Thought Experiment Analysis

 Q: If a very, very coherent photon from a quasar (say, with ) exists, it seems like theoretically it could be everywhere in the universe. 

Suppose two simultaneous double-slit experiments are performed (though simultaneity itself is a contradictory statement ), on opposite sides of the source. Will both have the probability of detecting the photon on their screens, even if they are millions of parsecs apart? 

What if the experiments are done at different times — will they both detect the photon? Is this related to the idea that photons don’t experience time?

Suppose only one photon starts its journey from a quasar with . Will it be seen by two observers if they are at the same distance from the source but on totally opposite sides, billions of parsecs apart? 

When we say we see a photon, it is the photon that has reached us — it appears as a faraway shining object light-years away, but the photon itself is here, not light-years away, correct? So when both observers see it, it means the photon is at their location. But since detection is required for it to be seen, only one can actually detect it. So if the same object is seen by billions of observers, those billions of photons have not each reached a billion different places. 

If the number of photons is equal to the number of observers, theoretically everyone could get one photon, but practically most would not see the object, and some would detect more than one. Does this mean increasing the number of observers would affect where the photon is detected, or which civilization detects it?

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A: If a photon from a quasar were perfectly coherent (), its wavefunction would in principle extend across the entire universe, but that does not mean two widely separated experiments could both register the same photon. A single photon can only be detected once: the act of detection collapses its wavefunction and localizes it. Until detection, we can only speak of probabilities for where it might appear. So even if two detectors are set up on opposite sides of the source, billions of parsecs apart, each has a certain probability of being the place where the photon is observed — but once one detector clicks, the photon ceases to exist elsewhere.

When we say we “see” light from a distant quasar, what we mean is that photons emitted long ago have traveled to us and are detected here; the photon is not “spread out” across all of space in a way that allows everyone to see the exact same one. Billions of observers can all see the same object only because the quasar emits an enormous number of photons. Detection is a sampling process: some civilizations might detect many photons, others none, but the number of observers does not influence where a given photon ends up. A single photon is indivisible — it cannot be seen in two places at once, whether detectors are separated by distance or by time.