# B How to explain "the right hand rule" to an alien universe

#### Rap

Suppose we are in communication with aliens who live in a different universe. I know, that's impossible, communication requires the exchange of mass or energy, which implies that we live in the same universe. But suppose it is true. I am wondering, can we and the aliens, via this communication alone, ever come to an agreement on what constitutes a "right handed rotation thru a 30 degree angle"?. I sit here and imagine a vector pointing upwards thru the center of my mouse. Using the right hand rule, I rotate the mouse +30 degrees about that axis. Is there any way, talking to the alien without exchanging any mass or energy, that I can get the alien to do the same?

We can't use beta decay, because they might be living in what we would call an antimatter universe.

I mean, if we could exchange photons, I could send the alien what I call a right-handed photon, spinning right-handedly about a vector axis pointing in the direction of propagation and say "this is what I call a right-handed rotation" and then we could come to an understanding. I might do the same with a massive particle, but if the alien lives in what I call an anti-matter universe, there would be an explosion, the alien could report this, and then I would have to send a massive antimatter particle. Or tell them to use beta decay.

But without a shared physical example, I don't see how it can be done using communication alone. Have I missed something?

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#### fresh_42

Mentor
2018 Award
The crucial question is: Can we agree on a common language? The rest of any answer depends on the answer to that!

#### Rap

The crucial question is: Can we agree on a common language? The rest of any answer depends on the answer to that!
Actually, that is what I am asking. I think a clever human and a clever alien could eventually establish a fairly sophisticated common technical language, but I wonder if it would be impossible for them to establish a common definition of the direction of rotation. Let's suppose that they have done their best and have established a common technical language to the extent possible without sharing physical examples.

#### anorlunda

Mentor
Easier than a written language is a simple monochrome bitmap picture. Several SF stories, including the movie Contact, used that idea. The disc we sent with the Voyager probe also included pictures.

#### Dale

Mentor
Agreed, just send a picture.

#### DrGreg

Gold Member
But how would the aliens know how to decode the digital data we send and turn it into a picture? Wouldn't we need to explain the order in which the pixels have been scanned, e.g from left to right? In which case we would first need to explain the difference between "left" and "right".

Staff Emeritus
Define the "electron" as the particle produced in the decay $K^0_L \rightarrow \pi^+ e^- \nu$ with lower probability than the "positron", produced in the conjugate reaction.
The weak interaction couples to left-handed electrons and right-handed positrons.
Now you have defined "left" and "right" from physical processes.

#### Rap

Define the "electron" as the particle produced in the decay $K^0_L \rightarrow \pi^+ e^- \nu$ with lower probability than the "positron", produced in the conjugate reaction.
The weak interaction couples to left-handed electrons and right-handed positrons.
Now you have defined "left" and "right" from physical processes.
Ah, I did not know that about that decay. (I'm not a particle physicist, so I ask for your patience). Can the $K^0_L$ particle be unambiguously identified without knowing left from right and not knowing which is matter and which is antimatter?

#### anorlunda

Mentor
But how would the aliens know how to decode the digital data we send and turn it into a picture? Wouldn't we need to explain the order in which the pixels have been scanned, e.g from left to right? In which case we would first need to explain the difference between "left" and "right".
Then maybe they would interpret our picture as the left hand rule. It's just a convention. Ditto if they defined + current as the same as the direction of electron flow. Or if we had chosen the opposite convention for left/right hand names.

What difference would it make to the aliens or to us?

Staff Emeritus
. Can the K0LK^0_L particle be unambiguously identified without knowing left from right and not knowing which is matter and which is antimatter?
Yes. It's the longer-lived neutral kaon.

#### Rap

Then maybe they would interpret our picture as the left hand rule. It's just a convention. Ditto if they defined + current as the same as the direction of electron flow. Or if we had chosen the opposite convention for left/right hand names.

What difference would it make to the aliens or to us?
I guess it wouldn't make any difference to them alone or us alone. But if there was an upcoming meeting between me and an alien and we shook hands, it would be embarrasing for both of us if we offered what the other considered the "wrong" hand. Not to mention the possible nanosecond of embarrassment if we couldn't sort out what each one meant by "matter" and "antimatter" before we exploded.

#### Rap

Yes. It's the longer-lived neutral kaon.
Ok, I'm studying up on K mesons, etc., please let me know if I get this right. The $K^0_L$ particle has an antiparticle and they are distinct. They are continually morphing into each other, but the $K^0_L$ meson is the one that spends a longer time being itself, and when it decays, tends to produce an electron. So without sharing an example, we could come to an agreement with the aliens on what constitutes matter and what constitutes antimatter. I'm not sure what a "left-handed electron" is, could you explain that? Also, I wonder if we could now tell the aliens to look at matter beta decay (or antimatter beta decay, if that's easier for them) to define left and right?

Staff Emeritus
The K0LK^0_L particle has an antiparticle and they are distinct. They are continually morphing into each other, but the K0LK^0_L meson is the one that spends a longer time being itself,
No.

we could come to an agreement with the aliens on what constitutes matter and what constitutes antimatter.
Yes.

I'm not sure what a "left-handed electron" is, could you explain that?
Not at B-level. But if you like, it's unnecessary. Once we have defined what positive and negative charge is, we can turn that into left and right handed.

#### Matt Benesi

Gold Member
Just send them some math, and use intersect language to ask "which way do you rotate?"

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#### sophiecentaur

Gold Member
Perhaps the mode of communication that we find successful would supply us with a clue / common reference. If EM were involved then both ends would be using the same Maxwellian Maths (no?).

#### Matt Benesi

Gold Member
That's interesting.

If you have GEM, you have a definite agreeable rotation direction measurement direction, unless you're dealing with stuff you can't measure.

"How to explain "the right hand rule" to an alien universe"

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