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A Galactic Challenge: How Would You Teach Left from Right to an Alien Civilization?

The views expressed are those of the author and are not necessarily those of Scientific American.

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spiral clockThe concept of handedness—of left and right, say, or of clockwise and anti-clockwise—is deceptively simple. In fact, I think it is among the most subtle in all of science and mathematics. In this post I will pose a challenge that I hope intrigue my readers.

This will not be a test or a quiz: I have not yet completely worked out a solution myself. Rather, it is the opening salvo for what I hope will be an interesting discussion. For a follow-up post, I plan to interview some physicists to see what their take is. But first, some preliminaries.

Imagine that you had to explain the meaning of the words “left” and “right” to someone who had never heard those words before. 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.

After giving it some thought, you would perhaps start by appealing to some shared experience. For example, to an American you could say that if they were to drive cross country from New York City to San Francisco, their right side would be where Canada is, while their left side would be the side of Mexico.

zodiacBut what if you were told to make no assumptions about where the person is coming from or how much geography he or she knows? You could hope that they knew something about the stars. You could explain, for example, that if one looks at the constellations of the Zodiac with their “up” side pointing north, then they would see the constellation of Gemini, say, to the left of Taurus and to the right of Cancer. (In the celestial sphere, the constellations of the Zodiac are those traversed by the sun in its apparent path in the sky during Earth’s yearly revolution.)

But constellations are purely conventional groupings of stars, and in fact the way they’re assembled is culture-dependent. Besides, how many of us can recognize them? In a dark, moonless sky I can perhaps spot the Big Dipper, Orion, and a couple more if I’m lucky.

Even if your friend had never learned about any constellations, though–or if they had never seen any because they spent their entire lives in L.A. or New York–you could still use astronomical references to explain left and right. Perhaps the simplest way to do so would be to tell them that if they’re facing north, “right” is where the sun will rise and “left” where it will set.

Of course, you’d first have to explain the word “north.” But that is easy to tell—at least if you live in the Northern Hemisphere—because the North Star is the only star that virtually doesn’t move during the Earth’s rotation. (There is no South Star.) In the long exposure below, where the stars form streaks in the sky over the Annapurna Range (Himalayas) as the Earth rotates, the North Star is the little dot in the middle.

North Star

Another way that you could introduce the concept of “left” and “right” could be by resorting to human anatomy. You could point out that that “left” is the side where the heart is, while the liver is on the right—and then you’d have to hope that your interlocutor is not among those rare people who have their entire bodies inverted, with their hearts on the right, their livers on the left and so on.

Z-DNA and B-DNAOr, you could take a biochemistry  approach. Have your friend learn how to purify and crystallize DNA and analyze its structure. Ordinarily, the DNA’s double helix is twisted in such a way that it looks like a spiral staircase that goes up as it goes from left to right: think of a forward slash, “/”. It is what people call a right-handed screw. A left-handed screw, on the other hand, is like a spiral staircase that goes down, like a backward slash, “\”. (You would also have to make sure that your friend does the experiment properly, so that their DNA doesn’t curl up the wrong way, in the so-called Z-DNA conformation, instead of in the usual conformation, called B-DNA. The two conformations however look very different to a structural biologist.)

Perhaps you are beginning to see my point. To communicate or even establish a notion of left and right, you have to appeal to some common experience, some tangible object or phenomenon. Or, as a physicist might say, you have to define that notion operationally. Absent that, there is no a priori way to orient yourself. This, by the way, is reflected in the way mathematicians describe handedness, also known as chirality or orientation, in an abstract space: In geometry there is no a priori notion of left and right the way that there is an a priori notion of positive and negative numbers. It is purely a convention.

So here is my challenge to you, dear reader. How would you communicate the concept of left and right via radio signals to an alien civilization?

First, of course, the two civilizations would have to detect each other and decrypt each other’s languages, which could take some back and forth; and messages would take tens or hundreds of years to get from here to their star system and back, depending how far their star system is. But let’s say you’ve had a few centuries to practice interstellar communication.

To spice things up a bit, let’s revert the situation–so that it is the aliens who have to teach left and right to us–and let’s say that the future of humanity is at stake. We have discovered that in a few decades the sun will explode, and that the only way our species can survive is to abandon the solar system. Fortunately, the aliens possess the technology to build a sort of cosmic gate to cross wormholes. A specially-built spacecraft could take four men and four women on board, cross the cosmic gate machine and almost instantaneously travel across the galaxy. There, these pioneers could find a hospitable planet to settle. They could then go forth and multiply, ensuring the survival of the species.

There isn’t enough time for the aliens to send a cosmic gate and a wormhole spacecraft our way, so we need them to teach us how to build one. We need them to send us the blueprint. So they begin to beam information to us in the form of a 3-D technical drawing, encoded as a set of 3-D coordinates in space.

But there’s a snag. To work properly, wormhole traveling relies on a network of cosmic gate machines the aliens have placed around the galaxy. All of those machines are built in such a way that a ship must cross them in a corkscrew motion. That motion must be like what we on Earth would call a right-handed screw. Any attempt to cross the gates in a left-handed motion will result in disintegration of the gate machine and of the ship and everything inside it.

You see, we can reconstruct a detailed 3-D drawing from the coordinates–all those x’s, y‘s, and z‘s they sent us. We earthlings have a convention to set up coordinates in such a way that if you stand up in the positive z direction and look down to the xy plane, the positive-x axis will be to the right of the positive-y axis. But we have no way to tell if that is the same convention that the aliens use. Thus, we have no way to tell if model we build, and its corscrew motion, are the correct ones or their mirror images.

