Beyond the God Particle (31 page)

Röntgen threw himself into to the detailed study of the penetrating power of his X-rays. Within a few weeks he had produced a ghostly photograph of the bones in his wife's hand and even saw his own skeleton as an X-ray shadow cast on a fluorescent card, to which he declared, “I have seen my own death!” Röntgen later discovered that ordinary lead was an effective barrier to X-rays.

Röntgen had single-handedly discovered a marvelous new phenomenon
of nature and then quickly developed most of the ingredients of the first effective medical/dental-imaging technology. It is no surprise that Wilhelm Conrad Röntgen received the
first
Nobel Prize in Physics in 1901. Today we know that X-rays are a very high-energy, ultra-short-wavelength form of light—they are very energetic and invisible photons.

INSPIRATION

Inspired by Röntgen's discovery of X-rays, a French scientist, Henri Becquerel, began to rethink “phosphorescence.” This is the stimulated emission of light from a material, following the material's exposure to an external source of light, where the resulting emitted light from the phosphorescent material is generally of a color different than that of the source light (for example, a clock dial that glows in the dark after the room lights are turned off is phosphorescent). Becquerel had reasoned that phosphorescent materials, such as uranium, a fairly common mineral that was found in a black, otherwise seemingly useless gravel-like material called “pitchblende,” might actually be coaxed to emit the newly discovered X-rays after being exposed to a source of bright sunlight. He did an experiment of placing the sunlight-exposed pitchblende onto a photographic plate, developing it, and finding that the plate had become “fogged,” indicating that the pitchblende was indeed emitting something. Becquerel had discovered that X-rays came from uranium salts found in pitchblende. However, his
raison d’être
, i.e., his initial hypothesis of phosphorescence, was wrong. Upon performing subsequent experiments, he found that the X-rays were
spontaneously coming from the uranium
and needed no sunlight exposure to stimulate their emission!

Becquerel had discovered natural “
radioactivity
.” Working with two brilliant doctoral students, Marie Curie and her husband Pierre, Becquerel subsequently discovered radioactivity in other “heavy” elements, such as thorium, polonium, and radium (the latter two elements were actually discovered by the Curies in the course of this research in Becquerel's lab). Together these three scientists shared one of the early Nobel Prizes for their discoveries of radioactivity and the new elements. In these experiments, they had observed three types of “rays” emitted by radioactive substances.
²

With one of these newfound rays, the so-called “beta rays” (“beta” is the Greek letter
β
), they had witnessed, unknowingly and for the first time, the
weak interactions
. We've told you this story because it is through the weak interactions that we have, today, uncovered the celebrated Higgs boson. The Higgs boson, in giving masses to the force carriers of the weak interaction, mainly the W
⁺
and W
^–
bosons, causes them to become “weak” and hard to detect. It is a rare quantum fluctuation that causes a beta ray to be emitted from a decaying atomic nucleus. Yet, our century-long ascent into the physical world of the weak interactions began with the first steps of Becquerel and the Curies, way back in 1896 with the discovery of radioactivity. What came next was the grandest revolution in our understanding of nature, the development of the quantum theory.
³

RUTHERFORD'S RADIOACTIVITY

We've previously met one of the greatest experimental physicists of all time, the New Zealander Ernest Rutherford. We described his most famous work: the discovery of the atomic nucleus, a discovery that was of paramount importance to the development of the entire quantum theory. However, this work was performed several years after he had already received his Nobel Prize, a rarity in the history physics. So what had Rutherford received his Nobel Prize for if it wasn't for the discovery of the atomic nucleus?

In 1898 J. J. Thomson, Rutherford's mentor at Cambridge University in England, had arranged an academic post for him at McGill University in Montreal, Canada. Here Rutherford set up his lab and explored the hot new topic of radioactivity. He soon discovered the concept of radioactive “half-life.”
⁴
The main business was understanding all the various “rays” that are emitted from substances displaying radioactivity. The situation was confusing and full of initial mistakes and false hypotheses by all the players—but it was finally sorted out.

Rutherford's initial hypothesis was that all the radioactivity “rays” were just X-rays. However, by using the much more radioactive elements of polonium and radium, discovered by the Curies, he was able to show that there were two different rays that were deflected in magnetic fields, and
therefore that these must be electrically charged particles. One of these was a slow and fairly non-penetrating form, which he called “alpha rays.” The alpha rays required a very strong magnetic field to deflect them, and later Rutherford would prove that alpha rays were actually the nuclei of helium atoms (i.e., helium atoms with no electrons). These are fragments that are produced when a very heavy nucleus falls apart into a lighter one, emitting alpha particles. Some of the other radiation was not bent at all in a magnetic field and was deeply penetrating through matter and must therefore be electrically neutral. Rutherford called these “gamma rays,” which are very energetic photons, of even higher energy than X-rays.

