Fermi paradox

'''Enrico Fermi's paradox** wonders why, with the age and size of the universe and billions of stars and planets that have existed for billions of years, we have not detected any other alien civilizations. There have been attempts to resolve the Fermi paradox by finding evidence of alien civilizations, along with thoughts that such life could exist without humans knowing.

The physicist Enrico Fermi first asked the question in an informal discussion in 1950. A paper by Michael H. Hart in 1975 made scientists more curious and they began to study the question in more detail. This is why some people call it the **Fermi–Hart paradox'*. Other common names for the same phenomenon are *Fermi's question* ("Where are they?"), the *Fermi Problem*, the *Great Silence*, (Latin for "the silence of the universe"; the misspelling *silencium universi'' is also common).

## Background 
Fermi first posed the paradox informally

* The universe is old, there are many stars. In the case that life developing on earth is not exceptional, life outside earth should be common.
* Life is no exception  among the planets, our solar system isn't special; therefore, there must be other technologically advanced civilisations in our galaxy.
* The galaxy has a width of about 100.000 light years. In a project, such as Project Icarus, a  propulsion has been developed  that can reach between 1 and 10 % of the speed of light. With such a propulsion, travelling from one edge to the other (in a straight line, without stopping) takes between one and ten million years. The galaxy has an age of about 10 billion years. This means that there is enough time to travel this distance, even multiple times.
* If there is or was at least one other civisation in the milky way, it must have left some traces.

The paradox is that no trace of any other civilization outside earth has been found.

## Why interstellar communication is difficult 
### Space is deep 
Extraterrestrial life might exist, but humans may not know about it. To send signals across long distances in our galaxy, and the universe, is difficult. This is because the power output needed to project the signal would be very large. Coherent (laser-like) signals travel at the speed of light, but our own galaxy is 100,000 light years across. Alien civilizations may not be able to detect the signal, or they may not understand the communication or the method used to communicate.  

### Space is dusty 
Space is full of dust clouds and magnetic fields, which would disturb or eliminate even a coherent (laser-like) beam of electromagnetic radiation. Thousands of tons of cosmic dust  are thought to reach the Earth's surface every year. A signal would face much interference in any long-distance travel across space.

If it is difficult or impossible for us, it may also be difficult for other forms of life. As for other galaxies, the Andromeda galaxy is 2.5 million light years away. 

## Time 
In our Galaxy, the time it takes for signals to travel is very large. Vast eras of time would pass before a reply could be received. There are theories about "worm-holes" in space in which communication could be sent through space more quickly, but these are just theories now. There is still a desire for space travel. Locally in the Solar System, the main goal is to get minerals which are scarce on Earth.

## Related pages 
* Carl Sagan
* Drake equation

## References