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Thank you to Displate for supporting PBS. Nothing is complicated. By which I mean everything is complicated because in quantum physics, even the nothing of completely empty space has some pretty crazy properties. One prediction that's been around for a while is that under extreme conditions, the quantum vacuum can sort of crystallize. And there's good evidence that we've observed this for the first time. Hey everyone, before we get started, we wanted to let you know that we're experimenting with YouTube shorts starting next week. This is a signal of an expanding spaceime,
not a contracting spacetime. The shorts will be a mix of originals and clips from our library of over 400 episodes. We hope you watch and enjoy. A couple of quick announcements. First, we have some new data. Liking and commenting really does help get the episodes shared. So, you know, please do both of those things. And we've also learned that the number one reason that people support us on Patreon isn't actually the perks. It's simply to support the Spacetime community and the work that we do. So, for those of you who do support, thank you so much. But don't get me
wrong, there are perks and they're good. There's a link in the description if you'd like to join. This really would be a huge help. Next up, they say the mind once stretched by a new idea never regains its original dimensions. Which means after watching enough spaceime, your brain is apparently ruined forever. So celebrate the good damage with our new space-time shirt and hoodie featuring a psychedelic astronaut and one of our favorite descriptions of what learning actually does to you. Links to the merch store in the description. you, yourselves, your atoms, and ultimately your
subatomic particles come from excitations in the quantum fields that fill all of space. But those quantum fields exist without you or your subatomic particles. They exist in empty space. And this busy quantum vacuum does stuff. It interacts with actual particles, subtly changing things like magnetic moment or atomic energy levels. Between the plates of the Casemir experiment, we can coax the quantum vacuum into negative energies. And at extreme positive energies, like at the collision point of a particle collider, the empty fields of the quantum vacuum can be excited to produce
all sorts of exotic particles. The mathematics of quantum field theory describes these quantum phenomena with incredible precision. But how far can we really push this? Can we really think of the vacuum as this richly structured thing beyond the microscopic scope of our experiments? Well, there is one phenomenon that may reveal this structure over hundreds or even thousands of kilome. An early prediction of quantum field theory is that an extreme magnetic field may lock empty space into a grainy alignment almost like crystallizing the vacuum itself. A real crystal
is a solid whose atoms are locked into an ordered repeating arrangement. But empty space is never that. However, the crystal and the magnetized vacuum do share one exotic property. They change how light travels through them, sort of locking photons into their grainy structure in a phenomenon called breringent. We routinely observe breringents in crystals. Maybe you remember that episode we did with Mither from looking glass universe. She used crystal breringent to do a quantum eraser experiment. But to observe vacuum by refringence, we need magnetic fields billions
of times stronger than anything we've been able to create on Earth. Impossible for us, but not impossible for the universe. And in a paper published in Nature this August, scientists claim evidence that a magnetar has crystallized the empty space around it. To understand all of this madness, we're going to need to start much ser much more familiar with good old refractive index. Light slows down in glass, in water, in any transparent medium. The refractive index tells us how much this phenomenon is due to the photons's electromagnetic wave interacting with the electric
charges, mostly the electrons in the medium. I'll give you a slightly cartoonish picture. A photon in a transparent medium encounters an electron in an atom. The photon is an electromagnetic wave. So it causes the charged particle to wiggle a bit. We can imagine the photon being momentarily absorbed and then remitted by the electron which slows it down. Now this is an oversimplification really. We have a global em field of all the wiggling electrons inducing phase velocity shifts but that's harder to animate. The change in speed depends on a number of things including
the wavelength of the light. And this is why refraction allows us to separate wavelengths of light into a spectrum when passed through a prism. In certain materials, however, the refractive index also depends on polarization. Now, polarization is just the direction that a photon's electric field points. That polarization direction determines the wiggle direction of the electrons that the photon encounters. Now, the more the electron is able to wiggle, the more it will slow down or even stop an incoming photon. In most media, electrons are able to wiggle the
same in all directions. And so, polarization of the incoming light doesn't matter. But that's not always the case. In a crystal, for example, the atoms are in a fixed grid. the electrons may have more freedom to vibrate in some directions over others. So if a photon's polarization direction, in particular its electric field, happens to be aligned with the directions where the electrons have more wiggle capacity, then there'll be more of this absorption reission, and that will slow the photon down more. So photons with different polarization directions traveling the
same direction through a crystal will experience different refractive indexes and so different speeds. This is what breringences. It's pretty intuitive to see how we get this behavior in a crystal because we have real electrons whose behavior we can talk about admittedly in highly simplified ways. But it turns out that even the vacuum of space can be forced into by refringence. So empty space is anything but. It's filled with quantum fields in their lowest energy state. That includes the electron field, a ubiquitous layer of the possibility of electrons everywhere in space.
