Quantum Mechanics - the most successful description of all things tiny in the universe has some very weird results and implications. One central tenet is the fact that quantum events are not "actualized" until observed or interacted with in some way or the other... pretty much saying the universe exists in all possible states at one time until observed. Wacky shit.
Einstein despised this view of the nature of the universe therefore year after year during the 1920s and 1930s, he constructed elegant thought experiments that he felt could refute the quantum view of the world. Each thought experiment was eventually refuted - usually by Neils Bohr - until a famous paper from Einstein and 2 other collaborators - the EPR (Einstein-Rosen-Podolski) Paper - hit the presses. This "perfect" paper elaborated on a theoretical experiment and a set of arguments that posited the following (in simple terms)....
If quantum mechanics theory is correct about the nature of quantum events.... then 2 particles that are sourced from the same interaction (entangled) have a curious and "spooky" property... that any subsequent observations on the entangled properties of these particles are intimately linked and correlated. In other words, no matter how far these particles move from each other - a measurement of an entangled property will "force" the other particles entangled property to be disambiguated in a correlated sense - no matter how far they were from each other in space or time. Since Einstein's theory of special relativity disallows instantaneous communication across space - then this result is quite strange.
What transpired was probably Einstein's worst nightmare. He was right in the sense that Quantum Mechanics could not be complete without allowing this "spooky" non-locality based condition to exist. John Bell, a brilliant American scientist, produced a famous inequality and major clarifications of quantum mechanics theory that provided a gateway for experimentalists to create experiments sensitive enough to test entanglement.
Experiments subsequently showed that this "spooky" condition actually did exist - and this cemented Quantum Mechanics as the strangest, spookiest and most accurate description of reality ever constructed.
Einstein, a founder and the most fervent critic of quantum mechanics, never ceases to amaze me... even as he tries to disprove theories, he helped advance and discover new components - such as entanglement.
Interesting new technologies have been derived from quantum entanglement properties, such as quantum teleportation, quantum cryptography and quantum computation but besides these groundbreaking technologies - arguably quantum entanglement's greatest legacy is the insight it provides into the nature of our universe.
Quantum Entanglement review - Stanford link
EPR Paper - PDF link
47 minutes ago