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Read this passage, lightly redacted from one of this year's Hugo nominees, and tell me if you think "lightsecond" is being used (correctly) as a unit of distance or (incorrectly) as a unit of time.
A docking-mouth opened, a whirlpool of matter spinning out and away, and the comet plunged into this vast funnel. For the first lightsecond, magnetic fields induced its braking, absorbing a fraction of its massive kinetic energy. Then a web of lasers met it...
See what I mean? One doesn't normally talk of, say, the first ten metres of braking; rather of the first half-second or so. And if the extruded funnel is really meant to be more than 300,000 km in size, that raises all kinds of problems about just how quickly this can be happening. Let's be charitable and assume it is a misprint for "millisecond"; "lightsecond" really makes no sense here.

(PS: two posts I made by email yesterday turned up on LJ with time-stamps eight hours into the future. I'm emailing this at about 0840 local time.)


( 21 comments — Leave a comment )
May. 10th, 2009 06:56 am (UTC)
I think people do talk about the first half mile of braking, if that's what they want to emphasise.
May. 10th, 2009 08:40 am (UTC)
Yes, that was my thought too. But does that distance make sense in the context? It's hard to tell from an extract that short - and I haven't read either Saturn's Children or Zoe's Tale.
May. 10th, 2009 08:42 am (UTC)
My problem is exactly that: I don't think the distance does make sense in the context.

(And this is one of the Best Novella nominees, so you won't find the passage in Stross or Scalzi!)
May. 10th, 2009 08:51 am (UTC)
Ah, if it's from one of the novellas the odds are I'll never read it
May. 10th, 2009 07:41 am (UTC)
How fast is the comet travelling when it plunges in? How long would we expect it to take to travel one lightsecond in distance?
May. 10th, 2009 08:37 am (UTC)
A quick google search suggests that an "average" speed for a comet is 20 000 miles/hour. 1 lightsecond is 186 322 miles, which the comet would traverse in 9.3 hours. 9.3 hours doesn't seem so long in the cosmological timescale, but the quoted passage doesn't have the feel of something that's meant to be unfolding over 9+ hours.
May. 10th, 2009 08:44 am (UTC)
This is a greatly accelerated and largely artificial comet. (Otherwise it wouldn't be docking.)

Edited at 2009-05-10 12:34 pm (UTC)
May. 10th, 2009 03:24 pm (UTC)
How accelerated? A funnel one light-second deep could be very narrow. But what is it a funnel *in*?
And what does "magnetic fields induced its braking" mean?
May. 10th, 2009 03:25 pm (UTC)
And what use is a web of lasers supposed to be?
May. 11th, 2009 06:18 am (UTC)
Hi, hadn't seen this discussion before. I think I wrote the passage in question (though it might have been Cory) and I can tell you what I was thinking, although, of course, the author is dead and the story is the reader's, so it means whatever you think it means:
  1. Yes, I know a light-second is a unit of distance. :-) It's supposed to be analogous to "the first half mile of braking"
  2. the comet is moving quite fast because
    1. it's accelerated by its own propulsion systems, the ones it uses to cross the galaxy -- it's actually a ship of sorts, made out of a comet, after all. The book begins with Nadia having this Beebe fry the asteroid it used to live on, and setting out with the comet,
    2. it's further accelerated by Nadia blowing up part of the comet during her struggle with Paquette, and
    3. it is falling straight into a massive body, Byzantium, which is composed of the matter of several solar systems. Your google search is, I reckon, discussing comets in orbit around stars -- which are the ones we generally see, right? -- not comets falling straight into stars.

  3. Byzantium is huge, as noted above. The fact that it has a docking tunnel over 300,000 km in length is meant to give an impression of its scale. (Of course, the docking tunnel may be made of relatively sparse material.)
  4. "Magnetic fields induced its breaking" is meant to suggest that something like Eddy current braking is used to initially slow the comet -- that is, magnetic fields acting on ferrous components in the comet's body (comets, I know, mostly aren't ferrous, but perhaps they don't need that much mass inside the comet, and after all its designed by posthumans...)
  5. the web of lasers is to vaporize the comet's actual mass, while carefully recording its information content -- since its inhabitants are actually composed of information. The idea is that the lasers are precise enough to not only take apart the comet atom by atom, but actually to record the state of those atoms. I'm not sure how this is done -- perhaps by noting the diffraction of the individual photons, etc., or perhaps by observing other effects -- but it doesn't seem impossible in principle. The lasers can also act as a final stage of braking, as per laser cooling. Once the atoms that used to be the comet are cooled in this manner, at the bottom of the funnel, if I recall correctly, lots of tiny little machines enter the tunnel to examine the atoms themselves in greater detail, e.g. perhaps to observe quantum states at the subatomic level.
  6. I am surprised that the passage reads as if it's happening instantaneously. I'm not really sure how long it takes, but hours rather than milliseconds actually seems right to me. It's a docking of a ship come home from the stars. In the following scene the inhabitants of Byzantium are waiting impatiently for the process (including the "software" part, the reconstruction of the comets' inhabitants and their potential merging with their Byzantine counterparts) to finish...

