Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Let me make sure of something?

  1. Mar 22, 2005 #1
    Is the vacuum = quantum foam?
  2. jcsd
  3. Mar 23, 2005 #2
    I don't think that they are the same. The vacuum is a term employed in quantum physics, and is not really void, because the continuum apperarance of pairs of virtual particles and antiparticles
    The term quantum foam was first proposed by Wheeler and is his idea of what spacetime should resemble when observed at the Planck scale: should have a foamy appearance. Quantum foam is only a hypothesis and an extension to General Relativity, but is not framed in what we can call "Standard quantum physics"

    Recently, Quantum foam has emerged appeared within a theory called Process Physics
    http://en.wikipedia.org/wiki/Process_Physics [Broken]
    In Process Physics, gravity is described as an inflow of quantum foam into matter
    Last edited by a moderator: May 1, 2017
  4. Mar 23, 2005 #3


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    what "vacuum" means depends on the theory

    string theory has many possible vacuums and they havent settled on which they want to represent reality

    quantum field theory has a rigid vacuum with no gravity. the QFT vacuum is usually flat but sometimes given some fixed curved shape. QFT is good at modeling matter, but not gravity.

    Loop Quantum Gravity has a model of spacetime called a "spin foam" in which gravity (and the changing geometry of space) can be modeled,
    but LQG has not gotten very far along in the project of representing MATTER in that "foam" vacuum. they still are using highly simplified matter-fields in LQG analysis and trying to get the geometry to work.

    Probably there are several more physical models where they use the word "foam" to describe the mathematical framework representing space. Meteor has just mentioned something about foam "flowing", in some theory I have never heard of.
    In LQG the foam representing spacetime DOES NOT FLOW anywhere
    it is a way of describing a path along which the geometry of space can evolve

    some string theorists have also been using the term "quantum foam" too and I cant tell what they mean by it. the term was cropping up in a notable paper by Dijkgraaf, Gukov, Neitzke, and Vafa posted at the end of last year.

    People are still groping for mathematical models that can comprise both particle physics and gravity (i.e. changing geometry of space responding dynamically to the matter in it). they use all kind of WORDS to refer to their various attempted mathematical models
    the words are not the reality
    the mathematical models come a bit closer but they are not the reality either
    Last edited: Mar 23, 2005
  5. Mar 23, 2005 #4
    But both of them have "virtual particles" coming in and out existence...I think that the "quantum foam" is what is in the "vacuum" which appears to be empty but not really since it has virtual particles foaming in and out of existence
  6. Mar 23, 2005 #5
    I have been googling about this for a while and its vacuum fluctuations (energy fluctuations in the vacuum) causes quantum foam, the creation of virtual particles
  7. Mar 23, 2005 #6


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    well in LQG the term foam gets used an awful lot because spin foam is the name of a mathematical entity that is the basis of one of the two main directions that research is going nowadays
    and in LQG foam certainly does not mean what you say it means for you.
    (it doesnt have to do with particles "foaming in and out of existence")

    and John Archibald Wheeler one of the grand old men of 20th Cent. physics made a famous speculation that spacetime down at planck scale was a foam. but that wasnt particles "foaming in and out of existence" that was just how space was geometrically down at quantum level---full of holes and bubbles and tunnels

    but in some OTHER area or areas of physics the word foam may very well mean just what you are talking about! I think "foam" is a wonderfully descriptive word to apply to virtual particles. I'll bet you that Brian Greene has used that mental image some of his highly popular books, of the vacuum being full of particles which "foam in and out of existence".

    I guess in physics words like that have no meaning except as they refer to some specific mathematical object which somebody is using to model nature.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook