Chapter 6 – Entanglement

What is entanglement?

We will start with a simple kindergarten-level idea:

• Put a red and blue ball in each box without looking at them
• Move the boxes far apart
• Look in one box and check the color of the ball
• We now know the color of the ball in the other box

This isn’t surprizing!

Quantum entanglement is both similar and also very different – the color of the ball in the box isn’t set until you look!

We can do a similar experiment with elementary particles. Imagine two particles being emitted that have opposite up/down spin. We can send them in different directions without knowing what spin each particle has.

The particles have wave functions giving the probabilities of measuring their spin states. If there were separate wave functions, they would look like this:

However, some combinations are not allowed because they need opposite spin to conserve momentum. If we see the particle on the left with spin up, we know the particle on the right must have spin down. The particles’ wave functions are entangled. There is a single wave function covering all four options with some combinations have 0% probability.

If we observe the left particle having spin up, the wave function collapses which changes the probabilities for the other particle.

Spooky action at a distance

Imagine taking one of the entangled particles and taking it to a galaxy far way without looking at it. Does looking at the one of earth instantly affect the other?

This ability for observing one particle having an immediate effect on the other troubled Einstein who called it “Spooky action at a distance.”

This does not break the rules of information not traveling faster than the speed of light. The observer on earth knows what will be seen on the other galaxy but no information is transferred. They can share results but that information flow is limited to speed of light

Entanglement is weird

The weird part of entanglement is the particle doesn’t have spin direction until it is observed and the observation affects both particles. But how? There are three options:

• Hidden variables – what gets observed was pre-determined and is carried somehow in hidden variables in the object.
• Instantaneous communication – observing one immediately affects the other
• Something else?

We will look at how the details of entanglement have been confirmed.