Washington: If alien life really exists, they would have found humans by now, but consciously avoiding us for unknown reasons, a new study has suggested.
Researchers, who calculated the time a society would take to muster up the resources and know-how to travel to another world, found that alien civilisations starting from the oldest stars in the galaxy would have had more than enough time to reach Earth.
"We are either alone, or they are out there and leave us alone," lead study author Thomas Hair, a mathematician with Florida Gulf Coast University in Fort Myers said.

Light travels at about 186,000 miles per second. Figure another 500 years to build new ships, set out again, and so on and so on, and the calculations show that civilisations starting out from the oldest stars in our galaxy would have had epochs of time to reach us by now.
So where are they? "They've either passed us by, or they stay around their home star systems and contemplate their navels," Hair said.
There could be several reasons why we are not listed in intergalactic Yelp. Perhaps most important is that we don't have anything aliens need.
"Any ancient civilisation is probably not biological. They don't need a place like Earth. They don't need to come here and steal our water. There's plenty of it out in the outer solar system where the gravity is not so great and they can just take all they want," Hair said.
Or perhaps modern-day extraterrestrials are following routes laid out long ago, all of which bypass Earth, he added.   

Hair presented his research at the Mathematical Association of America in Boston recently.
Whatever the reason we're being ignored, there is no chance ET, if he exists, does not know we are here, Hair said, pointing to telescopes, such as NASA's Kepler observatory, which can detect planets around other stars.
If humans living on a planet that is roughly five billion years old have technology like Kepler, an alien civilization with another 10 million years of experience under its belt would have advanced much further, Hair maintained.
"I'm sure they'd be able to detect if this planet had life on it. Just the CFCs (chlorofluorocarbons) in our atmosphere would give us away," he said.
CFCs are compounds typically found in refrigerants and aerosol products that release chlorine atoms when exposed to ultraviolet light and erode Earth's ozone layer.