Question about quantum weirdness: Suppose you deflect one of a pair of electrons in correlated spin states ?

Question

so that it ends up on Mars. Then you measure the spin of the electron that stayed on Earth, thereby instantly altering (or "altering"?) the state of the electron on Mars. Does this illustrate "quantum weirdness" ? Why, or why not?

Answer 1

I am a casual reader of mostly lay media, rather than a physicist; yet I'll have a first go at your question.

Yes, it does represent quantum weirdness, or "spooky action at a distance" as Albert Einstein called it.  Where two particles which are  quantumly entangled interact, possibly instantly, across a distance, you thus have quantum weirdness.

As to the "Why or why not [does this represent quantum weirdness]" part of your question... I understand this effect is the very definition of "quantum weirdness".  

Now, if, by the second part of your question you mean HOW does quantum weirdness (quantum entanglement) occur, I have no real knowledge here; but I have wondered, as have many others, whether the entangled particles just SEEM to be spatially apart.  Kind of like what you'd find with a seemingly-spread-out BOSE-EINSTEIN CONDENSATE, and as some have wondered when considering the classic DUAL-SLIT EXPERIMENTS (Google those terms, if you have not yet read of them.) possibly the entangled particles are merely a single particle which appears, to us, a multiple.  "Space" as we know it would thus be an illusion, or, at least, real only to four-dimensional creatures like us.

The institution which experimentally resolves the riddle here - What is quantum entanglement? - will have led us into a major revolution in physics.  

Answer 2

Sure, it demonstrates some quantum weirdness.  It does demonstrate the 'spooky action at a distance', since the influence of your measurement on the electron on Mars is instantaneous, despite being so far away.

But there are other aspects of quantum mechanics that I find weirder.  For example, that a particle will behave differently if it is measured than it will if it is not, and somehow seems to know when it gets measured.  That is related to your situation, but really is a different aspect.  And, to me at least, much more weird.