By Joshua Winn, Princeton University
If the protons in a nucleus have positive charge, and the neutrons are neutral, then there aren’t any negative charges. So, what’s holding the cluster of marbles together? Shouldn’t the protons repel each other and fly apart? That brings us to consider the fundamental forces of nature: the strong and weak nuclear forces.
Strong Nuclear Force
This is a very short-range force that acts between nucleons: protons and neutrons. Even though short, it is a complicated force. It depends on how many nucleons are present, which kinds, whether they’re spinning, and many other things. And it only acts over femtometers, anything beyond that is negligible.
Think of it this way: in a stable nucleus, all the marbles are coated with a thin layer of glue, that is strong enough to withstand the electrical repulsion. That’s the strong force. The strong force is also why those marbles are rigid—the force is attractive up to the point of contact, but then it becomes repulsive. That’s why it’s very difficult to compress a nucleus.
Know the Neutrino
By this point, we already know the 3 particles: the electron, the proton, and the neutron. Now, let’s meet the neutrino, which as its name suggests, is a teensy little neutral particle. At first, it might sound like the neutrino brings a nice symmetry to the family of particles. The neutron is the proton’s neutral “buddy”: same mass, but no charge. Maybe the neutrino is a neutral “buddy” of the electron? No. Not at all.
For one thing, the neutrino has a much smaller mass than the electron, by at least a factor of a million. Another thing is that neutrinos interact mainly through the fourth fundamental force of nature: the weak nuclear force.
This article comes directly from content in the video series Introduction to Astrophysics. Watch it now, on Wondrium.
Weak Nuclear Force
The weak force is short-range, like the strong force, but it’s not like any sort of glue. In fact, it’s kind of a stretch to call it a force; it’s more like a special power that nucleons have to change identities. A neutron can change into a proton, or vice versa. For example, a neutron sitting all by itself will spontaneously turn into a proton, within about 10 minutes.
But, that can’t be all that happens. The total electrical charge has to be conserved. The new proton’s positive charge has to be balanced by negative charge somewhere else. So, what happens is that the weak force conjures up an electron along with the proton, and they sail away in nearly opposite directions.
Proton and Neutron: Equal in Momenta?
One would expect them to be exactly opposite, because in addition to charge, momentum has to be conserved. The initial momentum of a stationary neutron was zero, so we would think the proton and electron would have equal and opposite momenta. But, the funny thing is, when we measure them, they’re not exactly opposite.
The reason is that the weak force also produces a neutrino that sails away at nearly the speed of light, carrying just enough momentum so it all adds up to zero. It took a long time to figure this out, because neutrinos have such tiny masses and hardly interact with anything after they’re produced. You can fire a neutrino through a light-year of solid lead, and there’s a decent chance it’ll come out the other side, unharmed.
For completeness, it must be noted that there are at least 6 kinds of neutrinos; in the case of neutron decay, what pops out is an electron antineutrino. But for us the important thing is that whenever you see any kind of neutrino, you know the weak force has been up to something.
Zooming in further, inside the proton, things get very hectic. There are quarks, within a sea of particles called gluons and everything’s in motion, particles appearing and disappearing. But, that’s a step further than we need to go.
Comparing the Four Fundamental Forces of Nature
As we meet the four particles—the electron, proton, neutron, and neutrino—we also see the fundamental forces of nature: gravity, electromagnetism, the strong nuclear force, and the weak one. What sets the nuclear forces apart is you only notice them on femtometer scales. In contrast, gravity and electromagnetism are long range, acting on all scales. And their force laws look similar: they both go like one over r-squared.
In contrast, gravity and electromagnetism are long range, acting on all scales. And their force laws look similar: they both go like one over r-squared.
Common Questions about Strong and Weak Nuclear Forces
The strong nuclear force is a fundamental force of nature. This is a very short-range force that acts between nucleons: protons and neutrons. Even though short, it is a complicated force. It depends on how many nucleons are present, which kinds, whether they’re spinning, and many other things. And it only acts over femtometers, anything beyond that is negligible.
Neutrino is a teensy little neutral particle. At first, it might sound like the neutrino brings a nice symmetry to the family of particles. The neutron is the proton’s neutral “buddy”: same mass, but no charge. The neutrino has a much smaller mass than the electron, by at least a factor of a million. Another thing is that neutrinos interact mainly through the fourth fundamental force of nature: the weak nuclear force.
The weak nuclear force is short-range, like the strong force, but it’s not like any sort of glue. In fact, it’s kind of a stretch to call it a force; it’s more like a special power that nucleons have to change identities. A neutron can change into a proton, or vice versa. For example, a neutron sitting all by itself will spontaneously turn into a proton, within about 10 minutes.
