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!
Ask Ethan: If Matter Is Made Of Point Particles, Why Does Everything Have A Size?
“Many sources state that quarks are point particles… so one would think that objects composed of them — in this instance, neutrons — would also be points. Is my logic flawed? Or would they be bound to each other in such a way that they would cause the resulting neutron to have angular size?”
When we consider things like molecules, atoms, or even protons and neutrons, they all have finite, measurable sizes. Yet the fundamental particles that they’re made out of, like quarks, electrons, and gluons, are all inherently points, with no physical size to them at all. Why, then, does every composite particle not only have a size, but some of them, like atoms, grow to be relatively huge almost immediately, even with only a few fundamental particles involved? It’s due to three factors that all work together: forces, the quantum properties of the particles themselves, and energy. Since the strong and electromagnetic forces work against each other, quarks and gluons can form finite-sized protons; protons and neutrons assemble into nuclei larger than the protons and neutrons combined would make; electrons, with their low mass and high zero-point energy, orbit around nuclei only at great (relative) distances.
Matter doesn’t need to be made of finite-sized particles to wind up creating the macroscopic Universe we know and love. Find out how on this week’s Ask Ethan!
Nuclear Physics Might Hold The Key To Cracking Open The Standard Model
“Interestingly, this could also lead to a renewed interest in the search for glueballs, which would be the first ever direct evidence of a bound state of gluons in nature! If the exotic QCD predictions of tetraquarks and pentaquarks are borne out in our Universe, it stands to reason that glueballs should be there as well. Perhaps the existence of these composite particles will be verified at the LHC as well, with incredible implications for how our Universe works either way.”
Nuclear physics has, for decades now, been regarded less as a window into fundamental physics and more of a derived science. As we’ve discovered that nuclei, baryons, and mesons are all composite particles made out of quarks, antiquarks, and gluons, though, we’ve realized that there are other possible combinations that nature allows, that should exist. In recent years, we’ve discovered tetraquark and pentaquark states of quarks and antiquarks, and yet there should be even more. QCD, our theory of the strong interactions, predicts that a set of exotic states of bound gluons – known as a glueball – should exist. Finding them, or proving that they don’t exist, might be a way to crack open the Standard Model in an entirely new way.
Nuclear physics might, after all these years, hold the key to going beyond the current limitations of physics.