> "PD" gmail.com> wrote in message
>
> news:b185efaa-f965-4f32-bd6f-4e8589124729@l42g2000hsc.googlegroups.com...
>

>> ...

>>> This reminds me of a puzzle I heard once. We get
>>> in communication with a civilization from a distant
>>> stellar system. They want to send a visitor to earth.
>>> We reply "Wait - we want to be sure that you aren't
>>> made of what we call antimatter - and vice versa.
>>> You'll have to perform an experiment and tell us the
>>> results first." What experiment can they (and we)
>>> do to tell if they're made of the same matter as us?
>>> Remember, it can't be something like "what is the
>>> charge on one of your protons?" because they
>>> have their _own_ definitions of "positive" and
>>> "negative", we have no idea whether their "positive"
>>> charge is the same or different from our "positive"
>>> charge. No interchanging of samples or something
>>> like whether there are lots of 511 keV gammas
>>> when they approach our solar wind (this is also
>>> really interchanging samples) Also ignore the lags in
>>> communication time, as if they had a Star Trek type
>>> communicator.

>

>> I'd ask them whether their neutrinos are left-handed.
>> Now let me think a bit why this should work....

>

>> ---------------------------------------------------------------------------
>> --------

>

>> "Please define 'left'."

>
> This can be done with a drawing. What it means is
> the sense that your fingers of your left hand curl
> around when you line your thumb up along a line.

>> "It's the _army_ left."

>

>> (actually, though, that can be resolved, if you
>> know where their system is -- pick three distant
>> stars prominent in their sky as well as ours
>> because they're very big and very far away in
>> galactic terms, tell them to draw a line from
>> number 1 to number 2, defining number 1 as
>> "top", and then number 3 will be on the left)