Sympathetic Twins

[Stephen]

Quantum teleportation?

Sent from my HD7 T9292 using Board Express

[JulianO]

Quantum teleportation?

... doesn't allow information to be sent faster than the speed of light.

There is a speed limit to the universe, and its only massless particles that can reach this.

[Superewza]

... doesn't allow information to be sent faster than the speed of light.

There is a speed limit to the universe, and its only massless particles that can reach this.

They might have meant quantum entanglement. So you would get an entangled pair, keep one on Earth, take the other one on a ship and travel 15 billion light years away (). Then when you do something to one of them the other one does the same at exactly the same time.

[JulianO]

They might have meant quantum entanglement. So you would get an entangled pair, keep one on Earth, take the other one on a ship and travel 15 billion light years away (). Then when you do something to one of them the other one does the same at exactly the same time.

Well - I must admit to not fully understanding quantum entanglement, but it is my understanding that there isn't really any information transmitted in this system that travels faster than the speed of light. You can't do something to one of the particles, only discover what state it is in, which instantly collapses the state of its entangled partner into the opposite state - no matter the distance. You can't use this to transmit information though.

Also - you have to get the particle 15 billion light years away in the first place, which will take its own sweet time!

[toml42]

The key point with quantum entanglement is you don't know if your partner on the other side of the universe has done their experiment yet, you don't get a little flashing light saying "my wavefunction has collapsed!". You know you're either going to get result 'a' or result 'b', there is no additional information you can detect - it is only when you meet up with your friend later to exchange results that the entanglement becomes obvious.

You might think that if you do your experiment first, and get 'a', you are transmitting some information to your friend that is forcing them to get 'b'.

The problem you run into is that when you factor in relativity there are situations where you will both think you did the experiment first - and you will both be right! When there is no privileged frame of reference, it is impossible to tell who went first and thus 'transmitted the information' and who 'received' it.

Relativity

Causality

Faster than light communication

you can only choose two of the above...

[Superewza]

The thing with it being only in either 'state a' or 'state b' is that we've had a lot of practice with communicating like that. Morse code? The Binary Numeral System? Computing?

[toml42]

The thing with it being only in either 'state a' or 'state b' is that we've had a lot of practice with communicating like that. Morse code? The Binary Numeral System? Computing?

state 'a' or 'b' is selected in quantum mechanics entirely at random and you have no way of influencing the outcome one way or the other.

Unless our current formulation of quantum mechanics is completely wrong, it cannot be used to transmit information - it would be like trying to communicate by flipping a coin

[Superewza]

Oh i see, so is it a case of there being a law that says it must be random, or simply that we don't know what influences the changes?

[toml42]

All of the statistical evidence so far points to true randomness (look up bells inequality). It is at least conceivable that some variant of the "hidden variables" theory could exist that is true... from where we stand right now though, it looks very unlikely that it is anything but random

edit: this is the best explanation of bells inequality and its implications i've come across, much clearer than the wikipedia article, i think

http://www.upscale.utoronto.ca/PVB/Harrison/BellsTheorem/BellsTheorem.html