A Brief History of Time

A Brief History of Time

by

Stephen Hawking

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A Brief History of Time: Chapter 3 Summary & Analysis

Summary
Analysis
On a moonless night the brightest objects are most likely Venus, Mars, Jupiter, and Saturn. There are also many stars, similar to our sun, just farther away. But they are not fixed; they all move relative to each other, which we can see because these stars are relatively close to us. As the earth moves round the sun we these stars move against the background of more distant stars; the nearer the stars, the more they seem to move.
Ever since the earliest civilizations, humanity has wondered at the stars. Hawking opens this chapter by summarizing a model of the universe that might seem obvious to modern-day readers, but would have astounded readers in Aristotle or Ptolemy’s day. Essentially, Hawking reminds the reader how far humanity has come that such knowledge is now commonplace.
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Proxima Centauri is the nearest star and is four light-years, or 23 million million miles, away. Other visible stars are mostly within a few hundred light-years, while the sun is just eight light minutes away. These stars are concentrated in a band we call the Milky Way. Even in 1750, astronomers thought this must be because we are in a spiral galaxy. Sir William Herschel confirmed this some decades later, but the idea only gained traction in the 1900s.
Providing more details, Hawking begins to add scale to the model, with some astounding numbers. Modern scientists’ understanding of the universe far outranks that of the classical thinkers, and the addition of dates helps the reader to piece together the history of the modern model of the universe.
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In 1924, Hubble defined our modern understanding of the universe when he showed that ours was not the only galaxy, proving there were many others, with lots of space in between. To measure how far away they were, he measured their luminosity, which is affected by their distance from us.
Astronomer Edwin Hubble, after discerning that there were other galaxies, did not stop there. He set out to measure their distances, and used ingenious methods to do so.
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Hubble worked out distances to nine galaxies this way, showing how there are hundreds of thousands of millions of them. Our galaxy is 100,000 light-years wide and rotating. Our sun is an average star among one of the galaxy's spiral arms. We've come a long way from thinking we were the center of the universe.
Hubble’s findings were humbling for the human race, yet, it seems, widely accepted. Having built up to this moment from the opening lines of the chapter, Hawking notes just how far humanity has come from thinking the universe revolves around us.
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Newton discovered that by using a prism we can measure the different colors of the light spectrum. By using a telescope and a prism, we can see the light make up of whatever star or galaxy focused on. We can in turn tell a light's temperature from its spectrum. Missing colors indicate what chemicals are in each star.
Scientist have devised resourceful ways to determine the make up and temperature of far distant stars. Those who studied the chemical components of stars in other solar systems may well have never known how their work would have contributed to humanity. Perhaps they were simply curious.
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When in the 1920s scientists looked at stars in other galaxies, they had the same missing colors as similar, closer stars, but they were all shifted toward the red end of the light spectrum. The Doppler effect tells us that as something moves away from us, each wavelength will be longer, while if it is approaching us each wave would reach us more quickly. As light is a wave, its wavelength will lengthen as an object moves away from us, and our eyes see longer wavelengths as red light.
By understanding the properties of light, scientists studying distant stars could tell that they were moving away from us, just from the spectrum of the color of light that filtered through the prism set to the telescope. Studying these patterns of light that reached them on earth showed the scientists the movements of the wider cosmos.
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As Hubble catalogued the galaxies and their distances from us, he found most galaxies were red-shifted, meaning they were moving away from the earth. Indeed, the red shift is proportional to the galaxies’ distance from the earth, meaning the further it is the faster it is moving away. As such, the universe must be expanding.
This analysis lead to the next great revolutionary scientific upset: the universe is expanding in all directions. This discovery did not happen in isolation, but was based on the layers of prior discoveries that supported the work. Hawking’s point, therefore, is that the results of previous curiosity will both spur and support future curiosity.
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This was such a great intellectual revolution that people wondered how it had not been thought of before. Newton should have guessed it, as otherwise the universe would have contracted under the influence of gravity. But as the universe is expanding, this cancels out that gravitational pull. If the universe is expanding beyond what gravity can balance, it could expand forever, like a rocket bursting out of the atmosphere and continuing through space instead of falling back to earth.
While Hawking shows that many new ideas have met with incredulity or even aggression, Hubble’s discovery was so obvious it was almost embarrassing that it hadn’t been suggested before. This new discovery, as with all the ones preceding it, gave rise to more questions, such as whether the universe will always be expanding.
