WEBVTT 1 00:00:01.480 --> 00:00:05.599 Welcome to the quart Side Quantum Physics Podcast, an exploration 2 00:00:05.679 --> 00:00:10.880 of the fundamental structure of reality, where quantum laws govern matter, energy, 3 00:00:11.199 --> 00:00:16.239 and information. Here, uncertainty is a feature, not a flaw, 4 00:00:16.719 --> 00:00:19.239 and understanding begins at the smallest scales. 5 00:00:27.640 --> 00:00:32.520 So, uh, picture this. It's around May first, twenty twenty 6 00:00:32.560 --> 00:00:36.039 six and a team of physicists up in Toronto are 7 00:00:36.079 --> 00:00:39.079 looking at their lab instruments and they realize something that 8 00:00:39.159 --> 00:00:41.600 sounds well, literally impossible. 9 00:00:41.759 --> 00:00:43.479 Yeah it really does, right. 10 00:00:43.679 --> 00:00:46.920 They realized a particle had basically exited their experiment before 11 00:00:46.920 --> 00:00:47.960 it had fully entered it. 12 00:00:48.520 --> 00:00:51.799 Which I mean it completely shatters the most comforting constant 13 00:00:51.840 --> 00:00:55.439 in our macroscopic world, the ticking claw. Exactly. We all 14 00:00:55.439 --> 00:00:57.880 just expect the arrow of time to only point in 15 00:00:57.880 --> 00:01:01.119 one direction, you know, one second follows the next, cause 16 00:01:01.159 --> 00:01:04.599 always comes before effect. But down on the quantum realm, 17 00:01:04.879 --> 00:01:07.599 the universe operates under a totally different set of rules, 18 00:01:07.680 --> 00:01:10.439 and frankly, it doesn't owe us intuitive comfort. 19 00:01:10.760 --> 00:01:13.280 It definitely doesn't. And today we are taking you down 20 00:01:13.319 --> 00:01:17.840 into that microscopic landscape where the relentless forward march of 21 00:01:17.840 --> 00:01:20.760 the clock simply stops, or well, in this specific case, 22 00:01:20.760 --> 00:01:22.000 it turns completely around. 23 00:01:22.159 --> 00:01:23.959 Yeah, we're talking about negative. 24 00:01:23.599 --> 00:01:26.640 Time, right. We're going to figure out exactly what it 25 00:01:26.719 --> 00:01:30.079 means for a particle to spend a negative amount of 26 00:01:30.079 --> 00:01:33.519 time interacting with an atom. Like. We'll break down how 27 00:01:33.560 --> 00:01:36.799 scientists actually measured this without breaking the laws of physics, 28 00:01:37.200 --> 00:01:43.159 and why this discovery reshapes our entire fundamental understanding of causality. 29 00:01:43.239 --> 00:01:44.719 It's a huge paradigm shift. 30 00:01:44.799 --> 00:01:49.040 It is because while you experience time as this unstoppable 31 00:01:49.079 --> 00:01:53.079 one way street today, you're going to learn how time 32 00:01:53.480 --> 00:01:55.840 can effectively wear a negative sign. 33 00:01:55.920 --> 00:01:58.120 And to really grasp the mechanics of this, I mean 34 00:01:58.120 --> 00:02:00.000 we have to basically throw out all our every day 35 00:02:00.040 --> 00:02:01.920 assumptions about how objects move through. 36 00:02:01.760 --> 00:02:03.519 Space, just toss them right out the window. 37 00:02:03.640 --> 00:02:06.040 Yeah, pretty much. We have to look at the physical measurements, 38 00:02:06.079 --> 00:02:08.280 like the hard data from the laboratory and just trust 39 00:02:08.319 --> 00:02:10.879 them even when they scream against all our common sense. 40 00:02:11.000 --> 00:02:14.240 Okay, let's unpack this because understanding the architecture of the 41 00:02:14.280 --> 00:02:18.280 actual experiment is it's really the only way to understand 42 00:02:18.360 --> 00:02:19.439 this bizarre result. 43 00:02:19.599 --> 00:02:21.680 Definitely, you need the physical context, right. 44 00:02:22.120 --> 00:02:25.199 So, at the University of Toronto, researchers took a cloud 45 00:02:25.240 --> 00:02:31.240 of ultra cold atoms specifically rubidium atoms. 46 00:02:31.479 --> 00:02:34.000 Yeah, rubidium is great for this kind of optical work. 47 00:02:34.159 --> 00:02:36.439 And they cooled them down to just a fraction of 48 00:02:36.439 --> 00:02:40.360 a degree above absolute zero, just incredibly cold. And the 49 00:02:40.400 --> 00:02:43.240 reason they do that is so that thermal noise, you know, 50 00:02:43.439 --> 00:02:48.159 random heat vibrations, wouldn't wash out the really delicate quantum effects. 