The aliens’ language does include words for “left” and “right” and for “left-handed screw” and “right-handed screw”. But in all our communications with them, we were never able to figure out which was which. We need them to explain to us which of their words corresponds to our word “left,” and which to our word “right.” To do so, they will need to appeal to some common point of reference or phenomenon. I shall restate the challenge as follows:

How can an alien civilization instruct us via radio signals on how to build a spaceship that flies with a right-handed twist?

I can think of at least a couple of “easy” solutions to this problem, one of which was suggested to me by my friend David Harris. I will describe those solutions in a subsequent post, but in the meantime I am curious however to see what readers—who are a smart bunch—will come up with. Remember, you have to save the human species!

Later, I will “cheat” and I will modify the problem a bit to make it harder, and also more interesting. At that point, some rather deep physics concepts should come into play.

Read the follow up to this post, Galactic Challenge, Part II: The Richard Feynman Files

This post is part of a series on handedness. Here are the all the posts in the series:  

A Galactic Challenge: How Would You Teach Left from Right to an Alien Civilization?

Galactic Challenge, Part II: The Richard Feynman Files

Galactic Challenge Part III: The “Easy” Solutions

Why There’s No Such Thing as North and South

Suggested reading:
The New Ambidextrous Universe: Symmetry and Asymmetry from Mirror Reflections to Superstrings: Third Revised Edition. By Martin Gardner​. 2005.
The Handedness of the Universe. Roger A. Hegstrom and Dilip K. Kondepudi in Scientific American, Vol. 262, pages 108-115; January 1990.
Alien Pizza, Anyone? On other worlds, biochemistry could have taken a different turn. Davide Castelvecchi in Science News, Vol. 172, No. 7, pages 107-; August 18, 2007

About the Author: Davide Castelvecchi is a freelance science writer based in Rome and a contributing editor for Scientific American magazine. Follow on Twitter @dcastelvecchi.

The views expressed are those of the author and are not necessarily those of Scientific American.

Comments 62 Comments

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  1. 1. Jacqueline Windh, PhD 11:12 am 08/28/2011

    I love this topic: the intersection of language, science, and world view – how having a word for something changes our perception of it, and how not having a word for something can keep us from even conceiving of it.
    I was thinking of this issue of conventions, left/right, up/down, clockwise/counterclockwise, just the other day for a blog post I was writing. I was trying to build a mental picture of our solar system, of the planets spinning around the Sun, and thinking how much our mental picture depends upon our convention of looking at it from “above” – which is the direction that we have defined on our planet as “North.” And there is some basis for that convention, since we do mainly look down on our world rather than up on it. So it makes sense to choose a point-of-view that also looks “down” to us.
    But when you think of maps… the fact that we put North at the top of maps is entirely arbitrary. Yet that arbitrary choice still comes across in our language as going “up” north or “down” south.
    So, as I was trying to mentally image our solar system, getting the planets’ rotation direction and direction of revolution right in my head, and thinking of that point-of-view, from what we call “above” – I tried to think of an analogy for that that is entirely here on Earth.
    And I thought: what about an airplane. You tell the pilot to fly in a clockwise circle; he obeys. But to anyone looking up at the plane from Earth, it will appear that the plane is flying in a counterclockwise circle.
    I look forward to following this discussion!

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  2. 2. kimballk 11:46 am 08/28/2011

    If the alien civilization is as advanced as you describe, they are aware of the distribution of galaxies around us. Then is is easy to define left and right. Pick 7 galaxies and identify them by their shape and spectra. Choose them so that one is in what will be defined as “center”, or “base”, and the other 6 are roughly in 6 directions from the center: plus & minus x, y, z. Then define the first galaxy as “center”, a second arbitrarily as “up” or “zenith”, the one in the opposite direction as “down” or “nadir”. The again arbitrarily define one of the remaining galaxies as “north”, then, appropriately, “east”, “south”, and “west”. Imagine slight adjustment to the lines connecting these 7 galaxies so that they become 3 perpendicular lines. Now imagine a rectangular parallelepiped with its long axis oriented “up and down”, and one face facing “north”. “East” is to the right, and “west” to the left. Simple. This took one minute of thinking. Spectra of prominent stars in a galaxy make the initial identification possible, if not trivial. And surely among the myriad of galaxies one can find 7 in the needed configuration even if they are many lightyears apart and at different distances. Once you have 3 perpendicular lines identified, defining “left” and “right” are again, trivial.

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  3. 3. KWillets 2:08 pm 08/28/2011

    The aliens could impose a circular polarization on the radio signal to indicate the correct chirality.

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  4. 4. scottyo 3:44 pm 08/28/2011

    Read this web page. As you read the words in succession on a single line, your eyes are pointing more and more rightwards. When you reach the end of the line, your eyes will speed leftwards to the next word.

    Get a standard analog watch. Wait till the second hand points at 12. As it continues moving, for the next 15 seconds or so, till it points at 3, its tip will be moving rightwards (and increasingly downwards), After this the tip will start moving leftwards and downwards.

    Get a standard compass. Read the instructions. Point it so the N and the north end of the needle (roughly) coincide (so it’s pointing north). The E is to the right, the W is to the left.

    If you allow biochem equipment, surely you will allow a printed page, an analog watch, a standard compass and reasonable shared understanding.

