One of the world's most respected theoretical physicists Michio Kaku has predicted that humans will make contact with an alien civilisation within this century.

While he doesn't know if we'll be able to talk back to them by then, Kaku thinks that humans will detect aliens before 2100 by listening in to their radio communications.

He gave the prediction on reddit last week during an Ask Me Anything (AMA) question-and-answer session.

"Dr Kaku, If we make contact with alien civilizations, then what? And how will we talk to them?" asked redditor MalachiNorris.

Kaku responded:

"Let me stick my neck out. I personally feel that within this century, we will make contact with an alien civilization, by listening in on their radio communications. But talking to them will be difficult, since they could be tens of light years away. So, in the meantime, we must decipher their language to understand their level of technology. Are they Type I, II, or III??? And what are their intentions. Are they expansive and aggressive, or peaceful."

The "types" of civilisations he's referring to represent the three categories in the Kardashev scale - a scale that measures a civilisation's technological advancement based on the amount of energy they are able to use for communication.

For reference, humans aren't quite a Type I civilisation as yet, as that would require learning to harvest every ounce of energy from their home planet, including all solar, thermal, oceanic energy, and more.

As Kaku added to his response: "Another possibility is that they land on the White House lawn and announce their existence. But I think that is unlikely, since we would be like forest animals to them, i.e. not worth communicating with."

Ooh, burn.

Kaku's not the first scientist to suggest we're not far off making contact with extraterrestrial life.

Seth Shostak, a senior astronomer with the Search for Extra-Terrestrial Intelligence (SETI) Institute has placed a wager we'll find signs of alien life within just 20 years.

Even Stephen Hawking recently said he's "more convinced than ever that we are not alone" and warned humanity to stop reaching out to aliens before it's too late. 

Hawking is involved in the Breakthrough Listen project, which will scan planets around the nearest million stars for signs of life within radio signals - including Earth-like planet Gliese 832c.

"One day we might receive a signal from a planet like Gliese 832c, but we should be wary of answering back," said Hawking.

Shostak, Hawking, and Kaku are all predicting that alien contact will come through radio waves, a narrow frequency band of electromagnetic waves that are often used for long-distance communication.

Scientists are so focussed on this small part of the electromagnetic spectrum because it stands out from the naturally occurring background noise in the Universe, which typically extends across a much wider frequency range.

Radio telescopes around the world are constantly scanning the Universe for any signs of life, and although we've had some close calls, we haven't seen anything as yet to suggest aliens are out there.

But according to the Drake equation, based on the sheer number of stars out there, it's highly unlikely that we're the only life in the Universe.

Alternatively, another alien civilisation might end up detecting our radio wave communications first. If that's the case, let's just hope they're friendly.