A similar conundrum occurred in my own life. I was about 7, and my father yelled at me to turn off the backyard faucet. I didn’t know which way to turn it, so he yelled to turn it “to the right.” I knew left from right, but it was clear that whichever way I turned the round faucet handle, the top and bottom would be moving in opposite directions. Since I couldn’t decide, my father came and shoved me out of the way, cursing “I’ve gotta do every goddamn thing around here myself!”
Tip: Turn it either direction - if it stops, that was “right”; if not, that was “left”.
There MUST be a rule that we share a spoken or written language. Otherwise how does one convert between our air-powered grunts and their vibratory ultrasonic beeps. 
Or their chameleon color flashes?
Yeah, indeed, if they can read our language, it’s eminently simple to explain in the sense that you can just say ‘you are reading this message left-to-right.’ And if they can’t read our language, how are we even communicating?
Morse code. No “lefts and rights” in sequence communication.
But that’s a different problem. If your answer to every given response is “OK, then define those terms, too,” sure, you can make any problem unsolvable, but that’s a bit of a ridiculous position to take.
Yes, I think the OP is getting very close to a “move the goalposts” question.
“Describe A without using B or C.”
“Well, you could use X and Y.”
“Well, what if you couldn’t use X or Y either? How would you describe A?”
“Well, you could try Z.”
“You can’t use that either. How would you describe A without B, C, X, Y, or Z?”
Wait, is this the Star Trek universe? If so, then we know that nearly every civilization out there speaks English. Problem solved. Oh, wait, did you mean to say that you are communicating with an illiterate alien?
Martin Gardner was fond of talking/writing about this problem. In fact, he wrote a whole book about it: The Ambidextrous Universe which has gone through several editions, including at least one edition after Gardner died.
IANA expert on this problem. Plus, I didn’t read the Sci-Am article from post #4.
Explaining that we’re trying to label in 3-space the two opposite directions that are not up nor down & also are not towards nor away is trivial, given some common language and a communications mechanism. And thereby we reduce the problem to simply which of the remaining two orthogonal directions is left & which is right. What they are, how they relate to up, down, towards, and away, and which two they are would not by doubted by our aliens. The sole remaining non-trivial issue is which of the two is which?
ISTR there are lots of chiral molecules that only fold or coil or … one way. There might even be some macroscopic crystals that exhibit that behavior. So grab one of those molecules. Made up example: hold it so the Nitrogen is up, the carbon is away, and look at the oxygen; it’ll be on the right every time.
ISTR there are also chemicals that do come in stereo pairs. But one pair is significantly more likely than the other. So grab a sample and look at a bunch of molecules. Made up example: hold each so the Nitrogen is up, the carbon is away, and look at the oxygen; The direction the oxygen points in 75% of the samples is right.
What am I missing or screwing up here?
But doesn’t that depend on which way up the guy is holding it?
I remember Martin Gardner’s article, and I believe Isaac Asimov dealt with it too. They both said that, before the asymmetry of certain Weak Force interactions, there was no possible answer. But nowadays, we know that (as Scotty said in an old Star Trek novel) the universe has a left-hand twist.
Describe the location of the numbers on a clock face.
Describe the location of "N’, ‘S’, ‘E’ and ‘W’ on the compass.
And: just to preclude: describe the direction of the hands on a clock.
‘N’ is at the ‘top’ (which is legit), 'S; is at the bottom (ditto); ‘E’ is at/ to the _______.
It gets back to ‘right’ is ‘right’ and ‘left’ is ‘left’.
Now you know why it takes children so long to get it right.
Not really, it’s a pretty well known and well established thought experiment: based only on exchanging a series of messages with an alien civilization (i.e. you can’t send a polarized beam, you can’t point to some astronomical object you can both see, etc), can you communicate to the alien which of its hands is the “right” hand? It’s just an illustration of how symmetric our universe is, and how all the laws of nature work equally well when the orientation is reversed.
As already mentioned, the answer is, the only way to communicate right/left is to refer to the violation of symmetry in weak interactions. But if I remember correctly, even this is not 100% unambiguous - it only works if you’re sure that your aliens are not made of antimatter.
No, if a molecule is stable, its mirror image is equally stable. They may behave differently in chemical reactions, but only if the other molecules involved are asymmetric. They will behave exactly the same way if you replace all the other molecules involved in the reaction with their mirror images. (I think Asimov had a short story about a man who was thrown up into a higher dimension and came back reversed; he almost died of malnutrition until they figured out how to synthesize mirror images of certain nutrients.)
I think this is only true for certain organic molecules. And the only reason one orientation is more likely is because they are synthesized by organisms or molecules that are themselves all one of 2 possible orientations.
Really, and how are you going to convey the definition of magnetic north and south?
I see people have already mentioned the whole breaking-conservation-of-parity thing, but here’s another reference for that, the first place I heard of it:
http://www.feynmanlectures.caltech.edu/I_52.html
Sections 52.6 to 52.8, though 52.4-5 are also useful for background.
You don’t have to - you do it in terms of direction of electron flow in a wire.
Give it a go. But remember, left and right are currently not defined.
As to using this to define north and south poles, you might ask if the technique will even work consistently here on Earth. 800,000 years ago you would have got the opposite answer.
I probably don’t understand this correctly at all, but for it not to work, I think the aliens would also have to be moving backwards in time. Which seems unlikely to be an issue.
CPT symmetry.
Flip left and right, replace matter with antimatter, and flip time to the other direction, and you have a universe that looks identical to ours. Do only one or two of those, and you can tell the difference.
Again, if I understand this correctly at all. This stuff is mostly way, way over my head.
Interesting. This seems achievable.
We can define an anode and a cathode easily enough. That means we can construct a circuit wire with a known anode and cathode. We can also therefore build a magnet with a known north and south pole, and from that define a north and south pole of the planet. We can then align a compass along a circuit wire with a known orientation.
Then we just repeat the Orsted experiment. If the north pole of your compass is facing the cathode, then it will deflect to the right. If it’s facing the anode, it will deflect to the left.
Thus left is the direction that the north pole of a north-facing a compass needle will deflect when current is run under it from a cathode in the south.
Can anyone see why this wouldn’t work?
You are just transferring the problem to north and south… what does north mean ?