On 12 January 1998, astronomer Adam Riess received an email from a colleague. "In your heart, you know that this is wrong," it said.
The "it" in question was the observation, using state of the art telescopes and instrumentation, that the universe is not only expanding, but that the expansion is accelerating. According to Riess's observations, the universe was blowing up like a balloon, and whatever was doing the blowing was getting stronger and stronger.
Riess's reply to his colleague should be framed and hung on every scientist's wall. "Approach these results not with your heart or head but with your eyes," he said. "We are observers after all!"
The head-not-heart approach worked: this morning, Riess was awarded the Nobel Prize for Physics for the discovery of "dark energy", a mysterious force that is inflating the universe at an ever-increasing rate.
The above email exchange is detailed in Robert Kirshner's book The Extravagant Universe. Kirshner was the colleague who said Riess's result couldn't be correct - and he was right to: the observation was so strange that it had to be treated with extreme scepticism.
In fact, this exchange of emails shows science at its best. First, a scientist makes an observation that doesn't fit with any preconceptions of how the world should be. Colleagues then jump in, say it can't be right, and try to find the flaw. When they can't find anything wrong - even with a result of enormous significance - everyone has to accept it, and adopt a new view of how the world works, whatever their gut feeling. The heart's response is not the ultimate arbiter.
That's why, when the finding was confirmed, Kirshner was thrilled. "This is nutty-sounding," he told the Washington Post, "but it's the simplest explanation." Brian Schmidt, who shared the prize with Riess, said his reaction to the discovery of dark energy was "somewhere between amazement and horror". Nonetheless, he had to accept it was true. He is no doubt glad he did: Schmidt said today he is "weak at the knees" at being awarded the Nobel Prize.
We don't yet know what dark energy is, or where it comes from. We can't tax it, we can't use it to bolster economic growth and it is not going to solve rising fuel prices or alleviate climate change. But sometimes knowledge is its own reward. Even if it is nutty-sounding.
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