Question: Evaporation on Contact?

by Ron Smith

Could the Vurdalak have radiated enough heat during its flight through the atmosphere to have dropped below the critical mass and ‘instantaneously’ completely evaporated, creating the 40 Megaton explosion?

Doctor Jack's Answer

I’ve got to say that’s a pretty ingenious scenario. Not that it’s the first time someone’s used PBH evaporation to explain why the exit event predicted by the original Jackson-Ryan hypothesis never happened, of course. But all the other attempts I’ve seen rely on the coincidence of the primordial black hole (PBH) happening to evaporate just as it was passing through the Earth. And that’s one colossal coincidence! After all, what are the odds that — after 13 or so billion years of radiating away like crazy — the clock would conveniently run out right at that precise instant?

If I’m reading you right, your explanation at least has the advantage of suggesting a reason why Vurdalak should have chosen 7:14 a.m. LT on June 30th, 1908 to give up the ghost. Namely, that some sort of interaction with Earth’s atmosphere might have tipped the scales and caused it to evaporate just then.

Problem is, far as we know, the process of Hawking radiation isn’t affected by external environmental factors like that. (Oh, there might conceivably be some set of circumstances that would accelerate the creation of the virtual-particle pairs that drive the process, but we’re pretty sure passage through a planetary atmosphere isn’t one of them.) So, it’s hard to see how Vurdalak’s chance interaction with the air molecules along its flight path could have influenced its internal evaporation countdown one way or the other.

But even setting that aside, the Tunguska blast was way too small to have been the product of a PBH evaporation event. Based on the evidence of forest fall and globe-circling atmospheric shockwaves, Tunguska was “only” some 2 to 40 megatons in magnitude. On the other hand, Stephen Hawking estimated the energy released by a black hole in the final run-up to total evaporation at ten to the thirtieth ergs. At 4.2e22 ergs to the megaton, that works out to some 25 million megatons, or about a million Tunguska Events!

The late Charles Sheffield, former president of the American Astronautical Society, gave us a handy way to get our arms around just how big 25 million megatons is: the nuclear-war scale. According to Chuck an all-out thermonuclear exchange between superpowers (back when there still were two) would release about 25,000 megatons of energy. On that scale, Tunguska’s too small to even jiggle the needle, but a PBH evaporation event tops the 1,000 mark! Think of it: one World War III every day — for three straight years! Enough to wreck Earth’s whole biosphere.

So, it’s a neat idea (and one I personally had never seen before), but — no, the Tunguska blast itself couldn’t have been the result of PBH evaporation. Or we wouldn’t be around to talk about it.

Yours truly,
Jack Adler

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