The Disappearing Black Hole

Most of you may be generally familiar with black holes – bizarre objects with strange, unique properties. But did you know they are all going to disappear?

Where do they come from?

Black holes are created in the largest supernova explosions.  It’s a place where gravity is so strong that nothing –not matter, not even light, can escape.

Because they don’t emit light and don’t reflect light they are difficult to detect. We can detect them via their gravitational effects on other stars, or if they are actively ‘feeding’ we can see them by the intense light of the quasars they create. 

Supermassive Black Holes

Astronomers have been tracking the movement of 90 stars at the very center of our galaxy for the last 25 years.  Based on their paths, it appears they are orbiting something with a mass of over 2.6 million Suns but with a radius of less than 0.002 light years.  

The presence of a black hole is the only plausible explanation that fits the data.  We now expect that supermassive black holes are at the center of all galaxies, including our Milky Way. 

In 2016 the LIGO gravitational observatory detected the merger of two black hole via the gravitational waves created by the collision. Since then, 10 more such events have been captured.

In 2019 the Event Horizon Telescope accomplished a previously unimaginable task – imaging a supermassive black hole in galaxy M87.  (photo)

How Long Do Black Holes Live?

Black holes can exist longer than anything else in the universe, but not forever.  While they are absorbing material, and have an accretion disk, they are growing. Eventually however, all their ‘food’ supply is exhausted.

Our physics cannot describe what is happening inside the black hole. But just outside the event horizon we can do reliable calculations based on quantum field theory in curved spacetime, thanks to general relativity.

We know from Einstein of the mass/energy equivalency E=mc².  Stephen Hawking developed a new theory in 1974.  He postulated that the gravity is so intense just outside the event horizon, energy is spontaneously converted into matter, creating a particle and an antiparticle.

These particles move in opposite directions.  One is generally recaptured by the black hole, but the other one escapes. This effect is now called Hawking Radiation.  Even though the mass of the escaping particle compared to the mass of the black hole is laughably small, it is finite. 

Thus, every particle that escapes reduces the mass and rotational energy of the black hole by an infinitesimal amount.  Eventually, in the very long term, enough particles will escape the black hole so that it completely evaporates.

How long does it take?  A supermassive black hole with a mass of 100 billion Suns will evaporate due to Hawking Radiation in about 2 x 10¹⁰⁰ years – long after all stars and planets will be gone. In more familiar terms, that’s 20 billion quadrillion quadrillion quadrillion quadrillion quadrillion quadrillion years.  Mark your calendar!