1 00:00:00,160 --> 00:00:02,799 Speaker 1: What if every single decision you ever made, I mean, 2 00:00:02,839 --> 00:00:05,440 every left turn or right turn, every yes or no, 3 00:00:05,919 --> 00:00:10,400 didn't just disappear into the past, but actually created an 4 00:00:10,560 --> 00:00:13,160 entirely new, fully formed version of reality. 5 00:00:13,320 --> 00:00:16,519 Speaker 2: And what if those other realities, those other yous, aren't 6 00:00:16,559 --> 00:00:19,679 just a fun thought experiment. What if they're physically real 7 00:00:19,719 --> 00:00:22,519 in or influencing you. Right now, it. 8 00:00:22,480 --> 00:00:24,600 Speaker 1: Sounds like something straight out of a movie marathon, not 9 00:00:24,640 --> 00:00:25,399 a physics lab. 10 00:00:25,719 --> 00:00:31,039 Speaker 2: Exactly. Welcome to Thrilling Threads. Today, we're diving into source 11 00:00:31,079 --> 00:00:34,359 material that takes that very concept and argues it's not 12 00:00:34,399 --> 00:00:38,479 science fiction but a cold hard science. We're looking at 13 00:00:38,479 --> 00:00:40,560 the work of Oxford physicist David. 14 00:00:40,280 --> 00:00:42,759 Speaker 1: Deutsch, right, and his argument is pretty bold. He claims 15 00:00:42,759 --> 00:00:46,520 that parallel universes aren't just some speculative, optional theory. 16 00:00:46,759 --> 00:00:49,719 Speaker 2: No, he says they are the only necessary explanation for 17 00:00:49,799 --> 00:00:54,079 the most fundamental, most baffling experiments in all of quantum mechanics. 18 00:00:54,119 --> 00:00:55,920 Speaker 1: So that's our mission for this deep Dives. We're going 19 00:00:56,000 --> 00:00:59,280 to start with a surprisingly simple experiment, just a laser 20 00:00:59,320 --> 00:01:01,600 and a few holes, really the kind. 21 00:01:01,439 --> 00:01:03,679 Speaker 2: Of thing you might see at a high school physics. 22 00:01:03,280 --> 00:01:06,519 Speaker 1: Class, and we're going to show how It's bizarre results 23 00:01:06,599 --> 00:01:10,480 point to what Deutsch calls invisible elements of reality. 24 00:01:10,920 --> 00:01:13,599 Speaker 2: From there, we'll explore the implications. You know, what this 25 00:01:13,719 --> 00:01:16,680 means for who you are, your identity, and even the 26 00:01:16,719 --> 00:01:19,760 mind bending possibility of time travel, which gets a total 27 00:01:19,799 --> 00:01:21,040 rewrite in the multiverse. 28 00:01:21,319 --> 00:01:24,920 Speaker 1: The ideas here really do challenge common sense at every turn. 29 00:01:25,239 --> 00:01:28,079 They suggest our reality is just one layer in a 30 00:01:28,319 --> 00:01:31,120 much much larger structure of existence. 31 00:01:31,400 --> 00:01:33,680 Speaker 2: To even begin to get your head around that, you 32 00:01:33,760 --> 00:01:36,519 have to start where it all began with light. 33 00:01:36,840 --> 00:01:40,200 Speaker 1: Okay, let's set the stage with this core experiment. It's 34 00:01:40,239 --> 00:01:42,840 famously known as the double slit experiment, and if you 35 00:01:42,879 --> 00:01:44,480 haven't heard of it, well get ready. 36 00:01:44,719 --> 00:01:47,519 Speaker 2: It really is the most mysterious demonstration in all of science. 37 00:01:48,040 --> 00:01:51,439 So simple, but the implications are just vast. 38 00:01:51,599 --> 00:01:54,879 Speaker 1: It's the cornerstone of the whole multiverse argument we're discussing. 39 00:01:54,480 --> 00:01:56,439 Speaker 2: Today, and to understand it we actually have to go 40 00:01:56,480 --> 00:01:58,920 way back, all the way back to eighteen oh one. 41 00:01:58,959 --> 00:02:02,480 Speaker 1: Before electricity, before we even knew what an atom was exactly. 42 00:02:02,879 --> 00:02:06,439 Speaker 2: There was this English polymath Thomas Young, and he wanted 43 00:02:06,439 --> 00:02:09,680 to settle a very old debate. Is light made of 44 00:02:09,719 --> 00:02:12,520 tiny particles like little bullets or is it a wave 45 00:02:12,759 --> 00:02:13,879 like ripples in a pond. 46 00:02:14,240 --> 00:02:16,719 Speaker 1: Newton had argued for particles, so that was the dominant 47 00:02:16,759 --> 00:02:18,039 theory at the time it was. 48 00:02:18,479 --> 00:02:21,879 Speaker 2: So Young sets up this brilliantly simple experiment. He takes 49 00:02:22,000 --> 00:02:25,639 candlelight and shines it at a screen, But in between 50 00:02:25,680 --> 00:02:28,439 the candle and the screen he puts a barrier with 51 00:02:28,520 --> 00:02:31,759 two very thin, very close together slits in it. 52 00:02:32,039 --> 00:02:34,599 Speaker 1: Okay, so, if light is made of particles like tiny 53 00:02:34,599 --> 00:02:37,840 little paintballs, you'd expect to see two bright lines on 54 00:02:37,879 --> 00:02:39,240 the screen behind it, two. 55 00:02:39,080 --> 00:02:41,599 Speaker 2: Lines mirroring the two slits. Simple. 56 00:02:41,719 --> 00:02:43,639 Speaker 1: But that's not what he saw, not at all. 57 00:02:43,759 --> 00:02:46,120 Speaker 2: What he saw was a whole series of alternating bright 58 00:02:46,159 --> 00:02:50,159 and dark stripes spread across the screen, a classic interference pattern. 