scent@sask.UUCP (Scent Project) (06/06/85)
Mirrors flip images back to front, but not in the plane of the mirror. We think of mirrors as inverting left-to-right because we project our- selves into the scene or person we see in the mirror, and from that vantage point, left and right are flipped. (If you stand facing a friend, your left is their right, and vice-versa.) An interesting, related bit of trivia is the fact that "left" and "right" are concepts which can only be communicated among two parties if both parties can "see" each other. In other words, if we ever contact an alien civilization which is so far away that we can communicate with them only by radio, and there is no astronomical object which both we and they can view, then we would have no way to tell them what we mean by "left" as opposed to "right"--or, for that matter, how to tell the north pole of a magnet from the south pole. For those interested, Martin Gardner, of "mathematical recreations" fame (I THINK it was Martin Gardner) wrote a very interesting book about the handedness of the universe. You'll have to find the title yourself. Ken McDonald "Recent research indicates that 90% of all statistics are completely unreliable" :-)
gv@hou2e.UUCP (A.VANNUCCI) (06/07/85)
>An interesting, related bit of trivia is the fact that "left" and >"right" are concepts which can only be communicated among two >parties if both parties can "see" each other. In other words, if >we ever contact an alien civilization which is so far away that we >can communicate with them only by radio, and there is no astronomical >object which both we and they can view, then we would have no way to >tell them what we mean by "left" as opposed to "right"--or, for that >matter, how to tell the north pole of a magnet from the south pole. > > Ken McDonald Not exactly true! If you can send radio waves to the distant aliens, use right-handed circular polarization to let them know left from right. If you can only send them "information" as opposed to a physical signal (such as radio waves), then you could ask them to perform one of the parity-violating experiments involving weak interactions. These are experiments where an axial vector (such as a magnetic field) is aligned with a polar vector (such as the direction of emission of particles). Parity violation means that if you look at the experimental apparatus in a mirror you see a physical phenomenon that is impossible. Only experiments involving weak interactions have this characteristic. However, if in addition to reflecting the image the mirror exchanges matter with anti-matter the reflected experiment is all right. The moral of the story is that, when you finally meet your alien friend after many years of discussing parity-violating experiments and you are about to shake hands, if he stretches out his left hand you'd better run away as fast as you can! He's made of anti-matter! Giovanni Vannucci AT&T Bell Laboratories HOH R-207 Holmdel, NJ 07733 hou2e!gv
emh@bonnie.UUCP (Edward M. Hummel) (06/08/85)
>An interesting, related bit of trivia is the fact that "left" and >"right" are concepts which can only be communicated among two >parties if both parties can "see" each other. In other words, if >we ever contact an alien civilization which is so far away that we >can communicate with them only by radio, and there is no astronomical >object which both we and they can view, then we would have no way to >tell them what we mean by "left" as opposed to "right"--or, for that >matter, how to tell the north pole of a magnet from the south pole. Parity violation in the weak interactions has been known to exist since the mid-1950s. (Yang, Lee, Wu, et al.) Just tell your alien to measure the spin of any neutrino he has handy. All neutrinos in the universe seem to be "left-handed" (helicity=-1; spin counter-clockwise as they travel away from you; etc.). All anti-neutrinos seem to be "right-handed". This certainly gives you a way to define "clockwise" as opposed to "counterclockwise", or "left" as opposed to "right". Well, you ask, how can I be sure he is looking at a neutrino and not an antineutrino? CP violation has been around since 1964. (Cronin, Fitch, Christenson, et al.) Just tell your alien to study the decay of neutral K mesons. By comparing results you can unambiguously define the particle K0, as opposed to its antiparticle, K0-bar. This gives you a way to define "matter" vs. "antimatter", and thus neutrinos vs. antineutrinos. I haven't read Gardner's book so I can't comment on what he intended. Ed Hummel