While it would take trillions upon trillions of years for a regular black hole to shrink away to nothing, what if the universe did manufacture those multitudes of tiny black holes—mini–black holes—during the first moments of the Big Bang, as Hawking has suggested? Like a ball rolling down a hill, the evaporation of a mini–black hole would accelerate as time progresses. The more mass this tiny primordial object loses, the faster and faster it fizzles away, until it reaches a cataclysmic end.
If the Big Bang did forge such holes, the smallest would have vanished before their dying light could catch our attention; but objects containing the mass of a mountain, yet compressed to the size of a proton, would have continued shedding the last of their mass in brief, spectacular bursts of gamma rays.
That’s what Cline and his colleagues believe they might be seeing within the gamma-ray detector records. Others are not so sure. Such signals could also be arriving from a more mundane stellar activity, one not yet identified. As Carl Sagan liked to say, “Extraordinary claims require extraordinary evidence.” Cline agrees and is urging other researchers to start studying these events as well, to see if his team’s claim holds up to scrutiny. If the distinctive pop of a primordial black hole is at last verified, it will be a significant moment in astronomical history.
CHAPTER THIRTY-ONE
Meet the Multiverse
A theoretical physicist brings her bewildering
science down to Earth
IT began with Sir Isaac Newton. With the publication of his Principia in 1687, Newton became the first scientist to demonstrate that nature’s actions, from the path of a cannonball to the Moon’s orbit about the Earth, could be described by distinct mathematical laws. Mathematics became the key to unlocking the secrets of the heavens.
Continuing along this path, the Scottish theorist James Clerk Maxwell in the 1860s devised a concise set of eminently beautiful equations that united the forces of electricity and magnetism, showing them to be different sides of the same coin. Several decades later, Albert Einstein, spurred by his superb physical intuition but also by an astute mathematical rigor, extended Newton’s laws and showed that gravity was a geometric manifestation. Space-time became a palpable item—a flexible sheet—and objects that appear to be under a gravitational force are actually following the geometric curves that matter impresses upon this rubbery mat of space-time. Even before tests confirmed this view, Einstein was sure his theory was right because of what he called “its incomparable beauty.”
Mathematical beauty is a potent lure to physicists. In 1963, Murray Gell-Mann looked at the bewildering array of ephemeral particles discovered by physicists and found order by imagining a more fundamental group of building blocks called quarks, which combined by specific rules to generate the many particles. At that time, theoretical physicists were generally working side by side with experimentalists, but, encouraged by their successes, the theorists began to race ahead into unknown territory. The most ambitious, guided solely by the beauty and power of their mathematics, built a construct known as superstrings. This theory suggests that all the forces we experience and the particles we detect result from infinitesimally small strings vibrating within a space-time composed of ten or eleven dimensions.
The story of superstrings was skillfully told in Brian Greene’s best-selling The Elegant Universe, but there’s another aspect to this tale that Greene kept in the background. Not all theoretical physicists are happy with this dependence on mathematical splendor. Some are worried that the notorious celebrity of superstrings has diverted many of the best and brightest in physics from their science’s more traditional (and successful) strategy: teaming up with experimentalists. Just as journalists Bob Woodward and Carl Bernstein were advised in the movie All the President’s Men to “follow the money” to reach their goal—exposing the Watergate scandal—superstring critics would like to see theorists follow the data.
Superstring mavens are the top-downers. They flew up to the ethereal heights and are now looking back down at the real world, hoping to find experimental evidence for strings below them. But, as Harvard professor of physics Lisa Randall asked in Warped Passages, have they now found themselves “at the edge of a precipitous, isolated cliff, too remote for them to find their way back to base camp”?
Physicist Lisa Randall.
(Festival della Scienza/Wikimedia Commons)
Randall represents the other faction of theorists: those whose feet are firmly planted near an atom smasher and who make predictions that will be either accepted or rejected as particles are slammed together and the resulting debris sifted through. They are the “model builders,” who offer a healthy dose of caution to the grander claims of superstring theory. “So far,” writes Randall, “all attempts to make string theory realistic have had something of the flavor of cosmetic surgery. In order to make its predictions conform to our world, theorists have to find ways to cut away the pieces that shouldn’t be there, removing particles and tucking dimensions demurely away. String theory is captivating at first, but ultimately string theorists have to address these fundamental problems.” She says the model builders, on the other hand, are the “trailblazers who are trying to find the path that connects the solid ground below to the peak. They yield definite predictions for physical phenomena, giving experimenters a way to verify or contradict a model’s claims.”
The two camps are not totally at odds. Indeed, Randall acknowledges that the inroads made by string theorists have been inspirational in part for her and her colleagues. “String theory introduces new ideas, both mathematical and physical, that no one would otherwise have considered, such as extra-dimensional notions,” she notes.
String theory brought to the forefront the idea that there may be more to the universe than just three spatial dimensions—height, width, length—plus time. There could be six more dimensions that we fail to perceive, possibly because they are so tiny and curled up and hidden from view, or perhaps because some are infinite in extension. These new spatial directions are Randall’s “warped passages.”