59 00:02:49,879 --> 00:02:52,520 Speaker 1: Which is something only waves do. Right When two waves overlap, 60 00:02:52,599 --> 00:02:54,280 they can either add up and get bigger, or they 61 00:02:54,280 --> 00:02:55,639 can cancel each other out. 62 00:02:55,719 --> 00:02:58,759 Speaker 2: Precisely, the bright stripes are where the waves reinforce each 63 00:02:58,759 --> 00:03:01,879 other constructive interference, and the dark stripes are where they 64 00:03:01,879 --> 00:03:06,360 cancel out destructive interference. So Young proved it light is a. 65 00:03:06,319 --> 00:03:09,240 Speaker 1: Wave case closed for a while anyway, for. 66 00:03:09,159 --> 00:03:11,919 Speaker 2: About a century. Then you know Max Planck and Albert 67 00:03:11,919 --> 00:03:15,080 Einstein came along and showed that light also behaves like 68 00:03:15,120 --> 00:03:16,719 it's made of discrete packets. 69 00:03:16,400 --> 00:03:19,000 Speaker 1: Of energy particles, which they called photons. 70 00:03:19,080 --> 00:03:22,039 Speaker 2: So suddenly we have this paradox, this duality. Light is 71 00:03:22,120 --> 00:03:25,199 somehow both a wave and a particle depending on how 72 00:03:25,199 --> 00:03:28,520 you look at it. And that's where really our every 73 00:03:28,599 --> 00:03:30,719 day intuition just completely breaks down. 74 00:03:30,840 --> 00:03:35,319 Speaker 1: Einstein himself said, this simple setup contains the deepest mystery 75 00:03:35,360 --> 00:03:36,599 of physics. 76 00:03:36,560 --> 00:03:39,360 Speaker 2: Because the act of just looking at it seems to 77 00:03:39,439 --> 00:03:40,520 change what happens. 78 00:03:40,840 --> 00:03:43,039 Speaker 1: So let's bring that into the modern day. This is 79 00:03:43,080 --> 00:03:46,039 the demonstration David Dortch uses to build his entire case. 80 00:03:46,280 --> 00:03:48,960 Speaker 2: Right, instead of a candle, we use a laser, And 81 00:03:49,039 --> 00:03:50,919 instead of two slits, let's say we have a piece 82 00:03:50,960 --> 00:03:53,800 of cardboard with ten tiny pinprick. 83 00:03:53,439 --> 00:03:56,520 Speaker 1: Holes in it, and we shine the laser through those 84 00:03:56,639 --> 00:03:58,000 holes at a detector screen. 85 00:03:58,400 --> 00:04:01,199 Speaker 2: Now, even with just one hole open, the result is 86 00:04:01,240 --> 00:04:03,560 already a bit strange. You don't just get a single dot. 87 00:04:03,840 --> 00:04:06,240 Because light is a wave, it defracts, it spreads out, 88 00:04:06,280 --> 00:04:08,120 and you get this sort of fuzzy pattern of rings. 89 00:04:08,199 --> 00:04:10,120 Speaker 1: Okay, that still makes sense with a wave, But. 90 00:04:10,120 --> 00:04:13,000 Speaker 2: Then you open a second hole and the pattern on 91 00:04:13,039 --> 00:04:16,800 the screen changes completely. It becomes that classic interference pattern 92 00:04:16,839 --> 00:04:17,360 of stripes. 93 00:04:17,720 --> 00:04:20,360 Speaker 1: And if you open a third, or a fourth. 94 00:04:20,079 --> 00:04:23,040 Speaker 2: Or all ten, every single time you add a hole, 95 00:04:23,319 --> 00:04:27,759 you get a totally new, unique, and very complicated interference pattern. 96 00:04:27,839 --> 00:04:30,040 Speaker 1: So the key thing here is that the final pattern 97 00:04:30,079 --> 00:04:33,240 on the screen depends on the exact number and arrangement 98 00:04:33,360 --> 00:04:34,720 of all the holes that are open. 99 00:04:35,079 --> 00:04:39,000 Speaker 2: Yes, every available path of the like contributes to the 100 00:04:39,040 --> 00:04:42,680 final outcome, which again makes total sense if you're thinking 101 00:04:42,759 --> 00:04:45,839 of a big continuous waves washing through all ten holes 102 00:04:45,839 --> 00:04:46,240 at once. 103 00:04:46,560 --> 00:04:48,399 Speaker 1: But now we get to the part that just breaks 104 00:04:48,399 --> 00:04:51,120 your brain, the single photon paradox. 105 00:04:51,519 --> 00:04:54,480 Speaker 2: This is where physicists really push the experiment to its 106 00:04:54,480 --> 00:04:57,399 absolute limit. They decided to see what would happen if 107 00:04:57,399 --> 00:04:59,480 they sent light through one particle at a time. 108 00:04:59,759 --> 00:05:00,560 Speaker 1: Do you do that? 109 00:05:00,639 --> 00:05:04,160 Speaker 2: You use an incredibly dense filter, so dense that it 110 00:05:04,240 --> 00:05:07,360 only lets a single photon, one indivisible particle of light 111 00:05:07,399 --> 00:05:10,199 pass through at a time, sometimes with minutes or even 112 00:05:10,240 --> 00:05:11,959 hours between each one. 113 00:05:12,079 --> 00:05:14,879 Speaker 1: So there's absolutely no chance they are bumping into each 114 00:05:14,920 --> 00:05:18,319 other or interfering with each other. Each photon makes the 115 00:05:18,360 --> 00:05:20,240 journey completely alone. 116 00:05:20,160 --> 00:05:24,040 Speaker 2: Utterly alone. Now, when you start this. The first few 117 00:05:24,079 --> 00:05:27,399 photons hit the detector screen and they just look like 118 00:05:27,519 --> 00:05:29,959 random dots. One land's here, another one over there. 119 00:05:30,120 --> 00:05:32,839 Speaker 1: It's just static, as you'd expect. It's a particle. It 120 00:05:32,879 --> 00:05:34,600 has to land in one specific spot. 121 00:05:35,160 --> 00:05:38,720 Speaker 2: But you let this run for hours, days, and as 122 00:05:38,800 --> 00:05:43,240 thousands and thousands of these individual isolated dots accumulate, something 123 00:05:43,279 --> 00:05:44,639 impossible starts to happen. 124 00:05:44,720 --> 00:05:45,720 Speaker 1: They form a pattern. 125 00:05:45,879 --> 00:05:49,800 Speaker 2: They form the exact same complex wave like interference pattern 126 00:05:49,879 --> 00:05:52,399 you saw when the full laser beam was shining through 127 00:05:52,399 --> 00:05:53,079 all ten holes. 