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What is energy?

  1. Feb 23, 2014 #1
    Can someone tell me if there is a definition for energy. I don't mean a type of energy such as kinetic or potential etc. We talk about empty space having energy but what is it? Thanks
  2. jcsd
  3. Feb 23, 2014 #2


    Staff: Mentor

    There are several definitions of energy, depending on what theory or formalism is being used:
    Newtonian: energy is the capacity to do work
    Lagrangian: energy is the conserved quantity associated with time-translation symmetry of the Lagrangian
    thermodynamics: energy is 1/2 mv² or anything that can be converted to it
    general relativity: energy (density) is the time-time component of the stress-energy tensor

    My favorite is the thermodynamics version.
  4. Feb 23, 2014 #3
    Thanks and can we discuss this further..

    Thanks for the thermodynamic definition. It is thirty years ago or more that I studied this at uni. I will brush up on it. I wonder though if we could discuss this further. I am interested in this because I know that physicists see nothingness as a sort of “physical nothingness”, because after they have removed all of what exists in a space and create a vacuum they still observe that there is still something there, which is energy. However this is not something that is observed over time but which pops in and out of being if I understand it properly. Am I right so far?
  5. Feb 24, 2014 #4
    That's a very good question. Obviously, energy has some trasferential property, that is....observations have demonstrated that it could be converted from one thing to another. In the same way of how a car motor works. On a macroscopic level, heat seems to be commonly generated from this conversion process.

    I'm not too privy or knowledgeable about quantum energy; perhaps that's another theory that can be extended, but to answer your original question, I think energy is simply the relationship between atomic interactions, albeit on a micro and macroscopic level.
  6. Feb 24, 2014 #5
    But in a vaccuum there are no atomic interactions, that is what puzzles me if I am to accept a "physical nothingness". Do you know someone else I can ask who is more knowledgeable about quantum energy or can you recommend where I can post this again with your definition? Thanks for your help.
  7. Feb 24, 2014 #6
    I believe someone with more knowledge about quantum behavior will eventually answer your question. A sci adviser on this site who goes by the name of "bhobba" has answered my questions about quantum behavior related to light; I believe he can adequately answer your question on quantum energy if you care to message him.
  8. Feb 24, 2014 #7


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    Staff: Mentor

    You need to look at vacuum energy from the perspective of Quantum Field Theory. In QFT and related theories, particles are simply excitations of underlying fields. These fields have quantized energy values and, as all quantum systems, have a ground state with a finite amount of energy.


    One could say that the claim "remove all of what exists in space" to be impossible since particles and fields are effectively two forms of the same thing.
  9. Feb 24, 2014 #8
    Thanks llstanfield, I shall message bhobba after I’ve brushed up on some background stuff. And thanks Drakkith I’m looking into the reference you gave me.
  10. Mar 20, 2014 #9
    re vacuum energy

    Am I correct in saying that the casimir effect, the Lamb shift and spontaneous emission etc., are the effects that we see by which we assume the existence of the vacuum energy?
  11. Mar 20, 2014 #10


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    This is one of our most-often asked question.

    Exhibit A: https://www.physicsforums.com/blog.php?b=3203 [Broken]

    Last edited by a moderator: May 6, 2017
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