Imagine a world where, if you and I had once met, my missing the bus to work would automatically make you late too. Or where, if I put on odd socks, yours would be odd too. A great excuse, maybe 鈥� but also deeply weird.
The classical world we live in isn鈥檛 like that. I do X and Y happens, and what Z is doing over there generally has little influence on that. But these clear relationships disappear when we enter the quantum world, the world of subatomic particles that are the building blocks of the universe 鈥� and encounter the phenomenon of entanglement.
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Read more: Cosmic uncertainty: Five universal truths that might be wrong
In physics, things are as they are 鈥� until they鈥檙e not. Here are five cosmic impossibilities that just might turn out to be true
Described by Einstein in 1935, this is a kind of particle telepathy that defies complete characterisation even today. Particles can become entangled when they interact, and once they do, no matter how far apart they are, measuring the properties of one automatically fixes the properties of the other 鈥� changes its socks, as it were.
Einstein decried this 鈥渟pooky action at a distance鈥�, yet many experiments have shown it is an essential ingredient of our world. 鈥淲ithout quantum entanglement, we could not have quantum theory as we know it, and quantum theory is the basis of chemistry, our semiconductor industry, even life,鈥� says Caslav Brukner of the Institute for Quantum Optics and Quantum Information in Vienna, Austria.
But here鈥檚 the really weird thing. There鈥檚 nothing stopping the quantum world having different levels of underlying correlation 鈥� largely uncorrelated worlds are possible within the broad sweep of the theory, as are ones that are far more connected. But only a universe with the exact level of weirdness that corresponds to entanglement produces the rich tapestry of phenomena, including life, that ours does.
So we probably shouldn鈥檛 wish for any level of weirdness other than our own 鈥� but it would still be nice to know why things are as they are. Finding out how would probably mean deriving quantum theory from underlying principles like the constant speed of light which is the foundation of Einstein鈥檚 relativity (see 鈥�Cosmic uncertainty: Is the speed of light really constant?鈥�). But the sheer universality of quantum theory makes this a far-off prospect, says Brukner. 鈥淚鈥檓 not even sure that this goal can be achieved.鈥�
鈥淥nly a universe with the exactly right level of weirdness produces life鈥�
According to quantum physicist Sandu Popescu of the University of Bristol, UK, we may have to accept that such questions are not physical, but philosophical. 鈥淲e can predict exactly what will happen, but to say why it happens, we don鈥檛 have a clue,鈥� he says. 鈥淚t happens because nature is quantum mechanical 鈥� that is probably the best answer you will ever get.鈥�
This article appeared in print under the headline 鈥淲hat if鈥� quantum weirdness were weirder?鈥�
