by Katie Mack
Some of the symmetries we work with in physics are abstract and only obvious in the mathematics, but some are the usual stuff. Rotational symmetry is when something looks the same rotated by some angle (like a circle or a five-pointed star). Translational symmetry means something looks the same if you shift it to one side (e.g., a long picket fence moved over the distance of one picket, or a long straight line slid over by an inch). Breaking a symmetry involves doing something to the situation to make that symmetry no longer work. A wineglass has perfect rotational symmetry until a lipstick mark appears in one spot. A picket fence has translational symmetry until one of the slats is broken. Even a dinner party can include a symmetry breaking event, as is frequently observed by groups of physicists at conference banquets after the liquor comes out. As you wait patiently at the start of the meal, surrounded by a perplexing array of silverware, a small bread plate on either side of you, you are in a rotationally symmetric situation. As soon as one person reaches right or left to take up the bread plate, the symmetry is broken, and everyone else can follow their lead.VI
No matter what kind of symmetry we’re working with, as physicists, we’ll see it in the equations describing the interactions. There are ways to encode rotational, reflection, and translational symmetries in equations, so you know the physics stays the same no matter how you rotate, flip, or move the system in question. Equations can also encode subtler kinds of symmetries best described using group theory and abstract algebra that are FASCINATING but sadly pretty far outside the scope of this book.
When electroweak symmetry breaking occurred, way back when the universe reached the ripe old age of 0.1 nanosecond, it was a kind of rearranging of the structure of physics on a fundamental level.VII The rules particle interactions must follow are completely different in our post-electroweak-era universe. The previously vaporous Higgs field has become an ocean.
The water analogy isn’t perfect. When you move through water, you’re slowed down by drag, which means that you’ll come to a halt if you stop expending effort. In the case of massive particles interacting with the Higgs field, the interactions don’t slow them down over time. Anything moving through a vacuum tends to keep doing what it’s doing. In the case of massive particles, this frequently includes careening through the universe at very high (though sub-light) speeds. The main difference between massive and massless particles is that, in order to change speed, massive particles moving through a vacuum require a push, whereas massless ones travel at light speed effortlessly. In fact, massless particles can’t travel at anything but light speed.
So we, who enjoy the ability to sit still once in a while, should be grateful the Higgs field did what it did, and broke the electroweak symmetry. The Higgs field not only gives particles the ability to have mass, it also determines several of the fundamental constants of nature, like the charge of the electron, or the masses of particles. The particular physical state we live in, with the Higgs field nicely situated where it is, is referred to as our “Higgs vacuum” or “vacuum state.” If the Higgs field had some other value, or if the symmetry had broken in some other way, we might not be able to exist at all. We enjoy a universe in which the masses and charges of particles are perfectly set to allow them to come together in molecules, form structures, and carry out the chemical processes of life. If the field took some other value, this delicate balance might be off, potentially making these bonds impossible. We owe our entire corporeal existence to the fact that the Higgs has settled on the value it has.
And this is where things start to get a bit dicey.
Experiments like the LHC, which create extreme conditions that mimic those of the early universe, help us to see not just what the laws of physics are, but what they could be, under different circumstances. In 2012, when physicists finally were able to produce the Higgs boson in particle collisions, measuring its mass produced the final missing puzzle piece in the Standard Model of particle physics. It gave us a glimpse not only of the current value of the Higgs field, but of all the possible values it might take, given half a chance.
The good news is that the measurement of the Higgs mass is in perfect agreement with a nicely reasonable and mathematically consistent formulation of the Standard Model that has so far passed every experimental test with flying colors.
The bad news is that this consistent picture of the Standard Model also tells us that our Higgs vacuum—the perfectly balanced set of laws that govern the physical world—is not stable.
Our whole beautiful cosmos appears to be living on borrowed time.
A SLIPPERY-SLOPE COSMOS
The idea that our vacuum might not be stable isn’t a new one. Even in the 1960s and 1970s, physicists were gleefully writing papers imagining the possibilities for a universe that might undergo a catastrophic decay process, destroying all life as we know it and even all possibility of organized matter. Of course, at the time, vacuum decay was just a fun idea to play around with in the equations, with no experimental data to back it up.
Unlike now.
To understand vacuum decay, we have to understand the concept of a potential, a mathematical construct that represents how the value of a field can change, and where it “prefers” to be. You can think of the Higgs field as a pebble rolling down a slope into a valley, with the potential represented by the shape of that slope. Just as a pebble will settle in the bottom of a valley, the Higgs field will seek the lowest-energy state, where the potential is at its lowest value, and settle there, assuming nothing stops it. A sketch of the potential might look like a U shape, with the bottom of the U being the bottom of that valley. When electroweak symmetry breaking happened, it created the potential that the Higgs field is governed by, and as we generally imagine it, the Higgs is now safely settled at the bottom.
The problem is, that might not really be the bottom. There might be another vacuum state, at some even lower part of the potential. Imagine a kind of tilted rounded W shape, with one of the valleys, the one representing where our Higgs field doesn’t live, a bit lower than the other. If the Higgs potential has that second, lower valley, it suddenly goes from being a nice mathematical construct to an existential threat to the cosmos.
