We may be able to spot enormous alien spacecraft by the gravitational waves they would create. Gravitational waves are ripples in space-time formed when a massive object moves around, so if there are any extraterrestrials driving gigantic spacecraft around our galaxy, the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the US could potentially detect them.
Gianni Martire at , a research institute in New York, and his colleagues calculated how large such a craft would have to be, and how fast it would have to move, to create a gravitational wave big enough for LIGO to spot. They found that the craft would have to be about the mass of Jupiter, travelling at about one-tenth the speed of light – that is nearly 30,000 kilometres per second, faster than any star astronomers have discovered.
They found that LIGO could spot such a craft if it travelled within about 326,000 light years of Earth, and more sensitive planned gravitational wave detectors could extend that distance even further.
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“With trillions of stars out there, you’re telling me that one doesn’t have aliens that haven’t done this? Just one? I think the odds are in our favour,” says Martire. “I wouldn’t want to be on the team figuring out how to build a Jupiter-sized spacecraft, but the odds aren’t zero.”
This could also work for crafts using warp drives, theoretical engines that move by creating their own wrinkles in space-time. “There’s no way in hell we could detect a craft so far away in other ways, even if it’s as big as Jupiter,” says Martire. “The LIGO folks and the SETI [Search for Extraterrestrial Intelligence] folks should be best friends.”
But even though it is clear that extraordinarily massive spacecraft and warp drives should create gravitational waves, other researchers have expressed scepticism that we will ever be able to actually detect them.
“This is well and fine in principle, it’s just that I am surprised they find the result to be in the sensitivity range of LIGO,” says at the Munich Center for Mathematical Philosophy in Germany. “If the spacecraft was actually Jupiter, which is pretty close, maybe we’d actually be able to measure it, but if it was much further away, probably not.”
Even if we were to detect gravitational waves from a massive, fast-moving object, it would be difficult to discern whether it was an alien spacecraft or a natural phenomenon. It could be easier for warp drives because of the unique way they are expected to affect the gravitational field around them. “How do you know the difference between a comet and the starship Enterprise? You can tell between a rock and a warp drive the same way you can tell whether a jet ski went past or a boat,” says Martire. “They both create waves, but they have a particular signature in their wake.”
There is no downside in checking for an outlandishly enormous spacecraft in LIGO data, says Hossenfelder. “It’s there, it’s collected already and freely available, I don’t see why not,” she says. “I’m just not that excited about the idea that they’ll actually find anything.”
But if they do, even if it isn’t an alien spacecraft but simply a huge object moving far faster than we expect anything that big to go, it would be an important find. “Even if it’s not aliens, it’d be something new,” says Martire. “It would be exciting no matter what it is.”
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