h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I´m pretty sure the answer is no, since it´s a rather obvious method and someone would have done it. So I guess the question is really, why not?

Think about the double slit experiment, done using entangled photons. One has polarization up-down, the other left-right. They go off in two different directions, after having been split up by a crystal. Photon number 1 encounters a double slit, one slit allowing left-right polarized light through, the other up-down polarized light through. Now photon 2 encounters a singe slit, that only allows left-right polarization through.
As I understand it, this set up means I now know which slit of the double slit photon 1 travels through. This means a detector behind the double slit would pick up a normal pattern. If I remove the single slit so I don´t know wether photon 2 is left-right or up-down, an interference pattern will be picked up by the detector behind the double slit, created by photon 1.
Now imagine each photon travelled a distance of several light years, before encountering anything. They went off in different directions, and are now 5 light years apart when they encounter the slits and detectors. Assuming entangled photons keep coming, should the person at location 1, not be able to find out by looking for an interference pattern, wether there is single slit over at location 2?
Then what if I kept removing and replacing the single slit at location 2, shouldn´t that mean over at location 1, the interference pattern would also disappear and reappear? Wouldn´t this allow me to send a series of essentially 1s and 0s between location 1 and 2 rather immediately? What am I missing?

I seem to remember the answer to this is yes, you can use it for faster-than-light communication, but I don't know the details.

The trouble is that if you can send messages faster than light you can - because of special relativity - also send messages backwards in time, which could cause some nasty paradoxes...

I also don't remember the answer though.

Stephen Hawking says that time travel does not introduce logical paradoxes - there may be odd things such as someone being their own grandparent, but nothing which is mathematically contradictory.

at what sort of distances has this entanglment been observed???

mills, inches, feet, meters, couple of hundred yards, Kms, miles??

what is the point of sending a message faster than light if it is only to the other side of the room???

There's no theoretical limit on the distance over which entanglement operates.

""no theoretical limit on the distance""

how far has it been SEEN to work??

I think a few metres along an optic fibre.

hardly a viable application at the moment then??

The really interesting applications of entanglement at the moment are in secure communications. A message is sent and you can tell if anybody has intercepted the message along the way.

I believe entanglement using particles has been done on distances of many miles.

But the faster-than-light aspect of it only really becomes relevant when you're talking in astronomical units.

I know, but the principle of it should still stand. We just haven´t had the opportunity to test in on larger scales yet.

do you have to have these 2 entangled particles at the same place and them move them apart or do they just entangle at random??

As far as I know, they would be entangled if they were created together. I don´t think you could entangle particles at a distance.

so you have to take one across the galaxy with you to your new colony, then you tap it with a morse key and the one on earth taps the receiver, if your communicator gets damaged you cant repair it you have to ship a new one all the way from earth, so to talk to other colonies you have to share a pair of comunicators, or relay through earth??

It would be very impractical certainly, but the point was if it could be done. Then that would seem to go against what Einstein said about faster-than-light communication.

as long as there was a central hub acting as an exchange you could talk to the entire galaxy(of colonys) in real time

I think you can only transmit 1 bit of information in each pair of tangled particles. You don't just keep 1 in a box and keep using it.

Indeed, only the Cat is kept in a box