A Little Book about the Big Bang

Tony Rothman

• Verlag: Belknap Press
• Jahr: 2022
• ISBN:
  9780674276307
• Thema: Naturwissenschaften
• Thema: Kosmologie

A concise introduction to the greatest questions of modern cosmology. What came before the big bang? How will the universe evolve into the future? Will there be a big crunch? Questions like these have no definitive answers, but there are many contending theories. In A Little Book about the Big Bang, physicist and writer Tony Rothman guides expert and uninitiated readers alike through the most compelling mysteries surrounding the nature and origin of the universe.Cosmologists are busy these days, actively researching dark energy, dark matter, and quantum gravity, all at the foundation of our understanding of space, time, and the laws governing the universe. Enlisting thoughtful analogies and a step-by-step approach, Rothman breaks down what is known and what isn't and details the pioneering experimental techniques scientists are bringing to bear on riddles of nature at once utterly basic and stunningly complex. In Rothman's telling, modern cosmology proves to be an intricate web of theoretical predictions confirmed by exquisitely precise observations, all of which make the theory of the big bang one of the most solid edifices ever constructed in the history of science. At the same time, Rothman is careful to distinguish established physics from speculation, and in doing so highlights current controversies and avenues of future exploration.The idea of the big bang is now almost a century old, yet with each new year comes a fresh enigma. That is scientific progress in a nutshell: every groundbreaking discovery, every creative explanation, provokes new and more fundamental questions. Rothman takes stock of what we have learned and encourages readers to ponder the mysteries to come.

1

GRAVITY, PUMPKINS, AND COSMOLOGY

COSMOLOGY IS the study of how gravity determines the evolution of the entire universe, so to understand cosmology requires understanding gravity.

Gravity is by far the weakest of the known natural forces. To a physicist, a force is nothing more than a push or a pull exerted on an object—no “dark side” enters the picture—and one of the main reasons that physicists call their field the most fundamental of all sciences is that, over the centuries, they have learned that only four fundamental forces exist in nature. One of these, termed the strong nuclear force, is easily the strongest natural force and holds the nuclei of atoms together. Any atomic nucleus consists of neutrons and protons, and the electrical repulsion among the positively charged protons would cause the nucleus to fly apart were it not for the strong force binding it together. The energy associated with the strong force is what is released in atomic explosions. The strong force, however, operates only within the atomic nucleus, which is extremely small, as cosmology goes.

The second fundamental force is the weak nuclear force. Billions of times weaker than the strong force, it governs certain forms of radioactive decay. Tritium, the extra-heavy version of hydrogen, is radioactive and decays into a form of helium; its rate of decay is determined by the weak force. But like the strong force, the weak force operates only within the atomic nucleus, which is insignificant on the scale of cosmology.

In daily life the most important forces are the electric and magnetic forces, which are actually two aspects of a single electromagnetic force. This force is responsible for all of chemistry and operates in any device requiring electrical currents, from toasters to smartphones to everything we take for granted today. The electromagnetic force is the basis of modern civilization. But to produce electric or magnetic forces requires electric charges. Because astronomical bodies, such as planets, are electrically uncharged they exert no electrical or magnetic forces on each other.

All objects do gravitationally attract one another. Gravity, though, is almost unimaginably weak—that the gravitational tug of the entire earth cannot budge a refrigerator magnet is a hint of how weak it is compared to the electromagnetic force. The way physicists tend to state it is that the gravitational attraction between two hydrogen nuclei, protons, is about thirty-six orders of magnitude smaller than the electrical repulsion between them. In designing consumer electronics, engineers pay no attention to gravity.

Yet, because nuclear forces operate only inside atomic nuclei and because astronomical bodies are electrically neutral, it is left to the weakest force in nature to determine the fate of the universe.

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Our modern theory of gravitation is Albert Einstein’s general theory of relativity, which is often called the most beautiful scientific theory. This is true.

On a superficial level, we might regard general relativity as merely a refinement of Newton’s theory of gravity, devised by Isaac Newton nearly four hundred years ago. It consists of a single immortal equation that shows how the gravitational force between two objects depends on their masses and the distance separating them. We don’t even need to write the equation down to understand its message: knowing just the masses of the objects and their separation allows us to determine exactly the gravitational force they exert on one another.^a

Above I said a force in physics is simply a push or a pull. More precisely, a force causes an object to change its velocity—in other words, to accelerate. If a piano is speeding up or slowing down, a force is acting on it. If the piano is moving at a constant velocity, no force is acting on it.