51 00:02:48.280 --> 00:02:50.800 You need it as quiet as possible in there, exactly. 52 00:02:51.400 --> 00:02:55.039 So then they fired this very weak laser pulse of 53 00:02:55.120 --> 00:02:59.280 photons straight into this freezing cloud of atoms, right, And. 54 00:02:59.240 --> 00:03:03.759 What's fascinating here is well, classically any object entering a 55 00:03:03.800 --> 00:03:06.639 medium has to spend a positive duration of time. 56 00:03:06.479 --> 00:03:08.319 Traversing it, like walking through a door. 57 00:03:08.439 --> 00:03:11.560 Yeah, exactly. A particle of light enters the cloud, it 58 00:03:11.599 --> 00:03:14.639 interacts with the atoms, it bounces around or gets delayed, 59 00:03:14.680 --> 00:03:16.639 and then you know, it exits the other side. 60 00:03:16.680 --> 00:03:19.159 And there are specific terms for this timing, right there are. 61 00:03:19.360 --> 00:03:21.199 So the time it takes for that whole pulse to 62 00:03:21.240 --> 00:03:23.400 travel through the material we call that the group deway 63 00:03:23.879 --> 00:03:27.280 and the actual physical duration the photon spends inside the 64 00:03:27.319 --> 00:03:30.680 atomic cloud, like physically interacting with the atoms, that is 65 00:03:30.719 --> 00:03:31.400 the dwell time. 66 00:03:31.599 --> 00:03:36.560 Okay, so dwell time, and in a normal everyday macroscopic world, 67 00:03:36.719 --> 00:03:38.919 that dwell time just has to be a positive number. 68 00:03:39.039 --> 00:03:40.680 It has to be like if you walk into a 69 00:03:40.680 --> 00:03:42.280 house to the front door and you leave through the 70 00:03:42.280 --> 00:03:45.960 back door, it takes a measurable positive amount of time. 71 00:03:46.439 --> 00:03:49.560 Even if you sprint like as fast as humanly possible, 72 00:03:49.919 --> 00:03:51.960 the duration is still greater than zero. 73 00:03:52.199 --> 00:03:54.879 Right, you can't leave before you arrive exactly. 74 00:03:55.240 --> 00:03:58.759 But in this experiment, the calculations for the photon's dwell 75 00:03:58.840 --> 00:04:01.400 time yielded and negative value, which. 76 00:04:01.199 --> 00:04:03.840 Is just it's so wild. The photon arrived at the 77 00:04:03.840 --> 00:04:06.520 detector earlier than it would have if it had just 78 00:04:06.599 --> 00:04:07.800 traveled through empty space. 79 00:04:07.960 --> 00:04:08.840 It took a shortcut. 80 00:04:08.919 --> 00:04:11.199 Yeah, it took a measurable shortcut through time. 81 00:04:11.360 --> 00:04:13.400 I know. A lot of the popular coverage tried to 82 00:04:13.400 --> 00:04:15.800 explain this using like a tunnel analogy. 83 00:04:15.919 --> 00:04:17.079 Oh yeah, I saw those. 84 00:04:17.279 --> 00:04:20.759 They said, imagine standing outside a long tunnel, right, and 85 00:04:20.800 --> 00:04:24.000 you're watching a car drive through it, and somehow the 86 00:04:24.040 --> 00:04:27.240 car exits the far end of the tunnel before the 87 00:04:27.319 --> 00:04:29.800 back bumper has even entered the front of the tunnel. 88 00:04:29.839 --> 00:04:32.839 Which is a fun image, but it's highly misleading. 89 00:04:33.079 --> 00:04:35.639 Yeah, it just describes the visual outcome. It doesn't explain 90 00:04:35.680 --> 00:04:38.079 the physics at all. It just makes it sound like magic. 91 00:04:38.439 --> 00:04:41.759 Yeah, the car analogy falls apart because a photon isn't 92 00:04:41.759 --> 00:04:44.839 a solid object like a car, right, It's a wave packet. 93 00:04:45.240 --> 00:04:49.160 And that distinction is basically the entire key to this phenomenon, 94 00:04:49.199 --> 00:04:49.720 isn't it. 95 00:04:49.720 --> 00:04:52.360 It is the whole ballgame. You have to think of 96 00:04:52.399 --> 00:04:56.399 the photon pulse as a bell curve of probability. It 97 00:04:56.439 --> 00:04:59.079 has this leading edge, a big peak in the middle, 98 00:04:59.079 --> 00:04:59.959 and then a trailing it. 99 00:05:00.199 --> 00:05:02.240 Okay, so it's smeared out exactly. 100 00:05:02.399 --> 00:05:05.360 So when this wave packet enters the resonant atomic cloud, 101 00:05:05.680 --> 00:05:09.000 the interaction isn't just uniform across the whole thing. The 102 00:05:09.079 --> 00:05:11.879 atoms and the cloud react to the light based on 103 00:05:11.959 --> 00:05:13.240 its specific frequency. 