If I Could Turn Back Time…

What if there was a machine that could change the past? And what if it was built nearly twenty years ago? And what if it was the subject of this article? The more astute among you may have noticed that this is going somewhere. The machine in question is known as a Quantum Eraser. Why it does what it does isn’t really important – this isn’t a physics lesson and frankly I don’t really know the answer myself. The exciting part is what the machine actually did, because it’s pretty fucked up and you don’t need to understand a textbook to appreciate said fucked up-ness.

Actually, first there are two tools we’ll be taking with us on our journey into the weird. And it’s not green bubbling slime, ten-year-old-boy weird. It’s wow-the-world-makes-no-sense-and-any-attempt-to-understand-it-is-doomed weird. It’s almost enough to lead one to think there’s a higher power that doesn’t want us to know certain things.

First: a photon is a packet of light energy. For our purposes, we can just think of it as a ‘particle of light’.
Second: light is a bit strange and is a wave as well as a particle, which means that it has a trait called ‘polarisation’. Think of polarisation as the comparison between two fish swimming forwards, where one is an eel that moves side to side and one is a weird flat disc fish that undulates up and down. Both fish are moving in the same direction, but with different polarisations.
So we have individual particles of light that can be polarised in different ways. That’s the extent of the physics knowledge that we will need. So tighten those seatbelts…

In the experiment, two identical photons are sent on different paths, and get bounced back to a half see-through mirror (which they pass through half the time and bounce back the other half) called a ‘Beam Splitter’. Then a detector measures where the photon ends up. In the first version of the experiment, the two photons always wind up at opposite detectors – each photon somehow knows what the other is doing. This spooky cooperation only happens if the setup is just right – if one photon has to travel a little further than the other to reach the beam splitter, it doesn’t happen.*

Both the photons are identical, but researchers wanted to be able to track them throughout their journey. They did this by flipping the polarisation of one of them (using a ‘tagger’) so that they could tell the photons apart. This makes it possible to ask whether the tagged photon wound up at the top detector or the bottom detector. The answer? The photons stopped cooperating. Now they would turn up at random detectors with no correlation between the behaviour of one photon and the behaviour of the other.

The next tweak in the experiment is where shit got real. A reverse polariser (the ‘untagger’) was placed at the end of the little circuit, to realign the photons so that they were identical again. There would be no way to distinguish one photon from another by the time they reached the detectors. So what happened? The photons resumed their cooperation. Once again, they would wind up at opposite detectors. Even though the alteration to the experiment is at the end, after the shenanigans with the beam splitter are all over, it affects the path the photons take. Somehow, the photons ‘know’ what’s coming up ahead, and ‘choose’ their path accordingly.

The polariser that made both photons equal again is called a quantum eraser – what it erased is information. Without knowing which photon is which, a ghostly cooperation is possible. But if you plan it so that you can tell them apart, they know. The tricksy part is that they also can tell if you’re going to erase the information later and then they’re allowed to cooperate.

Think of two children at home alone who get up to mischief, but you never know exactly which child does what. If they know you’re leaving a video camera running, they behave themselves so that you still can’t catch them. But if you erase the videotape after you arrive home before watching it, you’ll find the house messy and the labels on all of your canned food changed around. The same thing happens in this experiment, but with particles of light. How do they know which path to take? How does adding another polariser further down the track change what happened beforehand? Quantum Mechanics, of course, has a complete explanation for all of this weirdness, or so I’m told. But what is certain is that the world we live in is a very strange one indeed.

*although they will cooperate if the difference in distances travelled is any integer multiple of the wavelength.

Lewis Gurr

The author Lewis Gurr

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