Illustration by Soner Ön
Astronomy’s Quest to Peer Into an Event Horizon
Of all cosmic phenomena, the most extreme hold a particular fascination, and black holes are the ultimate extreme. Fantastical, even mythological in stature, they are a vital part of all we see around us.
There is good reason to think that hundreds of billions, perhaps trillions, of black holes are scattered throughout the universe. Imagine that we could sense directly the curvature and distortion of space-time as if it were a simple three- dimensional landscape. We would find a universe of gently undulating hills, valleys and small indentations -- peppered with the sharpest little pinholes, so fathomless that the walls plunge out of view as we peer in -- holes that flood the cosmos with radiation and particles.
Why does our universe make black holes? The laws of physics show us, first, how gravity builds dense structures out of normal matter. In some cases such gatherings of matter get hot enough to ignite nuclear fusion: A star is born. Eventually a few of these objects, stockpiling mass and succumbing to gravity, reach a density that distorts space and time. They drop, sink, burrow and implode all the way out of what we consider to be normal existence, leaving behind fearsome trails, like unplugged drains into the underworld, from which even light cannot escape.
The energy that these black holes spew back into the cosmos affects almost everything we see, including our own quite special Milky Way galaxy. Breaking just one of the crisscrossing strands of cosmic history and energy that connect us to black holes could subvert the entire pathway to life here on Earth.
We have much still to learn about the workings of black holes and the reasons for their existence. The laws of relativity, quantum mechanics and even thermodynamics are at play, but it is their complex manifestations that will explain black holes’ actual influence on the character of galaxies, stars and the matter within them.
Astrophysicists have struggled to understand how matter descends toward black holes, moving against the outpouring of radiation. Looking inside these systems, we have seen strange pulsations of energy. The rhythmic patterns of outflowing photons betray the ongoing fight between matter and radiation.
Scientists have also tried to figure out whether there is a maximum size for black holes. As they grow, they may simply become so good at generating energy that they push away new incoming material, limiting their own size. It’s like trying to feed a roaring bonfire. The more fuel you manage to throw on, the farther you need to back away. Such an impasse might occur when a black hole reaches 10 billion times the mass of the sun, roughly the size of the largest holes yet scrutinized. Pushing yet another limit, some of the most massive black holes appear to spin at close to the maximum rate allowed by physics.
Some data and theories hint at opportunities for stars to be born within the disk of material accreting into a hole. What a strange and alien environment this might be for the birth of a stellar system. Could there be planets around stars made there? If so, we can only imagine what their night skies might be like.
Intriguing new evidence also suggests that some black holes have been flung from their parent galaxies. Ejected during the final stages of merger between black hole pairs, they race out into intergalactic space.
Astronomers have not yet managed to look at a black hole up close. Is that even possible? The cleverest techniques still offer an incomplete view, but one that that can at least tell us about black hole mass, spin and the nature of the surrounding disk. What we see is not an image but a collection of the energies of these battered photons.
Physicists and astronomers have been devising systems to detect gravity waves, emanating from pairs of neutron stars or black holes that come to orbit each other closely enough to merge in a spectacular crescendo. Experiments such as the Laser Interferometer Gravitational-Wave Observatory try to detect changes in the underlying fabric of space as light traverses it, changes that reflect the influence of a passing gravity wave. A planned space-based gravity-wave detector -- currently on hold because of budgetary constraints -- would hear the deep rumbles of supermassive black holes merging in distant galaxies.
The ultimate goal is to observe the event horizon, the final gateway from our familiar universe to the one that is lost to the tremendous gravity within a black hole. We think that the event horizon is effectively dark nothingness. Nevertheless, the space immediately outside it is aglow with the final gasps of matter. And some astronomers, led by NASA scientist Keith Gendreau, hope it might be possible to observe a black hole by making an image of the intense X-ray light flooding out.
The catch is that for even a supermassive black hole, that innermost disk is perhaps only a few light-days across. If you want to look at the event horizon of the 4 million-solar-mass black hole at the center of the Milky Way, thousands of light- years away, you need to be able to see with extraordinary resolution. It’s like taking a photo of a coin on the surface of the moon with a camera here on Earth.
Building a telescope to do this is a phenomenal challenge. The physical properties of light are such that it is diffracted as it passes into apertures and lenses. The smaller a telescope’s diameter, the blurrier the image. This is why astronomers love to build big telescopes -- to make crisper, sharper pictures. Creating an image of a distant event horizon is going to require an enormous telescope.
The small wavelength of X-ray photons presents an additional obstacle. The kind of telescopes that can handle skittish X-rays cannot be built very large. The solution is to make many small telescopes that work together. We could place dozens of them in space, forming an array many tens of miles across. Each one would gather X-ray photons and beam them to a single detector, which would form an image of incredible resolution.
One design for such a system calls for two dozen small X- ray mirrors called periscopes to hover in a mile-wide swarm. The detector would sit in space 12,000 miles away and house a sensitive digital camera. The detector would need to be positioned precisely, and braced against the forces of solar radiation and gravity from other planets. But these hurdles would not be insurmountable. Spacecraft engineering and technology have come a very long way. In fact, it is breathtaking to think that such an observatory is within our reach, should we choose to place resources into it. The Black Hole Imager, as the proposed system is now called, would let us see through the dust and gas cloaking the core of the Milky Way, to peer into the very workings of a black hole.
Scientists have already found matter’s starting point. In the mottled haze of cosmic-background photons and the faintest recesses of electromagnetic radiation, we see the imprints of the primordial cosmos. And in our great particle accelerators we are re-creating the conditions of the universe mere instants after the Big Bang. Now, staring into the twisted chasms the universe has made in itself, we see the same matter leaving us behind. For all intents and purposes it is sinking into, but also out of, this cosmos. With a final glimmer, the particles are passing across the event horizon and releasing themselves to eternity. Yet just before the end, they give up what energy they can, and it surges back to sculpt and color the universe.
(Caleb Scharf, an astrophysicist, is the director of Columbia University’s Astrobiology Center and author of the blog Life, Unbounded. This is the last of five excerpts from his new book, “Gravity’s Engines: How Bubble-Blowing Black Holes Rule Galaxies, Stars and Life in the Cosmos,” which will be published on Aug. 14 by Farrar, Straus and Giroux. The opinions expressed are his own. Read Part 1, Part 2, Part 3 and Part 4.)
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