The cost of SETI

I think that things like SETI (the search for extraterrestrial intelligence) are extremely unlikely to find a signal: even if intelligent life is out there, it’s not at all clear that such beings would spend much of their time communicating with each other by sending radio signals that leak off into space at detectable levels.  Even if they do that for a while, they’ll probably quickly learn ways of communicating that are less wasteful and harder to eavesdrop on.  In other words, whatever the other numbers in the Drake equation are like, L is probably quite small.

Still, I’ve generally had positive, warm fuzzy feelings about SETI.  Even if the odds are terrible, I figured, the payoff is huge, and the cost is low, so why not go ahead?  The first part is certainly true: if SETI saw a signal, it would be about the most important discovery ever made.  But my friend Allen Downey (CS professor at Olin College) recently gave me a convincing argument that the costs are higher than I’d realized, and I think I’ve changed my mind and become anti-SETI as a result.

Here’s my summary of Allen’s argument.

A key part of SETI  is combing through vast amounts of data from radio telescopes, looking for signals that look like those of extraterrestrial intelligence.  This is a big computational project, and the SETI people have adopted a clever way to achieve it: they farm it out to huge numbers of supporters, who run the computations on their own computers during times when those computers would otherwise be idle.

The people who do this, of course, are paying a cost: they’re giving away free CPU cycles on their computers.  Aside from any other costs, this costs them money, because a computer that’s actually computing uses more power than one that’s idle.  I think that a decent estimate of that power difference is about 40 watts.  (You can find a bunch of estimates out there for this quantity.  There’s some variation, but this seems to be about right.)  Say a typical user has SETI@home running on one PC about 2/3 of the time for a month.  How much does it cost? To find out, multiply 40 watts times 20 days and convert the result into kilowatt-hours.  These days, 1 kWh costs about 12 cents, so multiply the result by $0.12.

If that’s starting to sound like work, it’s not.  Just ask Google.  (In case you didn’t know, Google’s also a calculator, and it knows a ton of unit conversions.)  The answer is that such a user is spending about $2.30 a month.

I’d bet that the typical SETI@home volunteer doesn’t know that it’s costing them that much, but that if they did they’d probably think it sounded like a reasonable cost.

So far, I probably haven’t convinced you that SETI costs too much.  But now let’s think globally.  In total, the project has used up 2 million years of computing time.  If you try the same calculation to get a total cost, you get  $84 million. I don’t know about you, but to me, that’s real money.  When you think about the things that could be done with $84 million, it’s hard to see that this incredible longshot is justified, in my opinion.

A few random notes:

1. Nobody’s actually paying this money directly, so it’s not noticeable. But just because the costs are hidden, that doesn’t mean they’re not real.
2. In fact, the cost is probably underestimated in a bunch of ways.  I was assuming that the computers would all be turned on anyway, and just considering the difference in power due to the extra computational load.  If people are leaving their computers on for SETI@home when they would otherwise be turning them off, then the cost is greater.  Also, the figure of 12 cents/kWh is the direct cost to the consumer, but there are externalities (greenhouse gas emissions, pollution, geopolitical problems due to resource competition) that that market price doesn’t include.
3. Instead of considering the cost savings, you could consider the other good things that people could be doing with all those CPU cycles.
4. Let’s come back for a minute to my original point about L in the Drake equation.  My reason for thinking it was small was simply that any communication method that sends radio power into space is wasteful.  A technologically advanced species will learn how to beam its communications directly to the intended recipient.  Allen pointed out a different reason, which I’d never thought of.  To prevent eavesdropping, you want to encrypt your communications.  The better a method of encryption is, the more the output looks just like noise.  If an advanced society is really good at encryption, we won’t notice its signals even if they’re out there, because they’ll look like noise.  I’m not sure I’m convinced by this, but it’s an interesting point.