Knocking on Heaven's Door (7 page)

Photons

Light is transmitted by photons, particles that can act like waves.

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FIGURE 5
]
Geometrical optics and waves were precursors to our modern understanding of light, and still apply under appropriate conditions.

Newton’s corpuscular theory reproduces results from optics. Nonetheless, Newton’s corpuscles, which don’t have any wavelike nature, are not the same as photons. So far as we now know, the theory of photons is the most basic and correct description of light, which consists of particles that can also accommodate a wave description. Quantum mechanics gives our currently most fundamental description of what light is and how it behaves. It is fundamentally correct and survives.

Quantum mechanics is now much more of a frontier research area than optics. If people continue to think about new science with optics, they are primarily thinking about new effects possible only with quantum mechanics. Modern science, though no longer advancing the science of classical optics, does therefore include a field of quantum optics, which studies the quantum mechanical properties of light. Lasers rely on quantum mechanics, as do light detectors such as photomultipliers, and photovoltaic cells that convert sunlight into electricity.

Modern particle physics also encompasses the theory of quantum electrodynamics (QED), which Richard Feynman and others developed and which includes not only quantum mechanics but also special relativity. With QED, we study individual particles including photons—particles of light—as well as electrons and other particles that carry electric charge. We can understand the rates at which such particles interact and at which they can be created and destroyed. QED is one of the theories that is heavily used in particle physics. It also has made the most accurately verified predictions in all of science. QED is a far cry from geometrical optics, yet both are true in their appropriate domain of validity.

Every area of physics reveals this effective theory idea at work. Science evolves as old ideas get incorporated into more fundamental theories. The old ideas still apply and can have practical applications. But they aren’t the domain of frontier research. Though the end of this chapter has focused on the particular example of the physical interpretation of light through the ages, all of physics has developed in this manner. Science proceeds with uncertainty at the edges, but it is advancing methodically overall. Effective theories at a given scale legitimately ignore effects that we can prove won’t make a difference for any particular measurement. The wisdom and methods we acquired in the past survive. But theories evolve as we better understand a larger range of distances and energies. Advances give us new insights into what fundamentally accounts for the phenomena we see.

Understanding this progression helps us better interpret the nature of science and appreciate some of the major questions that physicists (and others) are asking today. In the following chapter, we’ll see that in many respects, today’s methodology began in the seventeenth century.

UNLOCKING SECRETS

The methods scientists use today are the latest incarnation of a long history of measurements and observations that have been developed over time to verify and—as importantly—rule out scientific ideas. This need to go beyond our intuitive apprehension of the world to advance our understanding is reflected in our very language. The root used in Romance languages for the verb “to think”—
pensum
—comes from the Latin verb “to weigh.” English speakers, too, “weigh” ideas.

Many of the formative insights that ushered science into its modern expression were developed in Italy in the seventeenth century, and Galileo was a key player. He was among the first to fully appreciate and advance
indirect measurements
—measurements made with an intermediate device—as well as to design and use experiments as a means of establishing scientific truth. Moreover, he conceived abstract thought experiments that helped him create and consistently formulate his ideas.

I learned about Galileo’s many insights that fundamentally changed science when I visited Padua in the spring of 2009. One impetus for my visit was a physics conference that the Paduan physics professor Fabio Zwirner had organized. The other was to receive an honorary citizenship of the city. I was delighted to join my fellow physicist attendees as well as the esteemed group of fellow “citizens,” including the physicists Steven Weinberg, Stephen Hawking, and Ed Witten. And—as a bonus—I had a chance to learn some science history.

My trip was auspiciously timed, as 2009 was the 400th anniversary of Galileo’s first celestial observations. The citizens of Padua were particularly attentive, since Galileo had been lecturing at the university there at the time of his most significant research. To commemorate his famous observations, the town of Padua (as well as Pisa, Florence, and Venice—other towns that figured prominently in the scientific life of Galileo) had arranged exhibits and ceremonies in his honor. The physics talks took place in a hall in the Centro Culturale Altinate (or San Gaetano), the same building that housed a fascinating exhibit that celebrated Galileo’s many concrete accomplishments and highlighted his role in changing and defining what science means today.

Most people I met appreciated Galileo’s achievements and conveyed their enthusiasm for modern scientific developments. The interest and knowledge of the Paduan mayor, Flavio Zanonato, impressed even the local physicists. The head of the city not only actively engaged in scientific conversation at a dinner following the public lecture I gave, but during the lecture itself he surprised the audience with an astute question about charge flow at the LHC.

