E = mc2 Perhaps the most recognizable equation in the world. It says something with mass is equivalent to an enormous amount of energy. A kilogram of mass has 9 x 1016 Joules of energy. A quick back of the envelope says anything with the mass of three gallons of gasoline has enough energy to power the state of Utah for a year.
There's just one little problem. How do you tap it?
You could bring matter and antimatter together. That's 100% efficient. Three gallons of gasoline mixed with three gallons of anti-gasoline would liberate enough energy to power Utah for two years. Unfortunately antimatter isn't easy to come by. It's created in some nuclear decays (including some that happen in you body naturally) and in physics experiments, but the amounts are tiny and the cost of making it astronomical. More practically we're left with three basic methods: chemical, nuclear and gravitational.
I mention gasoline as it's a chemical reaction and we have a sense of what a gallon can do moving in a car. To get energy from chemical reactions you need to pick the right chemicals. But how efficient process when it comes to extracting the available mass energy? Doing a bit of math you find one of the most efficient chemical reactions - combusting hydrogen and oxygen, burning gasoline is not as good - converts 0.000000001% of the available mass into energy.1 This is so little we never worry about E = mc2 when we're using chemical reactions.
Nuclear reactions are much more efficient than chemical reactions. In regular fission, say uranium235 decaying to krypton and barium, only about 0.08% of the uranium's mass turns into energy. Fusion is somewhat better. Combining four hydrogens into one helium is about 0.7% efficient. Impression and potentially enough to end life on Earth, but peanuts compared to matter-antimatter.
What about gravity?
It turns out black holes are very efficient mass-energy converters. It might seem odd at first as most people have heard black holes as places where anything falling in don't escape. But not everything falls in. To see how that works consider a rock in space aimed at the Earth .. a meteorite for example. A typical speed as one hits the atmosphere about 50,000 kilometers per hour. Friction slows it down and the kinetic energy it loses turns into heat. Most objects smaller than marble size are vaporized leaving a brief streak in the sky. The heat is radiated away as infrared and visible light. Some of the mass is converted to energy. That works out to about 0.0000001% of its mass converted to energy. Not much different than a chemical reaction and a poor source of energy.
Black holes are special. They're really dense. Mind-numbingly dense. An Earth-mass black hole would be about the size of a grape. Something falling towards a black hole ends up moving very fast - often close to the speed of light. By the time a falling object makes it to the black hole's event horizon it has enormous kinetic energy -which would be lost to the rest of the Universe when it crosses over. But if it slowly spirals in, colliding with stuff as it goes, it heats up and radiates energy to the rest of the Universe.
The type of black hole is important. I won't get into the physics as it's reasonably thick, but an object spiraling in to a non-rotating black hole radiates about 6% of its mass to the Universe - about ten times better than nuclear fusion that powers stars. Not too shabby. Rotating black holes drag space-time around with their rotation and effectly allow the object to fall in further. If the rotation is as fast as garden variety black holes, about 42% of the mass is converted to energy. Douglas Adams would have been pleased with the result.
So if you use a rotating black hole you can power the state of Utah for a year on a bit over seven gallons of gasoline - or same mass of any matter.
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1 Physicists out there forgive me.. I'm being loose with the notion of converting mass into energy to stay out of the weeds. Sometimes being correct enough is ok.
I love this!
Posted by: Jheri | 04/10/2019 at 06:05 PM