At first, theorists postulated that it was the strings themselves that oscillated within these many dimensions, allegedly creating the various particles found in the cosmos. More recently, that idea has expanded to include membranes, or “branes” for short. A brane is essentially a slice out of that multidimensional world. According to this view, we might be living on a four-dimensional brane (space + time), which itself is immersed inside the full dimensional realm known in its entirety as the “bulk.” Such entities as light waves, electrons, and protons are confined to our specific brane, much like water droplets rolling down a shower curtain.
This setup introduces us to a new and mind-blowing take on the universe, or should we say “multiverse.” We may be residing amid other branes, other parallel universes, within this complex higher-dimensional domain. “Thinking about branes makes you aware of just how little we know about the space in which we live,” says Randall. “The universe might be a magnificent composition linking intermittent branes.” If there is life on those other branes, they likely experience different forces and possibly even different forms of matter. Despite the science-fiction quality to this notion, evidence for these higher dimensions might actually be obtainable in the foreseeable future. “Experimental tests of competing hypotheses are near at hand, and within a decade,” she predicts, “there should be a dramatic revision in our understanding of fundamental physical laws that will incorporate whatever is discovered.”
For the past few decades, many theorists have been focused on unifying the four forces of nature—gravity, electromagnetism, and the strong and weak nuclear forces. Just as Maxwell showed that electricity and magnetism were different features of the same force—electromagnetism—so, too, do theorists suspect that all the forces at some time were united, likely in the first moment of the Big Bang. As the primordial universe cooled and expanded, each force took on its own identity. But there might be more important questions to ans
wer first. Why are the masses of the elementary particles—such entities as the electrons, protons, and neutrons that make up atoms—so low (theory alone would predict masses much higher), and why is the force of gravity so weak, compared with the other forces? A toy magnet, for instance, can lift a paper clip off the ground, despite the entire Earth gravitationally pulling back on it.
An artistic imagining of the multiverse.
(GiroScience/Shutterstock)
The investigations of higher dimensions by Randall and her fellow model builders are centered upon these conundrums, and they offer several schemes for possible testing. In one model, for example, every particle we know and see around us has a partner in higher dimensional space—a KK particle (named after Theodor Kaluza and Oskar Klein, two physicists who first toyed with the idea of higher dimensions in the early twentieth century). According to Randall, these particles originate in the extra dimensions but make an appearance in our universe with measurable properties. In a way, they cast a three-dimensional “shadow” upon our world, much as an object would cast a two-dimensional shadow on a wall on a sunny day.
Finding a ghostly KK particle would not only be evidence of the higher dimensions, but would also provide an answer to gravity’s frailty. Whereas electromagnetism and the nuclear forces are confined to our brane, and so remain fairly strong, gravity could be the lone force that spans all the dimensions and, as a consequence, gets diluted. Or maybe, posits Randall, we live near a brane where gravity is intensely strong, but by the time the gravitational field extends through a fifth dimension, it arrives on our brane of space-time much weakened.
Most exciting for Randall and her colleagues in this endeavor is that testable predictions can be made, renewing the exhilarating time in particle physics of the late 1960s, when quarks were first detected at the Stanford Linear Accelerator as electrons slamming into protons revealed that the protons were built out of three smaller particles (as Gell-Mann had surmised theoretically).
CERN, the European particle-physics center situated on the Swiss-French border, recently installed the most powerful instrument ever built to investigate the properties of elementary particles. Two beams of protons smash together at energy levels so high that the resulting impact might nudge some KK particles into plain sight (or at least allow them to leave their calling cards within the collision debris). What is more, infinitesimally tiny black holes might form as well, quickly evaporating in a hail of energy. There’s even a small chance that strings themselves might be amplified and detected. Any of these occurrences would be evidence of higher dimensions.
How seriously should we take all this talk of vibrating strings and parallel universes? Hypotheses in high-energy physics rise and fall on the internet these days, sometimes in a matter of hours. But I can imagine getting comfortable with branes and higher dimensions, as some of us are already accustomed to black holes, relativity, and particle/wave dualities.
CHAPTER THIRTY-TWO
When the Universe Began,
What Time Was It?
To learn how the cosmos blossomed out of a subatomic
point, theorists must first settle a fundamental question: is
time, at the smallest of physical scales, irrelevant?
TIME is an elusive notion. Poets often think of time as a river, a free-flowing stream that carries us from the radiant morning of birth to the golden twilight of old age. It is the span that separates the delicate bud of spring from the lush flower of summer.
Physicists think of time in somewhat more practical terms. For them, time is a means of measuring change—an endless series of instants that, strung together like beads, turn an uncertain future into the present and the present into a definite past. The very concept of time allows researchers to calculate when a comet will round the Sun or how a signal traverses a silicon chip. Each step in time provides a peek at the evolution of nature’s myriad phenomena.
