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There can never exist a vacuum

  1. Apr 20, 2006 #1
    I was just wondering, when people say that there can never exist a vacuum due to "vacuum energy", black body radiation etc. Do they mean that there can´t exist a vacuum of a certain size/volume, or that a perfect vacuum (as in unoccupied space) can never exist, regardless of how small it is?

    I would REALLY appreciate it if someone could explain this to me.

  2. jcsd
  3. Apr 20, 2006 #2
    As far as I know (which isn't very far yet I'm afraid) there is at least a vacuum between all the particles. However, since we have Heisenbergs Principle of Uncertainty, and all the electrons being probability clouds rather than particles this isn't neccessarily true.
  4. Apr 20, 2006 #3
    What they mean is that a vacuum is a region of space with absolutely nothing in it. Quantum field theory tells us that there's always particles popping in and out of existence from the vacuum and consequently there's never truly empty space.

    However in quantum field theory the vacuum is a state of lowest energy, which isn't necessarilly, and usually isn't, zero. This state consists of no particles, but in real life particle excitations occur completely arbitrarilly so the state of a field is invariably never the vacuum state.
  5. Apr 20, 2006 #4
    I´m aware that the "vacuum" in QF-theory is the lowest possible energy state, but I´m not sure if I´m understanding the following correctly: These so called virtual particles come in and out of existence, and at no point in time there´s a part of space (however small) that´s completely unoccupied by these? My intuition tells me that a vacuum "between" these particles, or in the area previously occupied by one, the instant the particle "pop" out of existence, should at least be possible, but maybe my reasoning is faulty...
  6. Apr 20, 2006 #5


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    If you imagine any volume of space, no matter how small, and any duration of time, no matter how small, there is still a very small vacuum energy associated with it. (At least down to the Planck scale.) In effect, the vacuum is filled with potentials for particle creation. That potential is real and is observable (e.g. Casimir effect). Because of the Heisenberg Uncertainty Principle (HUP), you can't really localize the position of a particle anyway. And so the particles you are imagining a space "between" actually occupy that space in a sense anyway. At least in terms of the quantum field.
  7. Apr 20, 2006 #6
    Still, doesn´t the HUP merely say that you can not know BOTH the particle´s position and momentum exactly? (meaning you could theoretically know the particle´s position but the accuracy of the particle´s momentum would be ridiculously low?) I´m not sure if I´m getting this right, but maybe the problem lies in the wave-particle duality? So maybe my mistake is looking at the problem strictly from a particle view?
  8. Apr 20, 2006 #7


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    The only way to confine a particle's position is using other particles, thus negating your ability to have a vacuum.

    Besides, there are those who believe that the HUP implies that particles do not simultaneously have well defined positions and momentum values. You could say that is leaning towards the wave side of the duality you mention. (There is nothing wrong with that view.)
  9. Apr 22, 2006 #8
    Forgetting if there are particles or not, the vacuum is filled with lots of quantum fields! To my mind, a vacuum isn't a vacuum if it contains a field. And quantum fields are more real than particles ever will be (at least according to QFT).
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