 Now's a good time to review the particle sizes we've seen so far. In our first segment, we used an electron microscope to see a carbon atom with a diameter of 0.14 nanometers. That's a million times smaller than the width of a human hair. In our second segment, we probed the atom with alpha particles and found that the nucleus was very small compared to the atom. Here we have the carbon nucleus at around 26,000 times smaller than the carbon atom. The simplest nucleus is hydrogens. It's just a proton. At this level, the nanometer is way too large, so we'll move to femtometers instead. There are a million centimeters in one nanometer. Carbon experiments have produced excellent results and show that the diameter of a proton is 1.662 femtometers. In this segment, we probed the proton with high-velocity electrons and found that it contained three quarks. Powerful accelerators and hadron colliders have put the upper limit on the diameter of the cross-section of a quark at 0.001 femtometers. That's 1,760 times smaller than a proton and 140 million times smaller than a carbon atom. This is also the upper limit for the cross-section of an electron. The neutrino is the smallest elementary particle with a cross-section that is a thousand times smaller than an electron or a quark. That makes it 140,000 trillion times smaller than the diameter of a human hair.