As part of the citizenship ceremony, the mayor gave me the key to the city. The key was fantastic—it lived up to my movie images of what such a thing should be. Large and silver and nicely carved, it prompted one of my colleagues to ask if it was out of a Harry Potter story. It was a ceremonial key—it doesn’t actually open anything. Yet it was a beautiful symbol of entry—to a city of course but also, in my imagination, to a rich and textured portal of knowledge.

In addition to the key, Massimilla Baldo-Ceolin, a professor at the University of Padua, gave me a Venetian commemorative medal known as an
osella
. It is engraved with a quote from Galileo that is also on display at the physics department of the university: “Io stimo più li trovar un vero, benché di cosa leggiera, che ‘l disputar lungamente delle massime questioni senza conseguir verità nissuna.” This translates as “I deem it of more value to find out a truth about however light a matter than to engage in long disputes about the greatest questions without achieving any truth.”

I shared these words with many colleagues at our conference since this is in fact a guiding principle to this day. Creative advances often originate with tractable problems—a point we will return to later on. Not all the questions we answer have immediately radical implications. Yet advances, even seemingly incremental ones, occasionally lead to major shifts in our understanding.

This chapter describes how the current observations that this book presents are rooted in developments that occurred in the seventeenth century, and how the fundamental advances made at that time helped define the nature of theory and experiment that we employ today. The big questions are in some respects the same ones that scientists have been asking for 400 years, but because of technological and theoretical advances, the little questions we now ask have evolved tremendously.

GALILEO’S CONTRIBUTIONS TO SCIENCE

Scientists knock on heaven’s door in an attempt to cross the threshold separating the known from the unknown. At any moment we start with a set of rules and equations that predict phenomena we can currently measure. But we are always trying to move into regimes that we haven’t yet been able to explore with experiments. With technology and mathematics we systematically approach questions that in the past were the subject of mere speculation or faith. With better and more numerous observations and with improved theoretical frameworks that encompass newer measurements, scientists develop a more comprehensive understanding of the world.

I better understood the key role Galileo played in developing this way of thinking as I explored Padua and its historical landmarks. The Scrovegni Chapel is one of its most famous sites, housing Giotto’s frescoes from the early fourteenth century. These paintings are notable for many reasons, but to scientists the extremely realistic image of the 1301 passing of Halley’s comet (over the
Adoration of the Magi
) is a marvel. (See Figure 6.) The comet had been clearly visible to the naked eye at the time the painting was made.

But the images weren’t yet scientific. My tour guide pointed to an astral image in the Palazzo della Ragione that she had initially been told was the Milky Way. She remarked that a more expert guide had afterward explained to her the anachronistic nature of the interpretation. At the time the painting was made, people were just illustrating what they saw. It might have been a starry sky, but it was not anything so well defined as our galaxy. Science, as we understand it today, was yet to arrive.

[
FIGURE 6
]
Giotto painted this scene, which appears in the Scrovegni Chapel, in the early fourteenth century when Halley’s comet was visible to the naked eye.

Before Galileo, science relied on unmediated observations and pure thought. Aristotelian science was the model for the way people had tried to understand the world. Math could be used to make deductions, but the underlying assumptions were taken on faith or in accordance with direct observations.

Galileo explicitly refused to base his research on a “mondo di carta” (a world of paper)—he wanted to read and study the “libro della natura” (the book of nature). In achieving this goal, he changed the methodology of observation and, furthermore, recognized the power of experiments. Galileo understood how to construct and use these artificial situations to make deductions about the nature of physical law. With experiments, Galileo could test hypotheses about the laws of nature that he could prove—and, as importantly, disprove.

Some of his experiments involved inclined planes: the tilted flat surfaces that feature so prominently—and somewhat annoyingly—in every introductory physics text. For Galileo, inclined planes weren’t just some made-up classroom problem, as they sometimes appear to introductory physics students. They were a way to study the velocity of falling bodies by spreading out the descent of objects over a horizontal distance so that he could make careful measurements of how they “fell.” He measured time with a water chronometer, but he also cleverly added bells at specific points so that he could use his gifted musical ear to listen and establish speed as a ball rolled down, as illustrated in Figure 7. Through these and other experiments dealing with motion and gravity, Galileo, along with Johannes Kepler and René Descartes, laid the foundation for the classical mechanical laws that Isaac Newton so famously developed.

Bells Per Unit of Time