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  5. 5. Kapitano 4:45 pm 08/28/2011

    Our notions of left and right come from having a bilaterally symmetrical body, with a distinct ‘forward’ and ‘backward’, ‘up’ and ‘down’. Forward is the direction we can see in, backward the one we can’t. Feet are down and heads are up (at least usually).

    Do these aliens have a sense of vision? Is it binocular and facing one way? If they have three eyes spaced equally around their abdomen, they won’t have notions of forward and backward, but the notion of spinning should be easy for them.

    Do they live on a planet, or in space without gravity? If the latter, up and down won’t mean much to them – except ‘down’ as ‘whichever direction the gravity of something is pulling at the moment’.

    Without knowing what kind of bodies and senses they have, I don’t think we can know what kind of mathematics and geometry they’ve developed. Of course, mathematics is a giant tautology and presumably true everywhere, but your starting point (your grounding notions) for exploring the tautology surely depends on your practical experience – your body, your world and your problems to be solved.

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  6. 6. potomac79 5:15 pm 08/28/2011

    The common referent is a gravity well. From there you can define “up” and “down”. With that, since they understand 3-d space, you can use geometry to describe right, left, clockwise, etc.

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  7. 7. @dcastelvecchi 7:23 pm 08/28/2011

    Thanks for all the great comments. I will respond one by one.
    Jaqueline: Martin Garner wrote a whole book on the subject that I think you would enjoy. I just added it together with a couple of other suggested readings at the bottom of the story.
    In his book, Gardner devotes a whole chapter to the question of why mirrors appear to reverse your left and right sides but not your top and bottom. The explanation is actually rather subtle and involves psychology as much as physics or math.
    What you are saying about clockwise v. counter-clockwise is true. The issue I think is that there is no well-defined notion of clockwise in 3-D. Giving an orientation, or a handedness, to 3-D space requires giving an orientation of a plane plus specifying which way is up or down with respect to the plane itself. Once you do that, you don’t risk getting confused because you always know from which side you should look at your plane.
    The spiral motion of the spaceship is a 3-D orientation because it combines a rotation (which you could think of as the clock convention on the plane) plus a direction in which the spaceship is flying.

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  8. 8. @dcastelvecchi 7:31 pm 08/28/2011

    Kimballk: the galaxies sound like a very good idea. One of my ideas for an “easy” solution was similar: it was to figure out the positions of some stars that are close to the star system where our aliens live and tell them to use those. One could identify the stars by their spectra or just by ranking them by intrinsic luminosity. Your solution however seems much better because 1) it could be really difficult to estimate the relative positions of stars accurately enough, and 2) the galaxies could presumably be visible from most of the Milky Way, so you wouldn’t even need to know exactly where your aliens are located.

    However I don’t see why you would need 7. It seems to me that 3 should suffice, no?

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  9. 9. @dcastelvecchi 7:35 pm 08/28/2011

    KWillets: that sounds like a good idea, too — it was the solution David Harris had proposed. There could still be some subtleties though…

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  10. 10. @dcastelvecchi 7:37 pm 08/28/2011

    KWillets: that sounds like a good idea, too — it was the solution David Harris had proposed. There could still be some subtleties though.

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  11. 11. @dcastelvecchi 7:49 pm 08/28/2011

    scottyo: Sure, but that’s my point: you need to have a shared understanding, or some objects to refer to, or some phenomenon to appeal to.

    Kapitano: Those are great points. Aliens that had spherically symmetric bodies might have never had a need for a notion of orientation. But it also seems unlikely that they would have developed advanced technology! Animals that are rotationally symmetric, like jellyfish–which do have eyes all around their bellies, so to speak–still might want to know left and right because they may want to be able to know whether to spin clockwise or counter-clockwise.

    potomac79: you can surely define up and down if you live on a planet. But going from there to distinguishing left from right is not possible just by using geometry.

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  12. 12. kimballk 10:11 pm 08/28/2011

    @dcastelvecchi: You are correct; seven is overkill, but I don’t think three galaxies (points in space) are enough. Being inside the planar triangle they form and facing one does not tell you which one to turn to in order to make a right or left turn because you haven’t defined “up”. That implies 4 galaxies would do the job, with the 4th one out of the plane and defined as “above” it. Then facing one of the other three and turning towards it could define a “right” turn (or “left” of course). More clearly, pick 4 galaxies as close to a regular quadrahedron as possible. Label them A,B,C,D. “Stand” in the plane of A,B,C; define D as up; face A. B, or C, is to the “right” and C, or B, is to the “left.

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  13. 13. KWillets 10:54 pm 08/28/2011

    Glad I got it, but I confess Harris gave himself away with a domain name like From my researches I understand there are some possible problems with loss of polarization in the interstellar medium, but using multiple frequencies might help.

    Another thought if photons don’t work is that it’s easy to accelerate, eg electrons to .99c and shoot those over in only slightly more time than the photons. One could either get their spin axes aligned with the direction of travel, or come up with some entangly way to ensure that the axis we use to measure spin is aligned with the aliens’.

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  14. 14. werty22 11:04 pm 08/28/2011

    P-simmetry violation?

