One of the most famous experiments in physics is the double-slit experiment, in which a beam of light is shot through two slits to produce an interference pattern on a surface or detector on the other side. What's strange about it is, even if the beam of light is reduced to send only one photon at a time, the pattern still emerges, as though each individual photon is passing through both slits at once and interfering with itself.
About two decades ago, scientists further explored this phenomenon with the delayed choice experiment. Basically, a photon is sent through a beam splitter, off mirrors, and toward a (potential) second beam splitter. If the second beam splitter is there, the individual photons behave like waves that were split and recombined, and show an interference pattern. If it isn't, they behave like particles that chose only one of the two paths, and display no interference. It as if the photons know whether or not they should act as particles or waves when they hit the first splitter. Even more strange, it appears if the "choice" of whether or not to use the second splitter can be delayed until after the photon hits the first splitter, and reach back in time to determine what choice the photon made.
Now, scientists have tested this on a grand scale. Photons were split on earth, bounced off a satellite, and returned to earth to either be recombined or not. The choice in each test run was made after the photon had already reached the satellite in space, and yet the photons still behaved (i.e., showed an interference pattern or not) as though they already knew what the choice would be when the hit the first splitter, before ever heading off into space.
It's really difficult to wrap your head around what this really means. Much like entanglement, in which a paired particle reacts instantaneously to a measurement made on its partner at a distance, it seems to potentially violate our basic assumptions about relativity and causality. So, do we simply not quite understand the makeup of reality, or does reality somehow shift itself to match our measurements of it, even reaching back in time if necessary to do so? Creepy.
Edited for clarity
About two decades ago, scientists further explored this phenomenon with the delayed choice experiment. Basically, a photon is sent through a beam splitter, off mirrors, and toward a (potential) second beam splitter. If the second beam splitter is there, the individual photons behave like waves that were split and recombined, and show an interference pattern. If it isn't, they behave like particles that chose only one of the two paths, and display no interference. It as if the photons know whether or not they should act as particles or waves when they hit the first splitter. Even more strange, it appears if the "choice" of whether or not to use the second splitter can be delayed until after the photon hits the first splitter, and reach back in time to determine what choice the photon made.
Now, scientists have tested this on a grand scale. Photons were split on earth, bounced off a satellite, and returned to earth to either be recombined or not. The choice in each test run was made after the photon had already reached the satellite in space, and yet the photons still behaved (i.e., showed an interference pattern or not) as though they already knew what the choice would be when the hit the first splitter, before ever heading off into space.
It's really difficult to wrap your head around what this really means. Much like entanglement, in which a paired particle reacts instantaneously to a measurement made on its partner at a distance, it seems to potentially violate our basic assumptions about relativity and causality. So, do we simply not quite understand the makeup of reality, or does reality somehow shift itself to match our measurements of it, even reaching back in time if necessary to do so? Creepy.
Edited for clarity
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