by Harry Cliff
Despite this false dawn, there are good prospects that the imprints of gravitational waves in the cosmic microwave background will finally be detected in the coming years. A range of ambitious new telescopes, at the South Pole, high in the Atacama Desert, and orbiting the Earth, will produce maps of the cosmic microwave background so exquisitely detailed that they should be sensitive enough to finally spy the effects of primordial gravitational waves, if indeed they exist. If that were to happen, it could be our best chance to get actual data on the very first moments of the universe’s existence and the very highest energies imaginable.
Just before I left Joe’s office to head back to my rental car, he had a treat for me. “Take a listen to this,” he said, as he fiddled with his computer. After a few moments of quiet, I was startled by a loud, unearthly rumble coming from a subwoofer hidden at the back of the room. “This is the sound of the first gravitational wave.” As I listened, rising just above the deep rumbling came an abrupt thump, the sound of two black holes crashing headlong into each other, 1.3 billion years ago.
It’s easy to become inured to the achievements of modern science, but sitting in Joe’s office next to one of the most sensitive instruments ever built, listening to the echo of an event so remote in time and space and yet so vast and violent that it defies all description or imagination, I couldn’t help but feel a rising sense of optimism. Science is exploration, whether it’s done in the laboratory, the abstract world of mathematical theory, or by studying the signals from the universe itself. And as we explore we seem to endlessly stumble upon new phenomena and new mysteries that lead us farther and farther from where we started. Does this journey go on forever, or will we one day reach its end? That, perhaps, is the biggest question of all.
Skip Notes
*1 Einstein was famous for refusing to wear socks, complaining that his big toe invariably made holes in them.
*2 Just as a reminder, the electromagnetic, weak, and strong forces of the standard model appear to arise because of local symmetries in the laws of nature, which are called U(1), SU(2), and SU(3) respectively.
*3 Not to be confused with the experimental anomalies we were discussing in the previous chapter.
*4 I can recommend The Elegant Universe by Brian Greene.
*5 Let’s not get into an argument about Pluto.
*6 OK, to avoid being accused of physics imperialism I should say that strictly speaking it could describe any process involving fundamental particles or gravity. If you want to explain anything complicated like biology, economics or love then physics probably isn’t going to be much help.
CHAPTER 14
The End?
It is the year 843 million CE. After a hundred thousand millennia of construction, the Galactic Organization for Particle Physics (GOPP) calls a press conference to inaugurate its last and greatest scientific project. Hanging in the void of space, glistening like a circlet of silver around the center of the Milky Way, is the largest, most powerful, and most expensive machine ever built in the history of the observable universe—the Impossibly Large Hadron Collider. Three thousand light-years in circumference, the ILHC is the work of a pangalactic collaboration of more than eight hundred thousand intelligent species who have put aside their differences in an attempt to discover the fundamental nature of reality. Today is the day the whole galaxy has been waiting for, when at long, long last their gleaming new machine will collide particles with enough energy to probe the effects of quantum gravity. A complete understanding of the fundamental laws of nature is finally within reach.
It has been a long and bumpy road; centuries of grant proposals and funding applications, endless quibbling over which star systems would be awarded the key magnet contracts, not to mention more than one thousand court cases filed across the galaxy over fears that the collider will trigger the end of the universe. Even that very morning, the announcement had to be delayed after the French delegation demanded that the press release be made available in their own ancient language as well as in the far more widely spoken Galactic Creole.
Nonetheless, the moment has finally arrived. It has taken just over a million years to accelerate the protons to the required energy of 1019 GeV, and we are now just seconds from the first collisions. The director general of GOPP calls the room to attention with a wave of one of her twelve purple tentacles. “Ladies, gentlemen, incorporeal energy beings, and sentient fungi, the moment you’ve all been waiting for. I give you, the Planck scale!” Immediately, the screens around the large conference hall light up with fireworks of particles emanating from a point deep in the heart of the planet-sized detector. “Professor Splurg, the results if you please.”
A glowing orb of ethereal light approaches the lectern and hands a ticker-tape printout of the data analysis to the eager director general. “Ummm…well, this is interesting,” stutters the DG in a weak attempt to mask her alarm. “It seems we’ve made a black hole. Not to worry, though; perhaps if we put in a bit more energy…Professor Splurg, more power!”
With a straining of electromagnets, the ILHC pushes its protons beyond the Planck energy, to an incredible 1021 GeV. Yet more collisions flash across the screens surrounding the bemused journalists. “Ah, right, I see…,” stammers the DG. “Ladies, gentlemen, et cetera, my apologies…we will have to bring today’s proceedings to an end for now. I need some time to consult with my colleagues.”
This little bit of sci-fi silliness is an attempt to make a serious point: we may never be able to find out how the universe began. Even if we could build the ultimate collider to probe what happens down at the Planck length, we would end up squeezing so much energy into such a small space that we’d collapse our two particles into a black hole. The interior of a black hole is surrounded by a barrier known as the “event horizon,” from which nothing, not even light, can escape. As a result, what was happening down at the Planck length would be hidden behind the event horizon. Collide particles at even higher energy and the problem gets even worse—you just create a bigger black hole.
