How to make a black hole

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How to make a black hole

+Brian Koberlein, one of the most consistently energetic and interesting people here on G+, recently wrote about how to make a black hole.

His recipe works like this:

INGREDIENTS: one small neutron star, one solar mass of hydrogen.

Take a neutron star 2 weighing solar masses.  Gradually add one solar mass of hydrogen gas, letting it fall to the surface of the neutron star.  Be careful: if you add too much too quickly, you’ll create a huge nuclear explosion called a nova.  When your neutron star reaches 3 solar masses, it will collapse into a black hole.

This is the smallest type of black hole we see in nature.  The problem with this recipe is that we’d need to become at least a Kardashev Type II civilization, able to harness the power of an entire star, before we could carry it out.

Louis Crane, a mathematician at the University of Kansas, has studied other ways to make a black hole.  It’s slightly easier to make a smaller black hole – and perhaps more useful, since the Hawking radiation from a small black hole could be a good source of power.

Crane is interested in powering starships, but we could also use this power for anything else.  It’s the ultimate renewable energy source: you drop matter into your black hole, and it gets turned into electromagnetic radiation!

Unfortunately, even smaller black holes are tough to make.  Say you want to make a black hole whose mass equals that of the Earth.  Then you need to crush the Earth down to the size of a marble.  The final stage of this crushing process would probably take care of itself: gravity would do the job!  But crushing a planet to half its original size is not easy.  I have no idea how to do it.

Luckily, to make power with Hawking radiation, it’s best to make a much smaller black hole.  The smaller a black hole is, the more Hawking radiation it emits.  Louis Crane recommends making a black hole whose mass is a million tonnes.  This would put out 60,000 terawatts of Hawking radiation.  Right now human civilization uses only 20 terawatts of power.  So this is a healthy power source.

You have to be careful: the radiation emitted by such a black hole is incredibly intense.  And you have to keep feeding it.  You see, the smaller a black hole is, the more Hawking radiation it emits – and as it emits radiation, it shrinks!  Eventually it explodes in a blaze of glory: in the final second, it’s about 1/100 as bright as the Sun.  To keep your black hole from exploding, you need to keep feeding it.  But for a black hole a million tons in mass, you don’t need to rush: it will last about a century before it explodes if you don’t feed it.  

Unfortunately, to make a black hole that weighs a million tonnes, you need to put a million tonnes of mass in a region 1/1000 times the diameter of a proton.

This is about the wavelength of a gamma ray.  So, if we could make gamma ray lasers, and focus them well enough, we could in theory put enough energy in a small enough region to create a million-ton black hole.  He says:

Since a nuclear laser can convert on the order of 1/1000 of its rest mass to radiation, we would need a lasing mass of about a gigatonne to produce the pulse. This should correspond to a mass of order 10 gigatonnes for the whole structure (the size of a small asteroid). Such a structure would be assembled in space near the sun by an army of robots and built out of space-based materials. It is not larger than some structures human beings have already built. The precision required to focus the collapsing electromagnetic wave would be of an order already possible using interferometric methods, but on a truly massive scale. This is clearly extremely ambitious, but we do not see it as impossible.

I’m not holding my breath, but with luck our civilization will last long enough, and do well enough, to try this someday.

For details, see:

• Louis Crane and Shawn Westmoreland, Are black hole starships possible,

Here is Brian’s post on how to build a black hole:

#spnetwork arXiv:0908.1803 #blackhole

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