Here is a question that keeps me awake at nights: how do you define right versus left without referring to something, like the placement of our heart, an organ that is already tilted toward one side of the body (the left except in rare cases of situs inversus)?
For example, have a look at a bilaterally symmetrical organism below, in this case one of my favorites (Merriam-Webster defines bilateral symmetry as “symmetry in which similar anatomical parts are arranged on opposite sides of a median axis so that only one plane can divide the individual into essentially identical halves”.) We know left from right because we define them consistently, and that’s because humans are NOT bilaterally symmetrical so we can all agree on which side is which.
But now I’ll ask you to answer this. (i.e., by pointing) Assume you’re talking to a person (a Martian?) who has never heard about right vs. left sides. Tell them, using the diagram of one of my favorite organisms below, standing upright, which side is the right and which the left without referring to your own body, to any minute differences in the diagram, or to asymmetries in the environment (e.g. the world or the solar system). Since both sides are identical, how do you know which one is right without referring to how we’ve already defined it, presumably based on our own bodies? Explain to a Martian who is bilaterally symmetrical which side is its right and which its left, and how they would know it.
I’m not sure if I’m making myself clear here, so I looked in the Oxford English Dictyion for the definition of “right”. There are of course many definitions that don’t refer to the direction, but here’s what it gives for the direction:
a. of, relating to, situated on, or being the side of the body which is away from the side on which the heart is mostly located
b. located nearer to the right hand than to the leftc. located to the right of an observer facing the object specified or directed as the right arm would point when raised out to the side
d. located on the right of an observer facing in the same direction as the object specified
This didn’t help, because it all comes down to how humans have defined the sides based on our own asymmetries.
This problem is connected with something that’s always intrigued me: how do directional asymmetries evolve, in which an animals is predictably asymmetrical, like our hearts being more on one side or the others? (There are some creatures with “fluctuating asymmetry”, in which right is different from left, but it’s not consistent, like lobsters in which one claw is a crusher and the other a slicer, or flatfish that develop to lie randomly on its left or right side sides as adults. Evolving these doesn’t pose the problem I describe below.)
If we evolved from a bilaterally symmetrical (or radially symmetrical) organism, then even if front and back are genetically specified, as they are, how can you evolve from such a creature into an organism that has features consistently on the right (or left) sides? The chemical gradients in a bilaterally symmetrical ancestor are presumably the same on both sides, so how can a gene mutation arise that consistently recognizes a given side to give rise to a feature on that side? In other words, how can a mutation KNOW whether it is on the left or right side of the body? (Of course once an initial directional asymmetry has evolved, it creates a directional cue that can be used to evolve further directional asymmetries. It’s the evolution of the first directional feature that is the difficulty.)
I’ve discussed this more clearly in two old posts on this site (here and here), which gives some partial answers residing in how asymmetrical molecules or asymmetrical beating of cilia could lead to the evolution of directional asymmetry from bilateral asymmetry.
But the problem above still nags at me: how do you tell a bilaterally symmetrical Martian which side is right and which is left without referring to our own bodies? Can it be done?
Again, this may be a non-problem, but I’ve seen no definition of “right” or “left” independent of our own bodily asymmetries.
Facing the north pole of your planet, your right side is the one that faces sunrise and left the sunset.
This is close to what I was going to say. Assuming the Martian is aware of the general geographic configuration of Earth, and assuming that we and the Martian are clear on the ventral/dorsal and head/tail asymmetry of the fly, this should work: “North is the direction toward the planetary axial pole closest to our largest land mass. Orient the fly with its ventral surface facing north, and its head away from the ground. Then ‘right’ will be the side of the fly facing the direction of the Earth’s spin, and ‘left’ will be the side facing opposite the direction of the Earth’s spin.”
What we call north is arbitrary. In the movie Independence Day the aliens have a map of the earth that looks like our maps. I always thought it would be better if they had showed it upside down. Theres a 50% chance aliens would depict it like that
No, I don’t that’s correct.
