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Hank continues our series on the four fundamental forces of physics by describing the weak interaction, which operates at an infinitesimally small scale to cause particle decay.

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Hank Green: Today we continue our series on the four fundamental forces of physics, a mere four forces that make the whole universe work. And in this installment, we're talking about a force that acts over very very teeny teeny tiny distances: the weak force. [intro music] Without it, the sun would not shine, we would not have elements like radium or plutonium, and we wouldn't have carbon-14 dating. All of these things require one particle to turn into another particle through particle decay. Remember strong force? It acts on a proton or a neutron to hold its component quarks together, and one way that we describe quarks is by their color. It's a characteristic of a quarks sort of like a charge is a characteristic of an electron. Weak force acts on quarks just like strong force does, but instead of changing its color, it changes its flavor. There are six flavors of quarks: lemon, lime, root beer, hazelnut... sorry. Seriously, you've heard of these -- up, down, strange, charm, top and bottom. Up quarks have a slight positive charge while down quarks have a slight negative charge, and they both have the smallest masses of all the quarks. The other quarks have charges too, but they're all much heavier and much rarer because they quickly decay into up and down quarks. Up and down quarks are what most stuff is made of. Neutrons are made up of one up quark and two down quarks, while protons are two ups and one down. Now, the weak force changes quark flavor, so when a quark inside of a particle changes, that changes the whole identity of the particle. This is crazy stuff to me, to think that you can actually, like, just, through this force change a proton into a neutron. It's nuts. In addition to quarks, and thus neutrons and protons, weak force can also interact in a similar way with leptons, the most famous of which is the electron but also including neutrinos. And as with all fundamental forces, the weak force involves an exchange of particles called force carriers, which are these weird, barely-existing particles that convey forces between other particles. Weak force has two force carriers, the W bosons, which can be either positively or negatively charged, or the Z bosons, which have no charge. Now let's watch some weak interaction at work. How about we make a neutron change or decay into a proton. To do this, we're gonna need a neutrino passing by. So remember, neutrons are one up quark and two down quarks, and protons are two up quarks and one down quark. The weak force is called weak because it only operates within a teeny tiny tiny range; about 0.1% of the diameter of a proton. So, say our neutron comes close enough to a neutrino. A positively-charged W boson would travel from the neutrino to that neutron. That's the weak force, right there. The neutrino, having lost a positive boson, becomes negatively charged, turning it into an electron, and over on the neutron, meanwhile, the positive W boson encounters a down quark and changes its charge from slightly negative to slightly positive. And since neutrons and protons have a difference of just one quark flavor, this changes the neutron into a proton. With the composition of the nucleus that contained this neutron having changed, the atom itself has changed too, into an entirely new element. If this was a carbon-14 atom, with six protons and eight neutrons, through the weak force it just decayed into a nitrogen-14 atom, with seven protons and seven neutrons. And that actually happens. It happens all the time, and it's how carbon-14 dating works. So, there it is. The weak force is actually able to change the identity of particles when they come very very very very very close to each other. Thank you for watching this episode of SciShow. If you would like to know more about the four fundamental forces of physics, you should go to and subscribe. If you have any questions or ideas or comments, you can leave those for us on Facebook or Twitter or of course in the YouTube comments below. Goodbye. [outro music]