128 00:05:53,120 --> 00:05:56,519 Speaker 1: Wait, how that makes no sense. If each photon is 129 00:05:56,560 --> 00:05:59,319 a particle and it's traveling alone, it has to go 130 00:05:59,360 --> 00:06:01,040 through one hole, just one. 131 00:06:01,319 --> 00:06:03,920 Speaker 2: Logically, yes, it'd have to pick a path whole number one, 132 00:06:04,000 --> 00:06:05,800 or whole number seven, or whole number three. 133 00:06:06,000 --> 00:06:08,399 Speaker 1: So if that were true, the final pattern should just 134 00:06:08,439 --> 00:06:11,279 be ten slightly fuzzy clusters of dots on the screen 135 00:06:11,439 --> 00:06:12,959 right behind each of the ten holes. 136 00:06:13,199 --> 00:06:17,639 Speaker 2: That's what classical physics would predict. But that's not what happens. Instead, 137 00:06:18,199 --> 00:06:22,079 each individual particle lands in a position that is statistically 138 00:06:22,160 --> 00:06:25,079 determined by the existence of the other nine holes it 139 00:06:25,199 --> 00:06:26,120 did not go through. 140 00:06:26,279 --> 00:06:30,079 Speaker 1: It's as if each photon somehow knew that the other 141 00:06:30,199 --> 00:06:31,199 nine holes were open. 142 00:06:31,360 --> 00:06:33,480 Speaker 2: It acts as if it passed through every single hole, 143 00:06:33,560 --> 00:06:37,720 simultaneously interfered with itself, and then chose a final landing 144 00:06:37,759 --> 00:06:39,319 spot based on that interference. 145 00:06:39,600 --> 00:06:42,639 Speaker 1: That's the part that is just it's a logical contradiction. 146 00:06:42,839 --> 00:06:45,560 Speaker 2: It is. Think about it. If we have all ten 147 00:06:45,600 --> 00:06:48,600 holes open, we get pattern A. But if we close 148 00:06:48,720 --> 00:06:50,800 just one of them, say whole number five, and run 149 00:06:50,839 --> 00:06:54,120 the experiment again, we get a totally different pattern B. 150 00:06:54,519 --> 00:06:56,639 Speaker 1: Even though the photon we detect still had to go 151 00:06:56,720 --> 00:06:58,279 through one of the other nine holes. 152 00:06:58,360 --> 00:07:00,959 Speaker 2: Right, the presence or absence of a path that never 153 00:07:01,000 --> 00:07:04,680 took changes the outcome. So the source material forces a 154 00:07:04,720 --> 00:07:07,680 conclusion on us. Something must be going through those other. 155 00:07:07,600 --> 00:07:09,240 Speaker 1: Holes, something we can't see. 156 00:07:09,079 --> 00:07:12,160 Speaker 2: Something we can't see, but something that is physically real 157 00:07:12,279 --> 00:07:15,639 enough to shove our photon aside as Deutsch puts it, 158 00:07:15,720 --> 00:07:16,800 causing the interference. 159 00:07:17,160 --> 00:07:19,879 Speaker 1: So if our one visible photon went through hole one, 160 00:07:20,160 --> 00:07:23,800 there must have been nine other somethings, yeah, that went 161 00:07:23,839 --> 00:07:25,639 through the other nine holes exactly. 162 00:07:25,879 --> 00:07:28,480 Speaker 2: And when you do tests to figure out what this 163 00:07:28,560 --> 00:07:31,560 something is, you find it behaves exactly like light. It 164 00:07:31,600 --> 00:07:33,959 travels at the speed of light. It reflects, it defracts, 165 00:07:34,120 --> 00:07:37,800 it does everything a photon does, except except we can't 166 00:07:37,800 --> 00:07:41,439 detect it. It's invisible to our instruments in this reality. 167 00:07:41,560 --> 00:07:44,079 Speaker 1: So if it looks like light and acts like light, 168 00:07:44,319 --> 00:07:45,680 that is invisible, the. 169 00:07:45,600 --> 00:07:48,879 Speaker 2: Only reasonable conclusion, Deutsch argues, is that it is light, 170 00:07:49,399 --> 00:07:50,639 light of an invisible kind. 171 00:07:50,959 --> 00:07:53,079 Speaker 1: And this isn't just for light, is it. They've done 172 00:07:53,120 --> 00:07:54,160 this with other particles. 173 00:07:54,199 --> 00:07:58,920 Speaker 2: Oh yes, electrons, protons, even whole molecules, everything at the 174 00:07:59,000 --> 00:08:02,480 quantum level exis. It's this same behavior, which implies that 175 00:08:02,560 --> 00:08:05,480 all matter, including the atoms in our bodies, is constantly 176 00:08:05,519 --> 00:08:07,680 being influenced by these invisible. 177 00:08:07,120 --> 00:08:10,399 Speaker 1: Counterparts, which is a staggering thought. It means something that 178 00:08:10,439 --> 00:08:13,720 doesn't exist in our reality can't exert a physical force. 179 00:08:14,079 --> 00:08:16,839 So if our photon is being pushed, the thing doing 180 00:08:16,839 --> 00:08:18,439 the pushing must exist. 181 00:08:18,160 --> 00:08:20,839 Speaker 2: It must be real, and that propels us directly out 182 00:08:20,839 --> 00:08:22,399 of the lab and into the multiverse. 183 00:08:22,680 --> 00:08:26,199 Speaker 1: Okay, so the experiment leaves us with this unavoidable fact. 184 00:08:26,879 --> 00:08:30,879 Invisible things are interacting with the visible things in our universe. 185 00:08:31,439 --> 00:08:34,600 But that feels like a huge leap from invisible light 186 00:08:34,960 --> 00:08:37,039 to infinite parallel universes. 187 00:08:37,159 --> 00:08:39,960 Speaker 2: It is a huge leap, But is where the mathematics 188 00:08:39,960 --> 00:08:42,679 of quantum theory forces you to go. This is where 189 00:08:42,679 --> 00:08:45,159 we get to what's known as the measurement problem. 190 00:08:45,320 --> 00:08:48,519 Speaker 1: Right, So, in quantum mechanics, before you look at a particle, 191 00:08:48,679 --> 00:08:52,200 it doesn't have a definite position. It's described by something 192 00:08:52,240 --> 00:08:53,759 called a wave function. 