The contradiction
Our galaxy alone contains hundreds of billions of stars, most of them have planets, and some of those planets have conditions similar to Earth's. The galaxy is old enough that a civilization that arose 1 million years before ours would have had time to spread very far.
Yet decades of searching have produced not one confirmed signal. That raises the question attributed to a physicist who supposedly asked it over lunch: if there are so many of them, where is everybody?
The main explanations
- The filterSomewhere between simple life and civilization is a step that almost nobody gets through. The question is whether we've already passed it or still have it ahead of us.
- Rare EarthSo many conditions have to align so precisely that complex life arises extremely rarely.
- SilenceCivilizations exist, but they don't make noise: perhaps they aren't interested, it's unsafe, or they communicate in a way we don't know.
The first explanation is the most unsettling. If the difficult step is already behind us - for example, the emergence of life itself - we've been lucky. If it's still ahead, civilizations regularly fail to survive some stage, and we're only approaching it.
The lack of discoveries is data, but weak data. We've listened to a tiny fraction of the sky across a narrow range of frequencies and for a very short time by cosmic standards.
The scale of the search really is modest. A comparison people often use: it's like scooping a glass of water from the ocean, finding no fish in it, and concluding that there are no fish. It's too early to claim they're absent.
There's also a technical caveat. Our own radio signals have become weaker and more directional over the past decades: analog broadcasting has given way to digital and satellite transmission. Perhaps civilizations are noisy for only a short time, then become quiet.
The search has now shifted from radio signals to other things: signs of technology in the atmospheres of distant planets, unusual sources of light, and signs of chemical disequilibrium. It takes longer, but it doesn't require anyone to signal us deliberately.
What is the contradiction?
How strong a conclusion can we draw from finding nothing?
Sources
- The question of the contradiction between the likely prevalence of life and the lack of observable signs was formulated in the mid-20th century
- explanations include the great filter hypothesis, the rarity of the necessary conditions, and the low detectability of civilizations
Next in the “Life in the Universe” series
2 articles in full on the site, 3 more in the app
