If a tree falls in the woods and no one is there to hear it, does it make a sound? Do things only exist if they are perceived? What does physics tell us about reality? These are some of the greatest questions in physics and philosophy, still unanswered after decades, or even centuries, of debate. Some wonder if they can ever be answered, but one physicist was unafraid to try: John Archibald Wheeler.

How come existence?

John Archibald Wheeler (right) with colleague Eckehard W. Mielke (Photo by Emielke – CC BY-SA 3.0)

Wheeler (1911-2008) is a legendary figure in physics. He worked with Niels Bohr to explain nuclear fission, worked on the hydrogen bomb at Los Alamos, and taught many eminent physicists including Richard Feynman, Kip Thorne and Hugh Everett. He was the father of modern general relativity, was key in developing our understanding of black holes and, indeed, popularised the term "black hole" (after it was suggested to him by an audience member at a conference) and coined many others, including "worm hole" and "quantum foam".

Wheeler categorised his long and productive life in physics into three periods: "Everything is Particles", "Everything is Fields", and "Everything is Information". (You can read more about his life and work in his autobiography, Geons, Black Holes and Quantum Foam.) The driving idea behind the third period was spurred by his contemplation of the age-old question: "How come existence?" And his answer, first published in a brilliantly written (and very entertaining) paper in 1989, was it from bit:

"It from bit symbolises the idea that every item of the physical world has at bottom — at a very deep bottom, in most instances — an immaterial source and explanation; that what we call reality arises in the last analysis from the posing of yes-no questions and the registering of equipment-evoked responses; in short, that all things physical are information-theoretic in origin and this is a participatory universe."

No question? No answer!

Wheeler's "it from bit" concept implies that physics, particularly quantum physics, isn't really about reality, but just our best description of what we observe. There is no "quantum world", just the best description we have of how things will appear to us. As Niels Bohr, one of the founders of quantum theory, said:"It is wrong to think that the task of physics is to find out how Nature is. Physics concerns what we can say about Nature."

Anton Zeilinger is Professor of Physics at the University of Vienna. Image: Jaqueline Godany.

Anton Zeilinger, Director of the Institue for Quantum Optics and Quantum Information, explains: "My interpretation [of "it from bit"] is that in order to define reality, one has to take into account the role of information: mainly the fact that whatever we do in science is based on information which we receive by whatever means."

But can we go one step further? Can we say reality is information, that they are one and the same? Zeilinger thinks not: "No, we [need] both concepts. But the distinction between the two is very difficult on a rigorous basis, and maybe that tells us something." Instead, we need to think of reality and information together, with one influencing the other, both remaining consistent with each other.

Perhaps the answer will follow Einstein's great insight from one century ago when he showed that you can't make a distinction between space and time, instead they are instances of a broader concept: spacetime. In a similar way, perhaps we need a new concept that encompasses both reality and information, rather than focusing on distinguishing between them.

Does the act of observing the Universe create it? (Image by David Harrison, University of Toronto)

One clear consequence of "it from bit" is the importance of the observer: reality requires one. "I think [Wheeler] was very radical," says Zeilinger. "He talks about the participatory universe, where the observer is not only passive, but the observer in certain situations makes reality happen."

Before a quantum particle is observed, such as a single particle in a box, it is in superposition, simulatenously being in a number of different locations within the box. All we know (thanks to the solution to Schrodinger's equation for that system) is the probability we'll find the particle in any of these multiple locations. But when we make a measurement, we will find the particle in only one location. Quantum physics doesn't describe reality as it is, but as it is perceived by an observer. It simply can’t answer questions such as "what was the particle doing while no observation was being made?" (You can read more here.)

But Wheeler went further than saying we can only describe reality via our observations. "It's more than that," says Zeilinger. "He would have said that there is no reality beyond what can at least be observed. I don't know whether it's true or not but I like the radicality of this approach."

This idea may be radical but it is not new in philosophy. As far back as 1710, the philosopher George Berkeley wrote "to be means to be perceived". "I don't know whether Wheeler would have gone that far," says Zeilinger. Instead he thinks that Wheeler believed that carefully analysing what information is, and what it means, can teach us about reality.

The shore of our ignorance

"We live on an island surrounded by a sea of ignorance. As our island of knowledge grows, so does the shore of our ignorance." - John Wheeler, From Scientific American (1992), Vol. 267

The double slit experiment: The top picture shows the interference pattern created by waves passing though the slits, the middle picture shows what you'd expect to see when particles are fired through the slits, and the bottom picture shows what actually happens when you fire particles such as electrons through the slits: you get the interference pattern you expect from waves, but the electrons are registered as arriving as particles.

Wheeler's conjecture of "it from bit" has been very important in the development of modern ideas about the role of information in quantum mechanics. It has given theorists, philosophers and experimentalists new avenues to explore questions that have challenged quantum physicists for decades.

The role of the observer is already an area of debate in quantum physics. (You can read more here.) And a participatory universe brings up even more questions, for example: What is an observer? Most physicists agree that an observer doesn't have to be human, or even conscious. Measurement and observations happen in inanimate apparatus: for example, photographic film registers the presence of a photon. But if observations do make reality happen, why do our many observations all agree on one version of the Universe?

If Wheeler is correct and reality is based on answers to yes or no questions, then understanding how these measurements come about is very important. But what is a measurement? This is a straightforward question in classical physics: it is the registering, using some sort of equipment, of what is happening in physical space. The question "Is the object located at point x, yes or no?" has an answer regardless of any observation. But in quantum physics, as we saw above for the example of the quantum particle in a box, measurement is problematic. What happens when we make a measurement, and what constitutes a measurement, is a question that is still open to debate.

But Wheeler suggests that it doesn't make sense to talk of reality before the measurement is made: "The past has no evidence except which is recorded in the present." For example, in the double-slit experiment, whether the standard version or Wheeler's delayed choice experiment, a photon will act as a wave or a particle depending on how you decide to measure it. It doesn't make sense to ask what state it was in before you made the measurement.

And it is not just the theoretical side of quantum physics that has benefited from Wheeler's idea. Zeilinger is primarily an experimentalist and says Wheeler's "it from bit" has had an influence in his and his colleagues' thinking about designing experiments.

"He put out a great challenge for the development of quantum mechanics which is very important," says Zeilinger. "In some points his ideas might turn out to be too radical – it's too early to tell. But that's the process of science! In science it's necessary to have radical positions. And then when these positions are challenged, and we found out how far one can go, then that is progress. It's how science often operates."

About this article

Rachel Thomas is Editor of Plus.

This article is part of our Information about information project, run in collaboration with FQXi.Click here to read other articles related to John Wheeler's concept of "it from bit".