193 00:08:53,799 --> 00:08:57,279 Speaker 2: And that wave function is essentially a cloud of probabilities. 194 00:08:57,559 --> 00:08:59,600 It described all the places of the particle could be 195 00:09:00,039 --> 00:09:03,039 all at the same time. It's in a state of superposition. 196 00:09:02,720 --> 00:09:05,919 Speaker 1: Like our photon being simultaneously in all ten holes at once. 197 00:09:06,120 --> 00:09:09,240 Speaker 2: Exactly. That's the wave part of its behavior. But here's 198 00:09:09,279 --> 00:09:12,600 the problem that has stumped physicists for a century. The 199 00:09:12,679 --> 00:09:15,759 moment you measure it, the moment the photon hits the screen, 200 00:09:15,879 --> 00:09:18,840 that whole cloud of possibilities just vanishes. 201 00:09:18,799 --> 00:09:19,519 Speaker 1: It collapses. 202 00:09:19,559 --> 00:09:22,799 Speaker 2: It collapses, and the particle instantly snaps into one single 203 00:09:22,799 --> 00:09:24,039 definite location, which. 204 00:09:23,840 --> 00:09:27,440 Speaker 1: Feels a bit like magic. What what causes the collapse? 205 00:09:27,480 --> 00:09:29,120 Why does looking at it change everything? 206 00:09:29,399 --> 00:09:32,519 Speaker 2: That is the central mystery. The mainstream view for a 207 00:09:32,559 --> 00:09:36,440 long time was called the Copenhagen interpretation, and it basically 208 00:09:36,480 --> 00:09:40,159 just said the active measurement causes the collapse. 209 00:09:39,919 --> 00:09:42,240 Speaker 1: Which isn't really an explanation, is it. It's more of 210 00:09:42,240 --> 00:09:42,879 a description. 211 00:09:43,279 --> 00:09:46,120 Speaker 2: It's not. It doesn't explain how or why where do 212 00:09:46,120 --> 00:09:47,960 you draw the line? Does it collapse when it hits 213 00:09:47,960 --> 00:09:50,559 the screen or when the information from the screen hits 214 00:09:50,559 --> 00:09:55,200 a human eye or a computer. It requires this arbitrary, 215 00:09:55,320 --> 00:09:57,919 non physical event to happen just to make the math 216 00:09:58,279 --> 00:10:00,080 fit our single reality. 217 00:09:59,679 --> 00:10:03,519 Speaker 1: Exp and that's where David Deutsch and Many World's Interpretation 218 00:10:03,679 --> 00:10:04,759 or MWI come in. 219 00:10:04,919 --> 00:10:10,399 Speaker 2: Yes, MWI offers a much simpler, though admittedly much stranger explanation. 220 00:10:10,639 --> 00:10:12,919 Speaker 1: It says the wave function never collapses at all. 221 00:10:13,080 --> 00:10:15,879 Speaker 2: Never. Instead, every time a quantum measurement is made, every 222 00:10:15,879 --> 00:10:17,720 time a choice has to resolve, like which hole the 223 00:10:17,759 --> 00:10:21,080 photon goes through, the universe doesn't collapse. It branches. 224 00:10:21,279 --> 00:10:24,240 Speaker 1: So instead of all those possibilities and the probability cloud 225 00:10:24,279 --> 00:10:26,240 disappearing except for one, all of. 226 00:10:26,200 --> 00:10:30,279 Speaker 2: Them happen, every single one. The universe splits into multiple 227 00:10:30,559 --> 00:10:33,960 non interacting copies, and each possibility plays out in its 228 00:10:34,000 --> 00:10:35,080 own separate reality. 229 00:10:35,360 --> 00:10:39,080 Speaker 1: So in our experiment, the universe branched into ten versions. 230 00:10:39,480 --> 00:10:42,440 In one the photon went through hole, one in another hole, 231 00:10:42,440 --> 00:10:44,200 two in another whole. 232 00:10:44,080 --> 00:10:47,840 Speaker 2: Three and so on, all the possibilities described by the 233 00:10:47,840 --> 00:10:51,480 math are physically real. The invisible light we were talking about 234 00:10:51,679 --> 00:10:54,600 is just the photons in those other nearby branches, which 235 00:10:54,600 --> 00:10:56,759 are still close enough to interfere with our branch for 236 00:10:56,759 --> 00:10:57,440 a brief moment. 237 00:10:57,519 --> 00:11:01,639 Speaker 1: The implication, then, is just staggering entire parallel worlds with 238 00:11:01,720 --> 00:11:05,279 their own matter and energy existing right alongside ours. 239 00:11:05,440 --> 00:11:08,240 Speaker 2: And according to the theory, they all came into being 240 00:11:08,320 --> 00:11:11,480 at the Big Bang. They all started out together, almost identical. 241 00:11:11,840 --> 00:11:15,200 The source uses this great analogy of countless books, all 242 00:11:15,200 --> 00:11:16,519 with the same first chapter. 243 00:11:16,279 --> 00:11:21,200 Speaker 1: But then over time, tiny random quantum events and adam 244 00:11:21,240 --> 00:11:24,600 decaying here but not there cause them to fork. They 245 00:11:24,639 --> 00:11:26,120 start to diverge, and. 246 00:11:26,039 --> 00:11:29,080 Speaker 2: The stories in those books start to become different. Our 247 00:11:29,200 --> 00:11:32,440 universe is just one of those stories, one variation out 248 00:11:32,440 --> 00:11:33,480 of an infinite number. 249 00:11:33,600 --> 00:11:35,840 Speaker 1: This leads to Deutsch's core argument for why we have 250 00:11:35,919 --> 00:11:38,759 to take this seriously. He makes a distinction between prediction 251 00:11:38,919 --> 00:11:39,639 and explanation. 252 00:11:40,039 --> 00:11:42,200 Speaker 2: Right, if all you want to do is predict where 253 00:11:42,200 --> 00:11:44,240 the photon will land, you can just use the math 254 00:11:44,240 --> 00:11:49,159 a quantum mechanics. It's probabilistic, but it's incredibly accurate. You 255 00:11:49,200 --> 00:11:51,879 don't need to believe in anything outside this room to 256 00:11:51,919 --> 00:11:52,720 do the calculation. 