The Day of Doom
Suppose a far-off black hole is locked in a binary embrace with a star that goes supernova. Suddenly freed, the gravitational giant shoots our way at tens to hundreds of kilometers per second. How would we know?
The short answer is, we wouldn't -- at least, not until it interacted with something -- because a black hole's massive gravitation denies escape even to light. So, instead of trying to spot a peppercorn on a black carpet, let's look at a few ways we might identify a black hole indirectly.
First, matter ripped apart by a black hole emits radiation as it swirls into its accretion disk, causing the area around it to "shine" like a feather boa under klieg lights.
Second, the black hole's distortion of surrounding space, if spotted by earthlings, could also render it detectable. This gravitational lensing, predicted by Einstein's general theory of relativity, has been observed by astronomers near massive objects like galaxies, black holes and our sun [sources: STSI; University of Illinois].
Even under ideal circumstances, however, spotting a black hole this way would harder than finding a flea on a speckled dog at night -- with binoculars. And an eye patch. For gravitational lensing to be visible from Earth, the black hole must pass between us and a star; for us to spot it, it must transverse the star, so that astronomers have a normal view to compare it to. Even if this were to happen, which is unlikely, the size of both the black hole and of the lensing effect would be so miniscule that we'd be lucky to spot it even if we were looking for it [source: Unruh].
Finally, a black hole could make itself known by interacting gravitationally with celestial objects like planets, stars, asteroids or comets, which brings us to a key question: How close does our hypothetical black hole pass by our solar system?
Clearly, the closer it passes, the worse the damage. A near miss could severely perturb planetary and lunar orbits, like a sparrow slamming into a spiral spiderweb, dragging the curved orbits into a tangle of interactions.
From our perspective on Earth, the tides would change and the sky would alter. If the black hole's gravity kicked our orbit farther from the sun or closer inward, or made it more elliptical, we would suffer shifts in global temperatures and seasons, or possibly worse. In the worst case (short of becoming a black hole amuse-bouche), Earth might be thrown into the sun, or sent hurtling out into space on an escape trajectory, doomed to freeze and die.
As well-known astrophysicist Neil deGrasse Tyson once told news program "20/20" with characteristic understatement, "It would be a bad day for the solar system if we got visited by a black hole."
With that in mind, let's stop dancing around the event horizon and dive right in.