186. Video of the day: BBC on QED and Richard Feynman

8 Comments

1. Keith C says:

   January 18, 2009 at 8:53 pm

   Hi Dmitry
   I have heard of most of the stories before. But I was surprised and skeptical about the claim that Mayonnaise has something to do with QED. My impression is that the QED effects, like the Lamb shift in hydrogen atom is high order effect, maybe 1/alpha^3 (or 1/alpha^4,I don’t remember). And the Casimir force has to be measured in very careful designed experiment. So I don’t think it can be observed so easily, am I wrong?
   Keith

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2. Keith C says:

   January 18, 2009 at 9:44 pm

   Sorry I mean alpha^4 not 1/alpha^4

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3. Dmitry says:

   January 18, 2009 at 11:20 pm

   Hi Keith,

   in a sense, all you see around you is QED (simply because photons are the carriers of EM interaction), i.e., that’s the theory of nearly everything

   Cheers,
   Dmitry.

   Reply
4. Arjen Dijksman says:

   January 18, 2009 at 11:25 pm

   The stuff is interesting but the way of presenting adds to much weirdness to QM. Nice to have a look at Feynmans Caltech’s room… Btw, Shelter island is not at the coast of Pennsylvania, I thought, but of Connecticut(?)

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   • Dmitry says:

     January 18, 2009 at 11:34 pm

     Hi Arjen,

     but the way of presenting adds too much weirdness to QM.

     Yes, as too many BBC videos do

     Cheers,
     Dmitry.

     Reply
     • Arjen Dijksman says:

       January 21, 2009 at 9:41 am

       I just came across Murray Gell-Mann’s term Quantum Flapdoodle. That may qualify the spirit of such videos;-)

       Reply
       • Dmitry says:

         January 21, 2009 at 4:07 pm

         Well, not all of this is absolutely flapdoodly, but unprepared public will have hard time to separate flapdoodly talk from what actually makes sense

         Cheers,
         Dmitry.

         Reply
5. Lubos Motl says:

   January 19, 2009 at 12:25 am

   I’ve never heard a story that Dirac had a problem to believe the prediction of antimatter. Quite on the contrary.

   The story I know is that he was effectively ready for the Dirac sea prediction of antimatter since his childhood days and he did it almost immediately after he found the negative energy solutions.

   As a kid, Dirac was given the problem: three fishermen want to share their fish in the morning. The first one wakes up, wants to take 1/3, but the ensemble is not a multiple of 3, so he throws 1 away and takes 1/3 of the rest.

   The second one doesn’t know that the first one has left, wants to take 1/3, but it’s not a multiple, so he throws 1 away and takes 1/3 of the rest. The third also throws 1 and takes 1/3 of the rest. What’s the minimum number of fish to start with where it works?

   Dirac’s above-average classmates answered 25 (25, 24, 16, 15, 10, 9, 6). Dirac’s better answer was -2 fish. (-2, -3, -2, -3, -2, -3, it’s even a fixed point!).

   I am also surprised by all their bizarre statements that the interactions of electrons were the biggest problems in mid 20th century. Well, they knew “QED” since the early 1930s – they just didn’t understand all its loop and other aspects for almost two more decades.

   Dear Keith, the Casimir force took a long time for experimenters to measure but the difficulty is actually not its parametric smallness. The Casimir force comes from the (gradient of) Casimir energy which is just the uncancelled zero-point energy of the harmonic oscillators associated with the photon field. So it only depends on the geometry, not on interactions, and it has no alpha in it at all! We say it’s a 1-loop effect because it still needs some quantum mechanics but because it’s essentially a vacuum graph (vacuum energy density, no external legs), such a 1-loop effect is really as simple as other tree level effects.

   The Casimir energy density is -hbar.c.pi^2/240 a^4 where “a” is the distance between the plates. Parametrically it’s large. Still, for reasonably distant plates, it’s just very small.

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