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  15. 15. Kapitano 2:42 am 08/29/2011

    Having thought some more about it, I suspect there are six ‘forms of motion’ in 3D space:

    Change of position:
    * Shift: Up & down
    * Track: Left & right
    * Zoom: Forward-backward

    Change of orientation:
    * Tilt: Up & down
    * Swivel: Left-right
    * Spin: Clockwise-counterclockwise

    Remember how with Speed, Time and Distance, you can define (and discover) any one of these if you already know the other two. I’m thinking that if our aliens know Spin and Track, it should be possible to explain Swivel to them.

    If they know Shift and Swivel, then Track can be derived – together with the parabolic motion of a pendulum, which relates to Spin.

    I may be completely wrong here (being a liberal arts graduate :-) ) but if they have notions of any two changes of position and one change in orientation – or vice versa – then they can derive the full set. So we should send them a table of derivations, letting them choose the terms they already understand.

    There is also the question of what happens if they’re a plant species, unable to move about but able to change orientation. For them, a planet circling a star would be like the direction of the star’s ‘gaze’ ‘spinning’, with the planet on the end of it – much as we can think of the planet on the end of a string held by the star.

    They’ll have to conceive linear motion as being like that of a planet around a star, but the planet being an infinitely long distance away, so the curve is flattened to zero.

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  16. 16. jtdwyer 3:35 am 08/29/2011

    The galaxy orientation might help, depending on where your new friends came from and perhaps their visual spectral range. However, as I understand (not being an astronomer), disregarding all of the visual associations imbedded in our identification of the constellations (including human and animal body configurations), our view of them is particular to at least our current location within the Milky Way. Since their appearance is dependent on our perspective of them, an observer on the other side of the galaxy, for example, would not be able to find them. Even specific perspectives of the galaxies in the local group might be problematic…

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  17. 17. @dcastelvecchi 7:16 am 08/29/2011

    jtdwyer: That’s very cool, why didn’t I think of that? The Milky Way rotates. That by itself should not be enough to define a handedness of 3-D space. However if you know where the aliens’ star is located relative to the galactic plane, you can tell them to define that as “up”. Or, you could just decide that Andromeda (the nearest spiral galaxy) is “down”. Alternatively you could take the galaxy’s rotation together with the galaxy’s peculiar velocity (its motion with respect to the cosmic microwave background).
    The constellations are defnitely observer-dependent, but individual stars, their luminosities, and spectra are not.

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  18. 18. @dcastelvecchi 7:18 am 08/29/2011

    kimballk: I meant to say three galaxies plus the Milky Way. As long as the MW does not lie on the plane of the three galaxies, you could take it to be the origin and the other three to be the positive axes. (And yes, they’d have to be non-collinear.)

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  19. 19. @dcastelvecchi 7:19 am 08/29/2011

    werty22: You must be a physicist! We’ll dig more into that idea in the follow-up post.

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  20. 20. @dcastelvecchi 7:50 am 08/29/2011


    You may be a liberal arts major but you have good geometric intuition! I think that what you are referring to is what mathematicians would describe as the group of rigid transformations of 3-D space.
    There are essentially two types of rigid transformations: translations and (orientation-preserving) rotations. Each of those types has three degrees of freedom (yay!), so the total number of degrees of freedom is six. Translations have three because you have three directions in space, and rotations because any of them can be described in terms of three Euler angles, which Leonhard Euler described in the 18th century. (Modern geometry texts tend not include Euler angles but they do mention that the group of rotations of 3-D space is itself three-dimensional.)

    It is true that you can describe an orientation based on translations and deduce one for the rotations, and vice versa. Trouble is, this is just a reformulation of the problem, and it doesn’t really help to communicate your convention for the orientation of space to the aliens.

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  21. 21. jtdwyer 8:11 am 08/29/2011

    @dcastelvecchi – Quite right, except that the perceived luminosity of specific stars might depend somewhat on the observer’s location (the brightest stars in the sky are, to some extent, nearer ones), and their apparent luminosity and spectra also greatly depends on the spectral range being observed. An observer on Earth who could only perceive infrared spectra, for example, would see the sky on Earth much differently than we do.

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  22. 22. jtdwyer 8:27 am 08/29/2011

    …not even to mention visiting observers with compound eyes!

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  23. 23. GeekStatus 9:00 am 08/29/2011

    I dont see this being a huge problem for two groups that can communicate blue prints through radio waves. Cant you simply give a series of x,y,z coordinates and say if you follow this direction then it is this handedness….?

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  24. 24. GeekStatus 9:06 am 08/29/2011

    ^^ what im saying is that with all that blue print data to build this machine. Shouldn’t it become exceedingly obvious which system they are using?

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  25. 25. astro.pranu 9:33 am 08/29/2011

    Great honor to the article and author,but I don’t find any point in worrying bout this direction concept as long as we are going to know the direction of the civilization sending us that message !! Anyways, I would be very glad to know if I had missed any point in my Imagination :)

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  26. 26. @dcastelvecchi 10:15 am 08/29/2011

    jtdwyer: by luminosity I meant intrinsic luminosity over the entire EM spectrum. Advanced aliens should be able to estimate that accurately no matter what wavelengths their eyes can see, no?

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  27. 27. _jeremy 10:17 am 08/29/2011

    the wormhole builders should be able to set up broadcasting posts at 3 ends of the galaxy and communicate chirality with references to distinct signals from those points?

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  28. 28. johnverdon 11:14 am 08/29/2011

    I must assum that the aliens ‘live’ in a solar system. So take a finite plane (e.g. sheet of paper) fold it symmetrically in half, twice. There are now three folds, with one in the middle. Unfold the plane to observe the inside of the middle fold. On one side should be outside edge of one fold on the other side should be another inside fold (like the middle fold).

