## ← Why Hasn't the Universe Collapsed Into a Giant Black Hole? - Ask the Expert #13

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Showing Revision 4 created 05/22/2014 by Rita Logue.

1. Hi I'm George Musser. I'm a contributing editor at scientific American magazine
2. and author of : The complete idiot's guide to String Theory.
4. Ok, The first question is 'Let's say I'm an alien on a ship
5. 65 million or more light years away
6. Using a telescope I look at earth and I see
7. dinosaurs living their daily lives
8. If my ship started travelling towards earth
9. near the speed of light, Would I see the dinosaurs moving faster
10. fast forwarded? What a great question? It's a subtle question
11. It's the kind of thing comes up a lot
12. When you deal with relativity theory
13. Einstein's theory of relativity.
14. And actually asked a physicist to make sure
15. we get the answer right for this
16. So the bottom line is the dinosaurs are going to be moving a lot faster
17. as you see them on the ship
18. And the reasoning goes something like this
19. When you set out, you're seeing the dinosaurs 65 million years ago
20. When you arrive you're moving at something like the speed of
21. light. you see them 65 million years in the future
22. so 130 million years have passed in all
23. according to the dinosaurs time . Things look a little different on your ship.
24. On the ship the distance seems to be somewhat shortened by
25. the fact that you are moving so close to the speed of light.
26. If you're moving at 99% of the speed of light the distance
27. looks something like only 10 million light years
28. so you need 10 million years to cross that distance.
29. Ok 130 million years have passed on earth
30. 10 million years have passed on your ship, the dinosaurs
31. seem sped up by a factor of 13.
32. They're moving and they're stomping around and they're chomping on prey, 13 times faster from your view on the ship

33. than they are to them, to themselves.
34. Ok so question number 2 another great question
35. If all movement is relative how can there be a speed limit?
36. In order to measure speed you have to measure it against something.
37. So what is the speed limit measured against?
38. So it's really a great question
39. beacuse it gets at the heart of relativity theory.
40. Einstein's theory of relativity
41. and what does that theory mean and the weird predictions that it makes.
42. The bottom line is that the speed of light is the same for everybody.
43. it doesn't matter what you're measuring it against
44. you can measure it against a space ship
45. train ,car, boat, walking down the street, standing still, whatever
46. and you'll see the same speed of light for everybody.
47. And thats completely different from other things that we observe
48. in nature, from travelling in a car, and i threw a ball
49. off the car, i have to add or subtract the speed of the car
50. to the speed of the ball. But that's not true of light
51. if i shine a flashlight off that car, it would be moving at the speed of light
52. relative to the other car and the speed of light relative to someone
53. walking down the sidewalk, the speed of
54. light relative to someone in orbit looking on, an alien on a distant planet.
55. Everyone will see the same speed of light
56. for that light beam that I shine off the car.
57. And that's a weird fact that the speed of light
58. is the same for everybody but it underlies
59. everything that you've ever heard about with Einstein's theory.
60. It underlies the fact that time ticks differently for
61. different obeservers, lengths look different to different observers.
62. The space and time themselves are combined in something called "space time".
63. You can't really treat space and time
64. independently, they're united in this thing called "space time"
65. And it’s all consequence of this fact developed from the speed of light being the same for everybody
66. The third question goes in a somewhat different direction
67. but I really like it as well.
68. If a black hole event horizon gets bigger, every time its density gets bigger
69. i.e. it absorbs mass
70. Why hasn't the whole universe collapsed into an enormous black hole yet?
71. Shouldn't this kind of expanding black hole be getting bigger exponentially?
72. And will this be the end of our universe and possibly the birth of a new one?
73. or where ours began, a big bang?
74. Well it's a very perceptive question, because it's true.
75. As black holes take material in, they get bigger
76. so you think there is an exponential growth to them
77. that as they take more stuff in they get bigger, they're bigger target,
78. they suck more stuff in, they get bigger, bigger, bigger at an exponential rate.
79. True, black holes do grow at an exponential rate
80. if you keep feeding stuff into them, if there is material for them to eat.
81. In fact, there is something in astrophysics called the Salpeter time,
82. which is something like a few tens of millions of years
83. and that shows how fast they will double in size.
84. So keep doubling, and doubling and doubling in size every few tens of billions of years
85. If stuff is being fed into them. Now that 'if' is a big 'if'
86. Although black holes are monsters and they're vacuum cleaners
87. and they're sucking everything in
88. they're still pretty small on a galactic scale.
89. So they're not a huge target?
90. They're sitting at the centre of a galaxy , in this case typically
91. and it's hard for them to get enough material for them to feed on,
92. so they tend to choke off basically their own growth.
93. Not just because it's hard to feed stuff in but because the black hole is actively changing its environment
94. it's spraying stuff out in the space and actually pushing stuff back out.
95. It's heating stuff up, preventing it from falling in so they regulate their own growth.
96. So it's very hard for them to grow,
97. to continue growing at an exponential rate.
98. They probably did initially, but then they kind of level out
99. and they achieve something like a few
100. tens of a percent of the mass of their overall galaxy.
101. They have a self regulatory mechanism.
102. Thanks for the great questions.
103. If you have other questions for the next experts,