Sunday, April 28, 2013

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.


  1. Thanks for the explanation, Tsana. I get friction on earth - like how road cyclists have those very skinny, basically bald tyres on their bicycles to remove as much friction as possible, right? - but it's always hard to wrap your head around the concept of, well, nothing.

    So your options are A) use fuel to propel yourself/ship forward or B) do nothing and just drift. Is that about it? If you don't use any fuel to get forward or reverse momentum going, then you're not going anywhere? Can you drift endlessly in space and eventually, like a boat on a lake perhaps, end up somewhere else? Not that there are any currents in space! (Remember Red Dwarf? Sorry, no relation. Just made me think of it!) I know with boats, because I've driven them, that the water will propel you even when the motor's off, but the only real way of controlling the boat is to have the motor on. I guess in space, removing the bit about water and currents, it's more-or-less the same principle. You're not going to go anywhere or have any control unless the power's on. Have I got that right?

    1. Yeah, if you run out of fuel you would keep going at the speed you were at after the last time you did use your motor. You would go in a straight line unless you happened to pass close enough to a planet or a star to change your direction thanks to its gravity. The chances of being captured into orbit around a planet or a star are very small — you basically have to be going at a very exact speed and direction relative to the star/planet. And your chances of colliding with a planet or a star are even smaller. Space is very very big and very very empty. In terms of eventually ending up somewhere else, it's possible but, aside from the low probabilities because of the emptiness of space, it would also take a long time because of the vastness of space. It takes light a bit over 4 years to get to our nearest star and you would probably be drifting much slower than the speed of light.

      The only thing that could slow you down a bit is a nebula, a cloud of dust and gas (google for some pretty pictures if you're interested). But even then, the density of the dust and gas is pretty low in most of these so it would only make a difference if you were going quite fast (especially if you were close to the speed of light) when collisions with these dust and gas particles could do significant damage to your ship. In most of space, the density of random hydrogen atoms is extremely low and would only cause problems if you were going very very close to the speed of light, but in a nebula it'd become a problem sooner.

      So to get back to the point, if you were just drifting (= moving at the same speed in a straight line; I think "drifting" is a dangerous word to use in this context because it does evoke boats on water), you could eventually end up somewhere else, but it would probably take a very very long time to get there. I'd be concerned about the ship lasting that long just in terms of random bits of damage from interstellar radiation and general deterioration with age.

  2. This is pathetic. I don't want to read anything called science fiction written by anyone like that.

    That is the trouble called SF today. The reviewers don't even talk about the science in the story.

    1. No need to be rude, particularly to someone actively trying to learn.

      As for reviewers, there's more to science fiction — even hard science fiction — than just the science. While it would be nice if everyone were equipped to comment on the science, reviewers have every right to comment on whatever aspects they want.


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