    Align the middle fold with one end toward your Sun(s) and the other end away from your Sun(s). The plane should parallel the plane of your planets orbit. The direction of your planet’s orbit as it moves perpendicularly across the folds will define left and right. If your planet crosses the two inner folds before it crosses the outside edge of the third fold it is moving from right to left. If it crosses the outside edge of the one fold before it crosses the other two inside folds your planet is moving from left to right.

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  29. 29. @therealtinasky 11:25 am 08/29/2011

    Are we actually trying to explain “left” and “right” to them (or they to us) or are we just trying to get through the gate? If the former, it’s Quine’s problem of translation. Even if we think we know that they are saying “left” or “right,” we can’t be sure because we don’t know their orientation. And why bother verifying that unless we are interested in the concept, which means a great deal more interpretation of more complicated concepts than direction.

    But if we just want to get through the gate, provide two reference points outside the gate as observed from the direction of travel (pulsars would do). Orient the spin of the vessel toward the correct object. Don’t bother labeling it.

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  30. 30. jtdwyer 1:12 pm 08/29/2011

    @dcastelvecchi – Again, I’m no astronomer, but as I understand, the ‘intrinsic’ emission, or source, luminosity of most objects varies and cannot be definitively determined. The exceptions are those that are thought to be consistent and can qualify for use as standard candles, such as the peak period emission luminosities of type Ia supernovae (there is still some concern about them – see

    Other objects’ apparent, or observed, luminosities vary based on distance (the source luminosity is not precisely determinable), which can only be estimated based on apparent spectral redshift, using ‘standard’ cosmological models. Their cosmological models could be slightly different from our own, which vary…

    If astronomers could precisely determine distance from observed luminosity and/or redshift there’d be no need for standard candles to be used for calibration (error adjustment).

    As I understand, spectral luminosity also can vary depending on obscuring gas clouds of possibly varying composition.

    While the apparent luminosity and spectral redshift observed for a specific at a remote location could be used to estimate distance and emission luminosity in order to estimate its observable or apparent luminosity on Earth, I think it cannot be definitively determined.

    I doubt that our own Sun’s apparent luminosity can be definitive determined for some observer on the other side of the Milky Way – but then it would most likely be obscured from observation anyway.

    I think some positional information could probably be negotiated with a remote party, but I think that it would be much more difficult to achieve precision than it might seem.

    All this would be made so much simpler if we were attempting to communicate with someone in our own neighborhood of the Milky Way…

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  31. 31. GeekSMN 1:44 pm 08/29/2011

    I would presume that it would not be very difficult to convey the notion of magnetic north to such a species. Once that is established the next question would be should the rotation be the same as if our planet were entering the gateway from its magnetic North or South Pole.

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  32. 32. @dcastelvecchi 3:21 pm 08/29/2011

    GeekSMN: That is a very interesting idea. I’ll discuss it in the follow-up post.

    _jeremy: I like your idea a lot too. I think they wouldn’t even need to position their outposts too far away from them, just far enough that we can resolve them spatially. For example, an advanced civilization at a star in our galactic neighborhood should already be able to resolve radio sources that are spread around our solar system.

    jtdwyer: It’s true that in general it’s difficult to estimate the intrinsic luminosity of an object with great accuracy. That’s because you need a way to estimate distance and apparent luminosity independently. For relatively close-by stars, though, that can be done — for example by the parallax method. Thus, we have pretty good estimates of their intrinsic luminosity. And as you said, for standard candles — special objects such as cepheids or type 1a supernovae — it can be done regardless of how far they are.

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  33. 33. DarianJ 4:12 pm 08/29/2011

    I would think that the answer to this is should be done at a more basic level.

    If I remember my first year physics correctly (anyone feel fee to correct me)…

    There was a rule about electron flow in a magnetic field. Ironically (or conveniently) it was called the right hand thumb rule. Essentially a magnetic field, which can be set up by electrons flowing in a conductor wire in a circle, will then induce electrons to spiral in a given direction.

    Communicating that the electrons flow in a circle, producing this field, the perspective of the electrons flowing from the observer through the loop (away from the observer) will spiral in a specific direction. This could be used as the common frame of reference.

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  34. 34. jtdwyer 5:23 pm 08/29/2011

    @dcastelvecchi – Right – which is why I closed my previous comment with the statement:
    “All this would be made so much simpler if we were attempting to communicate with someone in our own neighborhood of the Milky Way…”

    However, that’d be changing the rules in the middle of the game, wouldn’t it?

    Your problem statement included:
    “But what if you were told to make no assumptions about where the person is coming from…”
    “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.”

    Those instructions seem to imply that the student could be remotely located, receiving an ancient message – like the Voyager ‘legend’, effectively precluding an instructional dialog…

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  35. 35. yahilli 5:25 pm 08/29/2011

    What about optical rotation of linearly polarized light sent through solutions of chiral molecules? That should be a clear method to define left and right.

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  36. 36. yahilli 6:47 pm 08/29/2011

    No sorry, that doesn’t work! In order to distinguish and define the two enantiomers for the solutions you already need left and right…

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  37. 37. infomebaby 9:43 pm 08/29/2011

    Kapitano wrote: 4:45pm 08/28/2011: “If they have three eyes spaced equally around their abdomen, they won’t have notions of forward and backward, but the notion of spinning should be easy for them.”

