Friction in space and on Earth

This post is in response to a comment I got on my previous post "More thoughts on the importance of science in science fiction" where Shannon commented/asked (I'm only quoting the question-y part of her comment):

It really is a hard concept to grasp, the no-friction-in-space thing. I don't think I really get it - I'm not sure how to visualise it, for a start - but I don't understand how a space ship - of the super-advanced, sci-fi kind - can't slow down. I mean, it's mechanical and computerised and runs on fuel; on Earth anything we build for transportation will slow down especially if there's a mechanical failure etc. I know in space you can't "stop", you'd only drift, right? I'm hoping you can explain this a bit more to me because I really do want to understand!

(The more time I have to let this concept dwell in my brain, the more I'm starting to get it. So what does happen when you, in sci-fi, go from "warp speed" or whatever they like to call it, to, well, not?)

On Earth (or really, anywhere that isn't the empty vacuum of space) moving objects slow down because they lose energy through friction — rubbing against other objects. Commonly on Earth, the source of friction would be land, water and/or air.

Some examples:

• The motor of a boat needs to stay on to keep the boat moving, because if the motor is turned off, the boat will be slowed down by the water pushing back against it.
• If you ski straight down a hill (let's say a small hill for safety reasons) you will accelerate (get faster) while you're going down hill, but once you reach the flat bit at the bottom you will eventually slow down and stop without having to stop yourself. This is because of the friction between the snow and your skis. Generally, skiing works because there's much less friction between snow and skis than, say, between shoes and dirt, but there isn't zero friction. When you were going down the hill and getting faster, there was still friction, but at that point gravity pulling you downwards was stronger.
• If you drop something from a great height (tall building, aeroplane), gravity will make it accelerate as it falls down. However, the air pushes back on it, upwards (or more generally, in the opposite direction to the movement) and eventually will prevent the object falling any faster. (With air, the friction is directly related to the size and shape of the object and how fast it's going, but I won't get into the maths.) The maximum speed the object can reach while falling is called terminal velocity.
• On the other hand, if there is no air — for example on the moon — there will of course be no friction from air and things like feathers which normally fall very slowly (because of all the little fuzzy bits catching on the air) will fall at the same speed and acceleration as a lead ball (or whatever). This will also work in a vacuum chamber where all the air has been removed. Here is a video of an astronaut on the last Apollo mission dropping a hammer and a feather at the same time:
  
  
  And a gif of the same if you can't be bothered watching and listening to the 47 second clip:
  
  
  
  
• Brakes on cars and whatnot work by intentionally increasing the friction on the axle to slow down the spinning speed of the wheels

Now let's talk about how spaceships slow down in space. I want to emphasis that my complaint with Across the Universe wasn't that the spaceship was slowing down, but that it was slowing down by itself. Things can only slow down by themselves if there is friction around (so really they're not slowing down by themselves but because of friction, but we don't usually think about or notice friction so it seems like its happening by itself).

In real life, spaceships slow down (and manoeuvre) by firing their engines in the other direction. It might be a bit easier to picture on a smaller scale. Consider an astronaut on a spacewalk. Let's pretend they're not tethered to their ship and that the ship is out in deep space away from the gravitational influence of any planets. To be able to move around, the astronaut will have a gas tank (or similar) that will allow them to press a button to move forward. The gas will shoot out backwards for a couple of seconds, and the astronaut will move forwards. At this point, if the astronaut does nothing, they will continue moving in a straight line indefinitely. Basically until they run into something. The same thing happens with a spaceship: gravity and obstacles not withstanding, after it fires its engines for a bit to accelerate, it will keep going in a straight line at the same speed until something else happens to stop it. This clip from WALL-E is a good example (thanks to Shaheen for the suggestion). Also note that once they start spinning, things will continue spinning until something else makes them change, which you can see a bit of in that clip.

That doesn't mean things can't stop or slow down in space. Our astronaut — assuming they're not unconscious — can fire their gas in the opposite direction (to manoeuvre properly they'd have to have several directional options, six for complete manoeuvrability) to slow down. The spaceship can also fire thrusters in the opposite direction to slow down (either by having two sets or by rotating the main ones). Coming to an absolute complete stop is a bit tricky because a) you would have to balance forces very exactly and b) there's not much to use as a reference for how fast you're going out in space, but matching speeds with another ship is doable. And the astronaut slowing down enough to not break a wrist colliding with his ship is also useful. My older post about turning around in space addresses some issues with why just stopping and going in the opposite direction isn't the most efficient way of doing it.

