4 ms·

Ok, interesting. The gravity part makes sense. I don't understand the difference with magnetism though. Let's say you have a positively charged item and you're

by zeven7 8y ago

Ok, interesting. The gravity part makes sense. I don't understand the difference with magnetism though. Let's say you have a positively charged item and you're standing on top of a large negatively charged surface. You "drop" the positively charged item. How does its velocity over time differ from the gravity example? And how is it different with time reversed? Seems like it would be the same to me... In forward motion, it would accelerate until it collided with the surface. With time reversed, it would start with a fast velocity and slow down over time. Edit: I googled this. It sounds like the motion of magnetized bodies is time symmetric, but when the charge of the electric field produced by the motion is taken into account it has to be reversed. Is this correct?

ridiculous_fish 8y ago
In classical electromagnetism, we separate the electric field E and the magnetic field B. If we isolate E, we get rest: electrostatics. Charged particles exert an attractive or repelling force in proportion to their charge, just like gravity, except charge instead of mass. So E is time-symmetric. This applies to your example. Now consider just B. A positively charged particle is moving to the left, through a magnetic field that points inwards. By the right-hand rule, the force on the particle is up. Reverse time: now the particle is moving to the right, so you rotate your hand 180 degrees and the force is downwards. This magnetic field produced an upwards force forwards in time and a downwards force backwards in time: it's antisymmetric. Put mathematically we have: F = qv x B where q is the charge, v is the velocity, B is the field. Reverse time and the velocity gets a minus sign, so F reverses. So E is symmetric and B is anti-symmetric.
- blueprint 8y ago
  What makes it the right-hand rule rather than the left-hand rule?
  - ridiculous_fish 8y ago
    Positive charges obey the right hand rule, while negative particles obey the left hand rule. Classical EM doesn't distinguish between these, and we might as well reverse the signs and hands. QM does so distinguish, for reasons way beyond me.
    - blueprint 8y ago
      Seems like a very important/foundational answer to know, though. Do you happen to know of anything I can read about why charged particles behave by that rule?
      - snarfy 8y ago
        https://www.youtube.com/watch?v=0mXW1zPlxEE https://www.youtube.com/watch?v=0mXW1zPlxEE https://en.wikipedia.org/wiki/Sterile_neutrino https://en.wikipedia.org/wiki/Sterile_neutrino

by zeven7 8y ago

ridiculous_fish 8y ago

In classical electromagnetism, we separate the electric field E and the magnetic field B. If we isolate E, we get rest: electrostatics. Charged particles exert an attractive or repelling force in proportion to their charge, just like gravity, except charge instead of mass. So E is time-symmetric. This applies to your example. Now consider just B. A positively charged particle is moving to the left, through a magnetic field that points inwards. By the right-hand rule, the force on the particle is up. Reverse time: now the particle is moving to the right, so you rotate your hand 180 degrees and the force is downwards. This magnetic field produced an upwards force forwards in time and a downwards force backwards in time: it's antisymmetric. Put mathematically we have: F = qv x B where q is the charge, v is the velocity, B is the field. Reverse time and the velocity gets a minus sign, so F reverses. So E is symmetric and B is anti-symmetric.

blueprint 8y ago
What makes it the right-hand rule rather than the left-hand rule?
- ridiculous_fish 8y ago
  Positive charges obey the right hand rule, while negative particles obey the left hand rule. Classical EM doesn't distinguish between these, and we might as well reverse the signs and hands. QM does so distinguish, for reasons way beyond me.
  - blueprint 8y ago
    Seems like a very important/foundational answer to know, though. Do you happen to know of anything I can read about why charged particles behave by that rule?
    - snarfy 8y ago
      https://www.youtube.com/watch?v=0mXW1zPlxEE https://www.youtube.com/watch?v=0mXW1zPlxEE https://en.wikipedia.org/wiki/Sterile_neutrino https://en.wikipedia.org/wiki/Sterile_neutrino