Surely you are joking Mr. Augustine!
[Subjective paradoxes and phoney entanglements]
Let's get some facts straight baby! I thought it's a pretty good idea to start with the Einstein, podolsky, and Rosen or the EPR paradox. Very soon you'll get an insight of the problem which I wish to address eventually. I'll try to keep this very non - technical. Whether I can do a good job or not - well, right now my guess is as good as yours. Heisenberg's uncertainty principle in Quantum Mechanics(QM) tells us that we cannot measure a particle's velocity and its exact location in space simultaneously. Instead, the best we can do is predict the probability that one of the particles is at any chosen location.But there are well - known physical processes whereby two particles emerge from a common location with properties that are related. For example, if an initial single particle should disintegrate into two particles of equal mass that fly off "back to back", something that is common in the realm of subatomic particle physics, the velocities of the two constituents will be equal and opposite. Moreover, the positions of the two constituent particles will also be closely related, and for simplicity the particles can be thought of as always being equidistant from their common origin. Thus while QM does not give definitive answers regarding particle speeds or positions, it does, in certain situations, give definitive statements regarding the relationships between the particle speeds and positions.
EPR sought to exploit these relationships to show that each of the particles actually has a definite position and a definite velocity at every given instant of time. Here's how: imagine you measure the position of the right moving particle and in this way learn, indirectly, the position of the left moving particle. EPR reasoned that nothing in your act of measuring the right moving particle could possibly have an effect on the left moving particle, because they are separate and distant entities. The left moving particle is totally oblivious to what you have done or could have done to the right moving particle. Note that the particles may be seperated by a distance of meters, kilometers or light - years apart when you carry out the measurements. Based on a similar logic EPR cleverly pointed out that you could have chosen instead to measure the velocity of the right moving particle. In that case you'd have determined the velocity of the left moving particle, albeit indirectly. Putting both together EPR concluded that the left moving particle has a definite position and a definite velocity at any given moment. Of course, this whole discussion could be carried out interchanging the roles of the left and right moving particles. This leads to the conclusion that both particles have definite positions and speeds. However there is still no conflict with the uncertainty principle. EPR realised that thay cannot identify both the location and velocity of any given particle. But, even without determining both the position and velocity of either particle, EPR's reasoning shows that each has a definite position and velocity.Reality, EPR maintained, was more than the readings on detectors. Thus, EPR concluded that QM is an incomplete description of reality. The core of the EPR argument is that an object over there does not care about what you do to an object over here.
We had to wait till the entry of John Bell (picture - right) into the picture to resolve the EPR paradox. The generalisation of which is known as the "Bell's Theorem". It is well known that calcium atom, as it returns to its less energetic state, will emit two photons, travelling back to back, whose spins are perfectly correlated. Bell found that there is a bona fide, testable consequence associated with a particle having definite spin values. QM tells us that we cannot simultaneously measure the spin of a particle with respect to more than a single axis. So we choose three axes oriented at three different angles in space. Then we want to measure the spins as detected by the detectors placed at these three locations - that is, up or down. Here's the experiment. Two detectors were placed 13 meters apart and a container of energetic calcium atoms was placed midway between them. Whenever the detector settings are the same, the two photons are measured to have spins that are perfectly aligned. If lights were hooked to the detectors to flash red in response to clockwise spin and blue in response to a counterclockwise spin, the incoming photons would cause the detectors to flash the same color. How will we make sense of the measurements now? Imagine I and Dr. have a box each with three doors. There's a kind of magic flash light inside the box which flashes only red and blue. Now I and Dr. try to open the doors of our respective boxes. Five (minimum) out of nine times we both agree on the flashes we observe if the respective doors are programmed blue, blue and red or red, red and blue - which is more than 50%. Every other combination of the program would give us a percentile more than the previous. This is the same situation we have at hand for the experiment. The experiment examined data from a large number of runs of the experiment - data in which the left and right detector settings were not always the same but, rather, were randomly and independently varied from run to run - the detectors did not agree more than 50% of the time!The EPR paradox required more than 50%. What went wrong with EPR? Their assumptions! The results of the experiment concludes that an object over there does care about what you do to another object over here. This is quite contrary to EPR assumption. QM shows that particles randomly acquire this or that property when measured, we learn that the randomness can be linked across space. Pairs of appropriately prepared particles called entangled particles - don't acquire their measured properties independently. in other words, the universe is not local. The outcome of what you do at one place can be linked with what happens at another place, even if nothing travels between the two locations. This unique correlation is called "entanglement".