Obviously we’re accepting that the Earth rotates on its axis and one point will be north and one will be south. So we’ve already removed almost all arbitrariness from the equation.
Now, given the Sun is in motion around the galactic centre we can define north as the point of the axis that’s at the front of the plane of motion. And south as in that which is bringing up the rear.
(The planets orbit the Sun at 60 degrees to the galactic plane.)
I think even a tilted planet like Uranus will have its North Pole aligned with Earth’s in the relative direction of travel. If not, and it’s tilted absolutely perpendicular to the Sun, we can define North thus: the pole that is directed toward its solar centre.
You are hinting at the fact, unmentioned above, that right and left are meaningless until we also define front and back. As far as organisms are concerned, front and back are generally more easily distinguished than right and left.
This problem is the same as defining dextro- and levo- isomers. They are chirally different, mirror images, and one cannot replace the other. It’s just a convention to call them left and right.
How do you define the North Pole? If you define it independently of the direction of spin e.g. by the magnetic pole, then the aliens might have a planet that spins the other way.
If you define the North pole in terms of the rotation then you have just changed the problem from one of defining left and right to one of defining clockwise and anti-clockwise.
Yes, I see the problem. It would have to be defined as the point of the axis of rotation closest to an observer in a position to observe the planet rotating counter-clockwise, but then, as mentioned above, one needs to define clockwise and counter-clockwise. I’m stumped.
Great question – didn’t this get a discussion a while back?
I’ll have to take time to let the thoughts percolate…
Dextro – right-rotation of plane polarized light
Levo – left rotation
Clock rotation forward in time is to the right on top of the clock, for clockwise
Sinister is Latin for left
Rectus is Latin for right
But where’d left/right originate…
.. at the moment the political is swamping it all out!
The portion of the organism with bilateral symmetry that lies within greater than 0 degrees and less than 180 degrees of the median axis is on the right and greater than 180 degrees and less than 360 degrees of the median axis is on the left.
Doesn’t that depend on whether you’re measuring degrees to the left or to the right? You’d have to tell the Martian which way, which presumes it would already know left from right.
Good question. I was going to edit but got distracted. The organism has bilateral symmetry, so has a top and a bottom. I should have said starting from the part of the organism where the median axis bisects the organism’s head.
I’m assuming My Favorite Martian would know what I mean by head and “he” understands the same geometry we do.
These are fun thought experiments.
If the aliens already understand the human way of measuring degrees with 0 and 360 at the top and proceeding clockwise, then the job is already done, but I bet you can’t define “clockwise” without relying on the terms “left” and “right”.
Here is my non-answer:
When facing an object, we wanted to differentiate direction relative to the subject facing said object. Up and down are easy because one is toward gravity and the other is away from gravity. But the horizontal directions are harder, so we just picked one and said “this is left, that is right”. This is different than east and west because left and right change as you face a different direction.
Describing left and right in objective terms is probably not possible, just like describing colors in absolute terms is impossible or describing the word “book” without refering to letters or phonetics is impossible.
To talk to the Martian you can always use the chirality of neutrinos versus antineutrinos–Feynman apparently once suggested this as a way to tell distant aliens what we mean by right and left. Though I guess you’d need to make sure they aren’t made of antimatter. The specifics of the situation are a little mind-boggling (to me, anyway), but apparently parity by itself is not symmetric in the universe with respect to–I think–the weak force, which governs beta decay (which is where the neutrinos mentioned come from, if memory serves). This is independent of our bodies, of course, which I don’t think became as they are because of beta decay. Also, I don’t think it’s WHY particular biological molecules are chiral in the way they are, either, but maybe some biochemist among the readers knows better?
Veritasium has a good video about the parity violation, though I don’t know if he goes into Feynman’s idea about how it could be used to provide a fully translatable description of right versus left: https://youtu.be/yArprk0q9eE?si=41WRhmCGTKpbR-1X
Feynman’s discussion is here. https://www.feynmanlectures.caltech.edu/I_52.html
Thank you!
