Q: If you are talking to a distant alien, how would you tell them which way is left and which way is right?

Physicist: Assuming this question isn’t about interstellar political leanings, the answer is: it’s tricky, but it can be done.

This is worth trying out just once in your life.  Try to define left and right for someone using only physical principles, and without making reference to anything else.  So you can define “down” by asking the other person to hold something out and then drop it, but you can’t define down as “the direction the Eiffel tower isn’t pointing” (for our Parisian readers).  You’ll find that any (successful) attempt you make to distinguish left from right involves citing example.

For example, "Left" can be defined as the relative position of Washington as compared to Lincoln when viewing Mt. Rushmore from the front.

If an alien is so distant that there are no common landmarks (spacemarks?) that you can both see, then you’re restricted to just using experiments.  You can cheat by sending your signal using circularly polarized light, but let’s not cheat.

For a long time it was assumed that the universe can’t tell the difference between right and left, since every physical law seemed to work exactly the same right-ways as left-ways (Also, since the universe has no thumbs it can’t make an “L” with its left hand).

However, the dawning of the nuclear age heralded a new age of handedness.  It turns out that if both you and your alien friend have access to decent laboratories, then you can describe left and right using only physical principles.

Some nuclear processes (specifically “β decay” in what follows) can tell between left and right.  Or more accurately, they can tell the difference between right and left-handed “chirality” (this post talks about chirality in passing).  So if you get something like radioactive cobalt and stick it into an extremely strong magnetic field you’ll find that the radiation it produces tends to stream out in one direction, which allows you to tell the difference between right-handedness and left-handedness.

One way is to notice that to produce the magnetic field you need to run electrical current in a loop, and the direction of the radiation stream allows you to define whether the current is running clockwise or counter-clockwise (in one case the radiation is toward the ring and in the other it points away).  Once you’ve defined clockwise and counter-clockwise, left and right is easy.

There is a caveat.  If the same experiment is done on anti-matter, then the results are reversed.  So, say you both run the experiment and find that the radiation comes out of the top of both of your devices.  While you both think that you’ve managed to communicate left/rightness, you’ll find that you’re mistaken.

But it won’t be until you invite your alien friend to Left-hander’s day that you’ll discover the mistake.  Hopefully you’ll manage to catch it before they land their anti-matter spaceship, which would be a bad day all around.  Bringing anti-matter and matter together results in the complete annihilation of equal amounts of both, and the release of all of their energy (which is a lot).

If after doing the experiment to establish right and left the alien shows up looking like this, then don't shake their hand. That dude's made of anti-matter.

Luckily, there doesn’t seem to be any anti-matter floating around the universe.  At least, not much.  So, if you know that you and the alien are both made of matter (which is pretty certain to be the case), then you can tell the difference between left and right.

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22 Responses to Q: If you are talking to a distant alien, how would you tell them which way is left and which way is right?

  1. Farhan says:

    I asked this question!

    Thanks for the reply sir 🙂

  2. Anthony Rose says:

    I’m not so sure, humbly. Our up/down, forward/back and left/right are all natural constructs of ours used to simplify our expression of the 3 dimensions, right? We could name those dimensions anything. Apart form our names for them they are in fact simply interdependent relationships, and in the absence of gravity for example, Up/Down is as difficult explain as Left/Right and as dependent on Left/Right and Fwd/Back as vice versa. So, I could call Up/Down A and B, Fwd/Back C and D, and Left/Right E and F. Now Left is E, not F. The question becomes, given the 3 axes, A/B, C/D, and E/F, how do I communicate which one is E relative to A/B and C/D. But if I can send any communication at all, surely I can send a description of the 3 dimensions, label each one, and use the labels for reference?

  3. Anthony Rose says:

    As in,
    A C
    | /
    | /
    / |
    / |
    D B

  4. The Physicist The Physicist says:

    You could certainly do all of that!
    However, without referencing anything, there’s no way to specify any direction in space, or even to uniquely specify a relationship between directions.
    You could describe to some alien, in detail, what a human hand looks like, but without references you wouldn’t be able to communicate what direction the hand is pointing, or even if it’s a left or right hand.
    There aren’t any clean proofs that I know of. So, try it yourself!

