Excerpt from min 99 ff of

Sleep on Space - What the Vacuum Really Is… And Why It Isn’t Empty

https://www.youtube.com/watch?v=PeZzD5jUf4I

To understand this [role of vacuum], we need to go back to the beginning. not 1:40:47 just to the big bang, but to what happened in the first unimaginably tiny 1:40:53 fraction of a second after it. Because something extraordinary happened during 1:40:58 that sliver of time. Something that determined the shape, structure, and 1:41:04 fate of everything that came after. The universe we observe today has 1:41:10 several striking features that require explanation. First, it's remarkably uniform. When you 1:41:18 look at the cosmic microwave background, radiation, the afterglow of the Big Bang that fills 1:41:25 all of space, you find that its temperature is almost exactly the same in every direction. It varies by only 1:41:33 about one part in 100,000. The universe looks the same to the left 1:41:38 as it does to the right, the same above as below, the same in every direction to 1:41:44 extraordinary precision. Second, the geometry of the universe is flat. 1:41:51 Not curved like a sphere, not curved like a saddle, but flat. Parallel lines 1:41:57 stay parallel. The angles of a triangle add up to 180° 1:42:03 even on cosmic scales. The geometry of the universe is the simplest possible geometry. Euclidian, the 1:42:11 geometry you learned in school. Third, despite being incredibly uniform on 1:42:17 large scales, the universe has structure on smaller scales. There are galaxies, 1:42:23 there are clusters of galaxies, there are vast walls and filaments stretching across billions of light 1:42:30 years, separated by enormous voids.

This structure exists. Something 1:42:37 seeded it. Something created the initial variations in density that gravity then 1:42:42 amplified over billions of years into the galaxies and cosmic web we see 1:42:48 today. In the 1980s, physicists proposed a bold idea to 1:42:54 explain all three of these features simultaneously. The idea is called __cosmic inflation__ and 1:43:02 it has become the leading theory for the very early universe. The basic proposal 1:43:08 is this. In the first tiny fraction of a second after the big bang, 1:43:13 the universe underwent a period of exponential expansion. Not the gradual steady expansion 1:43:21 we observe today, but an incredibly rapid explosive expansion that doubled 1:43:26 the size of the universe over and over again in an unimaginably short time. How 1:43:33 rapid? During inflation, the universe doubled in size roughly every 10 to the 1:43:39 -37 seconds. That's a decimal point followed by 36 zeros and then a one. And 1:43:48 this doubling happened many times perhaps 60 to 70 times or more. Each 1:43:54 doubling multiplied the size of the universe by two. After 60 doublings, 1:44:01 the universe would have expanded by a factor of roughly 2 to the 60th power, 1:44:06 which is about 10 to the 18th, a billion billion times larger. 1:44:13 And all of this happened in a time span shorter than anything you could possibly imagine. The entire inflationary period 1:44:21 may have lasted only about 10 to the 32 seconds.

