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.