More weird science: A quantum entity, such as a photon of light or an electron, behaves either like a particle or a wave, but not at the same time. Which way it behaves depends on how it is being observed. That’s weird enough, an example of what we have been talking about the last few days of the evidence that reality requires an observer (or Observer).
In 2007, a team of physicists found a way to change the mode of observation of beams of photons in the course of the experiment AFTER they have already entered the measuring apparatus. Somehow, the observation in the present affected how the photons behaved in the past. As one of the authors of the study put it, as quoted in a physics forum,
“In the present, one can change something that has already happened in the past.”
The experiment was published in Science, with this abstract:
Wave-particle duality is at the heart of quantum mechanics. Particles and photons can display both properties, and which property is measured depends on the type of measurement made. What if the experimental setup changes when the photon or particle is “in flight” and has already entered the experimental apparatus? Jacques et al. (p. 966) report an almost ideal realization of such a “delayed choice” experiment as formulated by Wheeler. A triggered single-photon source provides a mechanism for precise timing of the experiment within laboratory conditions. The behavior of the photon in the interferometer depends on the choice of the observable that is measured, even if that choice is made when the photon is already in the system.
Go here for a simple classroom experiment–that I have witnessed doing a class observation of our college’s excellent physics class–that shows how we know that light can be either a particle or a wave and that can help you visualize this more sophisticated experiment, in which the light apparently entered the two slits (as it were), which would involve starting to act like a wave, but then when one slit was closed reverted to acting like a particle as if the wave “decision” (the experimenters’ word) never happened.