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If a small black hole (say about .1 mm radius or 1% of Earth's mass) came flying along at the speed of a comet or higher and impacted the earth, what would happen? Would it pass through the earth (and atmosphere) with minimal damage? Would it leave a crater (entry or exit?) Would it create a big enough shock wave to significantly damage the earth? Would it be captured and wander around inside the earth?

I read a book a few years ago with this as the primary part of the plot, and I have wondered about the physics ever since.

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Not a duplicate, but related: physics.stackexchange.com/questions/2743/… –  Nathaniel Feb 26 '13 at 7:46
Thanks -- I read that and was really surprised how long that would take. –  xpda Feb 26 '13 at 7:50
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2 Answers

up vote 5 down vote accepted

There would, of course, be no "impact" since the black hole won't interact with the earth the same way that a solid object would.

However, the gravitational effects from the black hole would be catastrophic. Since the black hole is about as massive as the Moon, it would significantly deform the earth as it passes through, causing mega-earthquakes, mega-tsunamis, and mega-volcanoes.

(the actual accretion rate of matter into the black hole would be negligible, as mentioned in the other question)

Per Alan's comment: some of the matter that gets pulled near the black hole will be turned into extremely powerful radiation (in a relativistic jet along its axis of rotation) which will surely do additional damage.

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The matter lost to accretion would be small, but the corresponding energy liberated from the process could make some big explosions. But the gravitational effects you mention would likely be more apocalyptic. –  AlanSE Feb 26 '13 at 14:14
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Here is an archive paper where they calculate the effect of a small black hole from the primordial soup hitting the earth which gives a different estimate.

from a review of the paper

By calculating where the energy from the collision may come from, the researchers can estimate what effect the collision may have. The two main sources of energy will be from the PBH actually hitting Earth material (kinetic) and from black hole radiation. Assuming we have more likelihood of hitting a micro-black hole (i.e. much, much smaller than a black hole from a collapsed star) originating from the beginning of the Universe, it is going to be tiny. Using Hawking’s 1012kg black hole as an example, a black hole of this size will have a radius of 1.5×10-15 meters… that’s approximately the size of a proton!

This may be one tiny black hole, but it packs quite a punch. But is it measurable? PBHs are theorized to zip straight through matter as if it wasn’t there, but it will leave a mark. As the tiny entity flies through the Earth at a supersonic velocity, it will pump out radiation in the form of electrons and positrons. The total energy created by a PBH roughly equals the energy produced by the detonation of one tonne of TNT, but this energy is the total energy it deposits along its path through the Earths diameter, not the energy it produces on impact. So don’t expect a magnificent explosion, we’d be lucky to see a spark as it hits the ground.

Any hopes of detecting such a small black hole impact are slim, as the seismic waves generated would be negligible. In fact, the only evidence of a black hole of this size passing through the planet will be the radiation damage along the microscopic tunnel passing from one side of the Earth to the other. As boldly stated by the Russian/Swiss team:

“It creates a long tube of heavily radiative damaged material, which should stay recognizable for geological time.” – Khriplovich, Pomeransky, Produit and Ruban, from the paper: “Can one detect passage of small black hole through the Earth?“

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