I’ve heard a thousand times at least the argument that there must be intelligent life in the universe because of its great size. (I heard it several times after the Gazette published my column on the subject a couple of weeks ago.)

Surely, some folks argue, in the 300 billion stars of the Milky Way galaxy and in the trillions of galaxies in the universe, the conditions that led to intelligent life must have replicated itself many times.

Up to this point, we have no evidence that such life exists. Since it is difficult to prove a negative claim, neither logic nor evidence can prove that it doesn’t exist.

Given the supposed probability of such life, why don’t we have any hard evidence that such life exists? We’ve been looking for signals from alien civilizations for a while now, and we haven’t received any.

The usual rejoinder I get when I mention that fact to UFO aficionados is that there is an international conspiracy among scientists and governments to suppress knowledge of messages that we have already received. The conspirators also suppress the reality of alien visitation via spacecraft that, I am sorry to say, certainly violate the laws of physics as we know them.

I have more than once been accused of being a party in such a conspiracy. For the record: If we ever detect such a signal at Perkins Observatory, you will suffer serious harm if you happen to be standing between the telephone and me. Our funding problems will be done with forever because we will have made the most life-changing discovery in human history.

Such is the way of scientists and their hangers on. Even the most secretive of conspiracies cannot hold up against the promise of future research grants or an enormity of free publicity for their institutions.

Of the physical laws that prevent quick interstellar travel, I will save for another day.

Of the reasons we will probably never detect a signal from an extraterrestrial civilization, there are plenty. Here are 10 of them spread over the next two weeks of columns. The first three are given courtesy of the late, great astrophysicist Stephen Hawking.

1. The probability of primitive life developing might be low. There may be many planets, but the complex conditions for life may have developed on only a few of them.

2. The probability of primitive life developing into intelligent life might be low. We make the assumption that just because natural selection led to humans on Earth that it would do the same on other planets, and that may be a false assumption.

3. As civilizations develop the ability to send signals, they may also develop weapons of mass destruction. Civilizations may annihilate themselves before or at the same time they develop the ability to communicate at great distances.

4. We assume that extraterrestrial civilizations are enough like us that they want to communicate with other intelligent species. Many civilizations might never develop that desire.

5. Signal strength works against us. The stars that planets orbit around are enormous producers of energy. Trying to detect a signal, even a powerful one, is like trying to spot a speck of dust on an oncoming car headlight.

Still, stars do not generally produce energy at all frequencies of the electromagnetic spectrum of energy. There are quiet places where stars do not broadcast a strong signal. An intelligent species would, we assume, broadcast on those frequencies. However, the signal would have to be extremely tight and focused. No message in a bottle here. It would have to be aimed specifically at Earth and thus shift in complex ways to account for the complex motions of the alien planet, our sun, and Earth through space.

Such calculations are quite possible, of course, but they assume that the alien civilization knows about us already. Given the sheer number of targets available, such knowledge could probably not have happened randomly.

Perhaps, as some have suggested, an alien civilization could have detected our radio and TV transmissions. Sadly, those transmissions are weak to start with, and only a shadow of them could escape the barrier of Earth’s ionosphere. That which did escape would spread out and weaken at a rapid rate.

The upshot is that our transmissions would quickly fade to practically nothing and be lost in the noise that the universe and our own activities inevitably produce.

Even if a signal survived, the earliest signals would have made it only about 100 light years away in the century or so we have been broadcasting. Only a relatively few stars of the 300 billion stars of the Milky Way are within that perimeter. The chance of an alien civilization that is actively listening within that narrow frame of time and space is exceedingly small.

We are thus pretty much undetectable. Even if a civilization wanted to communicate, they would probably not be able to find us in the first place.

But wait. There’s more. Check this space next week.