  5. Anthony Rose says:

    Thanks for the prompt reply!
    I do appreciate that there is very little physical difference between Left/Right, which makes it hard to communicate in words, but I think you may have missed my point. Our ‘Left’ is just one unique direction in the 3 dimensions, and as such does not depend upon any physical difference in the universe in order to be referenced, as long as you can communicate the 3 dimensions with labels at all.
    A spherically uniform alien in an essentially gravity-less environment would have as much difficulty comprehending Up/Down, and possibly even Front/Back, as our previous alien would have had with Left/Right. Yet if he understood the 3 dimensions at all, he would almost certainly have worked out a spatial reference system for himself which involved naming the 6 directions in relation to one another and a central point. If THAT one is (his) ‘Up’, and THAT one is (his) ‘Left’, then THAT one is (his) ‘Front’ and THAT one (his) ‘Back’. Whatever those mean to him, however he chooses to label them, the directions exist intrinsically.
    To put it another way, it is only once you pin Up and Front that you can pin Left, and why? Because Left depends not on physics but is simply and purely the label of a direction on one of 3 axes which depends upon the labelling of the other 2 axes and upon nothing else, however difficult it may be to communicate.
    We cannot DEFINE Left and Right because they are axiomatic. The difficulty is only in DESCRIBING them in relation to Up and Front.
    Now it is easy to describe to the alien what Up/Down are (gravitational direction), and Front/Back, (direction of motion), but impossible to distinguish between Left/Right, USING WORDS ALONE without some physical chirality to describe it, I agree.
    But if you can do that much communicating with him, drawing a picture of the 3 axes and labelling the Left one is just as good.

  6. Anthony Rose says:

    Sorry, I meant, we cannot define Left and Right *using other concepts* because they are axiomatic. Even if there were no nuclear differences we could use to describe Left and Right, they would still exist and could still be referenced.
    Secondly, I am aware that I come across as dogmatic. I apologise! I am just very enthusiastic – and very willing to be corrected.

  7. The Physicist The Physicist says:

    Fair enough!
    I suppose that the idea of the post is more along the lines of: it you, for example, send some one instructions detailed instructions about how to build something, without referencing anything external or using “left” or “right”, what they send back to you is equally likely to be what you were describing, or its mirror image. Is that in line with what you’re saying?

  8. Anthony Rose says:

    I guess what I am saying is, it is great that the universe has some chirality, as this enables us to communicate left/right in words. But even if it did not and we could not describe left/right in words at all, if we can reference Up/Down and Front/Back then we can still communicate left/right diagrammatically with labels – which in fact may be the better method, anyway.
    In the case of the architectural blueprint, once we have indicated to the alien (whether by words referencing gravity or by labels on our diagram) what our Up/Down are, and similarly our Front/Back, then Left/Right, even having no characteristic in the universe allowing us to communicate the distinction between them other than their intrinsic axiomatic directions, can still be indicated easily on the blueprint in relation to the other two dimensions.
    This is necessarily so because the essential characteristic of left/right is a dimensional relation to the other two dimensions, and diagrams allow us to communicate direction.
    It is true that Up/Down is defined by gravity and Front/Back by direction of motion (or vision) but Left/ Right is not defined by anything in the universe, whether matter or energy, because it is an axiomatic labelling of the 3rd dimension in relation to the first two.
    As such it seems easier to draw its relation to the other two dimensions than find some chiral aspect of the universe to describe it in words.
    Not to take anything away from the fascinating information about the chirality of the atom described above and its usefulness. Just trying to say that useful as it is, it is secondary to what left/right is, and not necessary to define left/right.
    Apologies once again if I hhave sidetracked the topic. I guess I was just fascinated both by the atom’s chirality and by the concepts involved in the definition of left/right and wanted to dig in to see more as I tried to understand it and verify my understanding of it.
    Thank you very much for your polite and patient replies. I appreciate it very much.