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People could have realized the universe was expanding from Newton's theory of gravity, but everyone at the time seemed set on believing in a static universe. Even Einstein overlooked this idea in his general theory of relativity. Instead he thought up a kind of anti-gravitational force he called the cosmological constant.
Hubble’s discovery was based on fundamental laws known since Newton’s day, and the latter himself should have made the next logical jump, although he was too preoccupied with disliking the idea of nonabsolute space. Einstein had missed the idea too, as he was so certain the universe was static.
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Yet a Russian physicist called Alexander Friedmann tackled this head on. He assumed first that the universe looked uniformly the same in every direction on the large scale, and second that this would be true from wherever you looked in the universe. This alone suggested the universe is not static, and he suggested it before Hubble's landmark discovery.
Before Hubble’s announcement, however, Friedmann had made the same suggestion. The difference, it seems, was that he suggested it, but could not show it in the same way Hubble did.
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In nearby space, the universe does not look uniform, but further out it does. This was further backed up by two American physicists, Arno Penzias and Robert Wilson. They tested a very sensitive microwave detector, which picked up a lot more background noise than expected. After checking their equipment, then taking measurements in all directions as the earth traveled around the sun, they determined it was coming, uniformly, from the whole universe. They had confirmed Friedmann's first assumption.
Penzias and Wilson, albeit unwittingly, provided the observational evidence that Friedmann’s theories required. Although the two Americans at first thought their equipment was faulty, their curiosity got the better of them, and they were determined to discover where all the noise their detector was picking up was coming from. It turned out, it was from the entire universe.
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At the same time, two other American physicists, Bob Dicke and Jim Peebles, were setting themselves up to look into microwave radiation to examine George Gamow's idea the early universe was hot and glowed brightly. They said it was so long ago that energy would now have red-shifted to become microwave radiation. Penzias and Wilson heard about this, and saw they had already found the evidence. The latter won the Nobel Prize in 1978 for their work, which seems hard on those who suggested the theories in the first place.
Along with confirming Friedmann’s work, Penzias and Wilson also provided proof for numerous other scientific theories, simply because they pulled at a loose string. Hawking seems ambivalent about whether Penzias and Wilson ought to have won the Nobel Prize for their work over the other scientists, emphasizing the importance of asking questions and making suggestions, even if one cannot prove news ideas immediately.
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If the universe looks the same in every direction, it should look the same from any other point in the universe too. We argue this on the basis of modesty; we cannot prove it yet. In Friedmann's model, the universe is expanding, like a balloon with every point expanding from every other point. The further away two points are, the faster they'll be expanding, just as Hubble found. But despite his accuracy, Friedmann's work was not known in the West until Howard Robertson and Arthur Walker did similar work.
While philosophers used to argue that the earth was the center of all existence for mystical reasons, modern scientists seek to humble humanity in the interests of modesty, given the awesome scale of the universe they are studying. A little humility and open-mindedness seems to have brough humanity a long way.
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Three models obey Friedmann's assumptions, though he only suggested one himself. In the first, gravity can slow and eventually stop the expansion. Finally, the universe will contract again. In the second, gravity can slow the expansion a little, but not halt it, until it steadies to a constant rate. In the third, the gravity is just below where it can stop the expansion, so the universe continues to expand forever, but at an increasingly slow rate.
After realizing the universe must be expanding, Friedmann’s next question was just how fast that expansion was. If the universe was expanding too rapidly for gravity to balance it out, the universe might expand forever. If not, the universe would collapse back in on itself under its own gravity. Friedmann assumed the latter.
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In the first model, or theory, the universe is finite, yet does not have a boundary. Gravity bends space around itself, so space is curved, like the surface of the earth. When combining the general theory of relativity with quantum mechanics (as discussed later), space and time can be finite without a boundary. But this doesn't mean you could travel right around the universe back to where you started; it would collapse again before you managed it. You'd have to travel faster than light, and you can't. In the second model, the universe is bent like a saddle, so it is infinite in space. In the third model, space is flat and infinite.
While the first model might hold promise for circumnavigating the universe, it is only the wishful thinking of adventurous humans. It would take a normal object traveling under the speed of light too long, and the universe’s gravity would begin to draw everything back together, causing the universe to collapse in on itself. As Hawking explained, normal objects with mass require exponential energy to speed up, and infinite energy to reach the speed of light, so humans will most likely never circumnavigate the universe.
