Why QM (or rather, nature) is weird, in lay terms.

[georgir]

If I show you three face down cards (normal playing cards, so can be either a red or black suit), and tell you that no matter which two you pick, they would be different colors, I have no doubt you'll say I'm being weird (to put it mildly).

Well, nature does pretty much the same thing. This is the simplest explanation of why nature is weird.
To be more precise, nature does not tell you the two cards will 100% be different, so it is not quite as weird as that example. But it tells you they will be different some quite high percentage none the less.

I won't spoil it immediately, see if you can decide for yourself what percentages you think are weird and what aren't.

To add an extra pinch of weirdness, nature prevents you from ever checking the third card in this wicked game. But that's just a minor bonus to the main weirdness above.

 

[georgir]

The highest chance that I can claim in the above card trick situation, without hiding anything up my sleeve and swapping cards while distracting you with weird claims, is 66%.
But the fact is, nature can give us some "cards" that get 75% in an equivalent setup. For me that is weird. The only way I can explain that to myself is to blame nature in hiding something up its sleeves and swapping cards faster than light. And we can not trick nature to use this swapping for any practical FTL communication, which is yet another pinch of "weird" to me.

Eventually, I guess we'll just have to accept this property of nature, and stop calling it "weird". It is just a fact of reality after all. But I think this is a good summary of why at first it seems weird.

 

[georgir]

And for anyone that thinks the analogy with cards is inaccurate in some way, a more precise one:
Instead of cards, I give you a large set of identical device pairs with three labelled buttons on each device. The first time a button on a device is pressed, it produces a binary output (i.e. light up blue or red). I claim that if you press the same button on the two devices in a pair, they will always produce the same output, and if you press different buttons on the two devices in a pair, they will produce different output from each other in 75% of the cases.
The only way you can explain these devices is by a FTL communication between the pair. No pre-programmed behavior local to each individual device can explain this. In reality, such devices may be constructed from entangled particle pairs.

I make no claims of having come up with this example BTW, it was used by Brian Greene to trick Mulder and Scully into doing a new X-Files movie ;)

 

[ddd123]

I don't know if the cards analogy is sound, but it is good as a metaphor for the feeling we get with Bell pairs phenomenology. I think Hardy's theorem (violation of local realism without inequalities) shows it best in the realm of gedanken experiments: http://www.quantum3000.narod.ru/papers/edu/cakes.pdf .

But in the thread "An abstract long-distance correlation experiment" Neumaier pointed out that if we, somehow (experiments notwithstanding), could revert back to seeing fields instead of particles, this wouldn't seem weird because some classical fields were instantaneous. At most, I would agree with steveandaryl on weirdness: he says "what's weird is the lack of an answer to some basic questions about QM, particular to the EPR experiment". The electromagnetic field of Maxwell propagates at the speed of light, for instance. So nature would be playing us "card tricks" with wobbly spread out stuff instead of particles, it's the same for me.