Do we create the very reality that we observe?
Wheeler called it the Participatory Anthropic Principle. It goes something like this. The answers we get from posing questions to Nature depend very much on the questions we ask. Without any questions, nothing would be answered — hence we are participants in the bringing about of events. His most quotable quip was the following.
“No phenomenon is a real phenomenon until it is an observed phenomenon.”
John Wheeler was one of the most interesting characters of the 20th century. If you’re familiar with the idea of space as a seething boil of virtual particles popping in and out of existence, or if you’ve heard the words wormhole or black hole, then you’ve been in some way influenced by Wheeler. His story is ripe with legacy, which is why you can find it in many other places and also why I won’t retell it here. Besides, I wouldn’t do it justice, and I really only wanted to talk about one of his many ideas.
Starting at the end
The seeds of the participatory universe were sown in a thought experiment called the delayed choice experiment.
In the classic double-slit experiment, a single photon of light either “chooses” a path to take or interferes with itself, apparently taking both paths, depending on what experimental arrangement is used to capture it. This is the famous wave-particle duality of quantum physics — the photon acts as a particle if we interrogate the path it took and as a wave if we do not.
Sometimes it is said that photons behave as waves when we are not looking at them, and they behave as particles when we are looking at them. You can read more about the modern version of the double-slit experiment and its implications here:
How to confuse yourself with quantum physics in 3 easy steps
None of the experts got it — why not you, too?
Ignoring the fact that a photon — on account of traveling the speed of light — does not experience time and hence does not have agency, it is still a helpful metaphor to speak as if it makes choices. Anyway, the photon sees the apparatus and turns on the appropriate behavior as it is entering it. That is, if the photon enters an apparatus that checks which path it is going to take, it will turn on particle behavior. Otherwise, it will maintain wave behavior. Presumably, if the photon chooses a path, it does so at the moment (in our reference frame anyway) the possible paths split.
Now suppose you choose whether to detect the path of the photon after it has entered the experiment. In a double-slit experiment, simply placing an extra piece of glass right before the detector suffices for this purpose. Suppose you did this at the last possible moment.
One of two things must be true. Either your choice sends a message back in time to tell the photon how to behave, or the photon doesn’t really exist as a definite entity until it is observed. Within a small laboratory experiment, this all seems academic. But Wheeler was thinking on a cosmic scale.
If you point your telescope in the right direction, you can create a double-slit experiment the size of the universe! In the image below, the “ring” is actually a single star nearly as old as the universe. The line between that star and Earth is blocked by another galaxy somewhere within the many billions of light-years between the star and us. Yet, due to Einstein’s relativity, the light from the star bends around the massive galaxy to reach us. (This sort of gravitational lensing was the first experimental proof of Einstein’s theory, by the way.)
Every single photon from that star could have taken one of many paths to reach us here on Earth. The telescope used for the photo detects the path, and many photons project an image of a ring around the lensing galaxy. If instead, the light from all directions was combined before detection, then a wave-like pattern would be detected, implying the photon took all paths.
Surely, if the light chooses one path or another, it did so many billions of years ago when it encountered the intervening galaxy. Yet, how the photon manifests in the world is only decided up here and now, through the way we choose to arrange our telescope. From your point of view, that photon has been waiting for billions of years for you to come along and choose how it should manifest in the world.
Your questions create reality
Recall how the game 20 Questions works. I think of something, and you ask me yes or no questions with the hope of narrowing down my answers to reveal the one thing I was thinking about. Suppose I was thinking about a kangaroo. You ask, “is it bigger than a breadbox?”
“Is it a place?”
“Is it an animal?”
“Is it a mammal?”
“Does it live in Australia?”
“Is it an emu?”
“Is it a kangaroo?”
Yes! Wow, you are good at this game. But let’s change it up a bit. Suppose there were 20 of us “answerers” and you had to ask each question to one person in succession. You start with the first person: “Is it bigger than a breadbox?”
You ask the next person, “is it an animal?”
The third person thinks a bit harder when you ask, “is it electronic?”
And so on it goes down the line, each person seeming to think a bit harder to answer. Finally, you reach the last person, “is it a transistor radio?”
From your perspective, the game is no different. You assume the 20 people all agreed on the answer — transistor radio — before the game began. But here’s the kicker. In this game of 20 Questions, each of the 20 participants agreed beforehand to not think of something. The only thing they agreed to do was answer yes or no in a way that didn’t contradict any previous answer. Only when the last question was asked did “transistor radio” come into existence as the answer.
The point is subtle, but obvious in hindsight. Before you started asking the questions, there was no “answer.” It was only through your choice of question that an eventual answer materialized in the world. Wheeler called this “it from bit” — the physical world (“it”) is brought about through asking it yes or no questions (“bits”). Why is the universe the way it is? Turns out, it literally depends who’s asking.