104 00:05:13.439 --> 00:05:16.160 So as the wave packet pushes into the cloud, the 105 00:05:16.199 --> 00:05:18.319 atoms don't just act like a solid brick wall. 106 00:05:18.439 --> 00:05:21.519 No, not at all. They actively reshape the wave reshape 107 00:05:21.560 --> 00:05:24.480 it how so well. The atoms absorb and re emit 108 00:05:24.519 --> 00:05:28.199 the light. But because of the specific highly tuned frequency 109 00:05:28.199 --> 00:05:30.519 of the laser they used, the trailing edge of the 110 00:05:30.560 --> 00:05:33.519 wave packet, the back of the bell curve is highly 111 00:05:33.639 --> 00:05:36.480 likely to be absorbed by the atoms. Yeah, while the 112 00:05:36.560 --> 00:05:38.639 leading edge passes through much more easily. 113 00:05:38.759 --> 00:05:40.639 Okay, wait, I think I have a better analogy than 114 00:05:40.639 --> 00:05:41.519 the car and the tunnel. 115 00:05:41.600 --> 00:05:42.439 Let's hear it. 116 00:05:42.439 --> 00:05:45.600 It's like sending a long freight train through a station. Okay, 117 00:05:45.839 --> 00:05:48.959 if the station master uncouples the entire back half of 118 00:05:49.000 --> 00:05:51.800 the train and just holds it at the platform, the 119 00:05:51.879 --> 00:05:55.240 remaining front half just keeps rolling down the track. So 120 00:05:55.360 --> 00:05:59.439 if you were to somehow calculate the mathematical center of 121 00:05:59.439 --> 00:06:02.399 the train that left the station, that center has suddenly 122 00:06:02.439 --> 00:06:06.240 shifted way forward compared to the original full train. 123 00:06:06.360 --> 00:06:08.800 That is a much better analogy. Yeah, the peak of 124 00:06:08.839 --> 00:06:11.519 the wave packet shifts forward because the back half was 125 00:06:11.560 --> 00:06:13.759 basically eaten by the atomic cloud. 126 00:06:13.519 --> 00:06:16.319 So that shifting peak gives the illusion that the entire 127 00:06:16.399 --> 00:06:17.720 wave traveled faster. 128 00:06:17.519 --> 00:06:21.839 Than light, exactly arriving early. The speed of that peak 129 00:06:21.920 --> 00:06:24.839 is what we call the group velocity, and group velocity 130 00:06:24.879 --> 00:06:27.639 can mathematically speaking exceed speed of light in a. 131 00:06:27.639 --> 00:06:29.279 Vacuum, which always freaks people out. 132 00:06:29.360 --> 00:06:31.680 Oh totally. But the crucial detail here is that the 133 00:06:31.759 --> 00:06:34.639 true leading edge of the wave, like the very first 134 00:06:34.920 --> 00:06:38.800 faint ripple of the signal, that never exceeds the speed 135 00:06:38.800 --> 00:06:40.079 of light, which. 136 00:06:39.839 --> 00:06:43.319 Means no actual information traveled faster than light. 137 00:06:43.519 --> 00:06:46.279 Right, Einstein's theory of relativity is totally safe. 138 00:06:46.560 --> 00:06:48.800 Few good news for Einstein. 139 00:06:49.000 --> 00:06:51.639 Yeah, he can rest easy. We can't use this to 140 00:06:51.759 --> 00:06:54.720 build a time machine or you know, send a lottery 141 00:06:54.720 --> 00:06:58.399 ticket to our past selves or anything. Because the absolute 142 00:06:58.000 --> 00:07:02.439 frontier of the signal still obeys that cosmic speed limit. 143 00:07:02.519 --> 00:07:04.040 Causality remains intact. 144 00:07:04.160 --> 00:07:08.079 It does. But that realization actually led to this massive 145 00:07:08.120 --> 00:07:09.639 debate in the physics community. 146 00:07:09.920 --> 00:07:11.720 Oh really, why, well think about it. 147 00:07:11.759 --> 00:07:14.959 If the early arrival is just a reshaping of the 148 00:07:15.000 --> 00:07:18.800 wave packet, like just an optical illusion of absorption, then 149 00:07:18.839 --> 00:07:20.560 the negative time is just a math trick. 150 00:07:20.800 --> 00:07:23.519 Oh I see, Like it's just a statistical quirk. 151 00:07:23.680 --> 00:07:25.720 Right. A lot of physicists argued it shouldn't have any 152 00:07:25.800 --> 00:07:28.560 real physical significance to the atoms themselves. 