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  • [ 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-electronwhich 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 ofsoupmade 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.

  • Butwhy?

  • 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.

  • Unfortunatelythat 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.