Unknown Unknowns


On June 6, 2002, during a press conference at NATO Headquarters in Brussels, U.S. Defense Secretary Donald Rumsfeld recounted a particularly difficult episode in gathering intelligence during wartime. “Now what is the message there?” he said. “The message is that there are known knowns; there are things we know that we know. There are known unknowns; that is to say there are things that we now know we don’t know. But there are also unknown unknowns.” The snickering started then, in that briefing room, and it hasn’t stopped since.

In my previous LWON post, “The Beginning of the End of Science,” I discussed the dangerous, even tragic legacy of George W. Bush’s unfamiliarity with scientific logic. A friend wrote to me that the nonsensical ramblings of Bush resembled Rumsfeld’s own plunge down the rabbit hole. Not so, I wrote back. Rumsfeld (pause; wince; sigh) got it right.

I can provide an example of this kind of logic from my own research. In my current book,The 4% Universe, I recount how two rival teams of astronomers raced each other throughout the 1990s to discover the fate of the universe. One known known guiding them was that matter attracts matter through gravity. Another known known was that the universe is expanding. Put those two known knowns together, scientists reasoned, and you would assume that the cumulative effect of all the matter in the universe gravitationally attracting all the other matter in the universe is slowing down the expansion. Which brought them to the known unknown: How much?

Is the expansion slowing so much that the universe will eventually stretch as far as it can go, then reverse direction and collapse back on itself? Or is the expansion slowing just enough that the universe will keep on cruising, more and more slowly, until it reaches a virtual standstill?

To make this measurement, the two teams observed exploding stars from the far reaches of the universe, Type Ia supernovae. Assuming that the supernovae can serve as a sort of standard candle—a source of light with a reliably consistent magnitude—the astronomers would be able to calculate distance against dimness. The farther the supernova, the dimmer it would appear. Across enough distance, however, the slowing of the expansion would mean the supernovae were nearer than they would be if the universe were expanding uniformly. And because they would be nearer, they would be brighter than they would be if the universe were expanding uniformly. How much brighter would tell you how much the expansion of the universe was slowing down.

What the two teams found, instead, was that at a great enough distance, the supernovae were dimmer—more distant—than they would be in a uniformly expanding universe. The expansion wasn’t slowing down after all. It was speeding up.

Surprise! The unknown unknown: dark energy.

“It is,” Rumsfeld said of the unknown unknown, pushing back against the laughter of the press corps, “a very serious, important matter.” It certainly is in science. And it is in war, too, for all I know—or, I guess, for all I know I don’t know.

Illustrations: John Tenniel

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4 thoughts on “Unknown Unknowns

  1. Rumsfeld has corrected me only on one point. I had thought that all tautologies are trivially true. But “You don’t know what you don’t know” is clearly false. This is especially so at the Presidential level. If Bush is scientifically ignorant, then he needs scientifically literate staff. No excuses.

    Disclaimer: i’m not a scientist, exactly. When the Dark Energy news hit, i wondered about what else we know. There was a paradox that argued that the night sky should be bright. The Universe is infinite in size, and so every direction you look should end on the surface of a star. The paradox is broken because light has finite speed, and the Universe has finite age. There are stars whose light hasn’t had time to reach us. (I also expected that there could be a star density so low that there’d still be gaps, but i didn’t know calculus yet.) But also, the Universe is expanding, and if to bits are far enough away from each other, then they are moving apart faster than the speed of light. So the light will *never* reach us from some stars. And gravity moves at the speed of light. So two bits that are far enough apart will never feel each other’s gravity. And as the Universe expands, there will be fewer and fewer other bits within each light horizon. This means that less gravity is felt as the Universe gets older.

    I’m not at all sure about this argument these days. It probably doesn’t explain Dark Energy.

    Whatever Dark Energy is, it appears clear that it will never power your toaster, even in principal. So, i’m not comfortable with using e=mc^2 to give it 78% of the Universe. For me, we’re 20% of the Universe, and most of the rest is Dark Matter.

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