Don’t be frightened off by the title here. Quantum entanglement. I’m not going to get too deep into the science, which is beyond me. And don’t come here looking for the answers to your physics homework either.
I saw a few lines in an article that just mentioned that the “EPR paradox” came from a thought experiment that revealed a dichotomy that physical reality as described by quantum mechanics is incomplete. That sounds like big news. This was in 1935 and it was an early and influential critique against quantum mechanics.
Albert Einstein, Boris Podolsky and Nathan Rosen (known collectively as EPR) were not early fans of this new view of the universe. They designed a thought experiment – a term I have always liked – which revealed that the accepted formulation of quantum mechanics had a consequence which had not previously been noticed, but which looked unreasonable at the time.
That phenomenon is now known as quantum entanglement. That is when two particles act together in an entangled system where they behave like one object even though they are physically apart.
This suggests that space is just the construct that gives the illusion that there are separate objects. Kind of shocking to consider.
Scientists have repeatedly conducted experiments using particles like electrons or photons that interact physically together as one, but then when separated, they behave as if they are still together.
Does it mean the universe is an entangled system and everything in it is interconnected?
Back in the 1930s, Einstein and the guys had an explanation provided by Heisenberg’s uncertainty principle. That is one of those few bits of physics (like E = mc2) that almost everyone knows, even if it is just from watching The Big Bang Theory. Heisenberg explained this as a disturbance caused by measurement.
Like Einstein, Schrödinger – the one who is also famous in a pop way because of his cat – was dissatisfied with the concept of entanglement, because it seemed to violate the speed limit on the transmission of information implicit in the theory of relativity.
Einstein later said that this entanglement was “spukhafte Fernwirkung” which translates into another phrase that I like: “spooky action at a distance.”
This all explains things like your coffee cooling and things breaking and stars fizzling out eventually. Things in our universe are destined to degrade into that boring state of thermal equilibrium.
Back in 1927, astronomer-philosopher Sir Arthur Eddington theorized that the gradual dispersal of energy in the universe was evidence of – another phrase I like to think about – an irreversible “arrow of time.”
But physicists are not happy that the arrow of time does not seem to follow from the underlying laws of physics. Those laws work the same going forward in time as in reverse and following those laws you would be able to reverse the paths of all the particles in the universe. What then? That cold cup of coffee would spontaneously heat up. Broken things would reassemble. Sunlight would head back into the sun.
Scientists in these areas can wonder about why we can remember the past but not the future. That goes against time’s arrow too. I see you for the first time and my brain becomes correlated with you through the photons that reach my eyes. Now I can remember you.
In 1935, EPR published the article “Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?” and poor Dr. Einstein struggled to the end of his life looking for a theory that could better comply with his idea of causality.
That original EPR paradox challenges the prediction of quantum mechanics that it is impossible to know both the position and the momentum of a quantum particle.
Perhaps it is just impossible to know it all.