And a real electron is a persistent jiggle of energy in that field. Dump energy into the field and you can excite an electron posetron pair. You could do that with say a gamay photon as long as it had enough energy to cover the masses of the particles via E= MC^². But short of that energy, a photon will normally ignore the electron field. However, there are other ways to get the photon and the electron fields to notice each other. One of those is vacuum by refringence. I'm going to describe this in two ways. One more intuitive but less accurate then a bit more rigorously.
You may have heard of virtual particles. Even without sufficient energy to provide the particles their mass, we can imagine the quantum vacuum producing matter antimatter pairs extremely briefly with their energy borrowed from the intrinsic uncertainty of the energy of the vacuum. These virtual particles aren't like some cloud of electrons and posetrons everywhere in space. Rather, they're a way of representing the infinite ways that quantum fields can interact with real particles. Virtual particles appear in the mathematics of quantum field theories, most
notably in fineman diagrams. But most physicists consider them a mathematical artifact, albeit a very useful one, and useful for intuition, too. In a strong magnetic field, we can imagine that the flickering virtual electron posetron pairs are no longer randomly oriented. They're generated in a preferred direction, and that alignment mimics the alignment of the real charged particles in a crystal. And the result is vacuum polarization which leads to a polarization dependent refractive index and so do vacuum refringence in the passing photons. A more rigorous but less visually fun
way to say this is that an extreme magnetic field produces nonlinear corrections to electromagnetism through interaction of the electromagnetic field with the quantum electron field. These corrections alter the propagation of photons with the size of the effect depending on the photon's direction and polarization relative to the magnetic field. But yeah, it's more fun to say that space gets crystallized, even if that's not strictly true. But the resulting effect is very similar to crystals. Interestingly, this vacuum refractive index doesn't actually slow the wave.
The photons still travel at full light speed, but the phase of the wave picks up a lag as though it were slowed. Two photons with different polarizations will keep up with each other, but they'll fall out of phase with each other. The magnetic field strength where vacuum polarization gets strong is around 4.4 billion Tesla. This is called the quantum critical magnetic field. And that's not to say that vacuum by referring can't be seen below that field strength, just that the effect drops off very quickly at lower field strengths. Now, the strongest
magnetic field that we humans have ever made is 1,200 Tesla. And by refringence experiments are typically way lower than this. The effect is essentially unobservable in humanmade experiments, at least so far. But there are places in the universe where natural magnetic fields don't just approach the critical field strength, they far exceed it. By far the king of magnetic fields is the magnetar. These are a type of neutron star, the collapsed core of a massive star that's not quite massive enough to become a black hole. In its collapse, protons and neutrons are smooshed together to form neutrons, and the whole thing shrinks into
a citysized ball with the density of an atomic nucleus and the mass of a star. In that collapse, the magnetic field of the progenitor star is massively amplified. The shrinking core also spins itself up, powering surface dynamos that can boost the field to up to 100 billion Tesla, which is something like 20 times the critical field strength. So more than enough to crystallize the surrounding vacuum and cause vacuum polarization and breringence. And that's what we may just have seen in the recent Nature paper. Rachel Stewart and collaborators argue that vacuum by referring
is the best explanation for the extreme X-ray polarization observed in magnetar IE1547.0-5408. The key data comes from the imaging X-ray polarimetry explorer satellite which was launched in 2021 by NASA and the Italian space agency with the primary purpose of measuring X-ray polarization. Now X-ray polarization measurements are really hard due to the small number of X-ray photons and their extreme energies. IXP is really our first precision X-ray polarimeter. So this sort of measurement is only recently possible. The scientists though still needed
