(I need to add that there's a lot more math that goes into it, including the wave-particle duality that is another quantum facet of nature, but I think this is enough to get my point across.)
And just like for photons and radiation, the energy levels of electrons around a nucleus come in discrete steps. An electron can be on step 1 or step 14, just like you can eat chip 1 or chip 14, but it can't be between steps. So when you add or remove energy from that electron (say, by having it absorb or emit some radiation), it can only do so in discrete chunks.
In other words, the radiation that an atom or molecule will absorb or emit will only correspond to specific energies, which for radiation means that the light going in or coming out can only be specific wavelengths. So a hot gas of one particular element will emit very distinctive light, not everything in the rainbow. A distinct pattern of lines in the spectrum. If that same gas is blocking a light behind it, it won't absorb all of it, just a few frequencies here and there.
So the curious but incredibly useful phenomenon of spectral lines that astronomers had known about for decades, used to unlock the vast expanses of the cosmos from the motions of distant stars to the expansion of the universe itself, owes its unique properties to the subatomic quantum nature of energy.
Thanks, Max!
Once the quantum revolution got seriously underway in the early twentieth century, physicists really went to town exploring the weird and wonderful world lying in between our molecules. At first blush, it's an odd coincidence that at the same time physicists were busy unlocking quantum mysteries, astronomers were revealing cosmological profundity, but really this was standard operating procedure. Different tribes of scientists will explore different realms of nature, carving out little investigative niches for themselves, Sherlocking the clues wherever they may lead.
Scientists don't realize the full implications of their work, both for their own fields (often) and for disciplines outside their own (almost always). As we saw above, the chemists of the nineteenth century would have never realized that their investigations of hot gases would unlock the motions of galaxies, or that Max Planck's awkward studies of blackbody radiation would end up being applied to relic radiation from a distant epoch of our universe.
The same goes for Paul Adrien Maurice Dirac, whose fabulous antimatter hotel I introduced earlier and who has such a fantastic full name that I just had to type it again (plus he authored his papers as “P. A. M. Dirac,” so I'm guessing he wanted us to remember the whole thing). Dirac was practically diabolical in his studies of theoretical physics—he had a useful/nasty habit of simply making up from whole cloth mathematical functions and operations just to push his understanding of physical relationships.3
It seems he often went on hunches, and if a new formalism worked, it worked, and he left it at that. Later generations of mathematicians, much to their consternation, were frequently forced to go back over Dirac's work and provide the necessary proofs to validate his insights.
But enough on the awesomeness of Dirac, one of my favorite physicists of all time (the other is James Clerk Maxwell, whom we already met, if only briefly).
Dirac was interested in a lot of the same problems that other now-renowned physicists, including Planck, Einstein, Schrödinger, Heisenberg, and Pauli (and tons of others), were in the 1920s: the strange mechanics of subatomic particles. One by one, experiment by experiment, scientists were beginning to piece together the rules that particles lived by.
One of those rules was a newfound property called spin. If you shoot some neutral atoms through a magnetic field, those particles won't even notice, and they'll just sail on through unmolested—that's one of the benefits of being “neutral.” But in 1922, Otto Stern and Walther Gerlach (as a side note, you may notice a lot of German names associated with quantum mechanics; they were kind of dominating theoretical physics at the time) noticed something fishy. When they sent totally neutral silver atoms, with an equal complement of positively charged protons and negatively charged electrons, through an inhomogeneous magnetic field (that just means that the “north” direction in the apparatus was slightly stronger than the “south”), the atoms would get deflected.
Hmmm. Neutral atoms, affected by a magnetic field. The only way to explain this result was if the fundamental particles buried inside the atom had a new property, something like the usual mass and charge that allowed them to respond to magnetic fields. By analogy, the physicists turned to spinning metal balls—if you charge up a metal ball and set it spinning, it acts like a magnet, and if you throw a magnet through an inhomogeneous magnetic field, it gets deflected.
Spinning charged metal balls it was, and so the new property was dubbed “spin.” Let's ignore the fact that particles like electrons are modeled to be infinitesimally small and aren't really, you know, metal balls and don't really, you know, spin. Think of it more as “magnetic response” and you'll save yourself some sanity.
It gets even weirder. The property of “spin” possessed by our subatomic brethren itself is quantized in an odd way: for an electron, for example, it only comes in two varieties, dubbed “up” and “down,” probably because the deflected silver atoms shot through Stern and Gerlach's experiment split up into two distinct clumps, one on the upside and one on the downside.
Through a ridiculous number of follow-up experiments, the scientific community devised a set of “rules” to explain what nature was up to…down there. These rules of spin explained all the results (ahem, because they were designed to) but didn't explain where this property came from. The mystery of the behavior of the supposedly neutral silver atoms was “solved” by the newly devised rules of spin: electrons like to pair up spinwise up-to-down, canceling out their effects, but with silver, there's a loner odd-number electron hanging out. It's the unpaired spin of this single electron that's responsible for the experimental results.