Wherever the Higgs field is now in its potential, it’s given us a perfectly livable, comfortable universe. We have constants of nature that are nicely compatible with bound particles and solid, life-compatible structures. If another state is possible, lower down the potential, all that is at risk.
In such a situation, the Higgs vacuum is only metastable. Stable… ish… for now. The field is stuck in a part of the potential that looks like the bottom of a valley but is actually more like a divot in the valley wall. It can stay tucked in there for a long time—plenty long enough for the growth of galaxies, the birth of stars, the evolution of life, and the production and distribution of more superhero movies than anyone could ever really want—but the possibility looms that a large enough disturbance could kick it over the edge, and then there would be nothing to stop it from landing on the actual valley floor. And that would be really, really, apocalyptically bad. For reasons we will shortly discuss in gory detail.
Figure 17: The potential of the Higgs field with a false vacuum state. Each valley in the potential is a possible state of the universe. If our Higgs field lives in the higher valley (the false vacuum), it could transition to the other state (the true vacuum) via a high-energy event (marked “fluctuations” on the diagram) or via quantum tunneling. If we live in a false vacuum universe, a transition of the Higgs field to the true vacuum would be catastrophic.
Unfortunately, the best data we have, consistent with every measurement of the Standard Model of particle physics, suggests that our Higgs field is currently clinging onto just such a divot. This metastable state is also known as a “false vacuum,” as opposed to the “true” vacuum at the bottom of the valley floor.
What’s wrong with being in a false vacuum? Quite possibly, everything. A false vacuum is at best a temporary reprieve
from ultimate destruction. In a false vacuum, the laws of physics, including the ability of particles to exist at all, are contingent on a precarious balancing act that could be upset at any moment.
When this happens, it’s called vacuum decay. It’s quick, clean, painless, and capable of destroying absolutely everything.
A BUBBLE OF QUANTUM DEATH
In order for vacuum decay to occur, there has to be a trigger—something that will set the Higgs field wandering far enough to find the part of the potential corresponding to the “true” vacuum and realize it would rather be there.VIII An ultra-high-energy explosion, or the catastrophic final evaporation of a black hole, or even an unfortunate quantum tunneling event (more on these later) could set it off. If this happens anywhere in the cosmos, it creates an unstoppable apocalyptic cascade that nothing in the universe can withstand.
It starts with a bubble.
Wherever the event occurs, a tiny bubble of true vacuum forms. This bubble contains within it a drastically different kind of space—one in which the processes of physics follow different laws, and the particles of nature are rearranged. At the moment it forms, it’s an infinitesimal speck. But it is already surrounded by a bubble wall of extremely high energy that could incinerate anything it touches.
Then, the bubble begins to expand.
Because the true vacuum is the more stable state, the universe “prefers” it, and will revert to it if given the slightest chance, just like a pebble will roll down a slope if it’s placed on one. As soon as the bubble appears, the Higgs field all around it is suddenly being shaken down to the valley floor. It’s as though that first event knocks free every precariously balanced pebble near it, and then the avalanche spreads. More and more space succumbs to the true vacuum state. Anything unfortunate enough to be in the bubble’s path is first hit by the intensely energetic bubble wall, approaching at about the speed of light. Then it undergoes a process that could only be called total and complete dissociation, as the forces that previously held particles together in atoms and nuclei can no longer function.
Maybe it’s for the best that you don’t see it coming.
As dramatic as the process sounds from a bird’s-eye view, if you happen to be standing nearby when the bubble appears, you won’t notice it. Something coming at you at the speed of light is invisible—any little glint warning you of its approach arrives at the same time as the thing itself. There is no possible way to see it coming, or even to know that anything has gone wrong. If it approaches you from below, there will be a couple of nanoseconds during which your feet no longer exist while your brain still thinks it is looking at them. Fortunately, the process is also entirely painless: at no point will your nerve impulses be able to catch up with your disintegration by the bubble. It’s a mercy, really.
Of course, the bubble doesn’t stop with you. Any planet or star within its ever-expanding radius suffers the same fate, equally oblivious to what’s coming. Entire galaxies are engulfed and obliterated. The true vacuum cancels the universe entirely. The only regions able to escape are those that lie so far away that the accelerated expansion of the universe keeps them beyond the bubble’s horizon forever.
In fact, it’s entirely possible that, as we sit here now, calmly drinking our tea, vacuum decay has already occurred. Maybe we’re lucky and the bubble is beyond our cosmic horizon, swallowing up galaxies we would never have known. Or maybe it is, cosmically speaking, right next door, quietly approaching with relativistic stealth, destined to catch us unawares, between breaths.
Figure 18: The bubble of true vacuum. If a vacuum decay event happens at one place in the cosmos, it causes a bubble to expand outward at the speed of light, destroying everything in its path.
