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"Although by far the feeblest of the four universal forces, gravity is the only one we experience continuously. Every inquisitive person should read a book about it, preferably this one . . . Clegg's skills never flag and his account remains lucid and free of jargon, bad jokes, and math phobia."—Kirkus (starred review)
"Clegg's accessible presentation offers insight into everything from Aristotelian science to black holes and string theory as it reveals the complexities and surprise of a familiar force that continues to surprise scientists."—Publishers Weekly
WHAT GOES UP
So in all their procedinges … they shew themselffes to be men of gravyte and wisedom.
—State Papers of Henry VIII (1849), VII.614
Hold this book in your hand and let go. What will happen? It’s such an obvious question that it feels embarrassing to have to ask it. But humor me. What will happen? You don’t have to carry out the experiment to know the answer. The book will fall. Why? Just as embarrassingly obvious. Because of gravity.
This is the most directly obvious force of nature. Its influence is programmed into our expectations of the world around us. If we let go of something and it drifts upward instead of falling, it’s a double-take moment. Either we’re dealing with something special, like a helium balloon, or we’re not firmly planted on the Earth. When we drop things, they fall, simple as that. And yet, as we will discover, the story of gravity is anything but simple.
Gravity is so familiar and apparently obvious that we often miss seeing just how remarkable it is. Most rational people laugh at the idea of astrology. They may tolerate it as fun, but they accept that it is garbage. It’s bizarre, they say, that anyone should believe that our lives are influenced in any way by astronomical bodies that are millions of miles away. Yet we accept that gravity—an invisible force with no detectable mechanism for exerting an influence—can have a real effect across just such distances. After all, the only thing that keeps the Earth in orbit is the gravitational attraction between it and the Sun, 93 million miles away.
You will sometimes see this distant reach of gravity being used to try to give astrology a scientific basis. We are subject to the gravitational attraction of the planets, the argument goes, so they can have an influence on our lives. While this is strictly true, it is worth bearing in mind that the gravitational force between a human body and the distant planets is tiny. By comparison, the gravitational attraction between a baby and the midwife is greater. So if astrology really were based on this idea, we should have astrological charts including the position and mass of the midwife, and everything else that was present at the birth.
In the real world of science, gravity has a much greater effect on us than anything astrologers could even imagine. Without gravity there are just so many ways that we wouldn’t exist or be able to carry out our everyday activities. It isn’t just a convenient way of sticking to the surface of the Earth.
It is thanks to gravity that bodies like planets and stars came into existence in the first place. Just imagine you are visiting the site of the solar system before it formed, around 4.5 billion years ago. You are looking at a cloud of matter—gas and dust floating in space. There is no wind to disturb this collection of material, so it will not be blown from place to place, but there is gravity. Each of the specks of matter has a tiny influence on the others. Gradually, painfully slowly, the matter will be pulled together.
At the same time the whole thing is rotating. That’s the way it started out, and there is nothing to stop it. So as the matter bunches together, it is also whirling around, like the disk of a pizza as the dough is spun between the hands of the baker. Eventually, at the center of this whirling cloud will be a large clump of matter. As each new particle comes crashing in, it will add energy, producing heat. (Think of the way rubbing your hands together produces heat. It’s a much smaller effect for each particle but there are many billions of particles contributing.)
After millions of years of collecting particles in this central lump there will be enough heat and pressure from the gravitational pull of the accumulated mass for something remarkable to happen. Hydrogen atoms (or to be more precise hydrogen atoms each stripped of its electron to leave hydrogen ions, simple protons) will be forced closer and closer together with more and more energy. Eventually a reaction will occur. In a multistage process the hydrogen nuclei fuse to form helium, the next element up the chemical chain. In this process energy is released.
The released energy from the nuclear fusion gives even more oomph to the reaction, sending it snowballing through the mass of the central lump. What we are seeing is a star being kickstarted. The fusion process is the power source of the star. Without gravity, this could never have happened.
As described above, the process has one flaw. The positively charged hydrogen ions do not like to get near to each other. The closer you push them, the more the electrical charge fights back. The electromagnetic force causing this repulsion is much stronger than gravity. Even all the gravitational pressure of a star, plus the heat that has built up, is not enough to force the positively charged ions close enough together to fuse.
The final hurdle is overcome by quantum effects. Just as general relativity—the theory that explains gravity—deals with the very large, quantum theory explains the behavior of the very small. One of the oddities of quantum particles, like hydrogen nuclei, is that they don’t have a specific position. They just have a range of probabilities as to where they might be. So although a pair of hydrogen nuclei are most likely to be held too far away to fuse by repulsion, this quantum uncertainty enables some particles to perform a process called quantum tunneling.
The particles have a small probability of finding themselves on the other side of the gap separating them without traveling through the space in between. Although the chances of any particular particle undergoing tunneling are very low, there are so many particles in a star that vast quantities of them make this jump every second. The Sun, for example, converts around 4 million tons of matter into energy every second, all derived from the minute difference in mass that arises when particles fuse.
In a normal star there is a balance, an equilibrium between the inward gravitational pull that drags all the particles in the star toward its center, and outward pressure. This pressure is a combination of traditional gas pressure—the result of the gas particles in the star bouncing off each other and resisting collapse—and the pressure of the light emitted in the fusion process. This reaction is going on deep in the Sun. When mass converts to energy it comes out as light. But the light doesn’t come straight out—far from it.
