Why the Speed of Light Is NOT a Speed - Leonard Susskind — Transcript

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1. 0:00Why the Speed of Light Is NOT a Speed Let me tell you something that has bothered
2. 0:04me for fifty years. Something that every physics  textbook gets subtly, profoundly wrong. Not wrong
3. 0:10in the equations. The equations are fine. Wrong  in the interpretation. Wrong in what they tell you
4. 0:16it means. We call it the speed of light. We teach  it as the speed of light. We have built an entire
5. 0:22civilization of scientific communication around  the phrase the speed of light. And that phrase,
6. 0:28that single innocent phrase, is one of the most  misleading things in the history of science.
7. 0:33C is not a speed. Not in the way you think speed means.
8. 0:37Not in the way your car has a speed. Not in the  way a baseball has a speed. C is something deeper,
9. 0:43something stranger, something so fundamental that  calling it a speed is like calling gravity a push.
10. 0:50Technically you can make an argument.  But you've missed the entire point.
11. 0:53I'm Leonard Susskind. I've spent my career inside  the mathematics of spacetime, quantum fields,
12. 0:59black holes, the structure of reality at its  most basic level. And I want to tell you what
13. 1:04C actually is. Because once you understand it,  really understand it, the universe stops looking
14. 1:11like a place where things happen and starts  looking like something else entirely. Something
15. 1:16that has no good name in ordinary language. Let's start with what you think you know.
16. 1:21You were taught that light travels at  approximately 300,000 kilometers per second in
17. 1:25a vacuum. You were taught that nothing can go  faster than this. You were taught that Einstein
18. 1:31discovered this limit and built his theory of  relativity around it. And you probably walked
19. 1:36away thinking, okay, the universe has a  speed limit. Like a cosmic highway with
20. 1:40a maximum velocity. Light is just fast  enough to hit that limit perfectly.
21. 1:46That story is not wrong exactly. But it  is deeply, catastrophically incomplete.
22. 1:51Here is the first thing that should disturb you.  The number 300,000 kilometers per second is not a
23. 1:56fundamental fact about nature. It is an artifact  of how we chose to measure things. If we measured
24. 2:02distance in miles, C would be 186,000 miles per  second. If we measured distance in light-years
25. 2:08and time in years, C would be exactly 1.  Just the number 1. No units. Just 1.
26. 2:14That last one is the important one. Because  physicists who work in the right unit system
27. 2:19don't think of C as a large number. They  think of it as 1. A pure, dimensionless,
28. 2:24unit-free fact about the universe. Not a  speed. A ratio. A conversion factor.
29. 2:30A conversion factor between what? Between space and time.
30. 2:34This is where everything changes. When Einstein wrote down special relativity in
31. 2:381905, he wasn't fundamentally talking about light.  He was talking about the geometry of spacetime. He
32. 2:45was discovering that space and time are not two  separate things. They are two aspects of a single
33. 2:50four-dimensional structure. A manifold. A fabric.  Call it what you want, but the key is this:
34. 2:55space and time are made of the same stuff and  C is simply the exchange rate between them.
35. 3:00Think about currency. If you're traveling  between two countries, there's an exchange rate
36. 3:04that converts dollars to euros. That exchange  rate is not itself a dollar or a euro. It's a
37. 3:09relationship. A ratio. It tells you how much of  one thing equals how much of another thing.
38. 3:14C does the same thing for space and time. It  tells you how much space equals how much time. One
39. 3:20second of time equals 300,000 kilometers of space.  That's what C means. Not that light is fast.
40. 3:26That space and time are related by this ratio. And here is the truly disorienting implication.
41. 3:31If C is just a conversion factor between space  and time, then in some sense space and time are
42. 3:37the same dimension measured in different units.  The only reason we invented two different words,
43. 3:42space and time, is that we evolved brains that  experience them differently. We can move freely
44. 3:48through space in three directions. We seem  to be carried in one direction through time.
45. 3:52This asymmetry in our experience made us think  they were fundamentally different things.
