In my last entry
I discussed Kardashev's scale of civilizations and Dyson's insight into a completely general method of detecting distant alien civilizations. I gave a talk on all of this last Friday and before the talk Eric Feigelson mentioned to me that Nikolai Kardashev is alive and active: in fact he is currently the deputy director of the Russian Space Research Institute at the Russian Academy of Sciences at age 80.
Freeman Dyson, too, is an active scientist at 88 at the Institute for Advanced Study at Princeton. I heard Dr. Dyson speak when I was a graduate student at Berkeley (on the "garbage bag" theory of the origin of the cell, if I recall) and I may get to meet him in person next week in Philadelphia
The next scientist in my tale left us far too early. Carl Sagan took Kardashev's scale and extended it to include fractional numbers by noting that the fraction of all sunlight that strikes the Earth is (very) roughly the inverse of the number of stars in the Galaxy. That is,
This allowed him to redefine Kardashev's scale slightly by defining:
On this scale, a civilization with energy supply of 10 billion MegaWatts (which is roughly 5% of the total incident sunlight on the Earth) would have K=1. Humanity's current energy supply is about 10 million MegaWatts, or K=0.7, and if we collected and used all of the incident energy on the Earth we would have K=1.12.
A civilization that collected and used almost all of the radiant energy of a star like the Sun (4 x 1020 MW) would have K=2.06, and one with 100 billion solar luminosities of energy supply would have K=3.16, roughly consistent with Kardashev's scale.
It is interesting to note that humanity's energy supply has doubled in the last 30 years. At this exponentially increasing pace, we will achieve K=1 in 300 years, and have an energy supply equal to the incident sunlight on Earth in 400 years. At this point, we will have doubled the Earth's pre-industrial mid-infrared waste heat signature. In fact, this will be a new form of global warming that has nothing to do with greenhouse gasses: just by using energy for our own needs we will significantly warm the planet with the waste heat from our computers and electric cars and phones.
This gives a sense of how quickly we are approaching our Malthusian limits
on energy: unless we start colonizing space, we will hit hard energy limits in just a few human lifetimes. As I wrote before; every other Malthusian limit, from food supply to fresh water to material, we can, in principle, overcome with technology and energy expenditure, but energy itself will eventually limit us, and unless we want to seriously heat the Earth we're going to have to move to space to find the energy and waste-heat emission surfaces to keep expanding our economic activity. Given how poorly coordinated most of our energy use is (even when we know it's heating the planet, we continue to burn more and more fossil fuel), it seems very typical of our species that some fraction of our population will always be looking for new sources of energy. Viewed this way, the question is not whether we will ever be able to build a Dyson sphere; the question is why it isn't inevitable!
I like to generalize the Kardashev scale the way that Zubrin did in 2000 (in his book Entering Space, I think), not by the actual energy use, but by the extent of a civilization. A planet-wide species (like ours) would be a "K1", or civilization of the first Kardashev type. A solar-system-wide civilization would be a "K2", and this would include everything from a thriving moon colony to a full-on Dyson sphere. A "K3" civilization would have colonies between the stars.
I should note that the transitions between these types will actually be very quick, cosmologically speaking. Consider a space-faring civilization that can colonize nearby stars in ships that travel at "only" 0.1% the speed of light (our fastest spacecraft travel at about 1/10 this speed). Even if they stop for 1,000 years at each star before launching ships to colonize the next nearest stars, they will still spread to the entire galaxy in 100 million years, which is 1/100 of the age of the Milky Way. In other words, if you watched a galaxy in time-lapse from its formation to the present day in a movie 1 minute long, it would go from having a single K2 civilization to having a K3 civilization in under 1 second.
So if we scan the heavens for galaxies with aliens, we should not expect to find many that have only a few aliens: they should have either no K2's, or a K3. Interestingly, we can apply the same logic to the Milky Way: if there are aliens in the Milky Way, it is very unlikely that we would have come of age in an era where they were in transition between K2 and K3. Either the Galaxy is filled with spacefaring aliens, or we are the first. So we should either expect the galaxy to be filled with Dyson spheres, or totally empty of spacefaring life. This makes a null detection of Dyson spheres very interesting! (This is also a rephrasing of the so-called Fermi paradox
OK, practical numbers for Dyson spheres will have to wait for another entry. Next time, I'll look at the problem of extinction and perhaps why aliens might not have any waste heat to detect.