The Universe May Have Just Revealed the True Nature of Empty Space

Thank you to Displayed 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 space time, not a contracting space time. 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. 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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 moments or atomic energy levels. Between the plates of the Casimir 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 kilometers. 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 birefringence. We routinely observe birefringence in crystals. Maybe you remember that episode we did with Mithunur from Lookingglass Universe. She used crystal birefringence to do a quantum eraser experiment. But to observe vacuum birefringence, 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 saner. 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 photon'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 re-emitted 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. 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 re-emission, 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 birefringence is. 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 birefringence. 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-positron pair. You could do that with, say, a gamma ray photon, as long as it had enough energy to cover the masses of the particles via E equals mc squared. 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 birefringence. 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 could 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 positrons 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 Feynman 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-positron 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 to vacuum birefringence 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 birefringence 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 1200 tesla, and birefringence experiments are typically way lower than this. The effect is essentially unobservable in human-made 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 city-sized 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 birefringence. And that's what we may just have seen. In the recent Nature paper, Rachel Stewart and collaborators argue that vacuum birefringence is the best explanation for the extreme X-ray polarization observed in magnetar, i.e. 1547.0-5408. The key data comes from the Imaging X-ray Polarimetry Explorer Satellite, IXPE, 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. IXPE 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 Parkes radio telescope to constrain the magnetic geometry. Together, these new observations show extreme polarization of soft X-rays, which suggests extreme vacuum birefringence, 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 star's 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 magnetar's 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 a strongly polarized X-ray seen from Earth. The polarization directions of escaping light varies across the magnetar's surface, so from this distance all those different directions should mix together and partly cancel out. And this is where vacuum birefringence 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 birefringence 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 ordered dipole field, the polarizations of all X-rays follow and end up pointing in the same direction. And that's roughly what IXPE saw. In the 2-3 kV X-ray band, the polarization fraction reached as high as around 80% during certain parts of the star's 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 star's magnetic field and a somewhat different view of the X-ray emitting region. The result is exciting enough to warrant the nature paper, but it's not a definitive detection of vacuum birefringence. Another study published earlier this year argues that most of the X-rays could come from a single small hotspot 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 counter-argument. The new Parkes 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 IXPE sees. But if the models allow the polarization to keep following the magnetic field through vacuum birefringence, 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 radio-derived geometry at around the 2.5 sigma level, and the X-ray data allows a much wider range of possibilities. So even the authors stopped short of claiming a detection. Still, remember that vacuum birefringence isn't some speculative new phenomenon invented just to explain these data. It's a firm prediction of quantum electrodynamics, 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 birefringence. 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 are vibrant, high-resolution metal prints, including thousands of images of space and science. And if you're inspired by space exploration, whether real or imagined, Displate has real images from NASA, as well as space exploration inspired designs. 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