[ intro ] Here’s a weird thing: Based on our understanding of physics, the universe as we know it… should not exist. In fact, all matter — the stuff that makes up you and me and basically
everything you see around you — should have been destroyed not long after
it formed. As for why it wasn’t… well, that’s actually one of the biggest unsolved
mysteries in science. It’s something we’re very slowly starting
to unravel, and every year, scientists are uncovering
new leads that could help us figure out how and why
we’re all even here. Here’s what we know so far. The reason we’re so confident we shouldn’t
be here is based on our understanding of antimatter. When you hear that word, it might conjure
up visions of this hypothetical, mysterious substance, but that’s because
the name like stinks. Antimatter definitely isn’t hypothetical. And despite what it’s called, it isn’t
the opposite of matter, either. In reality, antimatter is just like normal
matter, but it has the opposite electric charge. For instance, if an electron has charge of
minus one, then an anti-electron — which is called
a positron — has charge of plus one. In every other way, they’re identical. We can study antimatter in radioactive decays
and particle accelerators, so we’ve been able to learn a decent amount
about it. And as far as we can tell, every kind of matter
particle has an antimatter counterpart. But when most people think of this stuff, they’re not thinking about cataloging particles
and misleading names. They’re thinking about explosions. If there’s one thing antimatter is known
for, it’s that when a particle of it touches its exact matter counterpart — like, say, an electron hitting a positron
— it all goes boom. It happens in a process called annihilation,
and when it’s over, the original matter and antimatter particles
are gone. Usually, the only things left behind are some photons of extremely energetic gamma-ray
light. And that’s where the problems start to come
in. See, as far as we can tell, the Big Bang should have created equal amounts
of matter and antimatter. It was all bunched up close together in a
sort of “soup” made of hot, dense plasma, and these particles were running into each
other constantly. Annihilation was happening all the time. Now, to be fair, matter and antimatter were also forming during
these early days. It happened during something called pair production It’s the reverse of annihilation, and it happens when a photon with enough energy
turns into two particles: one made of matter, and one made of antimatter. Pair production occasionally happens now, but it’s nowhere near as common as it was
billions of years ago. During the beginning of the universe Still, because it always produces equal amounts
of matter and antimatter, it shouldn’t have upset that fifty/fifty
balance. So when the universe cooled and expanded,
and pair production dramatically slowed, that ratio should have been locked in. Annihilation should have continued to rage
until there was nothing left but photons. But obviously that is not what happened, which
is great. Instead, we have good reason to believe that
essentially all of the particles left over from the beginning of the universe are made
of matter, and that almost none of them are antimatter. And that just doesn’t make sense. It’s not like there are secretly huge pockets
of antimatter out there, either. If there were, there would be a constant stream
of annihilation reactions wherever those pockets touched matter. And we’d be able to see a huge number of
gamma rays as a result. The only antimatter we actually see is the
occasional stray particle from space, the short-lived particles made in radioactive
decays on Earth, and the handful of particles made in accelerators. Technically, it isn’t a hundred percent
impossible, but it would be really hard for big chunks
of antimatter to fit in with our current evidence for the structure and evolution of the universe. So the more likely explanation is that the
modern universe contains essentially no antimatter. But… why? The best explanation we have right now is
that, for some reason, more matter survived those
early days of chaos. Evidence suggests that for every billion antimatter
particles made in the early universe, about a billion and one matter particles were
made. So for every billion annihilations that made
photons, one extra matter particle survived. That’s just bizarre, though. Based on what we’ve seen, matter and antimatter
should be identical. They should form the same way, and should
decay into other particles at the same rates. They should also be treated the same by what
we consider the three fundamental forces in particle physics: the strong nuclear force that holds together
atoms, the weak nuclear force that governs atomic
decay, and electromagnetism. So basically, if you imagine swapping every
matter particle with an antimatter one and vice-versa, your experiment should behave identically. Which means there should be no reason for
the imbalance that we see. Except, obviously, something had to have happened. Otherwise, everything would just be made of
photons, and we wouldn’t be having this discussion. The challenge physicists have now is figuring
out what caused that imbalance. And there are a few things they could look
for. For example, this mystery could be caused
by some reaction that produces more matter than antimatter. Or it could be caused by a decay reaction
that makes antimatter break down more rapidly. Or, more realistically, it could be due to a combination of extremely
rare production And decay processes that add up to a one-in-a-billion
difference. Unfortunately, we haven’t come up with a
hypothesis that explains anything for sure. But we have uncovered some interesting leads
