The black holes keep spinning in the dark, their secrets intact.
At the edge of what light itself can escape, black holes spin at velocities that challenge our deepest theories of physics — yet for decades, two competing visions of how fast they can turn have remained equally plausible, equally unresolved. A new study from the University of Virginia confirms that even our most powerful existing instruments cannot distinguish between these theories, because the black hole's accretion disk looks the same at either extreme. The answer may lie not in what we can see today, but in a razor-thin ring of trapped light that only a telescope placed in orbit could resolve — a mission called BHEX, still waiting to be built.
- Two rival theories about the maximum spin rate of black holes — 99.8% versus 93.75% the speed of light — have stood unresolved since the 1970s and 2004, representing a fundamental gap in our understanding of how black holes shape galaxies.
- Researchers ran the most sophisticated simulations available, modeling plasma and magnetic fields in extreme gravity, only to find that the Event Horizon Telescope cannot tell the two theories apart — the images it produces are virtually identical at either spin rate.
- Hidden inside the famous orange glow of a black hole's accretion disk is a far sharper signal: the photon ring, a hair-thin circle of light bent around the event horizon, which encodes the black hole's spin — but detecting it requires sensitivity five times beyond any ground-based instrument.
- NASA's Black Hole Explorer mission proposes to break this deadlock by placing a radio telescope in orbit and linking it with existing ground arrays, creating an interferometer powerful enough to directly image the photon ring around Sagittarius A*.
- The resolution of a decades-old argument in fundamental physics now rests on a mission not yet launched, technology not yet proven in space, and a future that remains unwritten.
Black holes spin — some approaching the very speed of light — yet physicists have argued for decades about just how fast they can actually turn. A new study from Tegan Thomas at the University of Virginia delivers both a sobering finding and a cautious hope: our current instruments cannot settle the question, but a future space mission might.
The debate has two camps. In the 1970s, Kip Thorne proposed a maximum spin of 99.8% light speed, braked by photons radiating from the surrounding disk. In 2004, Charles Gammie and colleagues at the University of Illinois offered a lower ceiling — 93.75% — with magnetized jets acting as the restraining force. The gap between these numbers is not merely academic; it touches on how black holes interact with their environments and influence the galaxies around them.
Thomas and her team ran three-dimensional simulations using general relativistic magnetohydrodynamics — models tracking plasma and magnetic fields in extreme gravity — and generated synthetic images as the Event Horizon Telescope would see them. The verdict was deflating: at either theoretical maximum, the accretion disk, jets, and polarization patterns look essentially the same. The EHT simply cannot resolve the difference.
The key may lie in the photon ring — a razor-thin circle of light composed of photons bent around the event horizon and released back toward Earth, sitting inside the blurry orange glow that made black holes famous. Reading it requires sensitivity of around five microarcseconds, beyond any ground-based instrument's reach.
This is the purpose of BHEX, NASA's Black Hole Explorer: a radio telescope placed in Earth's orbit and linked with ground arrays like the Green Bank Telescope and ALMA, creating an interferometer capable of directly imaging the photon ring around Sagittarius A*, our galaxy's central black hole. The ring's precise shape encodes spin. Even if Sagittarius A* doesn't spin at the theoretical maximum, its photon ring could reveal what signatures to seek elsewhere.
The argument between Thorne and Gammie may be approaching its end — but only if BHEX launches, succeeds, and delivers on its promise. Until then, the black holes keep spinning in the dark.
Black holes spin. Not slowly, either—they rotate at speeds approaching the velocity of light itself. Yet for decades, physicists have argued about just how fast they can actually turn, and our best telescopes have been powerless to settle the question. A new paper from Tegan Thomas at the University of Virginia and her collaborators offers both a sobering diagnosis and a glimmer of hope: we cannot measure black hole spin with current instruments, but within the next few years, we might finally have the tool to do it.
