33
Black Holes

Of all the odd creatures in the astronomical zoo, the "black hole" is the oddest.To understand it, concentrate on gravity.

Every piece of matter produces a gravitational field. The larger the piece, the larger the field. What's more, the field grows more intense the closer you move to its center. If a large object is squeezed into a smaller volume, its surface is nearer its center and the gravitational pull on that surface is stronger.

Anything on the surface of a large body is in the grip of its gravity, and in order to escape it must move rapidly. If it moves rapidly enough, then even though gravitational pull slows it down continually it can move sufficiently far away from the body so that the gravitational pull, weakened by distance, can never quite slow its motion to zero.

The minimum speed required for this is the "escape velocity." From the surface of the earth, the escape velocity is 7.0 miles per second.From Jupiter, which is larger, the escape velocity is 37.6 miles per second.From the sun, which is still larger, the escape velocity is 383.4 miles per second.

Imagine all the matter of the sun (which is a ball of hot gas 864,000 miles across) compressed tightly together. Imagine it compressed so tightly that its atoms smash and it becomes a ball of atomic nuclei and loose electrons, 30,000 miles across.The sun would then be a "white dwarf." Its surface would be nearer its center, the gravitational pull on that surface would be stronger, and escape velocity would now be 2,100 miles per second.

Compress the sun still more to the point where the electrons melt into the nuclei.There would then be nothing left but tiny neutrons, and they will move together till they touch. The sun would then be only 9 miles across,

-162-

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The Roving Mind
Table of contents

Table of contents

  • The Roving Mind *
  • Contents vii
  • Foreword Isaac Asimov: Science Popularizer, Skeptic, and Rationalist xi
  • A Celebration of Isaac Asimov — a Man for the Universe xv
  • Introduction 1
  • Part I the Religious Radicals *
  • 1: The Army of the Night *
  • 3: The Reagan Doctrine 20
  • 4: The Blind Who Would Lead 24
  • 5: Creeping Censorship 27
  • 6: Losing the Debate 29
  • Part II Other Aberrations 30
  • 7: The Harvest of Intelligence *
  • 8: That Old-Time Violence 37
  • 9: Little Green Men or Not? 40
  • 10: Don'T You Believe? 43
  • 11: Open Mind? 47
  • 12: The Role of the Heretic 49
  • Part III Population *
  • 13: The Good Earth is Dying *
  • 14: The Price of Survival 66
  • 15: Letter to a Newborn Child 72
  • Part IV Science: Opinion 74
  • 16: Technophobia *
  • 17: What Have You Done for Us Lately? 84
  • 18: Speculation 88
  • 19: Is It Wise for Us to Contact Advanced Civilizations? 92
  • 20: Pure and Impure 97
  • 21: Do We Regulate Science? 104
  • 22: For Public Understanding of Science 107
  • 23: Science Corps 110
  • 24: Science and Beauty 113
  • 25: Art and Science 116
  • 26: The Fascination of Science 120
  • 27: Sherlock Holmes as Chemist 127
  • Part V Science: Explanation 132
  • 28: The Global Jigsaw *
  • 29: The Inconstant Sun 139
  • 30: The Sky of the Satellites 151
  • 31: The Surprises of Pluto 157
  • 32: Neutron Stars 160
  • 33: Black Holes 162
  • 34: Faster Than Light 165
  • 35: Hyperspace 169
  • 36: Beyond the Universe 175
  • 37: Life on Earth 183
  • Part VI the Future 188
  • 38: Transportation and the Future *
  • 39: The Corporation of the Future 200
  • 40: The Future of Collecting 208
  • 41: The Computerized World 214
  • 42: The Individualism to Come 228
  • 43: The Coming Age of Age 237
  • 44: The Decade of Decision 244
  • 45: Do You Want to Be Cloned? 251
  • 46: The Hotel of the Future 256
  • 47: The Future of Plants 262
  • 48: Bacterial Engineering 266
  • 49: Flying in Time to Come 272
  • 50: The Ultimate in Communication 278
  • 51: His Own Particular Drummer 285
  • 52: The Future of Exploration 295
  • 53: Homo Obsoletus? 300
  • 54: Volatiles for the Life on Luna 307
  • 55: Touring the Moon 312
  • 56: Life on a Space Settlement 317
  • 57: The Payoff in Space 324
  • Part VII Personal *
  • 58: I Am a Signpost *
  • 59: The Word-Processor and I 334
  • 60: A Question of Speed 337
  • 61: A Question of Spelling 340
  • 62: My Father 344
  • Acknowledgments 348
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