WEBVTT 00:00.000 --> 00:03.360 Thank you to Displate for supporting PBS. Nothing 00:03.360 --> 00:08.160 is complicated. By which I mean everything  is complicated because in quantum physics, 00:08.160 --> 00:13.360 even the nothing of completely empty space has  some pretty crazy properties. One prediction 00:13.360 --> 00:18.160 that's been around for a while is that under  extreme conditions, the quantum vacuum can 00:18.160 --> 00:27.280 sort of crystallize. And there's good evidence  that we've observed this for the first time. 00:18.160 --> 00:27.280 sort of crystallize. And there's good evidence  that we've observed this for the first time. 00:22.542 --> 00:27.022 you. Links to the merch store in the description.  you, yourselves, your atoms, and ultimately your 00:27.022 --> 00:33.262 subatomic particles come from excitations in the  quantum fields that fill all of space. But those 00:33.262 --> 00:39.102 quantum fields exist without you or your subatomic  particles. They exist in empty space. And this 00:39.102 --> 00:44.462 busy quantum vacuum does stuff. It interacts with  actual particles, subtly changing things like 00:44.462 --> 00:50.142 magnetic moment or atomic energy levels. Between  the plates of the Casemir experiment, we can coax 00:50.142 --> 00:55.422 the quantum vacuum into negative energies. And at  extreme positive energies, like at the collision 00:55.422 --> 01:00.382 point of a particle collider, the empty fields  of the quantum vacuum can be excited to produce 01:00.382 --> 01:05.982 all sorts of exotic particles. The mathematics  of quantum field theory describes these quantum 01:05.982 --> 01:12.462 phenomena with incredible precision. But how  far can we really push this? Can we really think 01:12.462 --> 01:18.622 of the vacuum as this richly structured thing  beyond the microscopic scope of our experiments? 01:18.622 --> 01:23.182 Well, there is one phenomenon that may reveal  this structure over hundreds or even thousands 01:23.182 --> 01:29.422 of kilome. An early prediction of quantum field  theory is that an extreme magnetic field may lock 01:29.422 --> 01:36.462 empty space into a grainy alignment almost like  crystallizing the vacuum itself. A real crystal 01:36.462 --> 01:42.062 is a solid whose atoms are locked into an ordered  repeating arrangement. But empty space is never 01:42.062 --> 01:48.862 that. However, the crystal and the magnetized  vacuum do share one exotic property. They 01:48.862 --> 01:55.022 change how light travels through them, sort of  locking photons into their grainy structure in a 01:55.022 --> 02:03.022 phenomenon called breringent. We routinely observe  breringents in crystals. Maybe you remember that 02:03.022 --> 02:09.422 episode we did with Mither from looking glass  universe. She used crystal breringent to do a 02:09.422 --> 02:15.902 quantum eraser experiment. But to observe vacuum  by refringence, we need magnetic fields billions 02:15.902 --> 02:21.742 of times stronger than anything we've been  able to create on Earth. Impossible for us, 02:21.742 --> 02:27.582 but not impossible for the universe. And in a  paper published in Nature this August, scientists 02:27.582 --> 02:35.502 claim evidence that a magnetar has crystallized  the empty space around it. To understand all of 02:35.502 --> 02:42.142 this madness, we're going to need to start much  ser much more familiar with good old refractive 02:42.142 --> 02:48.302 index. Light slows down in glass, in water, in any  transparent medium. The refractive index tells us 02:48.302 --> 02:54.382 how much this phenomenon is due to the photons's  electromagnetic wave interacting with the electric 02:54.382 --> 02:59.902 charges, mostly the electrons in the medium. I'll  give you a slightly cartoonish picture. A photon 02:59.902 --> 03:05.502 in a transparent medium encounters an electron  in an atom. The photon is an electromagnetic 03:05.502 --> 03:11.022 wave. So it causes the charged particle to wiggle  a bit. We can imagine the photon being momentarily 03:11.022 --> 03:17.102 absorbed and then remitted by the electron which  slows it down. Now this is an oversimplification 03:17.102 --> 03:22.382 really. We have a global em field of all the  wiggling electrons inducing phase velocity 03:22.382 --> 03:28.062 shifts but that's harder to animate. The change  in speed depends on a number of things including 03:28.062 --> 03:32.942 the wavelength of the light. And this is why  refraction allows us to separate wavelengths of 03:32.942 --> 03:39.182 light into a spectrum when passed through  a prism. In certain materials, however, 03:39.182 --> 03:46.302 the refractive index also depends on polarization.  