“The magnet[‘s south pole] has grown hair!”!
>” . . . WHY particular biological molecules are chiral in the way they are?”
This may be a less deep answer than you seek. Any molecule with at least one carbon atom with four different functional groups attached at the four tetrahedral bonding axes is chiral by definition because it can’t be superimposed on its mirror image. That’s all it means to be chiral. We say, by convention, that the stereo-enantiomer that rotates plane-polarized light clockwise is dextro-rotatory. But this is just arbitrary because it requires a prior convention that the relevant left-right direction is the one the shadow of a sun-dial’s gnomon mostly moves when the sun is up. (In summer when the sun rises and sets north of due east and west, the shadow moves to the left during those early morning and late evening hours. Clock hands of course move to the left as often as they move to the right. Calling a screw thread right-handed that tightens clockwise is purely arbitrary. But because supination of the forearm is stronger than pronation, right-handers prefer clockwise threads. And thus we are back to human handedness.)
As to why a chiral compound rotates light in one direction while its mirror image rotates it in the other, I’m assigning that to myself as homework. To the best of my knowledge but I stand to be corrected, there is no way to predict from molecular structure alone (i.e., without reference to the known behaviour of related compounds) whether a given chiral molecule will rotate polarized light clockwise or counter-clockwise.
Biochemical systems need to take account of chirality simply because it exists. An amino acid’s mirror image will not fit where a levo-rotatory one needs to go for the protein to work property or even to be incorporated into the elongating polypeptide chain in the first place. (All amino acids used for protein synthesis, as opposed to merely as food, are levo-.) The crucial step essential for reading the mRNA template faithfully is the matching of each amino acid to the correct tRNA. The synthases that do this — one for each AA-tRNA dyad in eukaryotes — must recognize with high specificity the shape and charge distribution of the functional group of each amino acid, the same for the anti-codons of each tRNA, and then hold the two molecules in the correct spatial orientation so that the tRNA is esterified onto the carboxyl group at that “end” of the amino acid. To do this, the cell spends two high-energy phosphate bonds for each AA-tRNA made. It behooves it to not spend this energy until it is sure that the synthase has an L- and not a D- amino acid snared at its active site.
Physics is your friend here as current flowing through a wire will create a magnetic field in one direction and also a force in another specific direction.
That gives you a left/right standard which you can apply to chiral molecules or body parts.
I don’t think that’s quite right. Electromagnetism is parity invariant, so a mirror-image world obeys the same laws. You can’t tell which one you live in. (It follows that this is true of chemistry too.)
But physics is the right place to look. There’s a famous result from Lee-Yang-Wu that weak interactions break this symmetry. This was surprising as until then (1950s) everyone expected all of nature to be parity symmetric.
https://en.wikipedia.org/wiki/Wu_experiment
This is the P in CPT symmetry, which is the idea that simultaneously changing both parity and the direction of time (T) and the sign of charges (C) has no observable effect. This is still thought to be true.
Martians of course have the same sun & planets, so we could easily use which way Jupiter turns as our standard. And the questions you raise about development and evolution of body plans seem like they will be independent of both the solar system and the weak force.
AC Harper’s idea still works, because the observable universe is mostly matter, so we can establish which charge is positive/negative.
It’s just a fact of life, that there are no natural standards. All standards are human, and some are quite arbitrary.
All words are defined by referring to something in the universe. The reference can be an object, an action, or a relationship.
But the left/right dichotomy was certainly codified in language by people without knowledge of magnetic fields, neutrinos, planetary rotation, or even internal anatomy. Externally, humans ARE bilateral. Still no problem to distinguish what we call left and right sides.
As implied above, it is inherent in the term ‘bilateral’. Any bilaterally symmetrical animal by definition has an anterior/ posterior difference and a dorsal/ventral difference. Without either of these, symmetry is radial, not bilateral.