This point was made to me even more starkly by David Tong, a professor of theoretical physics and one of the stars of Cambridge’s Department of Applied Mathematics and Theoretical Physics, as we sat in his office on an overcast spring afternoon. Aside from being one of the world’s leading experts on quantum field theory, David is a captivating speaker, his excitement and curiosity shining through with every word, which coupled with his outward youth and thick-rimmed glasses puts you in mind of a real-life version of David Tennant’s Doctor Who.
David started out working on string theory but eventually got put off the subject by the fact that it’s almost impossible to test. “We’ve got to be extremely lucky to find any evidence for quantum gravity in an experiment,” he told me, “and it’s not going to happen in my lifetime, which makes it a little bit uninteresting.”
Then, with a mischievous sparkle in his eye, he went further: “If you really want a conspiracy theory, why is quantum gravity uninteresting? There are three things in nature, fundamental laws of physics, that suggest that quantum gravity is fundamentally something we cannot probe, or at least, nature does a very good job of hiding it.”
The first comes from the work of Kenneth Wilson, one of the greatest and possibly most underappreciated theoretical physicists of the twentieth century. Wilson is famous among particle physicists for his work on the renormalization group, a mathematical object that tells you how a system looks when you zoom in or out. Wilson’s insight was that in some sense it doesn’t matter what’s going on deep down if you want to understand how a system behaves at larger distances. Or as David put it, “Newton didn’t need to know about quarks to figure out how planets work.”
In other words, it doesn’t matter what the fundamental ingredients of the universe are down at the Planck scale; they’re very unlikely to leave any traces on the behavior of much bigger thin
gs that we can actually measure in the lab, like atoms or particles. Given that the whole subject of this book is an attempt to understand an apple pie by zooming down and down, that gave me some serious pause for thought.
“Number two, inflation in the early universe, what does inflation do? It just dilutes everything, makes sure any hint of what happens at the big bang is pushed way outside our cosmological horizon and just can never be seen.” Although inflation might conceivably create gravitational waves that we could detect evidence for, these would likely only ever allow us to see back to 10-36 seconds after the big bang, when inflation roared into action. The moment of the big bang itself, at time zero, gets dragged far beyond our view, over the horizon and out of sight, by the rapid expansion of space. Inflation hides the “b” of the big bang from us.
“Number three is cosmic censorship. Where can you hope to really learn about quantum gravity? Well, it’s at singularities at the centers of black holes, but they’re always hidden behind the event horizons! Gravity is weird; usually if you want to probe smaller distances, you build a bigger and bigger collider. But suppose you build a collider a hundred times the Planck scale, 1021 GeV, we know what’s going to happen, you collide them together, you form a big black hole.
“So, you wanna build an apple pie from scratch?” David asked. “Well, the scratch is hidden.”
* * *
—
Way back in the summer of 2011 when I was still a PhD student, I got to attend my first big international conference in the pretty midwestern city of Madison, Wisconsin. The LHC was only a year into its first run, the discovery of the Higgs was still another year in the future, and the presentations mostly consisted of preliminary results—No sign of supersymmetry yet, folks, but we’ll surely catch it soon—and speculative theoretical proposals. I admit to finding the long plenary sessions tedious at times, that is until I was shaken awake by the arrival onstage of the headline speaker of the conference, Nima Arkani-Hamed.*1
Nima speaks with a passion that makes you sit up and pay attention. Dressed head to toe in black, with a mane of black hair swept backward, he paced the stage like a lion eager to burst out of its cage, as a vision for the future of fundamental physics spilled out of him like a torrent. Barely pausing for breath, he spoke way over his allotted time and deep into the lunch break, but nobody seemed to mind. You couldn’t help but get swept along on the wave.
Based at the Institute for Advanced Study in Princeton, where Einstein spent his twilight years and which today is the Holy See of fundamental theoretical physics, Nima Arkani-Hamed is one of the world’s most influential physicists. He’s famous both for his many contributions to particle theory and for his charisma as a communicator, which puts him in high demand. So I was pretty pleased when I managed to get him on the phone for a conversation about apple pie, as he took a train from Princeton to New York. The first thing he said was characteristically startling: “I just want to mention one very cool thing that’s going on at the moment, one of those quiet intellectual revolutions that frames the subject for the next fifty years or more: we know the reductionist paradigm is false.”
“Oh,” I replied.
Reductionism is the idea that you can explain the world by breaking it down into its basic ingredients. It’s the philosophy that underpins particle physics. The entire story that I have been telling over the last fourteen chapters is the story of reductionism. It’s an approach to the world that has served us incredibly well for almost half a millennium. So the idea that it’s false is, to say the least, a big fucking deal.
