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View Full Version : Understanding quantum entanglement and non-locality.

hush36
2012-Nov-09, 07:47 AM
Yet another question !!Could somebody kindly explain why the following thought-experiment isn’t valid ? Send a spaceship to some far away destination (at non relativistic speeds), lets say close to a star that’s about to go supernova . On board are a vast supply of special “boxes” that contain entangled particles with, for example, spin ½ - their entangled partners are left in identical special boxes here on Earth, they too will have spin ½. . Each of the boxes is numbered progressively from 1 onwards. Part of the mission rules is that, once arrived at the star, at specific time intervals (which in turn are associated with relative boxes), the spacecraft is commanded to look at the star and assess whether or not it has gone supernova; depending on the answer it is then instructed to open or not one of the boxes. This is repeated until the stars goes supernova. For clarification : - Time interval # 1 – Take Special Box # 1- Look at star – Is it supernova ? If no, then move on to next time interval without opening the box- Time interval # 2 – Take Special Box # 2 - Look at star – Is it supernova ? If no, then move on to next time interval without opening the box and so on. Here on Earth, we open the boxes in sequence at specific times : - Time interval # 1 – Take Partner Special Box # 1 - Open it and measure the spin. If the value is as expected (in my case, spin ½ ) then the spacecraft did not open this box and therefore we can assume the star hasn’t gone supernova. And so on. When the star goes supernova, the spacecraft will take note, open the next box in sequence and measure the spin. Here’s the crux, as I understand non-locality and QE, simply taking that measurement will alter the spin not only of this particle but also of its partner here on Earth. Therefore, when that partner box is opened here on Earth, I will get a different result to what I’ve been getting so far which would lead me to the conclusion that the star had gone supernova. Now this star could in theory be so far away that the light-information travelling through space to reach us will take years but I just received instant FTL information thanks to QE. Surely this is wrong. Can you point out where ?Thanks.

John Mendenhall
2012-Nov-09, 02:17 PM
Yet another question !!Could somebody kindly explain why the following thought-experiment isn’t valid ? Send a spaceship to some far away destination (at non relativistic speeds), lets say close to a star that’s about to go supernova . On board are a vast supply of special “boxes” that contain entangled particles with, for example, spin ½ - their entangled partners are left in identical special boxes here on Earth, they too will have spin ½. . Each of the boxes is numbered progressively from 1 onwards. Part of the mission rules is that, once arrived at the star, at specific time intervals (which in turn are associated with relative boxes), the spacecraft is commanded to look at the star and assess whether or not it has gone supernova; depending on the answer it is then instructed to open or not one of the boxes. This is repeated until the stars goes supernova. For clarification : - Time interval # 1 – Take Special Box # 1- Look at star – Is it supernova ? If no, then move on to next time interval without opening the box- Time interval # 2 – Take Special Box # 2 - Look at star – Is it supernova ? If no, then move on to next time interval without opening the box and so on. Here on Earth, we open the boxes in sequence at specific times : - Time interval # 1 – Take Partner Special Box # 1 - Open it and measure the spin. If the value is as expected (in my case, spin ½ ) then the spacecraft did not open this box and therefore we can assume the star hasn’t gone supernova. And so on. When the star goes supernova, the spacecraft will take note, open the next box in sequence and measure the spin. Here’s the crux, as I understand non-locality and QE, simply taking that measurement will alter the spin not only of this particle but also of its partner here on Earth. Therefore, when that partner box is opened here on Earth, I will get a different result to what I’ve been getting so far which would lead me to the conclusion that the star had gone supernova. Now this star could in theory be so far away that the light-information travelling through space to reach us will take years but I just received instant FTL information thanks to QE. Surely this is wrong. Can you point out where ?Thanks.

Why do you think opening the box on the spaceship will alter the spin? You already know the spin from when the box was sealed.

More generally, information cannot be sent faster than the speed of light. Opening either box and measuring the spin fixes the other box. Unfortunately, it sends no information.