46. 3:57They are not. They are unified. And
47. 4:00C is the unification constant. Now let me push this further,
48. 4:04because there is a deeper strangeness  here that almost nobody talks about.
49. 4:08If C is a conversion factor and not a speed,  why does light specifically travel at exactly
50. 4:14C? Why not some other speed? Why does light hit  the cosmic maximum perfectly every single time,
51. 4:21in every direction, in every  vacuum, without exception?
52. 4:24The answer will sound too simple. It will  sound like a trick. But it is not a trick. It
53. 4:29is one of the most profound facts in physics. Light travels at C because light has no mass.
54. 4:36And massless things don't have a choice. Here is what I mean. In spacetime, everything
55. 4:40that exists is moving through the four-dimensional  structure at all times. Not just moving through
56. 4:45space. Moving through space and time combined.  And there is a rule built into the geometry of
57. 4:51spacetime about how this four-dimensional  motion works. Every object, every particle,
58. 4:57every field, moves through spacetime at a total  combined rate that is always exactly C.
59. 5:03Always. Without exception. But here is the crucial part.
60. 5:06The way that total motion is distributed  between space and time depends on mass.
61. 5:10When you are sitting still in your chair,  you are not moving through space at all. But
62. 5:14you are moving through time. And your rate of  motion through time is exactly C. All of your
63. 5:19spacetime motion is in the time direction.  None of it is in a space direction.
64. 5:24When you start moving through space, something  has to give. Because your total four-dimensional
65. 5:28speed must remain C. So as you gain speed  through space, you lose speed through time.
66. 5:34This is not a metaphor. This is the literal  geometric mechanism behind time dilation. The
67. 5:39faster you move through space, the slower you  move through time. The two are trading off.
68. 5:44They must trade off. Because C is fixed. Now push this to the extreme. Imagine an
69. 5:50object with zero mass. An object with no mass  has no resistance to being pushed to higher
70. 5:55and higher space-speeds. So it gets pushed  all the way. All the way to C in the space
71. 6:01direction. Which means it has zero speed left  in the time direction. It is moving entirely
72. 6:07through space and not at all through time. This is why photons don't age. This is why,
73. 6:12from a photon's perspective, no time passes  during its journey across the universe.
74. 6:17It has traded all of its time-motion for  space-motion. It has hit the geometric limit.
75. 6:22It is traveling at C through space because it has  nothing left to give to the time direction.
76. 6:27C is not a speed limit. It is a geometric  constraint. It is the total budget of
77. 6:33four-dimensional motion that every object in the  universe is allocated. And that budget is C.
78. 6:39You cannot exceed C in the space direction  because exceeding C would require borrowing
79. 6:43from a time budget that doesn't exist.  It would require negative motion through
80. 6:47time. It would require going backward in time  just to maintain the geometry. And causality,
81. 6:53the requirement that causes precede effects, makes  this impossible in a self-consistent universe.
82. 6:59So the speed of light is not a speed limit  the way a government posts a speed limit on
83. 7:04a highway. It's more like the constraint that  an angle cannot exceed 90 degrees while still
84. 7:10being an angle. It's not enforced by a cop. It's  enforced by the logic of the structure itself.
85. 7:15Let me now tell you the part  that most people never hear.
86. 7:18C is not even really about light. I said this earlier but I want to be
87. 7:23precise about it. C would exist in the universe  even if there were no such thing as light. C
88. 7:29would be the geometric conversion constant between  space and time even in a universe with no photons,
89. 7:35no electromagnetic field, no light of any  kind. Any massless particle, any particle
90. 7:41with no mass whatsoever, would travel at C.  It has no choice. The geometry demands it.
91. 7:48We call it the speed of light for purely  historical reasons. Because light was the
92. 7:53first massless thing we studied. Because Maxwell  worked out the equations of electromagnetism
93. 7:59in the 1860s and found that electromagnetic  waves propagate at a specific speed. And when
94. 8:05he calculated that speed from the properties of  electricity and magnetism, it came out matching
95. 8:11the measured speed of light exactly. Which told  him that light was an electromagnetic wave.
