Fear of Physics Read Online Free PDF Page A

energy of the neutrinos were also in good agreement with predictions.
    Whenever I think about this, I am still amazed. These neutrinos are emitted directly from the dense collapsing core, not from the surface of the star. They give us direct information about this
crucial period of seconds associated with the catastrophic collapse of the core. And they tell us that the theory of stellar collapse—worked out in the absence of direct empirical measurement over thirty-odd years, and based on extrapolating to its extreme limits the same physics of hydrostatic equilibrium responsible for determining the sun’s structure—is totally consistent with the data from the supernova. Confidence in our simple models led us to understand one of the most exotic processes in nature.
    There is yet one more example of the remarkable power of approximating the sun as a sphere. Even as the solar neutrino problem was being resolved, another puzzle regarding stellar structure seemed to remain. If we assume the same solar modeling procedure that we use to understand the sun to predict how stars evolve, we can use a comparison of theory and observation to date not only our sun (about 4.55 billion years old), but also the oldest stars in our galaxy. When this procedure was applied to stars in some isolated systems on the edges of our galaxy called globular clusters, it was found that such systems were greater than 15 billion years old.
    At the same time we can use the fact that our observed universe is expanding—and assuming this expansion is slowing down, as is natural given that gravity is an attractive force—to determine the age of our universe by measuring its expansion rate today. The argument is relatively simple: We observe how fast galaxies at a known distance from us today are moving away from us, and then assuming that they have been moving away from us at this speed or greater during the whole history of the universe, we can put an upper limit on how long it would have taken them to recede to their present distance since the big bang. After eighty years of trying, we were finally able to measure the expansion rate to about 10 percent accuracy, and we found that if the expansion
of the universe has been slowing down, then the universe had to be less than about 11 billion years old.
    This presented a problem, as it suggested that the stars in our galaxy were older than the universe! This was not the first time stellar ages had caused such a problem, and each time resolving the problem shed new insights into the universe. For example, in the 1800s an estimate was made of the age of the sun, by assuming it was a large ball of carbon, burning like coal. Given the mass of the sun, one could determine how long it would take for it to fully use up all its fuel, and the answer was about 10,000 years. While this meshed nicely with literal interpretations from the Bible about the age of the universe, by this time the evidence of fossils and geological strata had revealed the Earth to be far older than this. Near the end of the nineteenth century it was then pointed out by two well-known physicists, Lord Kelvin in the UK and Helmoltz in Germany, that if the material in the sun was collapsing due to the large gravitational field of the sun, this could provide energy to power the sun, and they worked out that this process could persist for perhaps 100 million years before exhausting all of the available energy. While this was a vast improvement, by this time geology and evolutionary biology had made it clear that the Earth was billions of years old, again causing a problem since it was hard to imagine how the Earth could be much older than the sun.
    In the 1920s this problem was so severe that the distinguished astrophysicist, Sir Arthur Stanley Eddington argued there had to be another, as of yet unknown, energy production mechanism that could keep the sun burning for billions of years. Many people were skeptical of this claim, as the 10 million