Any particular photon of light will only travel a tiny distance before colliding with another matter particle and being absorbed. The light is then reemitted. This process acts as if the light were another particle that has bounced off the matter. As a result it gives some energy to the matter. The reemitted photon is slightly lower energy and the extra energy of the matter particle results in extra pressure resisting the collapse of the star.
As the photons very gradually make their way out of the star, they reduce in energy all the way. This is reflected in the differences in temperature through the cross section of a star. The Sun, for example, has a core temperature of around 10 million degrees Celsius (18 million °F), while the outer layer that we see is only around 5,500 degrees Celsius (9,900 °F). There are so many absorptions and reemissions along the way that photons take somewhere between 10,000 and 1 million years to get out of the Sun.
Meanwhile, back at the newly formed solar system, other clumps, whirling around that young star will also be coalescing under the attractive force of gravity, producing the planets. Not all will succeed though. This is a complex interaction. If there are enough big planets nearby, the small bits of matter might never have a chance to coalesce into a planet. This is thought to be the source of the asteroid belt between Mars and Jupiter. Once assumed to be the wreck of a planet it is now thought to be pre-planetary material that never made it because of the disruptive gravitational fields of its neighbors.
All thanks to gravity. This omnipresent force might be a pain when we drop something or fall, but without it there would be no Earth. Even if gravity was somehow switched off after the formation of the planets, we still couldn’t live. Apart from anything else, the only thing that keeps the atmosphere in place is gravity—not to mention keeping our feet firmly planted on the Earth’s surface. You only have to watch the difficulties that astronauts on the International Space Station have undertaking basic tasks (including the familiar bodily functions) to realize that gravity is beneficial for everyday existence.
More subtly, if people stay without that familiar gravitational pull for too long, their muscles begin to waste and their bones deteriorate. All the evidence is that we couldn’t exist for a full lifetime in a weightless state. Evolution has developed us (and all the plants and animals around us) to work under the influence of gravity—it is as essential for our long-term existence as the air that we breathe.
If plants are grown in space, the roots head off randomly, struggling to find nutrition. Roots make use of the directive force of gravity to know which way to head, something that Charles Darwin was aware of, and that is easily demonstrated by turning a plant pot on its side and seeing how the roots grow. In zero gravity, plants get confused. It’s even worse for birds’ eggs. In an experiment on the space shuttle Discovery, bizarrely sponsored by the fast-food company KFC, it was discovered that a series of quail eggs failed to hatch without gravity to keep the yolks near the shell.1
No animals have lived their entire lifespans in space, but there are reasons to be concerned if humans were ever brought up in weightless conditions. Without gravity pulling down on the internal organs, lung capacity is reduced by a change in position of the diaphragm, while the liver floats higher in the body cavity leaving even less room for the lungs to function. A baby born in space may have seriously compromised ability to breathe, which combined with bone deterioration emphasizes just how much gravity is part of our natural environment.
It is impossible to escape the physical impact of gravity without traveling to space or enduring a zero-gravity flight, but even if we ignore its physical omnipresence, gravity still makes itself known in the way that it threads through our consciousness. The very word “gravity” stretches beyond a mere attractive force of nature. In my dictionary, the first definition of gravity is about being grave, serious, weighty, and important.
Looking back over the use of the word, this figurative sense provided its first noted use in English. It wasn’t until the seventeenth century that the scientific meaning of gravity as that mystical attractive force became common. The earliest scientific use of “gravity” contrasts two concepts of ancient Greek physics, gravity and levity. Gravity was a tendency to move downward toward the center of the Earth, and hence of the universe. Levity was a tendency to move up and away from the center. Solid matter had gravity, airy things had levity. (Of course, such gravity was referred to earlier than the seventeenth century, but it would usually have been in Latin.)
Strangely, while gravity was first used figuratively in English, levity seems to have started with the physical meaning and then moved into its more commonplace use, meaning something that is light, frivolous, and quite possibly funny. The descriptive opposite of the solid, meaningful, and serious gravity.
Gravity seems to have exercised the minds of writers before artists. The earliest art on the walls of prehistoric caves appears to ignore the effects of gravity, allowing figures to float in space at will. But long before the constraints of perspective were imposed to give paintings and other illustrations a pseudo three-dimensional appearance that better reflected how we see the world, painted feet had become planted firmly on solid ground—gravity was there by implication.
So it would remain. In the work of surrealist painters like Salvador Dali, gravity sometimes played a more visible role, acting to distort the shape of familiar objects, but largely it would be an unconscious inclusion in the arts. Only when cinema needed to portray adventures in space and made its own attempts, all too often inaccurate, to portray the impact of differing gravitational force away from the surface of the Earth, would it be considered more explicitly.
To understand the origins of the idea of gravity in the scientific sense, though, we need to return to the ancient Greeks with their concepts of gravity and levity. To minds like ours, brought up on a Newtonian picture of existence, the Greek view provides us with a surprisingly alien perspective.
Copyright © 2012 by Brian Clegg
Posted September 28, 2012