What drove this expansion? 1:44:29 This is where the vacuum enters the story in the most dramatic way possible. 1:44:35 According to inflationary theory, the early universe contained a quantum field, often called __the inflaton field__ 1:44:43 that was stuck in __a high energy vacuum state__. Not the true vacuum, not the 1:44:49 lowest possible energy state, but a false vacuum. A state that appeared to 1:44:56 be stable, but actually had a lower energy configuration available. Think of 1:45:01 it like a ball sitting in a shallow depression on the side of a hill. The 1:45:07 ball appears to be at rest. It's in a local minimum of energy, but it's not at 1:45:14 the bottom of the hill. There's a lower energy state available if the ball can 1:45:19 get over the lip of the depression and roll down to the valley below. 1:45:25 The ball in the depression is in a false vacuum. The valley below is the true 1:45:31 vacuum. __The inflated field in the early universe was in such a false vacuum__. It 1:45:38 had enormous energy density. Energy stored not in particles or radiation 1:45:45 but __in the configuration of the field itself__. This energy pervaded all of space 1:45:51 uniformly. And according to general relativity, a uniform energy density filling all of space doesn't 1:45:59 attract things together like normal gravity. It pushes them apart. It causes 1:46:05 space to expand. And the higher the energy density, the faster the expansion. The inflaton 1:46:13 field's energy density was colossal. It drove the exponential expansion of 1:46:19 space, blowing up the universe from something unimaginably tiny to something 1:46:25 macroscopic in a fraction of a second. The way inflation actually proceeded is 1:46:32 important for understanding the vacuum's role. In the R&B, most widely studied 1:46:38 models, the inflatant field didn't jump suddenly from the false vacuum to the 1:46:44 true vacuum. Instead, it rolled slowly down a gentle slope in its energy landscape. 1:46:52 Imagine a ball on a very shallow hillside. So shallow that the ball rolls 1:46:57 almost imperceptibly slowly. While the inflatant field was rolling slowly, its 1:47:04 energy remained nearly constant. And this nearly constant energy density 1:47:09 continued to drive exponential expansion. This is called __slow roll inflation__. 1:47:16 The expansion continued as long as the field was rolling. Only when the field 1:47:22 reached the bottom of the slope, the true minimum of its potential energy, 1:47:27 did inflation end. __At that point, the field oscillated rapidly around the 1:47:34 minimum. And these oscillations transferred energy from the inflaton field into all the other quantum fields 1:47:43 producing a flood of particles. This process is called reheating__. 1:47:50 __It's the moment when the energy stored in the vacuum configuration of the inflaton field was converted into the 1:47:58 hot dense plasma of particles and radiation that we associate with the big 1:48:04 bang__. __The universe went from being dominated by vacuum energy to being dominated by 1:48:10 matter and radiation in an extremely short time. Everything that exists 1:48:16 today, every atom, every photon, every particle <strike>of dark matter</strike> was produced 1:48:23 during reheating from the energy that had been stored in the inflaten fields vacuum state.

[1:48:30 Regions that were initially close together were flung apart to enormous distances. The universe expanded 1:48:38 so fast that any initial curvature was flattened out, like inflating a balloon 1:48:43 so large that any small patch of its surface appears perfectly flat. This 1:48:49 explains why the universe's geometry is flat. Any initial curvature was diluted 1:48:56 to insignificance by the enormous expansion. The uniformity of the universe is 1:49:02 explained the same way. Before inflation, the observable 1:49:08 universe was contained within a tiny region so small that everything within it could have been in thermal 1:49:14 equilibrium. Everything was at the same temperature because everything was close enough to exchange energy. 1:49:22 Then inflation stretched this tiny uniform region to cosmic proportions. 1:49:29 The uniformity we see in the cosmic microwave background today is a relic of 1:49:35 the uniformity that existed in that tiny pre-inflationary patch blown up to the 1:49:41 size of the observable universe. But here's where the vacuum's quantum nature 1:49:46 enters and changes everything. The inflaten field like all quantum 1:49:52 fields had vacuum fluctuations. Even during inflation, even while the 1:49:58 field was stuck in its high energy false vacuum, the uncertainty principle 1:50:04 ensured that the field's value fluctuated slightly from point to point. 1:50:09 At any given location, the inflatant field wasn't at exactly the same value. 1:50:15 It was jittering, varying randomly by tiny amounts. 1:50:20 These fluctuations were quantum mechanical. They were the irreducible 1:50:26 minimum variations that the uncertainty principle demands. 1:50:31 They were microscopic occurring on subatomic length scales. Under normal 1:50:38 circumstances, these fluctuations would be far too small to have any observable 1:50:44 effect on anything. But inflation changed that. The exponential expansion 1:50:50 of space stretched everything, including these quantum fluctuations. 1:50:56 Fluctuations that started on subatomic scales were stretched to astronomical 1:51:02 scales. By the end of inflation, variations that began as quantum jitters in the inflaton 1:51:09 field had been blown up to sizes spanning millions of light years or more. The quantum had become cosmic. ]

1:51:17 When inflation ended, when the inflaton field finally decayed from its false 1:51:22 vacuum to the true vacuum, its energy was released. This energy was converted 1:51:29 into a hot dense soup of particles and radiation. This is what we think of as 1:51:35 the big bang. The hot expanding universe filled with matter and radiation that 1:51:42 eventually cooled to form atoms, stars, and galaxies.