that suggest something weird is going on. Most notably, some experiments have found
that one of those three fundamental forces actually treats matter and antimatter a little
differently, after all. It’s the weak nuclear force, which governs how atoms decay. And figuring this out was such a big deal
that the first researchers to do it earned the 1980 Nobel Prize in Physics. These scientists worked with a particle called
a neutral kaon, which is composed of smaller particles called
quarks. Neutral kaons come in two forms, but both
are made of one matter and one antimatter quark. One kind is made of a down quark bound to
an anti-strange quark. And the other is made of a strange quark bound
to an anti-down quark. For the record, the quarks in the kaon don’t
annihilate each other because they’re not exact counterparts. If these things were made of, say, down and
anti-down quarks, that would be a different story. Regardless, in their Prize-winning experiment, the scientists found that something seemed
wrong about how these kaons decayed. Specifically, if you swapped all the matter
and antimatter in that experiment, you’d get slightly different results. And since the weak force is responsible for
decay, that meant it treated the two things differently. This isn’t the last time we observed something
like this, either. In fact, since that discovery, similar results
have been found in lots of other particles with different
types of quarks. For instance, in 2019, evidence from a large
particle accelerator suggested that the weak force treats a particle
made of charm quarks differently. We’ve also discovered similar results with
bottom quarks before. Still, these findings don’t exactly answer
our question. They’re interesting, sure, and the weak
force is a major player in physics. Like, I did just say that one thing that could
solve this antimatter mystery is if that stuff decayed faster. But these types of reactions still only involve
certain types of quarks. So as great as that would be, they aren’t enough to explain how even one
extra matter particle in a billion could have survived over the whole universe. Still, someday, maybe they could lead us toward
a real answer. Of course, scientists don’t think we’ll
be able to solve this antimatter problem just by testing and looking at particles in
the world around us. Unfortunately… that would just be too easy. Instead, it’s likely that we’ll need to
do a lot more theoretical work to answer this question. And that will probably mean coming up with
a whole new framework for physics. One of the things we might have to overturn
is called the Standard Model. It’s a really well-tested model that catalogs
all the types of particles we know of and also predicts how they should interact. But even though there’s a lot of evidence
supporting it and it’s been really good at explaining things, researchers have a few reasons to believe
there’s something beyond the Model, too. Because, among other things, it can’t explain
how gravity works. And gravity is definitely real, and kind of
a big deal. A bunch of researchers are currently trying
to find something called a Grand Unified Theory that will ultimately replace the Standard
Model. This theory, if we ever discover it, will be able to describe the three main forces
of particle physics as if they were the same force. Scientists think that unified force would
have been extremely important in very early universe, so maybe it could tell us what happened with
matter and antimatter. So, maybe that unified force treats the two
things differently like the weak force does, but on a much larger scale. That said, we might not need to fully understand
a Grand Unified Theory to solve this problem, either. We could figure it out just by making some
tweaks or extensions to the Standard Model — although what those might look like… is pretty up in the air at this point. Honestly, you could follow any of these questions
way into hypothetical science territory, but the truth is, we’re not really close
to getting an answer. We have a ton of fascinating leads and a bunch
of ideas we could keep testing, but at the end of the day, this problem with
antimatter isn’t just about antimatter. It’s about how all particles behave and
what the early universe was like, and it really highlights the fact that there’s
a ton we don’t know about how things work. So if we want to understand antimatter, we’ll need to roll up our sleeves and really
dig into the foundations of physics. There won’t be one magical experiment that
solves everything, and the first Grand Unified Theory to be widely
accepted may not explain it all, either. To figure out what’s going on, we’ll need out-of-the-box ideas and innovative
new experimental designs. We’ll need to know about fundamental forces,
and the early universe, and we’ll need to be able to round up enough
antimatter to do big experiments with it. Also, hopefully careful experiments with it Even one of these questions is something you
could make a whole career out of, and plenty of people have. But one thing is for sure: Someday, if we
do solve this problem, it will be huge. We’ll finally be able to answer why the
universe looks like it does, and why we’re all here. And along the way, we’ll have learned so
much. So in this matter-antimatter problem, it’s really about the journey just as much
as the destination. And this research about things like the weak
force and the Standard Model is a big part of that. Thanks for watching this episode of SciShow! If you want to learn about one way scientists
are actively trying to research Grand Unified Theories, you can check our episode about that over
on SciShow Space. [ outro ]