153 00:07:28.800 --> 00:07:31.399 But this is where the twenty twenty sixth experiment goes 154 00:07:31.439 --> 00:07:35.120 from a mathematical curiosity to a straight up mind bending 155 00:07:35.160 --> 00:07:38.519 physical reality. Because they didn't just measure the photon leaving 156 00:07:38.560 --> 00:07:39.000 the cloud. 157 00:07:39.240 --> 00:07:42.000 No, they didn't. They measured the atoms, which is incredibly 158 00:07:42.000 --> 00:07:42.759 hard to do. 159 00:07:43.120 --> 00:07:47.600 Yeah, it brings up this massive procedural wall because I mean, 160 00:07:47.639 --> 00:07:52.839 anyone who knows basic quantum mechanics knows about the observer. 161 00:07:52.480 --> 00:07:54.199 Effect, the good old observer effect. 162 00:07:54.360 --> 00:07:57.040 Right, you cannot look at a delicate quantum system without 163 00:07:57.040 --> 00:08:00.879 destroying it. Measuring a system collapse is its wave function. 164 00:08:01.040 --> 00:08:02.040 That's the textbook rule. 165 00:08:02.199 --> 00:08:06.439 Yeah, So how on earth do you ask a rubidium 166 00:08:06.480 --> 00:08:10.199 atom how long a photon hung out with it without 167 00:08:10.279 --> 00:08:11.879 ruining the entire experiment? 168 00:08:12.040 --> 00:08:15.879 And historically you know that exact barrier kept this entirely 169 00:08:15.879 --> 00:08:18.319 in the realm of theory because if you hit the 170 00:08:18.360 --> 00:08:21.319 atom with another laser to measure its state, you literally 171 00:08:21.360 --> 00:08:23.439 destroy the very interaction you're trying to observe. 172 00:08:23.519 --> 00:08:25.120 It's a catch twenty two totally. 173 00:08:25.399 --> 00:08:28.240 But the researchers overcame this using one of the most 174 00:08:28.279 --> 00:08:32.080 elegant techniques in modern physics. It's known as a weak measurement. 175 00:08:32.240 --> 00:08:34.440 Okay, weak measurement yea, which I have to say sounds 176 00:08:34.440 --> 00:08:35.879 like a total oxymoron. 177 00:08:36.039 --> 00:08:36.879 It does, doesn't it. 178 00:08:37.000 --> 00:08:39.120 Yeah, like either you measure something or you don't. 179 00:08:39.519 --> 00:08:43.200 Well, a standard, what we call a strong measurement is definitive. 180 00:08:43.519 --> 00:08:47.799 It collapses the state completely, But a weak measurement involves 181 00:08:48.000 --> 00:08:50.879 coupling the system. You want to measure to a sort 182 00:08:50.879 --> 00:08:57.000 of pointer system, but keeping that interaction incredibly almost absurdly. 183 00:08:56.399 --> 00:08:59.960 Subtle, subtle, how like I'm trying to think of it. 184 00:09:00.080 --> 00:09:02.200 Examples like trying to figure out the weight of a 185 00:09:02.200 --> 00:09:05.200 single snowflake. Oh, I like where this is going, right, 186 00:09:05.320 --> 00:09:08.440 Because you can't put one snowflake on a traditional mechanical 187 00:09:08.440 --> 00:09:11.679 scale without it melting or getting crushed. But if you 188 00:09:11.799 --> 00:09:16.000 let a million individual snowflakes fall onto an incredibly sensitive 189 00:09:16.320 --> 00:09:20.799 massive scale, you could like deduce the average weight of 190 00:09:20.840 --> 00:09:24.120 a single snowflake without destroying any specific one exactly. 191 00:09:24.159 --> 00:09:26.080 Wait, let me rephrase that a bit. But your snowflake 192 00:09:26.080 --> 00:09:29.240 analogy perfectly captures the statistical nature of the technique. 193 00:09:29.279 --> 00:09:29.600 Okay. 194 00:09:29.679 --> 00:09:32.519 Instead of a harsh spotlight that forces one single atom 195 00:09:32.600 --> 00:09:35.759 to declare its state, they let the passing photons slightly 196 00:09:35.879 --> 00:09:37.559 rotate the polarization of the light. 197 00:09:37.720 --> 00:09:40.039 Okay, so a tiny shift, right, It's. 198 00:09:39.919 --> 00:09:43.440 An almost imperceptible shift. But by doing this repeatedly over 199 00:09:43.480 --> 00:09:47.039 a massive on some we're talking millions of photons passing 200 00:09:47.120 --> 00:09:53.240 through millions of atoms, they extracted a statistical trend. They 201 00:09:53.360 --> 00:09:56.840 found the mean atomic excitation time, and they did it 202 00:09:56.879 --> 00:10:00.240 without destroying the quantum coherence of any individual interaction. 