A Long Class

I was in Quantum Mechanics from 9 AM till 10:15 AM and I am so glad that this class only meets 2 times a week.  Not so long ago we thought space and time were the absolute and unchanging scaffolding of the universe.  Then along came Albert Einstein, who showed that different observers can disagree about the length of objects and the timing of events.  His theory of relativity unified space and time into a single entity called space-time.  It meant the way we thought about the fabric of reality would never be the same again.  The EPR paradox of 1935 is an influential thought experiment in quantum mechanics with which Albert Einstein and his colleagues Nathan Rosen and Boris Podolsky “EPR” claimed to demonstrate that the wave function does not provide a complete description of physical reality, and hence that the Copenhagen interpretation (a collection of views about the meaning of quantum mechanics principally attributed to Niels Bohr and Werner Heisenberg) is unsatisfactory, so resolutions of the paradox have important implications for the interpretation of quantum mechanics.  Their article was entitled “Can Quantum Mechanical Description of Physical Reality Be Considered Complete?”

The essence of the paradox is that particles can interact in such a way that it is possible to measure both their position and their momentum more accurately than Heisenberg’s uncertainty principle allows, unless measuring one particle instantaneously affects the other to prevent it, which would involve information being transmitted faster than light as forbidden by the theory of relativity, so Einstein called this “spooky action at a distance”.  One of the strangest aspects of quantum physics is entanglement, a seemingly bizarre phenomenon that states if you can observe a particle in one place, and another particle, and even one that is light-years away, that it will instantly change its properties, as if the two are connected by a mysterious communication channel.  Scientists have observed this phenomenon in tiny objects such as atoms and electrons.  This consequence had not previously been noticed and seemed unreasonable at the time, but scientist know that quantum entanglement is a real principle, because the phenomenon passes test after test.  While EPR felt that the paradox showed that quantum theory was incomplete and should be extended with hidden variables, the usual modern resolution is to say that measuring one particle does instantaneously affect the other, but that this does not involve transmission of information.  The paper features a striking case where two quantum systems interact in such a way as to link both their spatial coordinates in a certain direction and also their linear momenta (in the same direction), even when the systems are widely separated in space.  As a result of this “entanglement”, determining either position or momentum for one system would fix (respectively) the position or the momentum of the other.  EPR prove a general lemma connecting such strict correlations between spatially separated systems to the possession of definite values.  On that basis they argue that one cannot maintain both an intuitive condition of local action and the completeness of the quantum description by means of the wave function.

God, I thought my head was going to explode while I was listening to that lecture.  I understand that in the quantum realm, quantum mechanical effects become significant, and particles can no longer be described using classical physics.  I knew about the wave function after reading about the Thomas Young double-slit experiment, and I knew that the photon is responsible for electromagnetic radiation, but I still couldn’t see why a pair of photons or electrons, would become entangled, and remain connected even when separated by vast distances.  I could see ballet or tango dancers becoming entangled, but this quantum entanglement connection between particles was entirely unexpected.  I know that electrons act as if they are spinning very rapidly, but they are so tiny, so there really isn’t much of anything to spin.  I feel that spin may actually be a part of the global conspiracy to keep quantum mechanics confusing.  My brain went into overload from taking in all of this new information, after my professor mentioned the positron, or the positive electron, a positively charged subatomic particle having the same mass and magnitude of charge as the electron and constituting the antiparticle of a negative electron.  Since the entangled particles are spinning, either up or down along a given axis, the pair are correlated, in such a way that if one particle’s spin is up, the other’s will be down (the spins may instead both be up or both be down, depending on how the experiment is designed, but there will always be a correlation).  If one of the particles (say the positron) is kept at the edge of the universe, as soon as you measure the spin of the other particle (electron), the spin of the particle at the edge of the universe becomes defined.  This gives the impression that the information between the two entangled particles is transmitted instantaneously or faster than light.

It is too late in the semester to drop this class, so I have to try and hang in there.  I am pretty sure that knowing how two particles interact and share their physical states for an instant is important, but I don’t think that this will ever get me a date with any girls, so I think I should concentrate on the things that are more important.  Walking across the campus, I began to wonder if I could create quantum entanglement, and I thought that if I met another student that she could be in one of two states, which would be indeterminate, but correlated.  If I was able to measure her individually, then I would get a random distribution of “0” and “1” answers, but if I repeated these measurements many times for many identically prepared pairs, I would find that the resulting lists of “0” and “1” measurements are identical.  The state of one of the two particles depends on the state of the other, and that correlation will hold even when they’re separated, and this may be how I can find true love.  What do you think?

Written for Reena’s Xploration Challenge 227, where we are to work with the photo above.