quantum entanglement - Why does measuring the spin of an entangled particle cause it to become unentangle?

I've been taught that measuring the spin of an entangled particle causes it to become unentangled, but not how or why, so I'd like to know by what process this occurs.

Is the cause known? If so, please explain why observation causes the particles to become unentangled. Thanks!

Answer

A situation like this already happens in classical physics.

Suppose that I write either "0" or "1" on two pieces of paper, the same thing on both pieces, and I put them into two separate envelopes. Then, if you don't know which thing I wrote, the contents of the two envelopes are "correlated": opening one tells you about the state of the other.

But once you've opened up one of the envelopes you then know what's inside both of them, so there's no remaining correlation between the envelopes. Making the observation has destroyed the correlation.

Now, quantum entanglement is different from classical correlation (Bell's Theorem shows that there are patterns of correlation that quantum mechanics can create but classical physics can't). However, the way in which quantum entanglement is destroyed upon making an observation is pretty much the same as the way that the classical correlation is destroyed when you open one of the envelopes.