Read Knocking on Heaven's Door Online
Authors: Lisa Randall
No matter what theory solves the hierarchy problem, it should provide experimentally accessible evidence at the weak energy scale. A train of theoretical logic will connect what we find at the LHC to whatever ultimately resolves this problem. It might be something we anticipate or it might be unforeseen, but it should be spectacular either way.
DARK MATTER
In addition to these particle physics issues, the LHC could also help illuminate the nature of the
dark matter
of the universe, the matter that exerts gravitational influence but does not absorb or emit light. Everything we see—the Earth, the chair you’re sitting on, your pet parakeet—is made up of Standard Model particles that interact with light. But visible stuff that interacts with light and whose interactions we understand constitutes only about four percent of the energy density of the universe. About 23 percent of its energy is carried by something known as dark matter that has yet to be positively ID’d.
Dark matter is indeed matter. That is, it clumps together through gravity’s influence and thereby (along with ordinary matter) contributes to structures—galaxies, for example. However, unlike familiar matter such as the stuff we’re made of and the stars in the sky, it doesn’t emit or absorb light. Because we generally see things through light that is emitted or absorbed, dark matter is hard to “see.”
Really, the term “dark matter” is a misnomer. So-called dark matter isn’t exactly dark. Dark stuff absorbs light. We can actually see dark stuff where light is absorbed. Dark matter, on the other hand, doesn’t interact with light of any kind in any observable way. Technically speaking, “dark” matter is transparent. But I’ll continue to use conventional terminology and refer to this elusive substance as dark.
We know dark matter exists because of its gravitational effects. But without seeing it directly, we won’t know what it is. Is it composed of many tiny identical particles? If so, what is the particle’s mass and how does it interact?
We might, however, soon learn much more. Remarkably, the LHC might in fact have the right energy to make particles that could be the dark matter. The key criterion for dark matter is that the universe contains the right amount to exert the measured gravitational effects. That is, the
relic density
—the amount of stored energy that our cosmological models predict survives to this day—has to agree with that measured value. The surprising fact is that if you have a stable particle whose mass corresponds to the weak energy scale that the LHC will explore (again via
E = mc
2
)
and whose interactions also involve particles with that energy, its relic density will be in the right ballpark to be dark matter.
The LHC could therefore not only give us insights into particle physics questions, but also give us clues to what is out there in the universe today and how it all began, questions that are incorporated into the science of cosmology, which tells us how the universe has evolved.
As with the elementary particles and their interactions, we understand a surprising amount about the universe’s history. Yet also as with particle physics, some very big questions remain. Chief among these difficult questions are these: What is the dark matter?, What is the even more mysterious entity called
dark energy?,
and What drove a period of exponential expansion of the early universe known as
cosmological inflation?
Today is a tremendous time for observations that might tell us the answers to these questions. Dark matter investigations are at the forefront of the overlap between particle physics and cosmology. Dark matter’s interactions with ordinary matter—matter we can make detectors from—are extremely weak, so weak that we are still looking for any evidence of dark matter aside from its gravitational effects.
Current searches therefore rely on the leap of faith that dark matter, despite its near invisibility, nonetheless interacts weakly—but not impossibly weakly—with matter that we know. This isn’t merely a wishful guess. It’s based on the calculation mentioned above that shows that stable particles whose interactions are connected to the energy scale that the LHC will explore have the right density to be dark matter. We hope that even though we haven’t yet identified dark matter, we have a good chance of detecting it in the near future.
However, most cosmology experiments don’t take place at accelerators. Dedicated outward-looking experiments on Earth and out in space are primarily responsible for addressing and advancing our understanding of potential solutions to cosmological questions.
For example, astrophysicists have sent satellites into space to observe the universe from an environment not obscured by dust and physical and chemical processes on or near the Earth’s surface. Telescopes and experiments here on Earth give us additional insights in an environment scientists can more directly control. These experiments in space and on Earth are poised to shed light on many aspects of how the universe has come to be.
We’re hoping that a sufficiently strong signal in any of these experiments (which we will describe in Chapter 21) will let us decipher the mysteries of dark matter. These experiments could tell us the nature of dark matter and illuminate its interactions and mass. In the meantime, theorists are thinking hard about all possible models of dark matter and how to use all these detection strategies to learn what dark matter really is.
DARK ENERGY
Ordinary matter and dark matter still do not provide the sum total of the energy in the universe—together they constitute only about 27 percent. Even more mysterious than dark matter is the substance that constitutes the remaining 73 percent and that has become known as dark energy.
The discovery of dark energy was the most profound physics wake-up call of the late twentieth century. Although there is much we don’t yet know about the evolution of the universe, we have a spectacularly successful understanding of the universe’s evolution based on the so-called Big Bang theory supplemented by a period of exponential expansion of the universe known as cosmological inflation.
This theory has agreed with a range of observations, including observations of the microwave radiation in the sky—the microwave background radiation left over from the time of the Big Bang. Originally the universe was a hot dense fireball. But during the 13.75 billion years of its existence it has diluted and cooled substantially, leaving this much cooler radiation that is a mere 2.7 degrees kelvin today—only a few degrees Celsius above absolute zero. Other evidence for the Big Bang theory of expansion can be found in detailed studies of the abundances of nuclei that were made during the universe’s early evolution and in measurements of the universe’s expansion itself.
The underlying equations we use to figure out how the universe evolves are the equations Einstein developed in the early twentieth century that tell us how to derive the gravitational field from a given distribution of matter or energy. These equations apply to the gravitational field between the Earth and the Sun but they also apply to the universe as a whole. In all cases, in order to derive the consequences of these equations, we need to know the matter and energy that surround us.