Now, polarization is just the direction that a 03:46.302 --> 03:52.542 photon's electric field points. That polarization  direction determines the wiggle direction of the 03:52.542 --> 03:59.022 electrons that the photon encounters. Now, the  more the electron is able to wiggle, the more it 03:59.022 --> 04:06.782 will slow down or even stop an incoming photon.  In most media, electrons are able to wiggle the 04:06.782 --> 04:13.102 same in all directions. And so, polarization of  the incoming light doesn't matter. But that's not 04:13.102 --> 04:19.422 always the case. In a crystal, for example, the  atoms are in a fixed grid. the electrons may have 04:19.422 --> 04:25.582 more freedom to vibrate in some directions over  others. So if a photon's polarization direction, 04:25.582 --> 04:32.382 in particular its electric field, happens to be  aligned with the directions where the electrons 04:32.382 --> 04:37.902 have more wiggle capacity, then there'll be  more of this absorption reission, and that 04:37.902 --> 04:45.422 will slow the photon down more. So photons with  different polarization directions traveling the 04:45.422 --> 04:50.622 same direction through a crystal will experience  different refractive indexes and so different 04:50.622 --> 04:57.262 speeds. This is what breringences. It's pretty  intuitive to see how we get this behavior in 04:57.262 --> 05:02.622 a crystal because we have real electrons whose  behavior we can talk about admittedly in highly 05:02.622 --> 05:10.302 simplified ways. But it turns out that even the  vacuum of space can be forced into by refringence. 05:10.302 --> 05:16.222 So empty space is anything but. It's filled with  quantum fields in their lowest energy state. That 05:16.222 --> 05:21.902 includes the electron field, a ubiquitous layer of  the possibility of electrons everywhere in space. 05:21.902 --> 05:28.142 And a real electron is a persistent jiggle of  energy in that field. Dump energy into the field 05:28.142 --> 05:33.262 and you can excite an electron posetron pair. You  could do that with say a gamay photon as long as 05:33.262 --> 05:38.382 it had enough energy to cover the masses of the  particles via E= MC^². But short of that energy, 05:38.382 --> 05:44.782 a photon will normally ignore the electron field.  However, there are other ways to get the photon 05:44.782 --> 05:52.462 and the electron fields to notice each other. One  of those is vacuum by refringence. I'm going to 05:52.462 --> 05:58.462 describe this in two ways. One more intuitive  but less accurate then a bit more rigorously. 05:58.462 --> 06:03.182 You may have heard of virtual particles. Even  without sufficient energy to provide the particles 06:03.182 --> 06:08.142 their mass, we can imagine the quantum vacuum  producing matter antimatter pairs extremely 06:08.142 --> 06:12.622 briefly with their energy borrowed from the  intrinsic uncertainty of the energy of the 06:12.622 --> 06:17.182 vacuum. These virtual particles aren't like some  cloud of electrons and posetrons everywhere in 06:17.182 --> 06:21.822 space. Rather, they're a way of representing the  infinite ways that quantum fields can interact 06:21.822 --> 06:27.262 with real particles. Virtual particles appear in  the mathematics of quantum field theories, most 06:27.262 --> 06:33.502 notably in fineman diagrams. But most physicists  consider them a mathematical artifact, albeit a 06:33.502 --> 06:39.582 very useful one, and useful for intuition, too.  In a strong magnetic field, we can imagine that 06:39.582 --> 06:45.342 the flickering virtual electron posetron pairs are  no longer randomly oriented. They're generated in 06:45.342 --> 06:50.622 a preferred direction, and that alignment mimics  the alignment of the real charged particles in 06:50.622 --> 06:57.662 a crystal. And the result is vacuum polarization  which leads to a polarization dependent refractive 06:57.662 --> 07:04.862 index and so do vacuum refringence in the passing  photons. A more rigorous but less visually fun 07:04.862 --> 07:11.022 way to say this is that an extreme magnetic field  produces nonlinear corrections to electromagnetism 07:11.022 --> 07:15.822 through interaction of the electromagnetic  field with the quantum electron field. These 07:15.822 --> 07:20.302 corrections alter the propagation of photons  with the size of the effect depending on the 