum" is different with the former pertain to the ground state of the vacuum fluctuations of the matter field while the the latter "vacuum" refers to the source of the matter fields.. or are they synonyms? If so, what term do you reserve for the source of matter and even gauge fields?

18. Nov 17, 2013

bapowell

What do you mean by "source" of matter/gauge fields? I have been using the terms interchangeably, which is convention.

19. Nov 17, 2013

kye

http://home.thep.lu.se/~torbjorn/seminars/111116-vacuum.pdf

Please check the above very interesting site about the Physical Vacuum. I'd like to know if the contents of it are standard usage or the author's own. If standard, then there is a distinction between the Quantum Vacuum and Physical Vacuum.. because Quantum Vacuum is only related to the quantum fluctuations/virtual particles but Physical Vacuum is home of the say Higgs, gluon condensate, source of inflation, etc. as mentioned in the site.

20. Nov 18, 2013

bapowell

What is "very interesting" about it? What do you understand it to be about?

The author's use of the term "physical vacuum" is not standard, and without more details (like published work in a peer-reviewed journal, for example), I can't make much sense of these slides. There is no commonly accepted notion of "physical vacuum" in mainstream particle physics or cosmology that is distinct from the quantum vacuum.

Again, what do you mean when you say that the "Physical Vacuum is home to the Higgs..."?