In other words, time is a tool. In fact, it was the first scientific tool. Ancient astronomers meticulously tracked the Sun’s march across the Zodiac in order to mark off the seasons and determine when to plant and harvest. In this day and age, solar timepieces have been replaced by atomic clocks that, thanks to the steady pulsing of hydrogen or other atoms, do not gain or lose a second in millions of years. Time can now be sliced into slivers as thin as one ten-trillionth of a second.
But what is being sliced? Unlike mass and distance, time cannot be perceived by our physical senses. We don’t see, hear, smell, touch, or taste time. And yet we somehow measure it. Captivated by this conundrum, physicists are beginning to explore the very origins of time. And on first look, they are wondering whether time is a fundamental property of the universe at all. Maybe it is solely a personal experience, set up by our minds to distinguish then from now. As the joke goes, “Time is nature’s way of preventing everything from happening all at once.”
Such thoughts are more than philosophic. As a cadre of theorists attempt to extend and refine the general theory of relativity, Einstein’s momentous law of gravitation, they have a problem with time. A big problem.
“It’s a crisis,” says mathematician John Baez, of the University of California at Riverside, “and the solution may take physics in a new direction.” Not the physics of our everyday world. Stopwatches, pendulums, and hydrogen maser clocks will continue to keep track of nature quite nicely here in our low-energy earthly environs. The crisis arises when physicists attempt to merge the macrocosm—the universe on its grandest scale—with the microcosm of subatomic particles.
Gravity is the weakest of nature’s forces, but gravity gains collective strength as masses accumulate and exert their effect over larger and larger distances. The force that causes one object to attract another eventually comes to control the motions of planets, stars, and galaxies. And the best description of how that happens is contained in Einstein’s general theory of relativity, introduced in 1915. But the domain in which this theory works is limited; it does not apply to problems at the subatomic scale. For decades, physicists have struggled to discern how gravity acts on the level of elementary particles, a realm governed by the quite different set of rules laid down by quantum mechanics. Arranging this rather curious marriage—an all-embracing theory of “quantum gravity”—is one of physics’ last great tasks.
There is a vital reason for physicists’ dogged pursuit of this problem. They believe that quantum gravity was the dominant force at the birth of the universe, during the first tiny 10–43 second (one ten-millionth of a trillionth of a trillionth of a trillionth of a second). It was an instant when all the matter and energy in the universe was squeezed into a space far smaller than a proton. The microcosm and the macrocosm, in effect, were crushed together in a “singularity,” a freakish state where density advances toward infinity and volume approaches zero.
By figuring out the physics of such a bizarre realm, theorists may at last find the key to the origins of the universe, how it came into existence. Simultaneously, they would be learning what lies at the heart of a black hole, the gravitational abyss that is thought to result when the core of an exploding star is crushed inward until its size becomes atomic rather than celestial.
A solution to this mystery, it turns out, lies in understanding the meaning of time: how it acts—and whether it even exists—at the moment of creation or deep within a black hole. Telling time, after all, involves picking out something in the world around you that is changing—the Sun rising and setting, pendulums swinging—and tracking those changes to establish a chronology. With a clock, one can determine the sequence of events; and with a sequence of events, one can properly analyze the behavior of a system—in other words, “do the physics.” But how do you register time, the most basic widget in a physicist’s toolbox, when the entire mass of a stellar core is squeezed into a subatomic speck? Or when the entire visible universe is in such a state? What kind of clock could physicists possibly use to deal with the crushing and featureless conditions that marked
the universe’s birth, when quantum gravity was in control?
The problem is really a mathematical one but can be visualized in this crude way: Imagine you could somehow shift a magical gear into reverse and travel back some 14 billion years to that moment of creation. For most of the trip, a wristwatch would work just fine in keeping track of time. But upon reaching the very cauldron of creation, the watch would melt in a nanosecond. You could still keep track of time through the constant vibrations of individual atoms, the basis of atomic clocks. But go back far enough and even atoms cease to exist. Soon there is no longer any means of measuring the progress of events. During that primordial moment when the force of quantum gravity was strongest and the cosmos was tinier than a nuclear particle, there was essentially no room to place a clock, safe from interference, and gauge how the universe was evolving.
This dilemma summarizes the problem of time in physics. Either theorists come up with a “quantum clock,” a means of understanding and dealing with the passage of time in that minuscule province where gravity and the quantum world mingle (at least on paper), or they do away with the concept of time altogether.
“The problem of time is one of the deepest issues in physics that must be addressed,” says theoretical physicist Christopher Isham of Imperial College in London. And more than timekeeping is at stake here. There will be no Theory of Everything—no peek at “the mind of God,” as the Cambridge University cosmologist Stephen Hawking so famously put it in A Brief History of Time—until this mystery is resolved. Time plays such an integral role in most laws of physics that physicists are starting to worry: without a sense of time, a definable clock at the moment of creation, will it be possible to explain all of nature’s varied forces with one unified law? The question has been lurking in the background, like some crazy relative hidden away in the attic, as physicists seek that Holy Grail.
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