EPR sought to exploit these relationships to show that each of the particles actually has a definite position and a definite velocity at every given instant of time. Here's how: imagine you measure the position of the right moving particle and in this way learn, indirectly, the position of the left moving particle. EPR reasoned that nothing in your act of measuring the right moving particle could possibly have an effect on the left moving particle, because they are separate and distant entities. The left moving particle is totally oblivious to what you have done or could have done to the right moving particle. Note that the particles may be seperated by a distance of meters, kilometers or light - years apart when you carry out the measurements. Based on a similar logic EPR cleverly pointed out that you could have chosen instead to measure the velocity of the right moving particle. In that case you'd have determined the velocity of the left moving particle, albeit indirectly. Putting both together EPR concluded that the left moving particle has a definite position and a definite velocity at any given moment. Of course, this whole discussion could be carried out interchanging the roles of the left and right moving particles. This leads to the conclusion that both particles have definite positions and speeds. However there is still no conflict with the uncertainty principle. EPR realised that thay cannot identify both the location and velocity of any given particle. But, even without determining both the position and velocity of either particle, EPR's reasoning shows that each has a definite position and velocity.Reality, EPR maintained, was more than the readings on detectors. Thus, EPR concluded that QM is an incomplete description of reality. The core of the EPR argument is that an object over there does not care about what you do to an object over here.
We had to wait till the entry of John Bell (picture - right) into the picture to resolve the EPR paradox. The generalisation of which is known as the "Bell's Theorem". It is well known that calcium atom, as it returns to its less energetic state, will emit two photons, travelling back to back, whose spins are perfectly correlated. Bell found that there is a bona fide, testable consequence associated with a particle having definite spin values. QM tells us that we cannot simultaneously measure the spin of a particle with respect to more than a single axis. So we choose three axes oriented at three different angles in space. Then we want to measure the spins as detected by the detectors placed at these three locations - that is, up or down. Here's the experiment. Two detectors were placed 13 meters apart and a container of energetic calcium atoms was placed midway between them. Whenever the detector settings are the same, the two photons are measured to have spins that are perfectly aligned. If lights were hooked to the detectors to flash red in response to clockwise spin and blue in response to a counterclockwise spin, the incoming photons would cause the detectors to flash the same color. How will we make sense of the measurements now? Imagine I and Dr. have a box each with three doors. There's a kind of magic flash light inside the box which flashes only red and blue. Now I and Dr. try to open the doors of our respective boxes. Five (minimum) out of nine times we both agree on the flashes we observe if the respective doors are programmed blue, blue and red or red, red and blue - which is more than 50%. Every other combination of the program would give us a percentile more than the previous. This is the same situation we have at hand for the experiment. The experiment examined data from a large number of runs of the experiment - data in which the left and right detector settings were not always the same but, rather, were randomly and independently varied from run to run - the detectors did not agree more than 50% of the time!The EPR paradox required more than 50%. What went wrong with EPR? Their assumptions! The results of the experiment concludes that an object over there does care about what you do to another object over here. This is quite contrary to EPR assumption. QM shows that particles randomly acquire this or that property when measured, we learn that the randomness can be linked across space. Pairs of appropriately prepared particles called entangled particles - don't acquire their measured properties independently. in other words, the universe is not local. The outcome of what you do at one place can be linked with what happens at another place, even if nothing travels between the two locations. This unique correlation is called "entanglement".
Now I'll come clean. This week's colloquium at the Physics department had a speaker who redefined entanglement. That was real "spooky" and not to mention "nonsense". I don't know why people didn't ask any good questions. The one question we had was spookier than the talk itself. My criticisms are very often ruthless and impolite. That's the reason why I refrain from asking questions, more often totally avoiding the colloquiums. This was a very busy week though. I was treated to some good vegetarian south indian food by Lakshmi over the weekend. It's almost three years since I had more than three dishes during a single meal and what can I say - was elated. I am from Kerala, which is a state, down south of India. The dinner reminded me of Kerala, filled with palm groves and where every meal has some form of coconut in it. Not only that - Kerala has other feathers to its cap also; it's the first place in the world to have a democratically elected communist government way back in 1958; people committing suicide on antisuicide day; record number of people getting killed in road accidents during road safety week and the list goes on. Wellesley invited me for the party once again. I think I've written too much.
posted by Biju R. Patla at 1:44 AM
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