Reading 13

This passage is excerpted from an article by Pankaj Joshi that originally appeared in “Scientific America” magazine (© 2009 by Pankaj Joshi).

  1. Modern science has introduced the
  2. world to plenty of strange ideas,
  3. but surely one of the strangest is
  4. the fate of a massive star that has
  5. reached the end of its life. Having
  6. exhausted the fuel that sustained
  7. it for millions of years, the star is
  8. no longer able to hold itself up
  9. under its own weight, and it starts
  10. collapsing catastrophically.
  11. Modest stars like the sun also
  12. collapse, but they stabilize again at
  13. a smaller size. Whereas if a star is
  14. massive enough, its gravity
  15. overwhelms all the forces that
  16. might halt the collapse. From a
  17. size of millions of kilometers
  18. across, the star crumples to a
  19. pinprick smaller than the dot
  20. on an “i.”
  21. Most physicists and astronomers
  22. think the result is a black hole,
  23. a body with such intense gravity
  24. that nothing can escape from its
  25. immediate vicinity. A black hole
  26. has two parts. At its core is a
  27. singularity, the infinitesimal point
  28. into which all the matter of the
  29. star gets crushed. Surrounding the
  30. singularity is the region of space
  31. from which escape is impossible,
  32. the perimeter of which is called
  33. the event horizon. Once something
  34. enters the event horizon, it loses
  35. all hope of exiting. Whatever light
  36. the falling body gives off is
  37. trapped, too, so an outside
  38. observer never sees it again.
  39. It ultimately crashes into
  40. the singularity.
  41. But is this picture really true?
  42. The known laws of physics are
  43. clear that a singularity forms, but
  44. they are hazy about the event
  45. horizon. Most physicists operate
  46. under the assumption that a
  47. horizon must indeed form, if only
  48. because the horizon is very
  49. appealing as a scientific fig
  50. leaf. Physicists have yet to figure
  51. out what exactly happens at a
  52. singularity: matter is crushed, but
  53. what becomes of it then? The event
  54. horizon, by hiding the singularity,
  55. isolates this gap in our knowledge.
  56. All kinds of processes unknown to
  57. science may occur at the
  58. singularity, yet they have no effect
  59. on the outside world. Astronomers
  60. plotting the orbits of planets and
  61. stars can safely ignore the
  62. uncertainties introduced by
  63. singularities and apply the
  64. standard laws of physics with
  65. confidence. Whatever happens in
  66. a black hole stays in a black hole.
  67. Yet a growing body of research
  68. calls this working assumption
  69. into question. Researchers have
  70. found a wide variety of stellar
  71. collapse scenarios in which an
  72. event horizon does not in fact
  73. form, so that the singularity
  74. remains exposed to our view.
  75. Physicists call it a naked
  76. singularity. Matter and radiation
  77. can both fall in and come out.
  78. Whereas visiting the singularity
  79. inside a black hole would be a
  80. one-way trip, you could in
  81. principle come as close as you like
  82. to a naked singularity and return
  83. to tell the tale.
  84. If naked singularities exist, the
  85. implications would be enormous
  86. and would touch on nearly every
  87. aspect of astrophysics and
  88. fundamental physics. The lack of
  89. horizons could mean that
  90. mysterious processes occurring
  91. near the singularities would
  92. impinge on the outside world.
  93. Naked singularities might
  94. account for unexplained high-
  95. energy phenomena that
  96. astronomers have seen, and they
  97. might offer a laboratory to explore
  98. the fabric of spacetime on its
  99. finest scales.
  100. Event horizons were supposed to
  101. have been the easy part about
  102. black holes. Singularities are
  103. clearly mysterious. They are places
  104. where the strength of gravity
  105. becomes infinite and the known
  106. laws of physics break down.
  107. According to physicists’ current
  108. understanding of gravity,
  109. encapsulated in Einstein’s general
  110. theory of relativity, singularities
  111. inevitably arise during the collapse
  112. of a giant star. General relativity
  113. does not account for the quantum
  114. effects that become important for
  115. microscopic objects, and those
  116. effects presumably intervene to
  117. prevent the strength of gravity
  118. from becoming truly infinite. But
  119. physicists are still struggling to
  120. develop the quantum theory of
  121. gravity they need to explain
  122. singularities.
  123. By comparison, what happens to
  124. the region of spacetime around
  125. the singularity seems as though it
  126. should be rather straightforward.
  127. Stellar event horizons are many
  128. kilometers in size, far larger than
  129. the typical scale of quantum effects.
  130. Assuming that no new forces of
  131. nature intervene, horizons should
  132. be governed purely by general
  133. relativity, a theory that is based on
  134. well-understood principles and has
  135. passed 90 years of observational
  136. tests.