203 00:10:00.480 --> 00:10:03.799 So they gently monitor the rubidium atoms while the photon 204 00:10:03.879 --> 00:10:07.960 passed through, taking this huge ensemble average, and the atoms 205 00:10:08.360 --> 00:10:11.159 revealed their actual physical experience. 206 00:10:10.600 --> 00:10:13.240 Of the event, and the hard data is just stunning. 207 00:10:13.759 --> 00:10:16.639 Depending on how far the light's frequency was from the 208 00:10:16.639 --> 00:10:20.600 atom's natural resonance, the mean atomic excitation time was a 209 00:10:20.679 --> 00:10:22.879 highly specific negative value negative. 210 00:10:23.000 --> 00:10:23.279 Yeah. 211 00:10:23.360 --> 00:10:26.200 Specifically, it was negative point eight two times the amount 212 00:10:26.240 --> 00:10:28.840 of time light normally takes to cross that empty. 213 00:10:28.519 --> 00:10:30.320 Space negative point eight two. 214 00:10:30.200 --> 00:10:31.759 Plus or mine is a tiny murlder bearer. 215 00:10:31.840 --> 00:10:34.200 Yeah, that is just I mean, the atoms themselves confirmed it. 216 00:10:34.559 --> 00:10:37.600 The weak measurements showed the atoms physically experienced a negative 217 00:10:37.600 --> 00:10:40.600 interaction time. It did. The atom recorded holding onto the 218 00:10:40.720 --> 00:10:42.679 energy for less than zero seconds. 219 00:10:42.480 --> 00:10:45.799 Which means the physical system agrees perfectly with the reshaped 220 00:10:45.799 --> 00:10:46.360 wave packet. 221 00:10:46.600 --> 00:10:49.279 That is wild, and this dual confirmation is really what 222 00:10:49.399 --> 00:10:50.919 makes this a watershed moment. 223 00:10:51.200 --> 00:10:54.799 Oh absolutely. I mean, you have the outgoing photons timing 224 00:10:54.960 --> 00:10:57.679 arriving early on one side of the experiment, and then 225 00:10:57.720 --> 00:11:01.080 you have the internal state of the atomic ense recording 226 00:11:01.120 --> 00:11:03.799 a negative duration on the other, and they match both 227 00:11:03.879 --> 00:11:07.360 point to the exact same negative value. It builds beautifully 228 00:11:07.399 --> 00:11:09.840 on this twenty twenty four work by Danielle Angulo a 229 00:11:09.919 --> 00:11:13.440 from Steinberg and their colleagues. Back then, they show that 230 00:11:13.679 --> 00:11:16.879 photons made atoms seem to linger in an excited state 231 00:11:16.960 --> 00:11:18.039 for a negative. 232 00:11:17.720 --> 00:11:19.440 Duration, Right I remember reading about that. 233 00:11:19.720 --> 00:11:23.000 Yeah, so this new data just cements it. The atoms 234 00:11:23.080 --> 00:11:27.519 effectively pre excite. The phenomenon is deeply, deeply tied to 235 00:11:27.600 --> 00:11:30.120 the non local nature of quantum wave functions. 236 00:11:30.279 --> 00:11:34.159 But wait, if the atom actually physically experiences the negative time, 237 00:11:34.519 --> 00:11:37.000 we have to talk about how it knows to do that. 238 00:11:37.159 --> 00:11:39.159 What do you mean, Well, if the trailing edge of 239 00:11:39.159 --> 00:11:42.279 the wave is getting absorbed and the peak is shifting forward, 240 00:11:42.879 --> 00:11:45.559 why does the atom register and negative time only when 241 00:11:45.559 --> 00:11:47.639 the photon successfully passes all the way through? 242 00:11:48.120 --> 00:11:52.200 Ah? Okay. That brings us to a concept called quantum 243 00:11:52.360 --> 00:11:57.799 post selection, which honestly is vital to understanding why causality 244 00:11:57.879 --> 00:11:58.639 isn't broken here. 245 00:11:58.759 --> 00:11:59.679 Okay, post selection. 246 00:12:00.600 --> 00:12:04.440 The negative values only emerge mathematically and physically when we 247 00:12:04.559 --> 00:12:05.519 filter the results. 248 00:12:06.000 --> 00:12:07.799 So it's like, okay, it's like we ran a massive 249 00:12:07.879 --> 00:12:10.799 survey of a million people walking through a giant building, 250 00:12:11.440 --> 00:12:13.320 but at the end of the day, we throughout all 251 00:12:13.360 --> 00:12:15.720 the survey data except for the people who walked out 252 00:12:15.759 --> 00:12:17.679 of one specific exit door. Yes. 253 00:12:18.000 --> 00:12:21.840 Perfect. The negative time only appears because we are aggressively 254 00:12:21.919 --> 00:12:24.200 filtering for a very specific outcome. 255 00:12:24.360 --> 00:12:26.440 The photons that made it completely through. 