Knocking on Heaven's Door (17 page)

The most popular candidate for such a theory is known as
string theory.
Originally string theory was formulated as a theory that replaces fundamental particles with fundamental strings. We now know that string theory also involves fundamental objects other than strings (which we’ll learn a little more about in Chapter 17), and the name is sometimes replaced with a broader (but less well-defined) term,
M-theory.
This theory is currently the most promising suggestion for addressing the problem of quantum gravity.

However, string theory poses enormous conceptual and mathematical challenges. No one yet knows how to formulate string theory to answer all the questions we would want a theory of quantum gravity to address. Furthermore, the string scale of 10-33 cm is likely to be beyond the reach of any experiment we can think about.

So a reasonable question is whether investigating string theory is a reasonable expenditure of time and resources. I am often asked this question. Why would anyone study a theory so unlikely to yield experimental consequences? Some physicists find mathematical and theoretical consistency reason enough. Those people think they can repeat the type of success Einstein had when he developed his general theory of relativity, based in large part on purely theoretical and mathematical investigations.

But another motivation for studying string theory—one that I think is very important—is that it can and has provided new ways of thinking about ideas that apply on measurable scales. Two of those ideas are
supersymmetry
and theories of
extra dimensions,
ideas that we will address in Chapter 17. These theories do have experimental consequences if they address problems in particle physics. In fact, if certain extra-dimensional theories prove correct and explain phenomena at LHC energies, even evidence of string theory could possibly appear at much lower energies. A discovery of supersymmetry or extra dimensions won’t be proof of string theory. But it will be a validation of the utility of working on abstract ideas, even those without direct experimental consequences. It will of course also be a testimony to the utility of experiments in probing even initially abstract-seeming ideas.

“SEEING” IS BELIEVING

Scientists could decipher what matter is made of only when tools were developed that let them look inside. The word “look” refers not to direct observations but to the indirect techniques that people use to probe the tiny sizes inaccessible to the naked eye.

It’s rarely easy. Yet despite the challenges and the counterintuitive results that experiments sometimes display, reality is real. Physical laws, even at tiny scales, can give rise to measurable consequences that eventually become accessible to cleverer investigations. Our current knowledge about matter and how it interacts is the culmination of many years of insight and innovation and theoretical development that permit us to consistently interpret a variety of experimental results. Through indirect observations, pioneered by Galileo centuries ago, physicists have deduced what is present at matter’s core.

We’ll now explore the current state of particle physics and the theoretical insights and experimental discoveries that have led us to where we are today. Inevitably, the description will have a rather list-like aspect to it as I enumerate the ingredients that compose the matter we know and how they were discovered. The list is a lot more interesting when we remember the very different behaviors of these diverse ingredients on different scales. The chair you are sitting on is ultimately reducible to these elements, but it’s quite a train of discoveries to get from here to there.

As Richard Feynman mischievously explained when talking about one of his theories, “If you don’t like it, go somewhere else—perhaps to another universe where the rules are simpler… I’m going to tell you what it looks like to human beings who have struggled as hard as they can to understand. If you don’t like it, that’s too bad.”
³¹
You may think that some of what we believe to be true is so crazy or cumbersome that you won’t want to accept it. But that won’t change the fact that it’s the way nature works.

SMALL WAVELENGTHS

Small distances seem strange because they are unfamiliar. We need tiny probes to observe what is happening on the smallest scales. The page (or screen) you are currently reading looks very different from what resides at matter’s core. That’s because the very act of seeing has to do with observing visible light. That light is emitted from electrons in orbits around nuclei at the center of atoms. As Figure 14 illustrated, the wavelength of that light is never small enough to let us probe inside nuclei.

We need to be more clever—or more ruthless, depending on how you look at it, to detect what is happening on the tiny scale of a nucleus. Small wavelengths are required. That shouldn’t be so hard to believe. Imagine a fictional wave with wavelength equal to the size of the universe. No interaction of this wave could possibly have sufficient information to locate anything in space. Unless there are smaller oscillations in this wave that can resolve structure in the universe, we would have no way, with only this enormous wavelength wave as our guide, to determine that anything is in any particular place. It would be like covering a pile of stuff with a net and asking where your wallet is located in the mess underneath. You can’t find it unless you have enough resolution to look inside on smaller scales.

With waves, you need peaks and troughs with the right spacing—variations on the scale of whatever it is we are trying to resolve—to be able to identify where something is or what its size or shape might be. You can think of a wavelength the size of the net. If all I know is that something is inside it, I can say with certainty only that something is within a region whose size is that of the net with which I caught it. To say anything more requires either a smaller net or some other way of searching for variations on a more sensitive scale.

Quantum mechanics tells us that waves characterize the probability of finding a particle in any given location. Those waves might be waves associated with light. Or they might be the waves that quantum mechanics tells us are secretly carried by any individual particle. The wavelength of those waves tells us the possible resolution one can hope to attain when we use a particle or radiation to probe small distances.

