Sciencefail rant: Across the Universe by Beth Revis

First things first: sorry it's been a while between posts. Life has been busier of late and I haven't quite had the brain space to devote to writing a serious sciencey blog post. Until now. I was reading Across the Universe by Beth Revis, a recent YA science fiction book set on a generation ship and just as I got up to the "ooh, things are getting interesting" plot-thickening part, I was smacked in the face by an epic science fail. This is what I am now going to rant about.

There will be spoilers. Many crucial spoilers. If you'd rather read a spoiler-free review and live in science fail ignorance, then you can read my ordinary review here.

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I mentioned spoiler-warning, right?

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Don't read on if you don't want to be spoiled.

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The Fail of the Science

Some background

In Across the Universe we have two main characters: an American teenage girl and the future leader of the generation ship. The girl gets frozen and loaded onto the ship as cargo because her parents are part of the colonisation mission on the new planet they're going to. For reasons unimportant to science fail (and which I hence won't spoil), she is accidentally unfrozen early, supposedly 50 years before they're due to land. The entire journey was supposed to take 300 years.

When she wakes up, she finds herself in a very different world to the Earth she left behind. Blah, blah, dystopia -- if you're interested in that aspect, go read my proper review. In the course of events, the two main characters discover that among all the secrets and lies aboard ship is the secret of what's going on with the ship's engine.

The parts I don't have a problem with is that the engine nuclear and they have some sort of process which is supposed to recycle the uranium so that it keeps running long enough. I mean, I'm sceptical of the whole re-enriching uranium part -- entropy, conservation of energy, the lack of a particle accelerator on board, etc -- but I'm willing to buy future technology with fancy engines. If we didn't have future tech with better stuff than our present tech, then science fiction would be a little dull.

Unfortunately for the residents (and cryo-residents, I suppose) of the generation ship, there is something wrong with the engine. It is losing efficiency. Given the magic physics that was making it run in the first place, this isn't surprising either. Do you know what is surprising? The fact that the engine failing is somehow slowing the ship down.

SPACE IS NOT AN OCEAN!

I know, I know, I don't usually actually scream when I'm ranting, but this blatant disregard for one of the most basic and fundamental ideas in physics infuriated me. I shouted, in real life, and bashed the book on the couch in my frustration* at this neglect of research. Ask anyone who's taken a first year university physics subject, gosh, even anyone who passed high school physics, and they should be able to tell you what happens on a spaceship when the engines fail.

It keeps going in a straight line until it hits something.

IT DOES NOT SLOW DOWN.

Galileo, who lived way back in the 1500s–1600s, worked out that an object will continue to move in a straight line. Newton, in 1687, appropriated this concept and dubbed it his first law of motion:

A body in motion will continue moving with a constant velocity unless an external force is applied to it.

On Earth, friction is usually that external force. Your car's engine has to keep running while you drive because if it doesn't, you're car will eventually roll to a stop as the friction between the axle holding the wheel in and whatever's on the other end of the axle. A boat slows down because of the drag force of the water around it -- drag force being a type of friction. Your bike might roll down a hill, but if you don't pedal, friction in the axles will eventually bring you to a stop. An aeroplane needs to keep firing its engines because of the drag force of the air slowing it down (they can sometimes coast down to a landing if the engines fail, but they need to over come the drag to maintain a constant speed so that they can stay in the air because of other physics I'm not going to go into right now).

You get the idea.

The drag force happens because something -- air particles, water molecules, etc -- collide with the moving object and push it slightly in the opposite direction. If only one particle hit the much larger object, it wouldn't make a difference, but there are very many particles in the air around you right now. There are even more surrounding a boat (or person) in water. That's why it's harder to move underwater than in air. The fewer particles around to collide with an object down, the less it will be slowed down.

Space, unlike the surface and atmosphere of Earth, is characterised by its vacuum. It's lack of anything substantial. There is no air in space. Sure, there are a few stray molecules and atoms floating around but, except in the densest of nebulae/molecular clouds, they are far sparser than even the best industrial vacuum we can create on Earth.

In space, there is no drag force. There is nothing to slow you down. If your engine failed, you wouldn't slow down, you would just keep going, indefinitely, until you collided with something, or came close enough to a gravitational field (of a star, for example) to change direction. Then you would keep going in that direction unless you were particularly well aimed to go into orbit around that star.

So when the engine of the generation ship in Across the Universe starts to fail, their problem isn't that it will take them longer to reach their destination. If it fails completely, they will not be "dead in the water". There is no water. It might be called a spaceship, but that doesn't mean it shares the same watery drag force as an ocean liner.

Their problems are more likely to be related to not being able to land or go into orbit around their destination, or not being able to make course corrections, or not being able to slow down and zooming straight past their destination.

By a similar token, people or things ejected out of the airlock wouldn't get left behind. Again, space is not an ocean. Once the airlock is opened and the air rushes out (pushing any lose objects out with it, perhaps), the ejected objects would appear to float close to the ship, continuing to move in the same direction along with the ship. If someone was thrown out an airlock, their body would only stop shadowing the ship when the ship did one of: speed up, slow down or change direction.

