It's likely that life happened elsewhere. We are completely alone as far as we can tell. Where is everyone?
A new analysis suggests an alternative solution to the problem of being impatient. It's possible that alien civilizations would wait for one to start broadcasting their presence before they send a probe. Someone might come knocking if we wait a long time.
"Where is everybody?" is said to have been asked by renowned physicist Enrico Fermi during a casual lunch conversation in 1950. We are the only planet in the universe that is intelligent. The Milky Way is one of up to 2 trillion galaxies in the entire universe. Physical processes don't happen just once on most planets. It should happen somewhere else if nature can make life here.
Why are we still looking for aliens?
Some of the intelligent animals would explore their neighborhood and build spaceships. They could spread themselves far and wide. They could swamp the entire universe in a few million years if they averaged only a small percentage of light's speed.
Even if most intelligent civilizations fail in the attempt, the fact that our universe has existed for over 10 billion years means that at least one civilization should have seen it.
Nothing yet. We don't have any evidence for an alien civilization. If life can happen, it should be common, and if it isn't, we should know about it. We do not.
Astronomers have come up with many solutions over the years. The rare Earth hypothesis suggests that life is special and unique on the scale. Life is so rare that we may be the first creatures to arise in the universe. The circumstances that led to the emergence of life on Earth are so special that it happened only once.
The great filter hypothesis postulates that intelligent life is not easy. It took billions of years for intelligence to arise after life appeared relatively early in the history of our planet. Nuclear weapons and climate change put the future of our species in question. We aren't likely to meet anyone capable of conversation if we findbacteria or other simple organisms.
A new paper written by Amri Wandel at the Hebrew University of Jerusalem and published in the preprint database arXiv suggests that we have only recently arrived on the Cosmic scene.
The powers of advanced alien civilizations are not infinite. Energy capture and storage, waste heat, information processing, and a finite amount of time are some of the mundane issues they have to deal with. It's reasonable to assume that alien civilizations wouldn't be able to send active probes or messages to every single planet in the universe.
It is easier to build large, sophisticated listening stations than it is to build active probes. The magic of radio will eventually be discovered by an intelligent civilization. Civilization will broadcast their presence through artificial signals.
If the aliens received a signal, they would use it to craft a message of their own or a probe to visit their new friends.
It takes a while. A large amount of it. Our radio bubble is less than 200 light-years wide, compared to the 100,000-light-year width of the entire Milky Way, because we have been broadcasting for less than 100 years. It may take a long time for our signals to reach an alien civilization. If they respond with a signal of their own, we could get it in a few thousand years because we would have to be looking in the right direction.
It will take even longer to get here if the aliens decide to send a probe.
Our universe is home to many other advanced civilizations. They haven't responded yet.
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