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O.K. this will be the last one on quantum physics, at least for a good long while. It makes my head hurt. Next time, something that’s bendy without bending to the breaking point.

Right, so here’s a video on today’s topic:

Remember Mind-Bender #2, about Schrödinger’s Cat, the thought experiment designed to prove that the Quantum Mafia was off its collective gourd? Here’s a link to that discussion – if you haven’t read it yet, you’ll love it, trust me, there’s nothing as entertaining as reading a fumbling attempt to grapple in lay terms with the nuances of Quantum Theory, as written by somebody using only a lay brain as his explicatory tool:

Anyway, and I never fully appreciated this before reading up on it a bit for the purposes of these posts, Schrödinger didn’t come up with his theoretical cat in order to illustrate how things worked in the quantum world, by putting it in terms we could better understand – bringing out the hand puppets, as it were – he designed it as a thought experiment to show that the Quantum Mafia was clearly on the wrong track. The paradox illustrated by the cat experiment is that if the cat is neither alive nor dead until we pop the lid on the box and take a look, but exists rather in a state of superposition in which it is both, and neither (alive/dead you might say), and then we do pop the lid, only to observe that oops, kitty’s been dead for five hours already, the quantum logic drives us to accept that an action just taken now has somehow crystallized the possibilities and determined events that happened in the past. Kitty can’t be both long-since expired and just-now expired. Right? Quantum theory, with its insistence that nothing is real until observed and measured, paints you into a corner in which you upend the rules of causality and the linear operation of time, immutable rules we observe working to govern our world every day, in everything we do.

It’s a compelling argument: we all know that’s not the way things work. Up here, the poor cat died before we opened the box, and by looking now, we’ve simply discovered what already happened – we didn’t suddenly cause an adjustment of the whole timeline, so that the cat, which was alive/dead just a second ago, is now, sadly, an ex-cat who was poisoned a few hours back.

Yes. Granted. However, and I’m actually a little sorry to tell you this, but at the quantum level, using only tiny things like photons, rather than big macro objects like cats, the Quantum Mafia can demonstrate that the obvious paradoxical impossibility that Schrödinger was touting as proof that they’re all nuts is, in fact, exactly what happens. They do this through what’s known as a “delayed choice” experiment, which uses as part of its apparatus – get this – a quantum eraser (sounds like something out of Ren and Stimpy: Don’t touch that you fool! That’s the History Eraser Button!!!) This type of experiment was first suggested by an extraordinarily brilliant shit disturber named John Wheeler, whose writings initially envisioned mere thought experiments, hypotheticals, as he posed questions along the lines of “Imagine if we could devise an experiment that did A, wouldn’t it be wild if the result was B?”. Then somebody figured out how to actually do the experiments. Hence today’s post. Spoiler alert: the result is “B”, and it is indeed wild.

First, remember the old “double-slit” experiment? If not, I refer you to Issue 1:

Long story short, this elegant experiment proved that the things we measured arriving at a photo-receptive plate as photons, distinct little bullets of light, must have been nothing more than waves of probability before they hit the screen to be observed and measured. Until then, there were no photons as such, only wave functions expressing a tendency of the photons to exist across a broad set of possibilities – nothing really exists until you look at it. The clever twist proposed by Wheeler is to modify the double-slit experiment by inserting extra instruments into the flow of light, allowing the experimenter to tinker with when, precisely, measurements are taken, and how much of the light is actually measured. Such tinkering allows you to reach some startling conclusions about how new information changes the overall quantum state of the system, when those changes occur, and how well that dovetails with our real-world notions about time’s arrow and causality.

At first blush it’s quite difficult to understand the experimental apparatus, but it’s just a riff on the original double-slit set-up. What they do is add stuff downstream of the slits that intercepts the light, splits it in two, and gives you observational evidence about only one half of the divided whole, while still more stuff is added even further downstream that subsequently interferes to take that observational data away. Thus, as one set of probability waves travels toward the final destination of a screen, just like in the original double-slit experiment, the other heads towards a sequence of devices at which information about it is, in a sense, unscrambled, but then re-scrambled, with the re-scrambler acting as the so-called quantum eraser. You can see how it works in the attached video above, but here’s a couple of screen captures, showing the rig. In the first, what they’re doing is intercepting the waves as they come out of the two slits, and further splitting them:

One set of waves now travels as usual toward the final screen. The other, though, heads towards a set of detectors (labelled “A” and “B” in the picture), and those detectors make a measurement of the things that were sent their way, revealing enough information that you can now tell what must have happened to the stream that went the other way. It’s a bit like this: imagine there were two marbles in two boxes, and one marble was black, the other white. I need only open one box to tell what has to be in the other. If I find a white marble, it must be the black one in the other box. It’s a similar process of discovery. What happens in the experiment is that when you start measuring the streams that travelled towards the detectors, the other streams heading towards the screen stop behaving like waves of probabilities. The interference pattern you get in the simple double-slit experiment disappears. That means that the non-measured streams travelling as waves suddenly turned into discreet photons without anything in those streams being observed. It had to work that way, because by measuring one set of streams, we found out enough that the probabilities of all of them, measured or not, had been reduced to a certainty. Observing one set determined the other, just as opening the box with the white marble settled the matter about which marble was in the other box, even though we never opened the other box.