There are always two repeatably different sides. Call them ‘blart’ and ‘blyrt’ if you want,
I was going to say something along those lines. Many (most) organisms have fronts and backs that differ from each other. We can define the front in a number of ways (eg where the majority of the organs in the body are in front of the backbone or notochord). If we then place the organism on its ‘front’, we have two sides that are bilaterally symmetrical. By convention, we give these sides the labels ‘left’ and ‘right’.
So we would have to invite the Martian to lie down on its ‘front’, and we would then indicate (eg by patting it on the ‘shoulder’) which side is labelled which. Whether it would accept such treatment is another question entirely. And we are scuppered if it has no difference at all between front, back and sides.
But they did know direction: east, west (sunrise, sunset). They were in the northern hemisphere so they knew north (pole star).
Left and right follow from direction.
Hmmm. Left is the direction from which the cars are coming when you step from the sidewalk onto the street in the U.S. 🙂
AC Harper’s (above) reference to the left hand/right hand rules of inductors, and Mark’s (above) reference to degrees in the unit circle, and pHilL’s and Robert Woolley’s (above) references to the earth’s rotation are all rules of thumb much like mine regarding stepping into the street. They’re all good, and workable in context, but they don’t seem to get to the “essence” of what it means to be left or right. Maybe a convention is just a convention and nothing more, and what is left and what is right cannot be stated without reference to our own bodies, or to unit circles, or to the left-hand rule, or to some other stated standard.
Of course, none of what I’m saying here touches the subject of deviations from bilateral symmetry in organisms.
Most humans are right-handed but left-handedness is not at all rare. This would have been apparent in prehistoric times as the preferred hand for punching, clubbing, and rock-throwing. By the time words were invented and soldiers were forming into battle lines with shields and swords it would have been obvious (and prejudicial to mutual protection) if a sword wielder found himself next to a neighbour with not his shield but his sword in his left hand. So the early words for “right” in all languages could have come to mean the side that most people are dominant on. (The homonyms “right”, “wright”, and “write” are surely a coincidence.). Mirror images in a pond of still water or a highly polished bronze shield would have got the smarter ones thinking.
While there might be a fundamental asymmetry in “right-ness” in the universe*, it’s hard to see how this would have given early cultures and languages the concept. So in explaining right and left to a Martian, I would just take him to a baseball game where one team starts a southpaw.
————————
* Something makes the embryonic tubes that become our internal organs rotate (breaking symmetry) nearly always in the “right” direction to put our unpaired organs on the side they are. Curiously, men with situs inversus are usually infertile because the flagella of their spermatozoa don’t beat. Genetic linkage of unrelated characters or causal connection?
A lot follows from humans being mostly right handed. Agree.
If fly genes can figure out how to put eyes on the top but not on the bottom, I don’t see why they shouldn’t be able to put a heart on the left but not on the right.
Because once the dorsoventral and anterior-posterior axes are specified (by the stuff in the egg), then there are no degrees of freedom for specifying left versus right. On either side of the body the chemical and genetic cues should be identical. How, then, can a gene tell which side it is on? That is the problem, and there are suggestions of solutions, but its still pretty much a mystery.
Isn’t that a bit like trying to define a table without any reference to a physical object? If we have defined right and left in relation to our bodies, if that’s what generates the definition, then trying to define it without reference to our bodies is obviously going to (potentially) fail, but that doesn’t create some logical or metaphysical conundrum.
I think the concepts of left and right require that you first define how top/down are related to front/back.
I would tell the martian to imagine that the fly is ‘standing’ in a space defined by the axes x, y, and z. Then, tell them to picture the head pointing in the positive z direction, and that its belly is pointed in the positive y direction. The side that is pointing in the positive x direction is right.
But, given an otherwise unlabelled triple of mutually perpendicular X,Y,Z axes, the directions that are called “positive” is just as arbitrary as left v right.
+1
I can comment on these questions.
