The quantum mechanics “bomb experiment”


For reference:

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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*PS: Sabine is not super well respected in the physics community, because of a lot of controversial takes that she presents in her videos*

This is a bit tough to do in an ELI5, but I’ll give it a shot.

This experiment is *only possible* with quantum mechanics because of a funny thing that happens with superposition. When something is random (like a coin flip for example) you can kind of think about it as “happening at the same time”.

*I know the video has an explanation for what that means, but it is slightly misleading*

We actually have hard evidence that this happens. You might’ve seen the double slit experiment, where light forms a strange pattern with multiple patches of light. You expect that the light can travel through one slit or the other forming two bright dots, but the pattern they form is the pattern pattern you would get of two light sources interfering with each other, not one. In a sense, the single photon travels through “both slits”, then forms a pattern with itself.

Now a strange thing happens if you try to figure out which hole it moves through. You can do this by adding a light detector at each hole. What you’ll find is that you’ll know where the photon goes, but it will no longer form the pattern, since after the detector you know exactly where it will go. By measuring it, you’ve also interacted with it, and changed the result after the measurement.

In this experiment, the real bomb is acting like a detector, meaning it’ll affect the path of the light. If it’s fake, nothing happens because there’s no detector. If the bomb is real, the light either hits it or it doesn’t, so there’s no superposition (similar to the no pattern case). If the bomb is fake, it’s free to move through both (pattern).

What you can do is look at all the outcomes (yes/no explosion, real/fake bomb) and you’ll find that no explosion + real bomb and no explosion + fake bomb has observably different results (pattern vs no pattern), light travels through *different* paths in these two cases. Meaning you can tell if the bomb is real or fake.

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.