Ask Ethan: What Is The Fine Structure Constant And Why Does It Matter?
“When we do our best to measure the Universe — to greater precisions, at higher energies, under various conditions, at lower temperatures, etc. — we often find details that are intricate, rich, and puzzling. It’s not the devil that’s in those details, though, but rather that’s where the deepest secrets of reality lie.
The particles in our Universe aren’t just points that attract, repel, and bind together with one another; they interact through every subtle means that the laws of nature permit. As we reach greater precisions in our measurements, we start uncovering these subtle effects, including intricacies to the structure of matter that are easy to miss at low precisions. Fine structure is a vital part of that, but learning where even our best predictions of fine structure break down might be where the next great revolution in particle physics comes from. Doing the right experiment is the only way we’ll ever know.“
This week, I was asked to explain the fine structure constant as simply as possible. It’s actually a story more than a century in the making, as the previously-observed fine structure of matter let us know that Niels Bohr’s model of the atom was insufficient from the outset! Today, our understanding of how the spin of matter, the relativistic effects that come from moving close to the speed of light, and the inherently fluctuating nature of the quantum fields permeating the Universe come together enables us to probe the structure and nature of matter more deeply than ever before.
The fine structure constant is so much more than almost anyone realizes. Come open your eyes to its wonders today.
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.
At Last, Physicists Understand Where Matter’s Mass Comes From
“The way quarks bind into protons is fundamentally different from all the other forces and interactions we know of. Instead of the force getting stronger when objects get closer — like the gravitational, electric or magnetic forces — the attractive force goes down to zero when quarks get arbitrarily close. And instead of the force getting weaker when objects get farther away, the force pulling quarks back together gets stronger the farther away they get.
This property of the strong nuclear force is known as asymptotic freedom, and the particles that mediate this force are known as gluons. Somehow, the energy binding the proton together, the other 99.8% of the proton’s mass, comes from these gluons.”
Matter seems pretty straightforward to understand. Take whatever system you want to understand, break it up into its constituents, and see how they bind together. You’d assume, for good reason, that the whole would equal the sum of its parts. Split apart a cell into its molecules, and the molecules add up to the same mass as the cell. Split up molecules into atoms, or atoms into nuclei and electrons, and the masses remain equal. But go inside an atomic nucleus, to the quarks and gluons, and suddenly you find that over 99% of the mass is missing. The discovery of QCD, our theory of the strong interactions, provided a solution to the puzzle, but for decades, calculating the masses in a predictive way was impossible. Thanks to supercomputer advances, though, and the technique of Lattice QCD, we’re finally beginning to truly understand where the mass of matter comes from.
Come get the scoop, and then tune in to a live-blog of a public lecture at 7 PM ET / 4 PM PT today to get the even deeper story!
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.
Ask Ethan: What’s So ‘Anti’ About Antimatter?
“On a fundamental level, what is the difference between matter and its counterpart antimatter? Is there some sort of intrinsic property that causes a particle to be matter or antimatter? Is there some intrinsic property (like spin) that distinguishes quarks and antiquarks? What what puts the ‘anti’ in anti matter?”
Every particle that exists has an antiparticle counterpart, with some particles behaving as their own antiparticles. Every time a particle collides with its antiparticle, it can annihilate away into pure energy; every time two particles collide with enough free energy, they can create particle/antiparticle pairs. But the key is that there are a whole slew of quantum numbers that must be conserved, and conserving energy is just the start of the story. Not every particle is matter; not every antiparticle is antimatter. Only fermions can have that designation. In fact, every fermion is a matter particle, and every anti-fermion is an antimatter particle, while the bosons, regardless of their other properties, are neither matter nor antimatter. Why is that? Because there are only two quantum numbers that are relevant to the question of whether you’re matter or not: baryon number and lepton number.
How do these come together to put the “matter” in normal matter, and the “anti” in antimatter? Find out on this edition of Ask Ethan!
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!
The Youngest, Most Massive Black Hole Is A Puzzle For Astronomy
“Recently, a new black hole, J1342+0928, was discovered to originate from 13.1 billion years ago: when the Universe was 690 million years old, just 5% of its current age.
It has a mass of 800 million Suns, an exceedingly high figure for such early times.
Even if this black hole formed from the very first stars, it would have to accrete matter and grow at the maximum rate possible — the Eddington limit — to reach this size so rapidly.
Fortunately, there are other ways to grow a supermassive black hole.”
We did it! We found our most massive, most distant quasar of all-time, telling us we’ve got a supermassive black hole that’s 800 million times as massive as our Sun when the Universe was just 5% of its current age. Even factoring in all we know about the formation and growth of black holes, from the early Universe and throughout all of time, we expect that there will only be around 20 black holes this large existing this early. Are there going to be more? Will we have to revise our current theories of cosmology and structure formation? Or is this simply an indication that we’re beginning to discover the brightest, most massive objects that are out there at any distance at all? As always, more and better data will decide, but here’s what we know so far.
Come get the full story, and some spectacular visuals (and video!) on today’s Mostly Mute Monday.
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.