According to Newton, if we know the forces on an object, we know its acceleration, and can then completely predict its future behavior. Thus, if we knew the masses and present separations of all the stars in the universe, we would know everything there is to know about the universe’s future—and its past, as well. For this reason, the Newtonian universe is often compared to clockwork. For the most part, it is.

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Newton’s theory of gravity works so well in ordinary circumstances that for two centuries astronomers believed it completely explained the motions of the solar system. In the mid-nineteenth century the first hints appeared that this might not be so. Like all the planets, Mercury travels around the sun in an elliptical orbit. If Mercury and the sun constituted the entire solar system, the point of Mercury’s closest approach to the sun, called its perihelion, would always remain at a fixed point in space. Astronomers observed instead that the perihelion was gradually shifting its position over time. Calculations indicated that the gravitational tug from the other planets in the solar system could account for most of this shift, but a tiny amount was stubbornly left over. Many theories were proposed to explain the anomaly, but the ghost in the machine remained a mystery for over half a century.

When Einstein began work on general relativity in the early twentieth century, apart from Mercury’s perihelion shift there was no observational evidence that Newtonian gravity might be inadequate. There was, however, James Clerk Maxwell’s theory of the electromagnetic field.

You should first realize that Newton’s theory is one of particles and forces. Two pumpkins sit in a pumpkin patch. We can think of them as two particles exerting a gravitational force on each other across the patch. Likewise, we can idealize the earth and moon as particles exerting a gravitational attraction on each other across space. In neither case does Newton’s theory explain how the force travels from one particle to the other. For this reason, Newtonian gravitation is often called an action at a distance theory, action being the word for force in Newton’s day.

Equally important is that the gravitational force between the two objects is evidently transmitted instantaneously; if the sun disappeared, nothing would be left for the planets to orbit and they would fly off into space with no delay whatsoever.

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Instead of a pumpkin patch, imagine that the pumpkins are floating in a pond. We immediately feel the picture has changed. The water in the pond is composed of an enormous number of molecules, but they are so tiny we forget about them and instead think of the water as having a certain density and pressure at each point. Density and pressure are “bulk” quantities, making no reference to individual particles. This is a signature characteristic of a field. The air in a room can be regarded as a field. So can the elastic surface of a trampoline. A swarm of bees in many respects resembles a field.

The field picture provides a natural mechanism for transmitting forces. If the pumpkins are bobbed up and down, they create small disturbances that propagate across the pond as water waves. These waves are local disturbances traveling through the water field at finite velocities. By contrast, in Newtonian gravity, one needs to imagine forces that are somehow transmitted across great voids, infinitely fast.

“Objection!” you cry, politely: the gravitational attraction between the earth and the moon does not involve waves. True. All analogies break down. When thinking about the permanent gravitational attraction between bodies, whether we imagine forces or fields doesn’t much matter. Nevertheless, fields exist; if you have ever sprinkled iron filings onto a piece of paper above a magnet, you have perceived the shape of its magnetic field fairly directly. On the whole, the field picture is so powerful that essentially all modern theories of fundamental physics are field theories. Without the field concept it becomes virtually impossible to describe electromagnetic and gravitational waves.

To be sure, when Maxwell considered the laws governing electric and magnetic fields, he was able to show that these fields could propagate through the vacuum of space in the form of an electromagnetic wave traveling at 3 × 10⁸ meters per second.^b His discovery, published in 1865, astounded Maxwell, because that number was almost the exact speed of light, which by then had already been accurately measured. The conclusion was “scarcely avoidable,” he wrote, that light itself must be an electromagnetic wave traveling not infinitely fast but at the finite velocity of 3 × 10⁸ meters per second. Maxwell’s prediction, the greatest theoretical triumph of nineteenth-century physics, was confirmed several decades later by the discovery of radio waves.

At the opening of the twentieth century, a number of physicists attempted to create field theories of gravity based on Maxwell’s electromagnetism. They all failed, because gravity doesn’t behave exactly like electromagnetism. Einstein was the first to understand the difference and the first to get gravity right. To appreciate how his theory, which he called general relativity, describes the gravitational field, we must first get a f...