07:20.302 --> 07:25.662 photon's direction and polarization relative to  the magnetic field. But yeah, it's more fun to say 07:25.662 --> 07:31.262 that space gets crystallized, even if that's not  strictly true. But the resulting effect is very 07:31.262 --> 07:38.942 similar to crystals. Interestingly, this vacuum  refractive index doesn't actually slow the wave. 07:38.942 --> 07:43.982 The photons still travel at full light speed, but  the phase of the wave picks up a lag as though 07:43.982 --> 07:50.142 it were slowed. Two photons with different  polarizations will keep up with each other, 07:50.142 --> 07:56.062 but they'll fall out of phase with each other. The  magnetic field strength where vacuum polarization 07:56.062 --> 08:02.062 gets strong is around 4.4 billion Tesla. This  is called the quantum critical magnetic field. 08:02.062 --> 08:06.942 And that's not to say that vacuum by referring  can't be seen below that field strength, 08:06.942 --> 08:11.742 just that the effect drops off very quickly  at lower field strengths. Now, the strongest 08:11.742 --> 08:18.142 magnetic field that we humans have ever made  is 1,200 Tesla. And by refringence experiments 08:18.142 --> 08:25.022 are typically way lower than this. The effect is  essentially unobservable in humanmade experiments, 08:25.022 --> 08:29.662 at least so far. But there are places  in the universe where natural magnetic 08:29.662 --> 08:35.422 fields don't just approach the critical field  strength, they far exceed it. By far the king 08:35.422 --> 08:41.102 of magnetic fields is the magnetar. These  are a type of neutron star, the collapsed 08:41.102 --> 08:46.062 core of a massive star that's not quite massive  enough to become a black hole. In its collapse, 08:46.062 --> 08:50.222 protons and neutrons are smooshed together to  form neutrons, and the whole thing shrinks into 08:50.222 --> 08:55.982 a citysized ball with the density of an atomic  nucleus and the mass of a star. In that collapse, 08:55.982 --> 09:01.982 the magnetic field of the progenitor star is  massively amplified. The shrinking core also 09:01.982 --> 09:08.862 spins itself up, powering surface dynamos that can  boost the field to up to 100 billion Tesla, which 09:08.862 --> 09:15.182 is something like 20 times the critical field  strength. So more than enough to crystallize the 09:15.182 --> 09:21.502 surrounding vacuum and cause vacuum polarization  and breringence. And that's what we may just have 09:21.502 --> 09:27.262 seen in the recent Nature paper. Rachel Stewart  and collaborators argue that vacuum by referring 09:27.262 --> 09:37.422 is the best explanation for the extreme X-ray  polarization observed in magnetar IE1547.0-5408. 09:37.422 --> 09:43.102 The key data comes from the imaging X-ray  polarimetry explorer satellite which was 09:43.102 --> 09:49.022 launched in 2021 by NASA and the Italian space  agency with the primary purpose of measuring X-ray 09:49.022 --> 09:54.862 polarization. Now X-ray polarization measurements  are really hard due to the small number of X-ray 09:54.862 --> 10:00.302 photons and their extreme energies. IXP is  really our first precision X-ray polarimeter. 10:00.302 --> 10:05.182 So this sort of measurement is only recently  possible. The scientists though still needed 10:05.182 --> 10:10.622 help from NASA's neutron star interior composition  explorer for improving X-ray timing and spectral 10:10.622 --> 10:16.382 information and from the parks radio telescope  to constrain the magnetic geometry. Together, 10:16.382 --> 10:23.342 these new observations show extreme polarization  of soft X-rays, which suggests extreme vacuum by 10:23.342 --> 10:29.182 refringence, at least so the scientists argue.  Let's see how this works by following X-ray 10:29.182 --> 10:34.462 photons from the surface of the magnetar.  At temperatures of millions of degrees, 10:34.462 --> 10:39.822 the surface and atmosphere produce huge numbers  of X-rays. Normally, those X-rays would have to 10:39.822 --> 10:44.862 fight their way through the electrons in the stars  atmosphere. And remember, the electric field of 10:44.862 --> 10:51.502 photons pushes on charged particles. And electrons  can absorb or scatter light when they're able to 10:51.502 --> 10:57.582 move in the direction of that push. But these  electrons in the magnetized atmosphere are locked 10:57.582 --> 11:02.862 to an insanely strong magnetic field. They can  only move relatively easily in the same direction 11:02.862 --> 11:08.462 of that field. At right angles to the field, the  electrons are locked down, so they can't even 11:08.462 --> 11:15.582 interact with photons that try to push them in  that direction. X-rays polarized in that direction 11:15.582 --> 11:22.142 barely see the electrons and so they can escape  from deep within that thick blocking atmosphere. 