[1:51:47 But the matter and radiation produced in this process weren't perfectly uniform. 1:51:53 They carried the imprint of the inflating fields quantum fluctuations. 1:51:58 Regions where the inflating field had fluctuated to slightly higher values ended up with slightly more energy, 1:52:06 slightly higher density. Regions where it had fluctuated to slightly 1:52:12 lower values ended up with slightly less density. These density variations were 1:52:18 tiny, only about one part in 100,000, but they were real and they were 1:52:25 everywhere. These variations are the seeds of all structure in the universe. Over the 1:52:32 following billions of years, gravity went to work on them. Regions that were 1:52:37 slightly denser than average attracted more matter through gravity, becoming 1:52:42 denser still. Regions that were slightly less dense lost matter to their denser 1:52:49 neighbors, becoming emptier. The tiny density fluctuations were amplified by 1:52:55 gravity into the galaxies, galaxy clusters, and cosmic web we observe 1:53:01 today. ]

This is one of the most remarkable ideas in all of science. The 1:53:07 galaxies you see through a telescope, the vast filaments of the cosmic web 1:53:13 stretching across billions of light years, the enormous voids between 1:53:18 them, all of it originated as quantum fluctuations in the vacuum during the 1:53:23 first fraction of a second after the big bang. The largest structures in the 1:53:29 universe grew from the smallest possible variations in the quantum vacuum. 1:53:35 The quantum world and the cosmic world are connected in the most intimate way 1:53:41 imaginable. And we have evidence for this. Strong evidence.  

[The 1:53:47 cosmic microwave background radiation, the afterglow of the Big Bang carries an 1:53:53 imprint of those primordial density fluctuations. When we map the temperature of the 1:54:00 cosmic microwave background across the sky, we see tiny variations. 1:54:05 spots that are very slightly warmer or cooler than average. These variations 1:54:11 are the direct descendants of the quantum fluctuations that occurred during inflation. 1:54:18 They're a snapshot of the universe when it was only about 380,000 1:54:24 years old, showing us the density variations that would eventually grow 1:54:29 into galaxies and galaxy clusters. The pattern of these variations, 1:54:35 their statistical properties, their distribution across different angular scales has been measured with 1:54:43 extraordinary precision by satellite missions. The cosmic background 1:54:49 explorer measured them first in 1992. The Wilkinson microwave anosotropy probe 1:54:57 refined the measurements starting in 2001. And the plank satellite launched 1:55:03 in 2009 mapped the cosmic microwave background with exquisite detail. The results are 1:55:12 remarkable. The statistical properties of the temperature variations in the cosmic microwave background match 1:55:20 the predictions of inflationary theory with great accuracy. 1:55:25 The fluctuations are nearly scale invariant, meaning they have roughly the 1:55:31 same amplitude at all scales, which is exactly what inflation predicts. They 1:55:37 follow a Gaussian distribution, meaning they're random in the specific mathematical way that quantum 1:55:44 fluctuation should be. The pattern we see in the cosmic microwave background 1:55:50 is consistent with having originated as quantum vacuum fluctuations stretched to 1:55:56 cosmic proportions by inflation. This is extraordinary. 1:56:01 We're looking at the sky and seeing evidence of quantum mechanics operating at the dawn of time. 1:56:08 The spots in the cosmic microwave background, each one spanning enormous 1:56:13 distances, are the stretched out remnants of subatomic quantum fluctuations 1:56:19 from the earliest moments of the universe. The vacuum's quantum jitters 1:56:25 amplified by inflation and sculpted by gravity became everything. 1:56:32 Now, let's connect this back to something we discussed earlier, the vacuum energy. 1:56:38 the 0 point energy of the quantum fields that fill all of space. Because 1:56:43 this energy has another profound implication for the universe, one that 1:56:49 was discovered much more recently and that remains one of the deepest mysteries in all of physics. 1:56:56 In 1998, two teams of astronomers made a discovery that shocked the world. They 1:57:04 were studying distant supernova, exploding stars that serve as cosmic 1:57:11 distance markers because their peak brightness is known. By measuring 1:57:17 how bright these supernova appeared and comparing that to how bright they actually were, the astronomers 1:57:25 could determine their distances. And by measuring the red shift of the light from these supernova, 1:57:32 they could determine how fast the universe was expanding at the time the light was emitted. What 1:57:39 they expected to find was that the expansion of the universe was slowing down. After all, gravity pulls 1:57:48 matter together. Every galaxy in the universe attracts every other galaxy. 1:57:54 This mutual gravitational attraction should be decelerating the expansion 1:57:59 like a ball thrown upward that gradually slows as a gravity pulls it back. What 1:58:04 they actually found was the opposite. The expansion of the universe is 1:58:10 speeding up, not slowing down, accelerating. Distant galaxies are receding from us 1:58:16 faster and faster, not slower and slower. Something is pushing the 1:58:22 universe apart, overcoming the gravitational attraction of all the matter in the cosmos. 1:58:29 This discovery earned Saul Pearlmutter, Brian Schmidt, and Adam Ree the 1:58:35 Nobel Prize in physics in 2011.]

