A radio signal detected by an Australian telescope in 2019, which seemed to be coming from the star closest to the Sun, was not from aliens, researchers report today in two papers in Nature Astronomy.

“It is human-made radio interference (干扰) from some technology, probably on the surface of the Earth,” says Sofia Sheikh, an astronomer at the University of Berkeley. But the signal, detected by Breakthrough Listen — an organization in search of aliens — looked interesting enough at first that it sent astronomers on a nearly year-long research to understand its origins.

“It’s really valuable for us to have these experiences,” says Jason Wright, the leader astronomer of the organization. “We need these signals so we can learn how we will deal with them — how to prove they are alien or human-made.”

Since 2016, Breakthrough Listen has used telescopes around the world to listen for possible broadcasts from alien civilizations. The program has picked up millions of signals of unknown origin, nearly all of which could be classified as coming from radio interference on Earth, from sources such as mobile-phone towers or aircraft radar.

The 2019 signal was different. It was detected by the radio telescope in southeastern Australia and came from the direction of Proxima Centauri — the nearest star to the Sun. Proxima Centauri is of great interest to alien researchers not just because it is nearby. The star has at least two planets, one of which orbits at the right distance for liquid water to be on its surface. Breakthrough Listen even plans to send a tiny spacecraft to this planet in the future to look for life there.

In November 2020, and in January and April this year, the researchers pointed telescope at Proxima Centauri to see if they could pick up the signal again. They could not. Instead, the team spotted other “look alike signals”. These signals had been proved out by the team’s analysis as being earthly interference. Further analysis showed that these signals were all interference from an unknown source, maybe electronic equipment that got shut down or fixed

The Universe gives us a “haystack (干草堆)”, says Sofia Sheikh. “It is our need to find the needle in it, and make sure that it is actually a needle that we found.”

“We are running out of space and the only places to go to are other worlds.... Spreading out may be the only thing that saves us from ourselves. I am convinced that humans need to leave Earth.” These are the words of the famous scientist Stephen Hawking, spoken at a science festival in Norway in 2017, a year before his death.

Hawking was not alone in this view. Many experts feel that the only way for humanity to last far into the future is to colonize （移民） other planets. That way, if an asteroid （小行星）, a terrible disease, nuclear war, or some other disaster strikes Earth, civilization would still have a chance. Mars is one of the ideal destinations. NASA, SpaceX, and Mars One all have plans to send humans there. “Either we spread Earth to other planets, or we risk going extinct,” SpaceX founder Elon Musk said at a conference in 2013.

But not everyone agrees that colonizing Mars or any other planet is such a great plan. The most common argument against going is that it’s just too expensive or dangerous. It will take huge amounts of money and other resources just to get people there, let alone set up a place for them to live. It’s not even clear if humans could survive on Mars.

Maybe all the time and money people would pour into a Mars mission would be better spent on more urgent projects here on Earth, like dealing with poverty or climate change. Some experts argue that handling a problem like an asteroid strike or disease outbreak while staying here on Earth would be much easier and less expensive than surviving on a new planet.

In addition, moving to a new planet could harm or destroy anything that already lives there. Human visitors change or damage the Martian environment. Some feel that’s too much of a risk to take.

Until recently, gravitational waves could have been the stuff of Einstein’s imagination. Before they were detected, these waves in space time existed only in the physicist’s general theory of relativity, as far as scientists knew. Now, researchers are on the hunt for more ways to detect the waves. “The study of gravitational waves is booming,” says astrophysicist Karan Jani of Vanderbilt University in Nashville. “This is just remarkable. No field I can think of in fundamental physics has seen progress this fast.”

Just as light comes in a variety of wavelengths, so do gravitational waves. Different wave lengths point to different types of origins of the universe and require different kinds of detectors. Gravitational waves with wavelengths of a few thousand kilometers—like those detected by the United States, Italy and Japan—come mostly from pairs of black holes 10 or so times the mass of the sun, or from collisions of dense cosmic blocks called neutron stars (中子星). These detectors could also spot waves from certain types of exploding stars and rapidly moving neutron stars.

In contrast, huge waves that span light-years are thought to be created by orbiting pairs of bigger black holes with masses billions of times that of the sun. In June, scientists reported the first strong evidence of these types of waves by turning the entire galaxy (星系) into a detector, watching how the waves make slight changes to the timing of regular blinks from neutron stars throughout the Milky Way.

Physicists now hope to dive into a vast, cosmic ocean of gravitational waves of all sorts of sizes. These waves could reveal new details about the secret lives of exotic objects such as black holes and unknown parts of the universe.

Physicist Jason Hogan of Stanford University thinks there are still a lot of gaps in the coverage of wavelengths. “But it makes sense to cover all the bases. Who knows what else we may find?” he says. The search for capturing the full complement of the universe’s gravitational waves exactly could take observatories out into the moon, to the atomic area and elsewhere.