Welcome back to round two of the four fundamental forces of physics! If you haven’t already read it, we strongly recommend that you check out our first post in this series – The Strong Force. As we mentioned in the last post, these four fundamental forces is what make the universe work!
In comparison to the last post, where we talked about the strongest force of them all, we are going to look into a much smaller force that works over very small distances – the weak force.
As we remember from last post, one way we describe quarks is by their color. Just like strong force, weak force also acts on quarks, but not by changing its color, but by changing its flavor. You can have six different flavors: Up, down, strange, charm, top and bottom. In 1947, there was a study of cosmic ray interactions, where proton collision with a nucleus was found to live much longer than expected, 10-10 seconds instead of 10-23 seconds. This particular particle was name a lambda particle, where the property that caused the particle to live so long was named strange. Yeah, that’s how these names are constructed.
But why does a quark have different flavors, and why does these flavors have different characteristics, such as different mass? I have no idea, and neither does most of the scientific community (source). This is known as the flavor problem.
Up and down quarks are the most common quarks in the universe (source), and is what most stuff is made of. A neutron is made up of one up quark and two down quarks, and a proton is made up of two ups and one down. So, what does this have to do with the weak force? Well, the weak force changes the quark flavor, which means that when a quark inside a particle changes, the entire particle changes. As an example, this could change a proton into a neutron, thus changing the composition of a nucleus (source).
The Weak Force: How it works
Just like the other fundamental forces, the weak force is a force of interactions, meaning that it is an exchange of particles called force carriers. The weak force has three types of force carriers, two W bosons, positively or negatively charged, and one electrically neutral Z bosons . Let’s try to explain these force carriers through an example.
Imagine that we have a neutron, as we see in frame 1 (up, down, down). Now, if this neutron meets a particle called a neutrino, which carries negatively charged W boson, the W boson can change a down to an up, thus changing a neutron to a proton. This is the weak force. Since a neutron now has changed into a proton, the atom itself has also changed, making it a new element. Please keep in mind that this is a very simplified way of explaining it, so check out sources such as this for a more thorough explanation.
If you’re new to physics, everything so far could seem a bit “Greek,” so let’s try to think of a more practical example of the weak force. Perhaps you’ve heard of carbon-dating.
If you have a radioactive particle, such as carbon 14 (6 protons, 8 neutrons), the weak force helps it to decay into a nitrogen 14 atom (7 protons, 7 neutrons). Carbon 14, like all radioactive particles, have what is known as a half-life. The half-life is the amount of time it takes for a certain number of radioactive particles to decrease by half. Carbon 14 has a half-life of 5730 years, which means that if you found a fossil of an organic material that was 5730 years old, the amount of carbon-14 would be half of what it was last time. This decay of particles is the work of the weak force.
Let’s wrap it up
So, there you have it, the ferocious weak force has the ability to change the characteristics of particles when they come really close together. Check out this video for references and great animations!