And the mysterious force driving the 1:58:41 acceleration was dubbed dark energy. __The simplest explanation for dark energy 1:58:48 is that it's the energy of the vacuum itself__. A constant energy density pervading all 1:58:54 of space, present everywhere at all times, 1:58:59 unchanging as the universe expands. This is exactly what you'd expect from 1:59:05 the 0 point energy of quantum fields. The vacuum has energy. That 1:59:11 energy fills all of space. It should have gravitational effects and 1:59:16 those effects according to general relativity would cause space to expand 1:59:21 faster and faster. Observationally, dark energy accounts for about 68% of 1:59:29 the total energy content of the universe. It's the dominant component more than ordinary matter, 1:59:37 more than dark matter, the energy of the vacuum or something behaving exactly 1:59:44 like it is the single largest contribution to the energy budget of the 1:59:50 cosmos. But as we discussed earlier, there's a catastrophic mismatch between the 1:59:56 predicted value and the observed value. Quantum field theory predicts a vacuum 2:00:03 energy density roughly 120 orders of magnitude larger than what 2:00:09 astronomers observe. This cosmological constant problem remains unsolved. 2:00:16 It's possible that some unknown mechanism cancels almost all of the vacuum energy, leaving just the tiny 2:00:23 residual that drives the observed acceleration. It's possible that 2:00:29 our understanding of how gravity responds to vacuum energy is incomplete. 2:00:35 It's possible that the answer lies in a theory of quantum gravity that we 2:00:40 haven't yet developed. Nobody knows. This single problem, the question of why 2:00:47 the vacuum energy is so much smaller than our best theories predict 2:00:52 may be the most important unsolved problem in fundamental physics. But even 2:00:58 with this mystery unresolved, the basic picture is clear. 2:01:04 The energy of the vacuum, whatever its exact value, is shaping the fate of the 2:01:11 universe. If dark energy continues to behave as it currently does, the 2:01:16 universe will expand forever.

[Galaxies will move apart from each other 2:01:22 faster and faster. Eventually, distant galaxies will be carried beyond the 2:01:27 observable horizon. Their light unable to reach us because the space between us 2:01:33 is expanding faster than light can cross it. The universe will become emptier, darker, and lonier. Trillions 2:01:42 of years from now, the only galaxies visible from any given point will be those in the immediate local group, 2:01:50 bound together by gravity tightly enough to resist the expansion. 2:01:55 Everything else will have disappeared beyond the cosmic horizon. The vacuum's energy is determining the 2:02:02 long-term fate of everything that exists.

