Monday, November 4, 2013

Quantum Mechanics

Just watched for about the third or fourth time the third DVD in the Fabric of the Cosmos series that was on PBS and done by Brian Greene along his book,  This series is similar to that which was done by PBS on Greene's book The Elegant Universe which is on string theory.  The third program is on quantum mechanics and I thought I'd write some comments about the program and general comments about quantum mechanics.

I wish I could write better.

I have been exposed to quantum mechanics more than the random person im pretty sure.  First off as I used to say to my students in an undergraduate physics course on quantum mechanics (QM), no one really understands quantum mechanics, if they tell you that they do understand it, they really don't understand it.  This is one of the really big questions in physics.  What does quantum mechanics really mean?  There are people both physicists, mathematicians, philosophers and others who study this question.  In an international meeting of such folks a survey was done on which interpretation  of QM these people believe. There was a variety of opinions, with the most folks (42%) there agreeing with the Copenhagen interpretation (that taught to most undergraduates). The Many Worlds interpretation I think got around 20%.  Plus there are other interpretations.  All of modern physics is based on quantum mechanics and relativity. Yet we really don't understand OM on a fundamental level, think Schrodinger's cat..  You really don't hear much discussion on the fundamental understanding of relativity.

Big problem.

The interesting thing about QM is that you can calculate a large variety of phenomenon from it and in all instances it agrees with experiment.  Every experiment, down to the last decimal point in the measurement that the experimental uncertainties allow.  That is amazing.  I used to tell students that I have read and heard in a variety of places about QM, just calculate, don't think about the deep meaning, just calculate. Maybe this is similar to the situation in the late 1600's with Newton and his law gravity.  As far as I know Newton was never happy about gravity in that he didn't understand how the force acted without the two bodies touching.  He had a problem with action at a distance.  I don't know if they called it that back in his time, but the idea of how gravity worked bothered him.  But still just go ahead and calculate with his Universal Law of Gravity and his three laws of motion and you have classical mechanics which explains almost all motion in our everyday day experience.  That is incredible if you really think about it.  I hope that's why he was Knighted.  Classical mechanics is now the first subject taught in introductory physics classes in college.  It is the subject of a year long class for physics students at the undergraduate level.  In my time it was also a year long class at the graduate level, I don't know if it still is that way now.  But still Newton didn't understand why gravity worked.  Commenting on his understanding of his laws of motion could be another post.

Einstein never liked QM and is suppose to have said that "God does not play dice with the universe".   The wave function is the all important thing that you calculate in the Schrodinger equation.  The wave function was describing a wave of probability and Einstein didn't like that physics now was based on probabilities.  I'm wondering however, in his time didn't statistical mechanics also have probabilities?  Quantum mechanics is the physics describing the atomic and smaller world.  Applying classical mechanics didn't work in that realm. Up until QM when physics calculated  quantities such as position and velocity of a particle the results were exact  Not probabilities.  Classical physics calculated exact positions not probable positions.  Einstein didn't like QM.  He thought that something was wrong with QM.   In 1935 he and two other guys (a nicer way would be to say collaborators), Podolsky and Rosen published a paper known as EPR on what they thought were problems with QM.  I don't know all the details about such paper.  John Bell published a paper (need to look up when) looking at QM, in which we have something called Bell's inequalities (which I really don't know much about).

An interesting phenomenon in QM is the concept of entanglement which Einstein called spooky action at a distance.  Remember Newton never liked the idea of action at a distance his law of gravity implied.    The NOVA program then talked about implications of entanglement like teleportation, which has been done in the lab. I've read about such stuff years ago and really didn't understand what they were doing. More on this subject later.  Especially,when a person is teleported using quantum entanglement would that person have the same consciousness?.

A question about QM is the measurement problem.  What really happens when we measure something about say an individual electron in the laboratory.  The collapse of the wave function?  What does the Many World interpretation of QM say about this type of measurement?  What does the Path Integral approach to QM have to say? Are the Many World interpretation and the Path Integral approach similar?

Quantum Mechanics, the stuff that you can lie in bed late at night and wonder about. A tad different than: head hits pillow, pillow hits head.

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