The very last part of the question was:

So what does happen when you, in sci-fi, go from "warp speed" or whatever they like to call it, to, well, not?

The short answer to this is, whatever you want. Warp speed and hyperspace and other "let's cheat to go faster than the speed of light devices" aren't real. They're generally not based on real physics, or if they are, it's very extrapolated and speculative and could well turn out to be just as implausible. That said, faster than light travel is a staple of science fiction and I'm not suggesting we should eliminate it because it's implausible. If all science fiction stories used only slow or relativistic (which means close to the speed of light, when weird things happen. My post about it) then there'd be a lot of very slow stories which would get boring. Variety is nice.

As long as the rest of the science is plausible, then I don't have a problem with a bit of faster than light travel and faster than light communication. If the writer doesn't feel up to making up a semi-plausible sciencey explanation, then my personal preference is not to try explaining how the FTL works at all. Because they usually stuff up some minor point which annoys me disproportionately.

Turning around in space

Another ask Tsana question today. (And a relatively shortish response, sort of. Gasp!) Keep 'em coming, guys :-)

Anon asked:

How hard would it be to turn around in space... Say for some reason, Curiosity needed to turn around midflight and return to earth. Would BURNING fuel on some sort of reverse thruster work or would it have to make the trip to Mars, orbit the planet and break orbit to return

This is for a picture book that I feel impelled to be at least somewhat based in reality... which may be dumb.

Hi Anon,

It's absolutely NOT dumb to try to make picture books or any sort of books for kids plausible or semi-plausible. Especially when it comes to these sorts of areas where they can't possibly have any hands-on experience. Hollywood bombards them (and all of us) with so much inaccuracy that any little bit of truth helps. If they remember your book when they come to learn about these things later on, it will help the science stick. If all they have to go on are poorly researched movies which have given them wrong "intuition" about these things, it makes it a lot harder for them since they have to unlearn the rubbish first.

On to the actual question part!

It's pretty tricky to turn around in space. Because there's no friction, you have to use the same amount of energy it took to speed up to slow down by the same amount (so to come to a stop, say). This is a huge waste of fuel. Changing course more subtly isn't as difficult, however.

Apollo 13 Movie poster. (Nabbed from Wiki)

For something specifically like Curiosity: an unmanned probe sent to another planet, I can't think of a reason they'd try to get it back to Earth (unless a sample return was specifically part of the mission plan, but I don't think that's what you're asking). If something went wrong, they'd be more likely to cut their losses and abandon it. Also, almost all of that kind of probe's fuel is used up during take off, leaving only enough for minor course corrections and landing. In that case, plausibility would dictate that attempting a gravitational slingshot around Mars would be the only way to maybe get it back. You'd also have the issue of how to collect it from Earth's orbit since a) Earth would have moved a lot while it was travelling and b) if you were lucky enough to get it to pass close to Earth, it would be travelling quite fast and probably wouldn't have enough fuel to go into orbit around Earth for collection. It would definitely be tricky.

A very good example of a scenario relating to your question is the movie Apollo 13. If you haven't seen it, I recommend that you do. As far as I can remember (and I freely admit it's been many years since I watched it, so don't hold me to this), the physics in it was pretty accurate. In that, things go wrong with the (real life) 70s moon mission and, among other fixes, the astronauts have to slingshot around the moon to get safely back to Earth.

In the end, I'd say it depends on the nature of your mission as to what would be done. If it was a manned mission to Mars, for example, they might try harder to bring them back early, but physics would not be on their side.

Hope that answers your question!

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.)

Masers in Space, or Cool Stuff You Didn't Know Existed

I think the title of this post pretty well sums up what it's going to be about.

What is a maser?

I am going to go ahead and assume that you've all at least heard of lasers and probably experienced them in a pointing at things from a distance context. Did you know Einstein developed the theoretical underpinnings of lasers in 1917? He sure did a lot of fundamental physics for someone who didn't like the idea of quantum uncertainty.

