MadSci Network: Physics

Re: is there anything smaller than a quark?

Date: Thu Jan 4 13:30:35 2001
Posted By: Benjamin Monreal, Grad student, Physics, MIT
Area of science: Physics
ID: 978575233.Ph

Hello Nick,

It's not always meaningful to talk about the "sizes" of particles. Quarks are thought to be fundamental particles - they cannot be broken down into smaller pieces - and as such we treat them as being little geometric points. Electrons, photons, neutrinos are also treated as being geometric points. When using quantum mechanics, that "geometric point" is blurred out and has some extent in space, but not in a way that lets you call it "the size of the quark."

We can talk about the proton having some size; there are three quarks in it, and the quarks are some average distance apart. (The proton has a radius around 10^-15 meters.) But fundamental particles, without any subcomponents, have no such internal distances.

There is a context in which physicists do talk about particle "sizes"; this is a concept called the "cross section". At a particle collider, you can never really pick up two protons and aim them precisely at one another and have them collide; instead you take a few hundred billion protons, pack them as densely as possible, and throw them at one another; some of them will collide, even though billions more will miss one another by a wide margin. Exactly how many head-on collisions do you expect? Obviously a set of really big particles will experience more impacts than a set of really small ones. For example, trying to collide two basketballs has a higher hit rate than trying to collide two golf balls. Similarly, a beam of "small cross section" particles will generate fewer collisions than a beam of "large cross section" particles, so you can sort of interpret the "cross section" as a sort of size. (Although particle cross sections must specify both of the projectiles, as well as the amount of energy they are carrying! Which isn't the case for basketballs and golf balls.) In this sense, quarks are (usually) much bigger than electrons! And the particle with the (usually) smallest cross section is the neutrino.

Of course, maybe the universe is full of particles which do not interact at all (zero cross section), but we do not know about them yet. :)

There is more info on particle physics in the MadSci library.


-Ben Monreal

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