5 ms·
Feynman wrote a exceptionally clear explanation of this experiment and what it meant: - "To illustrate the whole problem still more clearly, imagine that we we
by perihelions 2y ago
Feynman wrote a exceptionally clear explanation of this experiment and what it meant:
- "To illustrate the whole problem still more clearly, imagine that we were talking to a Martian, or someone very far away, by telephone. We are not allowed to send him any actual samples to inspect; for instance, if we could send light, we could send him right-hand circularly polarized light and say, “That is right-hand light—just watch the way it is going.” But we cannot give him anything, we can only talk to him. He is far away, or in some strange location, and he cannot see anything we can see. For instance, we cannot say, “Look at Ursa major; now see how those stars are arranged. What we mean by ‘right’ is …” We are only allowed to telephone him."
- "Now we want to tell him all about us. Of course, first we start defining numbers, and say, “Tick, tick, two, tick, tick, tick, three, …,” so that gradually he can understand a couple of words, and so on. After a while we may become very familiar with this fellow, and he says, “What do you guys look like?” We start to describe ourselves, and say, “Well, we are six feet tall.” He says, “Wait a minute, what is six feet?” Is it possible to tell him what six feet is? Certainly! We say, “You know about the diameter of hydrogen atoms—we are 17,000,000,000 hydrogen atoms high!” That is possible because physical laws are not invariant under change of scale, and therefore we can define an absolute length. And so we define the size of the body, and tell him what the general shape is—it has prongs with five bumps sticking out on the ends, and so on, and he follows us along, and we finish describing how we look on the outside, presumably without encountering any particular difficulties. He is even making a model of us as we go along. He says, “My, you are certainly very handsome fellows; now what is on the inside?” So we start to describe the various organs on the inside, and we come to the heart, and we carefully describe the shape of it, and say, “Now put the heart on the left side.” He says, “Duhhh—the left side?” Now our problem is to describe to him which side the heart goes on without his ever seeing anything that we see, and without our ever sending any sample to him of what we mean by “right”—no standard right-handed object. Can we do it?"
https://www.feynmanlectures.caltech.edu/I_52.html https://www.feynmanlectures.caltech.edu/I_52.html
- Modified3019 2y ago“Try to teach an alien on a telephone about left and right” actually sounds like a fun game to try with a class full of kids. I would solve this particular circumstance by using the andromeda galaxy (“the closest galaxy”) as a reference, but it definitely stumped me for a bit.
- samanator 2y agoYou can't use cancellation or heavenly bodies: "He is far away, or in some strange location, and he cannot see anything we can see. For instance, we cannot say, “Look at Ursa major; now see how those stars are arranged. What we mean by ‘right’ is …” We are only allowed to telephone him."
- Modified3019 2y agoAh good catch, I somehow still assumed same galaxy. Thinking about it, ironically I think the direct mention of Ursa Major might have played a part in that.
- noworld 2y ago"imagine that we were talking to a Martian"
- indoordin0saur 2y agoI think by Martian he just meant "alien". Obviously if you're on Mars you could use all sorts of celestial bodies to demonstrate chirality.
- verzali 2y ago" he cannot see anything we can see. For instance, we cannot say, “Look at Ursa major; now see how those stars are arranged"
- lern_too_spel 2y agoThe rest of the story about developing a way to communicate with the alien is a significant part of Andy Weir's Project Hail Mary, which is also a good book for kids.
- Gunax 2y agoI would love to try it, but I think the problem is hard enough to explain to an adult. I think it's difficult to describe the constraints. I've tried to explain this problem to adults and they just didn't really get it and the problem sounds like either a riddle or a bunch of nonsense rather than a physics problem with deep implications for the laws of nature.
- deleted 2y ago[deleted]
- AaronM 2y agoI thought about this for a bit. One easy to use explanation for left and right is a number line. Smaller numbers are on the left and larger numbers are on the right. It's still an assumption that alien species would count that way.
- jodacola 2y agoI think this still illustrates the problem, though: where is the start of the number line? Perhaps this alien species has a right-to-left written language. Or top to bottom. How would one help orient the alien to the left to right numbering system used in this language? Very interesting thought experiment while drinking my coffee this morning!
