How do you explain it without any common experience?
Well, that introduces a further problem - if you have no common experience... how do you communicate with them at all?
Well, that introduces a further problem - if you have no common experience... how do you communicate with them at all?
The "Contact" model of an enormous transmission that can be progressively decoded can not be ruled out, though we have experimental evidence that it's harder than it seems at first blush. (Still, those experiments tend to focus on having our 'top minds' decode pictures that amount to a few hundred bits; arguably it would actually be easier to decode an entire mathematical codex as we would have a lot more to work with, much like cyphertexts actually get somewhat easier to crack as they get larger.)
Remember that the rosetta stone was so useful because it provided a reference point to help decode a previously incomprehensible language. In order to communicate with someone, you must have a common experience somewhere.
The whole "But they might be incomprehensible and we might have zero common reference frames" is not really applicable here, just a fashionable knee-jerk reaction to "the alien question". As a possible outcome, sure, as the only outcome, no way.
And imagine, moreover, that you had to do so in a purely verbal manner, that is, without drawing pictures or pointing at things or otherwise making gestures.
With diagrams or pointing, showing what left and right mean becomes trivial.
On the knee-jerk reaction thing, it's more pointing out that it's a matter of moving the goalposts as mentioned below. It's making an arbitrary line of what can be communicated
I agree with you that the incomprehensible alien form is pointless, but as soon as we have a common frame of reference, we can drill down to defining right and left. All you need is four points: Point A is 'up', Point B is 'down', put your viewing position so that Point C is between you and the line AB. With A being 'up', Point D is on the left side of the line AB. Whether you use stars, a mountainside, a tree, galaxies, or jellyfish, that's all you need - but you still need a common point to refer to.
I think a better way of phrasing the problem would be: You have a communications conduit to a parallel universe and we only have the ability to speak through it. The alien understands us but has no prior concept of right and left. We can't point or refer to bodies in our universe as they can't see them, but apart from that, all physical rules are the same and they have the same level of science knowledge as us. How do we tell them right from left in this instance? We can get the concept across with left- and right-handed molecules (though DNA may be out as they may not have it), but how to indicate which is left and right in verbiage only, I'm not sure.
Phrased this way, you don't have to do silly logical twists to have the alien living in our universe yet still somehow preventing them from seeing the same things we do.