u/babayaga_3k

Physics nerds assemble and take a shot at this please. New physics thought experiment alert, at least I think.

I just spent like 6 hours researching and formulating a seemingly paradoxical thought experiment that shows the utter non-intuitiveness of quantum mechanics’ non-locality, wave-particle duality of light and superposition.

I call it the Light-Year Fiber Paradox

Imagine a perfect one-light-year-long fiber-optic channel in empty space:

Three scientists conduct an experiment; A is at the start, B is halfway, C is at the end.

A sends one single photon with a one-light-year-long wave packet toward C. The photon does not interact with anything except detectors.

After one year as the photon is arriving, C secretly chooses either to put a detector in the path or leave the path open. One second later, at the mid point of the channel, B puts a detector in the path halfway down the fiber.

C and B are half a light-year apart, so any normal message from C to B takes six months.

QM states that if C detects the photon, the photon is gone, so B should detect nothing. But if C chooses not to detect it, maybe B can still have a chance to.

So it seems there’s a chance that B can learn what C chose to do before any light-speed communication about their choice from C could arrive 6 months later?

In other words:

Can C communicate faster than light by choosing whether or not to absorb the single photon?

(Obviously the answer is no, but have fun refuting the paradox)

Physics nerds assemble and take a shot at this please. New physics thought experiment alert, at least I think.

I just spent like 6 hours researching and formulating a seemingly paradoxical thought experiment that shows the utter non-intuitiveness of quantum mechanics’ non-locality, wave-particle duality of light and superposition.

I call it the Light-Year Fiber Paradox

Imagine a perfect one-light-year-long fiber-optic channel in empty space:

Three scientists conduct an experiment; A is at the start, B is halfway, C is at the end.

A sends one single photon with a one-light-year-long wave packet toward C. The photon does not interact with anything except detectors.

After one year as the photon is arriving, C secretly chooses either to put a detector in the path or leave the path open. One second later, at the mid point of the channel, B puts a detector in the path halfway down the fiber.

C and B are half a light-year apart, so any normal message from C to B takes six months.

QM states that if C detects the photon, the photon is essentially gone right, so B should detect nothing. But if C chooses not to detect it, maybe B can still have a chance to.

So it seems there’s a chance that B can learn what C chose to do before any light-speed communication about their choice from C could arrive 6 months later?

In other words:

Can C communicate faster than light by choosing whether or not to absorb the single photon?

(Obviously the answer is no, but I’d like to understand why it still feels off. Thanks :)