Knocking on Heaven's Door: How Physics and Scientific Thinking Illuminate the Universe and the Modern World - Lisa Randall (2011)



On December 1, 2009, I reluctantly woke up at 6:00 A.M. at the Marriott near the Barcelona airport in order to catch a plane. I was visiting to attend the Spanish premiere of a small opera—for which I’d written a libretto—about physics and discovery. The weekend had been enormously satisfying, but I was exhausted and eager to get home. However, I was briefly delayed by a lovely surprise.

The lead story in the newspaper that the hotel provided at my door that morning was “Atom-smasher Sets Record Levels.” Rather than the usual headline reporting a horrible disaster or some temporary curiosity, a story about the record energies that the Large Hadron Collider had achieved a couple of days before was the most important news of the day. The excitement in the article about the milestone for the LHC was palpable.

A couple of weeks later, when the two high-energy beams of protons actually collided with each other, the New York Times ran a front-page news article titled “Collider Sets Record, and Europe Takes U.S.’s Lead.”35 The record energy reported by the earlier news was now on track to be only the first of a series of milestones to be set by the LHC during this decade.

The LHC is now probing the tiniest distances ever studied. At the same time, satellite and telescope observations are exploring the largest scales in the cosmos, studying the rate at which its expansion accelerates and investigating details of the relic cosmic microwave background radiation left over from the time of the Big Bang.

We currently understand a lot about the makeup of the universe. Yet as with most progress, further questions have emerged as our knowledge has grown. Some have exposed crucial gaps in our theoretical frameworks. Nonetheless, in many cases, we understand the nature of the missing links well enough to know what we need to look for and how.

So let’s take a closer look at what’s on the horizon—what experiments are out there and what we anticipate they might find. This chapter is about some of the chief questions and physics investigations that the rest of the book will explore.


The Standard Model of particle physics tells us how to make predictions about the light particles we’re made of. It also describes other heavier particles with similar interactions. These heavy particles interact with light and nuclei through the same forces the particles that constitute our bodies and our solar system experience.

Physicists know about the electron, and heavier similarly charged particles called the muon and the tau. We know that these particles—called leptons—are paired with neutral particles (particles with no charge that don’t directly experience electromagnetic interactions) called neutrinos, which interact only via the prosaically named weak force. The weak force is responsible for radioactive beta decay of neutrons into protons (and beta decay of nuclei in general) and to some of the nuclear processes that occur in the Sun. All Standard Model matter experiences the weak force.

We also know about quarks, which are found inside protons and neutrons. Quarks experience both the weak and electromagnetic forces, as well as the strong nuclear force, which holds light quarks together inside protons and neutrons. The strong force poses calculational challenges, but we understand its basic structure.

The quarks and leptons, together with the strong, weak, and electromagnetic forces, form the essence of the Standard Model. (See Figure 23 for a summary of the particle physics Standard Model.) With these ingredients, physicists have been able to successfully predict the results of all particle physics experiments to date. We understand the Standard Model’s particles and how its forces act very well.


FIGURE 23 ] The elements of the Standard Model of particle physics, which describe matter’s most basic known elements and their interactions. Up-and down-type quarks experience the strong, weak, and electromagnetic forces. Charged leptons experience the weak and electromagnetic forces, while neutrinos experience only the weak force. Gluons, weak gauge bosons, and the photon communicate these forces. The Higgs boson is yet to be found.

However, some big puzzles remain.

Chief among these challenges is how gravity fits in. That’s a big question that the LHC has some chance to explore but is far from guaranteed to resolve. The LHC’s energy—though high from the perspective of what has been previously achieved here on Earth and from the requirement of what it will take to address some of the big puzzles that come next on this list—is much too low to definitively answer the questions relating to quantum gravity. To do so, we would need to study the infinitesimally tiny lengths where both quantum mechanical and gravitational effects can emerge—and that is far beyond the reach of the LHC. If we’re lucky, and gravity plays a big role in addressing the particle problems that we’ll soon consider related to mass, then we will be in a much better position to answer this question and the LHC might reveal important information about gravity and space itself. Otherwise, experimental tests of any quantum theory of gravity—including string theory—are most likely a long way off.

However, gravity’s relation to the other forces isn’t the only major question left unanswered at this point. Another critical gap in our understanding—one that the LHC is definitively poised to resolve—is the way in which the masses of the fundamental particles arise. That probably sounds like a pretty strange question (unless of course you read my first book) since we tend to think of the mass of something as a given—an intrinsic inalienable property of the particle.

And in some sense that is correct. Mass is one of the properties—along with charge and interactions—that define a particle. Particles always carry nonzero energy, but mass is an intrinsic property that can take many possible values including zero. One of Einstein’s major insights was to recognize that the value of a particle’s mass tells how much energy it has when it’s at rest. But particles don’t always have a nonvanishing value for their masses. And those that have zero mass, like the photon, are never at rest.

