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Physics theory posits, and observations confirm time dilation for reference frames. In physics, this phenomenon is often described:

Moving clocks run slow.

Shouldn't it be slowly?

"Drive safe!" is clearly wrong to me: as the adverb/verb pairing is unquestionable. But the above has two verbs, and despite my instincts telling me it should be "slowly" the overwhelming appearances of the "... run slow." in print leads me to question it here.

Jamie
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    Incidentally, at least some people would say My clock runs slow even if their mains-powered clock has consistently shown a time exactly 60 seconds earlier than "true" time for years. But if you say My clock runs slowly, that probably always implies the clock loses some measurable amount every day (so the discrepancy gets bigger with the passage of time). – FumbleFingers Aug 03 '16 at 18:31
  • Google Ngrams seem to indicate that 'run slow' is much preferred over 'run slowly' when applied to timepieces. It might even be valid to interpret this as a link-like verb structure (as opposed to 'slow' = flat adverb), by comparison with 'this clock is slow' and 'the well is running dry'. – Edwin Ashworth Aug 03 '16 at 18:59
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    Of a clock, runs slow = is slow. It's not a manner adverb, it's a predicate adjective. – John Lawler Aug 03 '16 at 19:06
  • @JohnLawler Um, why? To me, runs slow = is running slow and is slow = is behind. These are two different usages of slow, the second one idiomatic. As an engineer, I will always pay attention to keeping ideas of position and velocity separate and differentiated. In the OP's title, the well-known Special Relativity prediction refers to run = rate, not run = is. – Phil Sweet Aug 04 '16 at 02:14
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    @PhilSweet If a clock's hands are moving, it is running. This is the mechanical sense of run, like a car or a pump or a meter. If a clock's hands take 59 minutes to complete one hour, it is running fast; if they take 61 minutes, it is running slow. The reference is to the displayed time, not to the theoretical speed of any piece of the machinery. If the clock displays 5:00 and it's known to be 5:01, the clock is (or is running) slow. – John Lawler Aug 04 '16 at 02:23
  • @JohnLawler Don't you think you are using two entirely different usages of slow? In your last sentence above, the statement is correct, but it is not related to the earlier ones. It it just idiomatic. We happen to have a usage of slow that means behind. Slow = behind is an adjective as in your earlier example, But this does not imply that slow, used to refer to a rate, is an adjective. It certainly looks like a manner adverb to me. And in the context of the relativity example, the slow = behind usage is simply off the table. That is not how the effect is canonically described. – Phil Sweet Aug 04 '16 at 02:42
  • That's the point. This use of slow with run is idiomatic. Whether the deficit in time registry is constant (that clock runs 5 minutes slow) or variable (the clock is running slow, so I have to reset it every morning), we use run and slow to describe it. "Behind" is not used much, since it's metaphorical and it's not clear what is "behind" what, and which direction is "front". – John Lawler Aug 04 '16 at 02:52
  • @JohnLawler. Okay, I think we are in 99% agreement. I agree that wrt real, nonrelativistic clocks, the language works just as you say. But the relativist clock is just a metaphor for time itself. You wrote "The reference is to the displayed time, not to the theoretical speed of any piece of the machinery." This is emphatically wrong wrt relativity. It is precisely "the theoretical speed of any piece of the machinery" which is being considered. That is why I balked at the road you went down. – Phil Sweet Aug 04 '16 at 03:18
  • English is not required (nor equipped) to deal with even special relativity, never mind an accelerated frame of reference. Natural language can only describe phenomena that humans can experience. Hence, no relativistic velocities, no neutrino-antineutrino collisions, no black holes, etc. We can name them, but we can't experience them, and we can't expect to talk about them in a human language without equations and boundary conditions. – John Lawler Aug 04 '16 at 03:35

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