Why can't quantum entanglement be used to send information faster than the speed of light?

Question

I never understood why quantum entanglement couldn't be used to send information. The usual answer is that knowing what a wave function collapsed to doesn't change the fact that it does so randomly, and thus cannot transmit information. But that's not what I mean. Why can't you use the fact that the waves collapse to send information? When I asked my undergraduate physics professor about it (a long time ago), he just gave me a weird look and told me it can't work. Still, I've never understood why that is, and I've always been curious.

To be specific, I'll link a YouTube video explaining the quantum eraser experiment. You only need to watch the first minute and a half to understand what I am talking about:

https://www.youtube.com/watch?v=sQfSm6o-KlQ

Using the experimental set up in the video, I do not see what would prevent someone at M1 from sending information via Morse code to someone at M2. Specifically, have switching M1 on and off play the role of the on-off tone for Morse code.

This would be sending information at a rate that is nearly instantaneous, only being limited by the rate at which the Morse code representation of the information can be encoded/decoded.

Am I missing something?

kumorifox · Accepted Answer

"Anyone who isn't shocked by quantum mechanics doesn't understand it." - Niels Bohr

A problem is that you don't get an interference pattern from just one particle. You need many particles to form interference patterns, be they electrons, photons, or other particles. In addition, you'd have to figure out a way to wipe the receiver of the previous message, which cannot be done at light speed, and you'd have to know the order in which the messages were sent, which also takes time to process. Finally, you'd have to be completely sure of which particle is the twin of another particle.

It's doable in theory; at least, I'd like to think so after watching that video. But practically, it would be fraught with problems.

Steve4Physics · Answer

Herre's something I wrote a while back:

"For illustration, suppose 2 coins, A and B, are entangled in some way so that if both are thrown (not necessarily at the same time) you get opposite results - if A comes up heads then B must come up tails and vice versa.

You now separate the two entangled coins Take A to the moon say and keep B on earth, but ensure they remain entangled.

If you toss A (say), it can come down heads or tails - BUT YOU CAN'T PREDICT WHICH. However If you toss B (at some time later) it will come down the opposite to whatever A was. It doesn't matter how far apart the coins are, you always get this result."

But if you deliberately try and force coin A to be heads, this destroys the entanglement and B is equally likely to be heads or tails.

Entanglement only works when the outcome of the first measurement you do is random. Trying to control the state of one of the entangled object will destroy the entanglement - so you can't control what information to send."

Anonymous · Answer

"Why can't quantum entanglement be used to send information faster than the speed of light?"

What you are missing is this:
- quantum entanglement is, I hand you either an ace of spades, or a joker.  Until one of us views his card, the two cards remain entangled.

How can you viewing your card, tell *me* anything?

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Why can't quantum entanglement be used to send information faster than the speed of light?

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