Knocking on Heaven's Door (53 page)

People are often puzzled when I tell them that I work on many different models when I know that they can’t all be correct and that the LHC should tell us more about which could be right. They are even more surprised when I explain that I don’t necessarily assign huge probabilities to any particular model I am thinking about. Nonetheless, I choose projects that illuminate a genuinely new explanatory principle or new type of experimental search. The models I consider generally have some interesting feature or mechanism that provides interesting potential explanations for mysterious phenomena. Given the many unknowns—and uncertain criteria for progress—predicting and interpreting reality poses formidable challenges. It would be miraculous to get it all right from the get-go.

One of the beautiful aspects of the extra-dimensional theories is that ideas from both the top-down and bottom-up camps converged to produce them. String theorists recognized the critical role of branes in their theoretical formulations. And model builders realized that by reinterpreting the hierarchy problem as a question about gravity, they could find alternative solutions.

The Large Hadron Collider is now testing such ideas. Whatever the LHC discovers will guide and constrain model building in the future. With its higher-energy experimental results, we’ll be able to piece together observations to determine what is right. Even if observations don’t conform to any one particular proposal, the lessons we learned from constructing those models will help narrow down the possibilities for which theory is ultimately correct.

Model building helps us recognize the possibilities, suggest experimental searches, and interpret data once they are available. We might be lucky and get it right. But model building also gives us insights into what to look for. Even if no particular model’s predictions turn out to be completely correct, they will help us deduce the implications of any new experimental result. The results will distinguish among the many ideas and determine which—if any—of the specific implementations correctly describes reality. If no current proposal works, data will nonetheless help determine what the right model might be.

High-energy experiments are not merely searching for new particles. They are searching for the structure of underlying physical laws with even greater explanatory power. Until experiments help determine the answers, we are all just making guesses. For now we’ll apply aesthetic criteria (or prejudice) to favor certain models. But when experiments reach the energies or distances and statistics necessary to distinguish among models, we will know much more. Experimental results, such as those we hope that the LHC will provide, will determine which of our conjectures are correct and help us establish the underlying nature of reality.

SCALING THE UNIVERSE

INSIDE OUT

Back when I was in elementary school, I woke up one morning to read the bewildering news that the universe (at least in our understanding) had suddenly aged by a factor of two. I was astonished by this revision. How could something as important as the universe’s age be at liberty to change so radically without destroying everything else about it that we knew?

Today my surprise works in the opposite direction. I am stunned by how much we can precisely measure now about the universe and its history. Not only do we know the universe’s age much more accurately than ever before, but we know how the universe grew with time, how nuclei were formed, and how galaxies and clusters of galaxies began their evolution. Before, we had a qualitative picture of what had happened. Now we have an accurate scientific picture.

Cosmology has recently entered a remarkable era in which revolutionary advances, both experimental and theoretical, have precipitated a more extensive and detailed description than anyone would have believed possible even 20 years ago. By combining improved experimental methods with calculations rooted in general relativity and particle physics, physicists have established a detailed picture of what the universe looked like in its earlier stages and how it evolved into its form today.

So far, this book has focused primarily on smaller scales at which we examine the inner nature of matter. Having reached the current limit of our inward journey, let’s now complete the tour over distance scales we began in Chapter 5 and turn our attention outward to consider the sizes of objects in the outer universe.

We need to be wary of one big difference in this journey to cosmic scales since we can’t neatly characterize all aspects of the universe according to size alone. Observations don’t just record the universe today. Because of the finite speed of light, they also look back in time. Structures we observe today can be early universe occupants whose light reached our telescopes only billions of years after being emitted. The size of the current greatly expanded universe we now see encompasses many times the size of the universe earlier on.

Size nevertheless plays a critical role in characterizing our observations—both of the current universe and its history over time, and this chapter explores both. In the second half, we’ll consider the evolution of the universe as a whole, from its tiny initial size to the vast structure we now observe. But first we’ll look out at the universe as it appears today in order to familiarize ourselves with some of the lengths that characterize what surrounds us. We’ll work our way up in scales to consider larger sizes and more distant objects—on Earth and in the cosmos—to get a feeling for the bigger types of structures that are out there to explore. This tour of large scales will be briefer than our earlier tour of matter’s interior. Despite the richness of structure in the universe, most of what we see can be explained with known physical laws—not fundamental, new ones. Star and galaxy formation rely on known laws of chemistry and electromagnetism—science rooted in the small scales we have already discussed. Gravity, however, now plays a critical role as well, and the best description will depend on the speed and density of the objects it is acting on, leading to varying theoretical descriptions in this case too.

TOUR OF THE UNIVERSE

The book and film
Powers of Ten,
⁶⁷
one of the iconic tours of distance scales, starts and ends with a couple sitting in Grant Park in Chicago—as good a place to begin our journey as any. Let’s momentarily pause on (what we now know to be largely empty) solid ground to view the familiar lengths and sizes around us. After momentarily reflecting on their human scale of about a couple of meters’ height, let’s take leave of this comfortable resting place and ascend to greater heights and sizes. (Refer to Figure 70 for a sampling of the scales this chapter explores.)

[
FIGURE 70
]
A tour of large scales, and the length units that are used to describe them.