[But there's another way the vacuum 2:02:09 shaped the universe that we haven't discussed yet. And it involves a concept called spontaneous 2:02:16 symmetry breaking in the very early universe when temperatures were 2:02:22 extraordinarily high. The quantum fields existed in a symmetric state. 2:02:29 The electromagnetic force and the weak nuclear force which appear as completely 2:02:35 different forces today were unified into a single electroeak force. The vacuum 2:02:42 state at those extreme temperatures was different from the vacuum state today. 2:02:48 It had a higher symmetry. As the universe expanded and cooled 2:02:54 about a trillionth of a second after the Big Bang, the temperature dropped below 2:02:59 a critical threshold. __The Higs field, which had been fluctuating around 2:03:05 zero in the high temperature symmetric vacuum, suddenly settled into a nonzero 2:03:11 value everywhere in space. This was a phase transition analogous to 2:03:18 water freezing into ice__. Just as liquid water has rotational 2:03:23 symmetry that ice crystals break, the high temperature vacuum had a symmetry 2:03:28 that the new Higs vacuum broke. When the Higs field settled into its new 2:03:34 nonzero vacuum state, the consequences were dramatic. 2:03:40 The W and Z bosons, which had been massless, 2:03:46 suddenly acquired mass. The electromagnetic force and the weak 2:03:51 force which had been identical became distinct. __The physics of the universe 2:03:57 fundamentally changed because the vacuum changed. The rules governing particle 2:04:03 interactions after this phase transition were different from the rules 2:04:09 before it. The universe we live in with its specific forces and particle masses 2:04:16 is a direct consequence of which vacuum state the Higs field settled into__. A 2:04:23 different vacuum would have meant different physics, different chemistry, 2:04:28 different possibilities for structure and complexity. The vacuum didn't just contain the 2:04:35 universe. It defined what kind of universe was possible. The ultimate 2:04:40 future of the universe, whether it expands forever or eventually reverses 2:04:46 or does something else entirely, depends on the properties of the vacuum. 2:04:52

And there's one more possibility we need to consider, one that is speculative but 2:04:58 grounded in real physics. And that is perhaps the most dramatic implication of 2:05:04 everything we've discussed tonight. It's the idea that the vacuum itself might 2:05:09 not be stable. Remember the inflaton field during cosmic inflation? It was 2:05:16 stuck in a false vacuum, a state that looks stable, but wasn't truly the 2:05:21 lowest energy state. Eventually, it decayed to the true vacuum, releasing 2:05:28 its energy and creating the matter and radiation that fills the universe. The 2:05:34 question is, could the same thing happen again? Could the vacuum we live in today, the vacuum 2:05:42 state of the quantum fields that fill our universe right now, be another false 2:05:48 vacuum?

[This isn't idle speculation. There are reasons from real physics to take this 2:05:55 possibility seriously. The key involves the Higs field, the 2:06:00 quantum field responsible for giving mass to fundamental particles. 2:06:05 The Higs Boson was discovered at the Large Hadron Collider in 2012, 2:06:12 confirming the existence of the Higs field and measurements of the Higs 2:06:18 Boson's mass combined with measurements of the top quark's mass allow physicists 2:06:24 to calculate the stability properties of the Higsfield's vacuum state. 2:06:29 The result is concerning, at least from a theoretical perspective. 2:06:35 Based on the measured values, the Higs vacuum appears to be metastable. 2:06:41 Not absolutely stable, not clearly unstable, but metastable. It's in a 2:06:48 state that could in principle decay to a lower energy state, but the probability of such a decay is 2:06:55 astronomically low. The expected lifetime of the metastable Higs vacuum 2:07:00 is far longer than the current age of the universe. probably much longer than 10 to the 100 2:07:07 years. So there's no practical danger, but

[the theoretical possibility is there. If the 2:07:15 __Higs vacuum__ were to decay, the consequences would be extraordinary and 2:07:22 catastrophic. The decay would begin at a single point in space, a quantum fluctuation that 2:07:29 pushes the Higs field over the energy barrier, separating the current metastable state from the true vacuum. 2:07:36 At that point, a bubble of true vacuum would form and begin expanding outward 2:07:42 at the speed of light. Inside this bubble, the Higs field would 2:07:48 have a different value. And because the Higs field determines the masses of 2:07:53 fundamental particles inside the bubble, the masses of electrons, quarks, and W 2:07:59 and Z bosons would all be different. The laws of physics as they appear to us 2:08:06 would change. ]

Chemistry would stop working. Atoms as we know them couldn't 2:08:12 exist. Stars couldn't form. Life couldn't survive. The bubble of true 2:08:18 vacuum would be a region of space where the rules are fundamentally different, 2:08:24 __spreading outward at light speed__, converting everything it touches into a 2:08:29 completely different physical regime. And because it travels at the speed of 2:08:35 light, there would be no warning, no advanced signal. The bubble would arrive 2:08:41 at the same time as any light that could warn of its approach. One moment everything is normal, the 2:08:49 next everything changes.