96. 8:17And then Einstein came along and realized  that C wasn't a property of light or even
97. 8:21of electromagnetism. It was a property of  spacetime itself. The geometry of the universe.
98. 8:28The conversion factor between its dimensions. But the name stuck. The speed of light. Even
99. 8:32though it was never fundamentally about light. There is a way of writing physics where C never
100. 8:37appears at all. Where you choose your units  so that C equals 1, and then every equation
101. 8:42becomes cleaner, every relationship becomes more  transparent, and light disappears from the story
102. 8:47entirely. What's left is just geometry.  Pure, clean, four-dimensional geometry.
103. 8:55And the geometry tells you everything. But I want to dwell on this for a moment
104. 8:59longer because the implications are genuinely  staggering and most people rush past them.
105. 9:05If C is a geometric conversion factor and not  a speed, then faster than light travel is not
106. 9:11forbidden the way speeding is forbidden on  a highway. It is forbidden the way drawing
107. 9:16a square circle is forbidden. It is not  a rule. It is a geometric impossibility.
108. 9:22The geometry of spacetime simply does not contain  the category of things moving faster than C
109. 9:27through space. There is no slot in the structure  for such a thing. It would be like asking for a
110. 9:33direction that is simultaneously north and south.  The question isn't illegal. It's incoherent.
111. 9:38Wormholes and warp drives, those  beloved staples of science fiction,
112. 9:43don't get around this by going faster than C. They  attempt to get around it by changing the geometry
113. 9:49itself. By bending spacetime so that two distant  points become locally close. By folding the fabric
114. 9:56so the gap disappears. This is technically not  forbidden by the C constraint because you're not
115. 10:01moving through space at C plus something. You're  changing what space means in that region.
116. 10:06But here is the brutal reality. To do this  in general relativity requires something
117. 10:11called exotic matter. Matter with negative energy  density. We have never observed such a thing. The
118. 10:17quantum vacuum produces something that looks  superficially similar in the Casimir effect,
119. 10:22but the numbers don't come close to what you would  need to hold a macroscopic wormhole open. Not even
120. 10:27in the same universe of possibility. So while  the equations permit wormholes as mathematical
121. 10:34solutions, actually creating one remains, as far  as we understand, physically out of reach.
122. 10:40The geometry permits the idea. Nature  refuses to provide the tools.
123. 10:44And notice something beautiful and terrible  about this. The very fact that C is a geometric
124. 10:49constraint rather than a speed limit means that  the prohibition on faster than light travel is
125. 10:55deeper than we usually describe it. It doesn't  matter how advanced your technology becomes. It
126. 11:00doesn't matter how much energy your civilization  can harness. You are not fighting against a limit
127. 11:05that better engineering might overcome. You are  fighting against the shape of spacetime itself.
128. 11:11The universe is not saying no because you  haven't tried hard enough. The universe is
129. 11:16saying no because the concept you're reaching for  doesn't fit inside the structure of reality.
130. 11:21Now let me go even deeper. Because there is a  question that bothers physicists in a way they
131. 11:26don't always admit publicly. Why is C the value it is?
132. 11:30Why 300,000 kilometers per second?  Why not twice that? Why not half?
133. 11:34What determined this particular  exchange rate between space and time?
134. 11:38The honest answer is we don't fully know. We know that if C were different, the universe
135. 11:43would be profoundly different. The fine  structure constant, which governs how
136. 11:48strongly light interacts with matter, depends  on C. If C were significantly different,
137. 11:54atoms would have different sizes. Chemical  bonds would have different strengths. Stars
138. 11:59would burn at different rates. The universe as  we know it, with its particular chemistry and
139. 12:06its particular stars and its particular  possibility of life, would not exist.
140. 12:12This is what physicists call a fine-tuning  problem. The constants of nature appear
141. 12:17to be tuned to values that allow for  complexity. For structure. For us.
142. 12:22Some physicists think there is a deeper  theory that will explain where C comes from.