- AaronM 2y agoI added a reply further up that attempts to explain my thought process
- DonaldFisk 2y agoWhat if they draw a number line with smaller numbers on the right and larger numbers on the left?
- perihelions 2y agoIndeed. There's nothing you possibly draw on paper that answers the question, in isolation. A diagram on paper means exactly the same thing if you simultaneously (i) flip it, and (ii) flip your mental interpretation if it. Or: if you run a software program on a raster image, there exists a different program that returns identical output given an input image which is mirror-flipped copy of the first one. (It's just the first program, plus a pre-processing step that flips its input (which is computable). If f is a computable program, and f(img) = "left" and f(flip(img)) = "right", there exists a computable program g=f∘flip such that g(img) = "right" and g(flip(img)) = "left"). Similarly: there's nothing you can do in complex analysis that can distinguish the case where all +sqrt(-1)'s are swapped with -sqrt(-1)'s. Nor can you invent any math whatsoever that has an isomorphism to the plane, that works differently if the plane is mirror-reversed.
- dotancohen 2y agoWhy couldn't we just tell the aliens to observe the direction of the field lines of electromagnetism? We could define for them "lower potential energy direction" by describing gravity, then use that phrase for describing the motion of the electrons in a conducting wire around a magnetic coil.
- griffzhowl 2y agoHow would that allow you to distinguish the coil from its mirror image?
- dotancohen 2y agoNo coil, see my other comment here: https://news.ycombinator.com/item?id=41195886 https://news.ycombinator.com/item?id=41195886
- jxy 2y agoWe chose a random direction as a convention, just as we chose a random charge as positive.
- dotancohen 2y agoNo, the electrons actually move in the same direction always, just as a free falling object always moves towards the locally dominant mass. We could call the direction of electron movement "right".
- snarkconjecture 2y agoIt's the magnetic field that has the arbitrary sign convention. You can't determine the direction of a magnetic field from observations without using the right hand rule.
- dotancohen 2y agoYou don't have to know the sign, you could tell which way the electrons are going. We tell the aliens: you take all sorts of organics with liquid inside (on earth we typically use a lemon, but other organics will do), and put a stake of element 29 in one side and a stake of element 30 in the other. You then connect them with a length of element 29. Keep trying different organics until you find one that works. They might not have lemons, but there is a good chance that something will eventually work. You'd need to predefine what the atomic numbers mean and some other things, but we're already assuming some level of communication already being established so this aspect is not far-fetched.
- luxuryballs 2y agophysicist L100, human anatomy L1 (the heart is in the center)
- blipvert 2y agoHa, I remember this from an old Open University TV lecture. They suggested looking at the spin of a particular particle. So eventually the alien arrives and it goes to shake hands/tentacles, but it’s not holding out the right one as it has been taught is customary … BOOM annihilation Ah, the alien was made from antimatter, where the spin is different. Oops.
- altruios 2y ago... but we can determine left and right via the chirality in chemistry? sugar water in a tube has a 'right-handed' chirality (where it twists polarized light to the right)... "left is the handedness of the rotation of the neutrino" Steve Mold and 3b1b have videos on the subject.
- perihelions 2y agoThe inverse enantiomers of sugar function exactly the same way, with the correspondingly inverted enzymes. The chirality of sugars isn't a universe-wide property inherited from fundamental physics, but just a path-dependent quirk of the evolutionary history of biochemistry on Earth. "Universal" for Earth-life only. (The strength of the weak force is very certainly far too... weak... for it to have plausibly influenced this selection process. It was 100% chance).
- altruios 2y agoFair point about biological selection playing a part. neutrinos (the only ones we can detect), however, are left handed. https://physics.stackexchange.com/questions/240502/why-only-left-handed-neutrinos-have-been-observed-yet#:~:text=Thus%2C%20the%20right%20handed%20neutrino,of%20the%20standard%20model%20forces https://physics.stackexchange.com/questions/240502/why-only-.... We simply point to the neutrino. left v right solved?
- Xen9 2y agoAs long as he exists within a planet instead of unknown dimension, we can simply give them a slightly modified version of Wikipedia (removing arbitrary data, arbitrary metadata; language does not matter given it is a plaintext we can encode into transmission). They will eventually figure it out.