As a result of some precise experimental data, we now know that the mass of the quark is not naively 1/3 the mass of the proton. The most recent estimates for the mass of the quark is: Masses of the current quarks: = 2 - 8 MeV/c2 = 1.0 - 1.6 GeV/c2 = 168 - 192 GeV/c2 = 5 - 15 MeV/c2 = 0.1 - 0.3 GeV/c2 = 4.1 - 4.5 GeV/c2 reference: http://en.wikipedia.org/wiki/Current_quark_mass Also see: Researchers reported in Physical Review Letters, April 2010, the mass of the 3 lightest quarks. The team finds that an up quark weighs 2.01 +/- 0.14 megaelectron-volts, whereas a down quark weighs 4.79 +/- 0.16 MeV. That’s 0.214% and 0.510% of the mass of the proton, respectively. reference: http://news.sciencemag.org/sciencenow/2010/04/mass-of-the-common-quark-finally.html [Broken] Even though there is a bit of disagreement on the range of the quark mass, let’s be generous and assume the heavier mass is right, at 8 MeV for the up quark, and 15 MeV for the down. We also know the quarks can not go faster than the speed of light. Let's assume that their velocity is between 0 and close to the speed of light. And we know the radius of the nucleon is 0.841 fm, the accurate measurement made as of January 2013. How do these experimentally-observed facts relate to the Heisenberg uncertainty principle? Converting MeV into MKS mass, the up quark is 1.43E-29 kg. If it s going close to the speed of light, its momentum is 3E8*1.43E-26=4.28E-21, in MKS units. This its uncertainty in the momentum. Its position is within the nucleon, which is a diameter of 1.68E-15. This is its uncertainty in position. This gives: (Delta x) (Delta p)= (4.28E-21)*(1.68E-15)=7.2E-36 This is way below h-bar, which is 1.05E-34 in MKS units. A violation of the uncertainty principle. How is this reconciled?