Kill the sun! How wild thought experiments drive scientific discovery

Listen, it’s not that I actually want to kill the sun – I just want to figure out how. But when I told my colleagues at New Scientist that I was recruiting scientists to do just this, I was met with baffled looks. I write about space every single day, an­d I find it endlessly fascinating. I feel at home with the mysteries of the cosmos, so why would I want to ruin any part of it, let alone kill our beloved and essential star?

Despite their confusion, my colleagues indulged me and my partner in destruction, our US editor Chelsea Whyte. We started reaching out to free-thinking astrophysicists and planetary scientists, asking them to join us on our podcast, Dead Planets Society. With them we began tinkering with the universe – in our minds, at least – not only killing the sun but imagining a gravitational wave apocalypse, what would happen if we sliced the moon in half or chiseled the Earth into a cube.

As we thought about questions to ask the guests we had on the show, who are all university professors and proper scientists, we found ourselves looking up tidbits about gravity and planetary science, doing calculations of escape velocities and Roche limits. As much as the podcast was a flight of fancy – a fun game to play – it also started to feel a bit like we were doing science. We realised that seemingly absurd thought experiments have always been at heart of the scientific method.

Science began with thought experiments rather than empirical experiments that are carried out with lab benches or telescopes, says philosopher H. Peter Steeves at DePaul University. Galileo Galilei, one of the founders of the modern scientific method in the 16th century, is remembered for dropping a feather and a hammer from the Leaning Tower of Pisa. By demonstrating they fell at the same rate, the story goes, Galileo overturned a 2000-year-old idea of how gravity worked. “[It] is as fanciful a story as Newton getting hit on the head with an apple,” says Steeves. “But there is evidence that he engaged in a thought experiment to demonstrate how Aristotle’s conception of gravity was incorrect.”

Over the course of history, we have become far better at performing practical experiments, but thought experiments remain important. For example, Albert Einstein, who also transformed our view of the universe by grappling with gravity, is renowned for conjuring absurd scenarios in his head. One evening as he was riding in a streetcar, he imagined what the world would look like if he were travelling at the speed of light. After years of frustration trying to explain the behaviour of light, this was the seed that grew into special relativity in 1905. “Imagining things on this grand scale presents the familiar in a different way,” says philosopher Guy Kahane at the University of Oxford.

This is no less true today. Indulging in thought experiments is common practice among theorists trying to understand black holes and what these extreme objects reveal about the nature of reality. For instance, the intense gravity of a black holes mean that you can’t place any scientific instruments right next to one or inside one in order transmit data back out. So theorists spend a whole lot of time thinking about and calculating what might happen to an observer in one of those positions – leading to all kinds of surprising insights about concepts like time and causality. “Once you’re thinking in this playful way, you start to see things that you wouldn’t see otherwise,” says Kahane.

In the first episode of season two of Dead Planets Society, we carry out our own black hole thought experiment. Black holes are often thought of as massive voids that swallow everything that comes near them – they are the ultimate destroyers. So what would we learn by trying to demolish one? Searching for the black hole’s weak spot, we considered using infinitely fast spacecraft to escape a black hole with some of its mass, or unrealistically powerful magnets to rip it apart. We can’t actually build these cosmic tools, but imagining them reframed how we thought about black holes.

Over the course of recording the episode, this led us to think of black holes in new ways. Using quantum mechanics, we can picture them as incredibly massive objects that happen to have escape velocities higher than the speed of light, or according to general relativity, they are infinitely deep divots in space-time itself. The latter, for the record, is much harder to destroy.

Freedom from seriousness is an opportunity that Chelsea and I run wild with in the podcast. Cosmologists can benefit from thinking like this too, says Wendy Freedman at the University of Chicago. As we observe more and more astronomical anomalies, jarring with the standard model of cosmology, it is becoming evident that our best empirical theory of the universe is due an overhaul. “As the data get better and better and the theories get more and more creative, something will fit,” says Freedman. “We need wacky ideas right now, because there are so many things that we don’t understand.”

Nobel prizewinning cosmologist Jim Peebles, one of the architects of the standard model, agrees that this sort of playful thinking “is an important part of science”, so long as you get the balance right. “I indulge in blue-sky thinking; it’s… a time sink if overdone and a loss if suppressed,” he says.

Now, I am not claiming that Chelsea and I are going to solve the problems with the standard model of cosmology by considering how to give the Milky Way more arms. But I do think that something is lost when scientists take themselves too seriously. Sure, the conversations we had while making Dead Planets Society are a little goofy, but they are also some of the most thought-provoking exchanges I have ever had.

“If you cut the moon in half, blew up the sun or suddenly turned the Earth into a cube, well, this is all interesting – and not just to Dr Evil, a Bond villain or the Borg,” says Steeves. “It pushes us both to think about limit cases given our current understanding of science and to have fun while doing it. Both of these are important: the pushing and the fun.”

If we weren’t having fun, we never would have realised that if the sun were to disappear, whales would outlive humans. It turns out this is true for most other types of apocalypses too, so underwater life may have a better chance out there in the universe than land-based organisms.  We certainly would never have thought of using aerogel as a sort of cosmic fly strip to catch asteroids.

Steeves quotes Rob Reiner’s cult movie This Is Spinal Tap, which he describes as a font of scientific truths: “It’s such a fine line between stupid and clever.” In other words, turning our silliness up to 11 doesn’t mean we won’t end up with clever or interesting ideas.

The universe is big and messy and sometimes it feels like anything that can happen, cosmically, probably is happening somewhere out there. That is the beauty of it. So idle speculation, no matter how outlandish, is not necessarily useless. It can help reveal the secrets of the universe – even if it does mean thinking like a cartoon villain and, sometimes, trying to kill the sun.

And as for my colleagues’ bafflement, I will let Steeves respond to that: “The sanity question is hard. Are you a maniac, Leah? Perhaps. But in the very best way.” I’ll carry that compliment with me as I continue to imagine exploring and occasionally ruining the cosmos.

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