The End of Everything: (Astrophysically Speaking)

by Katie Mack

  IX. Simple for stars, anyway.

  X. Based on current estimates, the Sun is already increasing in radius at about an inch per year. But at the same time, the Earth is expanding its orbit, such that we’re moving away from the Sun at about 15 centimeters per year (I make no apology for the mixed units here), so it’s not like the surface of the Sun is getting closer to us at the moment.

  XI. Supernova 1006, seen between April 30 and May 1, 1006, was likely a Type Ia supernova caused by the collision of two white dwarf stars about 7,000 light-years away, in our own galaxy. The remnant, which in astronomical images looks a lot like a colorful ball of smoke, can still be seen today.

  XII. They’re “dwarfs,” not “dwarves,” for reasons that are not entirely clear.

  XIII. If Eddington’s name sounds familiar, it might be because he made an eclipse expedition in 1919 that provided some of the first observational evidence for Einstein’s general theory of relativity. The observation of stars whose light grazed past the Sun en route to us showed that the light was being bent by the Sun’s distortion of space. (This is the kind of observation that can only happen when the Sun is eclipsed.) A famous headline at the time proclaimed, “LIGHTS ALL ASKEW IN THE HEAVENS—MEN OF SCIENCE MORE OR LESS AGOG OVER RESULTS OF ECLIPSE OBSERVATIONS.” Women of science were, presumably, unimpressed.

  XIV. This would be an excellent pun if Type Ia were likely to be able to create gold. While they can create other elements during the explosion (impressive amounts of nickel, for instance), due to the extreme temperatures and pressures involved, gold is probably mostly made in collisions of neutron stars. Alas.

  CHAPTER 6: Vacuum Decay

  None of the things one frets about ever happen. Something one’s never thought of does.

  Connie Willis, Doomsday Book

  In March 2008, a retired nuclear safety officer named Walter Wagner filed a lawsuit against the U.S. government to prevent scientists from starting up the Large Hadron Collider. From Wagner’s point of view, it was a desperate bid to save the world. The lawsuit was, of course, doomed to fail. For one thing, the LHC is controlled by the European Organization for Nuclear Research (abbreviated as CERN; the acronym comes from the French), not the U.S. government. And Wagner’s scientific worries, though presumably sincerely felt, were unfounded. In the end, CERN leadership put out some reassuring press releases about the safety of its collider technology, and the construction and operation of the LHC continued.

  That didn’t stop some segments of the public from ramping up their panic as the date of the first scheduled particle collisions approached. The LHC would be the most powerful particle physics experiment in history, colliding protons in four places along a giant circular, supercooled, vacuum-sealed underground track 27 kilometers in circumference. These collisions would produce, within the detectors, momentary bursts of energy so powerful they could re-create the conditions of the Hot Big Bang mere nanoseconds after the first moment of creation. The LHC would, the scientists hoped, lend us insight not only into the conditions of the early universe, but into the very structure of matter and energy itself. Earlier experiments had shown us that the laws of physics are energy-dependent—altering how particles and forces interact depending on the conditions in which they’re found—so creating collisions of higher and higher energies would allow scientists to probe the edges of our understanding of how physics works.

  And there was an even more tantalizing prize in sight. Decades before, physicists had theorized the existence of a new particle—one so central to the behavior of matter that it would be the final piece completing the Standard Model of particle physics. The Higgs boson, if it were discovered, would, for reasons we’ll get into shortly, finally confirm the leading theory explaining how fundamental particles were able to acquire mass in the early universe. And it would hopefully give us clues to the structure of physical law in regions beyond our current realm of exploration.

  But that very prospect—probing the unknown reaches of reality—was enough to strike fear into the hearts of onlookers. No one had ever created collisions at these energies. No one knew how the laws of physics might shift and re-form themselves in such an environment.

  Worst-case scenarios swirled around the internet. Perhaps the machine would open some kind of portal to another dimension, tearing apart the fabric of space itself. Perhaps it would create a tiny black hole that would grow, engulfing the entire planet. Perhaps it would create “strange matter”—a kind of composite material made of up-, down-, and strange-flavored quarksI that, some supposed, could lead to an ice-nine-style chain reaction,II converting all the matter it touched. But the physicists pressed on, apparently unconcerned. The LHC produced its first high-energy proton collisions in November 2009.

  Given that life on this planet still exists, it’s not too much of a spoiler to point out that none of the hypothesized existential disasters happened. (If you’re still worried, there is a live-update website: www.hasthelargehadroncolliderdestroyedtheworldyet.com.) But did we just get lucky? Was the experiment really warranted, given the potential risks?

  Physicists are not always cautious people, but exploring “what if” scenarios is kind of our bread and butter, and a chance to think deeply about the real physics behind hypothetical possibilities for ultimate destruction is too hard to pass up.III In fact, in 2000, four physicists (including one who would later win a Nobel Prize) wrote a sixteen-page paper for Reviews of Modern Physics called “Review of Speculative ‘Disaster Scenarios’ at RHIC.” RHIC was the Relativistic Heavy Ion Collider, a Brookhaven National Lab collider that predated the LHC and that was built to collide the nuclei of heavy elements such as gold at high energies. A pioneering experiment in its own right, it was also the subject of worries that it might create unforeseen consequences that could endanger the planet (or the universe), and the paper was written to fully explore, and hopefully dispel, those rumors.