However, the nonzero masses of elementary particles, which are an intrinsic property they possess, are an enormous mystery. Not only quarks and leptons, but also weak gauge bosons—the particles that communicate the weak force—have nonzero mass. Experimenters have measured these masses, but the simplest physics rules simply don’t allow them. Standard Model predictions work if we just assume particles have these masses. But we don’t know where they came from in the first place. Clearly the simplest rules don’t apply and something more subtle is afoot.

Particle physicists believe these nonvanishing masses arise only because something very dramatic occurred in the early universe in a process that is most commonly called the Higgs mechanism in honor of the Scottish physicist Peter Higgs who was among the first to show how masses could arise. At least six authors contributed similar ideas, however, so you might also hear about the Englert-Brout-Higgs-Guralnik-Hagen-Kibble mechanism, though I will stick with the name Higgs.36 The idea—whatever we call it—is that a phase transition (perhaps like the phase transition of liquid water bubbling into gaseous steam) took place that actually changed the nature of the universe. Whereas early on, particles had no mass and zipped around at the speed of light, later on—after this phase transition involving the so-called Higgs field—particles had masses and traveled more slowly. The Higgs mechanism tells how elementary particles go from having zero mass in the absence of the Higgs field to the nonzero masses we have measured in experiments.

If particle physicists are correct and the Higgs mechanism is at work in the universe, the LHC will reveal telltale signs that betray the universe’s history. In its simplest implementation, the evidence is a particle—the eponymous Higgs boson. In more elaborate physical theories in which the Higgs mechanism is nonetheless at work, the Higgs boson might be accompanied by other particles with about the same mass, or the Higgs might be replaced by some other particle altogether.

Independently of how the Higgs mechanism is implemented, we expect the LHC to produce something interesting. It might be a Higgs boson. It might be evidence of a more exotic theory such as technicolor that we will discuss later on. Or it could be something completely unforeseen. If all goes as planned, experiments at the LHC will discern what it was that implemented the Higgs mechanism. No matter what is found, the discovery will tell us something interesting about how particles acquire their masses.

The Standard Model of particle physics, which describes matter’s most basic elements and their interactions, works beautifully. Its predictions have been confirmed many times at a high level of precision. This Higgs particle is the last remaining piece of the Standard Model puzzle.37 We now assume particles have masses. But when we understand the Higgs mechanism, we’ll know how those masses came about. The Higgs mechanism, which is explored further in Chapter 16, is essential to a more satisfactory understanding of mass.

And there is another, even bigger, puzzle in particle physics where the LHC should help. Experiments at the Large Hadron Collider are likely to illuminate the solution to a question known as the hierarchy problem of particle physics. The Higgs mechanism addresses the question of why fundamental particles have mass. The hierarchy problem asks the question why those masses are what they are.

Not only do particle physicists believe that masses arose because of a so-called Higgs field that permeates the universe, we also believe we know the energy at which the transition from massless to massive particles occurred. That’s because the Higgs mechanism gives masses to some particles in a predictable manner that depends only on the strength of the weak nuclear force and the energy at which the transition occurs.

The peculiar thing is that this transition energy doesn’t really make sense from an underlying theoretical perspective. If you put together what we know from quantum mechanics and special relativity, you can actually calculate contributions to particle masses, and they are far bigger than what is measured. Calculations based on quantum mechanics and special relativity tell us that without a richer theory, masses should be much greater—in fact, 10 quadrillion, or 1016, times as big. The theory only hangs together with an enormous fudge physicists unabashedly call “fine-tuning.”

The hierarchy problem of particle physics poses one of the biggest challenges to the underlying description of matter. We want to know why the masses are so different from what we would have expected. Quantum mechanical calculations would lead us to believe they should be much bigger than the weak energy scale that determines their masses. Our inability to understand the weak energy scale in the superficially simplest version of the Standard Model is a real stumbling block to a fully complete theory.

The likely possibility is that a more interesting, more subtle theory subsumes the most naive model—a possibility we physicists find much more compelling than a fine-tuned theory of nature. Despite the ambitious scope of the question of what theory solves the hierarchy problem, the Large Hadron Collider is likely to shed light on it. Quantum mechanics and relativity dictate not only contributions to masses, but also the energy at which new phenomena must appear. That energy scale is the one the LHC will probe.

We anticipate that at the LHC a more interesting theory will emerge. This theory, which will address these mysteries about masses, should reveal itself when new particles and forces or symmetries show up. It’s one of the big secrets we hope LHC experiments will unmask.

The answer is interesting in itself. But it is likely to be the key to deep insights into other aspects of nature as well. Two of the most compelling suggested answers to the problem involve either extensions of symmetries of space and time, or revisions of our notion of space itself.

Scenarios that are further explained in Chapter 17 tell us that space might contain more than the three dimensions we know about: up-down, forward-backward, and left-right. In particular, it could contain entirely unseen dimensions that hold the key to understanding particle properties and masses. If that’s the case, the LHC will provide evidence of these dimensions in the form of particles known as Kaluza-Klein particles that travel throughout the full higher-dimensional spacetime.

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.


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 = mc2) 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.


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?


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.