Quantum Entanglement: Bohr-Einstein debates

 

 “The most bizarre, the most absurd, the most crazy prediction that quantum mechanics makes is, Entanglement.”

~ Walter Lewin, 2014 

Everyone of us taking some interest in quantum mechanics, must have heard of this perplexive phenomenon, “Entanglement”. What is it simply? Does it really happen? How does it help? Let’s dive right in to find out! Do checkout our previous introductory article on quantum entanglement for more learning!

What is Entanglement?

 

Entanglement is a theoretical prediction that comes from the equation of quantum mechanics! Any two particles can become entangled if they are close together and then their properties become linked. That is, any actions on one particle would not only affect that particle, but also counter-affect it’s entangled partner no matter how far they are! This makes sense when the particles are placed very close. But remarkably, quantum mechanics says that even if you separated those particles, sending them in opposite directions, they would still remain entangled, no matter how much the distance is between them and whether or not there is a direct medium connecting both the particles! How weird is it? It’s like you turn on your bluetooth in India and your brother’s airpods disconnects from his phone in America! (action and counter-action)

  

Two school of thoughts for Entanglement

 

Niels Bohr and Albert Einstein were two among the many significant scientists researching on quantum mechanics. While Bohr was confident in the mathematics of quantum mechanics and thus, laid out all the results of it, Einstein always believed that quantum mechanics lacked something that makes it incompetent to define the reality, unlike classical physics does. Let’s take a look at both of their perceptions of quantum mechanics.

Bohr - Einstein debates

 

Niels Bohr

Observation implies distinct action, is what Bohr emphasized on since the beginning. We have already talked about the role of “observation” in quantum mechanics. Any event or particle is a distribution of probabilities in the quantum world. And only when an observation attempt is made on the particle/event, it acquires a particular state! So, entanglement states that if you measure a particle here, not only would you affect it’s state (one out of the thousand possibilities), but your measurement would also affect the state of it’s entangled partner no matter how distant they are! There is no force, or no wires or anything connecting them! There’s no way they could communicate with each other. Sounds so unacceptable right? More like a philosophy rather than a science! But this is what Bohr excellently derived from the equations and maths of quantum mechanics!

 

Albert Einstein

This kind of weird long-range connection between particles sounded so absurd to Einstein that he called it spooky, spooky action at a distance! Amidst these situations, Einstein suggested that Entanglement would be possible, but not the way this is described as a spooky action at a distance. Let’s take an example to understand the ideology of Niels Bohr and Albert Einstein.

 

Examples

 

A pair of entangled gloves

 

Entangled particles can be like a pair of gloves. Imagine someone separates the two gloves and puts them one in each box. Then, he delivers one of the box to me, in India and the other to you, in Antarctica (well you would be a penguin that way then xD). So before I looked inside my box, I know it has either a left hand glove or a right hand glove. And the moment when I open my box, let’s say I found a left hand glove, then at that particular instant, I’m sure that the box in Antarctica would contain a right hand glove, even though you have not looked inside it yet!

There is nothing mysterious about this, says Einstein, because obviously by looking inside the box, I haven’t affected either of the glove boxes. My box always had a left hand glove and thus the box in Antarctica always had a right hand glove, since the moment they were separated! But Bohr would believe that the glove in my box was equally happening to be a left handed glove and a right handed glove. And only in the instant where I took the observation by opening the box, the glove resolved into a specific configuration. And thus, this affected the other glove box in Antarctica, making it to be it’s opposite configuration glove anyhow. 

 

Electron Spin

Einstein believed this exactly applied to particles as well. Whatever configuration the separated entangled particles are in, must have been fully determined from the moment they flew apart! Einstein and Bohr would often make their contradicting points regarding Entanglement! Electron can have two spins right; clockwise and counter-clockwise.

Imagine two entangled electrons are separated away very far. And then the spin of one of them is measured to be clockwise. At this instant, the spin of the other electron is bound to be counter-clockwise. So Einstein would say that this electron has a definite spin before you measure it. Bohr would ask how would you check that? Einstein says, measure it! You will find the definite spin! Bohr would nod, but its the act of measurement that brought the spin of the electron to a definite state. Boom, mindblowing right?

  

Who was right?

So, the important question, who between Bohr and Einstein was right regarding Entanglement?

Niels Bohr and Albert Einstein have both spent a good amount of time in researching quantum mechanics. They were two different school of thoughts. Einstein always believed that quantum mechanics is lacking something which doesn’t make it acceptable, and even persisted this thought till his demise in 1955! 

In 1967, John Clauser, a PhD student as a part for his academics, carried out an experiment to prove Entanglement and hence, the whole quantum mechanics wrong! He devised a machine that could measure thousands of pairs of entangled particles and compare their spins in different directions. Finally something to settle the debate between Bohr and Einstein! As the result started to come in, Clauser was surprised! All of the thousand pairs of entangled particles followed the rule of entanglement! Measuring spin of one of the particles in the device instantly affected the spin of the other particle! Thus, Clauser in an attempt to rule out quantum mechanics, proved that all the maths of quantum mechanics is correct! Entanglement is real!

So, Niels Bohr, who championed the equations, was absolutely right with all of his mathematics about quantum mechanics! Entanglement, is real. The one thing that Einstein thought was impossible, spooky action at a distance, actually happens :O

 

How does Entanglement help?

When was the last time you saw teleportation in movies? Ever thought if it was possible in real life? Well, Entanglement! Entanglement promises some progress in quantum teleportation. Apart from that, it also supports many fields like quantum computing, quantum cryptography, quantum dense coding etc. I would definitely take all of these interesting topics next, and share my thoughts on them! Stay tuned for the next one, hope you liked it :)

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To know the basics of the quantum world, astronomy and space exploration you can check out the book "Through the wormhole" on amazon kindle by Ratnadeep Das Choudhury.

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                                                                             -Beleswar Prasad Padhi

                                                             Chief Quantum-verse Writer

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