143. 12:26A theory of everything that derives C from first  principles. A theory where C is not an input but
144. 12:32an output. Where the geometry of spacetime  is itself explained rather than assumed.
145. 12:37We don't have that theory yet. Others think we live in a multiverse,
146. 12:41a landscape of possible universes with different  values of the constants. And we find ourselves
147. 12:47in one with this particular value of C simply  because this is the value that allows us to
148. 12:53exist. No deeper explanation. Just selection. I have gone back and forth on this for decades. I
149. 12:59helped develop the string theory landscape, which  is one version of the multiverse idea. I find it
150. 13:05intellectually uncomfortable but possibly correct.  The universe doesn't owe us an explanation of its
151. 13:10constants. It just has them. But here is what I want you
152. 13:13to take away from all of this. When you look at a beam of light, you are
153. 13:17not looking at something traveling fast. You are  looking at geometry in motion. You are looking at
154. 13:22a thing with no mass and therefore no choice but  to move through space at the full geometric budget
155. 13:27of the universe. You are looking at the conversion  factor between time and space made visible.
156. 13:32When GPS satellites need to correct for the  fact that clocks run faster in orbit than on
157. 13:37the ground, they are correcting for the geometry  of spacetime. For the fact that moving through
158. 13:42space trades off against moving through time at  an exchange rate of exactly C. This is not an
159. 13:47abstract theoretical nicety. It is a practical  engineering reality. Without accounting for
160. 13:52C as a geometric conversion factor, GPS  would drift by kilometers within hours.
161. 13:58When physicists write E equals MC squared, the  C squared is not telling you about light. It
162. 14:04is telling you that energy and mass are the  same thing, expressed in different units,
163. 14:09and that C squared is the conversion  factor between them. Between mass-units
164. 14:14and energy-units. Between a kilogram and a  joule. C is doing unit conversion, not speed.
165. 14:19This is what C really is. Not a cosmic speed  limit. Not a property of light. A geometric
166. 14:25fact about spacetime. The exchange rate  between dimensions. The conversion constant
167. 14:31that tells you how much space equals how much  time, how much mass equals how much energy,
168. 14:36how fast a massless particle must travel  because geometry leaves it no choice.
169. 14:41We gave it the wrong name. We called it the speed of light because that's how
170. 14:44we stumbled onto it. Historically. Accidentally.  The way humans often stumble onto deep truths, not
171. 14:52by deduction from first principles but by tripping  over them in the dark and then slowly, painfully,
172. 14:57recognizing what they actually found. What we found was the geometry of the universe.
173. 15:02And that geometry is stranger than any speed. It  is stranger than any limit. It is the structure
174. 15:07inside which space and time and mass and energy  and everything we have ever observed are all
175. 15:12embedded, all unified, all connected by a  single number that we happen to call C.
176. 15:18Not because it is the speed of light. But because it is the shape of everything.
177. 15:21There is one more thing I  want to say before I close,
178. 15:23because it connects to something deeply human. We measure things by giving them names. We name
179. 15:28them after the first context in which we noticed  them. The speed of light. The force of gravity.
180. 15:33The laws of physics. Each of these names  carries a shadow of the original confusion,
181. 15:39the original limited perspective from  which we stumbled onto something much
182. 15:42bigger than we realized at the time. Gravity is not really a force. It is the
183. 15:48curvature of spacetime. But we called it a force  because that's what it felt like to Newton when
184. 15:53the apple fell. We have been dragging that  misleading name around for centuries.
185. 15:57The laws of physics are not really laws. They  are descriptions of regularities in a structure
186. 16:02we don't fully understand. But we called them  laws because that's the language of authority
187. 16:07and certainty that 17th century scientists  reached for when they wanted to sound like
188. 16:12they knew what they were talking about. And C is not really a speed. It is the
189. 16:16geometric signature of the spacetime  we inhabit. But we called it the speed
190. 16:20of light because James Clerk Maxwell computed  it from electromagnetic theory and it matched
191. 16:26the measured velocity of light and that felt  like a sufficient description at the time.
