Entanglement

Sometimes I encounter books that fundamentally alter my conception of the world. It does not happen very often but when it does, it almost feels like I am in a "Matrix" of sorts, like I've lived my entire life believing something only for the book to reveal how wrong, or sometimes ignorant, I have been.

When my dad gave me On the Edge of Infinity, by German physicist and philosopher Stefan Klein, I never thought it would join the "books to fundamentally change Isabella's world view" club. My dad loves poetry (I don't) and the first chapter is titled "The Poetry of Reality" so I thought the book would be some sort of lyrical musing on the universe and left it on my to-read list for a while.

Klein does write lyrically (the full title of the book is On the Edge of Infinity: Encounters with the Beauty of the Universe) but he also does an exceptional job at taking readers on a journey through thought-provoking theories and ideas about physics and the universe while making them accessible. One of the most fascinating concepts he introduced me to was quantum entanglement.

Quantum Entanglement

In 1982, scientist Alain Aspect conducted a famous experiment in which he found that two distinct particles can share a mysterious and seemingly inextricable internal connection, such that whatever happens to one particle immediately affects the other, regardless of the distance between the two.

In his experiment, Aspect studied photons (the basic units of light) and released, from the same atom, one couple of photons at a time. The photons got separated from each other and traveled a distance of 6 meters. Aspect then proceeded to check a fundamental property (e.g. momentum, position, polarization, spin) in one of the photons. To his surprise, he found that the second photon would always have the same property as the first one, no matter what property he chose.

Once two particles become entangled, by observing one of the particles you will instantly know the properties of the other. Given the generally chaotic nature of quantum physics, this is really strange. It's as if person A and person B had an encounter, went their separate ways, and then you could accurately tell what person B was doing just by observing person A. Is person A sleeping? Yes, then so is person B. Is person A singing? No, then person B isn't either. But as soon as person A starts to sing, you know that person B is singing too. The connection is so mysterious that Einstein dubbed it "spooky action at a distance".

Aspect's experiment has been repeated multiple times, using longer distances (a photon in one experiment traveled almost 150 km) and bigger particles (Oxford scientists entangled two diamonds the size of a fingernail), and the results have not changed. It seems that two particles that become entangled will always remain intrinsically connected to each other. Why and how does this happen? Nobody knows yet. This "shared destiny" cannot happen by chance (probabilistically, it should happen only 50% of the times). Moreover, any exchange of information between the two particles has been ruled out because the information would have to travel as fast or faster than light which, as far as we know, is impossible.

Moreover, entanglement isn't just some obscure quantum physics concept, it happens everywhere around us. We just never notice it because it is ubiquitous. Entanglement could also potentially have real life applications. Just this summer, researchers at DTU Fotonik successfully teleported information between two microchips using quantum entanglement between two photons.

This raises so many questions. One of the first that comes to mind is, could there be internal connections between particles that are independent from time and space? And if yes, is reality really as we perceive it? Are we completely missing some important structure/property/connection among particles (and among ourselves, since we are made of particles)? And, as Klein suggests, given that entangled objects behave as if there were no space separating them, are "near" and "far" just an illusion? Is space irrelevant at a deeper level of reality?

It is so daunting and humbling to think that, despite all our technological advances, the world around us continues to astonish us and remains such a big mystery.