I grant that if the ship has magic artificial gravity (which the generation ship in Across the Universe does), some strange things might happen to throw the body further away from the ship, or make it somehow react unusually with the artificial gravitational field, but there was absolutely no indication of that being the case in this book.
 
* Don't worry, the book was unharmed.

Acceleration?

My first thought, in my brain's desperate attempt to fix the gaping science fail hole in Across the Universe, was that maybe the ship was accelerating and that's why they needed the engine to maintain efficiency and why things thrown out of the air lock got left behind.

Unfortunately, it can't have been.

According to the original mission plan (which the book gives us no reason to believe is a trick), the voyage is supposed to take 300 years. Also, their destination is called Centauri-Earth (and our world is referred to as Sol-Earth). This could refer to Alpha Centauri, the closest star, but given the fact that the planet they're headed for is supposed to be habitable, that doesn't seem likely (Alpha Centauri is a triple star system and the chances of conveniently habitable planet being there are slim). So it must be another star with Centauri in the name. There are lots. Here is Wiki's list of stars in the Centaurus constellation. If you sort that list by distance, you see that there aren't that many stars within 300 light years.

Since no relativistic effects are ever mentioned (see this blog about travelling close to the speed of light, and this one about accelerating up to fractions of the speed of light), it seems fair to assume that they never reach an appreciable fraction of the speed of light. Let's say that means less than around five percent time dilation goes on (see aforementioned links for previous posts if you're lost at this point). Well, travelling at a third of the speed of light gives us six percent time dilation, so close enough. So the maximum speed we're allowing is 0.33c. If we ignore acceleration, that limits us to stars within 100 light years. Habitability is probably limited to F, G, K and maybe M stars. Within 100 light years in the Centaurus constellation, that leaves us with... 14 viable stars (11 of which don't actually have Centauri in their name...). The furthest with Centauri in the name (not an unreasonable requirement, given the context of the book. If they were going to a star with a dull designation, surely they would have given it their own name?) is about 60 light years away.

If we assume they're accelerating until they get half way, then decelerating the rest of the way (the fastest way of getting there and also the main way to require the engine running the entire time), that requires a very low acceleration of 0.0013g or 1.3 cm/s². Which at least explains why a uranium engine might be the fuel source of choice. (For the record, if their destination was Alpha Centauri, then this value wouldn't change appreciably - it would be about 0.05 cm/s² less. Furthermore, for Alpha Centauri it would make much more sense to accelerate a bit and then spend most of the journey coasting until they needed to slow down at the other end.) The maximum velocity the ship would reach would be 0.37c, so that's not too far above my imposed limit of 0.33.

This low acceleration means that my point about bodies not being left behind when ejected from the airlock still stands. They still wouldn't appear to drift away that quickly.

The final piece of information we're given in the book is that the engine started failing when they were about halfway through their journey. What does this mean? It means that they wouldn't be able to decelerate, would reach their destination faster not slower and would zoom straight past it too quickly to go into orbit. Pretty much the exact opposite of the problems described in the book.

Over-reaction?

No. For two reasons. The first is just it's bad writing -- the science fail error jolted me completely out of the story and undermined my suspension of disbelief and plausibility of the whole setting. To achieve the same plot-mandated end, the author could have had the engine start to fail while accelerating or, without much consequence to the plot (as far as book 1 in the trilogy goes, at any rate) the ship could be unable to slow down, unable to correct its course or they could have found out that the planet wasn't as viable as they originally thought. Each of these things would have got the job done, but no, the author chose to not check physics.

The second reason is twofold. From a personal point of view, when learning physics for the first time, in high school or university, it's usual to relate everyday situations to the concepts you learn. In this way, you can intuitively predict basic mechanics based on experience. However, everyday situations tend to take place on the surface of Earth, so when trying to predict the mechanics of what happens in space (which, yes, does come up in physics classes -- take some if you don't believe me) the situations the student has to fall back on are what's portrayed in various media -- books, movies, perhaps computer games. However, thanks to the the generalised scientific illiteracy of most of society, half of these portrayals are plain wrong. They're why I have this blog, in fact. Honestly, having taught physics to new students, I have seen a lot of evidence for this sort of thing contributing to poor understanding and requiring a lot of unlearning.

Hollywood, poorly researched books, and other media undermine what little science education kids get. At least if the media surrounding us strived for some semblance of accuracy, perhaps people would pick up some science by osmosis. Then the climate debate wouldn't be so controversial, US presidential candidates wouldn't think a moon colony in 20 years was a viable idea, and we wouldn't have an anti-vaccination movement. Scientific literacy is important and, really, science fiction as a genre is uniquely positioned to encourage an interest in science. Sure, this wasn't a hard SF tech-centric book, but it was a giant spaceship. That's the sort of thing that can capture an imagination and ingraining wrong science while doing so is just irresponsible.

And it makes me angry.

(Other than the science fail aspect, this isn't a terrible book. I give it 3.5 / 5 stars -- half a star subtracted for the science fail. For a less science-oriented discussion of the book -- y'know, an actual review -- see my book reviews blog.)