[This sounds terrifying, but 2:08:54 there are important caveats. The probability is incredibly low. The 2:09:01 expected time scale for such a decay is so far beyond the current age of the 2:09:07 universe that it's effectively zero for any practical purpose. Furthermore, 2:09:14 these calculations depend sensitively on the precise values of the Higs mass and 2:09:20 the top quark mass and small changes in those values could shift the result from 2:09:25 metastable to fully stable. Future more precise measurements might show that the 2:09:31 Higs vacuum is absolutely stable after all. ]

But the 2:09:36 conceptual point is profound. The vacuum state, the ground state of the quantum 2:09:42 fields that fill our universe might not be permanent. It might be a temporary 2:09:48 arrangement, a false floor that could give way to something deeper. the very 2:09:54 fabric of physical reality as we know it, the properties of particles, the the 2:10:01 strengths of forces, the rules that govern chemistry and biology. 2:10:07 All of these depend on a vacuum being in its current configuration. If the vacuum 2:10:12 changes, everything changes.


So, let's step back and look at the full picture 2:10:17 of what we've explored tonight. We started with a simple question. What is 2:10:23 the vacuum? What is empty space? And the answer turned out to be far more profound and 2:10:30 far more strange than anything everyday intuition would suggest. The vacuum is 2:10:35 not empty. It cannot be empty. Space is filled with quantum fields, at 2:10:42 least 17 of them, all stretching across every point in the universe. [music] 2:10:48 These fields are the fundamental fabric of reality. Everything that exists, 2:10:54 [music] every particle, every force, every interaction is a manifestation of 2:10:59 these fields. When all exitations are removed, when there are no particles 2:11:05 present, the fields remain. They're always there. And in their lowest energy 2:11:11 state, the vacuum [music] state, they still fluctuate. They still carry energy. The uncertainty principle 2:11:19 guarantees that the fields can never be perfectly still. There's always a 2:11:25 residual jitter, a minimum vibration, a 0 point energy that persists no matter 2:11:32 what you do. This 0 point energy is real. It creates measurable forces 2:11:39 between metal plates. It shifts the energy levels of atoms. It modifies the 2:11:45 magnetic properties of electrons with a precision confirmed to more than 10 2:11:51 decimal places. Under extreme conditions, near black holes or in 2:11:57 enormously strong fields, vacuum fluctuations could be converted into 2:12:02 real particles. The vacuum shaped the universe. During 2:12:07 cosmic inflation, quantum fluctuations in the vacuum were stretched to cosmic 2:12:12 proportions, [music] becoming the seeds of all structure. Every galaxy, every 2:12:18 cluster, every filament of the cosmic web traces its origin to a quantum 2:12:24 jitter in the vacuum during the first fraction of a second of cosmic history. 2:12:31 The evidence for this is imprinted on the cosmic microwave background radiation and matches theoretical 2:12:38 predictions with remarkable precision. There's another piece of evidence that 2:12:44 confirms the role of vacuum fluctuations in shaping the universe [music] and it 2:12:49 comes from the distribution of galaxies themselves. When the primordial density fluctuations 2:12:56 from inflation were imprinted on the hot plasma of the early universe, they set 2:13:02 up sound waves, pressure waves that traveled [music] through the plasma at about half the speed of light. These 2:13:09 waves oscillated, creating regions of compression and rarifaction, 2:13:15 denser and less dense spots. When the universe cooled enough for atoms to form 2:13:22 about 380,000 years after the Big Bang, [music] these sound waves froze in place. The 2:13:30 pattern they left behind is called barriian acoustic oscillations. It shows up as a slight preference for 2:13:37 galaxies [music] to be separated by a specific distance about 500 million 2:13:43 light years in [music] today's expanded universe. This distance corresponds to how far the 2:13:50 sound waves traveled before they froze. Multiple galaxy surveys, including the 2:13:57 Sloan digital sky survey and the dark energy spectroscopic instrument, have 2:14:03 detected this pattern in the threedimensional distribution of galaxies across the cosmos. 