257 00:11:53,039 --> 00:11:57,480 Speaker 1: But science isn't just about predicting. It's about explaining why exactly. 258 00:11:57,840 --> 00:12:00,879 Speaker 2: If you want to explain why that interference pattern shows up, 259 00:12:01,200 --> 00:12:04,320 if you want to causal reason for what is physically 260 00:12:04,399 --> 00:12:07,720 shoving that photon around, then you need an explanation that 261 00:12:07,759 --> 00:12:08,960 fits the facts we observe. 262 00:12:09,200 --> 00:12:12,279 Speaker 1: And the fact is the photon is being affected by something. 263 00:12:12,639 --> 00:12:14,519 We can see the result of that interaction. 264 00:12:14,799 --> 00:12:16,919 Speaker 2: So you have two choices. You can either say our 265 00:12:16,960 --> 00:12:20,000 photon is being influenced by things that don't exist, which 266 00:12:20,039 --> 00:12:21,559 is just a logical absurdity, or. 267 00:12:21,519 --> 00:12:23,240 Speaker 1: You have to say it's being affected by things that 268 00:12:23,320 --> 00:12:23,879 really do. 269 00:12:23,840 --> 00:12:28,559 Speaker 2: Exist, and that's the only scientifically coherent path. Those invisible 270 00:12:28,559 --> 00:12:33,039 photons and the universes they inhabit must be real. So 271 00:12:33,120 --> 00:12:36,279 the many world's view isn't just an interpretation. It's the 272 00:12:36,320 --> 00:12:40,399 only one that takes the evidence seriously without inventing a mysterious, 273 00:12:40,559 --> 00:12:41,679 unexplained collapse. 274 00:12:42,000 --> 00:12:44,080 Speaker 1: So it's not that we're adding extra stuff to the 275 00:12:44,080 --> 00:12:46,480 theory for fun. We're just taking the math that we 276 00:12:46,519 --> 00:12:49,840 already know works and accepting what it literally tells us 277 00:12:49,879 --> 00:12:50,559 about reality. 278 00:12:50,600 --> 00:12:53,919 Speaker 2: That's the elegance of it. It simplifies the physics by 279 00:12:53,960 --> 00:12:56,960 complicating our picture of reality. It says, if the math 280 00:12:57,039 --> 00:13:01,720 describes a superposition of ten possibility, then ten real things 281 00:13:01,759 --> 00:13:04,919 are happening. The universe just does it all, okay. 282 00:13:05,000 --> 00:13:08,320 Speaker 1: So if we're living in this vast multiverse constantly being 283 00:13:08,399 --> 00:13:13,000 nudged by other realities, the question immediately becomes personal. What 284 00:13:13,039 --> 00:13:13,799 does this mean for me? 285 00:13:14,000 --> 00:13:16,799 Speaker 2: Yeah, this is where it stops being an abstract physics 286 00:13:16,799 --> 00:13:20,200 problem and starts feeling deeply philosophical. 287 00:13:20,279 --> 00:13:22,399 Speaker 1: The version of me who didn't sell his bitcoin in 288 00:13:22,399 --> 00:13:25,360 twenty seventeen is living a very different life right now 289 00:13:25,360 --> 00:13:27,600 in another branch, and he's just as real as I am. 290 00:13:27,759 --> 00:13:30,879 Speaker 2: That's the required mental shift. You have to stop thinking 291 00:13:30,960 --> 00:13:34,919 of yourself as this single entity in one universe. Deutsche says, 292 00:13:34,960 --> 00:13:37,279 we are an entity existing in a multiverse as a whole, 293 00:13:37,679 --> 00:13:39,559 where a collection of all our possible selves. 294 00:13:39,639 --> 00:13:41,519 Speaker 1: So, right at the moment a choice is made, the 295 00:13:41,559 --> 00:13:45,879 copies of us are identical, same memory is same thoughts exactly. 296 00:13:45,960 --> 00:13:48,200 Speaker 2: The source material points out that if two copies are 297 00:13:48,240 --> 00:13:51,639 completely indistinguishable, it's really just a matter of semantics, whether 298 00:13:51,679 --> 00:13:53,440 you call them two separate universes or not. 299 00:13:53,639 --> 00:13:56,080 Speaker 1: But most of them are already slightly different, right, even 300 00:13:56,120 --> 00:13:59,080 by the position of a single atom somewhere in the cosmos. 301 00:13:59,200 --> 00:14:02,200 Speaker 2: Yes, and those tiny differences are what allow for the 302 00:14:02,240 --> 00:14:05,799 ongoing interference at the quantum level. It's why our atoms 303 00:14:05,799 --> 00:14:07,919 are being influenced by their counterparts. 304 00:14:08,399 --> 00:14:12,120 Speaker 1: The source uses a really helpful analogy to explain how 305 00:14:12,120 --> 00:14:16,039 these identical copies grow apart, the identical twin analogy. 306 00:14:16,200 --> 00:14:18,000 Speaker 2: It's a great way to ground it. You have two 307 00:14:18,039 --> 00:14:23,279 identical twins born with the exact same genetic code, indistinguishable. 308 00:14:22,639 --> 00:14:25,840 Speaker 1: But as they live their lives, they make different choices, 309 00:14:25,960 --> 00:14:29,879 have different experiences. One moves to another city, one gets 310 00:14:29,919 --> 00:14:30,679 a different job. 311 00:14:30,799 --> 00:14:33,799 Speaker 2: They row apart, and they each become their own unique person. 312 00:14:34,200 --> 00:14:37,320 They're both equally real, but they have different stories. 313 00:14:37,519 --> 00:14:40,080 Speaker 1: The same thing is happening to all the versions of 314 00:14:40,320 --> 00:14:44,120 you across the multiverse, but on a scale that's almost 315 00:14:44,159 --> 00:14:45,559 impossible to comprehend. 