Recently, I made a video about whether gravity
was a force. I was scrolling through the comments – always an eye-opening experience – and
I noticed one question in a specific comment. This individual asked if the weak nuclear force
was a force in the usual sense of the word. It turns out that this is a common question, with
some interesting surprises, and that means you should sit back, because I have a story to tell.
(intro music) To determine whether something is a force, you
first need to know what it means to be a force. While there are technical definitions, for this
video I want to use a common sense one. A force is something that either causes an object to move
or would cause something to move if it wasn’t held in place.
There are several known fundamental forces.
While the question of how many forces exist is more subtle than you’d think, it is common to say
that scientists know of four forces. They are, gravity, electromagnetism, the strong
nuclear force, and the weak nuclear force.
Gravity, of course, holds us here on Earth. It’s a force, because if a cat misses a jump, it
falls. Gravity satisfies the simple definition and brings us thousands of funny cat videos.
Electromagnetism is also a
force. After all, a magnet, which is part of electromagnetism,
can pick up small metal objects. So, it’s a force too.
The strong nuclear force holds the nucleus
of atoms together. We know this because, a nucleus usually contains several protons, all
of which have a positive electrical charge. In electromagnetism, when you have two charges
with the same sign, they push away from one another, which would blow the nucleus apart.
Therefore, nuclei wouldn’t exist if there wasn’t a stronger force holding the nucleus together.
So that’s three of the four forces. However,
when scientists like me talk about the weak nuclear force, we usually just say “and the weak
nuclear force is responsible for some forms of radioactivity.”
And that’s completely true, but causing things to
decay doesn’t seem to be satisfy the simple force definition we’re using here.
So that’s the origin of that YouTube comment that
made me decide to do this video. Does the weak nuclear force cause something to move?
Well, to begin with, let’s talk about how the
weak nuclear force works at the quantum level. Like all quantum forces, the weak force
occurs when a matter particle emits a force particle that then flies off to
another matter particle that absorbs it.
In the weak nuclear force, there are actually two carrying particles.
There is the electrically neutral Z boson and the electrically charged W boson.
In the case of a weak nuclear force interaction,
some subatomic particle – say a quark – can emit a Z boson. The Z boson then zooms over to another
quark, which absorbs it. That’s at least one way the weak nuclear force is transmitted.
So now let’s get down to brass tacks. When the
quark emits the Z boson, the quark will recoil. If it’s initially stationary, it will move in the
direction opposite the motion of the Z boson. It’s not very different from when you’re in a boat
and throw a heavy sack off one side. If you do, the boat moves.
And when the other quark absorbs the Z boson,
it also recoils, not so different from someone catching that sack tossed from a boat.
So, if a Z boson is exchanged between two quarks,
the two quarks can move away from one another and, therefore the weak nuclear force definitely
satisfies the classical, intuitive, definition of a force. That’s the basic answer.
However, the weak nuclear interaction is much
more interesting than just being a force. For one thing both the W and Z bosons are very
heavy. They are in the ballpark of a hundred times heavier than a proton.
It turns out that the mass of the W and Z
bosons is why the weak force is so weak. It’s weak because weak force interactions are
rare, not because it only pushes a little.
To see that, let’s dig a bit into this. And, I should warn you, there’s some quantum
stuff involved. It’s all kinda mind-blowing.
Let’s take the W boson as an example. If you look up the mass of W boson,
you’ll see that it is 80.35 GeV or just shy of 86 times as heavy as a proton.
But when you’re talking about quantum particles,
stating the mass is only part of the story. In reality, every subatomic particle has a range of
masses – with some having a large range and some having a small one. In the case of the W boson,
the range is generally between 78.3 and 82.4 GeV. If you find a W boson, there’s a good chance
that it will have a mass in that range. You can see here a curve which kind of
demonstrates this. Where the curve is high, it’s likely the mass you find is there, where
it’s low, it’s unlikely that you can find a W boson with that mass.
However, the numbers I mentioned just gives the
typical range. Other masses are possible with rapidly decreasing probability
In the kinds of radioactivity that involves the
W boson, what’s needed isn’t a W boson with a mass of about 80 GeV. What’s needed is a
W boson with a mass more like 0.001 GeV. And, as we can see from the graph here, that’s
in “you gotta be kidding me” territory. W bosons with that mass are crazy rare.
So, this explains why the weak force is
weak. It’s just that W bosons with the required mass are super rare. If a rare
weak force interaction actually happens, it isn’t really all that weak – it has a similar
effect as the other known quantum interactions.
Since I’m talking about the weak force, I should probably tell you something that is
unique about it. It turns out that the weak force is the only one that can change a particle’s
identity. For example, when the top quark decays, it does so via the weak nuclear force. What
happens is the top quark emits a W boson and, when it does, it turns into a bottom quark. When
the bottom quark decays, it also spits out another W boson and becomes a charm quark.
This identity changing behavior is usually
what scientists talk about when they discuss the weak force. It’s not that the weak
force doesn’t push particles around like all the forces do – it’s that only the weak
force can change particles’ identities – so that’s the thing that we mention. 'Cause- come
one- that’s just kinda awesome, you know?
So, what’s the bottom line? The weak force can push particles around, so it’s
definitely a force like the others are. It’s also not really weak, so much as it’s rare. And it’s
also the only force that can change a particle’s identity when it decays. When you get right down
to it, the weak force is just very, very, cool. (phasing sound effect) Okay- that was an interesting topic. It’s
easy for non-experts to get the wrong idea about various particle physics concepts. After
all, when guys like me make science videos, we sometimes cut corners and don’t always say
everything we know. If you liked this deeper dive into the nature of the weak force, please
like the video and smash that subscribe button down there. And come back often to hear
more about the mysteries of physics – which makes good sense to me because, as I think
you’ll all agree, physics is everything. (outro music)