    Evolution wouldn’t mess up crumbs getting into your eyes. Eyes are always on top mouth on the bottom. That just sounds like a pattern throughout our spieces…? That solves a visual signal of “up” and “down” easily with a simple “happy face”. Owls can spin their heads to ‘see from the back’ so ‘forward’ and ‘backward’ are problematic and not fool proof…gravity has to affect in some form the evolution of an organism (I suppose) so down is the direction where gravity pulls you. A happy face moving around a block will easily explain ‘clockwise’ and ‘counterclockwise’ but you are saying only explain via radio waves.

    How about black and violet to show down and white to pink to show up? I mean don’t we know how far stars are based on this gathering of wave lights?

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  38. 38. infomebaby 10:02 pm 08/29/2011

    Footnote on the above: meant to use the electromagnetic spectrum which should be the same anywhere and a reference point

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  39. 39. jeffj900 12:32 pm 08/30/2011

    This is copied from the Wikipedia page on “Relative direction”. It’s an interesting twist on this discussion that many human cultures do not include the notion of left and right in their language, but use a geographically fixed reference frame.

    Cultures not using relative directions

    Many human cultures use relative directions for reference. However, some do not. The Australian Aboriginal people the Guugu Yimithirr for instance, in their language have no words denoting the egocentric directions, but instead exclusively refer to cardinal directions, even when describing small-scale spaces. For instance, if they want someone to move over on the car seat to make room, they would say “move a bit to the east.” To tell someone where exactly they left something in their house, they would say, “I left it on the southern edge of the western table.” Or they would warn a person to “look out for that big ant just north of your foot.” Other peoples “from Polynesia to Mexico and from Namibia to Bali” similarly have predominantly “geographic languages”.[1] American Sign Language makes heavy use of geographical direction through absolute orientation.[clarification needed] When speaking Mongolian, speakers will typically use the words for “front”, “back”, “left”, and “right” to mean “south”, “north”, “east”, and “west”, respectively.

    It seems we cannot create a system of meaning without starting with some fixed and arbitrary definition as a point of reference.

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  40. 40. infomebaby 2:36 pm 08/30/2011

    @jeffj90 : so true but wouldn’t you always have 1)yourself as a reference point and 2)forward (the side of your head with the vision) 3)backward 4) therefore upward (towards the sky) so would you always have enough reference points for an accurate left to right coordinate plane? Don’t laugh at my homemade graphics, okay giggle if you will, but there is what I meant: There has to be a way to explain left and right…

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  41. 41. vindu 5:27 pm 08/30/2011

    @DarianJ: I like your idea of relating left/right to the way electrons move in a magnetic field. However, for interesting reasons, it doesn’t provide an answer to the problem! Let me try to explain.

    The gist of it is that the direction of the “arrows” of a magnetic field is purely a matter of convention. There are two physical effects to consider:

    (1) an electric current generates a magnetic field;
    (2) a magnetic field affects the motion of a charged particle.

    For (1), there is a “right-hand rule” which tells us the direction of the magnetic flow lines generated by a current moving in a wire (perhaps in a wire loop, as you suggest). For (2), there is a “left-hand rule” which tells us how the motion of a charged particle becomes curved in the presence of a magnetic field.

    It would be nice to think that one or other of these rules would give us a way of describing left/right to our alien friends. We imagine we could just say:

    (1) “Look at the way an electric current generates a magnetic field. The direction of the current and the magnetic field together will give you enough information to reconstruct our earth notion of left/right.”

    or (2) “Look at the way a magnetic field affects the movement of a proton. The direction of the magnetic field and of the curvature of the particle trajectory together will give you enough information to reconstruct our earth notion of left/right.”

    Unfortunately, this doesn’t work. This is because our representation of the magnetic field as a collection of flow lines with arrows (technically: as a vector field) depends on a convention: do we point our arrows north or south? Perhaps the aliens have the opposite convention, in which they point their magnetic flow lines the other way. What would that do their theory of physics? They would now need a “left-hand rule” to explain behaviour (1), and a “right-hand rule” for behaviour (2). Other than that, the theory is fully consistent: everything remains unchanged, and they would have the same theory of physics as we do except for these two small changes. Now when they try to decode our instructions, they will reach exactly the opposite interpretation of left/right as our interpretation.

    Technically speaking, a magnetic field should be (and is, in more theoretical settings) viewed as a “2-form”, not a “vector field”. When we view it as a vector field we have to introduce a sign convention to do so, and then as a consequence we have to introduce left- and right-hand rules to explain phenomena (1) and (2). This makes us think that left/right is encoded in the physics of charged particles and magnetic fields; which it is not. That arises purely because of our choice of sign.

    On the other hand, the 2-form interpretation doesn’t require any conventions; we can explain phenomena (1) and (2) without any reference to left or right, which is how it should be. Orientation is not encoded in the physics of electromagnetism.

    I hope that is helpful.


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  42. 42. zer0mh 9:30 pm 08/30/2011

    If you allow that we can define north by our hemisphere, then the aliens can relate clockwise, counterclockwise, left and right to us simply by describing the rotation of the earth. Or we can relate it to them the same way assuming they can ‘see’ us.

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  43. 43. zer0mh 9:36 pm 08/30/2011

    I mean I know you basically already said that in the article but if the earth were doomed that would be my go-to strategy. I don’t see why that wouldn’t work. If they can build us a wormhole they must know where our planet is, and if they have 3-d coordinates we can understand then we can tell them where polaris is. So that seem the easiest way to me. Actually I really want to know if and why that won’t work…now I can’t stop thinking about it.