  9. The Physicist The Physicist says:

    Left/right-ness is certainly definable, but it’s not communicable without an example. If you draw a diagram like you suggest and physically hand it to someone, they’ll instantly understand which side is left and which side is right. But, if you send a description, even a detail description of the same diagram, to someone, then that description will apply equally well to either the actual diagram or its mirror image (which flips left and right, but keeps up-ness and forward-ness).
    You’re certainly right about each direction existing in an existential sense, however every useful definition of left/right always contains an example (in your case, a physical diagram). Without an example, there’s no way communicate left vs. right.

  10. Anthony Rose says:

    I see what you are saying. Still, I would hesitate to call a diagram an example of left/right-handedness. It is more precisely a communication via depiction of the actual dimension itself. I agree however that you have to reference something to resolve which side of the mirror is which! If that makes the diagram an example for reference, then so be it.

  11. Robert James Newton says:

    I haven’t read all this stuff, but here’s one little point. I’ve seen it many times.
    This has annoyed me for years. (I am easily annoyed.)
    Don’t shake hands with an alien (of course he has hands) made of antimatter!
    So, matter and antimatter only react when you shake hands?
    Come on guys. You can do better than that.
    Best wishes,

  12. Robert James Newton says:

    I’ve read the question properly now (always a good idea).
    You do say that it would be best to stop the alien before the spaceship landed.
    So, that deals with the point that I made yesterday.
    All you need to say is that the cartoon shouldn’t be taken too literally, because the alien would not get as far as the handshake before the spaceship was annihilated.

  13. instant grat says:

    While I can grasp some of the reasoning behind using examples to identify left and right to a distant alien, it seems to me to have jumped past at least one important step that must be taken beforehand.

    You – and the distant alien – must first know something about each other.

    Otherwise, the mere concept of left and right hand might be entirely mysterious in itself.

    We could make a start by solving another problem. What that might be is up for debate, but it might be something as “How do you describe a cloud to a fish?”

  14. Daniel Fritz says:

    To describe right and left we can use a sundial, celestial mechanics, and extent of visible stars for determining the hemisphere. This will require sometime to determine on both parties parts. We will require the knowledge of the rotation of the planet relative to its orbit i.e. is it like Earth to the Sun or Venus (backwards to orbit), and the extent of visible stars from our “friends” vantage point so as to determine the hemisphere, as the hemisphere is how we can explain left from right. A sundial’s shadow when it is pointing away from you in the “Northern” hemisphere will always start from left to right and vice versa for the southern hemisphere if the planet is rotating in the same direction (dark side center point towards orbit path). From this we can establish the arbitrary notion of left and right.

  15. Robert James Newton says:

    Daniel, your answer uses the stars as common reference points. So that can’t be a correct answer. Sorry.

  16. Carl Corey says:

    let them make a railgun and explain which way to pump electrons through that. Projectile will only move one direction. I think that is easier then radiation stream.

  17. Charles Rainville says:

    Hi Physicist,

    Your answer makes out that there is no way of communicating left/right to an anti-matter alien. Have you thought about the consequences of CP violation in neutral kaon decays, which allows matter to be distinguished from anti-matter?

  18. Abdullah says:

    Couldn’t you just figure out the anatomy of your alien and tell them that certain organs are on your right side, and certain organs are on your left side, therefore telling them what is left and right.

  19. Abdullah says:

    or you could tell him to send you a picture of his feet with a circle drawn on one foot and tell him if it is right or left

  20. Evgeni Kolarov, Bulgaria says:

    We can use left hand rule for magnetic field. So if the current is toward the alien we can define up and down with force direction, and left and right with magnetic field.
    Sorry for my English

  21. Robert James Newton says:

    Your English is good enough for me to understand your meaning, Evgeni.
    However, I don’t think that the left -hand rule could be used to communicate the meaning of left and right to the alien.

    That’s because the magnetic field direction we use is just a convention, based on our definitions of North and South poles. The magnetic field direction could have been be defined as the opposite direction. It could be defined either way by aliens, of course.

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