Take this from the view of trying to tell a completely
alien race what we look like. For instance, tell them that our left side of our bodies is
symmetric to our right side. "What's
Left? What's
Right?" Well, if you face north, then your-- "What's
north?"
The truth is that the only way we can differenciate left from right is by
particle physics, as
snol already mentioned. It has something to do with beta-decay.
However, the wierd thing is, that every particle has an
anti-particle. When the two are combined, they anniliate each other and turn into pure energy. The other wierd thing is, everything about anti-particles is opposite of the regular particles.
When you explain to this alien about beta-decay and how left is different than right, if he carries out some experiments with beta-decay, he can know what we are talking about when we say "left." However, if his entire world is made of
anti-particles, his "
left" will end up being our "
right". The particle physics only works if we assume both parties are using our "regular" particles, and not the anti-particles. (If his world is made of anti-particles, he wouldn't know it, they would be normal to him, and he would think
we were the ones made of the anti-particles!)
Which means, after explaining our left, and our right, and our
cultures, if we ever rush out in space to meet each other and you put our your right hand to shake his hand, and he put's out his
left hand instead of his right, you better watch out!
I only know this after reading some Richard P. Feynman lectures. Amazing guy that Feynman is...