Dr. Nathalie Cabrol works for the Search for Extra-Terrestrial Intelligence Institute (or SETI), which is a non-profit organization that aims to explore, understand and explain the origin and nature of life in the universe. Cabrol is leading SETI's hunt for aliens, and she says the fault lies in our technology. Here are some ways aliens could be trying to get in touch that we keep missing:
Right now, our instruments can only detect radio waves and visible light. However, it is possible that a more highly advanced alien civilization could produce coded messages at other frequencies on the electromagnetic spectrum. We know these are possible, but we don't have the technology to look for them.
Stars (like our sun) give off an enormous amount of energy. Most of it goes out into space, but if we were able to capture all of it, we would have... an enormous amount of energy. This would also cause the sun to exhibit unnatural light fluctuations, and form something called a Dyson Sphere. It's possible that extra-terrestrial creatures have managed to create a Dyson Sphere using their own stars. Unfortunately, we don't have the technology to see the telltale light fluctuations.
We've sent our own robots (such as satellites and rovers) out into space. What's to say that other advanced civilizations haven't done the same? Maybe they have, but knowing how vast the distances between stars and planets are, we know it would take eons for anything to reach us.
We've estimated there are about 10,000 stars for every grain of sand on every beach on Earth. A conservative estimate put the number of Earth-like planets at 100 per every grain of sand on Earth. If 1% of those planets have developed intelligent life like ours, that would mean there are 10 quadrillion civilizations in the observable universe. So where is everybody? Are the distances just too vast? Is there a point in time past which civilizations just don't survive? Are we at that point now?
There are even some people who believe that we have already been visited by aliens. They theorize that humanity's evolution, both biological and socio-cultural, can be attributed to extraterrestrial beings. These are only theories, though, and not accepted by most scholars.
Most humans ”” scientists and laypeople alike ”” are excited about the prospect of contacting another intelligent civilization. However, not everyone is as enthusiastic about the idea. Physicist Stephen Hawking has spoken publicly about his fears that an advanced alien civilization would have no problem wiping out all humanity if they ever found us. Speaking at the Royal Society in London, Hawking said, "We don't know much about aliens, but we know about humans. If you look at history ... encounters between civilizations with advanced versus primitive technologies have gone badly for the less advanced."