4 ms·

It turns out you can define it from looking at radioactive decay (or any other interaction governed by the weak force). Feynman famously had a short description

by dguest 4y ago

It turns out you can define it from looking at radioactive decay (or any other interaction governed by the weak force). Feynman famously had a short description that ended with the punch line > Suppose, after lots of communication you finally can go into space and meet your alien counterpart. If, as you approach one another, the alien extends its left hand to shake, watch out! He’s made of antimatter! because even the physics of weak decays of will invert in an antimatter universe.

cubefox 4y ago
That claim needs to be substantiated.
- qubidt 4y ago
  The assertion that it is possible or the attribution to Feynman? Scientific American [1] references Feynman's 4th Messenger lecture at Cornell: "Symmetry in Physical Law" (1964) [2] [3] [1] https://blogs.scientificamerican.com/degrees-of-freedom/galactic-challenge-part-ii-the-richard-feynman-files/ https://blogs.scientificamerican.com/degrees-of-freedom/gala... [2] https://www.feynmanlectures.caltech.edu/messenger.html https://www.feynmanlectures.caltech.edu/messenger.html [3] https://www.youtube.com/watch?v=W7K1kXeK-tY https://www.youtube.com/watch?v=W7K1kXeK-tY
  - cubefox 4y ago
    Sorry, I meant a hypothetical definition of "left".
- dguest 4y ago
  Which claim? The full transcript of the Feynman story is available online [1], you can search for "martian" to get right to the story. It's presented in the context of explaining CP symmetry, which says that physics is conserved under a parity (mirror reflection) plus charge inversion (matter -> antimatter) transformation. Of course it's only approximate [2]. I got the quote slightly differently from what is recorded on the caltech sight, the one they have is > So if our Martian is made of antimatter and we give him instructions to make this “right” handed model like us, it will, of course, come out the other way around. What would happen when, after much conversation back and forth, we each have taught the other to make space ships and we meet halfway in empty space? We have instructed each other on our traditions, and so forth, and the two of us come rushing out to shake hands. Well, if he puts out his left hand, watch out! [1]: https://www.feynmanlectures.caltech.edu/I_52.html https://www.feynmanlectures.caltech.edu/I_52.html [2]: https://en.wikipedia.org/wiki/CP_violation https://en.wikipedia.org/wiki/CP_violation
  - cubefox 4y ago
    So the claim is that CP violation would make it possible to define "left" without ever pointing at things? Of course then the question would be what this definition actually is.
_fizz_buzz_ 4y ago
I think it should also be possible with electro magnetism. By first defining the sign convention for electron flow and then deriving left hand and right hand rules of magnetism or something like that. It’s a bit tricky to explain the sign convention, but I think if one sticks with the movement of elementary particles, it should work.

by dguest 4y ago

cubefox 4y ago

That claim needs to be substantiated.

qubidt 4y ago
The assertion that it is possible or the attribution to Feynman? Scientific American [1] references Feynman's 4th Messenger lecture at Cornell: "Symmetry in Physical Law" (1964) [2] [3] [1] https://blogs.scientificamerican.com/degrees-of-freedom/galactic-challenge-part-ii-the-richard-feynman-files/ https://blogs.scientificamerican.com/degrees-of-freedom/gala... [2] https://www.feynmanlectures.caltech.edu/messenger.html https://www.feynmanlectures.caltech.edu/messenger.html [3] https://www.youtube.com/watch?v=W7K1kXeK-tY https://www.youtube.com/watch?v=W7K1kXeK-tY
- cubefox 4y ago
  Sorry, I meant a hypothetical definition of "left".
dguest 4y ago
Which claim? The full transcript of the Feynman story is available online [1], you can search for "martian" to get right to the story. It's presented in the context of explaining CP symmetry, which says that physics is conserved under a parity (mirror reflection) plus charge inversion (matter -> antimatter) transformation. Of course it's only approximate [2]. I got the quote slightly differently from what is recorded on the caltech sight, the one they have is > So if our Martian is made of antimatter and we give him instructions to make this “right” handed model like us, it will, of course, come out the other way around. What would happen when, after much conversation back and forth, we each have taught the other to make space ships and we meet halfway in empty space? We have instructed each other on our traditions, and so forth, and the two of us come rushing out to shake hands. Well, if he puts out his left hand, watch out! [1]: https://www.feynmanlectures.caltech.edu/I_52.html https://www.feynmanlectures.caltech.edu/I_52.html [2]: https://en.wikipedia.org/wiki/CP_violation https://en.wikipedia.org/wiki/CP_violation
- cubefox 4y ago
  So the claim is that CP violation would make it possible to define "left" without ever pointing at things? Of course then the question would be what this definition actually is.

_fizz_buzz_ 4y ago

I think it should also be possible with electro magnetism. By first defining the sign convention for electron flow and then deriving left hand and right hand rules of magnetism or something like that. It’s a bit tricky to explain the sign convention, but I think if one sticks with the movement of elementary particles, it should work.