One of the more spectacular demonstrations of human response to height that I’ve seen occurred during a performance of Elizabeth Streb’s dance company. Her dancers (or “action engineers”) fall onto their stomachs from a rail raised higher and higher until the final dancer falls a full 30 feet. That is definitely beyond our comfort zone as the many gasps in the audience make abundantly clear. People shouldn’t fall from that height—certainly not onto their faces.

Though maybe not so dramatic, most tall buildings inspire strong reactions too, ranging from awe to alienation. One of the challenges architects face is to humanize structures that are so much bigger than we are. Buildings and structures vary in size and shape, but our response to them inevitably reflects our psychological and physiological attitudes toward size.

The world’s tallest man-made structure is Burj Khalifa in Dubai, United Arab Emirates, which stands 828 meters (2,717 ft.) tall. That is dauntingly high, but it’s largely empty and the movie
Mission Impossible 4
probably won’t confer on it the same cultural status that
King Kong
gave to the Empire State Building. New York’s iconic 381-meter building stands at less than half the height of Burj Khalifa. However, to its credit, it has a much higher occupancy rate.

We live in a world surrounded by much larger natural entities, many of which inspire awe. In the vertical direction, Mt. Everest, at 8.8 kilometers, is the highest peak on Earth. Mt. Blanc, the tallest mountain in Europe (at least if you’re not from the country of Georgia), is only about half as high—but I was still pretty happy years ago when I made it to the summit—though my friend and I look pretty miserable in the photo we took at the top. At 11 kilometers deep, the Mariana Trench is the deepest known place in the ocean, and the lowest elevation of the Earth’s surface crust. This otherworldly trench was the director James Cameron’s destination once he had successfully conquered three-dimensional imagery with his successful movie
Avatar
.

Natural bodies spread on the Earth’s surface over far more extended regions. The Pacific Ocean, for example, is about 20 million meters wide, while Russia—at nearly eight million meters across—is almost half the extent of that. The nearly spherical Earth itself is some 12 million meters in diameter, with a circumference about three times as big. The United States, at 4.2 million meters across, is about a tenth this wide, but is still bigger than the diameter of the Moon, which measures about 3.6 million meters.

Objects in outer space have a large range of sizes as well. Asteroids, for example, vary quite a bit—tiny ones can be as small as pebbles, while bigger ones are far greater than any feature on Earth. At approximately a billion meters across, the Sun is about 100 times the size of the Earth. And the solar system, which I’ll take to be roughly the distance from the Sun to Pluto (which is in the solar system whether or not it merits planet status) is about 7,000 times the radius of the Sun.

The distance from the Earth to the Sun is considerably smaller—a mere 100 billion meters—a hundredth of a thousandth of a light-year. A light-year is the distance light can travel in a year—the product of 300 million meters/second (the speed of light) and 30 million seconds (the number of seconds in a year). Because of this finite speed of light, the illumination we see from the Sun is already about eight minutes old.

Many visible structures, of varying shapes and sizes, exist within our vast universe. Astronomers have organized most astral bodies according to type. To set some scales, galaxies are typically about 30,000 light-years or 3 × 10
²⁰
meters across. That includes our galaxy—the Milky Way—which is about three times that size. Galaxy clusters, which contain from tens to thousands of galaxies, are about 1023 meters in size, or 10 million light-years big. Light takes about 10 million years to traverse from one end of a galaxy cluster to the other.

Yet despite the huge range of sizes, most of these bodies act in accordance with Newton’s laws. The orbit of the Moon, like the orbit of Pluto, or the orbit of the Earth itself, can be explained in terms of Newtonian gravity. Based on the planet’s distances from the Sun, its orbit can be predicted with Newton’s gravitational force law. That’s the same law that caused Newton’s apple to fall to Earth.

Nonetheless, more precise measurements of planetary orbits revealed that Newton’s laws were not the final word. General relativity was needed to explain the precession of the perihelion of Mercury, which is the observed change in its orbit around the Sun over time. General relativity is a more comprehensive theory that includes Newton’s laws when densities are low and speeds are small, but also works outside these restrictions.

General relativity isn’t needed to describe most objects however. But its effects can accumulate over time, and are prominent when objects are sufficiently dense, as with black holes. The black hole at the center of our galaxy is about 10 trillion (1013) meters in radius. The enclosed mass is very large—about 4 million times the mass of the Sun—and, as with all other black holes, requires general relativity to describe its gravitational properties.

The entire visible universe is currently 100 billion light-years across—1027 meters, a million times the size of our galaxy. That is enormous and superficially surprising since it is bigger than the distance we can actually observe, 13.75 billion years since the time of the Big Bang. Nothing is supposed to travel faster than light speed so with the universe only 13.75 billion years old, this size might seem impossible.

However, no such contradiction exists. The reason the universe as a whole is bigger than the distance a signal could have traveled given its age is that space itself has expanded. General relativity plays a big role in understanding this phenomenon. Its equations tell us that the very fabric of space has expanded. We can observe places in the universe that are that far apart, even though they cannot see each other.

Given the finite speed of light and the finite age of the universe, this section has now taken us to the limit of observable sizes. The visible universe is all our telescopes can access. Nonetheless, the size of the universe is almost certainly not limited to what we can see. As with small scales, where we can make conjectures that extend beyond current experimental constraints, we can also consider what exists beyond the observable universe. The only limit to the largest sizes we can think about is our imagination, and our patience for contemplating structure that we can’t hope to observe.