help from NASA's neutron star interior composition explorer for improving X-ray timing and spectral information and from the parks radio telescope to constrain the magnetic geometry. Together, these new observations show extreme polarization of soft X-rays, which suggests extreme vacuum by refringence, at least so the scientists argue. Let's see how this works by following X-ray photons from the surface of the magnetar. At temperatures of millions of degrees, the surface and atmosphere produce huge numbers of X-rays. Normally, those X-rays would have to
fight their way through the electrons in the stars atmosphere. And remember, the electric field of photons pushes on charged particles. And electrons can absorb or scatter light when they're able to move in the direction of that push. But these electrons in the magnetized atmosphere are locked to an insanely strong magnetic field. They can only move relatively easily in the same direction of that field. At right angles to the field, the electrons are locked down, so they can't even interact with photons that try to push them in that direction. X-rays polarized in that direction
barely see the electrons and so they can escape from deep within that thick blocking atmosphere. But this by itself isn't enough to give us strongly polarized X-rays seen from Earth. The polarization directions of escaping light varies across the magnetar surface. So from this distance all of those different directions should mix together and partly cancel out. And this is where vacuum by refringence comes in. The polarized X-rays escaping from the atmosphere still have to travel through hundreds or even thousands of kilometers of intensely magnetized empty space.
Vacuum by refringence makes different polarization components of those X-rays accumulate phases at different rates which keeps each X-ray locked to a preferred polarization direction set by the local magnetic field. As the magnetic field gradually shifts into a large scale order dipole field, the polarizations of all X-rays follow and end up pointing in the same direction. And that's roughly what saw in the 2 to 3 kilo electron volt X-ray band. The polarization fraction reached as high as around 80% during certain parts of the stars rotation. And that polarization fraction changed
as the magnetar spun, which makes sense. At each point in the rotation, we're seeing a different orientation of the stars magnetic field and a somewhat different view of the X-ray emmitting region. The result is exciting enough to warrant the Nature paper, but it's not a definitive detection of vacuum by refringence. Another study published earlier this year argues that most of the X-rays could come from a single small hot spot on the surface of the star. Under the geometry favored by that analysis, the photons start out with fairly similar polarization directions. So,
we don't need the surrounding magnetic field to organize them nearly as much. But the latest study has an important counterargument. The new parks radio observations suggest that we are looking almost straight down the rotational axis of the magnetar with its magnetic axis also nearly aligned with that axis. And when the authors model the X-ray emission using that geometry, a hotspot alone has a much harder time reproducing what EXP sees. But if the models allow the polarization to keep following the magnetic field through vacuum by refringence, then the
observations are reproduced much more naturally. There's still some tension here. The geometry inferred from the X-rays alone is only consistent with the radiived geometry at around the 2.5 sigma level and the X-ray data allows a much wider range of possibilities. So even the authors stop short of claiming a detection. Still remember that vacuum by refringence isn't some speculative new phenomenon invented just to explain these data. It's a firm prediction of quantum electronamics and magnetars have magnetic fields easily strong enough for the effect to actually matter. So this
observation is worth taking seriously as perhaps our clearest glimpse of vacuum by refringence. The universe is weird and normally we need to build incredibly precise and/or gigantic experiments to explore its weirdest properties. But the universe is also awesome and sometimes it generously provides us with dead stars crushed to near black hole densities with magnetic fields so strong that they grip the quantum vacuum itself. Perfect for checking if we were right all along about magnetically crystallized spacetime. Thank you to Displate for supporting PBS. Displate posters
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