Units of Measurement

Wayne Mack

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[The definition of a second] does not change when you look at another person in a different relativistic/accelerating frame of reference. "It's not the units that are flexible and changing, it's spacetime itself, the actual physical reality of the universe, that changes to keep the speed of light constant." - quoting from Physics Stack Exchange
Doesn't this still present the problem of synchronizing time across frames of reference?

If i understand the statement, it is saying that the unit of time does not change, but the time paths it measures may vary. This shift is already a noticeable issue in communications satellites. If one imagines spaceflight at anything approaching light speed, then trying to define rendezvous times becomes a challenge. Having a ship arrive at a space station with both of their day times seemingly aligned becomes very unlikely.
 

farntfar

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A challenge perhaps. But not beyond our ability to calculate, surely.
We cope with flying to a different time zone on earth.
We will adapt to flying to a different time-frame.
 

Ursa major

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Having a ship arrive at a space station with both of their day times seemingly aligned becomes very unlikely.
They seem to have had that "problem" after rail travel was invented but before time zones were created.
 

Venusian Broon

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Doesn't this still present the problem of synchronizing time across frames of reference?

If i understand the statement, it is saying that the unit of time does not change, but the time paths it measures may vary. This shift is already a noticeable issue in communications satellites. If one imagines spaceflight at anything approaching light speed, then trying to define rendezvous times becomes a challenge. Having a ship arrive at a space station with both of their day times seemingly aligned becomes very unlikely.

With GPS the issue is that satellites are above us in a weaker gravitational field, hence 'invoking' general relativity, whereas they are moving much faster than anything on the ground, 'invoking' special relativity. If we did not know anything about such issues at all, the clocks on the GPS satellites would drift, and would really mess up our calculations on the data we would receive, as they would go out of synch with what they were communicating with on Earth.

However.

Because we know about the theory of relativity, special and general, we are able to make corrections to accurately take account of the various effects between the satellites and the infrastructure on the ground.

In both cases, above and on ground, a tight fixed definition of what a second is, would be essential!

Thus if we needed to make a spacecraft and space station rendezvous, which had different velocities, we should, in a very similar way, be able to work out the optimum course using relativity.
 

Cthulhu.Science

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With GPS the issue is that satellites are above us in a weaker gravitational field, hence 'invoking' general relativity, whereas they are moving much faster than anything on the ground, 'invoking' special relativity. If we did not know anything about such issues at all, the clocks on the GPS satellites would drift, and would really mess up our calculations on the data we would receive, as they would go out of synch with what they were communicating with on Earth.

However.

Because we know about the theory of relativity, special and general, we are able to make corrections to accurately take account of the various effects between the satellites and the infrastructure on the ground.

In both cases, above and on ground, a tight fixed definition of what a second is, would be essential!

Thus if we needed to make a spacecraft and space station rendezvous, which had different velocities, we should, in a very similar way, be able to work out the optimum course using relativity.
Skylab & KSP figured out how to dock spacecraft together in 1974
sddefault.jpg


But there is an important question of communication. It is all well and good in near earth orbit when direct communication is feasible. But what about over much, much larger distances where speed of light communication means weeks, months, centuries?

Is clock on the New Horizon spaceship - out beyond Pluto - still in synch with Earth clocks?
Why shouldn't it be?

But this is all fun to ponder for space stories. In the future in space, I imagine that time would be generally calculated as a "day" based on human biorhythms. Here is a report from the Navy Postgraduate School in Monterrey from way back in 1974. But it is interesting how the various Military Navy's are very concerned about biorhythm on board ships which are constantly shifting time zones.

Here is the time on the International Space Station. Do you suppose that this will be the basis of time for all space exploration into the future.
International Space Station Time - exact time now - Time.is

Fun thoughts
 

Swank

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Time suffers the biggest challenges away from Earth. Longer frame constructs such as day, month, year lose relevance on another planet. More problematic, though, is that time is a variable, a second is not a unit and has different durations in different relativistic frames.
While true during travel, I don't know if there is anywhere in our galaxy (aside from the close orbit of a black hole) that has a significantly different relativistic frame then our own. We're close to the outer rim and we're only going 230 km/sec - a very small fraction of the 300,000 km/sec that light goes (.07%). Andromeda is coming at us at even less - 113 km/sec.
 

Thaumechanic

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Don't burden your audience with math.

Metric units make sense if you're doing math, and the more we have to do math on measurements, the more we learn to appreciate metric. Use metric on worlds where measurements have to be communicated more precisely than the weight of a sack of grain. This usually becomes an issue when fast-moving vehicles start colliding on a regular basis. Thus, any society with fast-moving vehicles will move to a measuring system that makes math easier.

On worlds where science and math are limited to a select few, use local measurements. If you have to mention the measurements too often, you're trying to pack too much quantitative into your story, and you should shift it to qualitative.

Base 10 makes sense when people are in the habit of using it for math. Try this: Write down a five-digit number. Divide that number by three. Now divide that number by 10. See the difference? Base 12 is better for measuring because it's easier to cut a measurement in half than it is to cut it in 10ths, but as soon as you start doing math on it, base 12 becomes cumbersome.
 

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