192. 16:31The names we give things shape how we think  about them. The name speed of light makes
193. 16:36you imagine something zooming through space  very quickly. It puts your intuition in the
194. 16:40wrong place. It makes you think the mystery is  about velocity rather than about geometry.
195. 16:46Once you see it as geometry,  everything reorganizes.
196. 16:49Time dilation is not a paradox. It is a geometric  consequence. The Lorentz contraction of objects
197. 16:56at high speeds is not a physical compression. It  is a geometric rotation in spacetime. E equals MC
198. 17:03squared is not a mysterious formula about nuclear  explosions. It is a statement about two different
199. 17:08ways of measuring the same geometric quantity. All of it falls into place when you understand
200. 17:14that C is not a speed. That it is instead  the conversion factor between the dimensions
201. 17:20of a unified structure. The number that  tells space how much it equals in time.
202. 17:25The number that tells mass how much it equals  in energy. The exchange rate of reality.
203. 17:31And here is something I find quietly extraordinary  about this fact. The universe did not have to be
204. 17:36this way. You can write down mathematically  consistent geometries where space and time
205. 17:40do not unify. Where there is no C. Where the  exchange rate between dimensions simply does
206. 17:46not exist. In such a universe, mass could not  convert to energy. Massless particles would
207. 17:51not be constrained to a particular velocity.  Time and space would be forever separate. The
208. 17:56universe would be utterly unlike ours. The fact that our universe has a C,
209. 18:01a single clean conversion constant connecting its  dimensions, means our universe has a particular
210. 18:06kind of geometry. Minkowski geometry, we call it  after the mathematician who first wrote it down
211. 18:12clearly. It is a geometry with a very specific  symmetry, a symmetry between certain spatial
212. 18:18directions and the time direction. And that  symmetry is why the universe looks the same
213. 18:24to all observers regardless of how fast they're  moving. Why the laws of physics don't change when
214. 18:29you're in a car versus standing still versus  orbiting in a satellite. The symmetry is in
215. 18:34the geometry, and C is its numerical signature. We did not choose this geometry. We were born into
216. 18:40it. We evolved inside it. Our brains are shaped  by it without knowing it. When you throw a ball
217. 18:45and intuitively know where it will land, you  are doing geometry in your head, geometry of a
218. 18:50spacetime with a specific C, even though you have  never consciously thought about any of this.
219. 18:56C is in you too. Your atoms are held together  by electromagnetic forces that propagate at C.
220. 19:02The nuclear reactions in the sun that produce  the light hitting your face right now run at
221. 19:07rates determined by C. The information in  your neurons travels at speeds far below
222. 19:12C but within a universe whose structure  C defines. You are not just observing a
223. 19:17universe shaped by C. You are made of processes  that happen within the structure C describes.
224. 19:22And light, the thing we named C after, is simply  what happens when you have a massless excitation
225. 19:28of the electromagnetic field. A ripple in a  field with no mass, constrained by geometry
226. 19:33to travel at the only rate a massless thing can  travel. It is beautiful and it is strange and
227. 19:40it has been misnamed for a century and a half. I have spent fifty years inside the mathematics of
228. 19:45this structure. And I still find it astonishing.  Not the number itself. The number is just a
229. 19:50conversion factor. What is astonishing is  that space and time can be converted into
230. 19:56each other at all. That the universe is built  from a single unified fabric rather than two
231. 20:01separate stages. That the fact we experience  time as different from space is a feature of
232. 20:07our biology and not a feature of reality. Reality is a four-dimensional geometry. C is its
233. 20:13signature. Its fingerprint. The number that tells  you what kind of geometry you're living in.
234. 20:19And light, massless and eternal from its  own perspective, moving through space at
235. 20:25the full geometric rate because it has no mass  to slow it down, light is just the most visible
236. 20:32consequence of that geometry. The messenger  that carries the news of C across the cosmos.
237. 20:38It is not moving fast. It is moving at the only rate the
238. 20:41geometry allows for something with no mass. And that, finally, is what C actually is.
239. 20:46Not a speed. Never was.