Why hasn't ET made contact yet?
He's absolutely convinced. Frank Drake has been scouring the sky for 50 years, looking for signs of extraterrestrial intelligence.
He's heard nothing... but he's in no doubt they're out there.
Drake was a founder-member of Seti, the Search for Extraterrestrial Intelligence.
Since 1960, this activity has been pointing radio telescopes at the stars hoping to receive some indication - perhaps even a message - that an advanced alien lifeform exists elsewhere in our galaxy.
Many of us have been involved directly in this search by allowing Seti to use downtime on our PCs to crunch the radio data for any interesting signals.
Frank Drake himself has been in London this week to attend the Royal Society's discussion meeting - The Detection of Extraterrestrial Life and the Consequences for Science and Society.
The scientist is most famous for the equation that bears his name.
Drake's Equation is an attempt to express the potential number of intelligent civilizations that might exist in the Milky Way.
The number is dependent on several factors, such as the scale of star formation, the presence of planets around those stars that might harbour life capable of sophisticated communication, and the length of time over which any evolved species would be able to get in touch.
Remember, although we've existed as a species for at least 200,000 years, it's only in the last 100 years that we've developed the technology necessary to send messages into space.
There are a lot of assumptions in this game but when Drake plugs numbers he describes as conservative into his own equation, he comes out with a figure of 10,000 civilizations.
It sounds a lot until you consider there are at least 200 billion stars in the Milky Way.
So, here's one reason why we've not heard a dickybird out of ET yet: our searches so far have been puny compared with the scale of effort that would be required to do a thorough audit of the Milky Way.
The search field is simply enormous and the distances over which signals have to travel are colossal - tens of thousands of light-years potentially.
Talking with Drake today, he also raises another complicating factor:
"In searching for extraterrestrial life, we are both guided and hindered by our own experience. We have to use ourselves as a model for what a technological civilisation must be, and this gives us guidance for what technologies might be present in the Universe.
"At the same time, this limits us because we are well aware that all the technologies that might be invented have not been invented; and in using ourselves as a model, we may not be paying attention to alternatives, as yet undiscovered and as yet unappreciated by us."
In other words, we've been listening for extraterrestrials' radio signals but this may not be how they're trying to announce their presence.
It's one of the reasons why Seti, in the last few years, has also started to look for the optical flashes that might originate from powerful alien lasers systems.
Drake highlights another fascinating issue - and that is that we ourselves are becoming invisible to the extraterrestrials searching for us.
The signals emanating from Earth most likely to reach distant civilisations are our TV broadcasts. But the switchover from analogue to digital television means "our voice" is being diminished.
Part of this is down to the TV satellites which deliver targeted beams to the Earth's surface; and also to cable TV which runs direct to the home underground. Both don't "bleed" as much into space as the old high-power analogue TV transmitters; and the digital signal itself requires a good deal more sophistication to interpret it.
Drake says this may mean in future we have to establish a dedicated system of beacons to broadcast our presence.
"There are people who're saying we should be running a beacon - a simple message that we send to one star after another, pointing out that we exist.
"When you think about that, you quickly reach the conclusion that there should be two beacons - one that's easy to detect and has only the information on it that tells you what radio frequency you should tune to to get the other beacon with a great deal more information.
"Right now, we don't have to do this because we're sending all this information through our television, but when the Earth goes quiet it makes much more sense."
I'm fascinated by people's reactions to this search - whether it has any point, and what we'd do if we got a contact.
Would we be filled with fear or excitement? How should we initiate a dialogue, knowing a reply might not come back for hundreds of years because of the light-travel time between our two locations?
Or would shocked and scared earthlings immediately want to "hang up" as if to say "sorry, wrong number"?