316 00:14:45,919 --> 00:14:48,480 Speaker 2: Every choice, from what you eat for breakfast to who 317 00:14:48,559 --> 00:14:52,039 you marry, and every tiny random quantum event in your 318 00:14:52,080 --> 00:14:56,120 brain causes a divergence. The versions of you start to 319 00:14:56,120 --> 00:14:57,200 branch away from each other. 320 00:14:57,279 --> 00:14:59,600 Speaker 1: Some of those other us are almost identical to you, 321 00:15:00,240 --> 00:15:01,960 just I don't know, blink a second later. 322 00:15:02,399 --> 00:15:05,399 Speaker 2: But other versions, ones that have accumulated many different choices 323 00:15:05,480 --> 00:15:10,080 over many years, could be unrecognizable. They might have completely 324 00:15:10,159 --> 00:15:14,320 different beliefs, a different personality. They might not even like 325 00:15:14,399 --> 00:15:15,919 the same things you do anymore. 326 00:15:16,039 --> 00:15:18,360 Speaker 1: It redefines what a self even is. It's not a 327 00:15:18,399 --> 00:15:22,039 single point. It's this enormous branching tree of all your 328 00:15:22,039 --> 00:15:23,840 possibilities being played out, which is a. 329 00:15:23,799 --> 00:15:26,600 Speaker 2: Beautiful idea in a way. All that potential you feel 330 00:15:26,639 --> 00:15:29,200 you have is actually being realized somewhere. 331 00:15:29,240 --> 00:15:31,879 Speaker 1: But that brings us to the big skeptical question, the 332 00:15:31,919 --> 00:15:35,120 one everyone asks, if all these other worlds and all 333 00:15:35,120 --> 00:15:37,840 these other means are real, why can't I feel them? 334 00:15:38,039 --> 00:15:39,000 Why can't I see them? 335 00:15:39,159 --> 00:15:42,679 Speaker 2: It's the most common objection, and Deutsche's answer is that 336 00:15:42,720 --> 00:15:47,440 our senses are just limited. They haven't evolved to perceive 337 00:15:47,960 --> 00:15:49,279 quantum reality directly. 338 00:15:49,399 --> 00:15:52,200 Speaker 1: I mean, you'd think with infinite universes we'd feel something 339 00:15:52,919 --> 00:15:55,480 like shouldn't there combine gravity? Just crush us. 340 00:15:55,759 --> 00:15:59,080 Speaker 2: That's where a really key concept comes in, called decoherence. 341 00:15:59,559 --> 00:16:02,120 Think of it as the process that makes the universes 342 00:16:02,240 --> 00:16:05,039 separate and unable to interact on a large scale. 343 00:16:05,120 --> 00:16:07,480 Speaker 1: So they interfere at first, like in the experiment, but 344 00:16:07,519 --> 00:16:08,399 then they stop. 345 00:16:08,600 --> 00:16:11,720 Speaker 2: Why for two branches to keep interfering, they need to 346 00:16:11,720 --> 00:16:16,320 stay in sync, like two perfectly timed waves. But as 347 00:16:16,320 --> 00:16:19,200 soon as a quantum event happens in the macroscopic world, 348 00:16:19,320 --> 00:16:22,919 like you choosing to open a door, that choice involves 349 00:16:23,000 --> 00:16:28,840 interacting with billions and billions of atoms, air, molecules, the doorknob, photons. 350 00:16:28,279 --> 00:16:32,159 Speaker 1: And all those interactions are like tiny measurement countless tiny measurements, 351 00:16:32,320 --> 00:16:36,279 and that complexity immediately scrambles the synchronization between. 352 00:16:36,240 --> 00:16:38,320 Speaker 2: Your universe and the one where you didn't open the door. 353 00:16:38,679 --> 00:16:41,039 They decohere, they become uncorrelated. 354 00:16:41,159 --> 00:16:43,799 Speaker 1: Ah, so the walls between the universes go up, so 355 00:16:43,840 --> 00:16:46,200 to speak. They're still there, but they can't influence each 356 00:16:46,240 --> 00:16:48,000 other in any way we can perceive. 357 00:16:48,080 --> 00:16:51,360 Speaker 2: Precisely, we are only sensitive to the one branch we 358 00:16:51,440 --> 00:16:54,080 happen to be on. The interference we see in the 359 00:16:54,159 --> 00:16:58,120 lab is a glimpse into that brief moment before decoherence 360 00:16:58,159 --> 00:16:58,919 takes full effect. 361 00:16:59,440 --> 00:17:03,440 Speaker 1: Deutsch has a great analogy for this sensory argument, fucos pendulum. 362 00:17:03,559 --> 00:17:06,799 Speaker 2: Yes, our common sense tells us the ground we're standing 363 00:17:06,839 --> 00:17:09,799 on is stationary. We can't feel the Earth spinning in 364 00:17:09,880 --> 00:17:11,240 one thousand miles an hour. 365 00:17:11,279 --> 00:17:14,279 Speaker 1: Right, If you relied only on your senses, you'd conclude 366 00:17:14,359 --> 00:17:17,160 the Earth is still. That was the argument against Galileo 367 00:17:17,240 --> 00:17:19,720 for centuries. If the Earth is moving, why don't we 368 00:17:19,720 --> 00:17:20,480 get thrown off? 369 00:17:20,640 --> 00:17:23,720 Speaker 2: They were using their biological limits as proof. But we 370 00:17:23,799 --> 00:17:27,480 know better. We can build an instrument like a Fuco's pendulum, 371 00:17:27,680 --> 00:17:30,799 which shows the Earth's rotation over time, and we can 372 00:17:30,880 --> 00:17:32,279 indirectly detect that motion. 373 00:17:32,680 --> 00:17:35,400 Speaker 1: So the pendulum is like an extension of our senses, and. 374 00:17:35,359 --> 00:17:37,920 Speaker 2: That's exactly what the quantum experiment is for the multiverse. 375 00:17:38,359 --> 00:17:41,359 Our senses don't operate based on quantum interference, so we 376 00:17:41,400 --> 00:17:45,039 don't feel the other world. But if they did, Deutsch 377 00:17:45,079 --> 00:17:46,599 arg use, we would feel them. 378 00:17:46,680 --> 00:17:49,240 Speaker 1: So we have to build these sophisticated experiments to act 379 00:17:49,279 --> 00:17:50,559 as our quantum senses. 