How do you know anything is real? Some things you can see directly, like your fingers. Other things, like your chin, you need a mirror or a camera to see. Other things can't be seen, but you believe in them because a parent or a teacher told you, or you read it in a book. As a physicist , I use sensitive scientific instruments and complicated math to try to figure out what's real and what's not. But none of these sources of information is entirely reliable: Scientific measurements can be wrong,