Velocity blindness

[gfish]

When I was researching clock design extensively last year, I had an odd linguistic revelation. All my life I'd been saying a clock was "fast" or "slow" to indicate it was ahead or behind the correct time. This purely meant a fixed offset -- a clock was 5 minutes fast, and would still be 5 minutes fast next week. In fact, if a clock really WAS fast, getting further and further ahead I'd have to explicitly specify that. It was a sloppy use of "fast", but, eh, natural language is full of that. I never really thought about it. It wasn't until getting into the history of clocks that I realized this hadn't always been an idiosyncratic phrasing. It used to be quite literal. Clocks used to suck, even very expensive ones. If your clock was fast, it would be noticeably gaining time day after day. We live in an era of incredibly cheap, incredibly accurate clocks. Even the cheapest quartz wristwatch might only gain a few seconds over an entire year, a level accuracy that used to only be found in the best observatory regulators. So while the immediate observation is the same (a clock is 5 minutes ahead), the underlying condition is completely different. It's like seeing a plane taking off over a continental plate boundary and thinking they both have the same velocity since they're the same distance away from you. We take it so granted that clocks are (for >99.99% of human activity) perfectly accurate that we have taken the indefinite integral of a common figure of speech without even realizing it. That's pretty cool. I can't think of any other examples of that happening.

Comments

[anansi133]

This reminds me of the insanely accurate values of pi that people like to play with- where you need a circle the size of the solar system to talk about how far out of round the number would get you.

So now I'm suddenly curious how accurate is close enough so that no one notices. In sailing terms I guess it's being able to get within eyeshot of a small island, but in urban terms it's probably making a bus rendezvous.

When I got my first GPS my favorite part was the super accurate clock. didn't need it, but it was fun anyway.

[neuro42.livejournal.com]

I think not. What you're missing is the implied presumption that (a) the clock was once set and (b) at that time it was correct.

Therefore, in order for it to *now* be five minutes ahead, it *must* be running at fast by (5m/dT) seconds per second, and the reasonable expectation is that it will continue to do so.

Noboody (that I know of) thinks a clock that's 5m ahead will stay 5m ahead forever. It might take more than next week, but one can only expect it to keep gaining.

[gfish.livejournal.com]

I think the assumption now is that it was set incorrectly. In fact, if someone sets a clock ahead on purpose to help them not be late, they would say they "set it fast".

[neuro42.livejournal.com]

The latter, yes, but I consider that a flawed backformation.

The majority of people know set their clocks 'ahead', not fast.

I suppose I've never conducted a poll, but I can't imagine anyone I know thinking that a clock that is fast is not going to get further ahead over time, although it might easily be on the scale of minutes per year and not minutes per week.

[memegarden.livejournal.com]

If I hear someone say "that clock is five minutes fast", I assume it's five minutes ahead but reasonably accurate in tracking the passage of time. This is in fact true for the clock in my hallway, which has been about five minutes ahead for the past year or so since it was last set, and I just haven't bothered to reset it correctly.

If someone meant that the clock actually ran faster than time does, I'd expect to hear something like "that clock runs a little fast" rather than "is five minutes fast".

[neuro42.livejournal.com]

why on earth did whoever went to the trouble of setting the clock set it wrong in the first place? that's what I don't get.

[gfish.livejournal.com]

I know this will come as a surprise, but not everyone sets their clocks from tier 1 reference signals.

[memegarden.livejournal.com]

That would be me, I think. I probably just looked at another clock and then went up and set it, and took longer than I thought I had. Having a clock there that's five minutes wrong has been immensely more useful than not having one. And others in the house have noticed it's five minutes off, and not bothered to reset it themselves, so I think we're all pretty much on the same page on that.

[neuro42.livejournal.com]

I suppose I have to admit that there are a few people in the world still who don't have an ntp client in their pants, but the idea of bothering to set a clock but not bothering to set it correctly makes my brain hurt. :)

[peteralway.livejournal.com]

It actually took me some time to accept that when people said a watch was "fast" that it was really shorthand for the watch being ahead at the moment. Just seemed wrong. Especially because my motivation for setting my cheap timex 5 minutes ahead was because it was in fact slow, and I wanted a safety margin as it fell behind.

[solarbird]

My battery backup bedroom clock - digital! - actually is fast in the old sense. It's about four minutes fast across six months, which is still not horrible, but it does mean every DST adjustment involves individual minute changes. I still use it because I love the way I push a button and it unfolds from a slab into this clock-and-calculator thing, but honestly, by modern standards? It's a pretty poor clock.

[sistawendy.livejournal.com]

Yes! Yes! Somebody else who noticed the inaccuracy! Fish, you are my people.

And yeah, I knew that about the old clocks sucking. Shoot, among spring-wound clocks, even recent ones suck if they're cheap enough or the spring is shot.

[randomdreams.livejournal.com]

I'd internalized this without thinking of it: I say 'ahead' when the clock's set 5 minutes fast but is otherwise reliable, because I've spent a lot of time messing about inside vintage mechanical wristwatches. The mainspring attaches to a central pivot, which has an arm on it, allowing it to be varied. That changes the spring tension, which varies the watch speed. The arm end has a pointer on an inscribed arc, that's labeled "fast" and "slow" on the two ends.
Thing is: until you pointed this out, I had never consciously thought about my usage of ahead/behind vs fast/slow, so I must have been interpolating what other people were saying, unconsciously.

[zzyzx-xyzzy.livejournal.com]

The clocks on desktop PCs are surprisingly drifty on short timescales. I see drift rates on the order of 0.1 msec per second around here.

[arjache.livejournal.com]

...and I'm reminded of how difficult it is to keep clocks accurate on virtual machines. Depending on your workflow, if you don't use NTP or something like it, you're going to see major skew within a few days if not hours.

(Then again, perhaps there have been some major clock virtualization advances that I'm not aware of. I'm a few years out of date.)