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Casimir Effect and Perpetual Motion

  1. Mar 13, 2005 #1
    I read about the Casimir effect right away, and the first thing that came to me was a question. If quantum fluctuations can in empty space can move two completely uncharged metal plates at small distances, then wouldn't that give the potential for a perpetual motion machine?

    I realize perpetual motion machines are against the laws of physics, but then again, classical physics often don't apply to quantum mechanics, so would this be a possibility?

    I realize that the machine would be sustained off the fluctuations of space, but as long as space is there--as long as we exist--then it would theoretically run, would it not? For that purpose, would it not be perpetual? Perhaps I don't have a deep enough understanding of the Casimir effect?

    If someone could explain that'd be great :)
     
  2. jcsd
  3. Mar 13, 2005 #2
    When you separate the plates to do it again you will have to fight against the very same force. You would not gain any energy.
     
  4. Mar 14, 2005 #3
    But could we not "hold" those plates at those positions, and the energy pulling them in would subsequently be the energy gained?
     
  5. Mar 14, 2005 #4
    There is a force pulling them in not an energy. In order to do work(gain energy) you must let them move inwards.
     
  6. Mar 14, 2005 #5
    But would I be correct to assume that this creates energy? If so, this still provides a basis for a perpetual motion machine?
     
  7. Mar 14, 2005 #6
    No because eventually the plates would touch and you would not be able to let them move closer together. Hence no more energy gain. Then you would need new A)plates or B)separate the plate you have. A) is not economical, B) you don’t gain any net energy.

    Also the work(energy) that you could gain from letting the two plates come together is very small.
     
  8. Mar 14, 2005 #7
    My question isn't if it's practicle or not, but, is it possible?
     
  9. Mar 14, 2005 #8
    Yes I know. I have never heard any evidence that this is possible. There is no free ride, when you get energy it comes from somewhere. Earlier Physicsforms discussions:
    https://www.physicsforums.com/archive/t-4777_Free_energy_and_anti-gravity?.html
    https://www.physicsforums.com/archive/t-39619_Zero-point_energy_-_a_question_for_you_all..html
    https://www.physicsforums.com/archive/t-39619_Zero-point_energy_-_a_question_for_you_all..html
    https://www.physicsforums.com/archive/t-35190_Negative_energy_density_in_a_Casimir_vacuum.html

    This topic has been discussed many times before. I think that is why you are only getting me to respond.
     
  10. Mar 14, 2005 #9
    Ahh I see. Thank you :)
     
  11. Mar 15, 2005 #10
    https://www.physicsforums.com/archive/t-4777_Free_energy_and_anti-gravity?.html

    First, I would like you to look at the a quote from the very first message "...Casmir effect, be attracted to one another with 0.13 grams of force." Force is not calculated in grams; force is calculated in newtons(N). Now, lets look at what work is. Work is force times distance moved. A 10N weight moved 5 meters would produce 50J(fifty joules) of work. A brick held away from your body produces no work. You would get tired, but in physics you are not working! Why? The brick is not being moved. All the brick is doing is creating a force. Force times zero distance equals zero work.
    The Casimir effect produces a force between two plates just like gravity does with you and the ground. The means by which the force is produced is unimportant for this discussion. Now like a brick being dropped to the ground, a metal plate will be attracted to another metal plate and will fall to the stationary plate (it is odd thinking of an object falling upwards). Once the brick has released its energy on the ground, you must raise it again to get more energy, but in raising the block you undid all your work! Total energy gained is zero in zero friction. The Casimir effect is no different. Metal plate falls. Metal plate is picked up undoing all work gained.

    Davorak said
    No because eventually the plates would touch and you would not be able to let them move closer together. Hence no more energy gain. Then you would need new A)plates or B)separate the plate you have. A) is not economical, B) you don’t gain any net energy.
     
  12. Mar 15, 2005 #11
    Good example binarybob0001.

    Let me make clear that I was not trying to point out the correctness in those threads only that this has been discussed many times before. Many threads have been started with mistaken beliefs about the Casmir effect. Hence why few people have responded to this thread.
     
  13. Mar 15, 2005 #12
    Hi,

    I have read (I forget where) that some practical and theoretical work done on the Casimir effect suggest that under some conditions the force can be negative.

    These conditions are dependent on temperature, light, e/m fields and other variables.

    juju
     
  14. Mar 15, 2005 #13
    So the plates would push apart? Mind giving a reference if you can, it sounds like an interesting article.
     
  15. Sep 27, 2008 #14
    Sorry if I'm being dense here, but how do we know that the force caused by the Casimir Effect is conservative in all directions? As I understand it, two parallel plates will be pulled together in a vacuum in the direction of their normal vectors. However, I have seen no argument for why "sliding" the plates apart in a direction perpendicular to their normals would cost energy.
     
  16. Sep 28, 2008 #15

    atyy

    User Avatar
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    The Casimir effect is a fluctuation induced force, predicted by statistical mechanics, which is consistent with classical thermodynamics, which claims that no perpetual motion machines are possible.

    The `Friction' of Vacuum, and other Fluctuation-Induced Forces
    Mehran Kardar, Ramin Golestanian
    http://arxiv.org/abs/cond-mat/9711071
     
  17. Dec 11, 2008 #16
    I'm blatently being dense too but I cant see why sliding the plates apart in a direction perpendicular to their normals would cost any energy either, if u could then arrange the plates in some sort of loop, could u not get them to move around perpetually? I know it wont work but i cant work out why.
     
  18. Dec 11, 2008 #17
    Casimir force is an attractive force which depends on the distance.

    Left call it a 'Casimir Gravitational field' :)
    This field is a potential field
    Hence, no tricks with loops would work.

    (I know, it is an oversimplification)
     
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