Q: If you are talking to a distant alien, how would you tell them which way is left and which way is right?
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Try to define left and right for someone using only physical principles, and without making reference to anything else. You’ll find that any (successful) attempt you make to distinguish left from right involves citing example. It turns out that if both you and your alien friend have access to decent laboratories, then you can describe left and right using only physical principles. Some nuclear processes (specifically “β– decay” in what follows) can tell between left and right. Or more accurately, they can tell the difference between right and left-handed “chirality” (this post talks about chirality in passing).