# Size of particles

1. May 30, 2012

### alvarogz

What does a particle be small or big (in relative terms). Do particles have a defined structure? I thought that it is probably because in the particle accelerators they collide, so they have a material structure.
My doubt is related with the concept behind what Matter really represents. Is a electron smaller than a proton or a neutron? Is that question well proposed?.
The inclusion of the wave function of this tiny particles sometimes makes think if particles, at that level, are allowed to define a concept of size.

2. May 30, 2012

### Jano L.

The answer depends on the kind of particle.

Atoms and molecules are very tiny, but have size of order 10^-10 m (for hydrogen atom) and this is confirmed by many experiments.

On the other hand, there is no convincing experiment which would suggest the size of an electron. Therefore, the most simple and used view of electron is that it is point-like.
There are also some attempts to build theories in which it has some small radius, but these are very complicated and so far not very useful.

For proton, the mainstream view is that is has size of order 10^-15 m, but the exact value is less clear than in the case of the atom.

3. May 30, 2012

### mathman

Protons and neutrons are composite particles, made from quarks and gluons. Electrons are elementary.

4. May 31, 2012

### RGevo

I think it is quite tricky to define the 'size' of particles when talking about very small things due to their tricky quantum nature.

One way of thinking about it could be that when you are looking at the particle ( ie probing it with something - photons of light or electrons in a particle collider ) you are essentially probing it with something that has a wave-particle duality. So there is a de-broglie wavelength associated to your particle.

\lambda_{DB} = \frac{h}{p}

So you can tell whether the particle looks point like or not up to a certain length scale ( \lambda_{DB} ) which is defined by how powerful your probe is (its momentum).

Aside;
In reality it is a bit more complicated I think. For example an electron could be thought of a small ball of electric charge. But the little ball of charge will look differently depending on how energetic a particle you probe it with ( like a high energy photon vs a very very high energy photon )