# Why does quantum entanglement not allow ftl communication

#### Ken G

Gold Member
Which makes the answer to the first question "only if Alice tells him her outcomes", and that makes the answer to the second "no."

#### DrChinese

Gold Member
Let's do a thought experiment.

...

q: Does Bob learn the direction Alice is measuring at?
q: Does this violate the no-cummunication theorem?
Welcome to PhysicsForums, realbart!

Bob does NOT learn anything from Alice. The "no-signaling" theorem remains intact.

As Heinera says, Alice must first send her results by traditional means (signals propagating at c) to Bob in order for Bob to prepare your graph.

What Bob sees is the same 50-50 statistical result at any angle setting he chooses. Ie no different than flipping coins.

#### realbart

The graph you have enclosed is a plot of the correlations between Alice's and Bob's measurement results. Bob (without knowing Alice's results) can not make this plot by himself.
You're absolutely right, Heinera. I expected to see the graph as function of the angle between the measurement and the Z-axis.

So let's say Alice will measure every particle spin along the Z-axis.
What will Bobs measurements be along the horizontal plane? (perpendicular to the Z-axis)

#### Heinera

You're absolutely right, Heinera. I expected to see the graph as function of the angle between the measurement and the Z-axis.

So let's say Alice will measure every particle spin along the Z-axis.
What will Bobs measurements be along the horizontal plane? (perpendicular to the Z-axis)
Bob's measurements will be completely random to him, no matter what axis he chooses. They will be "up" or "down" with 50/50 % probability.

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