Black holes are cosmic vacuum cleaners – massive objects so big that not even light can escape them.
Most people imagine black holes do nothing but sit and devour stray bits of gas or dust.
But could black holes actually have more interesting inner lives? Could they, for example, explode? If an “explosion” is “a sudden, brief release of enormous amounts of energy”, then the answer is unequivocally yes. And the best part is that they can explode in several interesting ways, either by exploding themselves or their nearby surroundings.
There is a way black holes can explode. The process behind this has to do with the fact that black holes are not entirely black, which was discovered by famous astrophysicist Stephen Hawking in 1976.
“In classical physics, nothing can get out of the hole,” Ohio State University physicist Samir Mathur told Live Science in an email. “But Hawking discovered that with Quantum mechanics, the hole slowly loses its energy to infinity by emitting low-energy radiation; this is called Hawking radiation.”
Related: Are black holes wormholes?
Until a black hole sucks in new material, it will slowly lose mass by emitting Hawking radiation. However, Hawking radiation is emitted slowly. A normal black hole with a mass a few times that of the sun emits about one photon, or packet of light, every year. At this rate, the typical black hole would take 10^100 years to completely evaporate.
But Hawking realized that small black holes evaporate much faster. As a black hole gets smaller and smaller, it emits more and more radiation. In the last moments of its life, the black hole emits so much radiation, so rapidly that it effectively acts like a bomb, releasing a torrent of radiation and high-energy particles.
If small black holes (about the size of Earth) formed in the extremely early universe, they would take a few billion years to evaporate, which means that these “primordial” black holes, if they exist, would currently be exploding throughout the universe.
To date, astronomers have found no evidence of primordial black holes exploding, but they could be out there.
Black holes explode with a different type of explosion not found anywhere else in the universe, thanks to the fact that they spin. Rotating black holes – also named Kerr black holes after New Zealand mathematician Roy Kerr, who was the first to figure out how they work – create an ergosphere around their event horizons. An ergosphere is an elongated region of space where nothing can stand still. Anything that falls toward the spinning black hole begins to orbit around it as the particle enters the ergosphere.
The rotation space-time around a black hole can also attract photons. If there are enough photons, they can bounce off each other or any stray particle. Sometimes the rebound knocks the photons out of the ergosphere. But other times, the bounce causes the photons to fall deeper toward the black hole, where they gain energy. They can then disperse again to a higher orbit and then fall back down.
With each repetition of the process and each trip around the black hole, the photon gains energy. This process is called “superradiance”. If the photon eventually breaks free, it will have a huge amount of energy compared to when it started its journey.
If enough photons participate in the process, they can all burst at once with incredible energy, becoming what is called a “black hole bomb”. Even if the black hole itself does not explode, this superradiant effect once again shows how black holes can affect their surroundings.
Discs and jets
The most common way black holes cause explosions is not through their own self-destruction, but through the sheer power of their overwhelming gravitational force. Supermassive black holes are found at the center of galaxies, and sometimes large clumps of matter, like stars, pass too close. When this happens, the star is torn apart by tidal effects, and this tearing process releases a burst of energy. Astronomers on Earth can witness this release of energy as a brief but intense flare of x-ray and gamma ray radiation.
In addition to shredding stars, these giant black holes frequently collect swarms of matter that constantly swirl around them in giant accretion disks. Accretion disks reach temperatures of quadrillions of degrees, making them the brightest objects in the universe – a single glowing disk can eclipse more than a million galaxies at once.
At the height of their power, the discs wind up electrically and magnetic fields which funnel some of the disc material around black holes and in the form of long, thin jets that reach tens of thousands of light-years away.
While these jets don’t technically count as explosions, they’re still pretty intense.
Originally posted on Live Science.
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