When black holes collide

Ken Tapping, April 18, 2018

In the sky this week…

  • Venus is becoming more visible in the after-sunset glow, as a bright, star like object.
  • Jupiter around 10 pm, Mars at 2 am and Saturn at 3 am.
  • The Moon will reach Frist Quarter on the 23rd.

The highest energy events in the modern universe are probably collisions between big black holes. These crashes are so violent they cause ripples in space-time we can detect from millions or billions of light years away. A light year is the distance light travels in a year, almost 1e13 (1 with thirteen zeroes after it) kilometres. Try to imagine two very compact, dense objects, having masses ranging from a few to millions of times the mass of the Sun slamming into each other at almost the speed of light. In order to imagine such a dramatic event, we first need to look at what black holes are, and how they affect the fabric of space-time.

A black hole is made by compressing matter to the ultimate degree. For example, the Sun is a ball of hot gas with a diameter of about 1.5 million kilometres.  When the Sun runs out of fuel the pressure supporting its outer layers will drop, allowing them to collapse inward. The pressure due to the infalling material will compress the core until all the atoms are jammed tightly together and every atom is in its most compact form. The Sun will have become a white dwarf star, about the size of the Earth, where a teaspoonful would weigh a few tonnes. This is how the Sun will end its life. However, if we apply additional pressure, perhaps due to shock waves from a huge explosion in the outer layers, the atoms, which are mostly empty space, can completely collapse into a lump of neutrons – a neutron star, a few kilometres in diameter. A teaspoonful of this material would weigh billions of tonnes.

Surprisingly, if there is enough pressure, we can push the compressed material to a point where its self-gravity exceeds its resistance to compression and shrinkage starts again, and accelerates. Eventually the sheer concentration of a huge mass in a tiny volume, maybe about the size of an atom, distorts space-time to the point where our long-suffering object vanishes from view, hidden within an “event horizon”. It is now a black hole. The black hole can continue to grow by swallowing any gas, planet or star that gets too close. Anything going in can never come out.

Every object in the universe causes little distortions in space-time, like bowling balls sitting on a trampoline. If these bodies move around, they make ripples, which spread out like ripples on the surface of a pond. We call these waves in space-time gravity waves. For objects like the Sun, the bow waves and ripples they make are small and the amount of energy they carry away is negligible. For black holes this is not true. They lose lots of energy making gravity waves.

In most cases, stars have close encounters without affecting each other much. Because little energy has been lost they just keep on going. However, in close encounters between black holes enough energy can be lost through making gravity waves to get them trapped, orbiting each other. Once this happens their fates are sealed. They continue to lose energy, spiralling closer and closer together, circling faster and faster, reaching almost the speed of light before smashing into each other, merging into a bigger black hole. The size of the bang when this happens is hard to imagine, however our ability to detect those gravitational waves at enormous distances is an indication of the energy released.

Galaxies usually host supermassive black holes in their centres. Our galaxy has one a few million times the mass of the Sun. Latest research suggests the big black hole at the centre of the Milky Way has some smaller black holes orbiting around it, in which case at some point we will have a grandstand seat for a black hole collision or two.

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory.

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