Wheeler’s delayed-choice experiments demonstrate that extracting “which path” information after a particle passes through the slits can seem to retroactively alter its previous behavior at the slits.
If you read through Wheeler’s delayed choice experiments, all the variations he went through to try to pin this down, well… it’s hard to avoid the conclusion that the waveform doesn’t collapse until the moment that someone looks at the data. In fact, it’s hard to avoid the conclusion that the universe is laughing at us every time we try to get a specific answer. This statement from the conclusion is absolutely bonkers if you think about it:
The fact that it is possible to decide whether a wave or particle feature manifests itself long after—and even space-like separated from—the measurement teaches us that we should not have any naive realistic picture for interpreting quantum phenomena.
The method of observation determines whether the photon behaved as a wave or a particle, after the measurement is done.
Our results demonstrate that the viewpoint that the system photon behaves either definitely as a wave or definitely as a particle would require faster-than-light communication. Because this would be in strong tension with the special theory of relativity, we believe that such a viewpoint should be given up entirely.
The photon behavior as recorded changes depending on how you examine the record, even “long after” the record is made and the interpretation should be fixed. It quite literally depends on how you look at it.
from the Wikipedia article on the double-slit experiment
If you read through Wheeler’s delayed choice experiments, all the variations he went through to try to pin this down, well… it’s hard to avoid the conclusion that the waveform doesn’t collapse until the moment that someone looks at the data. In fact, it’s hard to avoid the conclusion that the universe is laughing at us every time we try to get a specific answer. This statement from the conclusion is absolutely bonkers if you think about it:
The method of observation determines whether the photon behaved as a wave or a particle, after the measurement is done.
The photon behavior as recorded changes depending on how you examine the record, even “long after” the record is made and the interpretation should be fixed. It quite literally depends on how you look at it.