Although it's now written all in lower-case, LASER was originally an acronym: Light Amplification by Stimulated Emission of Radiation. The acronym is actually a fairly good summary of how lasers work; they emit light (a form of radiation), which has been amplified thanks to stimulation. Yeah, OK, rephrasing it like that doesn't actually help. On the other hand, going into the quantum mechanics of it won't either. Briefly, a laser works thusly:

1. You need an optical cavity, which is just a fancy way of saying a container (usually a metal tube) with mirrors at either end and filled with the right kind of material (which varies depending on the colour/wavelength you want to get out).
2. You put some photons (particles of light) into it (or some other form of energy, like electrical, which will ultimately lead to photons) and they bounce back and forth thanks to the mirrors. The input light should be of a similar wavelength to what you want to emit.
3. While the photons are bouncing around, the material inside your cavity — called the "gain medium" — absorbs some of them and then re-emits them at a very specific frequency/wavelength. The reason for the specific part is because quantum physics dictates that the intervals between different energy states (in the gain medium, in this case) have very specific and discrete values. The properties of your laser will dictate exactly which energy transition (and hence which
4. So you end up with a whole lot of photons of the same frequency* bouncing around inside your cavity. At some point, you have more photons being emitted than are being absorbed and you can reap the rewards of your lasering. 
5. When you let these photons out of the cavity, you are releasing a whole lot of light which is exactly the same wavelength/frequency/colour and, thanks to the properties of the cavity, which is perfectly in synch (you can think of it as focussed, which is also true but actually a different property). That's why lasers are singularly coloured and why more energetic lasers can burn.

That's nice, you may be thinking, but what in space does this have to do with masers? Well, dear readers, it is no coincidence that "maser" sounds very much like "laser". It's a bastardisation of the acronym, standing instead for Microwave Amplification by Stimulated Emission of Radiation. The principle is the same but with microwave radiation instead of optical** (and because of the longer wavelengths involved, the industrial construction is a bit different). Interestingly enough, the first laser built was actually a maser. (See the history section on the laser wiki article I also linked to above.)

* If you're wondering why I keep randomly switching between frequency and wavelength, it's because they're interchangeable via a simple formula and we scientists tend to just use whichever we feel like in the context. The formula, if you're wondering is speed of light = frequency x wavelength. And the speed of light is constant (more or less).

** All forms of electromagnetic radiation are in principle the same, just longer or shorter wavelengths. From the shortest — gamma rays, X-rays, UV light — to optical light that we can see with our eyes, to the longer wavelengths — infrared, microwave, radio waves — it all works the same way just with different amounts of energy involved.  

Um, so where's the space part?

Glad you asked! This is where it gets really cool. (Arguably lasers are already cool, but bear with me.)

 In space, anywhere where there are large collections of molecules — for example in molecular clouds (d'uh), around dying stars, and some planetary atmospheres — the following sequence of events isn't uncommon:

1. Some external source excites a molecule. The source is probably starlight, but there are a few other things that could also do it. Beside the point at the moment. The term excitation refers to the molecule absorbing a single particle of light (photon) and thus increasing its internal energy level.
2. At some point, the molecule will spontaneously de-excite, emitting a photon of exactly the energy difference between the excited and less excited energy states. (This could be the energy of the original photon it absorbed, but in the case of masers often the exciting photon jumps the molecule up several energy levels and it drops down them one by one.)
3. The emitted photon goes on to be absorbed by another molecule of the same type (where there's one there's almost always going to be a bunch more) or it can stimulate another molecule to de-excite by exactly the same amount. (It's a bit of quantum magic, but it does happen.) And now you have two photons of the same energy.
4. Loop steps 2–3.
5. A whole lot of light of exactly the same frequency will escape the cloud (and some of it will shine at Earth, so that we can see it).

Sound familiar? Yep. There are natural masers in space.

If you are curious, some of the sorts of molecules which often exhibit masing* are water, carbon monoxide, methanol, hydroxide, molecular hydrogen, formaldehyde, ammonia, and a bunch more. We can easily tell the difference between these molecules because the exact energies of the light they emit is specific to the molecule. It's even possible to tell the difference between different isotopes of oxygen, nitrogen, carbon, etc. And yes, that means all those molecules are floating around in space or surrounding dying stars with other stuff ejected during their death throes.

* Science likes making up words.

One more thing...

Image credit: NASA/ESA

A cool link: Giant reservoir of water found surrounding a quasar (discovered thanks to water masers, although it doesn't explicitly say so in that article). Above is the image to go with it, because NASA/ESA are good at finding artists to draw cool things in space.