380 00:17:50,759 --> 00:17:53,759 Speaker 2: Our inability to feel the multiverse is a failure of 381 00:17:53,799 --> 00:17:57,359 our biology, not a failure of the physics. The lab 382 00:17:57,440 --> 00:18:00,839 is our telescope for seeing these other realities, and it's 383 00:18:00,880 --> 00:18:01,759 telling us they're there. 384 00:18:02,519 --> 00:18:05,880 Speaker 1: So if we accept this foundation that countless other histories 385 00:18:05,920 --> 00:18:09,119 are playing out in parallel, it completely changes the game 386 00:18:09,240 --> 00:18:12,519 for one of the biggest ideas in science fiction time travel. 387 00:18:12,599 --> 00:18:15,160 Speaker 2: It really does. It takes it from the realm of 388 00:18:15,359 --> 00:18:20,160 logical paradox and makes it well theoretically possible. 389 00:18:20,160 --> 00:18:22,200 Speaker 1: At least, let's start with the physics of it, leaving 390 00:18:22,200 --> 00:18:24,920 aside the paradoxes for a moment. Could we even build 391 00:18:24,960 --> 00:18:26,039 a time machine? 392 00:18:26,119 --> 00:18:29,160 Speaker 2: It's still a very open question, but based on Einstein's 393 00:18:29,200 --> 00:18:33,319 general relativity, the theory that describes gravity and space time, 394 00:18:33,319 --> 00:18:35,000 the answer is maybe this. 395 00:18:34,920 --> 00:18:37,160 Speaker 1: Isn't just about going really fast, this is about actually 396 00:18:37,200 --> 00:18:39,720 warping the fabric of space time itself exactly. 397 00:18:40,079 --> 00:18:42,799 Speaker 2: The math of general relativity allows for solutions where space 398 00:18:42,799 --> 00:18:47,039 time can be so extremely curved, say near a rapidly 399 00:18:47,039 --> 00:18:50,160 spinning black hole or involving some kind of theoretical exotic matter, 400 00:18:50,480 --> 00:18:52,359 that it could actually loop back on itself. 401 00:18:52,400 --> 00:18:55,519 Speaker 1: These are called closed timelike curves CTCs. 402 00:18:55,960 --> 00:18:59,319 Speaker 2: Right, a CTC is basically a path through space time 403 00:18:59,359 --> 00:19:02,359 that ends where it began, not just in space, but 404 00:19:02,480 --> 00:19:04,400 in time. It's a tunnel to the past. 405 00:19:05,039 --> 00:19:08,240 Speaker 1: So if the laws of physics don't explicitly forbid these 406 00:19:08,279 --> 00:19:11,880 from forming, then a pathway to the past is possible. 407 00:19:12,119 --> 00:19:15,359 Speaker 2: In theory, yes, but the moment you say that, you 408 00:19:15,440 --> 00:19:17,960 run headfirst into the paradoxes. 409 00:19:17,480 --> 00:19:19,720 Speaker 1: The most famous one being the grandfather. 410 00:19:19,319 --> 00:19:21,880 Speaker 2: Paradox, the classic I build a time machine, I go 411 00:19:21,960 --> 00:19:25,440 back in time, and I prevent my grandfather from ever 412 00:19:25,480 --> 00:19:26,400 meeting my grandmother. 413 00:19:26,519 --> 00:19:29,640 Speaker 1: If you succeed, your father is never born, which means 414 00:19:29,839 --> 00:19:31,000 you are never born. 415 00:19:30,799 --> 00:19:33,480 Speaker 2: Which means I never existed to build the time machine 416 00:19:33,519 --> 00:19:34,599 to go back in the first place. 417 00:19:34,720 --> 00:19:37,640 Speaker 1: The effect erases its own cause it's a contradiction. 418 00:19:37,880 --> 00:19:40,880 Speaker 2: It's a complete logical breakdown. And that's why in a 419 00:19:40,920 --> 00:19:44,440 single universe model, most physicists conclude that time travel into 420 00:19:44,480 --> 00:19:47,559 the past must be impossible. The universe must have some 421 00:19:47,759 --> 00:19:49,400 law that prevents these paradoxes. 422 00:19:49,519 --> 00:19:52,480 Speaker 1: But the Many World's interpretation offers an escape patch. 423 00:19:52,559 --> 00:19:56,079 Speaker 2: It provides this perfect, elegant solution. David Deutsch argues, the 424 00:19:56,079 --> 00:19:59,279 paradox only exists if you assume there's only one timeline. 425 00:20:00,000 --> 00:20:02,559 The multiverse is real. You can't change your own past. 426 00:20:02,880 --> 00:20:05,400 Speaker 1: So time travel isn't about rewriting history. 427 00:20:05,480 --> 00:20:09,720 Speaker 2: It's about jumping tracks. It's about entering the past of 428 00:20:09,759 --> 00:20:10,720 a different universe. 429 00:20:10,960 --> 00:20:14,200 Speaker 1: So if I travel back to, say nineteen fifty, to 430 00:20:14,240 --> 00:20:17,880 stop my grandfather, the moment I arrive, I am not 431 00:20:18,000 --> 00:20:19,400 in my own past anymore. 432 00:20:19,599 --> 00:20:22,960 Speaker 2: You've stepped sideways into a parallel branch, a universe that 433 00:20:23,240 --> 00:20:26,400 was up until that moment identical to your past, but 434 00:20:26,559 --> 00:20:28,599 is now going to have a different future because you're. 435 00:20:28,480 --> 00:20:31,799 Speaker 1: In it, and my original timeline, the one I left. 436 00:20:31,640 --> 00:20:36,759 Speaker 2: It continues on, completely untouched, consistent in that universe. You 437 00:20:36,759 --> 00:20:39,680 were born, you built the time machine, you stepped into it, 438 00:20:39,920 --> 00:20:43,519 and you vanished. The cause and effect are perfectly preserved. 