# Transferring information faster than the speed of light?

You perform a double slit experiment, detecting the photons at the slit, and storing, but not observing, that data on a memory stick. The pattern on the screen is also stored, but not observed.

Person A travels with the memory stick to New Zealand and person B travels to London with the unobserved screen. Person A then decides wheter to destroy the memory stick or to observe its data.

Person B then, as simultaneously as possible, observes the screen. As I understand it, as long as the memory stick still exists out there, observed yet or not, person B will see a particle pattern on the screen. But if person A destroys the data, person B will see an interference pattern. I suppose that when A makes his choice, B would instantly be able to see what that choice was when he observes the screen, no matter if the information about A:s choice has had time to reach B?

In that case there is a convenient way for us to transfer information faster than the speed of light, and we could start sending binary data instantly in this way over endless distances?

Sharath K

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mfb
Mentor
The photon detectors at the slits are observations, if they in principle allow to record the information. And the memory stick does an observation. You don't need humans.

B will see the same no matter what you do with the memory stick. Two single-slit patterns in this case. Or a double-slit pattern if you detectors were broken and didn't do anything.
Even if you make a magical memory stick (delayed choice quantum eraser), the pattern someone sees never depends on the choice of others.

Another way to see this: there are local interpretations of quantum mechanics. Those would not exist if a local description would be impossible.

mfb:

If the data is destroyed before you observe it, you get an interference pattern. Watch this:

Watch the clip From 12:16 and onwards for a couple of minutes

mfb
Mentor
mfb:

If the data is destroyed before you observe it, you get an interference pattern. Watch this:
But you do not destroy the data before it is observed. Storing it on the memory stick is an observation already. The memory stick would have to be some clever quantum memory in order to avoid observations. But even then the idea does not work.

This has been discussed thousands of times, and the Wikipedia article I linked to is a proper realization of what you want to do. No superluminal signal transfer.

By the way: I studied physics, I don't need youtube videos attempting to describe it to laypeople, thanks.

berkeman
I apologize. I got fooled by that clip