It’s been over 100 years since the first solution for a black hole was discovered in General Relativity. For generations, scientists argued over whether these objects were physical, existing all throughout our Universe, or whether they were mere mathematical artifacts. In the 1960s, Roger Penrose’s Nobel-winning work demonstrated how black holes could realistically form in our Universe, and shortly thereafter, the first black hole — Cygnus X-1 — was discovered.
Black holes are now known to range from just a few times the mass of our Sun up to many billions of solar masses, with most galaxies housing supermassive black holes at their centers. In 2017, a tremendous observing campaign was coordinated between a large number of radio telescopes around the world in an attempt to directly image a black hole’s event horizon for the first time. That first image was released in 2019, revealing a donut-like shape surrounding the interior void. Now, a new series of papers has improved upon that image, and we can see it’s not a donut, but rather a cruller, with sweeping magnetic “lines” tracing out the hot plasma. Here’s the new science behind this epic image, and why black holes are crullers, not donuts.
This animation shows the event horizon, singularity, and other features for rotating black holes. In … [+] the vicinity of a black hole, space flows like either a moving walkway or a waterfall, depending on how you want to visualize it. At the event horizon, even if you ran (or swam) at the speed of light, there would be no overcoming the flow of spacetime, which drags you into the singularity at the center. Outside the event horizon, though, other forces (like electromagnetism) can frequently overcome the pull of gravity, causing even infalling matter to escape.
Andrew Hamilton / JILA / University of Colorado
In our Universe, black holes aren’t merely clumps of mass that have collapsed under their own gravity to a single point. In space, all forms of matter exert gravitational forces on one another, and whenever objects interact in this way, they attract the “closer” parts of the object by a greater amount than the “farther” parts of it. This type of force — known as a tidal force — isn’t just responsible for tides, but also for causing a torque: a change in the angular momentum of an object. As a result, everything that exists in the Universe rotates, or spins, rather than remaining stationary.
This means that the black holes that we form aren’t stationary and non-rotating, but rather spin about some axis. Indirect measurements had previously indicated that black holes spin relativistically: close to the speed of light. However, the major idea of the Event Horizon Telescope is that, regardless of how this spinning black hole is oriented, there will be light emitted from the surrounding matter that just “grazes” the event horizon, and goes off in a straight line, creating a photon ring for us to observe that encircles the dark center, from where no light can escape. (For reasons related to the curvature of space, the size of this dark center is actually more like ~250% the diameter of the physical event horizon.)
This artist’s impression depicts the paths of photons in the vicinity of a black hole. The … [+] gravitational bending and capture of light by the event horizon is the cause of the shadow captured by the Event Horizon Telescope. The photons that aren’t captured create a characteristic sphere, and that helps us confirm General Relativity’s validity in this newly-tested regime.
Nicolle R. Fuller/NSF
The way we went about imaging this was a tremendous technological achievement. We needed to take an array of radio images (at millimeter-submillimeter wavelengths) from all around the globe at once. This gave us the light-gathering power of all the telescopes that were part of the array, combined, but gave us the resolution of the maximum separation between the various telescopes, which was roughly the diameter of Earth.
In order to see anything, then, we had to look for black holes that were simultaneously very large, with a large angular diameter as seen from our perspective on Earth, and were also active: emitting copious amounts of radiation at radio wavelengths. There are only two that fit the bill:
Sagittarius A*, the four million solar mass black hole at the center of our galaxy, just ~27,000 light-years away.
And the black hole at the center of the massive elliptical galaxy M87, which comes in at 6.5 billion solar masses (some 1500 times the mass of Sagittarius A*), but some 50-60 million light-years distant (about 2000 times as far).
In April of 2019, after two years of analysis, the first images were released: a map of the radio light that traced out the emitted photons from around the black hole in the distant galaxy M87.
The Event Horizon Telescope’s first released image achieved resolutions of 22.5 microarcseconds, … [+] enabling the array to resolve the event horizon of the black hole at the center of M87. A single-dish telescope would have to be 12,000 km in diameter to achieve this same sharpness. Note the differing appearances between the April 5/6 images and the April 10/11 images, which show that the features around the black hole are changing over time. This helps demonstrate the importance of syncing the different observations, rather than just time-averaging them.
Event Horizon Telescope Collaboration
Even though this is usually depicted as a single image — where the four images from the four different days are added and averaged together — it’s important to recognize what’s actually happening here. Light from a very distant source is striking our telescopes at many different locations on Earth. In order to make sure we’re adding the data from the same exact times together, we have to sync up the various observatories with atomic clocks, and then account for the light-travel time to each unique point on Earth’s surface. In other words, we need to make sure that the telescopes are properly synchronized: a tremendously difficult task.
