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Friction in empty space

  1. Dec 9, 2013 #1
    Since there are quantum fluctuations popping in and out of existence then there should be an friction in the space though negligible(i know that they disappear in less than planks time but even then there should be a frictional force all round the universe)
     
  2. jcsd
  3. Dec 9, 2013 #2

    Mentz114

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    These fluctuation do not affect macroscopic bodies enough to be measured. But at the atomic/sub-atomic level they can cause spectral lines to split ( see the Lamb effect) and also affect the trajectories of particles ( see Feynman diagrams).
     
  4. Dec 9, 2013 #3

    K^2

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    That's not actually true. Not in vacuum. These interactions can affect how a particle interacts with another particle, but you'll never get "friction" in vacuum.

    The reason for that is that the total cross-section of QFT vacuum is zero. The simple way of picturing this is that these fluctuations are not just random, but as any QM effect, you get all possibilities at once. So for every electron that pops up into existence, there is a positron, so that the net electric field is zero, and there is no net interaction.

    The key to that is that particles are really fields at these scales, so you are not going to just "bump" into them. You have to interact with them via one of the fundamental forces. And all of these cancel out on vacuum.
     
  5. Dec 9, 2013 #4

    Bill_K

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    Friction would imply that an object in motion would be slowed down and eventually come to rest. But since the vacuum state is Lorentz invariant, there is no preferred rest frame, and "coming to rest" is meaningless.

    Affecting the trajectories of particles would imply that an object could exchange momentum with the vacuum. But the vacuum state is translation invariant, so its energy-momentum vector is zero, and this cannot be changed by interaction with a particle.
     
  6. Dec 9, 2013 #5

    ChrisVer

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    However doesn't that imply there would be no vacuum polarization?
     
  7. Dec 9, 2013 #6
    Would it be fair to say that it whatever is given up is quickly given back? Or are there just no interactions at all?
     
  8. Dec 9, 2013 #7
    anubodh...that's a reasonable inference.....just doesn't turn out to be accurate.

    Code (Text):
    Since there are quantum fluctuations popping in and out of existence
    Not really a good, yet common enough, description.

    From prior discussions in these forums:

    Bill_k:
    These virtual particles are not physical observables...so potential 'friction' is not either.....
     
  9. Dec 9, 2013 #8
    But you might consider the hydrogen molecules floating around....an average, is it?, of something like 6 per cubic meter....but that's probably not a good count 'all around the universe'.....I suspect, just an average.
     
    Last edited: Dec 9, 2013
  10. Dec 9, 2013 #9

    Mentz114

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    Thanks for pointing this out. 'Trajectories' is not a good choice of word for the perturbative loop calculation ( what does that mean, exactly ?)
     
  11. Dec 9, 2013 #10

    K^2

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    And that's why it's just an analogy. Naty1's reply points out the actual physics of it. I think I oversimplified it too much in attempt to make it more intuitive.
     
  12. Dec 10, 2013 #11
    This is what Dirac writes in his book The Principles of Quantum Mechanics:

     
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