John Archibald Wheeler

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1. 01

   In quantum physics, ‘observing’ means any interaction that leaves a lasting trace in the world: a detector, an air molecule, heat. No person or consciousness is needed. The more things an object touches around it, the faster it loses its bands; this is called decoherence.

   Law Without Law · 1983

   Observing means leaving a trace in the world, and the world leaves traces all the time, with or without us.

   In the last lesson we saw that the bands vanish when the electron's path leaves a trace. Let us take the idea all the way. A trace does not have to be a screen or a device. It can be an air molecule bumping into the particle, or a tiny ray of heat the particle gives off. Each such touch carries a little information about where the particle was. The information spreads out and can never be gathered back. Physicists call this decoherence. That is why the everyday world is not full of bands. A chair is touched every moment by billions of air molecules and by light. Any chance of being ‘in two places at once’ is lost almost instantly. Not because the quantum laws stop at big objects, but because big objects cannot stay unobserved by the rest of the world for even a fraction of a second. There is also an experiment that seems to turn back time: the quantum eraser. You mark a particle's path, the bands vanish, then you erase the mark and the bands ‘return’. But they return only after you sort the data, using information that arrives over an ordinary channel. On the screen, with all the points taken together, bands never appear. Nothing is sent back in time. Wheeler put it simply: a phenomenon becomes a phenomenon only when it is registered by an irreversible act.

   “It makes no sense to talk of the phenomenon until it has been brought to a close by an irreversible act of amplification: “No elementary phenomenon is a phenomenon until it is a registered (observed) phenomenon.””

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