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Zero-point energy, dark energy and space

  1. Apr 11, 2010 #1

    I've looked on the internet a lot about this: what is the zero-point energy and dark energy? From what I gathered, they both mean the same thing. Is that so?

    Also these two energies represent minimal energy densities *in* space. But what about energy *of* space itself. I mean here the constitution of space itself. This is what physicists used to call the ether, I think. This idea has been abandonned. But don't you think that space has to be constituted of something, for it certainly cannot a void. If it was a void, then how could matter be 'floating' is a void?

    I'd appreciate a lot if someone could clarify this for me. Thanks.

  2. jcsd
  3. Apr 12, 2010 #2


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    They are not the same thing.

    In quantum mechanics, we often talk about the harmonic oscillator. The quantum oscillator is similar to the classical oscillator, like a pendulum or a mass on a spring. It turns out though that the quantum harmonic oscillator's lowest energy state is non-zero. This is also called the zero-point energy though I typically hate to use that term now that it has been used as a buzz word so much. This is different from the classical oscillator where the lowest energy state is zero.

    It turns out that when we analyze the electromagnetic fields using quantum field theory that the fields behave similar to quantum oscillators. Long story short, and a lot of simplification, it means that the ground state for the vacuum for electromagnetic fields has a non-zero energy. In the vacuum state (not exactly the same as the vacuum of space, we mean just an absence of any photons), the fields fluctuate about a mean value of zero and have an energy greater than zero. So the energy density of the vacuum, if you do not allow for an upper limit of the frequency that electromagnetic waves can have, is infinite. However, this is not useful in anyway because we can only work with the change in energy, which gives rise to a force. In fact, we can renormalize the vacuum energy to be zero and the physics ends up being the same. In addition, this energy is the energy of the electromagnetic field states. In the vacuum state, there isn't any real photons so this energy does not represent any existing mass. You may hear about virtual photons popping out of the vacuum. This is different though. This is just a mathematical tool and it is based upon a situation where the energy of the system may have a large range of values such that the energy may be large enough to momentarily create and destroy a photon. But again this is viewed as a mathematical technique. So right now we do not see a physical consequence of the potentially infinite vacuum energy.

    Dark energy/matter, which is out of my area of expertise, is a means by which astronomers account for the apparent deviation in the mass of the Universe. Dark matter, unlike the zero-point energy, is treated as a real thing though its properties are beyond my knowledge. One of the many astrophysics experts can expand on this in more detail.

    However, I will comment on ether. Ether is a specific theory, the luminous aether, and it was the medium which mediated the propagation of light. However, the consequences of a luminous aether were not seen in experiments. This caused the theory to become more and more convoluted in attempt to keep current. In the end, it was dropped because it became overly complex and unsustainable. While you are not the first to use the term in this manner, you should understand the baggage that comes with the term and why it was rejected originally. Nothing in current physics has allowed the idea of a luminous aether to be supported.
  4. Apr 12, 2010 #3
    VERY well put. I just wish you could convince all of the perpetual motion enthusiasts out there.

    Dark Energy is a term used to identify the unknown force that accounts for the constant acceleration of everything we can see in the Universe. The Doppler Effect has been used to calculate the speed at which everything in the Universe is moving away from us. From this calculation, they have determined that everything, (outside of our Galaxy), seems to be moving away from us - faster and faster every moment we observe those objects.

    Dark Matter has been mostly narrowed down to a type of particle, called a WIMP.
    Last edited: Apr 12, 2010
  5. Apr 15, 2010 #4
    Thank you for the clarification. I didn't understand all the fine details, but I understand now that zero-point energy and dark energy aren't the same. And I suspected that much already before. I was seeking confirmation.

    But you have not answered my other question: the constitution of space, or the fabric of space. What makes up space? I don't mean what is inside space, but what is space itself. The way I see it, there are two concepts: the energy *in* space, and then energy *of* space. Is dark energy the energy of space, the energy that space itself is made of? Think of it this way: after the Big Bang, space had to be created first, then second, energy went there. It's like if you wish to go across a river. You build first a bridge, then people can go across. Do you see what I'm trying to get across?

    Looking forward for some more clarification. Thank you.

  6. Apr 15, 2010 #5


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    Dark energy and zero point energy are very similar. Dark energy behaves gravitationally just like quantum vacuum energy -- it leads to accelerated expansion of space. After the accelerated expansion of the universe was discovered, cosmologists thought that it was the result of the nonzero quantum vacuum energy! However, when they added up all those fluctuations, they got a number 120 orders of magnitude larger than the observed expansion rate! So, we're not sure where the dark energy is coming from, but it behaves just like quantum vacuum energy, and very well could be some form of it.

    Space itself isn't made out of anything as far as we know, however, the geometry of spacetime is manifested as gravity. There is no energy *of* space -- if you remove all sources of energy density, then space becomes truly empty and has no dynamics (you recover special relativity). Dark energy would be energy that you put *in* space.

    EDIT: Since writing this, it occurred to me that perhaps we should touch on gravity waves. These are quadrupolar waves in spacetime itself, just as light is a dipolar wave resulting from oscillating electric and magnetic fields. Gravitational radiation is an example of spacetime storing energy, and you can even think of gravity waves as spin 2 particles just as we now know that light is actually made up of photons. One can take this analogy further. Just as magnetic and electrostatic forces are often described as mediated by 'virtual photons', some have suggested that the manifold of spacetime actually be made up of virtual gravitons. However, these are by definition unobservable, and any energy the virtual gravitons have themselves won't gravitate, so it really becomes a question of interpretation. Sorry if I've terribly confused things...but it's an interesting question without an easy answer (that I can come up with anyway...)
    Last edited: Apr 16, 2010
  7. Apr 16, 2010 #6
    Thank you for your reply.

    You explain:
    " Space itself isn't made out of anything as far as we know, however, the geometry of spacetime is manifested as gravity. There is no energy *of* space -- if you remove all sources of energy density, then space becomes truly empty and has no dynamics (you recover special relativity). Dark energy would be energy that you put *in* space. "

    If space isn't made of anything -- not even energy -- then space isn't anything at all. The reason is that deep down in the quantum world, everything comes down to energy, correct? One way to see that is that only energy came out of the Big Bang. So, if it doesn't possess energy, it's nothing--more precisely, it's nothingness. So then space isn't made of energy, it's nothingness. But then there is space-time that explains gravitation. Gravitation occurs in space. However since space is made of nothing, gravitation--and everything else--takes place in nothingness. Could space be an illusion?

    Also the space-time model suggests that space-time has a 'shape'. But how can space which is nothingness have a shape? This is not only a philosophical impasse, but a physical one too. I cannot see how space cannot have a constitution made of energy.

    Can someone clarify this for me, please? This impasse makes my head spin. My hunch is that dark energy is what the fabric of space is made of. I read today an article that says that dark energy was discovered around 1998, and that physicists admit of not knowning much about this energy. So then the question is still open to debate.

  8. Apr 16, 2010 #7


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    I'd like to address some of your questions but I don't have time now. But I'd like to comment on the above now. As I explained, dark energy is not what space is 'made out of'. We don't know where the dark energy comes from, but we understand how dark energy behaves -- how it gravitates and how it might behave quantum mechanically. This particular part of your question is not open for debate.
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