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Science Forum

  1. Standard member sonhouse
    Fast and Curious
    17 Jan '11 00:25 / 1 edit
    Here is a link to a new technology in electron microscopy:

    http://www.physorg.com/news/2011-01-electron-vortices-potential-conventional-microscopes.html

    My question is this: What are the forces involved that would make an electron beam spiral around itself like that? I can see if there was a magnetic field line, that's what they do in regards to magnetic fields, but if there is no magnetic field, how can it spiral? Can electrons spiral around electric fields? I suspect there is more to the show than appears in the article.

    Anyone know about this kind of thing?
  2. 17 Jan '11 10:38
    From the article:
    "Passing electrons through a nanometer-scale grating, the scientists imparted the resulting electron waves with so much orbital momentum that they maintained a corkscrew shape in free space."

    I.e. the "vortex" motion is caused by conservation of angular momentum.
  3. Standard member sonhouse
    Fast and Curious
    17 Jan '11 16:16 / 1 edit
    Originally posted by KazetNagorra
    From the article:
    "Passing electrons through a nanometer-scale grating, the scientists imparted the resulting electron waves with so much orbital momentum that they maintained a corkscrew shape in free space."

    I.e. the "vortex" motion is caused by conservation of angular momentum.
    But if it is just spinning, if you did that with some kind of weight, say a 100 mg sphere, and imparted the same motion, as soon as it was let loose it would just continue on a straight line. How can you manage that feat with electrons?

    Even if the sphere was spinning at 10,000 rpm, when it was let loose it would not continue in a spiral motion.
  4. 17 Jan '11 19:39
    Originally posted by sonhouse
    But if it is just spinning, if you did that with some kind of weight, say a 100 mg sphere, and imparted the same motion, as soon as it was let loose it would just continue on a straight line. How can you manage that feat with electrons?

    Even if the sphere was spinning at 10,000 rpm, when it was let loose it would not continue in a spiral motion.
    Erm. It would, somewhat, if it were in a medium. That's how baseball pitchers, tennis players, golfers and so on play effect shots. I must admit that I don't see how this could affect an electron, though, unless its spin causes a quantum-dynamic force in nearby particles (and thus also in itself).

    Richard
  5. 17 Jan '11 22:24
    Originally posted by sonhouse
    But if it is just spinning, if you did that with some kind of weight, say a 100 mg sphere, and imparted the same motion, as soon as it was let loose it would just continue on a straight line. How can you manage that feat with electrons?

    Even if the sphere was spinning at 10,000 rpm, when it was let loose it would not continue in a spiral motion.
    Apparently they manage that feat with that nano something thingy. Don't ask me how it works though!
  6. Standard member sonhouse
    Fast and Curious
    18 Jan '11 16:52
    Originally posted by KazetNagorra
    Apparently they manage that feat with that nano something thingy. Don't ask me how it works though!
    Yeah, that nano thingy is just a kind of filter if I have it right, then the electrons leave and continue spiraling. Would like to know how.
  7. 18 Jan '11 20:55
    Originally posted by sonhouse
    Yeah, that nano thingy is just a kind of filter if I have it right, then the electrons leave and continue spiraling. Would like to know how.
    I suppose it's analogous to gratings used in (ye olde) experiments with light.
  8. Standard member sonhouse
    Fast and Curious
    18 Jan '11 21:55
    Originally posted by KazetNagorra
    I suppose it's analogous to gratings used in (ye olde) experiments with light.
    Yeah but there you get polarization, circular, linear, etc. That does not change the path the photon takes through space. Electrons, having a negative charge, would tend to fly apart in reaction to their own electric field, that might have something to do with it but just how that would be involved I haven't a clue.
  9. 19 Jan '11 10:46
    Originally posted by sonhouse
    Yeah but there you get polarization, circular, linear, etc. That does not change the path the photon takes through space. Electrons, having a negative charge, would tend to fly apart in reaction to their own electric field, that might have something to do with it but just how that would be involved I haven't a clue.
    Gratings do change the path the photons take through space.

    Electrons do repel each other, but in these kind of experiments this expansion of the cloud/beam is limited due to the speed of the electrons.
  10. Standard member sonhouse
    Fast and Curious
    19 Jan '11 17:25 / 2 edits
    Originally posted by KazetNagorra
    Gratings do change the path the photons take through space.

    Electrons do repel each other, but in these kind of experiments this expansion of the cloud/beam is limited due to the speed of the electrons.
    Not sure what the actual velocity of the electron beam, it can't be going more than about 25 KEV. We regularly accelerate arsenic, boron or phos to 200 KEV in our ion implanters but of course electrons, being so much lighter will go a hell of a lot faster volt for volt of accel. If I remember right, Arsenic, AMU 75, at 200 KEV is going about a half million miles per hour, 800,000 km/hr, so I imagine electrons at 25 KEV would be going a lot faster than that considering the huge difference in mass, about 140,000 to one compared to arsenic. That would indicate some kind of restoring force to keep the path of the beam in a spiral.
  11. 21 Jan '11 07:17
    As far as I can tell, the electrons are travelling so fast that there is a wave effect as a result of quantum dynamics. If so, then the spiraling too may be a quantum wave effect rather than a Newtonian type spiral motion. I am not sure though where the 'angular momentum' comes in as I don't think that should apply in quantum dynamics (but then I could be wrong).
  12. 21 Jan '11 07:42
    Originally posted by twhitehead
    As far as I can tell, the electrons are travelling so fast that there is a wave effect as a result of quantum dynamics. If so, then the spiraling too may be a quantum wave effect rather than a Newtonian type spiral motion. I am not sure though where the 'angular momentum' comes in as I don't think that should apply in quantum dynamics (but then I could be wrong).
    Yep, you're wrong. First of all, the wavelength of electrons decreases as they move faster, so you will notice less "wavelike" effects in general. Secondly, conservation of angular momentum holds both in quantum and in classical physics.
  13. 21 Jan '11 09:09
    Originally posted by KazetNagorra
    Yep, you're wrong. First of all, the wavelength of electrons decreases as they move faster, so you will notice less "wavelike" effects in general. Secondly, conservation of angular momentum holds both in quantum and in classical physics.
    Please explain more.
    Are these waves quantum waves? If so, surely the slower the electron is going the greater the certainty of its position and thus the smaller the wave length? Or am I wrong again?
    Is there any momentum involved in quantum waves? Is the electron moving side to side or forwards and backwards in a wave like motion, or is it really a probability function of its location?
  14. 21 Jan '11 12:13
    Originally posted by twhitehead
    Please explain more.
    Are these waves quantum waves? If so, surely the slower the electron is going the greater the certainty of its position and thus the smaller the wave length? Or am I wrong again?
    Is there any momentum involved in quantum waves? Is the electron moving side to side or forwards and backwards in a wave like motion, or is it really a probability function of its location?
    Read this:

    http://en.wikipedia.org/wiki/De_broglie_wavelength
    http://en.wikipedia.org/wiki/Angular_momentum#Angular_momentum_in_quantum_mechanics

    Note that angular momentum is not the same as momentum. Angular momentum is given by the operator L = r x p, with p the momentum operator and r the position.
  15. 24 Jan '11 05:09
    Originally posted by KazetNagorra
    Read this:

    http://en.wikipedia.org/wiki/De_broglie_wavelength
    http://en.wikipedia.org/wiki/Angular_momentum#Angular_momentum_in_quantum_mechanics

    Note that angular momentum is not the same as momentum. Angular momentum is given by the operator L = r x p, with p the momentum operator and r the position.
    Thanks for the links. I don't think I understand it all, but I did learn something new.