Black Holes: What’s their temperature?

Black holes are regions where there is so much mass that the gravitational forces are too powerful for even light to escape.  With the recent combined observations of gravity waves and light from the combination of two neutron stars we now have physical evidence of the likely creation of a black hole.  For it to be formed two very hot and very dense objects merged, disturbing the space about them.  That gravitational radiation has been detected by several of the LIGO instruments here on Earth.  In the case of the kilonova, the popular name for the combining of these two stars, the observation was able to be followed up from several observatories all around the planet that looked at the massive emission of light from many different spots of the spectrum.  This large amount of energy released was in part due to the absurdly high temperatures of the massive event!

So why are black holes so unbelievably cold?

A simple explanation can be seen around some of the same concepts that you run into every day in your kitchen.  When you turn on the stove and place a pan on top of it you can begin to feel the heat radiating off of the pan.  This is helpful in the context of cooking.  It allows for heat to be transferred from your heating element (natural gas, electric burners, magnetic induction, etc.) to your food!  Heat travels from hot things to colder things.

This can also be felt when you go to grab your favorite frozen treats out of the freezer.  The feeling of cold is the transferring of heat from yourself to your frozen delicacies.  In a very simplified way, this is what a black hole is doing.  It is absorbing heat from the surrounding environment because it is much colder than it, allowing the energy to be passed into black hole and preventing it to be radiated to the much hotter empty space that surrounds it.

But how do we know that the surrounding space is hotter than black holes?  Also, what evidence do we have that black holes are cold?

I will be continuing this explanation over the next week with more details relating to the microwave background radiation and thermodynamics use of entropy with references to Stephen Hawking’s work.  For now you’ll just have to find a way to chill out.

Best,

Josh