Perpetual Motion

  1. I guess this wouldn't be perpetual motion. But if you could lasso a photon wouldn't it keep pulling the object? Purly hypothetical of course. Maybe make a chamber that keeps the photons directed going one way, constantly pushing. That could turn a trubine in a genorator constantly creating a flow of electricity.

    Hey, why not.
  2. jcsd
  3. My guess is that the light energy would change to the kinetic or mechanical energ, so that it gives the thing one good tug, and that is all.
  4. russ_watters

    Staff: Mentor

    Light has momentum and if you take momentum from it (by lassoing it), it would lose momentum until it disappeared.
  5. Poy - I thought that photons were kenetic matter. So the energy will always stay the same. And yes it would only give one good tug in one direction, then it would bounce back and equalize the force that was given. The photon will have to be constantly directed moving one way.

    russ_watters - I saw the consept of lassoing the photon no different than having it bounce of a wall. And I don't fully understand what you mean by having it dissapear. Wouldn't all photon eventually loose all momentum from constantly bouncing off objects?
  6. light acts in a similar respect to matter in that in a collision energy is lost. The velocity of a photon cannot however change so either the frequency or the amplitude changes(I can't be sure which right now). As well if you think about it in order to lasso a photon without slowing it down the lasso and whatever was attached to it would need to be instantaneously accelerated to the speed of light. This is impossible.
  7. I'm sorry if I gave the impression that the photon wouldn't slow down. It would. What I'm trying to say is that the photon is complete kenetic energy, now just convert that to mechanical energy. What the "lasso" will be is unknown to me, yet. It would be like roping a bunch of cattle that could never stop running. The increased mass would simply slow the photon down.

    As I understand the photon creates 'push' when it colides with something. This is due to the electromagnetic waves it posseses. And the photon is always moving at c (the fastest speed possible through space). Now let those waves push an object one way without having the photons rebound and push the opposite direction.

    I believe that electrons already do this in a way. The electron travels at close to c, but not at. And when something collides with them, electrons kick off photons and slow down. But if it bombarded with photons it 'absorbs' some and kicks to a higher energy level. Maybe the electron has a core that has some type of wave that attracts photons.
  8. russ_watters

    Staff: Mentor

    Momentum is a vector property - it has magnitude and DIRECTION. So if a photon bounces off of something, the thing it bounces off of gets TWICE the initial momentum of the photon and the photon travels in the oppostie direction with the same scalar momentum value and loses no energy.
  9. So you're saying that if the photon wasn't able to bounce back, then it would disappear? If it did, where would it go?
  10. Integral

    Integral 7,288
    Staff Emeritus
    Science Advisor
    Gold Member

    Photons do NOT bounce.

    I am sure it is quite a lot of fun imagining that a photon is like a little ball that you can play catch with on sunny fall afternoon, but it just isn't so. As long as you continue to carelessly apply classical concepts to the quantum arena you are sure to be wasting your time.

    Once again photons do NOT bounce. When a photon interacts with a surface it is adsorbed. Its energy is transfered to that of the atom it encounters. In response to adsorbing the photon an electron is excited to a higher energy level. If this is an unstable level the electron will cascade back to its original level. The path it takes determines what photons are re-emitted. It may be same, it may be several different photons, it will depend upon the incident photon and the material.

    If you wish to learn how to harness (lasso?) the energy contained in a photon you MUST learn Quantum Mechanics, thinking of it as a classical problem will get you no where.
  11. So you're saying that the photons that I see bouncing off the mirror are being absorbed. That then brings the electron to an unstable level, so the electron in turn kicks off those same photons just so I can see myself brush my teeth every morning. Whoa... I wonder what it takes to turn the door knob?

    Oh, and if the photon is the electromagnetic carrier or whatever, then wouldn't there be positive and negative photons?
  12. russ_watters

    Staff: Mentor

    Sorry. When just talking about momentum, I didn't feel the need to go into the QM - though clearly the concept of it 'disappearing' requires talking about absorption.
  13. Hurkyl

    Hurkyl 15,987
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    You don't want to know. :wink:
  14. Integral

    Integral 7,288
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    No, Why should it?
  15. Because there are positive and negative electromagnetic waves. And wouldn't the carrier have to have all the wave properties?
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