This Is Why Neutrinos Are The Standard Model’s Greatest Puzzle
“But this is where the big puzzle comes in: if neutrinos and antineutrinos have mass, then it should be possible to turn a left-handed neutrino into a right-handed particle simply by either slowing the neutrino down or speeding yourself up. If you curl your fingers around your left thumb and point your thumb towards you, your fingers curl clockwise around your thumb. If you point your left thumb away from you, though, your fingers appear to curl counterclockwise instead.
In other words, we can change the perceived spin of a neutrino or antineutrino simply by changing our motion relative to it. Since all neutrinos are left-handed and all antineutrinos are right-handed, does this mean that you can transform a left-handed neutrino into a right-handed antineutrino simply by changing your perspective? Or does this mean that left-handed anti-neutrinos and right-handed neutrinos exist, but are beyond our current detection capabilities?”
Every fermion in the Standard Model has a number of properties inherent to it. Mass, charge, baryon number, lepton number, lepton family number, etc. All the fermions that exist in the Standard Model have non-zero masses, even the neutrino, and all of them can have their intrinsic angular momentum go in any direction… except the neutrino. Unlike all the other fermions, we’ve only ever seen left-handed neutrinos and right-handed antineutrinos. But if we’re clever enough, we can design an experiment that will reverse the perceived spin of these neutrinos.
What will we see then? Believe it or not, the answer could unlock the mystery of why our Universe is filled with matter and not anti-matter. Let’s do what we can to solve this puzzle; the entire Universe may be at stake!
No, Physicists Still Don’t Know Why Matter (And Not Antimatter) Dominates Our Universe
“According to the hot Big Bang, the Universe as we know it today was born 13.8 billion years ago, and was filled with energy in the form of photons, particles, and antiparticles. The Universe was hot, dense, and expanding extremely rapidly under those early conditions, which caused the Universe to cool. By the time less than a single second had passed, practically all of the antimatter had annihilated away, leaving approximately 1 proton and 1 electron for every 1 billion photons.
The Universe was thought to be born matter-antimatter symmetric, as the laws of physics dictate. But something must have happened during that first fraction-of-a-second to preferentially create matter and/or destroy antimatter, leaving an overall imbalance. By the time we get to today, only the matter survives.”
In a really cool experiment at CERN, enormous numbers of particles called mesons, containing one quark and one antiquark, are produced and studied. Some of those particles contained charm quarks, while others contained charm antiquarks. When they decayed, there was a slight difference in the ratios of what their decay products were, indicating a fundamental asymmetry between matter and antimatter.
But this was already seen in both strange quark and bottom quark systems, and doesn’t explain our Universe’s matter-antimatter asymmetry. Get the full story here.
There’s Almost No Antimatter In The Universe, And No One Knows Why
“So how did we get here today, with a Universe made of a lot of matter and practically no antimatter, if the laws of nature are completely symmetric between matter and antimatter? Well, there are two options: either the Universe was born with more matter than antimatter, or something happened early on, when the Universe was very hot and dense, to create a matter/antimatter asymmetry where there was none initially.
That first idea is scientifically untestable without recreating the entire Universe, but the second one is quite compelling. If our Universe somehow created a matter/antimatter asymmetry where there initially wasn’t one, then the rules that were at play back then should remain unchanged today. If we’re clever enough, we can devise experimental tests to uncover the origin of the matter in our Universe.”
The laws of physics, to the best of our knowledge, are highly symmetric between matter and antimatter. There are no known interactions that have ever resulted in the net creation or destruction of matter particles versus antimatter particles; you cannot make one without making the other. Yet, quite clearly, our Universe is made of matter and not antimatter! Where did all our matter come from? Or, conversely, where did all our antimatter go?
This is one of the greatest unsolved mysteries in physics, but one we’ve got a tremendous shot at figuring out. Here’s the story behind it.
What Was It Like When The Universe First Created More Matter Than Antimatter?
“This is only one of three known, viable scenarios that could lead to the matter-rich Universe we inhabit today, with the other two involving new neutrino physics or new physics at the electroweak scale, respectively. Yet in all cases, it’s the out-of-equilibrium nature of the early Universe, which creates everything allowable at high energies and then cools to an unstable state, which enables the creation of more matter than antimatter. We can start with a completely symmetric Universe in an extremely hot state, and just by cooling and expanding, wind up with one that becomes matter-dominated. The Universe didn’t need to be born with an excess of matter over antimatter; the Big Bang can spontaneously make one from nothing. The only open question, exactly, is how.”
