Does empty space have energy?

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Particles are not regarded as bits of curved spacetime, but rather as excitations of quantum fields.

It has been suggested that spacetime curvature can cause structures that behave a bit like objects (though not fundamental particles) and these objects are called geons. However it remains unclear whether these would be stable. It also isn't known what impact quantum gravity effects would have on the formation and stability of geons.

The vacuum has a precise definition in quantum field theory and it has a mean energy of zero by definition. However we can have gravitatinal waves propagating where no matter is present, and there is an energy associated with these gravitational waves. You could regard this as a vacuum having energy, though normally we wouldn't call it a vacuum if gravitational waves are present. This energy is in effect the energy of the spacetime curvature, though this turns out to be a rather elusive quantity.

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Updated on August 23, 2020

Comments

  • whatwhatwhat
    whatwhatwhat about 3 years

    My physics friend suggested that "the answer to why matter exists in the universe" is because all massive particles are just the fabric of space excited into little packets. To illustrate, imagine a blanket on the ground. Then, pinch a small bit of the blanket and twist it. This is a particle that has mass.

    It was intriguing to hear this (he's only studied up through Freshman year of college physics), but there are clear flaws (i.e. angular momentum of a "particle" tied to a "blanket"??).

    Regardless, it made me wonder about vacuums. Is there any theory that suggests that a vacuum actually has energy in some form or another?

    • Hasan
      Hasan about 7 years
    • CuriousOne
      CuriousOne about 7 years
      Energy is the ability to perform work. What is a vacuum to perform work on? Even if we know nothing about general relativity and quantum field theory, the logical implications of the definitions of energy and vacuum are already pointing to a rather difficult problem to make such a statement in some way consistent.
    • Bill Alsept
      Bill Alsept about 7 years
      Is there a such thing as empty space?
    • whatwhatwhat
      whatwhatwhat about 7 years
      @BillAlsept in the universe? I couldn't say. In theory? Yes, remove all particles and fields.
  • kpv
    kpv about 7 years
    "though normally we wouldn't call it a vacuum if gravitational waves are present" - By this logic, even if empty space has energy, we could not call it vacuum because energy is present.
  • John Rennie
    John Rennie about 7 years
    @kpv: well yes. The QFT vacuum has zero (mean) energy by definition. If energy is present it isn't the vacuum state.
  • kpv
    kpv about 7 years
    So does vacuum exist at all anywhere in the universe then? Because of CMB..
  • John Rennie
    John Rennie about 7 years
    @kpv: quite so, since the CMB is present everywhere there is no such thing as completely empty space. But the CMB has such a low energy density that it can be ignored for most purposes. You'd have to be an awfully zealous physics pedant to object to the phrase empty space on those grounds.
  • kpv
    kpv about 7 years
    John, that was a question, not an objection. Thanks
  • Ján Lalinský
    Ján Lalinský about 7 years
    "...it has a mean energy of zero by definition." What definition do you mean? The precise one that defines vacuum or the one that defines energy?
  • Bill Alsept
    Bill Alsept about 7 years
    @kpv that's why up above I asked "is there a such thing as empty space?" Where in space can you find no photons or no gravity?