Me and my friend were arguing. I think there could theoretically be different types of protons, but he says not. He says that if you have a different type of proton, it isn't a proton, it's something else. That doesn't make sense to me! There are different types of apples, but they're still called apples!
He says that's how protons work, but can we really know that?
Answer
It is an experimental fact that all electrons and also all protons (but this often applies also to nuclei, atoms and even molecules) are indistinguishable from one another, i.e. they both are identical particles.
Imagine to perform the following experiment: you take two objects A and B, perform as many measurements as you want on them, put them into a "black box", shake the box and then take them out. At this point, you want to be able to tell which object is A and which is B.
Let's say that A and B are two...apples. You can then measure their mass, their volume, take photographs of them etc.: you will obtain different results (taking into account experimental errors). Therefore, the only thing you have to do is take note of these results and you will be able to tell which is A and which is B.
However, if you try to do the same thing with two electrons, you will discover that all the quantities you can measure (mass,charge,spin etc.) are identical within experimental error. Therefore, you will not be able to tell one electron from the other.
This is an experimental fact, and as far as I know there is not a theoretical reason why it should be so. Maybe one day we will be able to perform more precise measurements and we will discover that electron charges are actually slightly different from each other!
PS I would like to stress that it is pointless to say that protons are identical because they are made of identical quarks, because this only shifts the problem from proton to quarks (we could then ask "why are all quarks identical?").
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