physics

Why Quantum Entanglement Is Useless for Faster than Light Communication?

Imagine that you are Alice who has a box that is black on the outside. On the inside it can be either red or green, you don’t know. The probabilities are 1/2 for each. There is a button on the outside. By pressing it, you can change the inside color,  from green to red or vice versa. Pressing the button just flips the color, but doesn’t tell you what color it is. You may press as many time as you wish, but only while the box is closed. And you can also open it and see, what the actual color is.

Bob in a galaxy far, far away has exactly the same kind of a black box, except that his is red on the inside, if and only if yours is green. When you press the button both boxes change color. The same goes for his box.

The two interiors are entangled. Your boxes will always be of different colors on the inside, at least as long as quantum entanglement holds.

Opening one box means the end of the entanglement and you will see nothing more than the result of the last change.

Finally, Alice and Bob are in possession of 100 pairs of such boxes. Alice has one half of each  entangled pair, the other half belongs to Bob. Now Alice is going to send number 42 to Bob. It shouldn’t be a problem to code the number 42 using 100 bits, should it? When Bob opens all the boxes he will see the number 42 inscribed in a red-green pattern, influenced by Alice from a distance. The coding protocol has been agreed upon, long before.

Now, how could Alice code the number with those boxes? She can’t. She can switch any box’s interior color as she pleases and the corresponding box at Bob’s place will change accordingly. The problem is, Alice is blind. She doesn’t know, what the pattern inside her box collection is. She can change it, but she cannot see it. Peeking inside would kill the entanglement, so the pattern inside is unknown until the boxes are opened. It could be exactly 42 or any other number with equal probability. Alice can write, but she can’t know what she is writing.

Therefore, when Bob opens his box, he will see a mirror of what Alice has written. But it will be a random 100 bit number, with no particular correlation to the 42 she had in mind.

When writing, Alice is blind and clueless about what she is typing. Therefore, Bob will not get her message.

It’s the same story with bees and flowers and polarized photons. Alice can’t see and can’t write. So the spooky action at a distance is useless for communication. You can’t write your message, and even if you try you won’t be able to know what is written inside.

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2 thoughts on “Why Quantum Entanglement Is Useless for Faster than Light Communication?”

1. msjr says:

Thomas, thank you for this. It’s clear and understandable.

• Thank you.

All the fog in Quantum Mechanics is of this nature, I am sure. I’ll try to clear a few more cases here in the future.