Category: particle physics

This Is The One Symmetry That The Universe Must Never Violate

“In physics, we have to be willing to challenge our assumptions, and to probe all possibilities, no matter how unlikely they seem. But our default should be that the laws of physics that have stood up to every experimental test, that compose a self-consistent theoretical framework, and that accurately describe our reality, are indeed correct until proven otherwise. In this case, it means that the laws of physics are the same everywhere and for all observers until proven otherwise.

Sometimes, particles behave differently than antiparticles, and that’s okay. Sometimes, physical systems behave differently than their mirror-image reflections, and that’s also okay. And sometimes, physical systems behave differently depending on whether the clock runs forwards or backwards. But particles moving forwards in time must behave the same as antiparticles reflected in a mirror moving backwards in time; that’s a consequence of the CPT theorem. That’s the one symmetry, as long as the physical laws that we know of are correct, that must never be broken.”

Are the laws of physics the same, here and now, as they are at all other times and places in the Universe? If they are, then that means a certain symmetry exists. It appears, to up to 18 orders of magnitude in some experimental cases, that specific symmetry is good and respected by our Universe. If a theory demands its violation, that theory is presently unsupported by all of the evidence. The reason? 

One particular symmetry, known as CPT, that the Universe forbids us from violating.

Ask Ethan: It’s Absurd To Think Dark Matter Might Be Made Of Hexaquarks, Right?

“Lots of science headlines [are] telling me dark matter might be a Bose-Einstein condensate of d* hexaquarks. Only problem I see? When notionally detected d* hexaquarks lived for 10-23 seconds. What’s your take?”

There are lots of ideas out there in physics: some good and some bad; some popular and unpopular. One of the recent ideas that’s caught on, at least among the general public, is that dark matter might be made out of hexaquark particles: the d*(2380) excited state resonance of the deuteron. The idea is pretty wild, but the excited state particle is real and observed, and many of the objections that you’d think of right away (that too much binding energy is required; that this would destroy the successes of Big Bang Nucleosynthesis) can be evaded. But even so, there are good physics reasons to think not only isn’t this possible, but it’s a bad idea from the get go. 

Come take a deep dive into the physics of hexaquarks, and learn why the d* particle is heavily unlikely to be dark matter.

Ask Ethan: Why Can’t The Large Hadron Collider Put More Energy Into Its Particles?

“Why can’t the LHC create particles with the energy of the OMG particle? What’s the limitation? Why can’t such a vast, incredibly powerful machine pump a mere 51 joules into a single subatomic particle?”

Over in Europe, the Large Hadron Collider reigns supreme as humanity’s most powerful particle accelerator of all-time. With particles achieving energies of approximately 7 TeV apiece and reaching speeds of 99.9999988% the speed of light, it’s already reached its limit. Try to accelerate these particles inside to higher energies, and your magnets won’t be able to keep them on course in this 27 km ring.

You could try building a bigger ring or applying a stronger magnetic field, but both of those are expensive and have limits themselves. However, in space, there are objects like pulsars, magnetars, and potentially even black holes that have field strengths we cannot even dream of reaching here on Earth. The LHC runs into the limits of physics, just like everything else does, and getting that extra energy into a particle means you’d lose control over it.

But there are ways to achieve that goal, if only we dream big enough. The LHC can’t put more energy into its particles, but a future collider just might!

How Certain Are We That Protons Don’t Decay?

“There is no arguing, however, that in all our endeavors to measure the stability of the proton, we’ve never observed even one event of a proton spontaneously decaying into lighter particles and violating the conservation of baryon number. If the proton is truly stable and will never decay, it means that a whole lot of proposed extensions to the Standard Model — Grand Unification Theories, supersymmetry, supergravity and string theory among them — cannot describe our Universe.

Regardless of whether the proton is truly stable forever and ever or “only” stable for a septillion times the current age of the Universe, the only way we’ll figure it out is by performing the critical experiments and watching how the Universe behaves. We have a matter-filled Universe almost completely devoid of antimatter, and nobody knows why. If the proton turns out to be truly stable, many of our best ideas for what could cause it will be ruled out.

The secrets of nature may remain a mystery for a little while longer, but as long as we keep looking, there’s always the hope of a new, revolutionary discovery.”

Do protons decay? If they do, we’d have a hint of where our matter-antimatter asymmetry comes from. We’d have an idea that grand unification might be correct, either with or without supersymmetry, extra dimensions, or string theory. And we’d learn that nothing, not even the humble atom, will be stable forever; much like galaxies and black holes, they’ll also eventually decay.

But if the proton is stable, as we’ve observed so far, it’s back to the drawing board on all of it. Here’s how certain we are, at the start of 2020, that the proton really is stable.

The ‘Strong CP Problem’ Is The Most Underrated Puzzle In All Of Physics

“At almost every frontier in theoretical physics, scientists are struggling to explain what we observe. We don’t know what composes dark matter; we don’t know what’s responsible for dark energy; we don’t know how matter won out over antimatter in the early stages of the Universe. But the strong CP problem is different: it’s a puzzle not because of something we observe, but because of the observed absence of something that’s so thoroughly expected.

Why, in the strong interactions, do particles that decay match exactly the decays of antiparticles in a mirror-image configuration? Why does the neutron not have an electric dipole moment? Many alternative solutions to a new symmetry, such as one of the quarks being massless, are now ruled out. Does nature just exist this way, in defiance of our expectations?

Through the right developments in theoretical and experimental physics, and with a little help from nature, we just might find out.”

The list of unsolved problems in physics is long and glorious, reflecting our curiosity about every yet-to-be-understood facet of our Universe. Most of what remains to be understood is because we observe something that we can’t explain: we see a phenomenon that our best current theories won’t predict without adding something new. But the strong CP problem is different: our best current theories predict that there should be CP violation in the strong interactions, but we don’t see any at all!