2:14:10 The signal is exactly where inflationary theory predicts it should be. Barry and 2:14:17 acoustic oscillations provide completely independent confirmation that the 2:14:22 primordial density fluctuations really existed [music] and had the properties that inflation 2:14:28 predicts. Combined with the cosmic microwave background measurements, [music] this 2:14:34 gives us two separate lines of evidence, both pointing to the same conclusion. 2:14:40 Quantum fluctuations in the vacuum during inflation seeded everything. The 2:14:47 vacuum's energy is driving the accelerating expansion of the universe right now. 2:14:53 Dark energy, the dominant component of the universe's energy budget, behaves 2:14:59 exactly like the energy of the vacuum. And the mismatch between the predicted 2:15:05 and observed values of this energy is one of the biggest unsolved problems in 2:15:10 all of physics. And the vacuum might not even be permanent. [music] The current 2:15:15 vacuum state of the Higs field may be metastable, a temporary configuration that could in 2:15:23 principle decay to a different lower energy state, 2:15:28 changing the laws of physics as we know them. The vacuum is not nothing. It 2:15:34 never was nothing. It's the most fundamental state of reality. The canvas 2:15:40 on which everything is painted. The ocean from which all waves arise. The 2:15:46 silence from which all music emerges. It has energy. It has structure. It has 2:15:52 fluctuations. It has physical consequences that span from the subatomic to the cosmic. from 2:16:00 the interior of atoms to the large scale structure of the entire observable 2:16:06 universe. And here's what I find most remarkable about all of this. We figured it out. 2:16:13 Human beings living on a small planet orbiting an ordinary star using nothing 2:16:19 but our minds and our instruments discovered that empty space isn't empty. 2:16:26 We measured forces that arise from nothingness. We predicted the magnetic properties of 2:16:32 electrons to 10 decimal places by accounting for the vacuum's influence. 2:16:38 We detected the afterlow of the big bang and found in its patterns the fingerprints of quantum fluctuations 2:16:45 [music] from the dawn of time. The vacuum is the deepest layer of 2:16:52 physical reality we've ever probed. It's the foundation beneath the foundation. 2:16:59 [music] and understanding it, truly understanding it, solving the cosmological constant problem, 2:17:06 understanding why the vacuum energy has the value it does, figuring out whether 2:17:12 the vacuum is truly stable. These [music] challenges represent some of the deepest questions human beings have ever 2:17:19 asked about the nature of existence. When you look out at the night sky 2:17:25 tonight, at the stars, at the darkness between them, remember 2:17:31 what you've learned. That darkness isn't empty. The space 2:17:37 between the stars, the gaps between galaxies, the vast voids that stretch 2:17:42 across hundreds of millions of light years, none of it is truly empty. 2:17:48 It's filled with quantum fields in their ground state. [music] Filled with fluctuations that can never 2:17:54 be silenced, filled with energy that shapes the fate of the cosmos. 2:18:01 What we call nothing is actually the foundation of everything. The particles 2:18:06 that make up your body, the light that lets you see the world around you, the 2:18:13 gravity that holds you to the earth, all of it arises from quantum fields. 2:18:18 [music] And those fields exist everywhere, even in the emptiest regions of space, fluctuating quietly, carrying 2:18:27 their zero point energy, shaping the universe from the very beginning to the 2:18:32 very end. The vacuum is not the absence of reality. [music] It is reality in its 2:18:39 most fundamental, most stripped down, most essential form. And the fact that 2:18:45 we can understand this, that we can measure it, predict it, and test it is 2:18:52 one of the most extraordinary achievements of the human mind. 2:18:58 Empty space isn't empty. It never was. 2:19:03 And now you know Why?