While we marvel about the possibility of millions, even billions of intelligent civilizations in the universe, we really only know for sure of one: our own. It has, as a sentient culture only existed for a couple of hundred thousand years. If I can classify civilization as a group of sentient beings that records its history, then we’re only about 5,000 years old as a civilization. If technology is the defining factor, we’re only about 150 years old. All these time spans are very short in the context of cosmological terms, where time periods are counted in millions of years, even billions. We also don’t know how long a civilization lasts. Our own has so far not lasted long, and there are some signs that we’ll do something stupid soon and it will have been a very short period indeed.

So let’s speculate that an intelligent civilization lasts about 10,000 years from first recording its history until flaming out and dying off.

Our universe is 13.77 billion years old and the Earth is 4.54 billion years old. There are almost half a million 10,000 year spans in 4.5 billion years. So our civilization, based on my assumptions here, lasts about half-a-millionth of the time span of the earth.

If other planets on other stars had similar timescales, and if there were half a million such planets in our galaxy, all forming about at the same time the Earth formed, we could conceivably have had half a million civilizations on these planets alone without overlap. This means every one of those civilizations could have existed throughout its entire life-cycle without ever knowing about the existence any of the others. All of them could have been advanced technological civilizations with active programs in place to scan the sky for signs of life. They still would never have seen a trace of any. They existed, but separated from each other by time.

This makes me think of camera flashes in a stadium:

Watching the short video above I can’t help but think of each of the flashes to be a 10,000 year civilization somewhere in our galactic neighborhood. The short video spans perhaps 10 million years of time. We’re one of them flashing right now, but we never saw those before us or after us. Yet they all exist.

Taking this thought process further: If the universe is 13.77 billion years old, and the Earth only 4.54 billion, there could have been several full solar systems that came and went before ours even started.

Let’s say a solar system formed when the universe was 5 billion years old, and matured to the current state of ours at 9 billion years. There could have been highly advanced civilizations in that solar system that never knew about ours, since our own sun had not even formed at that time.

So when we think about civilizations in the universe apart from ours, we have to think not only about those that may exist right now, but all of those that have ever existed, and now we can multiply the current estimates of possibly trillions (see my post about this here) to millions of trillions.

Meanwhile, it would be nice if we could finally find just one flash of consciousness in the vast dark. Just one.