Grant Hatch

2017-Feb-03, 11:00 PM

I .just read a confusing to me article by Corey Powell at Discover which claims that the radius of black holes increases in direct proportion to its mass. Double the mass and the diameter of the event horizon doubles as well. This leads to the density of the black hole dropping off rapidly as its diameter increases.

"The direct relationship between size and mass has a funny effect. The more massive a black hole is, the less dense it is–and the dropoff happens rapidly, as the square of the radius. (Again, I’m using the event horizon to define the surface of the black hole.) A solar-mass black hole crams the sun’s entire 865,000-mile-wide bulk into that 4-mile wide sphere, corresponding to a density 18 quadrillion times the density of water. It’s a staggering number. The black hole at the center of the Milky Way has a mass of 3.6 million suns, which means its density is (3.6 million x 3.6 million) times lower. That translates to about 1,400 times the density of water–still very high, and more than 100 times the density of lead, but no longer so incomprehensible.

Other black holes are much more massive than the central one in our galaxy, though, which means they are also much puffier. The galaxy M87 contains a monster black hole that astronomers have measured as having the mass of 6.6 billion suns. Its density is about 1/3,000th the density of water. That is similar to the density of the air you are breathing right now!

Now to the most mind-blowing part. If you keep going to higher masses, the radius of the black hole keeps growing and the density keeps shrinking. Let’s examine the most extreme case: What is the radius of a black hole with the mass of the entire visible universe? Turns out that its radius is…the same as the radius of the visible universe. Almost as if the entire universe is just one huge black hole." http://blogs.discovermagazine.com/outthere/2013/08/20/the-baffling-simplicity-of-black-holes/#.WJT_UfkrLIU

Really? How can something the density of air even have an event horizon?

Does this mean that mathematically (at least in this limited sense) there is no difference between our Universe and a black hole of our universe's mass?

"The direct relationship between size and mass has a funny effect. The more massive a black hole is, the less dense it is–and the dropoff happens rapidly, as the square of the radius. (Again, I’m using the event horizon to define the surface of the black hole.) A solar-mass black hole crams the sun’s entire 865,000-mile-wide bulk into that 4-mile wide sphere, corresponding to a density 18 quadrillion times the density of water. It’s a staggering number. The black hole at the center of the Milky Way has a mass of 3.6 million suns, which means its density is (3.6 million x 3.6 million) times lower. That translates to about 1,400 times the density of water–still very high, and more than 100 times the density of lead, but no longer so incomprehensible.

Other black holes are much more massive than the central one in our galaxy, though, which means they are also much puffier. The galaxy M87 contains a monster black hole that astronomers have measured as having the mass of 6.6 billion suns. Its density is about 1/3,000th the density of water. That is similar to the density of the air you are breathing right now!

Now to the most mind-blowing part. If you keep going to higher masses, the radius of the black hole keeps growing and the density keeps shrinking. Let’s examine the most extreme case: What is the radius of a black hole with the mass of the entire visible universe? Turns out that its radius is…the same as the radius of the visible universe. Almost as if the entire universe is just one huge black hole." http://blogs.discovermagazine.com/outthere/2013/08/20/the-baffling-simplicity-of-black-holes/#.WJT_UfkrLIU

Really? How can something the density of air even have an event horizon?

Does this mean that mathematically (at least in this limited sense) there is no difference between our Universe and a black hole of our universe's mass?