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

Negative energy?

  1. Jan 26, 2009 #1
    I want to know what the status of gravity constituting "negative energy" is. This theory proposes that the net energy of the universe is zero, presumably with the electro-weak and strong forces exerting "positive" energy over their respective ranges (distance). Is this idea still viable?
  2. jcsd
  3. Jan 26, 2009 #2


    User Avatar
    Science Advisor
    Gold Member

    It is [according to Sean Carroll] a convenient way to balance the energy content of the universe. Matter introduces a positive energy that is countered by gravity - when you strictly apply the laws of thermodynamics.
  4. Jan 27, 2009 #3


    User Avatar
    Science Advisor

  5. Jan 27, 2009 #4


    User Avatar
    Gold Member

    If gravity can counter possitive energy, where is the energy of gravity stored, if gravity is the particle, (graviton) i could understand it, but what if there is no graviton?


    The rubber sheet analogy, (energy stored in the rubber) would only work if space/time was a material.
    Last edited: Jan 27, 2009
  6. Jan 28, 2009 #5
    Thanks, but his thread wanders off into philosophical issues. My question is how regarding gravity as negative energy impacts our understanding of physical theory. For example, are there "negative fields" (gravity) and "positive" fields (EMF?, strong force?). Is the Higgs field a positive field? In what way should we regard the Higgs field as a positive field and gravity as a negative field? Why wouldn't they cancel each other out? This whole idea doesn't make much sense to me, not that it should since I'm not a physicist. However, the only time I've encountered this idea is in some of Hawkings' books. What is the place of this idea in mainstream physics?
    Last edited: Jan 28, 2009
  7. Jan 29, 2009 #6


    User Avatar
    Science Advisor

    It's just a feature of the always-attractive nature of the force (that is, any two test masses are always attracted to one another). I honestly don't know what more there is to say about this, as there is no fundamental requirement that energy be positive. Perhaps one must recognize that energy isn't such a simple thing that it can always be thought of as the kinetic energy plus rest energy (mass) of a particle.
  8. Jan 29, 2009 #7

  9. Jan 29, 2009 #8


    User Avatar
    Science Advisor

    Bear in mind that conservation of energy isn't absolute. Conservation of energy only holds when certain properties of the system in question are independent of time. There is, therefore, no requirement that energy always be conserved.

    Now, that said, it is probably always going to be possible to re-formulate your system so that the rules that govern its behavior are independent of time, which means that it is probably always possible to write things in a way that energy is conserved. But there are many entirely accurate ways of writing down the behavior of a system where the conservation of energy simply does not hold.

    Look up http://en.wikipedia.org/wiki/Noether's_theorem]Noether's[/PLAIN] [Broken] Theorem for some more detail on where the conservation of energy comes from.
    Last edited by a moderator: Apr 24, 2017 at 10:43 AM
  10. Jan 29, 2009 #9
    OK. Matter-energy may be or may not be fully conserved, but that's a long way from saying that the total (net) energy of the universe is zero. This is, I think, a very profound statement, not a mere issue of nomenclature. It seems to fly in the face of any conservation law, and invalidate most of, if not all of physics. I say "seems" because I'm not a physicist. Nevertheless the suggestion is that zero must be "something"; "something" being in some sense observable and/or testable, at least in principle if we're talking about anything scientific.
    Last edited by a moderator: Apr 24, 2017 at 10:44 AM
  11. Jan 29, 2009 #10


    User Avatar
    Science Advisor

    Actually, it is, because this statement only appears in the context of a closed universe described with the Hamiltonian formalism. It does not appear in the more common formulation of gravity, where the energy in the gravitational fields is not considered.

    Instead what is conserved in General Relativity is the stress-energy tensor. This tensor relates to the energy, pressure, and shear that exists in the matter content of the universe (it includes stuff like normal matter, dark matter, and dark energy). And this tensor is absolutely conserved within General Relativity due to the symmetries that it obeys. But because the stress-energy tensor includes more components than just energy, its conservation, under a variety of situations, means that the energy component of the tensor is forced to change while other components change. This is, fundamentally, why it is said that dark energy has to have negative pressure: in order for the energy of a comoving volume to increase, the relationship between the pressure and energy density within the stress-energy tensor has to be negative. If this is the case, then the conservation of the stress-energy tensor forces the energy of a comoving volume to increase as it expands.

    Well, the difficulty is that there are many different ways of describing the exact same physics. They're all correct, however, so it is perfectly valid to state that the energy of a closed universe is indeed zero, provided you qualify what you mean by that. It's also equally valid to merely state that the energy of an expanding universe is not conserved, due to the conservation of the stress-energy tensor, provided you qualify what you mean by that. These two scenarios are just two different ways of looking at the same physics.
  12. Jan 29, 2009 #11
    if by that you mean that 'negative energy' is just a way of saying that energy isnt 'really' conserved in gravitational interactions then I tend to agree.
  13. Jan 29, 2009 #12


    User Avatar
    Science Advisor

    Well, that just depends upon what you mean by "energy".
  14. Jan 30, 2009 #13
    I frankly don't understand why energy isn't conserved in gravitational interactions. Take this simple example: A weight is lifted to some arbitrary height in earth gravity and then dropped to its original position on the surface. The potential energy gained by the hoisted object is converted to the kinetic energy of the falling object. This energy is of course dissipated when the object returns to its original position possibly deforming the object and the surface, but energy (ie gravitational energy) is conserved.

    On a cosmic scale, you (Chalnoth) say that the stress-energy tensor is conserved. I can see this, because conservation laws allow for matter-energy conversion and pressure is dimensionally equivalent to energy density. I'm not sure how 'stress' is defined in this context, but I'm used to thinking of it as "stored energy", a kind of potential energy. Clearly, in an expanding universe, we have to think about these concepts in a non-classical way. The concept of "total energy" as I understand it, considers energy as basic, including all its possible forms. If it's the total energy of the universe that's supposed to be zero, why is this not creation ex nihlo?
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Negative energy?
  1. Negative energy universe (Replies: 20)