256 00:12:26.480 --> 00:12:29.279 Exactly the ones that made it through without being fully scattered. 257 00:12:29.879 --> 00:12:33.000 That filtering process is the actual engine of the phenomenon. 258 00:12:33.320 --> 00:12:38.279 Interesting in quantum mechanics, a particle explores multiple probability paths simultaneously. 259 00:12:39.120 --> 00:12:41.200 So when we pre select the initial state you know, 260 00:12:41.399 --> 00:12:43.559 firing the laser, and then we post select the final 261 00:12:43.639 --> 00:12:47.559 state only looking at the photons that transmitted through, we 262 00:12:47.639 --> 00:12:51.080 are looking at a highly specific subgroup of quantum histories. 263 00:12:51.559 --> 00:12:54.399 We are essentially isolating the weirdest possible path. 264 00:12:54.799 --> 00:12:58.919 Exactly, we are isolating a path where virtual processes dominate 265 00:12:59.039 --> 00:13:02.879 the interaction. In this post selected group. The quantum interference 266 00:13:03.000 --> 00:13:07.919 creates a situation where the probability amplitudes mathematically demand a 267 00:13:08.039 --> 00:13:09.320 negative interaction. 268 00:13:09.039 --> 00:13:10.240 Time demand it. Wow. 269 00:13:10.720 --> 00:13:13.120 Yeah, And if we connect this to the bigger picture, 270 00:13:13.600 --> 00:13:17.360 Leading quantum theorist Howard Wiseman helped confirm this. He showed 271 00:13:17.480 --> 00:13:21.080 that these post selected negative values aren't just artifacts. They 272 00:13:21.240 --> 00:13:25.440 dictate the actual real physical kick the atoms receive. 273 00:13:25.679 --> 00:13:27.559 I just keep thinking about the reaction to this. When 274 00:13:27.639 --> 00:13:31.279 the news broke place like Reddit, you know, our science 275 00:13:31.559 --> 00:13:32.639 they just exploded. 276 00:13:32.759 --> 00:13:33.720 Oh it was everywhere. 277 00:13:33.840 --> 00:13:36.320 People were calling it a bomb on the standard view 278 00:13:36.399 --> 00:13:39.039 of temporal flow. But I imagine this is also an 279 00:13:39.080 --> 00:13:42.360 absolute nightmare for quantum computing. How so, Well, if you're 280 00:13:42.399 --> 00:13:45.000 trying to build a perfectly timed quantum logic gait and 281 00:13:45.080 --> 00:13:47.919 one of your photons takes a statistical shortcut and arrives 282 00:13:48.000 --> 00:13:50.960 before it left, I mean that throws the whole synchronization 283 00:13:51.080 --> 00:13:53.000 completely out of whackry Yeah, oh yeah. 284 00:13:53.360 --> 00:13:55.879 Managing that kind of temporal jitter is one of the 285 00:13:55.960 --> 00:13:59.399 hardest parts of scaling quantum processors. But at the same time, 286 00:13:59.600 --> 00:14:03.360 it's also a massive opportunity, really an opportunity, Yeah, because 287 00:14:03.360 --> 00:14:06.759 if we can harness and manipulate these negative dwell times, 288 00:14:07.360 --> 00:14:13.799 it gives us unprecedented control. Understanding these ultrafast, counterintuitive interactions 289 00:14:13.919 --> 00:14:18.720 allows us to design photonic quantum computers that manage entanglement 290 00:14:18.879 --> 00:14:20.440 with vastly greater precision. 291 00:14:20.519 --> 00:14:22.240 Oh I see, so what does this all mean for 292 00:14:22.360 --> 00:14:26.320 the tech? We could like intentionally use the negative delay 293 00:14:26.480 --> 00:14:28.440 to correct synchronization errors. 294 00:14:28.559 --> 00:14:32.879 Theoretically, yes, if one entangled particle is lagging behind, you 295 00:14:32.919 --> 00:14:36.399 could pass it through a specifically tuned resonant atomic cloud 296 00:14:36.519 --> 00:14:37.919 to give it a negative time kick. 297 00:14:37.879 --> 00:14:39.559 Just snap it right back into perfect sync with its 298 00:14:39.600 --> 00:14:40.679 partners exactly. Now. 299 00:14:41.080 --> 00:14:43.639 That level of engineering is at the absolute limit of 300 00:14:43.639 --> 00:14:46.559 physics right now, but it could also revolutionize things like 301 00:14:46.759 --> 00:14:51.039 slow light technologies and quantum memories member. Yeah, systems where 302 00:14:51.120 --> 00:14:53.519 we need to store light in an atomic state and 303 00:14:53.600 --> 00:14:57.720 retrieve it on demand. Factoring in these negative time scales 304 00:14:57.799 --> 00:15:02.279 allows for theoretically perfect fidelity in those memory transfers. 