Another in a nearly infinite set of reasons why I'm not a physicist: I fail utterly at understanding quantum entanglement, aka "spooky action at a distance". Here is some very rudimentary background. Here is an explanation by Gary Felder, brother of Raleigh Charter High School's Kenny Felder, of Bell's Theorem. It's quite nice but it ends this way:
Nonetheless, we have not explained our result. It's one thing to say the electrons must affect each other instantly, but you might still wonder how an electron here instantly knows what is happening millions of miles away. Moreover, in order to explain the results we got, we had to say that the measurement of one electron somehow changed the other one. Why should the electron, either one, be affected at all by my measuring it? My intent was simply to measure a property of the electron, not to change it. This result demonstrates one of the other strange results of modern physics, which is that the act of measuring a property always changes the system you are measuring. In this case the "system" apparently includes not only the electron you are measuring, but also the other one which isn't even there at the time. Physicists have been trying for over fifty years to understand these results, and there is no consensus on how to interpret them. There is clear agreement, however, that the results occur. Spooky action at a distance is part of nature.