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How to answer cosmic queries

23 Aug 2018
Taken from the August 2018 issue of Physics World

Jillian Scudder answers tricky questions about astrophysics

Stars as a lion's head
(Courtesy: Shutterstock / Anton Khrupin)

When you create a blog (Astroquizzical) whose sole raison d’être is answering questions from the general public about space, you anticipate that there will be some unusual questions coming your way. But some of these are so unexpected and stray so far afield from the sorts of questions that, as a trained scientist, I would pose, that you really have to stop and think about how best to answer them.

These unanticipated queries come in three broad forms. The first are questions that have mixed up or jumbled concepts, and what brings you pause is how to best untangle the knot of confusion. This gets us into questions like “How come the signal from our spacecraft doesn’t get lost on the way back to Earth? Wouldn’t objects in between them and us block the signal?” To answer this takes an understanding that spacecraft must ping information back to Earth, that astronomical distances make this task harder, and that a solid object (like a planet) must do something to that signal. What is missing from this picture, though, is a grasp of how fundamentally empty most of space is. To answer this, I must go into a combination of the emptiness of space, even in the relatively dense solar system, and the limitations of our solar-system explorations so far. It’s certainly true that we can’t hear from Mars when it’s on the other side of the Sun. But for spacecraft like NASA’s New Horizons probe, which is much further away than Mars, the signal from the craft can spend its time zooming through the mostly empty solar system, arriving safely at Earth without ever having to bounce off a planet.

The second type of perplexing questions are those from children, who have, in their deep curiosity, invented a theory, but with no background material whatsoever. These questions are often the most unexpected, but can usually be answered using a relatively straightforward path – children want to know how it works, so there’s not so much untangling of existing knowledge to do. These questions can, however, be the most amazing scenarios you’ve ever heard. One of my all-time favourite questions was passed on by the parent of a five-year-old, who wanted to know if the universe was tiger-shaped, surrounded by dinosaur bones. I unabashedly love this cosmological hypothesis, which, in this case, even the child was not so certain of. Fortunately, the tests which have told us that the universe is fundamentally flat also (alas) rule out a tiger-shaped universe.

A five-year-old wanted to know if the universe was tiger-shaped, surrounded by dinosaur bones. I unabashedly love this cosmological hypothesis

The last are people who aren’t super-confused, have a little bit of information in their heads and have come up with some fantastical scenario, but can’t figure out how that scenario would work given their existing information. This is the set of questions for which I usually have to do the most work. All of the submitted questions require a degree of research to answer them, but these hypotheticals often require me to extract numbers from technical papers, make some assumptions, and work out some values that are useful for that scenario. This is where “if you didn’t burn up, could you surf on the Sun?” fits. “If you could divide the Sun into two half-sized Suns, what would happen to the Earth?” “Can you actually take an entire star and pull it into a planet sized object, the way they did in Star Wars?”

While these ones are my favourites to answer, they are simultaneously the trickiest. How do you figure out if you could surf on the Sun? What’s the density of the surface of the Sun relative to water? A billion times less dense. Ok, so what if you make the surfboard bigger? That’s a possible solution – increase the surface area, and you can increase the buoyant force from the Sun. But you’d need to have a surfboard 26 m long, and it would need to weigh barely as much as a mosquito in order to float on the surface – a technologically impossible task. To actually balance a human on board, you’d have a surfboard the size of Manhattan.

What about half-sized suns? This question boils down to “are two suns of 50% the mass of the Sun equal in brightness to 1 Sun?” The answer to this is no – 50% of the mass only gets you 10% of the light, so at a maximum, the Earth would be getting 20% of the light it currently receives. If you don’t like winter now, you’d really dislike the planetary deep freeze that the Earth would endure if we split the Sun in half. Mercury would start to look balmy at that point, but with Mercury’s strange rotation, both the days and the nights would be lengthy.

Can you compress a star into something smaller? Sure, but that would be at the expense of whatever you hope to use to contain the star, because if you compress a star, you wind up with some kind of stellar remnant – a neutron star or black hole. The gravitational forces surrounding these objects are extreme, and would rupture any rocky structure surrounding it. And if you cut deeply into a planet the way that Starkiller Base did, you’d have a lava trench, as the mantle of the planet would be exposed.

These are only a few of the many questions that have been answered. There’s a backlog of equally excellent questions waiting for me to get to them, and even more answered in my new book Astroquizzical: a Curious Journey Through Our Cosmic Family Tree.

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