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What is quantum entanglement?

📓 The short answer

Quantum entanglement is a phenomenon in which two particles are linked together in such a way that the state of one particle is dependent on the state of the other, no matter how far apart they are.

📚 The long answer

Let's start with some vocabulary.

What do these words mean?

Quantum mechanics is a branch of physics that studies the tiniest building blocks of matter, like atoms and even smaller particles. In other words, it's the physics of outrageously tiny things.

Quantum superposition is a principle in quantum mechanics that states that a particle can exist in multiple states at the same time until the particle is observed or measured.

For a simple metaphor, imagine a spinning coin. It is both heads and tails until it is observed. Then, the coin is either heads or tails, but not both. Basically at the quantum level, particles exist in all possible states until an observation or measurement forces the particle to be a single state. (Seriously, just take a moment to consider how counterintuitive this is. Something is all conditions until we measure it and then it is one condition.)

How do non-entangled particles behave?

Now that we have an understanding of quantum superposition, consider what would happen if we observed the spin of two non-entangled particles. Let's continue to use the coin metaphor:

If you observe two different spinning coins, there's a 50-50 chance Coin A will be heads or tails and a separate 50-50 chance that Coin B will be heads or tails. The coins' heads or tails outcomes are not connected. This means mathematically there's a:

• 25% chance they're both heads
• 25% chance they're both tails
• 25% chance Coin A is heads and Coin B is tails
• 25% chance Coin A is tails and Coin B is heads

How do entangled particles behave?

With entangled particles, the state of one particle is dependent on the state of its entangled particle. The likelihood that a coin will be heads or tails when measured is still 50-50. But let's say Coins A and B are entangled. Now if you measure Coin A to be heads, Coin B would always be tails. And if you measured Coin B to be heads, Coin A would always be tails. The condition of one coin is now connected to the condition of its entangled coin.

The wildest part about quantum entanglement is that the phenomenon happens instantaneously and doesn't depend on the two particles being close to one another. They can be theoretically be lightyears apart and still be entangled.

Quantum entanglement is still quite a mystery and shows us that our classical physics principles cannot be reliably applied at the teeny tiny level of matter. Albert Einstein said it best when he called quantum entanglement "spooky action at a distance."

🎥 Recommended viewing

I have tried my best to provide an easy-to-digest overview of quantum entanglement, but I'm hardly doing the subject justice! If you're interested, I highly recommend watching this video to learn more about the weird science of quantum entanglement:

​Sources​

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Book of the Week

​Slaughterhouse-Five: Or the Children's Crusade, a Duty-Dance with Death by Kurt Vonnegut​

Kurt Vonnegut tells the story of Billy Pilgrim, a man who becomes "unstuck in time" and travels through different moments of his life, including his experience as a prisoner of war during World War II. Vonnegut's dark humor and non-linear narrative style challenge readers to think critically about the impact of war on individuals and society.

It's also one of my favorite books by one of my favorite authors. Please read this (or re-read it) so I have more people to geek out about Kurt Vonnegut with.

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