This is a puzzle that never gets the prominence it’s due based on its merits. Don’t be intimidated; find out what all the fuss is about today!

This One Award Was The Biggest Injustice In Nobel Prize History

“Every October, the Nobel foundation awards prizes celebrating the greatest advances in numerous scientific fields. With a maximum of three winners per prize, many of history’s most deserving candidates have gone unrewarded. However, the greatest injustices occurred when the scientists behind the most worthy contributions were snubbed.”

Imagine this scenario: you work hard all your life investigating some aspect of reality with as much scientific rigor as anyone ever has. You make a great breakthrough working on a very hard problem, and you push your scientific field forward in a novel, important, and unprecedented way. And then, when the time comes to evaluate the quality and impact of your work, it’s chosen as being Nobel-worthy. 

Only, when they announce the winners of the Nobel Prize, your name isn’t called at all. Instead, other scientists are awarded the prize, while both your name and your decisive work are omitted from every aspect of the award. Sounds like a pretty big injustice, yes? 

Well, it’s happened to many people over the years, including Chien-Shiung Wu in perhaps the greatest injustice of them all. Come get the full story on the eve of the 2019 Nobel Prize in physics being awarded!

Ask Ethan: Why Are There Only Three Generations Of Particles?

“It is eminently possible that there are more particles out there than the Standard Model, as we know it, presently predicts. In fact, given all the components of the Universe that aren’t accounted for in the Standard Model, from dark matter to dark energy to inflation to the origin of the matter-antimatter asymmetry, it’s practically unreasonable to conclude that there aren’t additional particles.

But if the additional particles fit into the structure of the Standard Model as an additional generation, there are tremendous constraints. They could not have been created in great abundance during the early Universe. None of them can be less massive than 45.6 GeV/c^2. And they could not imprint an observable signature on the cosmic microwave background or in the abundance of the light elements.

Experimental results are the way we learn about the Universe, but the way those results fit into our most successful theoretical frameworks is how we conclude what else does and doesn’t exist in our Universe. Unless a future accelerator result surprises us tremendously, three generations is all we get: no more, no less, and nobody knows why.”

There are three generations of (fermionic) particles in the Universe. In addition to the lightest quarks (up and down), the electron and positron, and the electron neutrino and anti-neutrino, there are two extra, heavy “copies” of this structure. The charm-and-strange quarks plus the top-and-bottom quarks fill the remaining generations of quarks, while the muon and muon neutrino and anti-neutrino plus the tau and tau neutrino and anti-neutrino comprise the next generation of leptons.

Theoretically, there’s nothing demanding three and only three generations, but experiments have shown that there are no more to within absurd constraints. Here’s the full story of how we know there are only three generations.

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!

Forget About Electrons And Protons; The Unstable Muon Could Be The Future Of Particle Physics

“Humanity can always choose to build a bigger ring or invest in producing stronger-field magnets; those are easy ways to go to higher energies in particle physics. But there’s no cure for synchrotron radiation with electrons and positrons; you’d have to use heavier particles instead. There’s no cure for energy being distributed among multiple constituent particles inside a proton; you’d have to use fundamental particles instead.

The muon is the one particle that could solve both of these issues. The only drawback is that they’re unstable, and difficult to keep alive for a long time. However, they’re easy to make: smash a proton beam into a piece of acrylic and you’ll produce pions, which will decay into both muons and anti-muons. Accelerate those muons to high energy and collimate them into beams, and you can put them in a circular collider.”

There are lots of possibilities being discussed for how we could build a next-generation particle collider, capable of pushing past the frontiers where the LHC will be fundamentally limited. We could go to a larger proton collider, we could go back to doing high-precision collisions of electrons and positrons to create large numbers of the known, existing particles, or we could push the frontiers in an entirely new way: by colliding muons with anti-muons.

“But they only live for 2.2 microseconds,” you correctly object. Good thing we understand physics. If we can get the technology there, it’s the best option imaginable.

This Is Why Two Higgs Bosons Don’t Have The Same Mass As One Another

“In this quantum Universe, every particle will have properties that are inherently uncertain, as many of the measurable properties are changed by the act of measurement itself, even if you measure a property other than the one you wish to know. While we might talk about photon or electron uncertainties most commonly, some particles are also unstable, which means their lifetime is not pre-determined from the moment of their creation. For those classes of particles, their inherent energy, and therefore their mass, is inherently variable, too.

While we might be able to state the mass of the average unstable particle of a particular variety, like the Higgs boson or the top quark, each individual particle of that type will have its own, unique value. Quantum uncertainty can now be convincingly extended all the way to the rest energy of an unstable, fundamental particle. In a quantum Universe, even a property as basic as mass itself can never be set in stone.”

Create an electron, and there will be a certain set of properties that you’ll know for certain, irrespective of any quantum uncertainty. You’ll know its mass, its electric charge, its intrinsic angular momentum, and many other properties as well. But that’s because the electron itself is a fundamentally stable particle: it’s lifetime is infinite, with no uncertainty. This isn’t true for many of the particles of the Standard Model, though, with the heaviest particles like the Higgs boson, the W and Z bosons, and the top quark having the shortest lifetime. Well, there’s also an energy-time uncertainty relation, and that means that the shorter your lifetime is, the bigger your inherent uncertainty in your energy is. Now, combine that with the knowledge that E = mc^2, and what do you get? 

An inherently uncertain mass. Yes, it’s true: every top quark you create has a unique mass that’s different from every other top quark. Come find out the science behind this remarkable property of nature!