305 00:15:02.399 --> 00:15:08.679 Wow. But beyond the immediate technology, this finding fundamentally broadens 306 00:15:08.720 --> 00:15:11.360 how we even think about the universe, oh completely, because 307 00:15:11.360 --> 00:15:16.279 it joins this very exclusive club of bizarre negative concepts 308 00:15:16.320 --> 00:15:20.720 and physics that describe completely real phenomena. We usually think 309 00:15:20.759 --> 00:15:23.360 of physical properties as having a hard floor at zero, right, 310 00:15:23.480 --> 00:15:26.120 zero is usually the bottom, But the universe just doesn't 311 00:15:26.120 --> 00:15:26.879 always agree with us. 312 00:15:27.000 --> 00:15:29.399 No, it doesn't. And actually temperature is a perfect example 313 00:15:29.480 --> 00:15:29.639 of this. 314 00:15:29.879 --> 00:15:33.559 Wait really, because I've always been taught that absolute zero 315 00:15:33.919 --> 00:15:36.320 is the hard limit most people are It's the point 316 00:15:36.360 --> 00:15:39.919 where all atomic motion completely stops. You physically can't have 317 00:15:40.120 --> 00:15:41.320 less motion than no motion. 318 00:15:41.960 --> 00:15:45.360 Classically, you're right. Temperature measures the average kinetic energy of 319 00:15:45.480 --> 00:15:49.720 moving particles. But in deep thermodynamics, temperature is actually defined 320 00:15:49.759 --> 00:15:51.360 by how energy relates. 321 00:15:51.039 --> 00:15:53.600 To entropy, entropy being disorder. Right. 322 00:15:54.000 --> 00:15:57.600 Usually, adding energy to a system increases its disorder. If 323 00:15:57.639 --> 00:16:00.399 you heat up in ice cube, the rigid st ructured 324 00:16:00.440 --> 00:16:04.200 water molecules break apart and start moving chaotically as liquid water. 325 00:16:04.399 --> 00:16:08.320 More energy equals more chaos positive temperature exactly. 326 00:16:08.919 --> 00:16:13.360 But in certain specialized quantum spin systems, like say atoms 327 00:16:13.440 --> 00:16:16.480 locked in a magnetic lattice, there is an upper limit 328 00:16:16.600 --> 00:16:19.240 to how much energy the system can physically hold. 329 00:16:19.440 --> 00:16:20.679 Okay, a ceiling right. 330 00:16:21.039 --> 00:16:24.279 If you keep pumping energy into this bounded system, the 331 00:16:24.360 --> 00:16:27.320 atoms eventually have no choice but to align their spins 332 00:16:27.399 --> 00:16:29.759 perfectly just to absorb that maximum energy. 333 00:16:29.919 --> 00:16:32.080 Oh wait, so as it reaches maximum energy, the system 334 00:16:32.120 --> 00:16:33.399 actually becomes highly ordered again. 335 00:16:33.600 --> 00:16:37.879 Yes, the disorder goes down. Adding energy decreases entropy. That's 336 00:16:38.000 --> 00:16:41.879 wild and mathematically, the only way the thermodynamic equations can 337 00:16:41.960 --> 00:16:45.320 handle a state where adding energy lowers entropy is for 338 00:16:45.399 --> 00:16:47.919 the temperature value to actually flip to a negative number. 339 00:16:48.000 --> 00:16:49.960 Okay, so wait, that means a system with a negative 340 00:16:50.000 --> 00:16:54.360 temperature is actually mathematically speaking hotter than a system at 341 00:16:54.399 --> 00:16:56.080 an infinite positive temperature. 342 00:16:56.240 --> 00:16:58.440 It is, if you put them right next to each other, 343 00:16:58.919 --> 00:17:01.960 heat would flow from and the negative temperature system into 344 00:17:02.039 --> 00:17:02.799 the positive one. 345 00:17:02.960 --> 00:17:04.440 My brain just melted a little bit. 346 00:17:04.559 --> 00:17:07.880 It's totally counterintuitive. The math creates a negative, but the 347 00:17:07.920 --> 00:17:12.799 physical reality is just a different extreme state of energy allocation. 