科学美国人60秒:Lasers Could Hide Earth from Prying Aliens

Lasers are the preferred weapons for interplanetary warfare in countless works of science fiction. But they might actually be better at keeping the peace. Because a new study finds that lasers offer an efficient, elegant way to conceal our planet, hiding us from any prying malevolent aliens throughout the cosmos.

“Let’s do it—let’s cloak the Earth.”

David Kipping, a professor of astronomy at Columbia University who came up with the idea along with a graduate student, Alex Teachey.

DK: “So why would a civilization want to hide?”

Teachey:

AT: “So a lot of scientists, including Stephen Hawking, have said it actually might be dangerous for other civilizations out there to know that we exist. If you look at the history of humanity there have been cases where two civilizations have come in contact for the first time, one of these civilizations with a slightly more advanced technology, and that group has ended up subjugating the other group. And so, ya know, something like that could happen on a planetary scale.”

Over the past two decades, astronomers have discovered thousands of planets orbiting other stars. Most of those were found by observing temporary small decreases in the light coming from those stars due to a planet passing. That mini-eclipse is called a transit. And it’s possible that from the point of view of some intelligent aliens the Earth causes small dips in our sun’s light—telling anyone watching that we’re here. So how can we hide? By making up for the sunlight we’re blocking.

AT: “And you can do this with lasers—moderately powered lasers. That might sound a little counter-intuitive because lasers are very narrow when they are first emitted, but if that light travels across many light-years that beam widens significantly, and so the beam width can be on the order of tens of millions of kilometers across or more, and so any planet lying within that beam would be unable to see our transit.”

Cloaking the Earth from the view of aliens would require firing a 30-megawatt monochromatic laser once per year towards the targeted star system for the duration of our planet’s transit across the sun—something not nearly as difficult as it may sound.

DK: “The earth’s transit takes about 10 hours, and it does that once every 365 days. So you could essentially have just a few meters squared of solar panels collecting sunlight, storing it up, and then being released at a peak intensity on the order of megawatts for these ten hour periods. So the actual energy requirements, when you think about it, over the entire year are very, very low…an alternative thing you could do is just build a satellite…you’d just have solar panels about the same sort of size that the ISS, the International Space Station, has. You would store up that energy over the course of a year and then you’d release it with a very high power laser over the course of ten hours once per year.”

The concept is in the Monthly Notices of the Royal Astronomical Society. [David M. Kipping and Alex Teachey, A Cloaking Device for Transiting Planets]

Of course, the laser cloak could not entirely conceal the Earth or its life from the rest of the universe. Aliens could always find us in other ways besides transits, for example, with telescopes so big they could snap pictures of our planet from light-years away like galactic paparazzi. Which is fine with Kipping and Teachey—their laser-cloaking system, they say, could also be repurposed for bolder, more flamboyant things: instead of hiding, we could create a cosmic beacon to attract the attention of intelligent aliens. Perhaps one day we’ll need to send out an SOS in the hopes that some good aliens will come to save us from evil ones—or, more likely, from ourselves.

—Lee Billings