Like I said, the early quantum mechanics couldn't explain this spin effect naturally; it didn't pop out of the equations. At least, until Dirac took a crack at it. He wasn't directly trying to solve the spin problem (notice a pattern?), but a more pernicious one: reconciling the burgeoning rules of the quantum world with the lessons from Einstein about special relativity, which by this point was almost a couple dozen years old and generally accepted to be really, really important.
Special relativity itself is almost like a metatheory of physics: the fundamental relationships between space and time and mass and energy must be integrated into any theory of physics for it to be considered fully correct and universally applicable. As we saw earlier, Erwin Schrödinger attempted a reconciliation of special relativity and the mathematics behind quantum mechanics and gave up; he couldn't make heads or tails of what the chimera would mean.
So he decided on a simpler approach, declaring the incorporation of relativity to be Somebody Else's Problem, and produced a not-fully-correct-but-still-really-useful equation that students still today learn in Quantum 101.
Dirac tackled the same spin problem and initially also couldn't make heads or tails of the mathematics, but he just forged ahead anyway because he was Dirac and Schrödinger wasn't. By pretty much inventing a formalism from whole cloth, his solution to the quantum dilemma naturally incorporated the concept of spin—it popped right out of the equations. If Dirac had come around to this problem a decade earlier, he would have predicted this new quantum mechanic property of fundamental particles.
There was another consequence of Dirac's solution. Or rather, solutions. He identified a symmetry in the equations that predicted that every particle had an evil twin: one with the same mass, spin, and all the other properties, but with a perfectly opposite charge.
Antimatter.
The ensuring decades saw a gold rush of physicists hunting for new particles—and striking gold. Unfortunately this isn't a book on fundamental physics, except where it intersects our story of the universe (please call Prometheus Books and beg for Your Place in the Quantum: Understanding Our Tiny, Messy Exis
tence), but since the early moments of the big bang were such a hot and crazy time, and the deep future of our universe will see a resurgence of some strange characters, we need a nanozoology lesson.
At the highest level, you have two general kinds of particles: the fermions and the bosons. Extremely generally, the fermions are the things, and the bosons are carriers of the forces. If I chopped up your molecules into atoms and cracked open the atoms, I would find a bunch of fermions: protons, neutrons, and electrons. To do the cracking I might employ a giant laser, which is made up of photons, the boson carrier of the electromagnetic force.
Hence the name W boson for the carrier of the Weak force. The pickup trucks of the strong nuclear force, the gluons, are also bosons and do exactly what their name suggests (if you recall). Gravity doesn't formally have a particle-based carrier known to current theories, but that boson is called the graviton anyway.
The only difference between the fermion camp and the boson gang—besides whom they're named after, Enrico Fermi and Satyendra Nath Bose—is their spin. The way the conventions in the mathematics worked out, fermions are defined to have “half-integer spin” (like 1/2, 3/2, 5/2, etc.) and the bosons have “whole-integer spin” (like 1, 2, 3, etc.). That may seem like splitting hairs, but for reasons I won't go into, this difference has profound implications for how the particles mix together—all the fundamental differences between how particles like photons behave and how particles like electrons behave can be attributed to their different spins.
And it turns out that protons and neutrons aren't fundamental; they're made of smaller dudes named quarks. There are six quarks: top, bottom, up, down, strange, and charm. Don't ask about the names.4 Quarks normally either pair up (and the buddy-system pair of quarks is now dubbed a “meson”) or run around as triplets (now called baryons).
Baryons are far, far more common than mesons, hence why we focused on them so much in the story of baryogenesis in the early universe and their ultimate victory over their evil, mustachioed antimatter twins.
Just in case you were wondering if the electron was feeling lonely, don't worry. There's a version of the electron that has the same charge and spin but is about two hundred times more massive; it's called the muon. And a heavier version still is called the tau. Paired with each of these is a related neutrino. Neutrinos themselves are nearly massless, ghostly particles that hardly ever interact with anything, ever. So there's an electron neutrino, muon neutrino, and tau neutrino.
The electron, its two heavier siblings, and the only-visit-for-Christmas neutrinos form a group called the leptons.
So there are six quarks and six leptons. The quarks feel the strong force, so they bind up tightly into balls that we call protons, neutrons, kaons, pions, and so-ons. The protons and neutrons are especially friendly and bunch up into atomic nuclei. While the strong force is, well, strong, its limited range confines its influence.
The leptons don't feel the strong force, so they just hang out by themselves.
The weak force transforms one kind of quark into another and, in the process, emits one of the electron triplets along with its associated neutrino. As we saw earlier it's a pretty intense force when conditions are right, which they hardly ever are in the present-day universe. It too has a rather short range, hence why it was the last force to be discovered.
The electromagnetic force will touch anything it can, and given that the photons are massless, it has infinite range. Think about it: that star you see twinkling in the night sky hurled its light thousands of years ago, and across the vastness of empty space, it was able to activate the rods and cones in the back end of your eyeball. That's a pretty impressive feat for a force, but electromagnetism does have one weakness: it can only care about you if you have an electric charge. If you're neutral, you're invisible to photons.