348 00:17:13.559 --> 00:17:17.680 We see similar mathematical leaps with effective negative mass in 349 00:17:17.880 --> 00:17:22.119 certain superfluids, where pushing an object makes it accelerate backward 350 00:17:22.160 --> 00:17:22.559 towards you. 351 00:17:22.720 --> 00:17:26.319 Which brings us to honestly the biggest philosophical ramification of 352 00:17:26.359 --> 00:17:26.559 all this. 353 00:17:26.839 --> 00:17:27.279 Let's hear it. 354 00:17:27.720 --> 00:17:31.359 If time can be negative in these quantum measurements, if 355 00:17:31.400 --> 00:17:35.359 the rubidium atoms and the photons are literally trading negative durations, 356 00:17:36.480 --> 00:17:39.480 what does that actually say about our everyday experience of time. 357 00:17:39.599 --> 00:17:40.599 It's a deep question. 358 00:17:41.039 --> 00:17:44.880 We treat the ticking clock as a fundamental, unbreakable property 359 00:17:44.920 --> 00:17:48.680 of the universe, but this experiment makes time look incredibly fluid. 360 00:17:48.839 --> 00:17:51.839 It does, and it heavily reinforces the theory that time 361 00:17:52.000 --> 00:17:54.640 is just emergent, emergent like wetness. 362 00:17:54.839 --> 00:17:57.400 What do you mean, well, a single molecule of H 363 00:17:57.480 --> 00:18:00.240 two O isn't wet, oh right, right, Wetness is an 364 00:18:00.279 --> 00:18:04.079 emergent property of billions of water molecules all interacting together. 365 00:18:04.240 --> 00:18:05.759 That's a great way to look at it, and time 366 00:18:05.839 --> 00:18:09.079 might operate the exact same way. Really, Yeah, the relentless 367 00:18:09.119 --> 00:18:11.640 forward earrow of time, the strict flow of cause and effect, 368 00:18:12.039 --> 00:18:15.640 it's likely a macroscopic illusion. It's created by the overwhelming 369 00:18:15.680 --> 00:18:18.680 statistics of entropy. At the human scale, there are so 370 00:18:18.839 --> 00:18:22.960 many particles interacting that the statistical probability of time flowing 371 00:18:23.039 --> 00:18:25.559 backward is essentially functionally zero. 372 00:18:25.920 --> 00:18:27.720 But when you zoom all the way down to a 373 00:18:27.880 --> 00:18:31.240 single photon interacting with an ultra. 374 00:18:31.079 --> 00:18:34.039 Coold atom, the rigid rules just wash away. At the 375 00:18:34.119 --> 00:18:37.279 fundamental quantum level, time isn't a rigid track at all. 376 00:18:37.559 --> 00:18:42.759 It's a localized statistical measurement, and sometimes those statistics yield 377 00:18:42.839 --> 00:18:46.480 a negative number. Negative time joins things like superposition and 378 00:18:46.599 --> 00:18:49.839 quantum entanglement as just another profound reality that proves our 379 00:18:49.880 --> 00:18:53.599 macroscopic intuition is just a tiny, tiny, simplified slice of 380 00:18:53.640 --> 00:18:54.279 the whole picture. 381 00:18:54.640 --> 00:18:56.559 So we started this with the assumption that time is 382 00:18:56.599 --> 00:19:00.000 an unstoppable one way street. But down at the microscype 383 00:19:00.000 --> 00:19:04.000 topic scale, looking at a cloud of ultra cold rubidium atoms, 384 00:19:04.319 --> 00:19:06.079 that streak completely dissolves. 385 00:19:06.440 --> 00:19:07.039 It really does. 386 00:19:07.680 --> 00:19:11.599 Using the delicate art of weak measurements, scientists allowed us 387 00:19:11.640 --> 00:19:14.640 to look closely at a wave packet traversing a medium. 388 00:19:15.359 --> 00:19:18.559 The leading edge pushes through, the trailing edge gets absorbed, 389 00:19:18.880 --> 00:19:22.240 and the peak leaps forward, seemingly outrunning the speed of 390 00:19:22.319 --> 00:19:22.920 light itself. 391 00:19:23.319 --> 00:19:25.759 It's an incredible feat of observation, and. 392 00:19:25.799 --> 00:19:29.400 By actually measuring the atoms themselves, we saw the physical 393 00:19:29.480 --> 00:19:33.559 reality of a negative dwell time. The atom literally records 394 00:19:33.640 --> 00:19:37.039 holding the photon for less than zero seconds. 395 00:19:36.960 --> 00:19:39.799 And it does all of this without breaking causality, and 396 00:19:39.880 --> 00:19:41.680 it doesn't violate relativity. 397 00:19:41.920 --> 00:19:44.640 Information is still bound by the speed of light exactly. 398 00:19:44.920 --> 00:19:47.440 But it forces us to recognize that our physical models 399 00:19:47.480 --> 00:19:51.200 are vastly more complex than a simple ticking clock. Physics 400 00:19:51.319 --> 00:19:54.240 just marches on revealing these hidden layers of strangeness that 401 00:19:54.319 --> 00:19:56.960