Conveying left/right to alien by description

I remember hearing of a puzzle some time ago that went along the lines:

There is a humanoid alien race on a far distant planet that has never encountered the human race.

Purely by spoken instruction (i.e. no visual communication, and you cannot make use of scientific attributes such as polarisation), you need to inform this race how to identify left and right.

Since they are humanoid you can assume that they share the concepts of up/down front/rear, and left/right - they just do not know which name for the latter describes which side. You can also assume they can do the same everyday functions that we can.

Now, it was stated that this is impossible, but I think I’ve missed something as it seems a fairly simple thing to do.

Does anyone know the full rules to the problem that make it impossible?

Tell us how you would do it and no doubt Dopers will be happy to tell you where you went wrong.

This is going to come down to what you consider a “scientific attribute.”

If that phrase covers all of observation and physics, it’s impossible because you can no longer use any words that are in any way related to the topic you’re talking about. Heck, “left” and “right” are explanatory descriptions of the world, which makes the very words themselves “scientific attributes” and hence verboten.

It’s like trying to describe the number “2” without being allowed to directly or indirectly reference counting of anything, numbers, mathematics, etc.

This article might be useful

If you were knowledgeable about the astronomy of the alien’s home planet, you could describe the planets star as apparent motion across the sky from right to left or left to right, if standing in one or the other of the planets hemispheres.

An alien explaining it to an earthling in London could just say that when facing the sun, it moves from left to right across the sky. Or in Cape Town from right to left.

There’s the Right Hand Rule, and something about a substance that only forms on the left legs of aphids or something like that.

Until failure of conservation of parity was discovered, there would have been thought to be no way. But of course, the experiments needed to demonstrate that (which does give an unambiguous definition of right and left) would require some fairly subtle physics, banned by the OP.

Except there’s no way to convey what “northern hemisphere” and “southern hemisphere” mean.

The southern hemisphere is the one that has Christmas in the summer.

Which way do their clocks turn?

I’m not sure I think this is an interesting question. In reality we could easily define left and right by defining up and down in relation to some astronomical features above and below the galactic plane, then define left and right in relation to the rotation of the galaxy. Any alien species in our galaxy we are communicating with is going to be able to check the same astronomical features as us and we can quickly agree on common terms.

How would you define which is above and which is below the galactic plane?

Even races in other galaxies can be communicated with in a similar manner. As long as you can correctly reference a particular (spiral) galaxy (including ours) and there’s some astronomical feature outside that galaxy distinctive enough to be clearly identified, it seems simple: ‘left’ is the direction of the rotation of <galaxy> if observed from <astronomical feature>.

Whether you can identify the galaxy and the astronomical feature depends, I suppose, on the exact details of what you’re not allowed to communicate.

Below (Galactic South) is the hemisphere with the Magellanic Clouds.

The question becomes more interesting if you can’t see their galaxy at all; the communication might be occurring through a wormhole to a galaxy outside the Hubble Volume, for example. This would mean there would be no possibility of visual cues.

If I understand the OP correctly, he/she is not saying that there are any scientific concepts which you are not allowed to use in your explanations, but only that the medium of communication itself is not allowed to convey left/rightness. E.g. you cannot transmit them a polarized light beam and use the properties of that light beam to define “left”.

That doesn’t mean you cannot use the concept of polarized light in your explanation; just that the explanation itself must be pure abstract data. You could transmit a picture as a series of bits, but then you’d run into a chicken-and-egg problem in trying to explain to them which side of the picture is the left side.

This reminds me of Sunday school in the 70’s, during which a popular assignment was explaining Christmas (from a religious standpoint, of course) to a Martian. Wish I still had the story I wrote.

This is a classic.

It is one of my favs:

“Describe left and right without using “left” or “right””.

I’ve asked this a few dozen times in the office (IT).

A few actually try responses. The “too sad to even debunk”: “Your left hand is the one which forms an ‘L’ when outstretched like this!”.

(if anyone actually does NOT see the problem: "Describe the shape of the letter “L”.)


If the planet has a magnetic field, it can be defined.

As for the OP’s scenario, its simply arbitrary. You explain that you can call one side right and the other left, and as long as everyone agrees wihch is which, there is useful handedness.

With no further qualifications/restrictions, I would just use clockface descriptions (9 o’clock and 3 o’clock) or compass points (if ahead is north, then left is west and right is east.) This sort of explanation, though, is disqualified by the full version of the question. Or the side where the low notes are on the piano if you’re facing the piano (OK, that only works with people who are familiar with the piano). Or, if speaking to IT guys, I assume they know math, so the side that is negative on the X-axis. Or, if English or European language speakers, the side we start writing from to the side we start writing to. Etc. Asked that simply, it seems there’s a bunch of answers unless you make some qualifications.