The reason we have an image of the black hole at the center of M87 and not one of the black hole in our own galaxy’s center is because of its remarkable size. At 6.5 billion solar masses, its diameter is approximately one light-day, meaning that the features in the photon ring take about ~1 day to change appreciably. At just 0.15% of that black hole’s mass, our black hole’s features change by that same amount every single minute, making the image much more difficult to construct.
However, while the Event Horizon Telescope team is still working on our black hole’s first image, the one at the center of M87 has just gotten a far more detailed image thanks to a special set of measurements that were also taken: polarization measurements.
Light is nothing more than an electromagnetic wave, with in-phase oscillating electric and magnetic … [+] fields perpendicular to the direction of light’s propagation. The shorter the wavelength, the more energetic the photon, but the more susceptible it is to changes in the speed of light through a medium. The direction of the electric and magnetic fields defines light’s polarization.
And1mu / Wikimedia Commons
Whether you view them in a quantum way (as photons) or in a classical way (as waves), the phenomenon of light behaves with intrinsic electromagnetic properties. As an electromagnetic wave, light is made of oscillating, in-phase, mutually perpendicular electric and magnetic fields. Whenever light either, passes through a magnetized plasma or reflects off of a material, it can become partially or completely polarized: where instead of having the electric and magnetic fields oriented randomly, they’re preferentially oriented in a particular direction.
Around pulsars — radio-emitting neutron stars with very strong magnetic fields — light can be almost 100% polarized. We’d never measured the polarization of photons from around a black hole before, but in addition to simply measuring the flux and density of photons, the Event Horizon Telescope also measured the information required to reconstruct the polarization data for the black hole at the center of M87.
Just as we were able to reconstruct images of the black hole’s photon ring that evolved with time, so too could we reconstruct polarization data on that individual, day-by-day basis.
These lines trace out the polarization of the hot plasma surrounding M87’s black hole. The … [+] polarization is strongest along the southern and western limbs of the black hole, and clearly migrate over time. Only about 15% of the light is polarized, which is significant but not as large as it is for other extreme objects, such as pulsars.
EHT Collaboration, ApJL, Vol. 910, L13, 24 March 2021
The polarization data is completely complementary to the direct light received, as it gives information that’s independent of the shape and density of the light emitted from around the black hole. Instead, polarization data is useful for teaching us about the matter that surrounds the black hole, including what the strength of the magnetic field is in that region, the number density of free electrons, the temperature of that hot plasma, and how much mass the black hole is consuming over time.
What we learn is fascinating, and perhaps not what many expected.
The magnetic field strength in the vicinity of the black hole is between 1 and 30 Gauss, where ~1 Gauss is the strength of Earth’s magnetic field at the surface. Compared to neutron stars, where fields can reach more than 1015 Gauss, this is minuscule, but on much larger scales.
There are between ten thousand and ten million free electrons in every cubic centimeter around this black hole.
The temperature of the plasma that has accreted around this black hole is enormous: between 10 and 120 billion K, or more than 1000 times the temperature at the center of the Sun.
And finally, this black hole is consuming mass at a rate that’s between 100 and 700 Earth masses every single year.
Still, as exciting as this is, the greatest sight of all was the new image of the radiation around the black hole, with the magnetic fields (traced out by the polarization data) included.
Polarized view of the black hole in M87. The lines mark the orientation of polarization, which is … [+] related to the magnetic field around the shadow of the black hole. Note how much swirlier this image appears than the original, which was more blob-like.
The first thing you’ll notice — and you might even worry about it — is that these swirling features appear so much sharper than the original image, which looked more like a blurry ring than anything else. Why would this polarization data, which was taken with the same instruments as the regular light data, have such a high resolution?
The answer is: surprisingly, it doesn’t. The polarization data has the same resolution as the regular data, meaning it can resolve features down to about ~20 micro-arc-seconds. There are 360 degrees in a full circle, 60 arc-minutes in each degree, 60 arc-seconds in each arc-minute, and one million micro-arc-seconds in each arc-second. If you were able to view the Apollo mission manual that was left on the Moon from Earth, 20 micro-arc-seconds would span roughly the “Ap” from the word Apollo.
What the polarization data tells us, however, is how much the light twists and in which direction, which basically traces out the magnetic field around the black hole. Just as we see the light and the polarization data evolving over time, we can put those results together, and determine how the photon ring around the black hole’s event horizon has changed and evolved during the course of our observations.
This 8-panel image shows the inferred polarization (top) and the reconstructed photons (bottom) for … [+] the black hole at the center of the galaxy M87. Note how the polarization evolves over time, and how, along with the light data, the structure of the photon ring (or photon sphere, if you prefer) changes over the period of the observations.
