[ISL]

To tell an alien about handedness definitively, without an artifact, may require that both parties are aware of the CP-violation present in the weak interaction.

Put another way, how can we know that aliens won't reconstruct an electromagnetic message as a mirror image of what we sent?

[raldi]

Make use of the right-hand rule, which will suffice so long as they're not made of antimatter.

[infogulch]

GP is right. If aliens don't know about CP violation, there is no way to communicate rotational direction that preserves the absolute direction without some shared reference.

Because CP-violation is actually the only detectable way to discern between right handedness and left handedness. That is, without CPv, we would have no way to know that we aren't in a hypothetical mirror universe with all rotations reversed. Every other physical interaction behaves identically left or right.

So we have a few options to deal with our rotationally-challenged alien friends:

1. Hope they can parse far down enough into the dictionary to understand what CP is and either know it or can test it.

2. Communicate using a shared reference, like pointing out two quasars that rotate relative to each other. (Quasars are pretty good galactic reference points.)

3. As long as it doesn't affect giving directions, just don't worry about it, the physics works out the same. If we ever meet and they try to shake our left hand, well, they get their very own "oops I guess electrons are negative then" situation. "And that, kids, is why you always negate earth-radians before using them in a formula."

[sp332]

Feynman suggested shining polarized light through a solution of glucose and water. This will rotate the plane of the polarization clockwise. https://en.wikipedia.org/wiki/Glucose#History

[infogulch]

Cool, I didn't know that! The glucose molecule has a handedness though, which means it has a rotationally symmetric twin L-Glucose [1] with the same properties. The relative abundance of glucose over l-glucose is purely a property of which one life on earth happened to choose (they're not biologically compatible). So if you communicated blueprints for life on earth including glucose without an absolute rotation reference, the aliens could reconstruct everything two ways: biology with our glucose, or biology with l-glucose, and there's no way to differentiate between them since they would both work the same.

Veritasium makes a great video about symmetries that's very relevant here [2], and might suggest a slightly more practical way to correctly communicate handedness to aliens. The full CP violation experiment isn't necessary, as long as we assume that the aliens live in a universe made of mostly matter (as opposed to antimatter) like ours, and we can communicate the parity violation experiment (~measuring the preferential atomic decay direction of cobalt atoms near 0K in a magnetic field [3]), they should be able to reconstruct an absolute reference for rotation.

[1]: https://en.wikipedia.org/wiki/L-Glucose

[2]: https://www.youtube.com/watch?v=yArprk0q9eE

[3]: http://www.physics.utah.edu/~belz/phys5110/PhysRev.104.254.p...

[raldi]

I don't follow. If we're communicating with aliens that we somehow know are made of matter, we tell them that right is the direction a horizontal wire carrying positive current away from us gets pushed by a vertical magnetic field.

[yorwba]

That just shifts the problem to specifying the direction of the magnetic field, and you can't use the concept of left vs. right in that definition without creating a circular dependency.

[pbhjpbhj]

Vertically up it's just too a higher local gravitational potential, no? Sure, we flip right-left when we look at things upside down, but that's a nuanced usage.

[yorwba]

How do you determine whether your magnetic field is pointing up and not down?

[ISL]

How will they know we're not made of antimatter?

[FreeFull]

As far as astronomers can tell, there is very little antimatter in the observable universe (If there was more, we'd see it anihilating with matter more).

[raldi]

If we're not sure both sides of the conversation are made of the same particles, the situation is hopeless.

[benj111]

Aren't matter and antimatter a matter of perspective? If so haven't we just reintroduced the left/right problem.

[ISL]

Precisely.

[naturlich]

They didn't explode when we shook hands (/tentacles/general-graspy-bits)