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Two protons next to each other in an  atomic nucleus are repelling each other   electromagnetically with enough force to lift  a medium-sized labradoodle off the ground.

Release this energy and you have,  well, you have a nuclear explosion.

Just as well there's an even stronger force than  the electromagnetism holding our nuclei together.

But it's not the strong force, as  you might have imagined.

At least   not directly.

Nuclei are held  together by a quirk of nature,   without which we would have no complex atoms,  no chemistry, and certainly no labradoodles.

Quantum chromodynamics is a complicated  sounding name, but the fundamental force   of nature it describes - the strong force - is  actually even more complicated than it.

sounds.

In our previous episode on this topic we saw how  the strong force binds the elementary quarks into   protons and neutrons.

But that’s where we left  it.

Based on what we learned in that episode,   the strong force is entirely confined to the  nucleons, and yet somehow it’s also the force that   binds protons and neutrons together to form most  atomic nuclei.

That process is strange enough that   it deserves its own episode - especially because  it was this very question - how are atomic nuclei   held together, that led to the discovery of both  the strong and weak nuclear forces in the first   place.

But this is also the story of the meson -  the obscure cousin to the proton and neutron that,   while less well known, is just as essential to the  existence of all complex matter in the universe.

Our story starts with Hideki Yukawa, a young  physicist who graduated from the University   of Osaka in 1929, before moving back in with his  parents and working for no money as a university   assistant.

Due to the Great Depression, not  because he wasn’t a great physicist.

All the   while, Yukawa was focused on the problem  of how nucleons stuck together.

He was   perplexed about why a force so strong  shouldn’t be observable outside atomic nuclei.

Yukawa had one intriguing clue  to the inner lives of nuclei,   and that was beta decay - the tendency of large  nuclei to occasionally spit out an electron,   and in the process convert one  of their neutrons into a proton.

He knew that the electromagnetic force is  communicated by the exchange of photons,   and it stood to reason that this nuclear  force should be mediated by some particle.

So what if this particle was the electron,   and beta decay was the side effect of the  exchange of electrons between nucleons?

Like, if protons and neutrons were playing catch  with the electron and sometimes they fumble the   ball.

It turns out this is totally not what’s  happening, but the insight that forces should be   mediated by particles was new in its time, and  it eventually led Yukawa to the right answer.

Having an exchange particle with  mass, unlike the massless photon,   is a great first step for  making a short-range force.

These exchange particles are what we call  virtual particles - which we’ve discussed   before.

Virtual particles can sort of break the  laws of physics, in the sense that they can pluck   the energy needed for their existence out of  nowhere, as long as they give it back again.

This is a result of the Heisenberg uncertainty  principle, which in one form says that we can   never know both the energy and the duration of a  phenomenon to better than a certain tiny accuracy.

That means, for very short periods of time, the  amount of energy in a patch of space can vary.

Mass is a form of energy, so a  virtual particle can briefly gain   mass from nothing - but the more mass  it has, the less time it can exist.

Yukawa used the electron mass to calculate the  strength and range of a force mediated by this   particle, assuming it’s traveling near the speed  of light.

The resulting force turned out to be   around 1/200th of the strength needed to hold the  nucleus together, and had a range much larger than   the size of the atomic nucleus.

So the electron  couldn't really be the particle mediating this   nuclear force.

But Yukawa now had an equation  that described the type of force he wanted.

He adjusted the equation to describe a force  with the appropriate strength and range,   and found that it required a much  more massive exchange particle.

He called this hypothetical particle the "meson"  from the Greek “mesos” for middle - because it   had to have a mass somewhere between  that of the proton and the electron.

Yukawa also predicted that these particles had to  have electric charge.

Just like with Beta Decay,   a neutron could emit a negative  meson and transform into a proton,   a proton could emit a positive  meson and transform into a neutron,   and the neutral meson could be exchanged  between protons or between neutrons.

As a result, Yukawa predicted 3 mesons  types, one for each possible charge value.

Unfortunately this new theory didn’t explain  beta decay at all.

In fact, Yukawa reasoned   that beta decay required a completely different  nuclear force, much weaker than the one holding   the nucleus together.

In fact, he correctly  predicted the existence of both the strong and   the weak force at the same time.

Although the  weak force isn’t really mediated by electrons,   and we have videos explaining everything  you probably want to know about it.

Yukawa published his theory in 1935, and it  was almost completely ignored.

To be fair,   proposing the existence of two new forces of  nature was an extraordinary claim, and such claims   require extraordinary evidence.

No one was going  to believe this until we’d detected a meson.

These days we routinely create countless  mesons by smashing atoms together in particle   accelerators.

But back then particle  accelerators were just being developed,   so we had to rely on cosmic rays.

These are high energy particles that rain down on   the Earth from natural space particle  accelerators like supernovae and quasars.

Yukawa wrote a letter to Nature magazine  explaining why cosmic radiation could contain   mesons, hoping to inspire other researchers  to look for these, but his letter it was rejected Nonetheless, others had independently thought  of searching for new particles in cosmic   radiation.

We have Bibha Chowdhuri  and Debendra Mohan Bose for example,   who developed a method using photographic  plates.

They set these up at high altitudes   in India and Tibet due to the fact that our  atmosphere is so good at blocking cosmic   rays - which is annoying if you’re a particle  physicist, but great if you’re anyone else.

And Chowdhuri and Bose did indeed spot the meson  at around the mass Yukawa predicted.