Quantum mechanics also tells us that short wavelengths require high energies. That’s because it relates frequencies to energies, and the waves with the highest frequencies and shortest wavelengths carry the most energy. Quantum mechanics thereby connects high energies and short distances, telling us that only experiments operating at high energies can probe into the inner workings of matter. That is the fundamental reason we need machines that accelerate particles to high energy if we want to probe matter’s fundamental core.

Quantum mechanical wave relations tell us that high energies allow us to probe tiny distances and the interactions that occur there. Only with higher energies, and hence shorter wavelengths, can we study these smaller sizes. The quantum mechanical uncertainty relation that tells us small distances connect to large momenta combined with connections among energy, mass, and momenta provided by special relativity make these connections precise.

On top of that, Einstein taught us that energy and mass are interconvertible. When particles collide, their mass can turn into energy. So at higher energies, heavier matter can be produced, since
E = mc
²
.
This equation means that larger energy—
E
—permits the creation of heavier particles with bigger mass—
m.
And that energy is ecumenical—capable of creating any type of particle that is kinematically accessible (which is to say light enough).

This tells us that the higher energies we currently explore are taking us to smaller sizes, and the particles that get created are our key to understanding the fundamental laws of physics that apply at these scales. Any new high-energy particles and interactions that emerge at short distances hold the clues to decoding the underpinnings of the so-called Standard Model of particle physics, which describes our current understanding of matter’s most basic elements and their interactions. We’ll now consider a few key Standard Model discoveries, and the methods we now use to advance our knowledge some more.

THE DISCOVERIES OF ELECTRONS AND QUARKS

Each of the destinations on our initial tour of the atom—the electrons circulating around a nucleus and the quarks held together by gluons inside the protons and neutrons—were experimentally discovered with successively higher-energy and hence shorter-distance probes. We’ve seen that the electrons in an atom are bound to a nucleus through the mutual attraction due to their opposite charges. The attractive force gives the bound system—the atom—lower energy than the charged ingredients in isolation. Therefore, to isolate and study electrons, someone had to add enough energy to
ionize
them, which is to say to free the electrons by ripping them off. Once isolated, physicists could learn more about the electron by studying its properties, such as its charge and its mass.

The discovery of the nucleus, the other part of the atom, was more surprising still. In an experiment analogous to particle experiments today, Ernest Rutherford and his students discovered the nucleus by shooting Helium nuclei (then called alpha particles since nuclei hadn’t been discovered) at a thin gold foil. The alpha particles turned out to have enough energy for Rutherford to identify the structure inside the nucleus. He and his colleagues found that the alpha particles they shot at the foil sometimes scattered at much greater angles than they would have anticipated. (See Figure 20.) They expected scatterings like those from tissue paper and instead discovered ones seeming more like they were ricocheting off marbles inside. In Rutherford’s own words:

[
FIGURE 20
]
Rutherford’s experiment scattered alpha particles (which we now know to be Helium nuclei) off gold foil. The unexpectedly large deflections of some of the alpha particles demonstrated the existence of concentrated masses at the centers of the atoms—atomic nuclei.

“It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you. On consideration, I realized that this scattering backward must be the result of a single collision, and when I made calculations I saw that it was impossible to get anything of that order of magnitude unless you took a system in which the greater part of the mass of the atom was concentrated in a minute nucleus. It was then that I had the idea of an atom with a minute massive centre carrying a charge.”
³²

The experimental discovery of quarks inside protons and neutrons used methods in some respects similar to Rutherford’s but required even higher energies than that of the alpha particles he had used. Those higher energies required a particle accelerator that could accelerate electrons and the photons they radiated to sufficiently high energies.

The first circular particle accelerator was named a
cyclotron,
due to the circular paths along which the particles were accelerated. Ernest Lawrence built the first cyclotron at the University of California in 1932. It was less than a foot in diameter and was very feeble by modern standards. It produced nowhere near the energy needed to discover quarks. That milestone could happen only with a number of improvements in accelerator technology (that nicely gave rise to a couple of important discoveries along the way).

Well before quarks and the inner structure of the nucleus could be explored, Emilio Segre and Owen Chamberlain received the 1959 Nobel Prize for their discovery of antiprotons at Lawrence Berkeley Laboratory’s
Bevatron
in 1955. The Bevatron was a more sophisticated accelerator than a cyclotron and could raise the protons to energy more than six times their rest mass—more than enough to create proton-antiproton pairs. The proton beam at the Bevatron bombarded targets and (via the magic of
E = mc
²
)
produced exotic matter, which includes antiprotons and antineutrons.

Antimatter plays a big role in particle physics, so let’s take a brief detour to explore this remarkable counterpart to the matter we observe. Because the charges of matter and antimatter particles add up to zero, matter can annihilate with its associated antimatter when they meet. For example, antiprotons—one form of antimatter—can combine with protons to produce pure energy according to Einstein’s equation
E = mc
²
.

The British physicist Paul Dirac first “discovered” antimatter mathematically in 1927 when he tried to find the equation that describes the electron. The only equation he could write down consistent with known symmetry principles implied the existence of a particle with the same mass and opposite charge—a particle that no one had ever seen before.