N equals L
greg posted in Uncategorized on July 2nd, 2007
Image Source.
Last weekend, I participated in the “Future of Intelligence in the Cosmos” workshop at NASA Ames. In an age of ultra-specialized conferences, the focus for this one bucked the trend by pulling back for the really big picture:
The Future of Intelligence in the Cosmos” is an interdisciplinary two-day workshop that seeks to elucidate potential scenarios for the evolution of intelligent civilizations in our galaxy and thus, perhaps, to find a resolution for this seeming paradox. The probability that intelligent civilizations exist has been succinctly stated by the Drake Equation. While the first few terms in the equation, such as the number of stars in the Milky Way Galaxy, the fraction of stars that have planets, and the number of planets in the habitable zone, are becoming better known, the last three terms that depict the fraction of planets that evolve intelligent life, the fraction that communicate, and the fraction of the lifetime of the Milky Way Galaxy over which they communicate, are not well known. It is these last three terms in the Drake Equation that are the focus of the workshop.
In most venues, extrasolar planets veer toward the esoteric. At this workshop, however, the galactic planetary census was perhaps the most nuts-and-bolts topic on the agenda. We know that planet formation is common in the galaxy, and its increasingly clear that the “great silence” isn’t stemming from a lack of Earth-mass worlds.
Here’s a link to a .pdf document containing the slides from my talk.
In an upcoming post, I’ll try to pull together a synopsis of what emerged from the conference. Perhaps the most startling moment for me came in Paul Davies‘ talk, when he described the extent to which the simulation argument has been developed.
When I was in graduate school, Frank Drake was a faculty member in our Department. I noticed right away that the license plate on his car read “neqlsl”. I always read this as “n equals one”, until I finally asked him which term was responsible for thwarting all the alien civilizations.
“It’s not N equals one,” he said, “it’s N equals L”.
July 2nd, 2007 at 11:23 pm
N = R* • fp • ne • fl • fi • fc • L
Sometimes, when I get caught up with the technical aspects of what we do on Systemic, I forget that in large part we do what we do in attempt to try to start assigning values to a couple of the variables in the above equation, i.e., the Drake equation. Presently, we are at the phase in our investigations where we are trying to get some idea of two variables in particular: 1) fp: The fraction of those stars with planetary systems, and 2) ne: The number of planets, per solar system, with an environment suitable for life.
As Greg so eloquently pointed out in his blog “A Habitable Earth” are best chances of obtaining the first results for the latter variable (i.e. ne)lie within the habitable zone of red dwarfs, because 1) the low mass of red dwarfs makes the detection of an Earth-mass planet easier using radial velocity techniques and 2) because red dwarfs are so long-lived it is more likely that intelligent life would have enough time to become technologically sophisticated enough to be able to communicate by means of electromagnetic technology.
It’s nice to be reminded of the big picture once in a while, as Greg has done in this blog.
best,
Eric
July 3rd, 2007 at 5:01 am
What is the significance of “N equals L”?
July 3rd, 2007 at 10:37 am
L is the length of time a civilisation broadcasts detectable signals to space: most probably the weakest link in the equation, since we know of only one “civilisation”.
For ourselves, for example, we’ve been broadcasting high-power radio signals for about 50-80 years, but are now turning down the power - use of high-power ABM radar, we’re lowering the power of radio / TV stations as we use better technology and harder-to-detect coding schemes.
It could well be that within 50 years we no longer broadcast signals detectable throughout the milky way, even if we’re still here. N = 50-100 is a hard search in the milky way.
July 3rd, 2007 at 4:14 pm
The first term in the Drake equation, R*, (see Eric’s formula above) is the rate of stars formed per year in the galaxy, and thus has units of 1/time. The last term L, the lifetime of a civilization has units of time, which thus allows the expression as a whole to express a pure number N. Given that the Galaxy contains ~100 billion stars, this is implying that 1 star in 10 spawns a civilization. *Highly optimistic*
When teaching basic astronomy, the last lecture is usually spent talking about the Drake equation. After running through the usual arguments for the values of R, fp, ne, fl, fi, and fc, I poll the class on how long they think our civilization will last. It’s amazing how pessimistic they always are. The median value lifetime always lies between 10 and 50 years. Invariably, however, there’s usually one person who votes for something like 100,000 or even 1,000,000 years, which skews the mean up to 1000+ years. We thus usually get a final number for N of order 1-2 for the Milky Way.
July 3rd, 2007 at 7:10 pm
mckinstry, while we may only be broadcasting inadvertent signals into outer space for a few decades, it’s not difficult to envision a time when we will deliberately broadcast our presence to the rest of the galaxy (always assuming we avoid a civilization ending catastrophe, of course).
It might be targeted signals, directed at the tens of thousands of Earth-analogues we should have uncovered within the next century (assuming they’re there), or we might have the confidence as a species to simply call out to the cosmos declaring that “We are here, who else is out there?”
And if L is suitably large enough, then even if the product of fl, fi, and fc is vanishingly small, we may still have a chance to establish contact eventually. It could be that we’re just not there yet, technologically speaking.
July 3rd, 2007 at 7:22 pm
Regarding the simulation argument, I must admit that thinking about it makes my head hurt. To be honest, I don’t really see how we could even know for sure since if we are in a simulation the we have no way of knowing how to distinguish it from the real thing.
And even if we find out we are just all part of a grand simulation, what does it matter? If we have no existence beyond the simulation, then this is all there is, for us–this is our reality. It doesn’t even change the argument over religion. If “God” is just as much a part of the simulation as we are, then does that make that supernatural being any less real to us?
July 3rd, 2007 at 11:50 pm
I had a closer look at Bostrom’s “Are You Living in a Computer Simulation” paper. It seems to deemphasize the entire-universe-as-a-simulation idea in favor of a Matrix-type scenario, which to me doesn’t seem particularly likely.
The idea that the universe itself is a simulation seems to me to have a bit more merit, although it would take an awfully large computer to run it.
I don’t think I’m going to pursue this particular line of inquiry any further…
July 4th, 2007 at 7:36 pm
In my humble opinion, the weakest link in Drake’s equation is that idea of communication.
Consider that you go to Alpha CEN with the best equipment you can have, and point it to the Sun. What would you see? The Sun and none of the leakage from Earth.
This is true at any frequency you use.
July 6th, 2007 at 5:31 am
Mike-
Although outshining the sun is difficult, we can now do it regularly with radio telescopes. It is possible because the sun is relatively faint in the microwave/radio range and because we can put a lot of power into a short narrow-frequency radio signal. At that frequency, we easily outshine the sun (which would not even be detectable in the radio at interstellar distances). This has some practical use as well: these powerful radio bursts can be targeted at near earth asteroids and the return signal contains a wealth of information about the distance, size, shape, binarity, and surface properties of the asteroid in question.
There is also some SETI work at detecting signals in the optical. (http://seti.harvard.edu/oseti) Using a laser and a large (i.e. expensive) mirror, we currently have the technology available to send anoptical beam that outshines the sun in a narrow wavelength region.
This comes full circle: optical SETI searches for these signals by piggybacking on some of the same telescopes used for detecting planetary radial velocities!