How the 'delayed choice quantum eraser' experiment got us to rethink reality

In the late 1970s, legendary physicist John Wheeler proposed a radical question: Exactly when does the universe notice that we're paying attention to a quantum experiment? And does it really matter? The answer goes against everything we thought we knew.

Wheeler's thought experiment, which eventually became a real experiment, involved the famous double-slit experiment. Let's say you have a source of light and a screen with two thin, vertical slits. When you shine the light through the slits, the light acts like a wave. It interferes with itself, causing a ripple-like pattern on a far wall, with strips of brightness alternating with darkness. This is exactly how waves work, and if you ever find yourself in a harbor with two narrow openings, you'll see the waves washing up onshore with a similar pattern.

Now, let's say you make the light really weak — so weak that, eventually, only one photon at a time goes through the double slit. Amazingly, even though each individual photon acts like a particle — it hits the far wall in one specific spot — after enough photons arrive, the same interference pattern emerges. The usual conclusion is that the wave nature of a single photon interferes with itself to create the pattern.

Now, let's add one more layer. Let's say you introduce a detector to the slits, to figure out which slit the photon actually passed through on its way through the screen. When you do this, the wave nature of the photon goes away. You get to see which slit the photon passed through — but it only ever acts like a particle, and you never get an interference pattern on the far wall.

When we design a quantum experiment, we must choose to investigate either the wave nature or the particle nature of photons — but we can't do both. OK, it's weird. But so far, it's the standard sort of quantum weirdness.

Wheeler upped the ante. He asked what would happen if you were to introduce a delay. What if you were to insert a detector at the slits after the photon had already passed through?

Wheeler proposed a helpful analogy. Imagine a distant light source, like a quasar, that sends light traveling for billions of light-years. Some of that light heads right for us, while some beams follow a curved path through a gravitational lens, like a massive cluster. Both beams arrive on Earth at the same time, and we can set up an experiment to interfere with those beams. In that experiment, we can choose to study either the wave nature or the particle nature of light.

Wheeler guessed the answer. He was right, and his correctness was later borne out by experiments. Even when we make a delayed choice, the photons somehow keep track of that and alter whether they're going to make an interference pattern.

How does this work? We're making our choice at the final leg of the light's journey. How did the photons "know" what choice we were going to make ahead of time? It seems as if our choice in the future went back in time to alter how the photons behaved in the past.

An updated version of the experiment, known as the "delayed choice quantum eraser," makes this even crazier. In this experiment, the photons pass through the slits. Then, the experiment decides whether to monitor the slits. Well after the photons have struck the screen, the experimenter decides to read the information. If the experimenter reads the information about which slit the photon passed through, there will never be an interference pattern. If the experiment throws away the information, an interference pattern emerges.

Remember, all of this is after the photon has already hit the screen.

Wheeler taught us how to think about this. He argued that it doesn't make sense to talk about photons "in flight." We only have measurements and observations — the final results of our experiments. The order of the events and what happened during the experiment itself don't matter. Photons aren't really in flight in the way we think of it, and the wave-particle duality of photons doesn't make sense in the way we usually think about things.

What we get, whether particles or waves, is what we get. And it's only once we make that measurement that nature reveals what aspect of reality to show us.