EHT Collaboration, ApJL, Vol. 910, L12, 24 March 2021
One of the big surprises is how small the photon polarization is. If you have a magnetized plasma surrounding this black hole — and we’re pretty certain that we do — you’d naively expect that the light would arrive almost completely polarized: with polarization fractions of 80-90% or even more. And yet, what we see is that the polarization fraction is tiny: about ~15-20% at its peak, with the actual value being even smaller in most locations.
Why would this be the case?
Unlike pulsars, where the magnetic field can be coherent on scales comparable to the size of the neutron star (about ~10 kilometers), this black hole is absolutely enormous. At about 1 light-day in diameter (about 0.003 light-years) for the black hole, there’s almost certainly a complicated magnetic structure on smaller scales than that. When light passes through a magnetic field, its polarization direction rotates, and rotates proportionally to the strength of the field. (This is known as Faraday rotation.)
However, if that magnetic field is non-uniform, the rotating polarization should “scramble” the signal, reducing its magnitude significantly. If we want to accurately map out the magnetic field, we’d need to leave Earth: building a similar telescope array that was larger than the diameter of our planet.
This composite image shows three views of the central region of the Messier 87 (M87) galaxy in … [+] polarised light, namely, from top to bottom, with the Chile-based 5 Atacama Large Millimeter/submillimeter Array (ALMA), the National Radio Astronomy Observatory’s Very Long Baseline Array (VLBA) in the US, and with the Earth-sized telescope synthesized by the Event Horizon Telescope.
EHT Collaboration; ALMA (ESO/NAOJ/NRAO), Goddi et al.; VLBA (NRAO), Kravchenko et al.; J.C. Algaba, I. Martí-Vidal
Still, none of this should diminish just how remarkable an achievement this is. By combining the effects of the light we directly observed with the polarization data, we can more accurately map out the behavior of the light emitted from this supermassive black hole: quite possibly the most massive supermassive black hole within ~100 million light-years of Earth.
When the data from the black hole at our own galaxy’s center is finally put together properly, we should have an incredibly interesting comparison to make. Right now, there are a slew of open questions, including:
will the same parts of the black hole remain “bright” and “dark” over time, or will the accretion flows migrate to all directions in space?
how large is the magnetic substructure around the black hole compared to the event horizon, and is it consistent between supermassive and ultra-mega-supermassive black holes?
will we observe a larger polarization fraction for smaller mass black holes, and will that teach us anything about Faraday rotation?
will there be comparable temperatures, magnetic field strengths, and electron densities between these two black holes, or will they be different?
Perhaps most importantly, will our theoretical calculations, borne out through simulations that incorporate all the relevant physics, match the reconstructed data to the extraordinary degree that they aligned for the black hole at the center of M87?
The April 11, 2017 reconstructed image (left) and a modeled EHT image (right) line up remarkably … [+] well. This is an excellent indication that the model library the Event Horizon Telescope (EHT) collaboration put together can, in fact, model the physics of the matter surrounding these supermassive, rotating, plasma-rich black holes quite successfully.
Huib Jan van Langevelde (EHT Director) on behalf of the EHT Collaboration
Just a few years ago, we didn’t even know whether it was a certainty that black holes had an event horizon, as we’d never observed one directly. In 2017, a series of observations were finally taken that could settle the issue. After a wait of two years, the first direct image of a black hole was released, and it showed us that the event horizon was, in fact, real as predicted, and that its properties agreed with Einstein’s predictions.
Now, another two years later, the polarization data has been added into the fold, and we can now reconstruct the magnetic properties of the plasma surrounding the black hole, along with how those features are imprinted onto the emitted photons. We still only have the one black hole that’s been directly imaged, but we can see how the light, the polarization, and the magnetic properties of the plasma surrounding the event horizon all change over time.
From over 50 million light-years away, we’re finally beginning to understand how the most massive, active black holes in the Universe work: powered by over 100 Earth masses per year and driven by the combination of Einstein’s gravity and electromagnetism. With a little bit of luck, we’ll have a second black hole that’s very different to compare it to in only a few months.
NAIROBI, Kenya (AP) — The body of Ugandan Olympic athlete Rebecca Cheptegei — who died after being set on fire by her partner in Kenya — was received Friday by family and anti-femicide crusaders, ahead of her burial a day later.
Cheptegei’s family met with dozens of activists Friday who had marched to the Moi Teaching and Referral Hospital’s morgue in the western city of Eldoret while chanting anti-femicide slogans.
She is the fourth female athlete to have been killed by her partner in Kenya in yet another case of gender-based violence in recent years.
Viola Cheptoo, the founder of Tirop Angels – an organization that was formed in honor of athlete Agnes Tirop, who was stabbed to death in 2021, said stakeholders need to ensure this is the last death of an athlete due to gender-based violence.
“We are here to say that enough is enough, we are tired of burying our sisters due to GBV,” she said.