  The results were encouraging. Not only did the researchers find that the chances of producing strange matter or black holes were incredibly small based just on theoretical considerations, there actually was experimental data to back this up. Specifically: the existence of the Moon.

  The argument for any kind of weird collider-induced phenomenon destroying us rests on the notion that the extreme high-energy collisions in these colliders are so unprecedented that we couldn’t possibly know what might happen. Which is ignoring an important fact: while the energies reached by RHIC and LHC might be novel to us puny humans, cosmic rays cruising through the universe reach incredibly high energies all the time, and collide with other objects and each other constantly. In the words of the RHIC paper authors, “It is clear that cosmic rays have been carrying out RHIC-like ‘experiments’ throughout the universe since time out of mind.” Collisions at much higher energies than what any Earthly collider could reach have been occurring across the universe for billions of years, so if they could destroy the cosmos, surely we would have noticed.

  “Hang on,” you might say, “what if cosmic ray collisions in deep space really are incredibly destructive, but just too far away to affect us? What if strange matter clumps exist all over the cosmos, and we just don’t know?” It’s a valid concern. While most of the time, particles produced in a collider are expected to have enough leftover momentum to zip out of the lab as soon as they’re formed, it’s conceivable that we could create something dangerous that could more or less come to rest in the detector. What then?

  Fortunately, we can use the Moon as our canary in the coal mine. We have enough data from Earth-based detectors and space telescopes to know that high-energy cosmic rays slam into the Moon all the time. (In fact, with radio telescopes we can even use the Moon as a neutrino detector,IV which is kind of amazing in and of itself.) If high-energy particle collisions could convert nearby ordinary matter into strange matter, this would have happened on the Moon eons ago, and we would have a VERY different object in our sky. Likewise, the night sky would be pretty noticeably alte
red if a tiny black hole formed on the Moon and swallowed it up. Not to mention the fact that we humans have actually been there, walked around, hit a few golf balls, and brought back samples. The Moon is doing just fine. Ergo, the authors argued, the RHIC won’t kill us all.

  Strange matter and black holes weren’t the only apocalypses debunked, though. Another possibility, similarly dismissed by witnessing the superior firepower of cosmic rays, is the notion that a powerful enough collision could trigger a universe-destroying quantum event called vacuum decay. The whole idea of vacuum decay rests on the hypothesis that our universe has a kind of fatal instability built into it. While that might sound scary even as just as a remote possibility, at the time the RHIC was commissioned, there was no real evidence for such a flaw, so it wasn’t taken especially seriously.

  When the LHC discovered the Higgs boson, in 2012, that all changed.

  THE STATE OF THE UNIVERSE

  A good way to make a particle physicist cringe is to refer to the Higgs boson by the name that made it famous: the God Particle. Our collective grumpiness around this lofty moniker isn’t fueled entirely by a discomfort with the mixing of science and religion (though for many that’s a big part of it). It’s also that “God Particle” is just terribly imprecise, and sounds a bit presumptuous, frankly. Which is not to say that the Higgs boson isn’t a deeply important part of the Standard Model of particle physics. It could even be argued that the Higgs is key to everything else fitting together. But it’s really the Higgs field, not the particle, that plays a central role in the workings of particle physics and the nature of the cosmos.

  The short version of the story is that the Higgs is a kind of energy field that pervades all of space and has interactions with other particles in a way that allows them to have mass. The Higgs boson has the same relationship to the Higgs field that the photon, the carrier of the electromagnetic force (and light), has to the electromagnetic field—it’s a localized “excitation” of something that pervades a larger space. The long version of the story has to do with electroweak theory, the theory that unites the weak nuclear force with electricity and magnetism, and how a process called “spontaneous symmetry breaking” separates those forces.

  (This is the part of the book where I really really want to teach you all of quantum field theory, but where by a heroic effort I limit myself to just touching on a few key issues. You’ll just have to trust me that if you decide to go and learn the mathematics behind all this, it gets MUCH cooler.)

  We talked in Chapter 2 about the fact that physics works differently at different energies. Electromagnetism and the weak nuclear force, for instance, act like fully separate phenomena at the kinds of energies we deal with in everyday life, but in the very early universe, at very high energies, they were aspects of the same thing. The Higgs field was instrumental in that transition; when it changed, the laws of physics changed too.

  This is a big part of why we build colliders: to create, in tiny little spaces inside our detectors, the kinds of extreme conditions that existed at the beginning of the universe and that can give us insights into the underlying physical principles that dictate how everything in physics fits together. The basic idea is that there must be some kind of overarching mathematical theory that gives us a blueprint for particle interactions under all possible conditions, and by continually producing higher and higher energy interactions, we get an increasingly clear picture of what that larger framework looks like.