One of the biggest unsolved questions in physics today is how the Universe came to be filled with matter and not antimatter. After all, the laws of physics are completely matter-antimatter symmetric, and yet when we look at what we have today, every planet, star, and galaxy is made of matter and not antimatter. How did it come to be this way? The young, hot, but rapidly expanding-and-cooling Universe gives us all the ingredients we need for this to occur. We are certain of the exact mechanism, but theoretically, there are some enticing possibilities. Here’s a walk through one of those scenarios in great detail, but expressed so simply that even someone with no physics knowledge can follow it.
Here’s what the Universe was like when it was matter-antimatter symmetric, along with how it could have become matter-rich without breaking the laws of physics.
How Did The Matter In Our Universe Arise From Nothing?
“[Y]ou can start with a completely symmetric Universe, one that obeys all the known laws of physics and that spontaneously creates matter-and-antimatter only in equal-and-opposite pairs, and wind up with an excess of matter over antimatter in the end. We have multiple possible pathways to success, but it’s very likely that nature only needed one of them to give us our Universe.
The fact that we exist and are made of matter is indisputable; the question of why our Universe contains something (matter) instead of nothing (from an equal mix of matter and antimatter) is one that must have an answer. This century, advances in precision electroweak testing, collider technology, and experiments probing particle physics beyond the Standard Model may reveal exactly how it happened. And when it does, one of the greatest mysteries in all of existence will finally have a solution.”
On one hand, we have all the stars, galaxies, gas, plasma, and the great cosmic web, all made out of matter and not antimatter. On the other hand, we have the laws of physics, which are almost completely symmetric between matter and antimatter, so much so that we’ve never created or observed more matter than antimatter in any reaction throughout human history. Yet somehow, such a reaction must have occurred, since the Universe exists as we see it: made of matter. So how did it get to be this way? How did a completely symmetric, early Universe give rise to a matter-dominated existence, complete with two trillion galaxies, each containing billions of stars? Believe it or not, we’re closer than ever before to answering this question, and the 21st century is poised to be the one where the answer to this existential question goes from speculative to solid.
Come learn the science behind why we live in a matter-rich Universe instead of a matter/antimatter symmetric one!
How Neutrinos Could Solve The Three Greatest Open Questions In Physics
“Perhaps ironically, the greatest advance in particle physics — a great leap forward beyond the Standard Model — might not come from our greatest experiments and detectors at high-energies, but from a humble, patient look for an ultra-rare decay. We’ve constrained neutrinoless double beta decay to have a lifetime of more than 2 × 1025 years, but the next decade or two of experiments should measure this decay if it exists. So far, neutrinos are the only hint of particle physics beyond the Standard Model. If neutrinoless double beta decay turns out to be real, it might be the future of fundamental physics. It could solve the biggest cosmic questions plaguing humanity today. Our only choice is to look. If nature is kind to us, the future won’t be supersymmetry, extra dimensions, or string theory. We just might have a neutrino revolution on our hands.”
When we look at the Standard Model of particle physics, there’s a big problem that leaps out at us right away: it works well. It works so practically perfectly well that it leaves us very little room for solving some of the problems we have in the Universe: problems like dark matter, dark energy, and the origin of matter. The only hint we have that the Standard Model isn’t fully correct comes from our observations of neutrinos: they have very tiny masses, and they oscillate from one flavor into another. Furthermore, they only come in one chirality: neutrinos are always left-handed and anti-neutrinos are always right-handed. This could be explained if neutrinos weren’t normal (Dirac) fermions like the other Standard Model particles, but were their own antiparticles, and behaved as Majorana fermions. If this scenario is true, it could not only solve the dark matter, dark energy, and matter/antimatter asymmetry problems all at once, but there’s an experimental signature we can look for.
That experiment is running right now! While the LHC is all the rage as far as looking for new physics goes, neutrinoless double beta decay just might bring about the future of fundamental physics.
Ask Ethan: How Close Are We To A Theory Of Everything?
“Has science made any progress with regards to the Grand Unified Theory and the Theory of Everything? And could you elaborate on what it would mean if we did find a unified equation?”
Ever since we began uncovering the laws of nature, humanity has looked for a way to simplify them. We attempt to create an overarching framework that encapsulates all the different particles, interactions, forces, and concepts into a single, unified, simpler structure. From this, then, we can derive all the non-fundamental laws and rules, obtaining the complex Universe we see today. But this idea has its challenges: the complexity of what we know today requires that new symmetries must exist in order to create this unification we seek. And those symmetries, then, must be broken today, which predicts the existence of new particles and interactions in addition to the ones we already know. Yet we’ve searched for these with great rigor, and none of it has come to fruition. Still, the hope of unification exists.
What progress have we made towards unification, and what does our current knowledge mean for its existence (or non-existence)? Find out on this edition of Ask Ethan!