Ask Ethan: When Do Black Holes Become Unstable?
“Is there a critical size for black hole stability? [A] 1012 kg [black hole] is already stable for a couple of billion years. However, a [black hole] in the range of 105 kg, could explode in a second, thus, definitely not stable… I guess there is a critical mass for a [black hole] where the flow of gained matter will equal to the Hawking evaporation?”
Wherever you have a black hole in the Universe, you have two competing processes. On the one hand, anything that crosses the event horizon, whether it’s normal matter, dark matter, or even pure energy, can never escape. If you fall in, you just add to the overall mass of the black hole, and grow it in size, too. But on the other hand, all black holes radiate away energy in the form of Hawking radiation, and that subtracts mass over time, shrinking your black hole. For all realistic-mass black holes, the rate-of-growth far outstrips the rate of mass loss, meaning they’ll grow for a very long time before they start to shrink.
But eventually, they will shrink. And although we think they don’t exist, a low-enough mass black hole would start shrinking today. Find out when black holes become unstable today!
Watch your hot cup of coffee or tea carefully, and you may notice a white mist of tiny micron-sized droplets hovering near the surface. These microdroplets are a little understood part of evaporation. They form over a heated liquid, levitating on vapor that diffuses out from them and reflects off the liquid surface. (This is similar to the Leidenfrost effect, but the authors note it occurs at much lower temperatures. Unrelated research has suggested the Leidenfrost effect can occur at lower temperatures when there is very little surface roughness.)
One of the particularly peculiar behaviors of these tiny levitating microdroplets is that they can exist over dry surfaces as well. The image above shows microdroplets migrating from a liquid surface (right) to a dry surface (center and left). When the droplets near the contact line, they encounter a strong upward flow due to increased evaporation there. This launches the droplets upward and they sail to the dry area. There, their vapor layers continue creating levitation and provide a cushion between them and their neighbors, causing the drops to self-organize into arrays. (Image credit: D. Zaitsev et al.; via Physics World; submitted by Kam-Yung Soh)
Ask Ethan: Do Black Holes Grow Faster Than They Evaporate?
“Wondering why black holes wouldn’t be growing faster than they can evaporate due to [Hawking] radiation. If particle pairs are erupting everywhere in space, including inside [black hole] event horizons, and not all of them are annihilating one another shortly thereafter, why doesn’t a [black hole] slowly swell due to surviving particles that don’t get annihilated?”
So, you’ve got a black hole in the Universe, and you want to know what happens next. The space around it is curved due to the presence of the central mass, with greater curvature occurring closer to the center. There’s an event horizon, a location from which light cannot escape. And there’s the quantum nature of the Universe, which means that the zero-point-energy of empty space has a positive value: it’s greater than zero. Put them together, and you get some interesting consequences. One of these is Hawking radiation, where radiation is created and moves away from the black hole’s center. It occurs at a specific rate that’s dependent on the black hole’s mass. But another is black hole growth from the mass and energy that falls through the event horizon, causing that black hole to grow. At the present time, realistic black holes are all growing faster than they’re decaying, but that won’t be the case for always.
Eventually, all black holes will decay away. Come find out the story on when evaporation will win out on this week’s Ask Ethan!