The challenge to define L from R without an external reference or to minor asymmetries is neither not possible or not practical. The fruit fly, btw, is not perfectly bilateral since the gut has some torsion asymmetry and one can refer to that to define its L and R. I suppose there are bilateral organisms that are arguably without asymmetry (maybe Desmid algae), but in those cases you have to refer to something external like your left or right. I don’t think we need to define L from R without some reference.
It’s been ages since I’ve learned this, but during embryonic development left and right Asymmetry in vertebrate embryos becomes defined very early. A structure called Hensons’ node (it’s given different names in different embryos) has cilia that spin like a corkscrew with a certain handedness. This seems to create a delicate fluid current that causes a gradient of chemical signals that result in L/R asymmetries in various developing organs. Here is a review article with goes into this, although I have not done more than skimmed it:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6737901/ It gets into this subject starting with the paragraph that begins: “Over the last 20 years…”
Yes, the cilia beating seems to be one solution, and it is the one I favor. But I still muse about trying to get someone to be able to tell right from left unless they have learned it from others.
This may be a non-problem (not the evolutionary one), but I thought I would raise it.
The physics explanation in Comment 5 is correct, and this can be used as an absolute definition of right and left, at least in our universe which consists of matter (not antimatter).
Nowadays the mystery of how to define right and left does still remain, but only at a higher level. If you simultaneously interchange right and left, positive and negative charges, and the direction of time, then there is no physical experiment you can do to distinguish those two universes. This is called CPT (Charge-Parity-Time) symmetry.
Years ago Jerry wrote a long detailed science post on this subject, explaining research on it over decades, piece by piece until most of the picture was pretty clear. It was one of my favorite science posts by him. Actually, it was a series of three posts.
Directional asymmetry: how does it develop and how did it evolve? Part 1.
Directional asymmetry: how does it develop and how did it evolve? Part 2. Mechanisms for generating handedness
Directional asymmetry: how does it develop and how did it evolve? Part 3. Artificial selection for handedness
Another piece of the puzzle, vesicles filled with a signaling molecules are released from the floor of the nodal pit and the directional flow caused by the specialized cilia move them to ones side of the pit, where they open, thus establishing a gradient of the signaling compound.
Also cool, the direction of rotation of the cilia is determined by their chiral structure.
This is slightly off the discussion topic, but does refer to the ambiguity of left and right in some humans. As a grad student in German, I had a wonderful professor who had been born in Austria and was initially left-handed (this was in the 1930s). Since that was considered a mark of the devil, she was forced to do everything with the right hand when she started school. She was sent to a concentration camp when she was 11 and ended up at Auschwitz before she was sent to work in a steel plant (Krupp). During those years she apparently didn’t do much that required knowing right/left. When I knew her–when she was in her early 40s–it was pointless to give her any directions pertaining to left/right, especially related to driving, as she had no clue. It was also useless to try to tell her in relation to her heart, as that just confused her. I’ve known one other person who had left/right problems, but not as severe. If she was driving, however, a passenger had to point to the direction she should go, as left/right just confused her.
I’m like your professor in being unable to automatically distinguish right from left. It’s been a lifelong embarrassment. But I felt little better about it when I recently learned that a surprising number of other people have this problem. Here, the BBC describes it:
>While for some people, telling left from right is as easy as telling up from down, a significant minority – around one in six people, according to a recent study…
>”Nobody has difficulty in saying [something is] front and back, or top and bottom,” says Ineke van der Ham, professor of neuropsychology at Leiden University in the Netherlands. But telling left from right is different, she says. “It’s because of the symmetry, and because when you turn around, it’s the other way around, and that makes it so confusing.”…
>When the answer doesn’t come instantly, participants described various techniques, from making an L shape with their thumb and index finger, to thinking about which hand they use to write, or strum a guitar.
https://www.bbc.com/future/article/20230112-why-some-people-cant-tell-left-from-right
I use the technique of glancing at my hands. Since I know my right hand is the hand I write with, I know that I should turn to the same side as that hand if I need to go right or the opposite side if I need to go left. But this is hardly ideal because it means I have to think for a split second before I can answer or act correctly, and I’m apt to get flustered and mess up if I have to make an instantaneous decision.