The shocking observation was that measurements of the characteristics of the universe required the presence of this new form of energy that is not carried by matter. This energy is not carried by particles or other stuff, and it doesn’t clump like conventional matter. It doesn’t dilute as the universe expands but maintains a constant density. The expansion of the universe is slowly accelerating as a consequence of this mysterious energy, which resides throughout the universe, even if it were empty of matter.
Einstein had originally proposed such a form of energy in what he called the
universal constant,
which later became known to physicists as the
cosmological constant.
Shortly after, he thought it a mistake and, indeed, that his use of it to try to explain why the universe was static was misguided. The universe does in fact expand, as Edwin Hubble showed soon after Einstein proposed the idea. The expansion is not only real, but it now seems that its current acceleration is due to the funny type of energy that Einstein had introduced and quickly dismissed in the 1930s.
We want to understand this mysterious dark energy better. Observations at this point are designed to determine whether it is just the sort of background energy that Einstein first proposed or whether it is a new form of energy that changes with time. Or is it something entirely unanticipated that we don’t yet even know how to think about?
OTHER COSMOLOGICAL INVESTIGATIONS
This is only a sampling—albeit an important one—of what we are now investigating. In addition to what I have already described, many more cosmological investigations are in store. Gravity wave detectors will look for gravitational radiation from merging black holes and other exciting phenomena involving large amounts of mass and energy. Cosmic microwave experiments will tell us more about inflation. Cosmic ray searches will tell us new details about the content of the universe. And infrared radiation detectors could find new exotic objects in the sky.
In some cases, we will understand the observations sufficiently well to know what they imply about the underlying nature of matter and physical laws. In other cases, we’ll spend a lot of time unraveling the implications. Regardless of what happens, the interplay between theory and data will lead us to loftier interpretations of the universe around us and expand our knowledge into currently inaccessible domains.
Some experiments might yield results soon. Others could take many years. As data come in, theorists will be forced to revisit and sometimes even abandon suggested explanations so we can improve our theories and apply them correctly. That might sound discouraging, but it’s not as bad as you might think. We eagerly anticipate the clues that will help us answer our questions as experimental results guide our investigations and ensure that we make progress—even when new results might require abandoning old ideas. Our hypotheses are initially rooted in theoretical consistency and elegance, but, as we will see throughout this book, ultimately it is experiment—not rigid belief—that determines what is correct.
MACHINERY, MEASUREMENTS, AND PROBABILITY
I am not one prone to overstatement, since I usually find that great events or achievements speak for themselves. This reluctance to embellish can get me into trouble in America, where people overuse superlatives so much that mere praise without an “est” at the end is sometimes misinterpreted as slander by faint praise. I’m frequently encouraged to add a few buzzwords or adverbs to my statements of support to avoid any misunderstanding. But in the case of the LHC I’ll go out on a limb and say there is no question that it’s a stupendous achievement. The LHC has an uncanny authority and beauty. The technology overwhelms.
In this chapter, we’ll embark on our exploration of this incredible machine. In the chapter that follows, we’ll enter the roller coaster construction adventure and a few chapters later, the world of the experiments that record what the LHC creates. But for the time being, we’ll focus on the machine itself, which isolates, accelerates, and collides together the energetic protons that we hope will reveal new inner worlds.
THE LARGE HADRON COLLIDER
The first time I visited the LHC, I was surprised at the sense of awe it inspired—this in spite of my having visited particle colliders and detectors many times before. Its scale was simply different. We entered, put on our helmets, walked down into and through the LHC tunnel, stopped at an enormous pit into which the ATLAS (A Toroidal LHC ApparatuS) detector would ultimately be lowered, and finally arrived at the experimental apparatus itself. It was still under construction, which meant ATLAS was not yet covered up as it would be when running—but was instead on display in full view.
Although the scientist in me recoils at first in thinking of this incredibly precise technological miracle as an art project—even a major one—I couldn’t help taking out my camera and snapping away. The complexity, coherence, and magnitude, as well as the crisscrossing lines and colors, are hard to convey in words. The impression is simply awe-inspiring.
People from the art world have had similar reactions. When the art collector Francesca von Habsburg toured the site, she took along a professional photographer whose pictures were so beautiful they were published in the magazine
Vanity Fair.
When the filmmaker Jesse Dylan, who grew up in a world of culture, first visited the LHC, he viewed it as a remarkable art project—a “culminating achievement” whose beauty he wanted to share. Jesse embarked on a video to convey his impressions of the grandeur of the experiments and the machine.
The actor and science enthusiast Alan Alda, when moderating a panel about the LHC, likened it to one of the wonders of the ancient world. The physicist David Gross compared it to the pyramids. The engineer and entrepreneur Elon Musk—who cofounded PayPal, runs Tesla (the company that makes electric cars), and developed and operates SpaceX (which constructs rockets that will deliver machinery and products to the International Space Station)—said about the LHC, “Definitely one of humanity’s greatest achievements.”
I’ve heard such statements from people in all walks of life. The Internet, fast cars, green energy, and space travel are among the most exciting and active areas of applied research today. But going out and trying to understand the fundamental laws of the universe is in a category by itself that astounds and impresses. Art lovers and scientists alike want to understand the world and decipher its origins. You might debate the nature of humanity’s greatest achievement, but I don’t think anyone would question that one of the most remarkable things we do is to contemplate and investigate what lies beyond the easily accessible. Humans alone take on this challenge.