Okay, anyway, so the conclusion from this observation is that something has to be at both slits in order to produce interference. And the reason we know that is because we don’t actually need both of these detectors, one of them is enough, because if we have one detector and it fails to register that we know the particle went through the other slit. Now, that particle going through the other slit, it never interacted with anything, the particle never interacted with anything. But because it allows us to know where the particle was even though we didn’t actually measure it, that’s enough to destroy the interference. And so this particle that’s over here must somehow have known that we were looking over here even though the particle itself wasn’t there. So, something must have been there to be able to tell that we had a detector here, but we don’t know what that is.

Now, it turns out, this is again, this is a universal property of quantum mechanics. It holds for any kind of particle — in practice, that means photons and electrons because that’s all there is in this universe unless you start getting into nuclear physics. And any kind of measurement, and any kind two-slit experiment, any experiment where you provide two different paths for the particle to potentially go down and bring it back together without knowing which way it actually went will produce interference and any modification that you make that lets you figure you out where it went will destroy that interference.

Now, this is still not intractably weird because we can still tell a reasonable story about why this might happen. So, maybe measurement does something to this system. These are after all very small particles and very delicate systems and so maybe it’s just physically impossible to make a measurement without disturbing the system in a way that is the cause of the destruction of this interference. Maybe the wave function collapses and becomes a particle somehow. This is the famous *Copenhagen Interpretation* of quantum mechanics.

But it turns out that we can rule out that possibility as well. And the way we rule out that possibility is by asking: ** How and when does this collapse, this purported collapse, happen?** Collapse has a number of features that ought to make us very suspicious of it just at priority without even doing any experiments. It’s a discontinuous and non-reversible phenomenon, that once you know that a particle has gone through one slit or the other, you can’t go roll back time and undo that.

And if you look at the mathematics of quantum mechanics, which we’ll get to later, there’s nothing in the math that’s discontinuous. And more than that, all of the math is actually time reversible. So we can make a — hypothesize that this collapse happens, but this is fundamentally at odds with the mathematics of what quantum mechanics – of how quantum mechanics says that our universe works. So we can actually do better than that.

We can actually do an experiment to show that collapse, if it happens, is a much subtler phenomenon than it would’ve first appeared and this is the famous — this is the quantum mystery number two, the famous quantum eraser. Now, this is a two-slit experiment that I have now reduced to something more abstract. So we have some particle source, this can be photons or electrons. We have some abstract way of splitting up particles so that it has two different paths to go down and some abstract way of recombining that particle so that both of those paths end up in the same place so that we get interference.

And here notionally, we have one detector at one of these dark fringes and another one at a bright fringe so that if we introduce some kind of an abstract measurement on one of these branches, then the interference pattern fusses out, that we now have the path, the amount of light, at each detector. So now we don’t have fringes anymore, we have the spread-out pattern that we saw under the single slit version of the experiment.

Like I said, there are lots of different ways that you can do this — actually, let me go on to the next one. So it turns out that you can erase this, that there are physical ways that if this measurement, certain kinds of proto measurements, that you can do here, you can then go back and erase after the fact and restore the interference.

And here’s a concrete example of that. If we polarize light and — I’m actually going to show you this in just a second, so bear with me if you don’t understand what I’m about to say, you use polarized light and you do the measurement by rotating that light 90 degrees and then erase it by filtering it 45 degrees, that is an actual concrete example of a quantum eraser. And I can’t show you the interference part of it but I can show you the eraser part.

So what I have here is some Polaroid film, this is the same stuff that you find in polarized sunglasses. And, I first want to convince you, has anyone not played with this stuff before? Okay, so again, real quick. If the axis of the film are aligned then you can see through it. Can everybody see through this? And if I rotate it at 90 degrees, then you can’t see through it anymore and that effect is independent of the absolute orientation of the film. So the light that’s going through to your eyes starts out unpolarized over here, it gets — passes through this film and becomes polarized, let’s say in this direction. And I can demonstrate then that it has become polarized in that direction by filtering it out using a filter at 90 degrees.

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