杏吧原创

鈥淚T FROM bit.鈥� This phrase, coined by physicist John Wheeler, encapsulates what a lot of physicists have come to believe: that tangible physical reality, the 鈥渋t鈥�, is ultimately made from information, or bits.

Concepts such as entropy in thermodynamics, a measure of disorder whose irresistible rise seems to characterise our universe, have long been known to be connected with information. More recently, some efforts to unify general relativity, the theory that describes space and time, with quantum mechanics, the theory that describes particles and matter, have homed in on information as a common language.

Inside knowledge: The biggest questions about facts, truth, lies and belief

Forget alternative facts. To get to the bottom of what we know and how we know we know it, delve into our special report on epistemology 鈥� the science of knowledge itself

But what is this information? Is it 鈥渙ntological鈥� 鈥� a real thing from which space, time and matter emerge, just as an atom emerges from fundamental particles such as electrons and quarks and gluons? Or is it 鈥渆pistemic鈥� 鈥� something that just represents our state of knowledge about reality?

Here opinions are divided. Cosmologist Paul Davies argues in the book Information and the Nature of Reality that information 鈥渙ccupies the ontological basement鈥�. In other words, it is not about something, it is itself something. Sean Carroll at the California Institute of Technology in Pasadena disagrees. Even if all of reality emerges from information, he says, this information is just knowledge about the universe鈥檚 basic quantum state.

So we have to drill deeper. In quantum mechanics, an object鈥檚 state is encoded in an abstract mathematical entity known as a wave function. This wave function doesn鈥檛 itself say anything definite about that state. Rather, it gives you probabilities of finding the object in a particular state 鈥� of pinpointing a particle in a certain location, say.

But is the wave function itself ontological or epistemic? Is the uncertainty the wave function represents a true reflection of reality鈥檚 state 鈥� or just our state of knowledge about it? 鈥淥ne of the most controversial things about quantum mechanics is the status of the wave function,鈥� says , a theorist at Rutgers University in Piscataway, New Jersey. 鈥淭here鈥檚 total disagreement about that.鈥�

The answer depends on which of many different philosophical interpretations of quantum mechanics you plump for. In the most prevalent 鈥淐openhagen鈥� variation, the wave function is epistemic. Not only that, but it represents a state of knowledge that is fundamentally limited. There is no way we can know what鈥檚 happening in reality without looking: there is nothing in the mathematics that represents, for example, a particle鈥檚 trajectory over time. When we do look, by making a measurement, we find the particle somewhere, and the wave function is said to have 鈥渃ollapsed鈥� into a definite knowledge state.

Who collapsed the universe?

There are other interpretations of quantum mechanics in which the wave function is itself something real. In this case, though, it doesn鈥檛 exist in the familiar dimensions of space and time, but rather exists in some higher-dimensional 鈥渃onfiguration鈥� space that contains information about the positions of particles.

All of this creates immense problems 鈥� not least when we come to consider the quantum state of the universe. If the universe鈥檚 wave function only collapses into a definite state when someone makes a measurement, who or what is making that measurement on the universe, and what is the universe doing when no one鈥檚 looking?

It鈥檚 here that we begin to encounter the seemingly fundamental barrier of self-reference (see 鈥�Knowledge: Why we鈥檒l never know everything鈥�): we cannot achieve complete knowledge about the universe because we ourselves are part of it. Knowledge may be everything, but we鈥檙e stuck working out whether everything is knowledge.

This article appeared in print under the headline 鈥淚s everything knowledge?鈥