The last known force, gravity, is so weak it's hardly worth mentioning; and indeed, when it comes to particle physics it doesn't even enter into the calculations. As far as the inner workings of an atom are concerned, gravity might as well not even exist. But as we saw in the early years of the universe, it does have two superpowers. One: like the photon, it has infinite range. The gravity of that distant star isn't affecting you nearly as much as Kepler would have preferred, and at the barest fraction of what the photon can do, but technically it does have some influence.
This very book, sitting right in front of your face, has a greater gravitational influence on you than even the largest planets in our solar system. So there.
The second superpower is that gravity acts on everything. If you have energy or mass, gravity is going to touch you. So even though it's weak, it's persistent, and in the cosmological game you definitely get an A for effort.
Did I mention the antiparticles? Every lepton—every electron, muon, tau, and neutrino—and every quark—all six of them—has an associated antiparticle.
It's a zoo run by an insane zookeeper, so don't feel inadequate if it seems like it's just a torrent of names, jargon, and stats. I barely even scratched the surface of particle interactions, but I wanted to at least introduce them because it's about to get a lot funkier in here.
Following Dirac's lead, multiple scientists continued exploring the connection between relativity and quantum mechanics. The play of the game was simple—take each of the four forces in turn, shove special relativity up its rear end like a Thanksgiving turkey, and hope for the best.
First up to bat was electromagnetism (it was an obvious choice, given that it was this very theory that prompted Einstein to develop relativity in the first place). Unfortunately, the best was looking pretty peculiar. To make everything make sense, theorists had to revisit the concept of the field.
A field in physics is a lot like a cornfield, except mathematical. As you walk around the cornfield, you'll notice that the stalks are different heights. You could make a map, if you wanted, that assigned a particular cornstalk height to each position on the ground. That way, when you reported back to the homestead, if someone asked you about the height of corn at a given latitude and longitude, you could smartly and promptly return an answer.
Congratulations, you've made, uh, a field. A mathematical one. It's a set of values assigned to coordinates in space. The number of dimensions doesn't matter, and for our purposes we'll jump ahead from the farmer's back forty to the full four-dimensional space-time that we're now accustomed to.
Fields are used all over physics; you may have even heard of them used in casual conversation, depending on your definition of “casual.” Take, for example, the electric field. If you put a single isolated positive charge in the middle of space, we can, for the sake of mathematical convenience, assign a field to that electric charge. That field will tell you, wherever you may be located in respect to the isolated charge, how you might respond to it. If you yourself are positively charged and close by, the field will instruct you to be suitably repulsed. If you're negatively charged and far away, the field will whisper sweetly in your ear to come a little closer.
Electric fields, magnetic fields. Unified together into the electromagnetic field. The gravitational field. By the twentieth century, fields were about as common as…fields. They were already seen to be more than a mathematical convenience, and after the work of Dirac and others, they took on an entirely new property.
The fields associated with our normal everyday experience, like the electromagnetic field, aren't just passive conveyers of information—they are alive. They are dynamic. They flex and bend and wave. Waves of electricity and magnetism are already familiar to us: that's simply light. Good old radiation.
Here's the juicy bit. When quantum mechanics and special relativity unite to take a stab at fully explaining the electromagnetic force, it becomes all about the waves. In this picture the entire universe, all of space-time, is filled with a field, the electromagnetic field. Ripples can propagate through this field, which we correctly identify as light.
But it is at its heart a quantum field. And the nu
mber one rule of the quantum world is that nothing is smooth; everything comes in packets, or chunks, or bits, or units: quanta. Thus waves of the electromagnetic field can only be so small for a given amount of energy; there's a minimum wave size on the field: the photon. The familiar particle version of electromagnetic radiation.
Now take this concept that doesn't sound so radical for radiation and extend it. To everything. Look down at your hand. You know that if you could examine it closely enough, you would see tissues composed of cells composed of molecules composed of atoms composed of subatomic particles. Those particles, the electrons and quarks buzzing around, are simply vibrations: excitations of a field that permeates all of space-time. This is the world of quantum field theory, the crowning achievement of twentieth-century physics. In this picture the field is the ultimate physical object. There is one field associated with each type of particle, from the electromagnetic field for radiation to the suitably named Dirac field for electrons.
The fields fill up the universe and overlap each other, constantly awash with waves and counterwaves and excitations and propagations. A ripple rushing through one of these fields in space-time could represent anything: a cluster of protons racing from a supernova explosion, a brief weak force interaction, or the dance between gluons and quarks inside a nucleus.
These fields can support localized vibrations, but since they are distinct physical entities in their own right—arguably more “real” than the particles we associate with them—they can change and evolve under their own rules. Thus creatures like the hypothetical “inflaton” field can dynamically change, and since that field permeates all of space-time, it can radically impact the rate of expansion in the earliest moments of the universe—inflation.
It's admittedly hard to think about particles as anything but particles. But some of the peculiar properties of particles click into place once you adopt an appropriately quantum field worldview. Like the fact that they can be created and destroyed at will.
Your Place in the Universe Page 13