Will Earth Perish by Fire, Ice or Black Hole?

On last night’s news, veteran PBS news anchor Gwen Ifill interviewed a prominent astronomer to solicit comment on the recent discovery of two enormous black holes hiding in the bright central bulges of the giant elliptical galaxies NGC 3842 and NGC 4889.

The scientific news itself went “viral,” being picked up on BBC, The New York Times, Huffington and elsewhere that I can recall, as well as in the scientific journals. The Sky & Telescope article is much more oriented toward readers who are already familiar with cosmological objects and distances. It can be picked up at this link.

You can also read the PBS transcript of Ifill’s interview with Chung-Pei Ma. Ma is professor of astronomy at the University of California, Berkeley. She appeared visibly constrained by the problem of how to explain these concepts to a general viewing television audience.

But the item here concerns Gwen’s question to Chung-Pei Ma. Presumably Gwen had done her homework and knew the answer, but most viewers might not:

GWEN IFILL: Nearby, but not a threat? I mean, we’re not — you’re talking about black holes that suck in light and gases and everything in its path, but we’re not in its path?”

Ma tried to explain, in lay terms, why not. Breaking this question apart, the salient components of a better answer would be:

  • how far out do the effects of these monster black holes reach?
  • how far away are we now?
  • how long in years could an approach to within their spheres of gravitational influence take?

Facts:

  • Both galaxies in question are about 300 million light years away.
  • “For NGC 3842’s central monster, the team found a mass between 7 and 13 billion Suns; for NGC 4889 the range is much bigger: 6 to 37 billion solar masses” [Sky & Telescope].
  •  In other words, each black hole’s estimated bulk suggested it had already swallowed the mass equivalent of an entire “ordinary” galaxy.
  • The “event horizon” of each black hole – the boundary inside of which even light cannot escape the black hole’s unimaginable gravitational field – is estimated at around 3 to 5 solar system diameters.
  • Our Solar System has a diameter of about 0.001 light year. To put this into some kind of perspective, our Milky Way galaxy has a diameter of about 100,000 light years.

Discussion:

  • So, our Milky Way (which has a large black hole of its own) is about 3,000 Milky Way diameters away from NGC 3842 and NGC 4889.
  • Looking at the second illustration in the Sky & Telescope article, and the companion text, it appears that only the the motion of stars within 1,000 light years of their black holes NGC 3842 and NGC 4889 are affected by the nearby dark monsters.
  • We are 300,000 times further way than that.

Devil’s Advocate:

But … but … supposing some cataclysmic upheaval were to propel our solar system, or our planet, toward those monster black holes? How long might it take for them to tear us apart? How fast could an “object” like us move in that direction?

Obviously, we’d have to move really fast.

  • Let’s disregard the fact that any catastrophic event powerful enough to do that would also undoubtedly shred Earth to dust, if not elemental gases.
  • A supernova explosion of our Sun might propel an expanding sphere of gases and dust outward at 11 million miles an hour, though it’s a fact our Sun is way too small to go supernova.
  • According to a Stanford article  “THE MYSTERY OF THE FASTEST MOVING STAR STILL PUZZLING,” they mention a candidate speed in this question: “How do you accelerate 2.7 octillion tons (27 followed by 26 zeros) from a standstill to over 1,800 kilometers per second, about one- half of one percent of the speed of light? That could be as fast as 4 million miles per hour.”

So, even at the catastrophic speed of one percent of the speed of light (give or take), hurtling straight toward either of those two monster black holes, it would take us something like 30,000 million years to reach a destination 300 million light years distant. The universe is currently 13.7 billion years old. Cosmologists think it might be good for another 10 or 20 billion years or so before perishing in fire, or ice, or whatever.

In short: since 30,000 million years is 30 billion years, the universe may not even exist by the time a battered Earth arrives at NGC 3842 and NGC 4889 at the improbably high speed of only one percent of the speed of light. Any slower than that, we’d never arrive, nor would there be any destination to arrive to. I don’t think we have to worry about it too much.

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