# Define "left".

I saw something on TV a while back - someone suggested that you can’t define “left” or “right” only using words. The other people on the show couldn’t think of a way. I couldn’t think of a way (Other than “That way” points) So, I went looking to see how dictionaries handled this problem;

The problem with these is that it does require other knowledge - I know which side the heart is on, how north relates to west, but how could I explain “left” and “right” to someone who doesn’t know these concepts without using gestures?

Not really – since “left” and “right” are relative positions – they have meaning only relative to another object – they have no absolute meaning.

You just tell the person to face a local landmark that you know to be to their north. Now tell them that “left” is the side of their body in the direction that the sun sets.

Left: That part of you that is immediately counterclockwise to your front.
Right: Opposite of wrong.

Or, assuming the person lives in the northern hemisphere, you could use the moon to illustrate the concept. The side of the moon which is first illuminated after a new moon is the right side, the side which is last to be illuminated as it wanes is the left.

I presume that you’re ruling out referring to clockwise and anti-clockwise rotation?

If not, left would be: direction one faces after completing a 90’ rotation anti-clockwise.

Well, in the end, any concept can only be defined in relationship to other concepts. The question is how simple or reproducible you want to go.

For example, I’ve seen “left” defined as “the side of the body in which the heart is normally found in humans.” That’s a definition of “left” with regard to physical objects, and it’s reasonably easy to use (you can pretty much tell where your heart is located in your body).

Alternately, you could define “left” as “where the east lies when you are facing south.” Subsequently, you could define “south” as “the direction where the sun appears at its highest point in the sky,” and define the other directions in relation to south, which would give you left and right for free. (In fact, several languages derive their word for “south” from their word for “noon”; the south of France is called the Midi, and the word for “southerly” is “méridional.”)

I knew that but never thought about it before. Cool!

And you’d have some very confused Australians.

Slightly off topic: I remember a few threads around here where people claim they have trouble remembering left and right, even as adults. I’ve never experienced that, but I can remember my mother trying to teach me the concept of left and right when I was about six or so, and for the life of me I couldn’t understand how I could turn around 180 degrees and my left side was still my left side. I already understood compass points, so I thought this was the same deal.

I was thinking about that and I couldn’t recall whether the sun appears to be in the north in the Southern Hemisphere or not.

Thinking about it more, it occurs to me that between the Tropic of Cancer and (at least) the Equator, the sun is be in the northern half of the sky at noon during the summer months.

So it’s probably safer to define “east” as “the direction from which the sun rises at the equinoxes,” which is true everywhere.

Which was the reason we were given for having south “on top” in some XVIII and XIX centuries high-detail Spanish maps… locating “south” can be done easily without a compass.

“East is the direction from which the sun rises” is the definition I’ve always used. Equinoxes are a bit of a complicated concept for a kid, and most people only need to have a vague idea of East… the “east-ish” you get outside the equinoxes works fine for any uses other than topography and ship navigation.

Left = the lower-hanging testicle.
On me, anyway.

If we’re starting witha blank canvas of knowledge, then knowledge of whether one is in the northern or southern hemisphere is eliminated, so defining the highest point of the sun as north or south doesn’t work.

It would be possible to use constellations for orientation - just the a description of the positions and magnitude of the stars in the Southern Cross, for instance, could identify left from right.

I think it might be definable in terms of the Left Hand Rule. Or the simpler Right Hand Grip Rule

I’m not talking about defining what left and right hands are - instead, these rules define certain electromagnetic behaviours that could be used as a reference from which notions of ‘left’ and ‘right’ could be unambiguously derived.

No, because every right-hand-rule (or left-hand-rule) in math or physics involves pseudovectors, all of which have an arbitrary sign convention. There’s no particular reason we had to define magnetic fields the way we did: We could just as easily have defined them to point in the other direction, and aside from referring to left-hand-rules wherever we now use right-hand-rules, the physics would be identical.

If you really want to start from a clean slate, you have to go into a great deal of detail, with some very difficult and subtle particle physics measurements. The definition goes something like this:

Define “positive charge” to be the charge of the lepton preferrentially produced by the decay of the long-lived neutral kaon (this preference is very slight; something like 501 times out of 1000). Now consider a positive muon, which decays in such a manner that the electron neutrino produced is travelling upwards relative to the center-of-mass frame. Now consider a macroscopic object rotating such that its angular momentum is parallel to the angular momentum of this neutrino. We can define “left” as the side of that macroscopic object which is moving forward.

You’re assuming time flows in the same direction for the reader as for us?

Some other stabs at answers to this question showed up in Radio-only communications with Aliens: left and right?

Hold out your hands with your palms facing out. The hand on which the thumb and forefinger make an “L” is the left.

So what, now everybody uses the Latin alphabet?

What does “clockwise” mean?