The debate traces back to competing theories about the physical limits of black hole rotation. In the 1970s, Kip Thorne proposed that a black hole could spin up to 99.8 percent the speed of light, held in check only by photons radiating from the disk of material swirling around it—these photons essentially pushing back against the spin. Three decades later, in 2004, Charles Gammie and colleagues at the University of Illinois Urbana-Champaign offered a different ceiling: 93.75 percent light speed, with highly magnetized jets acting as a brake on the black hole's rotation. The difference is not trivial. It speaks to fundamental physics, to how black holes interact with their surroundings, and to how they shape the galaxies they inhabit.
The Event Horizon Telescope, that remarkable global network of radio dishes that produced the first direct photograph of a black hole roughly a decade ago, seemed like it might finally answer the question. But Thomas and her team ran sophisticated three-dimensional simulations—general relativistic magnetohydrodynamics models that track plasma and magnetic fields in extreme gravity—and then generated synthetic radio images as the EHT would see them. The results were disappointing. At either theoretical maximum spin rate, the black hole's accretion disk looks essentially identical. The jets are indistinguishable. The light curves, polarization patterns, everything the EHT can measure overlaps almost completely. The telescope simply lacks the resolution to tell them apart.
But there is another signature waiting to be read: the photon ring. Inside the fuzzy orange donut of plasma that made the black hole famous sits something far more delicate and far brighter—a razor-thin circle of light composed of photons that the black hole's gravity has trapped, bent around the event horizon at least once, and then released back toward Earth. Detecting it requires sensitivity on the order of five microarcseconds, a threshold no ground-based sensor can reach. This is where the Black Hole Explorer enters the picture.
BHEX is a NASA Small Explorer mission still in development, intended for launch sometime in the coming decade. The concept is elegant: place a radio telescope in Earth's orbit and link it with existing ground-based instruments like the Green Bank Telescope and the Atacama Large Millimeter Array. By extending the Event Horizon Telescope into space, BHEX would create an interferometer with enough resolving power to directly observe the photon ring around Sagittarius A*, the supermassive black hole at the center of our galaxy. The precise shape of that ring encodes information about the black hole's spin. Even if Sagittarius A* does not spin at the theoretical maximum, studying its photon ring could reveal what signatures to look for in other black holes that might.
The decades-old argument between Thorne and Gammie may finally be approaching resolution. But that resolution depends on a mission that has not yet launched, on technology that remains untested in orbit, on a future that is not yet certain. For now, the black holes keep spinning in the dark, their secrets intact.
Citas Notables
Kip Thorne theorized in the 1970s that black holes can spin up to 99.8 percent the speed of light, held back by photons from the accretion disk— Tegan Thomas and colleagues, via research findings
Charles Gammie and colleagues proposed in 2004 that the maximum spin is 93.75 percent light speed, with magnetized jets acting as a brake— University of Illinois Urbana-Champaign research
La Conversación del Hearth Otra perspectiva de la historia
Why does it matter how fast a black hole spins? What changes if it's 99.8 percent versus 93.75 percent?
The spin rate tells you how the black hole shapes everything around it—the jets it launches, the way it feeds, how it influences the galaxy. Different spin rates mean different physics at work. One theory says photons hold back the spin; the other says magnetic fields do. That's a fundamental disagreement about how nature works.
And we really can't tell the difference right now, even with the Event Horizon Telescope?
Not at all. The EHT is extraordinary—it's a planetary-scale instrument. But when you simulate what it would see at either spin rate, the images are identical. The accretion disk looks the same, the jets look the same. The telescope just doesn't have the resolution.
So what does BHEX do differently?
It puts a radio telescope in space and links it with ground-based dishes. That creates a much larger baseline, much finer resolution. Enough to see the photon ring—that thin circle of trapped light around the black hole. The ring's shape depends on spin.
How soon could this actually happen?
The mission is in development now, slated for the coming decade. So if all goes well, within a few years we might finally know. But space missions are complicated. Nothing is guaranteed.
And if BHEX works, does it settle the debate?
It should. At least for Sagittarius A*. And that would tell us which theory of black hole physics is correct—a question physicists have been arguing about for fifty years.