    But, of course, that's just my take, and I admit to not having done all the math it would take to really work it out in every detail. :-)

May. 11th, 2009 06:19 am (UTC)
"Magnetic fields induced its breaking" -> um, braking. The lasers are doing the breaking... :-)
May. 11th, 2009 09:50 am (UTC)
OK, cool.

The fact that the comet is falling into Byzantium is of relatively little consequence (given that Byzantium has several solar masses and is several light-seconds across); regular comets are essentially falling into the solar system - have parabolic or near-parabolic orbits - and have a maximum speed - at perihelion - equal to (hmm) root-2 times the speed of a circular orbit at perihelion distance. That is, maybe tens of km/s. A body such as this comet - in hyperbolic orbit with a large surplus velocity - will be accelerated by much less than this: delta-v = delta-E/2mv.

I really will get around to reading it some time....
May. 10th, 2009 07:48 am (UTC)
When I was taught relativity, I was taught that a lightsecond was a unit of interval in four-dimensional space, and could apply either to space or time. Physics may have moved on since then; certainly, I've seen enough physicists being sniffy about people using 'light year' as a measure of time.

Of course, this doesn't stop bad SF writers from putting 'light' in front of intervals of time to make them sound more 'outer-spacey', whether or not it could technically be correct.
May. 10th, 2009 08:24 am (UTC)
Honestly. That journey took light-ages.
May. 10th, 2009 08:36 am (UTC)
Of course, the story is set lightyears into the future!
May. 10th, 2009 08:35 am (UTC)
I suspect I did the same course as you, only a year earlier! But I don't remember that. I dug out my copy of Rindler's Essential Relativity, opening it for perhaps the first time in ten years, and found (chapter 4) much discussion of four-vectors, but nothing about the units which they might be measured in. My memory is probably at fault, though (or, less likely, perhaps they changed the syllabus the following year when you did it).

In any case, since the comet here is decelerating, the inertial frame that matters is that of the entity to which it is being docked; and if the intention is to stress the relativistic aspects of the event, choosing a unit which is better known as a unit of spatial distance rather than of relativisic displacement is an odd (and for me unsuccessful) way of doing it.
May. 10th, 2009 10:15 am (UTC)
'Twas light-years of time since his mission did start
Just to be clear, I was talking about A levels, supplemented by a certain reading around the subject. And from what I recall, the point was more that in terms of relativity, space and time should be measured in the same units; light-year was just a convenient one that the textbook author happened to choose.

I totally agree that the unit is probably nonsense when applied to the trajectory of the comet in question.
May. 10th, 2009 03:22 pm (UTC)
Tschah. To physicists of that water, c = 1, so a second is a unit of spacetime, of course.
But such a physicist would never say "light-second".

Other values often set equal to 1 by physicists include G and h-bar (and also pi, -1, and i).
May. 11th, 2009 09:40 am (UTC)
More on this: setting c=G=1 makes the second a unit of mass as well as interval; also setting ℏ=1 requires one to abandon the last SI unit and use Planck units (a.k.a. "God's units" [*]) instead, some of which are quite handy (my gas bill is about 1 Planck energy; a grain of sand has about 1 Planck mass; a running cat has about 1 Planck momentum), except for time and length, which are both much too tiny.

[*] Except God sets 4πG=1.
May. 11th, 2009 06:22 am (UTC)
This may be true as a matter of physics, but it would be totally misleading and annoying as a use of language. You could also give people's weight in joules divided by (meters per second) squared, but that doesn't make it a good idea.
May. 11th, 2009 07:02 am (UTC)
I'm about two-thirds of the way through "True Names" now, and enjoying it. I started reading after this post, so I was looking out for light-seconds and so on, and I spotted a "light-minute" that unambiguously referred to a distance. Also, lots of references to "light-cone", "timelike" etc. that didn't seem to be just technobabble.

So, given that there is a reading of that paragraph that easily supports distance, especially for those of us who internalised the "braking distances" diagram on the back of the Highway Code booklet, I can't make myself read that as a time.
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