Risu shrugged. "Most likely yes. The factors in the equation are supposed to be as accurate as possible, of course, but some of them are averaged and others can't ever be much better than an educated guess. Without investigating every single star and every planet in our galaxy for a definite yes or no, that's how it works. All we can hope to do, if we work with this equation, is to close in on the true values of those factors as much as possible. Now, as I said, I don't think most of them can be known to any degree of accuracy, but if you somehow figured out that the average developed civilisation lasted, say, 550 years before inevitably dying out, then that would be a good value for L."

Risu tapped at the letter on the blackboard. "Since this is an average this will mean that some civilisations will not be included in this factor. A particularly violent civilisation might blow up its planet within 100 years, for instance, and a peaceful alien race might exist for a thousand years, but this would mean that a bog-standard civilisation could be expected to transmit signals for 550 years." He paused and grinned. "I do hope I calculated that correctly. Of course, with just two examples, 550 years for L would still be nothing more than a guess - perhaps the majority of alien life is peaceful, which would mean the true value of L is closer to 1000 than 550, but if we knew about, say, 20 civilisations, then we could get a better average."

He shrugged again. "But the most important criticism still stands, in my opinion. It seems very, very improbable to me that we will figure out more than, perhaps, one or two of these factors. Not in my lifetime, at least," he added with another grin.

Risu pointed at the next student and listened to her question. He smiled and gave another little shrug. "That is the question. If you ask me, I'll say yes, I do think so, but no one can give a definite answer until we actually do find other forms of life."

He nodded. "I do agree. You might be aware, perhaps, of the rather famous story surrounding the discovery of pulsars."

Risu looked around at the students for any sign of recognition. As far as he could see, there was none. "Perhaps not as famous as it is in astronomer circles." He smiled. "Really quickly, then: A pulsar is a certain type of star that's spinning very, very fast, sending off a beam of radiation with every rotation. These beams come at very regular intervals, one every few seconds." He paused, and then an analogy occurred to him. "Think of a lighthouse. Now, because these pulses of energy were so regular and smooth, when they were first discovered they became known as LGM-1, LGM standing for little green men. At that time, it was unheard of that a natural object such as a pulsar could be as regular as clockwork.

"My point then is, and yours too, I presume, is that even if we discover a certain type of signal, we can't automatically assume that it is coming from," he smiled, "little green men. Perhaps a natural source is more likely."

"You, er, must have a very big interest in biology, I guess," Risu said. "Well. A lot of the search for alien life assumes that this life will be similar to life on Earth in some respects. It'll need water and an atmosphere, and so on. If that were the case, then yes, it would probably employ similar chemical components. Maybe not DNA, but another molecule. Of course, carbon might not be the only stable basis for life. Silicon, for example, has similar chemical properties. Perhaps we will one day find silicon lifeforms - they would not necessarily require the same things that we require, and therefore we'd need to be searching for them in different ways."

OOC: Please don't forget to cite the source if you're quoting from somewhere. :>

"Any other questions?" he asked.