The first challenge to reductionism comes from the expectation that if you collide two particles together with enough energy to probe the Planck scale you make a black hole, and if you try to keep going after that, go to even higher energies, you make even larger black holes. “This is one of the absolutely deepest things that we know about quantum mechanics and gravity,” Nima told me, “that in a very real sense, higher energies start turning into longer distances again, which is utterly mysterious from a reductionist point of view.”
Now arguably reductionism breaking down at the Planck scale might not worry us too much. After all, the Planck scale is far, far beyond our experimental reach at the moment. But what is surprising, shocking even, is that reductionism might fail us long before we get there. We could be watching it unravel right now at the LHC.
As we’ve seen, one of the biggest outstanding problems in fundamental physics is the fact that the Higgs field has a uniform value of 246 GeV everywhere, a Goldilocks number that gives particles nice sensible masses, allowing atoms—and thus our universe—to exist. Apart from the untestable multiverse, all the solutions to this problem imply that we should see new stuff as we zoom down to smaller and smaller distances, and therefore higher and higher energies. This new stuff could be superparticles, extra dimensions of space, or perhaps smaller building blocks inside the Higgs itself. However, so far at least, as the LHC has zoomed in on the vacuum, all we’ve seen is…the Higgs.
To borrow Ben Allanach’s metaphor, it’s akin to walking into a room and seeing a pencil standing upright on its tip. Confronted with such a weird situation, a reductionist would assume there must be something at shorter distances that we can’t see that’s keeping the pencil upright. Maybe there’s an ultrathin wire suspending the pencil from the ceiling, or a tiny invisible clamp that you can only see with a microscope. Not finding new stuff to stabilize the Higgs suggests that this approach is wrong; we can’t explain some features of the world by zooming in further.
“The real gauntlet that’s being thrown down here by the results of the LHC is a challenge to the reductionist paradigm,” Nima said, “but much closer to home, in a place that we didn’t expect.”
This is what’s really at stake now in fundamental physics—the very idea that we can continually learn more about the world by looking deeper and deeper. If reductionism turns out to break down when we try to understand the Higgs, it would be an enormous shake to the foundations of physics. For Nima, studying the Higgs “to death” is the most important task facing particle physics for the next half century.
“We have never seen anything like the Higgs before. This is not hyped, it’s not like we’re making a big deal about the latest particle. The Higgs is the first elementary particle of spin zero we’ve ever seen, it’s the simplest elementary particle we’ve ever seen, it doesn’t have any charge, the only property that it has is mass, and the very fact that it’s so simple is what makes it really theoretically perplexing.”
Since the Higgs’s discovery in 2012, ATLAS and CMS have been gradually improving our understanding of it, confirming that it really is spin 0 and measuring how it decays to the other particles. In the mid-2020s, the LHC will get a major upgrade to increase its collision rate, allowing physicists to zoom in on the Higgs even closer. However, by the time the LHC finally powers down around 2035, we will still only have a fairly fuzzy picture. To settle this issue once and for all may require an even more powerful microscope.
Nima has spent much of the last few years crisscrossing the globe, making the case for a successor to the LHC. Two potential projects have emerged as the leading contenders, one based at CERN and the other just outside Beijing. These machines would be true behemoths, around 100 kilometers in circumference, more than three times longer than the LHC and ultimately capable of accelerating particles to energies seven times higher. The CERN project is known as the Future Circular Collider (though presumably it’ll get a rebrand if it gets built) and comes in two stages. First a 100-kilometer tunnel would be bored through the Geneva-basin area—the largest that the geology can accommodate—running from the foothills of the Alps, under Lake Geneva, past the current CERN site, and all the way to the Jura Mountains. Into this enormous ring would go an electron-positron collider, designed to make huge numbers of Higgs bosons and study their properties in exquisite detail. Then comes the rea
l monster, a proton-proton collider like the LHC, which would be able to reach collision energies of 100 TeV (trillion electron volts).
Between them, these vast machines would offer a wealth of opportunities for new discoveries in the quantum world. To name just a couple, the proton collider would be so powerful that it would be able to more or less completely rule out the most popular form of dark matter*2 and be able to recreate the conditions that may have caused matter to form in the early universe. However, as far as Nima is concerned, studying the Higgs is far and away the most important target of these machines and more than enough justification (scientifically at least) on its own.
Of course, 100-kilometer particle colliders do not come cheap. The full Future Circular Collider project would cost an eye-watering €26 billion. But to put that in perspective, that’s significantly less than it cost to put humans on the Moon (around $152 billion, or €124 billion, in today’s money), and it would be spent over a period of around seventy years, with the proton machine completing its mission close to the start of the twenty-second century. And of course, such a project could only be realized by a collective global effort of dozens of countries pooling their resources over many decades. When spread out like this, it’s argued that the Future Circular Collider could fit within CERN’s existing annual budget, which costs each citizen of the United Kingdom around £2.30 per year, or as the physicist Andrew Steele pointed out, about the cost of a packet of peanuts.