  As an analogy, think of water. At the most fundamental level, it’s a collection of molecules made up of bonded hydrogen and oxygen atoms in a particular arrangement. But our everyday experience of water is as a uniform colorless liquid, or perhaps as a crystalline solid, or at certain unfortunate times as the kind of soul-crushing humidity that makes you wish your clothing were made of towels.V By examining water’s behavior in these different forms, we can make inferences about what it really is, even if we don’t have powerful microscopes at hand to see the individual atoms themselves. The shape of a snowflake, for example, tells us something about the shape of the molecules as they arrange into crystals. The way water evaporates tells us something about the bonds that hold the molecules together. If we only ever experienced water in one of its phases, we wouldn’t have a full picture of it, and it would be harder to get to that complete story. In the same way, our experience of subatomic particle interactions changes based on the energy (or temperature) of the experiment, and that allows us to get a better view of what’s really going on.

  What we want to know in particle physics is how the particles interact with each other and how their fundamental properties, like their masses, came to be what they are. The salient feature of any particle that has mass is that it can’t accelerate without an application of force, and it can never reach the speed of light. In the very early universe, the Higgs field underwent a transition that separated the electroweak force into electromagnetism and the weak force, and in the process gave some particles (though not the photon or gluon) the ability to interact with the Higgs field itself. The strength of that interaction determines the particle’s mass. The photon continues to zip through space at the speed of light, but the particles with mass move more slowly in proportion to the tugging they experience from the Higgs.

  Comparing the behavior of particles in the early universe to their behavior today is like comparing how you might interact with vapor versus liquid water. Imagine the vapor is the Higgs field—an energy field, present at every point in space. And imagine that at some point that Higgs field drastically changes character, as completely as vapor condensing into liquid water. If you’ve been used to encountering nothing but humid air, moving through a pool of water is a completely different prospect. When the Higgs field suddenly shifted in character, it was as though the laws of physics had condensed into a totally different form. Suddenly, particles that could move through space unimpeded at the speed of light were slowed down by their interactions with the Higgs field. They obtained mass.

  We call this process electroweak symmetry breaking.

  FEARFUL SYMMETRY

  Symmetry in physics is the kind of subtle, abstract concept that is extremely hard to explain without equations but which is also so absolutely vital to everything we think about as physicists that I can’t in good conscience just brush by it. Symmetry is central to how we describe theories of nature, and, more often than not, to how we develop new ones. If you happen to be someone who is used to thinking about the world in terms of the mathematical equations that govern it, you’re probably already comfortable with the idea that theories can be described in terms of the symmetries they obey; if you’re not, that’s complete gibberish to you, and understandably so. So let’s take a short detour for a moment and lay this all out, because it’s an incredibly beautiful thing, and once you know about it you see it everywhere.

  Symmetry isn’t just about whether or not something looks the same in a mirror. In physics, it’s all about patterns, and how those patterns can give you deeper insight into some underlying structure. Take, for instance, the periodic table of the elements. Why are the elements arranged in the rows and columns we are used to seeing them in today? If you’ve studied chemistry, you’ll know that certain columns collect elements that have things in common—the noble gases, the column on the far right, are all loath to chemically react, whereas the halogens, right next to them, are especially volatile. These patterns were discovered before the table was even complete, and in fact the creator of the table, Dmitri Mendeleev, left gaps for elements he knew should exist based on the pattern, even before they were discovered.

  The patterns in the periodic table led to theorizing about electron orbitals, which led to discoveries about the fundamental nature of subatomic matter. Over and over again, scientists have developed new theories of nature by recognizing patterns in their observations and then looking for a hidden property that could give them insight into what was really going on. We do this all the time, ourselves, withou
t noticing it. Watching highway traffic change over the course of the day can tell you standard business hours. The pattern of fading in a carpet can let you deduce which parts of the room get the most sunlight (and thereby, indirectly, tell you how the Earth and Sun are oriented in the Solar System).

  In the case of particle physics, using symmetry is often a lot like building new periodic tables, but for even smaller building blocks of nature. Similarities between particles—their charge, mass, or spin, for instance—can give clues about similarities in their formation or their connections to fundamental forces. Arranging these particles by their patterns lets physicists identify the symmetries that can be the defining features of entire theories.

  Sometimes those patterns are most easily seen mathematically. If you write down an equation to describe a physical process, and then find that you can swap some terms around without actually changing the physical phenomenon the equation describes, you’ve found a mathematical symmetry. And it’s probably telling you something deep about the particles or fields you’re describing.

  This symmetry-oriented way of looking at particles and the relationships between them is so prevalent in physics that we find ourselves using references to mathematical symmetries as shorthand for the theories themselves. For instance, electromagnetism is frequently called U(1) theory, because some aspects of the mathematics have the same kind of symmetry as a circle, and “U(1)” is shorthand for a mathematical group that describes rotations around a circle.

  A symmetry breaking event is when the conditions suddenly change such that the theory you would write down to describe how particles interact takes on a different, less symmetric, structure. After a symmetry breaking occurs, you can no longer swap around symbols in equations in the same way, and this change in symmetry expresses itself as altered behavior in the physical world.