# What determines a particle's size?

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1. Jul 25, 2015

### Ryan Reed

I know that particles differ in size such as the electron is smaller than the proton, but why is this?

2. Jul 26, 2015

### DeldotB

Its much more useful to talk about why different particles differ in mass. When physicists talk about "sizes" of particles, they are really talking about mass for the most part. A down quark is much "smaller" than a top (or truth) quark, but its smaller in the sense that it has less mass. Its sort of pointless to talk about particles as having a finite radius anyway since they arent even localized objects.

Now, physics does have an answer to why particle masses are different: the more a particle (like a quark) interacts witht the Higgs field, the more mass it aquires.

3. Jul 26, 2015

### Staff: Mentor

The electron does not have known size, it is expected to be point-like.
The proton is a composite particle, its size is the volume those particles are in - which is non-zero. This is a completely different "size" definition than the size of the electron. If you ask for the volume where an electron is in an atom, for example, you get the size of the atom (approximately), which is much larger than a proton.

I have never seen any physicist use "size" in that way.

4. Jul 26, 2015

### ChrisVer

@DeldotB The size in such a way wouldn't fit in the discription for the top/down... The energy is the reciprocal of distance, so a larger mass(larger energy) would imply a smaller distance. And that's true- you need larger energies to see "small distances"

The electrons at least as we know them today, have no size - they are fundamental particles and they are point-like.
The quarks are also considered pointlike.
The proton however is a soup of quarks and gluons. You could try in a similar (to the atom) manner to measure what's the distribution of its constituents and thus determine a size of it. I think the size is ~0.8fm (and so is the Hydrogen nucleus radius), that means that within that range you have the largest possibility to find a quark that is bound to the proton.

5. Jul 26, 2015

### DeldotB

Right. Im being informal about it.