Sadly their   research went mostly unnoticed until British  scientists independently found mesons in 1947.

Anyway, not only were these new particles in the  right mass range, there were three of them as   predicted by Yukawa, one positive, one negative,  and one neutral.

But then things got awkward.

Over the next couple of years, thanks to better  particle accelerators and improved understanding   of radiation, more and more mesons were  discovered, with a range of properties.

The first mesons, the ones found by  Bose and Chowduri, are called pions,   and these are the particles actually  exchanged by protons and neutrons in   the nucleus.

But there were many others like  kaons, eta mesons, D mesons, B mesons, etc.

And to make matters worse, scientists also  found more particles that could "exchange   mesons" just like protons and neutrons.

These particles were named baryons,   after the Greek word for "heavy".

There was the Lambda, Sigma, Xi,   Omega, and more.

Again, all with  different masses, charges and spins.

This endless collection of baryons and mesons  came to be known as the Particle Zoo and it   was a huge problem for physics.

According  to Yukawa's theory, the atomic nucleus only   needs two baryons - the proton and neutron  - and three mesons—the pions.

So what was   with all this extra junk that nature doesn’t  seem to use outside of particle accelerators?

This problem was finally solved  by Murray Gell-Mann, who realized   that mesons and baryons are not elementary  particles, but rather are made of quarks.

Hadron is the general name for a particle made  of multiple quarks.

The particle zoo just results   from all the quark combinations that are possible.

We covered this in our episode on quantum   chromodynamics, but here’s a quick refresher  so we can get to the punchline of this video.

With the discovery of quarks, it was clear that  we needed the strong force to not only hold the   nucleus together, but also hold the nucleons  together.

In fact, the latter is what the   strong force really.

Binding nucleons is sort of  an afterthought.

So just as electromagnetism works   on things with electric charge, the strong force  works on things with color charge.

There are 3   types of color charge, unlike the single type  of electric charge.

A quark, for example,   could have a colour charge of red, green, or  blue, or the opposite colour charge of antired,   antigreen, or antiblue.

These names by  the way are just analogies of course.

Composite particles need to be colour neutral,  so the 3-quark baryons have one of each colour   charge which cancel each other out, while 2 quarks  of a meson will have a colour and its anti-colour.

These quarks are bound by the strong  force, which is mediated by a massless   particle called the gluon.

Gluons each carry  one regular and one anti-colour charge,   and they cause the colours of  quarks to flip when absorbed.

Baryons and mesons are held together by  a continuous exchange of virtual gluons.

But wait a minute, haven't we been saying  the entire time that the reason the Strong   Force has a short range is because  it is transmitted mesons with mass?

Well that was Yukawa's whole deal, but now   it turns out mesons aren't the real exchange   particles of the strong force?

And the real ones  - the gluons - don't have mass?

What's going on?

Actually, the colour-neutral hadrons  can’t even feel the strong force directly,   just as electrically neutral objects don’t  exert an electrostatic force on each other.

But there is a workaround.

Atoms seem  electrically neutral from afar, but if   you are close enough to the nucleus you would feel  its positive electric charge.

Similarly if protons   or neutrons get close enough to each other, their  internal quarks will feel each other’s presence.

A quark from, say, a proton, will want to  tug at a quark from a neighboring neutron.

But they can’t simply exchange a gluon to  do that - because gluons have colour charge,   so exchanging a gluon would cause the hadrons to  no longer be colour neutral, which is not allowed.

In order to properly feel the strong  force the hadrons need to exchange a   neutral particle.

And nature has  found a way to make that happen.

Here’s how it goes.

A pair of nucleons get close  and a quark from one gets pulled towards the   other.

Its gluon connection to the other quarks in  its nucleus - what we call a flux tube - extends   and snaps, and in the process generates  two new quarks.

One remains in the nucleus,   while the other forms a quark-antiquark pair  with the original escaped quark.

This is our   meson - our pion.

It’s colour neutral and so it  can be absorbed by the second nucleus without   breaking any physics.

There, one of the meson’s  quarks annihilates with an antiquark counterpart   in that nucleus.

Ultimately we’re left with two  nucleons of the same type that we started with,   but they've now communicated the strong  force.

They did it a roundabout way,   but the process nonetheless exchanged energy  and momentum, binding the particles together.

This is how the strong force finds its  way around its lack of a neutral gluon.

It cobbles together a neutral  exchange particle - the meson.

But the meson has mass.

A virtual meson has  to borrow a lot of energy to create its mass,   and so the uncertainty principle says  that it can’t exist for very long.

Its   range defines the possible size of an atomic  nucleus.

If a nucleus gets too big then its   nucleons are too far apart to exchange mesons  effectively, and it’ll decay.

By the way,   when we talk about the force between  quarks we call it the strong force,   but this residual strong force that exists between  nucleons is called the strong nuclear force.

Ya know, this reminds me of our episode  about quasi particles.

In that episode   we talked about how fields can arise  as emergent behaviors of other fields,   and these quasifields will have their own  quasiforces and quasiparticles.

Well that's sort   of what's happening here.

The strong nuclear force  is a quasi-force mediated by a quasi-particle,   the meson.

Without this little quirk of nature  - this emergence of a force that’s not really   supposed to be there, the most complex  atom in the universe would be hydrogen.

Fortunately nature stumbled on a way to bind the  nuclei, which gave us chemistry, and biology,   and ourselves, including a young physicist  named Hideki Yukawa, who was able to figure   out some of the most important forces from  which emerges this richly complex spacetime.

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