This may relate to Jerry’s question. Or maybe not. I find anything to do with right/left hard to get my mind around.
“You write with your right.”
As a driver I sometimes have interactions with a navigating passenger that go something like:
[P] Turn right.
[Me] [turns]
[P] No! The OTHER right.
Nice to know I’m not uniquely cursed.
“Developmental Topographical Disorientation” (DTD) is a disease that causes people to lose their sense of direction. An NPR “Radiolab” podcast at “https://radiolab.org/podcast/110079-lost-found” describes a woman who by spinning around would find everything she viewed ninety degrees out of kilter. Imagine right becoming front and left becoming back in the blink of an eye and you see her problem.
It would say: If an electric current runs from the posterior to the anterior then the magnetic fields lines on the left will point towards the ventral side**
**Id have to think about whether the way we describe electric current is arbitrary
It is. Just ask Ben Franklin about “positive” and “negative”.
Assume you are hungry and wish to move toward your favorite food to eat it. Call the direction toward the food north. Then your right side is east.
By the way, I don’t think it’s possible to define left and right for a creature with say five-fold radial symmetry like a starfish even if a symmetrical line can be drawn through one arm bisecting it into two symmetrical halves.
This reminds me of another puzzler: why do mirrors reverse right and left, but not up and down?
Oy vey! I puzzled over that question for ages and finally grasped the solution, but now I have forgotten it again.
Perhaps mirrors reverse handedness 🙂
The answer is mirrors do not reflect neither up and down nor left and right.
Mirrors reflect front and back.
By the way, I think most comments conflate (at least) two different things, perhaps because JC post also did not make just one question but many. One question is: is there a definition of “absolute” left and right? And the answer to that is the one Feynman gave (and I originally came here to say that, because I’m a physicist :P). A different question is: how can we define left in a bilateral organism? (and I think it’s the same answer, provided we defined previosly top/bottom and front/back).
Then we have questions about embriology (how that asymmetry forms, and I thank Mark Sturtevant for the answer, that I did not know). Finally, there is the question of how that came about in evolution, and I think that too is touched by Feynman: it’s an historical accident that life uses mostly molecules of one chirality and not of the opposite one.
I am not at all sure that the evolution of directional asymmetry is the result of the chirality of molecules (I assume that you are referring to amino acids). Feynman may have suggested it, but we have NO idea whether that is the case.
Sorry, I implied too much / wasn’t clear enough. The very existence of situs inversus in fact seems to exclude a strong connection.
I just re-read the Feynman lecture linked above; he did not speak about the connection between the chirality of molecules and the evolution of asymmetry there. He just noted (using an example of sugar and bacteria) that “IT LOOKS as though the phenomena of life permit a distinction between “right” and “left,” or chemistry permits a distinction, because the two molecules are chemically different”. But then he notes that this is an historical accident that only permits to note that left and right are different. Which is which is then derived by parity violation, assuming you don’t exchange matter with anti-matter.
No reference to either embriology or evolution there, except to note that “Somewhere, once, one organic molecule was lopsided in a certain way, and from this particular thing the “right” happened to evolve in our particular geography; a particular historical accident was one-sided, and ever since then the lopsidedness has propagated itself”.
I don’t presume to know what drives the asymmetry in development, I simply think that once you have a molecular means to distinguish it, it doesn’t seem strange anymore: the evolution has something upon which it can build. Why should it be stranger than up/down and front/back? Or at least I don’t understand what you mean when you speak of no degrees of freedom left.
Mirrors do reverse up and down, if you stand on one.
😀
Hah! It reminds me of a meme where someone is holding an orange against a mirror, but there is a large piece of paper between them. You can still see the reflection of the orange in the mirror, and the caption asks: How does the mirror know about the orange?”
This is an interesting thought experiment. It seems you must find some other concept to relate it to. On a case by case basis, you could say something like: “When you are face to face with another person, their left side is the side you encounter first if you were to move your gaze clockwise around their body.” I think clockwise could be used in a lot of cases, if you could convey the meaning of clockwise.
