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Comment by zw123456 | Hacker News Reader

OK, another dumb question from a neophyte here, doesn't the Heisenberg uncertainty principle make it impossible to know the size of a proton for sure? It seems like if you cannot not know the position and momentum of a particle then measuring its size exactly would be impossible?

Thanks in advance to anyone willing to take the time to give me an ELI5 on this :)

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How big is the smell of a baking pie? The concentration of pie molecules in the air is not a binary yes/no function of position, but at the same time the volume is definitely smaller than the county, and larger than the kitchen. Fuzzy-edged objects have some sense of size but there is some freedom in where to define their edge.

For the proton, the "size" is defined as a length-valued parameter in the function that expresses the charge distribution through space. The parameter is objective - but its association with the word "size" is imprecise. It is necessarily imprecise, not because of anything quantum or even unfamiliar, but because there's not another English word for the scale of objects without hard edges.

If I'm blindly shooting a lot of bullets at a lot of ducks, I should be hitting some ducks. If I then count the number of shots fired and the number of ducks dead on the ground I should be able to calculate the size of ducks with high precision.

A better analogy would be to subtract the amount of bullets embedded in the wall behind the ducks from the total bullets fired.

Assuming ducks stop the bullets.

And assuming it's not just one immortal duck flying around super fast.

The Heisenberg principle only makes it impossible to know the exact size of a single proton in a particular moment in time. But it doesn't say that you can't calculate the average of millions of protons, for example.

Thanks for all the good discussion, this actually makes sense, of course, now that I think about it, I am sure they are crashing millions of particles together and doing some analytics, so based on that we know about what the size is. Sort of like how when we do analytics online we know the approximate height of say all Amazon customers who purchased jeans. OK dumb analogy but it works for me :)

I'm an ignorant layman here, but the uncertainty principal only says that if you're sure about the size/location, then you're very unsure about the momentum. It's possible that measuring its size does not need much certainty about its momentum, so they can get the accuracy they want.

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