It was a somber mood at the morgue as athletes and family members viewed Cheptegei’s body which sustained 80% of burns after she was doused with gasoline by her partner Dickson Ndiema. Ndiema sustained 30% burns on his body and later succumbed.
Ndiema and Cheptegei were said to have quarreled over a piece of land that the athlete bought in Kenya, according to a report filed by the local chief.
Cheptegei competed in the women’s marathon at the Paris Olympics less than a month before the attack. She finished in 44th place.
Cheptegei’s father, Joseph, said that the body will make a brief stop at their home in the Endebess area before proceeding to Bukwo in eastern Uganda for a night vigil and burial on Saturday.
“We are in the final part of giving my daughter the last respect,” a visibly distraught Joseph said.
He told reporters last week that Ndiema was stalking and threatening Cheptegei and the family had informed police.
Four in 10 women or an estimated 41% of dating or married Kenyan women have experienced physical or sexual violence perpetrated by their current or most recent partner, according to the Kenya Demographic and Health Survey 2022.
TEL AVIV, Israel (AP) — A rare Bronze-Era jar accidentally smashed by a 4-year-old visiting a museum was back on display Wednesday after restoration experts were able to carefully piece the artifact back together.
Last month, a family from northern Israel was visiting the museum when their youngest son tipped over the jar, which smashed into pieces.
Alex Geller, the boy’s father, said his son — the youngest of three — is exceptionally curious, and that the moment he heard the crash, “please let that not be my child” was the first thought that raced through his head.
The jar has been on display at the Hecht Museum in Haifa for 35 years. It was one of the only containers of its size and from that period still complete when it was discovered.
The Bronze Age jar is one of many artifacts exhibited out in the open, part of the Hecht Museum’s vision of letting visitors explore history without glass barriers, said Inbal Rivlin, the director of the museum, which is associated with Haifa University in northern Israel.
It was likely used to hold wine or oil, and dates back to between 2200 and 1500 B.C.
Rivlin and the museum decided to turn the moment, which captured international attention, into a teaching moment, inviting the Geller family back for a special visit and hands-on activity to illustrate the restoration process.
Rivlin added that the incident provided a welcome distraction from the ongoing war in Gaza. “Well, he’s just a kid. So I think that somehow it touches the heart of the people in Israel and around the world,“ said Rivlin.
Roee Shafir, a restoration expert at the museum, said the repairs would be fairly simple, as the pieces were from a single, complete jar. Archaeologists often face the more daunting task of sifting through piles of shards from multiple objects and trying to piece them together.
Experts used 3D technology, hi-resolution videos, and special glue to painstakingly reconstruct the large jar.
Less than two weeks after it broke, the jar went back on display at the museum. The gluing process left small hairline cracks, and a few pieces are missing, but the jar’s impressive size remains.
The only noticeable difference in the exhibit was a new sign reading “please don’t touch.”
VICTORIA – The British Columbia government is partnering with a bear welfare group to reduce the number of bears being euthanized in the province.
Nicholas Scapillati, executive director of Grizzly Bear Foundation, said Monday that it comes after months-long discussions with the province on how to protect bears, with the goal to give the animals a “better and second chance at life in the wild.”
Scapillati said what’s exciting about the project is that the government is open to working with outside experts and the public.
“So, they’ll be working through Indigenous knowledge and scientific understanding, bringing in the latest techniques and training expertise from leading experts,” he said in an interview.
B.C. government data show conservation officers destroyed 603 black bears and 23 grizzly bears in 2023, while 154 black bears were killed by officers in the first six months of this year.
Scapillati said the group will publish a report with recommendations by next spring, while an independent oversight committee will be set up to review all bear encounters with conservation officers to provide advice to the government.
Environment Minister George Heyman said in a statement that they are looking for new ways to ensure conservation officers “have the trust of the communities they serve,” and the panel will make recommendations to enhance officer training and improve policies.
Lesley Fox, with the wildlife protection group The Fur-Bearers, said they’ve been calling for such a committee for decades.
“This move demonstrates the government is listening,” said Fox. “I suspect, because of the impending election, their listening skills are potentially a little sharper than they normally are.”
Fox said the partnership came from “a place of long frustration” as provincial conservation officers kill more than 500 black bears every year on average, and the public is “no longer tolerating this kind of approach.”
“I think that the conservation officer service and the B.C. government are aware they need to change, and certainly the public has been asking for it,” said Fox.
Fox said there’s a lot of optimism about the new partnership, but, as with any government, there will likely be a lot of red tape to get through.
“I think speed is going to be important, whether or not the committee has the ability to make change and make change relatively quickly without having to study an issue to death, ” said Fox.
This report by The Canadian Press was first published Sept. 9, 2024.
