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.
Everywhere, all the time, pairs of positive and negative "virtual particles" pop briefly into existence, then recombine and annihilate one another. What would happen to such particle pairs at a black hole's event horizon? According to physicist Stephen Hawking's theory, the negatively charged particles would be caught by the black hole, whereas the positively charged ones would escape. This Hawking radiation, if it weren't too faint to detect, would provide another way to spot black holes in space [source: Economist].
Doomed -- there's no other word for it. The scientists have crunched the numbers, and we're in the black hole's path. Even Bruce Willis and a plucky crew of nuke-toting oilers can't save us now.
The tug of the black hole's gravity on Neptune provides our first solid clue that something dire is afoot. Astronomers know Neptune's orbit so well that they can detect a deviation in it as small as 1 arc second (a unit of angular measure). A typical 10-solar-mass black hole moving at a characteristic speed of 300 kilometers per second (671,081 mph, or about one-thousandth the speed of light) would cause that much ruckus while still one-tenth of a light-year out [source: Hamilton].Here's our last bit of good news:
A black hole of such a size moving at such a clip would give us around 100 years of warning in which to get our act together. A slower moving black hole might give us 10 times that long. Either way, it's time to start building that space ark, folks [source: Hamilton].
As it passes close by Neptune, the dark destroyer pulls the gas giant into its orbit. The planet looks strange now: As it moves away from us, its colors appear redshifted -- the wavelengths of its radiation, including light, stretch out, shifting toward the red end of the spectrum. As Neptune passes behind the black hole, gravitational lensing makes it appear to flatten and warp around the murky sphere. As the planet emerges again, heading toward us, its colors look blueshifted -- its emission wavelengths bunch up, shifting toward the blue end of the spectrum.
Redshifting and blueshifting usually result from a stellar object moving away from us or toward us, respectively, much like the Doppler shift of a siren as an ambulance passes us; around a black hole, they're also a consequence of the way extreme gravity warps time.
Soon, the black hole tears the gas giant apart, stripping its component gases off in a swirling gravity spiral, the accretion disk, like spun sugar in a cotton candy machine. From our perspective, it seems to take forever to spiral into the event horizon. Meanwhile, the light released by Neptune's death throes outlines the black hole in negative, like the sun's corona during a solar eclipse.
As the black hole closes on Earth, we can finally see its surrounding starfield bend and stretch from lensing, like something in a funhouse mirror. Every available telescope turns to watch this distortion and the black emptiness at its center.
If our dark destroyer is a supermassive black hole, the story is already over; its event horizon looms as much as five times as large as our solar system [source: Marder]. But what fun is that? Let's see what one of these monsters looks like on the inside.
.The End of the World, or Through the Looking Glass
You climb into your indestructible pod, knowing it will spare you only briefly, but hoping at least to survive long enough to experience the black hole's interior. Launching into space, you plot a gentle, decaying orbit inward.
Luckily for you, but unluckily for the solar system, this is a supermassive black hole. Yes, we're changing the rules, but everything would happen far too quickly if we didn't. Here's why:
In a small black hole -- say, around 30 solar masses -- the tidal forces caused by the steep intensification of gravity over distance would tear you apart long before you reached the event horizon. In fact, at the event horizon, the tidal force between your head and your feet would be around 1 million G's (Earth gravities). Even if you could survive, there would be no time to enjoy your victory, for you would encounter the singularity 0.0001 seconds after crossing the event horizon [source: Hamilton].
In a supermassive black hole sporting the mass of 5 million suns, like the one at the center of our galaxy, the experience is much different. Any black hole that bulks up to more than 30,000 solar masses exerts head-to-toe tidal forces of less than 1 G at its event horizon. There's also more time for sightseeing on the way to your doom: On a curved descent, it will take you 16 seconds (and change) to reach the singularity after crossing the event horizon (this "infall" time is a function of the black hole's mass; the more mass, the longer it takes) [source: Hamilton].
Falling through the event horizon of a black hole is a bit like falling asleep or falling in love: It's hard to pinpoint when it happens, but once it does, your sense of reality is significantly compromised. In the case of the black hole, you can still see the starfield (light can get into a black hole, it simply cannot leave), but the view reminds you of the whorls of color inside a soap bubble, and there is something else wrong, too: The curved space-time garbles and twists light, confusing your binocular vision; it's like peering through a kaleidoscope with your eyes crossed [source: Hamilton].
Tidal forces stretch your craft downward like taffy and crush inward on you from every side. As you near the singularity, you witness an extraordinary sight: The outside universe appears to compress into a bright, thin, blueshifted band around your waist, as the views above and below dim and redshift. After that, what's left of your shredded matter enters a point of infinite curvature, where known space and time end.
- NICHOLAS GERBIS
My take:
13. and the stars of the sky fell to the earth, like unripe figs dropping from a tree shaken by a great wind.
14. The sky receded like a scroll being rolled up, and every mountain and island was moved from its place.
22. Unless that time of calamity is shortened, not a single person will survive. But it will be shortened for the sake of God’s chosen ones.
25. There will be signs in the sun and moon and stars, and on the earth dismay among the nations, bewildered by the roaring of the sea and the surging of the waves.
26. Men will faint from fear and anxiety over what they see coming upon the earth, for the powers of the heavens will be shaken........
Note: Only today with modern technology can men actually see what is coming upon the earth.
There are many forces in the heavens acting upon each other and then there is the gravitational force of the presence of an approaching black hole, an awesome thing indeed and yet it is really nothing compared to the power of approaching prayer.
Prayer is not what you have left, it is what you have.
My take:
13. and the stars of the sky fell to the earth, like unripe figs dropping from a tree shaken by a great wind.
14. The sky receded like a scroll being rolled up, and every mountain and island was moved from its place.
22. Unless that time of calamity is shortened, not a single person will survive. But it will be shortened for the sake of God’s chosen ones.
25. There will be signs in the sun and moon and stars, and on the earth dismay among the nations, bewildered by the roaring of the sea and the surging of the waves.
26. Men will faint from fear and anxiety over what they see coming upon the earth, for the powers of the heavens will be shaken........
Note: Only today with modern technology can men actually see what is coming upon the earth.
There are many forces in the heavens acting upon each other and then there is the gravitational force of the presence of an approaching black hole, an awesome thing indeed and yet it is really nothing compared to the power of approaching prayer.
Prayer is not what you have left, it is what you have.
27. At that time they will see the Son of Man coming in a cloud with power and great glory.…
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