The search for aliens is proving to be quite the intergalactic slog.

Just ask Jill Tarter, one the most prominent figures in the field of science known as the Search for Extraterrestrial Intelligence (SETI). The inspiration for Jodie Foster's character in the 1997 sci-fi movie Contact, Tarter recently explained to TechCrunch that the primary method for trying to detect the presence of intelligent life in space involves looking for radio waves and laser signals that the natural world could not have created.

Here are three reasons humans have yet to answer the question of whether we're alone in the universe, as explained by Tarter to TechCrunch.

1. We haven't been looking that long.

Humans have been curious about the existence of other life forms in space for thousands of years, but it's only relatively recently that we've had a prayer of actually finding any. Computers and radio telescopes that could be used for SETI have been around for only about 60 years. 

2. The amount of space we've explored is miniscule.

Perhaps the best way to understand how large outer space is--and how little of it humans have actually searched--is to compare it to the size of Earth. As Tarter explained to TechCrunch, if the size of space were equal to the size of the Earth's oceans, humans would have explored an area equivalent to a single glass of water. While the rate at which we're exploring may sound discouragingly slow, Tarter notes that in the near future, scientists will have the ability to look at areas the size of "pools" and "lakes."

3. We may need technology that hasn't been invented yet.

The technology that exists today, while allowing humans to capture images of space billions of light years away, might still be inadequate for conducting a thorough search of space. "We may not have invented the right way to do this yet," Tarter told TechCrunch.

So what happens if aliens find us before we find them? Tarter points to research from Harvard psychology professor Steven Pinker suggesting that the more humans evolve, the kinder we become. Using that logic, Tarter posits that for aliens to develop technology advanced enough to bring them to Earth, perhaps they would have already evolved beyond the point of behaving aggressively. Still, "this is all speculation," Tarter explained. "No one can really know."