For reference: https://youtu.be/RhIf3Q_m0FQ
I think I grasp the concept, but why is this something unique to quantum mechanics? It just seems like a well thought-out method of testing for a result without affecting the original variable. I dunno… then again maybe this is all over my head. Someone, please ELI5.
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They key point about this experiment is that you specifically make use of the detector.
Just to clarify:
– The experiment only test whether the photon interact with the detector. It, obviously, cannot tell apart dud bomb that still have functional detector.
– It specifically directly attempt to trigger the mechanism it’s trying to detect. There are no roundabout method where you try to figure out whether it will work by examining the mechanism or anything.
To rephrase the experiment: can you obtain an information about whether an interaction is possible, only by attempting it, but without it being successful.
It’s interesting from a physics point of view, because it seems hard to do that. Generally speaking, observation requires interaction. Even if you just “look” at something, there are photons bouncing off that object, impacting a tiny amount of energy and momentum.
Of course, that’s not quite true. Even classically, we can obtain information without any interaction. For example, you can shoot balls at an unknown object in a dark room. There is a chance that you will hit the object causing an interaction; but there is also a chance that you don’t, and you can narrow down the location of the object, thus obtaining information about it, without any interactions.
However, doing that classically only give you information about the failure of the interaction. You obtain that fact that the interaction failed, derived from the failure of the interaction.
But the situation with this bomb experiment is opposite. You’re attempting to *confirm* that the interaction *can* succeed, by attempting to cause an interaction, while hoping that it does not.
This require you to be able to somehow physically obtain information from a hypothetical, if you take the classical view that a photon take a random path, or that the photon split itself and take both paths. You would be attempting to find out what would happen after the photon were to hypothetically move onto a path that get it to interact with the detector, even though the photon actually move onto the safe path. This classical view would make no senses. That’s why you need quantum mechanics to explain this.
What’s the key different between classic and quantum that allows this to work? In classical world, probability are additive. If multiple possibilities can happen with certain probability, and certain event can happen for each possibility at certain probability, then you can compute the final probability of that event happen by just add up all the weighted probability. Probability are additive, so the final probability (that the event happen at all) is always bigger than the probability of the event happen and one of the possibility happen at the same time.
But in the quantum world, probability is not additive, it can actually subtract. You can reduce the chance that something happen by setting your interference correctly. This is what happened in the experiment, you set it up so that the possibility that the photon successfully pass through the bomb path actually cause the probability that it will arrive at a detector to go down to 0.
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