11:22.142 --> 11:27.342 But this by itself isn't enough to give us  strongly polarized X-rays seen from Earth. The 11:27.342 --> 11:33.822 polarization directions of escaping light varies  across the magnetar surface. So from this distance 11:33.822 --> 11:38.942 all of those different directions should mix  together and partly cancel out. And this is where 11:38.942 --> 11:44.382 vacuum by refringence comes in. The polarized  X-rays escaping from the atmosphere still have 11:44.382 --> 11:50.062 to travel through hundreds or even thousands of  kilometers of intensely magnetized empty space. 11:50.062 --> 11:55.742 Vacuum by refringence makes different polarization  components of those X-rays accumulate phases at 11:55.742 --> 12:00.622 different rates which keeps each X-ray locked to a  preferred polarization direction set by the local 12:00.622 --> 12:06.302 magnetic field. As the magnetic field gradually  shifts into a large scale order dipole field, 12:06.302 --> 12:12.942 the polarizations of all X-rays follow and end up  pointing in the same direction. And that's roughly 12:12.942 --> 12:19.422 what saw in the 2 to 3 kilo electron volt X-ray  band. The polarization fraction reached as high 12:19.422 --> 12:25.422 as around 80% during certain parts of the stars  rotation. And that polarization fraction changed 12:25.422 --> 12:30.622 as the magnetar spun, which makes sense. At each  point in the rotation, we're seeing a different 12:30.622 --> 12:35.742 orientation of the stars magnetic field and a  somewhat different view of the X-ray emmitting 12:35.742 --> 12:42.142 region. The result is exciting enough to warrant  the Nature paper, but it's not a definitive 12:42.142 --> 12:47.742 detection of vacuum by refringence. Another study  published earlier this year argues that most of 12:47.742 --> 12:53.262 the X-rays could come from a single small hot spot  on the surface of the star. Under the geometry 12:53.262 --> 12:59.262 favored by that analysis, the photons start out  with fairly similar polarization directions. So, 12:59.262 --> 13:04.142 we don't need the surrounding magnetic field  to organize them nearly as much. But the latest 13:04.142 --> 13:09.742 study has an important counterargument. The new  parks radio observations suggest that we are 13:09.742 --> 13:15.502 looking almost straight down the rotational axis  of the magnetar with its magnetic axis also nearly 13:15.502 --> 13:20.942 aligned with that axis. And when the authors  model the X-ray emission using that geometry, 13:20.942 --> 13:28.222 a hotspot alone has a much harder time reproducing  what EXP sees. But if the models allow the 13:28.222 --> 13:33.502 polarization to keep following the magnetic  field through vacuum by refringence, then the 13:33.502 --> 13:39.182 observations are reproduced much more naturally.  There's still some tension here. The geometry 13:39.182 --> 13:45.902 inferred from the X-rays alone is only consistent  with the radiived geometry at around the 2.5 sigma 13:45.902 --> 13:51.182 level and the X-ray data allows a much wider  range of possibilities. So even the authors stop 13:51.182 --> 13:56.782 short of claiming a detection. Still remember that  vacuum by refringence isn't some speculative new 13:56.782 --> 14:01.742 phenomenon invented just to explain these data.  It's a firm prediction of quantum electronamics 14:01.742 --> 14:08.302 and magnetars have magnetic fields easily strong  enough for the effect to actually matter. So this 14:08.302 --> 14:15.182 observation is worth taking seriously as perhaps  our clearest glimpse of vacuum by refringence. The 14:15.182 --> 14:21.342 universe is weird and normally we need to build  incredibly precise and/or gigantic experiments to 14:21.342 --> 14:28.142 explore its weirdest properties. But the universe  is also awesome and sometimes it generously 14:28.142 --> 14:33.582 provides us with dead stars crushed to near black  hole densities with magnetic fields so strong that 14:33.582 --> 14:39.022 they grip the quantum vacuum itself. Perfect  for checking if we were right all along about 14:39.022 --> 14:45.182 magnetically crystallized spacetime. 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