This is strange enough, and illustrates the bizarre phenomenon of quantum entanglement, in which determining one outcome immediately determines the other; what’s weird, or “spooky”, as Einstein put it, is that unlike with the marbles, the two streams of quantum waves weren’t already fixed before we took the measurement. With the marbles, we know it’s a black marble in the other box because it was always a black marble. It was a black marble before we put it in there. It’s different with the probability waves. At the quantum level, the waves weren’t resolved down into photons yet when they exited the slits – that’s what the original double-slit experiment proved absolutely. Yet the instant you measure one set of waves, the other, travelling off in the other direction and not measured at all, settles down too. It’s as if by opening the box with the one marble, which is determined only in that instant to be white, we forced the marble in the other box to immediately turn black, even though before we popped the lid on the white one, there was no black marble yet (and no white one either!). As if, in fact, the two marbles somehow talked to each other across space and time, and communicated faster than the speed of light – because there’s no delay. None whatever. The transformation is instantaneous.

Still with me?

Because that’s not even the bizarre part, for the purposes of today’s discussion. The delayed choice experiment adds another wrinkle, when, having taken it to this point, they add in the dreaded quantum eraser (the bits labelled “C” and “D” in the picture):

Don’t trouble yourself about how the bits labelled “C” and “D” are physically working, if the picture doesn’t make it obvious. The thing to understand is that by further bouncing the light around downstream of the detectors at “A” and “B”, they render everything obscure again. We no longer know what “A” and “B” measured. We’ve been deprived of the information. This is why “C” and “D” get the ominous quantum eraser designation. So what happens then? The other streams, heading off in the other direction toward the screen, become unknowable again, and thus revert to being waves of probability, with the result that the old double-slit interference pattern re-emerges!


So here’s why this experiment, like all quantum experiments, takes a nine pound sledge hammer to reality. Light moves at a certain speed – very, very, fast, but at a fixed speed. Therefore, the streams that split off to head in the direction of the unscramblers and re-scramblers must be hitting them in sequence, the first before the second. By the time they’ve been re-scrambled, they were previously, for a little while, unscrambled. When they were unscrambled, information was obtained, however briefly, and all probability waves throughout the system were reduced down to photons, including the ones bound for the screen. Therefore all of them, heading in both directions, were already travelling in space as discreet photons when the one set subsequently hit the re-scrambler. Then, suddenly, they’re all back to being waves again, both sets, and the ones that headed towards the screen – which, remember, were never themselves observed – arrived and created the wave interference pattern as if the instant they hit the screen is the first and only time their state had been determined.

Why is that so brain-curdling? See the drawing above – the distance to the unscramblers is greater than the distance to the screen. Therefore, by the time the stream that first hit the unscramblers reached the devices that re-scrambled it, the other stream should already have arrived at the photo-reactive screen in a determined state as photons. Yet that’s not the result. Now that the information has all been taken away, and we observers never knew one way or the other, the interference pattern re-emerges. It’s as if something in the present affected a prior event, and the travelling waves have learned somehow that after all they never actually collapsed down into photons. The effect seems to be that the subsequent re-scrambling reaches back in time and undoes the prior unscrambling.

It’s what you’d observe if, at the quantum level, time doesn’t exist, and there is no sequence of events, only a continuous present in which the information we obtain (or fail to obtain) up here as observers determines what everything is, and always was. As if there is no was. Not down there. Down there, there’s only now.

Light travels very, very fast, of course – 300,000 Km a second – so the first qualm you might have is that the apparently bizarre violation of causality, with causes coming after events, is an illusion created in the lab by instruments not subtle enough to measure the tiny increments of time involved. Nope. Sorry. First, the lab instruments are subtle enough, and second, just to be sure, they’ve conducted the experiment by bouncing light off of distant satellites, making sure that the unscramble and re-scramble elements are separated by thousands of miles, and still it comes out the same. Changes in the amount of information known, wherever and whenever those changes occur within what we perceive as the sequence of events, are instantaneously reflected throughout the entire system, as if the observation in the now affects the state of the system as it was in the past.

Another way to look at it: the unscrambling never actually happens, even though the light hits the first set of detectors, because we never learn the results. Nothing is real until you look at it, right? If that makes you feel better about it, well, OK, I guess.

Are present actions really altering events in the past? Or what? There’s debate about all this going on among the illuminati, to be sure, and the relevant scientific papers can be found on the internet readily enough, but damned if I can understand them. What I can tell you is that many, many physicists are prepared to assert that yes, no other way to say it, present observation does change past reality at the quantum level. Not possible, of course, but there you have it. My suspicion is that those who are holding out, and claiming something else must be going on, are just like Einstein used to be, and committing the classic sin of begging the question, as that term is properly defined, by insisting that something is impossible because it can’t be possible.

As Niels Bohr might respond: well, there it is, so you tell me – and please show your work.

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