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To know which model fits best, we need to know the universe's rate of expansion and average density, to determine if gravity will slow or stop the expansion. We know from the Doppler effect that galaxies are moving away from us at a rate of 5 to 10 percent every billion years. Our estimate for density is even more vague. What we can see and measure is less than one percent of the mass required to halt the expansion. Even what we cannot see but think is there would not add up to enough. Right now, it seems the universe will expand forever, but even if it did recollapse, it would most likely be billions of years after humans had died out anyway.
Although the ultimate fate of the universe, if it were to collapse, would not have any bearing on the human race as the species would most likely have died out anyway, scientists still pursued the question of which of these models is right. The attempt to choose any Friedmann model with certainty only highlights just how much humans still do not know about the universe, including what humans don’t know that they don’t know.
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All the Friedmann models start out with a beginning where the space between everything was zero—the universe was infinitely dense and curved. Laws of science break down at this point. This big bang singularity means any previous events would not have any meaning to us now. In a sense, then, time began with the big bang.
As St. Augustine pointed out, time is a property of this universe and has no meaning outside of it. If the laws of science break down at the big bang, any events previous to that can be considered beyond the boundaries of this universe, with no effect on events taking place today. As such, the best option is to focus on what is discoverable and say time started with the big bang. There do have to be limits to human curiosity it seems, with the boundary being our own universe—as this is all humans can observe, measure,e and therefore truly know.
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While the church liked the big bang model because it leaves room for God, many dislike the idea. Hermann Bondi, Thomas Gold, and Fred Hoyle proposed the steady state theory, where matter spontaneously comes into being in the gaps between expanding galaxies. A group of astronomers headed up by Martin Ryle looked into radio waves from different galaxies, and found variations that disproved the theory. Penzias and Wilson’s earlier discovery also suggested the universe had been denser in the past, further disproving the theory.
Somewhat ironically given earlier examples Hawking uses, the church grasped at the big bang theory to support its own teachings, while scientists disliked the idea because it backed religious ideas. Hawking’s ironic tone dealing with both approaches shows his frustrations with those who cannot view new ideas objectively. Neither approach helps to ensure the continued progress in humans’ understanding of the universe.
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Russian scientists Evgenii Lifshitz and Isaac Khalatnikov also tried to disprove the big bang. They said that as galaxies do not move directly away from each other, perhaps they were simply nearby at the beginning, not in a singularity. They created many more models, and found that there were examples for both sides, retracting an earlier statement that said there were many more scenarios in which there was no big bang. They did show that a big bang was possible under the general theory of relativity.
With this example, Hawking shows there can be redemption for those who approach scientific undertakings with a biased agenda. While Lifshitz and Khalatnikov set out to disprove the big bang theory, rather than simply assess its validity, they ended up demonstrating that the big bang was a legimate theory within the realms of possibility, humbly eating their hats in the process.
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In 1965, Roger Penrose showed that stars can collapse in on themselves to become singularities, in this case, black holes. While Penrose only talked about stars, a young Stephen Hawking saw the relevance this had for the big bang theory. After surviving longer than expected after being diagnosed with Lou Gehrig’s disease, Hawking took the research matter up for his PhD. He suggested that if the universe is infinite and expanding too fast to recollapse, it should have started at a singularity. Penrose and Hawking’s resulting paper in 1970 faced opposition. But the math held out, and now everyone tends to assume there was a big bang. However, Hawking himself has since changed his mind, when taking quantum mechanics into account.
Stephen Hawking now enters the book as a character in his own right, in addition to being the narrative voice. He set out to prove that infinite Friedmann models necessitate a big bang singularity, but faced opposition from those who simply did not like the idea. Hawking’s derision for those who challenged his work does not arise from his bias toward his own work. In fact, he later changed his mind about the idea based on subsequent discoveries! Instead, Hawking criticizes his critics’ prejudice and lack of objective judgment.
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Over the millennia, our understanding has changed significantly. Penrose and Hawking’s work showed that Einstein’s general theory of relativity is only a partial theory. It breaks down at the beginning of the universe. When the universe was squeezed into infinite density, quantum mechanics comes into play. As such, their focus turned from the massive to the miniscule.
If Einstein’s general theory of relativity cannot describe what happens in singularities, and explains the phenomena only as the break down of scientific laws, then it follows that the laws themselves are not good enough, yet. General relativity, while crucial in solving some problems, cannot aid scientists to uncover the hidden truths in singularities such as black holes or the big bang. Time, then to move the discussion onto a new topic.
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