4 ms·

Actually his story was about trying to explain the difference between left and right in terms of nuclear spin experiments. You also explain that humans greet by

Actually his story was about trying to explain the difference between left and right in terms of nuclear spin experiments. You also explain that humans greet by shaking their right hands. If you meet the alien and he extends his left hand to shake, he must be made of antimatter (since the combination of parity and antimatter conjugation is a symmetry of the universe). edit: jess is correct, and that's the whole point of the story actually. if CP was invariant you wouldn't be able to tell they were antimatter by describing the nuclear physics decay experiment. instead the antimatter version of the parity experiment gives the opposite result. I shouldn't try to physics so early in the morning :)

jcoffland 10y ago
Assuming a two handed alien of course.
- hossbeast 10y ago
  Many handed
  - Semiapies 10y ago
    But only bilaterally symmetrical. Radially symmetric aliens with hands give no indication! (Well, aside from whether or not they blow up whatever local matter surface they're standing on.)
    - grkvlt 10y ago
      That's neither necessary or sufficient! Notice that they could be bilaterally symmetric and two arms/hands positioned vertically on their front (anterior?) so no left/right hand distinction (but top/bottom instead) or; they could be asymmetric but have two hands/arms on one side, and one on the other, still giving them a left/right side choice.
- grkvlt 10y ago
  And also attached to different sides, not in a vertical row straight up the front of the creature, for instance... (When asked "What do you look for in a woman?" the Mathematician answers "Bilateral symmetry, of course!")
- ianai 10y ago
  I'm just proud when a physicist assumes a cows not a perfect spheroid.
- stan_rogers 10y ago
  In Feynman's story, the "alien" we meet is an artificial human (a biological Locutus, if you will) built using a description we sent them rather than a coincidentally humanoid life form. For a throw-away illustration taking only a couple of minutes in the middle of a lecture about symmetry, the guy filled in a lot of the obvious gaping holes.
jessriedel 10y ago
> combination of parity and antimatter conjugation is a symmetry of the universe It's only an approximate symmetry. Look up CP symmetry violations.
- spott 10y ago
  CP symmetry violation is the reason that the universe appears to be made more of matter than antimatter. Though if I'm not mistaken, the scale of the violations that we have seen, and the violations that must exist in order for the observable universe to be mostly matter aren't in agreement. We don't see enough CP violating processes to explain this asymmetry in the universe. One of the big open problems in Physics.
- tbabb 10y ago
  CPT still holds, though, right?
  - jessriedel 10y ago
    Yes

jcoffland 10y ago

Assuming a two handed alien of course.

hossbeast 10y ago
Many handed
- Semiapies 10y ago
  But only bilaterally symmetrical. Radially symmetric aliens with hands give no indication! (Well, aside from whether or not they blow up whatever local matter surface they're standing on.)
  - grkvlt 10y ago
    That's neither necessary or sufficient! Notice that they could be bilaterally symmetric and two arms/hands positioned vertically on their front (anterior?) so no left/right hand distinction (but top/bottom instead) or; they could be asymmetric but have two hands/arms on one side, and one on the other, still giving them a left/right side choice.
grkvlt 10y ago
And also attached to different sides, not in a vertical row straight up the front of the creature, for instance... (When asked "What do you look for in a woman?" the Mathematician answers "Bilateral symmetry, of course!")
ianai 10y ago
I'm just proud when a physicist assumes a cows not a perfect spheroid.
stan_rogers 10y ago
In Feynman's story, the "alien" we meet is an artificial human (a biological Locutus, if you will) built using a description we sent them rather than a coincidentally humanoid life form. For a throw-away illustration taking only a couple of minutes in the middle of a lecture about symmetry, the guy filled in a lot of the obvious gaping holes.

jessriedel 10y ago

> combination of parity and antimatter conjugation is a symmetry of the universe It's only an approximate symmetry. Look up CP symmetry violations.

spott 10y ago
CP symmetry violation is the reason that the universe appears to be made more of matter than antimatter. Though if I'm not mistaken, the scale of the violations that we have seen, and the violations that must exist in order for the observable universe to be mostly matter aren't in agreement. We don't see enough CP violating processes to explain this asymmetry in the universe. One of the big open problems in Physics.
tbabb 10y ago
CPT still holds, though, right?
- jessriedel 10y ago
  Yes