KICKING THE HORNET’S NEST
You shouldn’t worry about vacuum decay. Really. For several reasons. There are the obvious ones, of course: there’s no way to stop it if it’s happening; and you can’t know it’s about to; and it’s not like it would hurt; and no one would be around to miss you anyway, so what’s the point of worrying about it? You’re better off double-checking your smoke alarm batteries and, I don’t know, lobbying to close down coal power plants or something. But if for some reason that isn’t sufficiently reassuring, I can also say with a reasonable degree of certainty that vacuum decay is extremely unlikely to happen—at least, anytime in the next many many many trillions of years.
There are a few ways vacuum decay could, in theory, occur. The most straightforward is some kind of high-energy event. Think of it as the equivalent of an earthquake, knocking the pebble out of its divot to send it plummeting to the valley floor. Fortunately, the “earthquake” in this case would have to be really unfathomably powerful. The best estimates we have suggest that the event would have to be much more energetic than the most devastating explosions we’ve witnessed in the cosmos, and certainly many orders of magnitude stronger than anything we could possibly do with a human-built machine like the Large Hadron Collider. If we’re ever worried about that, we can always appeal again to the fact that particle collisions in the cosmos are and have always been reaching much higher energies than the LHC or any other machine possibly could, so as long as we haven’t blinked out of existence yet, our modern equivalent of banging rocks together is really no threat at all.
The difficulty of creating a high enough energy event to directly trigger vacuum decay comes down to the height of the potential barrier between our false vacuum and the true one. Going back to the picture of the pebble stuck in a divot, the potential barrier is the bit of land that sticks up to make a divot pocket-shaped. In our current best guess at the true shape of the Higgs potential, the divot is a substantial one, separated from the deeper true-vacuum valley by a very high ridge. The amount of energy it would take to kick the pebble over that ridge (or push the Higgs field over its potential barrier) is so high, it’s hardly worth worrying about.
Except… we’re living in a universe that doesn’t follow those kinds of rules. Our cosmos is fundamentally based on quantum mechanics, and in quantum mechanics, if you’re living on a subatomic scale, the path you take to get from one place to another might, very rarely, send you sailing right through solid objects without missing a beat. If you’re standing in front of a wall, you might not need to get enough energy to jump over it. You might be able to step right through it instead. Especially if “you” are the Higgs field.
TUNNELING INTO THE ABYSS
Quantum tunneling sounds like science fiction or some obscure theoretical thing that physicists sit around chuckling over while writing down incomprehensible equations. Like, sure, quantum mechanics says that you can’t ever really say for sure exactly where a particle is, or what path it’s taking as it travels. That means that to get the math to work out, you have to write down and calculate things about all the paths, even the outlandish ones that send the particle from one side of the lab to the other by way of a coffee shop three cities over. But that doesn’t mean that the particle really does that, right?
As it happens, the question of what the particle really does is surprisingly hard to answer, and has spurred a decades-long debate about interpretations of quantum mechanics. Where the particle goes on the journey between Point A and B is still something of a mystery, as is what it actually means that particles are measured as small localized things but still manage to obey the mathematics of waves that are spread out through space.
The one thing everyone agrees on is what the data say, and those data make it very clear that tunneling through seemingly impassable barriers is something that particles are very happy to do on the regular. Wherever a particle really goes in the interim, it’s clear that a wall can’t stop it. This sort of escape artistry is such normal behavior for particles that people who design things like cell phones and microprocessors have to take into account the fact that every once in a while an otherwise well-behaved electron will suddenly materialize on the wrong side of a chip. Some technologies, including flash memory, occasionally
also use this to their advantage. Scanning tunneling microscopes use the expectation of tunneling almost like a valve to drip electrons slowly onto a surface and get images of individual atoms.
Letting electrons sneak across short gaps or squeeze through insulating barriers is a nice party trick, but it gets significantly more ominous when you realize that quantum tunneling can be performed not only by particles, but also by fields. Fields like the Higgs, separated from that big true-vacuum valley by a potential barrier it can tunnel right through. The only thing standing between our nice hospitable universe and ultimate cosmic disaster suddenly looks a lot less solid.
The (somewhat) good news is that even something as weird as quantum tunneling does actually follow certain rules, at least when it comes to the expected rate of its occurrence. The probability for a tunneling event is based on the physical characteristics of the system, which means that how likely it is to happen over a set period of time can be very well known. It isn’t a total free-for-all. As hard as quantum mechanics may be to fully understand or interpret, it is at least calculable.
But those “rules” that we calculate don’t come in any form more reassuring than probabilities. We can’t confidently say that the Higgs field won’t tunnel across the barrier and create a bubble of quantum death right next to you in the next 30 seconds, setting off a process of unthinkable destruction that will tear through space for all eternity. What we can say is: that scenario is extremely unlikely. (At least, the “in the next 30 seconds” part is. If our vacuum really is metastable, strictly speaking, the bubble has to show up eventually.)
The best calculations we have suggest that our nice pleasant vacuum is not likely to undergo a radical rearrangement anytime soon—at the time of writing, the latest estimate gives us more than 10100 years. By then, we’ll likely be well into the process of a Heat Death, or perhaps, if we’re very unlucky, being torn asunder by a Big Rip. At which point maybe an instant painless obliteration won’t seem so bad.