439 00:20:43,079 --> 00:20:45,079 Speaker 1: And in the new universe, I can do whatever I want. 440 00:20:45,319 --> 00:20:48,039 My actions there create a new history for that reality, 441 00:20:48,359 --> 00:20:50,640 but it has no effect on where I came from. 442 00:20:50,440 --> 00:20:53,119 Speaker 2: No contradiction. The effect of you traveling back in time 443 00:20:53,240 --> 00:20:56,039 happens in one universe, while the cause of your existence 444 00:20:56,039 --> 00:21:00,079 remains intact in another. It's a beautiful solution, it's logical, it's. 445 00:21:00,079 --> 00:21:03,000 Speaker 1: Clean, but when you think about the human cost, it's 446 00:21:03,039 --> 00:21:04,519 also incredibly sobering. 447 00:21:04,720 --> 00:21:07,359 Speaker 2: It really is, because if this is how it works, 448 00:21:07,440 --> 00:21:09,039 the journey is strictly one way. 449 00:21:09,119 --> 00:21:10,000 Speaker 1: You can never go home. 450 00:21:10,319 --> 00:21:14,440 Speaker 2: Never. The act of creating and using that pathway through 451 00:21:14,480 --> 00:21:17,960 space time causes such a massive divergence that you were 452 00:21:17,960 --> 00:21:20,160 permanently cut off from your universe of origin. 453 00:21:20,440 --> 00:21:22,920 Speaker 1: Think about what that means for the people you left behind, 454 00:21:22,960 --> 00:21:25,160 your family, your friends. They watch you get in the 455 00:21:25,160 --> 00:21:28,240 machine and you're just gone forever. 456 00:21:28,440 --> 00:21:33,039 Speaker 2: From their perspective, you've vanished, you're lost, you've effectively died. 457 00:21:33,279 --> 00:21:35,799 Speaker 1: And then in the past of the universe you arrive in, 458 00:21:36,039 --> 00:21:37,799 there's another set of problems. 459 00:21:37,400 --> 00:21:40,319 Speaker 2: Because there's already a version of you there, or well, 460 00:21:40,359 --> 00:21:40,839 there will be. 461 00:21:40,920 --> 00:21:43,720 Speaker 1: The original copy of me in that timeline still exists. 462 00:21:44,000 --> 00:21:47,400 So I, the Traveler, just appear out of nowhere. 463 00:21:47,519 --> 00:21:49,400 Speaker 2: Suddenly there are two of you in that world. You've 464 00:21:49,440 --> 00:21:54,000 become this chronological immigrant in a reality that wasn't expecting you. 465 00:21:54,079 --> 00:21:56,599 Speaker 1: So it's not about fixing your mistakes. It's about abandoning 466 00:21:56,640 --> 00:21:58,640 your entire life for a chance to start a new 467 00:21:58,680 --> 00:21:59,920 one somewhere. 468 00:21:59,599 --> 00:22:02,720 Speaker 2: Else, and in the process, leaving a mystery behind in 469 00:22:02,759 --> 00:22:06,160 your old world and creating a complication in your new one. 470 00:22:06,240 --> 00:22:08,799 Time travel becomes less of a fun adventure and more 471 00:22:08,839 --> 00:22:12,319 of a tragedy, the ultimate act of self exile. 472 00:22:12,640 --> 00:22:15,480 Speaker 1: What a journey. We started with a single particle of 473 00:22:15,519 --> 00:22:18,640 light behaving strangely in a simple experiment, and that. 474 00:22:18,599 --> 00:22:22,680 Speaker 2: One experiment, when you really follow its implications, serves as 475 00:22:22,720 --> 00:22:27,000 the bedrock evidence that our reality is being physically shaped 476 00:22:27,039 --> 00:22:28,440 by unseen ones. 477 00:22:28,440 --> 00:22:32,519 Speaker 1: That led us to the multiverse, not as some fun 478 00:22:32,640 --> 00:22:36,839 sci fi idea, but as David Deutsch's necessary conclusion, the 479 00:22:36,880 --> 00:22:39,480 only theory that actually explains the quantum map without just 480 00:22:39,519 --> 00:22:41,599 throwing up its hands and saying it collapses. 481 00:22:41,839 --> 00:22:44,400 Speaker 2: It provides the why, and from there we had to 482 00:22:44,400 --> 00:22:48,000 rethink everything. We realize our own identity isn't some single 483 00:22:48,119 --> 00:22:52,079 fixed thing, but this huge branching tree of all our 484 00:22:52,119 --> 00:22:53,039 potential selves. 485 00:22:53,119 --> 00:22:55,240 Speaker 1: And the reason we don't feel these other worlds is 486 00:22:55,279 --> 00:22:57,240 the same reason we don't feel the earth spinning beneath 487 00:22:57,240 --> 00:22:59,799 our feet. Our senses just aren't built for it. But 488 00:22:59,839 --> 00:23:02,440 the instruments, the experiments, they confirm it's real. 489 00:23:03,039 --> 00:23:05,720 Speaker 2: And finally we saw how all of this provides a stunning, 490 00:23:05,920 --> 00:23:09,319 if a bit tragic, solution to the paradoxes of time travel. 491 00:23:09,799 --> 00:23:12,039 You could visit the past, but it's not your own, 492 00:23:12,200 --> 00:23:13,240 and you can never return. 493 00:23:13,559 --> 00:23:17,559 Speaker 1: It means that every mistake you regret, every road not taken, 494 00:23:18,480 --> 00:23:21,799 it's not a ghost. It's a real story being lived 495 00:23:21,839 --> 00:23:24,799 out by another version of you in a universe just 496 00:23:24,880 --> 00:23:28,039 as real as this one. Our life is just one 497 00:23:28,079 --> 00:23:29,559 book in an infinite library. 498 00:23:29,640 --> 00:23:31,400 Speaker 2: A reality is just one variation. 499 00:23:31,799 --> 00:23:33,440 Speaker 1: So we want to leave you with a final thought 500 00:23:33,480 --> 00:23:36,400 to mull over if time travel really worked like this, 501 00:23:36,519 --> 00:23:39,079 if it were physically possible, But you knew the journey 502 00:23:39,160 --> 00:23:40,960 was a one way ticket, you knew you could never 503 00:23:41,000 --> 00:23:43,559 go back, that you'd abandon everyone you've ever known forever. 504 00:23:44,160 --> 00:23:46,319 Would the chance to step into a new history, maybe 505 00:23:46,319 --> 00:23:49,640 a better one, be worth that ultimate price. Let us 506 00:23:49,680 --> 00:23:50,720 know what you think in the comments.