Define "left".

[Leo_Bloom]

It’s the side you elicit lower frequencies in any gamut of any keyboard instrument since the 1300s AD.

It’s the side your cast is on when you’re about six and struggling to get it down pat and showing off your broken arm.

[Leo_Bloom]

Just remembered (and closer to heart examples): it’s the side 85% of men think it feels like someone else’s hand.

[Asympotically_fat]

All these definitions of left are ‘not right’

[MikeS]

@ZenBeam

Seriously, for anyone wondering how that jibes with what I posted above, mirror symmetry was expected to hold until 1956 when it was experimentally shown to be violated. In 1957 CP symmetry was proposed in its place. In 1964 CP symmetry was shown to be also violated, leading to proposed CPT symmetry (“They can’t do experiments with time reversed; let’s see them violate that one!”).

To be fair, CPT symmetry follows from a small number of fairly simple assumptions about the nature of the “vacuum state” (the lowest-energy possible state, without any “real” particles in it.) This was known before the neutral kaon decay experiments in 1964, so it’s not like CPT symmetry was just a fallback position.

[Peter_Morris]

Okay, lets try this. Find four stars in space which can be unambiguously identified, e.g. pulsars with particular frequencies. They form an irregular tetrahedron. Points ABC are a triangle on a particular plane, and point D is above that plane. You can imagine yourself at A, and draw an imaginary line from B to C. You can say that B is the leftmost point of the line, and C is the rightmost.

Would that work? I can’t see anything wrong with that.

[Colophon]

Sit them down in a room and tell them you’ll be back in 5 minutes to explain. Come back half an hour later and say: “That’s what left means.”

[MikeS]

@Peter_Morris

Okay, lets try this. Find four stars in space which can be unambiguously identified, e.g. pulsars with particular frequencies. They form an irregular tetrahedron. Points ABC are a triangle on a particular plane, and point D is above that plane. You can imagine yourself at A, and draw an imaginary line from B to C. You can say that B is the leftmost point of the line, and C is the rightmost.

Would that work? I can’t see anything wrong with that.

I don’t see why it wouldn’t work. You’re essentially using the stars to define the handedness of your coordinate system.

[Johanna]

@Peter_Morris

Okay, lets try this. Find four stars in space which can be unambiguously identified, e.g. pulsars with particular frequencies. They form an irregular tetrahedron. Points ABC are a triangle on a particular plane, and point D is above that plane. You can imagine yourself at A, and draw an imaginary line from B to C. You can say that B is the leftmost point of the line, and C is the rightmost.

Would that work? I can’t see anything wrong with that.

It’ll work only if you pre-define B as being on the left.
Your method says, in brief: Let us define B as the point on the left; in that case B is the left endpoint on the line BC.

@Leo_Bloom

Just remembered (and closer to heart examples): it’s the side 85% of men think it feels like someone else’s hand.

Could you clarify? I have no idea what you are trying to say.

[Bosstone]

@Johanna

Could you clarify? I have no idea what you are trying to say.

Masturbation joke.

[Leo_Bloom]

Originally Posted by Leo Bloom
Just remembered (and closer to heart examples): it’s the side 85% of men think it feels like someone else’s hand.
Could you clarify? I have no idea what you are trying to say.

It was a silly reference to the idea held by boys that the sensation of masturbating with the left hand–not the usual hand–is novel enough to imagine that a real other person is giving you a hand job.

The example, come to think of it, is interesting in how emotional memory has such a strong role in handedness.

[Chronos]

It’ll work only if you pre-define B as being on the left.
Your method says, in brief: Let us define B as the point on the left; in that case B is the left endpoint on the line BC.

No, he’s saying “Let us define B as the one that has a pulsar frequency of 73.92 Hz”, or whatever. This works, as long as the aliens are able to observe the relevant pulsars. But it’s fundamentally no different than “Left is the way I’m pointing right now”: It’s still just a reference to a specific object.

[Peter_Morris]

@Chronos

But it’s fundamentally no different than “Left is the way I’m pointing right now”

The difference is that the aliens can’t see which way you are pointing.

[Half_Man_Half_Wit]

@JWT_Kottekoe

This also raises the question of how a developing embryo knows how to break the symmetry to put the heart on the left side.

I wondered about this for many years and always assumed it had to do with the asymmetry of our biomolecules, which occurred by random selection early in our evolutionary history. In fascinating work done rather recently, the answer was finally determined and, indeed, it can be traced back directly to the handedness of biomolecules. Briefly, in embryogenesis, after the dorsal/ventral and anterior/posterior directions are chosen randomly, the left/right directions emanate from a center of assymetry created by the clockwise (or anticlockwise, I don’t remember) rotation of a cilium (or flagellum, I’m not sure of the difference). The cilium itself is handed because of the handedness of its microtubules, which are handed because of the handedness of their proteins, which are handed because of the handedness of their amino acids!

Which in turn may be handed due to supernova-produced neutrinos…

[JWT_Kottekoe]

@Half_Man_Half_Wit

Which in turn may be handed due to supernova-produced neutrinos…

I’ve never understood why people propose such things. There is no need for such a theory, so we can rule it out as highly improbable. There is no more mystery about why our molecules are handed than there is about why a particular three base pairs code for a specific amino acid. It is an accident of evolution.

[Half_Man_Half_Wit]

The issue the theory addresses is the enantioenrichment of amino acids found in meteorites.

[Chronos]

It was my understanding that there was no such enrichment, that amino acids found in meteorites have a 50-50 split between their mirror isomers.

[OldGuy]

@Chronos

To clarify: Neutrinos produced by beta decay processes do in fact always have their angular momentum in the same direction relative to their direction of motion (or the same chirality, if you prefer). Antineutrinos also all have the same chirality, the reverse of that of neutrinos (this may, in fact, be the only difference between neutrinos and antineutrinos, in which case there’s no real fundamental difference at all, since the chirality of a massive particle can be changed). This asymmetry is, so far as anyone can tell, absolute: Nobody has ever observed a neutrino to be produced the “wrong way”-- Just one more example of how the Weak Nuclear Force is weird.

But now that neutrinos are thought to have mass, they no longer can move at c. Doesn’t that mean there are inertial frames of reference in which a neutrino’s apparent direction of motion is reversed? An observer in that frame of reference would see the opposite chiralities for neutrinos. It would also be possible to have an inertial frame in which the neutrino was staionary – or am I missing something?

[Half_Man_Half_Wit]

@Chronos

It was my understanding that there was no such enrichment, that amino acids found in meteorites have a 50-50 split between their mirror isomers.

Can’t really say I’m equipped to argue the issue, but the paper cites some sources that claim there is; no idea how well supported those claims are, though.

[Chronos]

Yes, that’s correct. Neutrinos produced via any of the particle decays we know about all have the same chirality in the zero-momentum frame of the decay. But there are other frames (admittedly, difficult to access in practice) where the chirality is opposite. This is what I was referring to with my parenthetical in that post.

[initech]

Here are a couple of approaches:

Left is this side of the sentence, right is this side of the sentence.

• or -

Left is the hand you don’t write with, unless you’re a freak.