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The weak nuclear force gets short shrift among the four fundamental forces of nature.
People wax on about the marvels of gravity that holds entire worlds together.
They’ll detail electromagnetism, which lets us see and keeps us from falling through the floor.
They’ll even praise the strong nuclear force that fights to keep nuclei together.
Then they’ll say the weak nuclear force causes radioactivity and fusion, and then move on.
Which is a real shame, because the weak force has been causing trouble for a century,
ruining everything physicists thought was true.
And at the same time, it might be responsible for your very existence.
Fundamentally, the weak force acts on leptons, which includes electrons, neutrinos,
and their heavier cousins, and on the quarks that make up particles like protons and neutrons.
When it acts, it changes a particle’s identity by altering some of its properties.
But other forces change particle types, too, so that isn’t what makes the weak force special.
It’s special because it’s the exception to almost every rule.
It’s the Mongols of the forces.
Hints of trouble came around 1900, when scientists thought atoms of elements were conserved.
For example, they thought lead atoms were lead atoms, no matter how they moved or bonded,
and the same went for every other element.
Then, they discovered radioactive decays: Atoms of one element can transform into atoms
of another just by shooting some stuff out of their nuclei.
Nobody knew it at the time, but this alchemy was the result of the weak force.
And that wasn’t the last time it overturned scientific common sense.
The weak force also majorly messes with our ideas about symmetries,
which help guide working physicists.
These are things you might change about an experiment without changing the results of it.
Like, if I’m in front of an experiment and you’re behind it, we shouldn’t get different results.
Physics should just care that something is moving, not that your left is the same as my right.
Physicists in the early 1900s also figured that swapping left for right is no different
from swapping up and down, you can just turn your head, or from swapping forward and backward.
If all of those are symmetries, then swapping all three of them at once should be, too.
This is known as P symmetry to the cool kids, and parity symmetry to the rest of us.
And it says that if you flip all these directions, no experiment will act differently.
Experiments show that gravity would work the same way, and so would electromagnetism and
the strong nuclear force, making a good case for P symmetry being true.
But the weak force has to be different.
See, particles have a property called spin.
It’s more complicated than the direction they’re really spinning,
but it’s an okay mental picture to have here.
One of P symmetry’s consequences would be that if something with spin decays,
particles should fly out of it in a random direction.
But in 1957, physicists watching cobalt nuclei decay reported that the direction of those
decay particles depended on whether the cobalt spun clockwise or counterclockwise.
And since the weak force causes decays, that meant the weak force is the only
thing in the universe we’ve ever seen violate P symmetry.
Later experiments showed that spin really matters, too: The weak force only acts on
clockwise-spinning matter particles and counterclockwise antimatter ones.
That also makes it the only force that treats matter and antimatter any differently,
so it’s the only one that violates another symmetry called charge conjugation or C symmetry.
And in case you were curious, it violates both of them at once, too, known as CP symmetry.
Much more recent experiments have also revealed that the weak force is the only one violating
T, or time-reversal symmetry.
Gravitational, electromagnetic, and strong nuclear effects would all look
exactly the same if time ran backwards, which is weird to think about.
There are particles called B0 mesons that throw a wrench in things.
They can exist in two forms, called B0 and anti-B0,
and the weak force can freely switch a particle between forms over time.
Switching in either direction should be the same, but switching from B0 to anti-B0
actually takes longer than going the other direction.
The weak force’s effects are therefore the only ones that depend on the direction of time, too.
Today’s physicists have retreated one more time to CPT symmetry, a combination of all three.
Special relativity and the Standard Model of particle physics, two of the best-tested
ideas humanity has ever had, would both fall to pieces if it didn’t work.
And finally, so far at least, there’s never been a single experiment violating it.
Although we are looking.
Now, you might be wondering: So what?
Why does it matter if one force is different and doesn’t follow physicists’s made-up rules?
Well, for one thing, we want to know how all these forces are related to each other.
That’s, like, a particle physicist’s whole job.
Symmetries and conservation laws are also two of the most effective tools we’ve
ever found to guide us to new ideas.
If one force keeps being a weirdo, it makes understanding how the world works a lot harder.
But there is another reason you might care:
You could exist thanks to that CP symmetry violation.
One of the big problems in modern physics is called the matter-antimatter asymmetry:
Every reaction we’ve ever seen produces equal amounts of matter and antimatter,
but the Big Bang made more of one than the other.
Because otherwise, all the matter would have collided with antimatter and annihilated into nothing.
Of everything in the known universe, the weak force is the only one
that seems to differentiate between matter and antimatter.
So some modern scientists think that the weak force caused that asymmetry:
It may have resulted in about one extra matter particle per billion particles.
And that’s all it took to make everything, everything.
Nobody has worked out exactly how yet, but the weak force is the only game in town,
at least, among ideas with solid experimental evidence.
So it might not just be a pain in the metaphorical physics butt.
The weak nuclear force could also be the reason the universe has anything in it at all.
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