In the 1970s, John Wheeler proposed a “Delayed Choice Experiment” in which the experimenter can seemingly decide after a particle has entered an apparatus whether it will behave like a wave or a particle. This delayed choice suggests that the particle’s past behaviour – whether it “went through both slits” like a wave or “chose one path” like a particle – is not fixed until the observer makes a choice in the present. Thus, Wheeler entertained the provocative idea that the present can, in a very real sense, influence the past. He did not claim that the present literally changes a fixed past, but that the past is not fully determinate until the measurement context is specified.
Wheeler also introduced the Participatory Anthropic Principle, which speculates that the entire universe might require observers to exist at all. In this view, reality is not a pre-existing static container in which observers appear; rather, the very fabric of space, time, and matter emerges through acts of observation distributed across the history of the cosmos. He likened this to a self-excited circuit where the universe generates observers who then, in turn, help bring the universe into definable existence. Wheeler tried to dissolve the traditional boundary between epistemology and ontology: knowing is being. The observer is not outside the system, looking in, but is entangled with the universe's very genesis. He generally avoided explicit metaphysics in print, but in lectures and correspondence he pushed these ideas much further, hinting that the universe might be fundamentally relational, informational, and participatory.
Wheeler's "It from Bit" suggests physical reality ("it") arises from information ("bit") – that is, from acts of observation, measurement, or binary distinctions made by observers. Wheeler proposed that:
"Every it – every particle, every field of force, even the spacetime continuum itself –derives its function, its meaning, its very existence entirely... from the answers to yes-or-no questions, binary choices, bits."
In other words, reality is fundamentally made not of matter or energy, but of information. The bits here are not literal ones and zeroes in a computer, but any elementary quantum event that can be framed as a binary question, like: Is the photon detected here or there? Spin up or spin down? Every such distinction adds a "bit" to the universe’s informational history. Measurement isn't just revealing pre-existing properties, but helping bring the properties, or even the fabric of spacetime itself, into being. Wheeler's model suggests that without the registration of information, there is no definite reality. This concept places information, observation, and meaning at the heart of physics, shifting away from the classical view of a material substrate that exists independently. Note that he never explicitly claimed that consciousness is required; he spoke instead of ‘observer-participancy,’ a broader category that includes measurement interactions as well as conscious observation. John Wheeler believed that spacetime geometry itself arises from accumulated acts of information-registration. His vision of a universe in which information, observation, and reality are inseparable, marks the first major step toward the new metaphysical framework this book describes.