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  44. 44. @dcastelvecchi 5:33 am 08/31/2011


    You are definitely right. If the aliens had visited or observed our planet we could tell them 1) the N pole is the one without a continent and 2) standing on the N pole, the Earth around you rotates to the left.

    But that game would not be any different than that of explaining left and right to another human culture. The point of my game was to assume that the aliens have no way to directly see our planet. In fact, in a follow-up post I will make the game harder and I will assume that the aliens don’t even know which planet we live on, which star we orbit, or in fact which galaxy we come from.

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  45. 45. dtaparan 9:43 am 09/1/2011

    There is an issue with the assumption of the problem.

    First, RF signals experience decreasing power density when broadcasted.
    At extremely large distances, the power density can not be distinguished separately from thermal noise.

    Transmission and reception of broadcast signals intended to span over several light years require either an impractically large amount of transmission power or impractically large receive antennas…bigger than the solar system.

    So alien civilizations receiving TV broadcasts from earth is a myth…unless they are watching from Mars.

    Second, signals that are intended to span light years require use of extremely narrow beam transmissions to mitigate loss of power density.

    High energy laser, high frequency photons, neutron beams, etc.

    But by sending such a signal, you make your vector of origin known to the receiver.
    In fact, the sender must first know the exact location of the receiver.

    With these two implications, the left handedness of the universe can be communicated in several ways -

    Use two beam signals pointing at different locations at the receiving planet.
    The first signal is modulated to indicate left, the other right.

    Or a single signal is modulated to contain the constellation of pulsars in the target galaxy and use their unique frequencies as reference to indicate orientation.

    Or, by using two beams again, send them within the planet’s orbit but one is sent at a clockwise manner, and the other counter-clockwise. The signal is modulated with the info of their direction.


    Or if one insists that we can only send broadcasted signals and we don’t know the exact location of the receiver, we can use all the wormhole stations as transmitters. That is, all stations become a transmitter sending their unique reference clock and their orientation with respect to the rest of the stations….just like GPS.


    Or send the diagram of fermions in weak interactions :-)

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  46. 46. jtdwyer 1:53 pm 09/1/2011

    dtaparan – Good thinking! However, any signal is only effective for the duration of the transmission. Reliably directing even a transmission beam over distances of tens of thousands of light years would be challenging to say the very least!

    If we know the location and distance of the receiver we must have first received at least one prior transmission from them. Even for directed beam transmissions, signal propagation delay for communication dialogs becomes unfeasible as a function of distance. By the time we could respond to a locational beam transmission (perhaps randomly directed like a searchlight) from a planetary system on the other side of the Milky Way, nearly two hundred thousand years might have elapsed. Our own ability to decode sophisticated EM transmission signals has existed for less than 100 years. I wonder if we can survive this century?

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  47. 47. @dcastelvecchi 8:35 am 09/2/2011

    dtaparan: About beaming signals to different locations, you can’t do it with just two: you need at least three, together with the receiver’s knowledge of the sender’s (rough) position. As the first commenter to this post pointed out, it is not enough to orient a plane or a line: we want to fix an orientation of 3-D space.

    I like the idea of using pulsars, though!

    And about picking up radio signals from aliens, I take it you don’t believe the whole SETI effort can ever succeed?

    And yes, jtdwyer: our ability to survive as a civilization was perhaps one of the most optimistic of my assumptions :-/

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  48. 48. GeekSMN 3:45 pm 09/2/2011

    zer0mh (42)
    “If you allow that we can define north by our hemisphere, then the aliens can relate clockwise ….”

    zer0mh I suspect you are replying to my comment #31. However, I believe that you may have missed the nuance that I was not talking about geographic north. I was suggesting that we would be able to explain the concept of Magnetic north which is a very specific point on the magnetic field generated by our planet. In fact chances are that any species that live on a planet with a stable atmosphere will likely have a similar magnetosphere to that of earth. If so they’re almost certainly familiar with the concept of the magnetic poles of a planet. Which pole is North, and which is south is simply a convention, but it is defined by which direction the field points.


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  49. 49. gwmckenzie 5:56 pm 09/2/2011

    I haven’t read all the discussion, so my apologies if someone has suggested this. Assuming that you can get a response to an either/or question, it seems to me that viewed from one orientation (“above” arbitrarily) our galaxy has arms bent one way, and rotates so that the arms “trail” the rotation (clockwise?). Seen from the other orientation, the rotation would appear to be in the opposite (counterclockwise?) direction, with the arms leading. If you could get a choice between those two options, would that solve the rotation problem?

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  50. 50. gwmckenzie 6:01 pm 09/2/2011

    @Geek SMN: My understanding is that Earth’s magnetic field has reversed periodically through history while the direction of rotation (ignoring precession) being always the same. In other words, the current orientation is arbitrary and uninformative as to the direction of rotation.

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  51. 51. gwmckenzie 6:04 pm 09/2/2011

    Sorry, I just realized that thought is completely stupid … the arms always trail.

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  52. 52. gwmckenzie 6:07 pm 09/2/2011

    None the less, the galactic shape is unique, and should be enough to determine rotational direction. So it may still be that all you need is for the aliens to choose picture (a) or picture (b) to know which way the rotation should be.

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  53. 53. jtdwyer 2:25 am 09/3/2011

    gwmckenzie – Interesting idea, but don’t discs appear to be rotating in one direction from above and from the other direction from below?

