• Question: Is it possible to change the strength of the vacuum fluctuation locally, i.e. create an area in which the local density of virtual particles is lower than in other areas?

    Asked by anon-324403 on 1 Apr 2022.
    • Photo: Luke Humphrey

      Luke Humphrey answered on 28 Mar 2022:

      Hi sure451rue, excellent question. I’m not sure to be honest with you.

      Quantum is not my best subject, but I’ll try my best to answer.

      It’s my understanding is that vacuum fluctuations are random fluctuations in the fundamental force fields, characterised by virtual particle-antiparticle pairs.

      Given the randomness, I would assume there’s some reasonably flat virtual particle density distribution when zoomed out, but if you took a few samples from small enough regions you’d find varying densities.

      The space occupied by “real” particles would be an obvious contender for a space with a lower density of virtual particles, but I think you mean in “empty” space so I wont cheat with that answer.

      You’re using the word “create” , so are you asking whether we as people can manipulate this vaccum fluctuations?
      Again, I don’t know, but I imagine we could do so by using particles and fields to bias the location of annihilations.

      There is also zero energy universe theory which says that the whole universe is a quantum fluction, so by that theory then all the density variation of the known universe is emergant from random fluctuation alone.

      Hopefully someone who specialises in quantum physics is around to give you a better answer.

    • Photo: Judd Harrison

      Judd Harrison answered on 28 Mar 2022:

      Yes! For instance, around objects like black holes where the electric and gravitational fields can become very strong, the energy required to create a pair of virtual particles can be offset considerably by the potential energy of the pair after their creation. This offset in energy changes the structure of the vacuum fluctuations. In fact if the fields become strong enough then the particles don’t have to be virtual – strong enough electric fields will pull pairs of electrons and positrons from the vacuum. This mechanism also causes black holes to spontaneously emit what’s known as “Hawking radiation” where one particle of a created pair falls into the black hole past the event horizon and the other escapes.

    • Photo: Daisy Shearer

      Daisy Shearer answered on 28 Mar 2022: last edited 31 Mar 2022 1:40 pm

      Wow, it sounds like you know your stuff! I’m going to assume you know what vacuum fluctuations are and virtual particles since you used these terms in your question but do feel free to comment on this question if you need further clarification on anything as we can get a dialogue going if you want to!

      I’m coming at this from a quantum optics perspective as that’s my background. To me, this question sounds like you’re asking about a concept in cavity quantum electrodynamics. When we create a cavity (let’s say with a pair of mirrors), we impose boundary conditions on a system. When we put an atom into a cavity that is sufficiently reflective/resonant, its properties can change quite a bit and it’s primarily because of the vacuum fluctuation that you speak of. Examples of cavities we can use for this include a Fabry-Perot cavity and systems where we have a two-level quantum system such as a quantum well or quantum dot (a distributed feedback laser or VCSEL is a good example of this). I could get into the details of dressed states and Rabi oscillations if you like (just drop a comment on this question) but I’ll try to keep this relatively short!

      Backing up a bit, vacuum fluctuations are essentially virtual photon emission and reabsorption which usually just go about being created and reabsorbed in free space (Lamb shift is evidence for vacuum fluctuations existing). But when we confine a photon in our cavity, there’s an enhancement of the quantum fluctuations of the electric field as we’ve created a system where we have several modes of a harmonic oscillator. In this case, we can see zero-point vacuum fluctuations which can be demonstrated using the Casimir effect.

      So, to answer your question- yes, in my current understanding (which may be flawed- I’m constantly learning!) we can change the local density of virtual particles by using a cavity.

      We could also explore this question from an astrophysics and cosmology perspective too but that’s not really my area as I’m primarily a quantum physicist/engineer 😊

      [NOTE: I realise that this is quite a complex answer which includes some assumptions about prior knowledge which I inferred from the nature of your question. Please do ask for any follow-ups or clarifications in the comments if anything is unclear to you.]