Again, arbitrary. E.g. if sundials and clock faces had first been developed in the southern hemisphere then clockwise would be the other way. 🤯
See above, there is an absolute way to distinguish left and right in our universe made of ordinary matter. There are still some kinds of universes that are experimentally indistinguishable from ours, if we simultaneously interchange left and right, positive and negative charge, and forward/backward time.
“Arbitrary” is in the context of the replied-to post, in the macroscopic world. I agree that all of C, P, T, CP, CT, and PT are breakable symmetries in the subatomic world.
Sticking with the macroscopic world, I do find it surprising just how many of our apparently universal conventions, such as “clockwise” , are just that — arbitrary conventions; and in some cases (e.g. the direction of current flow) the original choice is now seen to have been made wrongly but we are stuck with it.
Yes, but this provides a way to tell a Martian how to distinguish right and left even at the macroscopic level, and that was the question posed in Jerry’s post.
Correct. Feynman describes the technique, tongue-in-cheek, that could be communicated to the Martian who knew nothing of left and right and had no external reference point shared with us on earth. (Develop a common language and send him the instructions to build a supercooled cobalt magnet and contrive for mesons to decay in its magnetic field…). This answers the question Jerry put (leaving anti-matter aside.)
Q.E.D.
But I wanted to make another point. Suppose we humans and fruit flies showed perfect bilateral symmetry, inside and out. The heart is really a midline structure, after all. We feel the apex beat near the left nipple because the left ventricle becomes much larger than the right owing to the demands of the high-impedance systemic circulation. (In the developing fetus the output of the right ventricle is shunted away from the airless lungs and experiences almost no afterload.) The liver, spleen, pancreas, and gut are definitely asymmetric because of embryonic rotation. But suppose the whole tract was just a long straight tube, as in earthworms, with tiny little “liver-lets” and “spleen-lets” budding off symmetrically from each side.
Even with perfect bilateral symmetry the left and right sides would still be non-superimposable mirror images of each other. Little gloves and booties made for the fly’s feet would only fit one side, just as in our imperfectly symmetrical selves. The fly, and we, would still have to tell right from left to get dressed in any “sided” garment. It’s only because we humans are mostly (but not universally) asymmetric in the same, albeit subtle, way that allows us to unambiguously define right and left in reference to our own bodies. Absent this, the cobblers’ guild would have had to come up with some other way to tell if they were making shoes in pairs and not over-producing one side. The glove-makers might have adopted a different convention in trying to ensure the same thing. This is the beauty of what Feynman is so eloquently getting at.
That little itchy spot on my back I just can’t reach and must use the nearest convenient rough surface to rub against before going mad from pain!
Martin Gardner wrote a book on the subject:
https://en.wikipedia.org/wiki/The_Ambidextrous_Universe#The_Ozma_Problem
In the abstract setting of a perfectly bilaterally symmetrical martian, you wouldn’t be able to define left and right for them, without reference to external asymmetric features (e.g., the spin of a galaxy). Bilateral symmetry would mean the martian would recognise two special directions, which we know as left and right, but you wouldn’t be able to tell them which was which. Any way the martian chose to label those two directions, as left or right, would be isomorphic. That is to say, anything they could say about their left-hand side would be equally true about their right-hand side.
You can take this problem even further. Imagine a perfectly spherical alien (a sentient soap bubble, perhaps) in a completely empty universe (just flat open space). Such a creature would be incapable of differenting between any directions, and so the very concept of direction would be meaningless to them. The bottom line: As you remove features that introduce asymmetries in your thought experiment, so the concept of direction loses its meaning.
Put your arms straight out with your palm and fingers up and thumb out. Your left index finger and thumb make an L, ergo…😅
It’s an interesting and old problem. Related are these. Why is a mirror image reversed left to right but not top to bottom? How does a fertilized egg know left from right during development? What is the etiology of handedness in humans? I don’t know if I still am the only person to demonstrate a genetic basis for varying asymmetry in a vertebrate (lateral asymmetry in a flounder), but I was when I published the work in the early 1980s.