    Also, describing rotation as proceeding from right to left, for example, might be difficult without first establishing what right and left were. Like clockwise and counter-clockwise, unless you know that, by convention, the hands of clocks spin in only one direction the terms may not be helpful…

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  54. 54. gwmckenzie 10:58 am 09/3/2011

    I reread the “problem” and realized that it is may be two problems, not one. I focused on the “saving humanity” part which, I submitt, does not actually require the concept of “right” and “left” for a solution. In fact, the “right” and “left” concepts only muddy the water. To build your rescue ship you only need to know which of two possible rotations is correct. Your point that a disk appears to rotate in opposite directions from above or below is exactly the point. Unlike a simple disk, the “arms” of the galaxy are clear indicators of the direction of rotation (and therefore, arguably, also indicate the “point of view” of the observer). So, send two pictures, one indicating a “right hand” rotation, and one the “left hand” rotation. As long as it’s clear that the pictures are of the galaxy, and that your question amounts to: which rotation is the correct one? – the aliens only have to choose and return one picture (or include a picture of the galaxy oriented to show the correct rotation in their original transmission saving a whole lot of time); we build our ship to rotate in the direction indicated and we’re saved – eight of us anyway. If there is a further need to establish a clear “point of reference” that could be accomplished by placing a “unique” astronomical body (a pulsar might work) within the glactic disk – positioned correctly relative to our location (the aliens clearly know our location since they know our plight) along with a spectral chart for positive identification. This establishes with certainty that we are referring to the same galaxy. Then, to one of the pictures, add another unique object outside the galaxy (arbitarily “north”) and similarly add a unique object from outside the galaxy to the second picture (arbitrarily “south”). You would now have our known location, the known location of a unique object within our galaxy and (for the picture returned) the known location of an object from outside the galactic plane (either “north” or “south” depending on the picture returned), along with the known (from the direction of the spiral arms) rotation, all of which should fit together geometrically to provide an unambiguous point of reference and rotation – which you could call “right”. Although, I don’t think you need all that for your spaceship.

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  55. 55. jtdwyer 3:04 pm 09/3/2011

    I had to go back and reread all the game stipulations specified that I’d skipped over because of fundamental difficulties. Now, having done so, I find the following texts (ironically having to change all of the left and right handed quotation marks to ambidextrous ones – maybe we’re not smart enough to communicate with aliens):

    “But let’s say you’ve had a few centuries to practice interstellar communication.

    “…We have discovered that in a few decades the sun will explode, and that the only way our species can survive is to abandon the solar system. Fortunately, the aliens possess the technology to build a sort of cosmic gate to cross wormholes.”

    “The aliens’ language does include words for “left” and “right” and for “left-handed screw” and “right-handed screw”. But in all our communications with them, we were never able to figure out which was which.”

    Regardless of how many centuries we’ve had to practice interstellar communications encoding and decoding (presumedly with these aliens), even assuming that these friendly aliens are located within our galaxy, unless some imaginary wormhole is employed in our communications, even if we’re using directed gamma-ray beams as our communications medium, the response to a simple question could easily take more than a hundred thousand years.

    A few centuries would not be enough time to chit-chat about language encoding, etc., but even if we had accomplished all of the protocol negotiation,s etc., before discovering that our Sun was to explode in a “a few decades”, there would not be enough time (perhaps more than a hundred thousand years) to even raise the handedness issue and receive an answer.

    Sorry I just can’t be more fun. By the way, I strongly suspect that any organized matter that enters anything resembling a wormhole will leave as a ‘decoded message’ consisting of highly energetic fundamental particles.

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  56. 56. LittleDragon 11:16 pm 09/3/2011

    Cool, we can show what left and right is but not tell (describe) it.
    Is that a property of physics or us?

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  57. 57. LittleDragon 10:37 pm 09/4/2011

    When the Budda returned from Nivana, he held a sermon for the priests to illustrate how the world was an illution. As subject he showed how impossible it was to explain what fire was to somebody that had never seen or experienced it.
    (ref: Not the Bagavagita, but the Lord of light by Roger Zelazny)

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  58. 58. Dr. Strangelove 11:23 pm 09/4/2011

    Dr. Castelvecchi,

    It’s really very simple. Tell the aliens to observe the beta decay of cobalt-60 and designate the direction where there is more electrons as either left or right.

    Both aliens and earthlings will agree on what is left and right because cobalt-60 violates the conservation of parity.

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  59. 59. yahilli 6:18 pm 09/6/2011

    @Dr. Strangelove: Imagine two aliens sitting one in front of the other, facing each other. They both observe the beta decay of cobalt-60 happen exactly between them. One of them sees more electrons on the right the other on left. Which is the “right” alien?

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  60. 60. Dr. Strangelove 10:52 pm 09/6/2011

    yahili, they are both correct so long as they agree which is “right” and “left.” That’s the whole point. There is no universal left and right. They are merely human convention.

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  61. 61. yahilli 2:21 am 09/7/2011

    Yes of course. But the problem here if it is possible to COMMUNICATE these conventions to someone with whom you have no common spatial reference point. The cobalt decay doesn’t help there (as well as the rotation of polarized light as I suggested before). It is as if you told the alien that right is the hand where the thumb points left :-)

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  62. 62. Dr. Strangelove 9:59 pm 09/7/2011

    yahilli, read my comment #10 in this link. That answers your question.

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