The universe has been around for more than 13.7 billion years, which seems like plenty of time for another civilization to develop. But we haven’t found any evidence of life, and nobody’s said hello to us either. That’s the Fermi Paradox.
So maybe other civilizations don’t exist. Or maybe … they’re hiding. Because, theoretically, we could hide the Earth from faraway telescopes, just by shooting some lasers.
Over 70% of the exoplanets we know of were found with the transit method. When we point a telescope at a star, we expect to see all the light that’s shining in our direction – which depends on the star’s temperature, size, and how far away it is. But sometimes a planet is orbiting that star, and transits, or crosses exactly between the star and our telescope. The planet blocks some of the light, creating a dip in the measurements called a transiting light curve. And a regular pattern of dips tells us that an exoplanet is probably there.
So what if another civilization was using the transit method to look for us? Two astronomers from Columbia University think that we could /hide/ the Earth. In theory, we could /cover up/ those light curve dips – by shooting the right amount of light into space, in the right direction, at the right time. Which sounds kind of complicated. But you wouldn’t expect hiding a whole planet to be easy.
So this theoretical cloaking device is based on a couple assumptions. Like, first of all, we’d be assuming that another civilization was looking for the Earth. And that they’re using the transit method. That means we’d have to cross between their planet and our Sun. For that arrangement to happen, their planet would have to be lying somewhere along the ecliptic plane, which is the path the Earth is travelling on, as it orbits the sun.
Not to mention, we have to think about the technology they’re using to survey the sky. Take our old friend Kepler, which is a spacecraft that has found thousands of exoplanets through the transit method. Kepler uses an instrument called a broadband photometer, which collects as many photons of light as possible in a chosen range of wavelengths.
Most Sun-like stars emit their light at wavelengths of around 600 nanometers, in the orange part of the visible spectrum. So to hide ourselves from similar technologies, we could shoot an array of lasers into space with a wavelength of 600 nanometers, and a peak power of about 30 megawatts. Big, orange lasers? Check.
Sometimes exoplanet surveys use spectroscopy instead of photometry, though, with telescopes that measure light at lots of different wavelengths. Astronomers can use spectroscopy to figure out what’s in an exoplanet atmosphere because different gases absorb and reflect different wavelengths of light. So if we wanted to hide from /these/ kinds of instruments, we’d need lasers blasting light at a bunch of different wavelengths, which would get more complicated. Or we could pick a couple – like the light signatures of oxygen and ozone, which suggest there’s life here – and use lasers to pretend that we’re just a simple, lifeless rock.
Now, firing all these lasers for a long time sounds energy-intensive. But here’s the thing, we only need them as long as the Earth is transiting. That depends on how the alien planet is oriented and moving relative to ours. But by these assumptions so far, the astronomers estimated a transit of around 13 hours. Which means our array of orange lasers would need to be on for 13 hours, once per year. They could be on the Earth’s surface, but then we’d have to consider how the Earth rotates as it transits, and the fact that our atmosphere is in the way. So our best bet might be launching this laser array into space.
On the plus side, we could capture energy from the Sun to power them. But that would also mean manned missions to get everything into space and put together, which isn’t exactly cheap or easy.
Basically, we could hide the Earth with lasers; if aliens exist, they’re looking for us, living in the ecliptic plane, using Kepler-like surveys, and recording data for multiple years.
Even if all that was true, would we want to hide?
If we don’t, the astronomers also mentioned that their idea could be used to broadcast a signal into space. Using similar lasers, we could distort parts of our transiting light curve to look weirdly artificial, kind of like sending a bat signal into space, on the off-chance that anyone is looking.
But for now, this idea from a couple astronomers remains broadcast across our internet … instead of into space.