But Rutherford also found another kind of “ray” that was easily bent in a magnetic field, which he called “beta rays.” Becquerel had also identified the “beta rays,” but Rutherford now found that they were a hundred times more penetrating of matter than the alpha rays. From the deflection of their motion in a magnetic field, the “charge-to-mass ratio” of beta rays could be determined, and it was found to be identical to that of the electron, discovered by J. J. Thompson a few years earlier.

Usually the emitted beta particles have the negative charge of the electron. But in some rare materials the emitted beta particle has a positive electric charge: that's the antielectron, or
positron
. So, we'll now take a brief side-excursion into the fascinating phenomenon of antimatter (and by the way, here, too, has emerged another billion-dollar industry—positron-emission tomography—better living through particle physics
⁵
).

ANTIMATTER

We have all seen and admired the famous equation E = mc
²
emblazoned upon T-shirts, opening graphics for TV shows like
The Twilight Zone
, corporate logos, commercial products, and countless
New Yorker
cartoons. “E = mc
²
” has become the universal emblem for “smart” in our culture today.

But this isn't exactly what Einstein said. What Einstein
really
said, is that for a particle at rest

E
²
= m
²
c
⁴

To get the energy, E, for the particle, we have to take the
square root
of both sides of this equation, and sure enough, we'll then get a solution: E=mc
²
. So what is the difference?

First, please just bear in mind a simple mathematical fact that you may have forgotten since you took high school algebra:
every number has two square roots
. For example, the number 4 has the two square roots
= 2 and
= –2; the latter is “negative 2.” Of course, we all know that 2 × 2 = 4, but we also know that (–2) × (–2) = 4 (two negatives make a positive when you multiply them together). The “other” square root of a positive number is always a negative number (and even negative numbers have square roots, which leads to imaginary numbers, but we digress).

So, then, here's the puzzle: If the true equation is E
²
= m
²
c
⁴
, then how do we know that the energy, E, that we derive from Einstein's formula should be a positive number? Which square root is it? Positive or negative? How does nature know?

Suppose
negative
-
energy particles
exist. These particles would have a
negative
rest energy of –mc
²
. If they move, their negative energy would become a still greater
negative
quantity, that is, they would
lose energy
as they accelerate, i.e., their energy becomes more and more negative as their
velocity increases
. In fact, in collisions their energy would become more negative, and after enough collisions, eventually, the negative-energy particles would have an infinitely negative energy. Such particles would continuously accelerate and fall down into an enormous sinkhole of negative infinite energy. The universe would be full of these negative infinite-energy oddball particles, constantly radiating energy as they fell deeper and deeper into the infinite negative-energy abyss.

In 1926, a young British genius named Paul Dirac sought an equation for the electron, one that would be consistent with Einstein's theory of special relativity, as the equations of the time were based only upon Newton's concepts of space and time and they only worked for slow electrons.
⁶
Dirac found a beautiful equation in the new quantum theory, combined with relativity, and he hoped that it would lead to E = +mc
²
. But he soon encountered a problem: his equation indeed had the “correct” solutions representing electrons that have spin 1/2 and positive energy, i.e., we do get E = +mc
²
. But for every positive energy solution there was also a negative energy solution with E = –mc
²
. The negative-energy electrons should be just as prevalent in nature as the positive-energy ones.

According to Dirac's equation, the universe should be full of these negative-energy electrons. The universe would become a purgatory, eternally collapsing into a great sinkhole of negative energy. Dirac became frustrated as seemingly nothing could be done about this conundrum—Einstein's theory of relativity combined with quantum physics predicted negative-energy electrons. This would imply that ordinary atoms, even simple hydrogen atoms, indeed, all of ordinary matter, could not possibly be stable. The positive-energy electron with E = mc
²
could emit a few photons, adding up to an energy of 2mc
²
, and could become a negative-energy electron with E = –mc
²
, and then begin its orbital descent, accelerating and radiating more photons into the abyss of infinite negative energy. The whole universe could not be stable if the negative energy states truly existed. The negative-energy electron solutions of Dirac's equation were now a prime headache for the baby quantum theory.

However, Dirac soon had a wild idea. Wolfgang Pauli had successfully, and brilliantly, explained the Periodic Table of the Elements with a “rule” that must be obeyed by all electrons, known as the Pauli exclusion principle. This principle says that
no two electrons can be put into exactly the same quantum state of motion at the same time
. That is, once an electron occupies a given quantum state of motion, like a quantum orbit in an atom—that
state is filled
. No more electrons can join in. Quantum states are like seats on an airplane—only one passenger per seat is allowed. This is more than a mere “ordinance” or “rule,” and Pauli actually proved it mathematically to apply for all spin-1/2 particles.
⁷