My limited understanding of the mirror problem is that there is no left right reversal, it only seems that way because of the left right symmetry.
If you wave at yourself in the mirror the same hand waves back not the reversed one.
I think that’s right.
I was going to link to Richard Feyman’s answer, but someone else already linked to it above. Beta decay, in which a neutron decays into an electron, a proton and an antineutrino, doesn’t occur in a symmetrical manner. It’s possible to provide an absolute reference about left vs. right based on the direction and spin of the three particles produced by the decay event.
As far as I know, beta decay is the only event in nature that can be used to define left and right without referring to some physical object (such as the geography or orbit of a specific planet). However, as someone else mentioned above, this method only works if the aliens you’re communicating with are made of matter, because beta decay in antimatter is the mirror image of what it is in regular matter.
There is a joke about this that I think originated from Feynman: You could use this method to explain the distinction between left and right to an alien, and then meet the alien and ask him to extend his right hand for a handshake. But if the alien extends his left hand instead, don’t shake it!
Electromagnetic Induction follows rules that can be used to infer handedness.
If I have a magnetic field, 1) I determine its direction, and
2) move a conductor perpendicular to the field, I will
3) get an induced current that has a direction.
Using up and down for reference for our alien’s sake, let’s say down is the direction of the induced current and forward (have to assume the alien can determine forward) is the direction of the magnetic field, then left will be the direction I moved the conductor (Fleming’s right hand rule…minus the finger references).
How do you convey the direction to the alien?
You can determine the direction of the current by observing the average movement of the electrons in the wire, but what about the magnetic field’s direction?
Ah – I see… relative to what?!
Well, based on positioning of visual organs on the ‘face’ – they face ‘forward’ so if we follow the apparent circle of the sun ‘clockwise’ we assume the degrees zero is where the nose is, directly forward, then right is 1-179 degrees, left 181-360.
In Andy Weir’s recent novel Project Hail Mary, there is an alien with no visual organs, that has multiple prehensile limbs like a spider. That creature would have no clear right or left I suppose.
I probably don’t understand the problem, but it the Martian understands gravity, I think it’s possible to explain what left and right are, because bellies and backs are different.
Next step is to build a molecular machine that always spin in the same predictable direction (is that possible?).
I’m going to assume that “asymmetries in the environment” means anything physical. Magnetic fields, chiral molecules, neutrinos, planets spinning etc are all part of the environment.
My answer only assumes that the alien can communicate in English and can read. You simply say that the beginning of the word “bilateral” in the diagram is on the left side.
“That’s cheating” you say, but the thing is that, once you eliminate the physical environment and the writing, and you specify that the diagram has reflective symmetry about the red line, there is no way to tell the difference. That’s what reflective symmetry means: if you can tell the difference, it’s not symmetrical.
So, if the only way to tell the difference is to introduce some asymmetry, you can do it by the most direct method. Right “left” on the left side of the diagram and “right” on the right side.
Huh. I’d never even considered defining left and right with respect to human anatomy. I just always considered left and right to be arbitrary conventions, kind of like defining X,Y, and Z coordinate systems. So, I’d simply define left and right with a picture or a diagram, something like this:
| ….Left….|….Right….|
Sure, it’s arbitrary, but that’s just what it is. And yes, it can be confusing depending on your perspective, but that ambiguity is something we deal with by specifying the point of view e.g. the left side of the house when looking at it from the street, vs. the left side of the housing when looking out of the front door.
I guess you could expand the diagram a bit more to something like this:
……Forward
Left…………….Right
……Backward
After skimming through the other answers, I guess maybe the challenge is to come up with a definition that doesn’t depend on convention. But why? Communication itself depends on convention. Besides, you could always just show the Martian the same diagrams.
Thanks for this post regarding an interesting question in biology.