Aliens could be spying on Earth’s historical landmarks from quadrillions of miles away.
Research suggests certain types of advanced ETs might be able to pick out buildings from a vast distance, but only from 3,000 years ago due to the time it takes for light to cross the universe.
A study to be published in scientific journal Acta Astronautica, which considers if aliens are watching humans, said: “In particular the question is: can the artefacts of our technological society be visible and potentially detectable by the telescopes of ETs?
“Since the question is to identify our society with civilisation the major focus should be on the search for large ships, buildings and space satellites etc.
“Such artefacts might easily be identified as artificial constructions. For this purpose it is natural to focus on the visible light reflected from the corresponding objects.
“To identify an observed object with an artificial one the best way is to spatially resolve it. Therefore, optical telescopes will be used.”
Study author Zaza Osmanov – a research affiliate at the Search for Extraterrestrial Intelligence Institute, also known as SETI – added according to the laws of physics the maximum distance for potential detection would be around 3,000 light-years.
It means aliens 17.6 quadrillion miles away could be using huge telescopes to pick out structures of the major civilisations of the time, which would include the Romans, Greeks and Egyptians.
Researchers suggested there could three types of alien civilisation based on how advanced different ETs may be.
Type one is an alien society that consumes the total energy of a planet with a host star, while the second is an extra-terrestrial civilisation that uses the total energy of a star.
Type three is an advanced alien society that consumes the total galactic energy.
Researchers said highly intelligent aliens could deploy several telescopes working together or use super computers to snoop on the Earth.
Their study added: “Instead of using large telescopes of astronomical sizes one can apply long baseline optical interferometry by using at least two telescopes separated by a huge distance.”