Teoria relativităţii pe înţelesul tuturor - 3 — Transcript

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1. 0:00[Music]
2. 0:02you may have heard the term Lightyear
3. 0:04used in astronomy or
4. 0:06cosmology a light year is the distance
5. 0:09that light travels in one year about 6
6. 0:12trillion miles so you could say that the
7. 0:16speed of light is one light year a
8. 0:18distance per year a measure of
9. 0:22time you could just as easily say a
10. 0:25light second is the distance that light
11. 0:27travels in 1 second about
12. 0:31186,000
13. 0:32M and so the speed of light is one light
14. 0:36second a distance per second a measure
15. 0:39of
16. 0:42[Music]
17. 0:47time here are two
18. 0:49Starships one at rest and the other in
19. 0:53motion when the moving ship passes the
20. 0:55stationary ship each fires its laser
21. 1:00now according to Einstein the speed of
22. 1:02the laser light doesn't depend on the
23. 1:04motion of the ship so we see both beams
24. 1:08traveling side by side at the same
25. 1:11speed the crew of the ship at rest
26. 1:14witness their laser beam traveling 12
27. 1:16light seconds and 12
28. 1:19seconds and report that light travels at
29. 1:22the speed of one light second per second
30. 1:26correct but what about the beam from the
31. 1:28moving chip
32. 1:30does it travel only six light seconds in
33. 1:3312 seconds if so the crew on board the
34. 1:36moving vessel would report that light
35. 1:38travels at only half speed no they
36. 1:40wouldn't but why not according to
37. 1:43Einstein light always travels at one
38. 1:46light second per second regardless of
39. 1:48the motion of the person who measures it
40. 1:51how do we fix our picture so that this
41. 1:53is right part of the answer lies in the
42. 1:55fact that the clocks on the moving ship
43. 1:57slow down as we stated ear earlier if we
44. 2:00must agree on the speed of light we may
45. 2:02have to disagree on the measure of time
46. 2:05if the clock's on board the mooning
47. 2:07vessel ran only half as fast as the ones
48. 2:09at rest our problem would be completely
49. 2:12solved the light beam from the moving
50. 2:14ship would still travel six light
51. 2:16seconds and with the slow clocks it
52. 2:18would take only 6 seconds for this to
53. 2:21happen and the crew would report that
54. 2:23light travels at precisely one light
55. 2:26second per second
56. 2:32but it turns out the clocks don't slow
57. 2:33down enough to be the whole answer so in
58. 2:37a minute we'll look at the second part
59. 2:39of the
60. 2:41[Music]
61. 2:42answer and of course this isn't a
62. 2:45statement about clocks it is time itself
63. 2:48that slows
64. 2:50down right over the last 100 years
65. 2:54countless experiments have shown without
66. 2:55a doubt that it is time itself that
67. 2:58slows down but why does it occur Jeeves
68. 3:07please a clock is any device that counts
69. 3:11events that occur at regular
70. 3:13intervals the event that drives a clock
71. 3:16can be just about anything the swing of
72. 3:19a
73. 3:19pendulum the bounce of a ball the
74. 3:23unwinding of a
75. 3:24spring or the changing current in an
76. 3:27electric circuit any regular event will
77. 3:34do let's imagine constructing a clock in
78. 3:37which a beam of light bounces between
79. 3:39two
80. 3:43[Music]
81. 3:48mirrors whenever the light strikes the
82. 3:50bottom
83. 3:51plate the clock
84. 3:54[Music]
85. 3:56advances the advantage of using this
86. 3:59kind of clock is that we can be sure
87. 4:01that the Observer who looks at the clock
88. 4:03will see the timing mechanism the beam
89. 4:06of light moving at the same speed if two
90. 4:10such clocks are stationary then we would
91. 4:13expect the time indicated on each clock
92. 4:15to be precisely the
93. 4:19[Music]
94. 4:24same but what happens when one clock is
95. 4:27in
96. 4:28motion notice although both clocks are
97. 4:31identical the moving clock ticks slower
98. 4:34than the one at
99. 4:35rest in order to understand why notice
100. 4:39the paths of the light
101. 4:41beams the light in the stationary clock
102. 4:43travels up and down
103. 4:45vertically but the light beam in the
104. 4:47moving clock must travel a longer
105. 4:49diagonal
106. 4:51path since both beams travel at the same
107. 4:55speed and the upper beam travels a
108. 4:58longer distance the stationary Observer
109. 5:01sees the moving clock run slower than
110. 5:04his own this slowing down of clocks in
111. 5:07motion is known as time
112. 5:09dilation it should be stressed that a
113. 5:12person moving along with the moving
114. 5:14clock would notice nothing unusual at
115. 5:17all the clock and the flow of time would
116. 5:20seem to progress normally according to
117. 5:23this Observer he is at rest and the
118. 5:25other Observer is moving past
119. 5:27him according to him it is the other
120. 5:30clock that is running
121. 5:32slowly the flow of time in Einstein's
122. 5:35Universe is entirely
123. 5:42relative now let's revisit our Starship
124. 5:45and Laser problem and reveal the second
125. 5:47part of that resolution
126. 5:51[Music]
127. 6:00similar to the effect that motion has on
128. 6:03time there's a relativistic effect
129. 6:06involving length as
130. 6:08well any moving object is shortened
131. 6:11along the direction of
132. 6:13motion this effect is known as length
133. 6:17contraction length contraction is the
134. 6:19second necessary consequence of
135. 6:22Einstein's postulate that the speed of
136. 6:24light is the same for all
137. 6:26observers we now know beyond all doubt
138. 6:29that both length contraction and time
139. 6:31dilation actually occur with time
140. 6:35dilation and length contraction together
141. 6:38we can now see the final solution of how
142. 6:40Einstein's postulate allows the two
143. 6:42Starships to measure the same speed of
144. 6:46light as the upper Starship passes we
145. 6:49see that the starship's clock runs
146. 6:51slower than the one at rest we also note
147. 6:55that the Starship itself is shortened or
148. 6:58contracted
149. 7:00after 12 seconds elapse on the
150. 7:02stationary clock we see that only 9
151. 7:05seconds elapse on the moving clock not
152. 7:08the 6 seconds we assumed in an earlier
153. 7:12example but due to length contraction
154. 7:16the moving ship and all the rulers
155. 7:17aboard are contracted just enough to
156. 7:20make the measurement
157. 7:22work according to the crew the beam of
158. 7:25light travels 9 light seconds and 9
159. 7:28seconds
160. 7:30they now report that the light beam
161. 7:31travels at the correct speed of one
162. 7:34light second per
163. 7:36[Music]
164. 7:49second does that really
165. 7:53happen sure does thousands of
166. 7:56experiments have been done since
167. 7:57Einstein stated his postulate
168. 7:59and everyone has shown time dilation and
169. 8:02length contraction are real and really
170. 8:08happen back in
171. 8:101971 four cesium atomic clocks were
172. 8:14synchronized and flown on Commercial
173. 8:16planes that circle the Earth twice and
174. 8:19then compared to the reference clock at
175. 8:22the US Naval
176. 8:24Observatory sure enough the moving
177. 8:26clocks differed from the reference clock
178. 8:29by exactly the amount predicted by
179. 8:32relativity let's look at it a little
180. 8:34differently
181. 8:36[Music]