The interference disappearing from measurement is not really because the instrument alters the state. Or, at least, putting it like that occludes the more fundamental reason.
Fundamentally, measurements are subject to the uncertainty principle, which dictates that one can not define precisely the values of two complementary observables at the same time. Position and momentum of any quantum object are such complementary observables, so measuring one – for example position – requires that the other (momentum) becomes less defined.
When the position of a particle is narrowed down to a pixel on a detector screen, its momentum becomes very uncertain and we must talk about all the possible paths for it to have arrived at that point.
The probability of a particle being measured at any given pixel is given by the probability of all possible paths combined[1], with this important quirk: when combining possible quantum states, they interfere with each other, constructively or destructively. Repeated measurements of positions give you what appears to be wave-like interference due to the way the probabilities of all paths interfere.
By checking which slit a particle passes through, you exclude all the possible paths through the other slit and end up not observing the same pattern because the two slits simply do not interfere.
To be more precise, by “combining” I mean state vector addition. Probability is magnitude squared of a quantum state vector. So for a given position, you take all possible paths there, sum their state vectors, then square the resulting vector’s norm (magnitude) to get its probability. The sum of all positions’ individual probabilities will be exactly one - meaning that it will always be somewhere. ↩︎
I think the issue is that quantum mechanics is hard to popularize without leading people into wrong conclusions, pop science clickbaits make this worse.
I find it easier to understand if you say that observing necessarily means there’s an interaction energy (for example a photon), otherwise no information can be retrieved, and however small that information retrieval energy is, quantum systems are so sensitive, that it is enough to modify their behavior.
Agreed completely, especially about the clickbait-y titles that are at best half-truths. Quantum mechanics are difficult to grasp even on a surface level for sure.
Perhaps it would be better to explain why instead of attempting a mic drop based on your superior knowledge?
It’s called the observer effect, and it happens because:
And particularly in the double-slit experiment:
https://en.wikipedia.org/wiki/Observer_effect_(physics)
So for anyone who wants to have a surface understanding of the observer effect, the wiki does a fair job of the basic explanation.
The interference disappearing from measurement is not really because the instrument alters the state. Or, at least, putting it like that occludes the more fundamental reason.
Fundamentally, measurements are subject to the uncertainty principle, which dictates that one can not define precisely the values of two complementary observables at the same time. Position and momentum of any quantum object are such complementary observables, so measuring one – for example position – requires that the other (momentum) becomes less defined.
When the position of a particle is narrowed down to a pixel on a detector screen, its momentum becomes very uncertain and we must talk about all the possible paths for it to have arrived at that point.
The probability of a particle being measured at any given pixel is given by the probability of all possible paths combined[1], with this important quirk: when combining possible quantum states, they interfere with each other, constructively or destructively. Repeated measurements of positions give you what appears to be wave-like interference due to the way the probabilities of all paths interfere.
By checking which slit a particle passes through, you exclude all the possible paths through the other slit and end up not observing the same pattern because the two slits simply do not interfere.
To be more precise, by “combining” I mean state vector addition. Probability is magnitude squared of a quantum state vector. So for a given position, you take all possible paths there, sum their state vectors, then square the resulting vector’s norm (magnitude) to get its probability. The sum of all positions’ individual probabilities will be exactly one - meaning that it will always be somewhere. ↩︎
I think the issue is that quantum mechanics is hard to popularize without leading people into wrong conclusions, pop science clickbaits make this worse.
I find it easier to understand if you say that observing necessarily means there’s an interaction energy (for example a photon), otherwise no information can be retrieved, and however small that information retrieval energy is, quantum systems are so sensitive, that it is enough to modify their behavior.
Agreed completely, especially about the clickbait-y titles that are at best half-truths. Quantum mechanics are difficult to grasp even on a surface level for sure.
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