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Einstein vs. Bohr

Einstein called the cosmological constant his “greatest blunder.” Einstein was wrong. The cosmological constant was a neat idea for General Relativity that’s still important today, and General Relativity was, IMO, his greatest accomplishment. The idea that space and time are curved by matter and energy, and that curvature is what makes gravitational force is profound and beautiful, and profoundly affects the way I look at everything that involves Gravity. 

                 But Einstein had his blunders, oh yes. The big thing Einstein was wrong about? Quantum mechanics. One of Einstein’s more memorable quotes was this: “God does not play dice with the Universe.”

       What was this in reference to? In good ol’ regular, classical physics (including General Relativity), if you know all the initial conditions of your system and you know the laws of physics, you can figure out exactly what’s going to happen. In quantum mechanics, though, if you know the initial conditions and you know the laws of physics, you can figure out the probability of various outcomes happening, but you can never know which one will definitely occur until after it’s over. Einstein didn’t believe it, and held a series of great debates with Neils Bohr over the issue. 

        But this isn’t Lincoln-Douglas. This is physics. You want to settle something? You do it with an experiment. So Einstein (and his grad student, Nathan Rosen) tried to show that the Universe had to be deterministic. Their hope was that there are variables that we just haven’t seen yet that determine what’s going to happen. It doesn’t, and there’s now a theorem that tells us why. So Bohr was right, and Einstein was wrong. The Universe isn’t deterministic, not even according to the laws of physics. 

       But this is abstract. Let’s give you a concrete example of an experiment that you can do (well, in principle) to help you better understand this. Imagine I’ve got a big screen with two narrow slits that are very close together. And I’ve got a Cathode Ray Tube that shoots out electrons. If I leave both slits open and shoot a whole myriad of electrons, the electrons go through and act like waves. 

          They interfere with one another, and produce a nice pattern where they have constructive interference (where lots of electrons land) and destructive interference (where no electrons land). You can keep track of where the electrons land over time, and here’s what you see when you add it all up. 

              Cover either slit up, and the interference pattern goes away. So it needs two slits. What about electrons? What if you fire them one-at-a-time? Sure, electrons can interfere with other electrons. But, can one electron interfere with itself? What do we see if we shoot the electrons through the double slit experiment one at a time? Well, it takes a long time to get enough electrons to see, but here’s what the results are: 

               Amazing. The electron must be interfering with itself! How does it know where to go? And how do you determine which slit it went through? 

              Now, here’s where things get interesting. You can set up some light sensors on each of the slits to figure out which one the electrons go through. When the electron passes through the slit, if a photon (a particle of light) hits the electron, you know which slit it goes through. If a photon doesn’t hit it, you don’t know. 

              Here’s the crazy part: if you hit the electron with a photon, the interference pattern goes away. You force it to go through only one slit, and you just get two bumps on your screen, one for each of the two slits. If you don’t hit the electron, though, the electron does interfere with itself, and you get the interference pattern above. 

            If you look, and you try to know, you will destroy the quantum mechanical effects. If you don’t look, though, God plays dice while your back is turned. 

          It’s messed up. And it’s awesome. Was Einstein wrong? About quantum mechanics, yes. Yes he was. And that, my friends, is what Einstein’s greatest blunder really was. Einstein never accepted quantum mechanics, never accepted that this is the way the Universe works. If you can accept and understand this, then at least about this one thing, you’ll have taken a step that Einstein never did.

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