Just what exactly is a G?

[Malenfant]

as for gas giant.. always gone more for a very high speed slingshot pass through the thin atmosphere, with a decent period before you can do it again (normally while you turn round) for the hull to cool down again

The problem with that, as always, is speed. You're gonna be coming in HOT if you use this approach - the gravity is only getting higher as you get closer to the GG, and if you're already going fast then by the time you hit the atmosphere you're going at several km/s. At those speeds, the heating and stresses on the hull is going to be obscene. if the hull isn't perfectly designed for such operations, then if you had one tiny fracture, flaw, or opening in the hull and it'd be Columbia all over again for sure.

 

[Anthony]

Originally posted by Malenfant:
The problem with that, as always, is speed. You're gonna be coming in HOT if you use this approach - the gravity is only getting higher as you get closer to the GG, and if you're already going fast then by the time you hit the atmosphere you're going at several km/s.

From what I remember last time we worked this out, for Jupiter you're hitting atmosphere at 60+ kps, with temperatures in the million kelvin range. Eight times faster, and sixty times hotter, than atmospheric re-entry on earth.

 

[TheDS]

Holy guacamole! That's HOT! That's fusion reactor hot! No wonder we use G-nullification.

Now that this discussion is over, can some one hijack this thread?

I'll start:

What if we wanted to DEFINE the speed of light as being 300,000 km/s? What would we have to change? C is constant, so we can't change that, so we have to change either km or s.

If we change km, that messes up a lot of stuff, not least of which is length, area, volume, mass, and who knows what else...

If we change time, make a second slightly longer, what does that change? All velocities, anything that's a change over time, the length of a minute...

Which is the better one to change, our arbitrary length unit or our arbitrary time unit? One benefit I see is that G will be closer to the value of 10 meters per second per second, making it less of an approximation.

It is my opinion that time is the better unit to change, because as we get into space and live on other planets with their own local days (nevermind the completely arbitrary day length of spacecraft), it will be necessary to redefine things from day and up, and possibly from minute and up. IMTU, I use seconds as an absolute time measurement. kS, MS, GS... well, I do have some simplifications, like a year is 40 MegaSeconds (vice 31.5576), a space combat round is 1 kilosecond (vice 1200-1800), and local days are always locally divided into 10 hours, each being when a particular face of an icosahedral projection faces the sun.

 

[veltyen]

Wouldn't it be easier to define the speed of light in vacuum as 1?

Thus you would have measurements of speed in the milliC (~300km/s) and microC (~300m/s) and nanoC (0.3 m/s). This has the advantage of being abstract to both time and distance.

An external measure of time is also fairly easy to standardise. The half life of tritium would be a fairly good one. The half life of any common decaying element can be used.

With time and velocity codified, distance just falls out of the equation, as does acceleration both being derived between distance and time.

There are problems carrying specific planetary units between systems. G, day, month, year are all such units, and are dependant on specific metrics that cannot be easily carried from the originating planet. None of these units make it as far as another planet in this system, let alone to interstellar space. Even the old definition of a metre (10^-7 of the distance from the equator to the pole) is based on terrestrial measurement.

From a gaming point of view you want to use terestrial units though. The units used by your characters are not going to be talking metres hours and days in the OTI, I have always assumed that they used Vilani based units, or some codified external measurement. I use earth based measurements so that my players will not be as confused.

For G you already have a non-terrestrial measurement, the newton.

Tangentally, I found an interesting site talking about what happens when you do travel faster then light. It creates something called Cerenkov
radiation. Interesting stuff.

 

[Malenfant]

There are problems carrying specific planetary units between systems. G, day, month, year are all such units, and are dependant on specific metrics that cannot be easily carried from the originating planet. None of these units make it as far as another planet in this system, let alone to interstellar space. Even the old definition of a metre (10^-7 of the distance from the equator to the pole) is based on terrestrial measurement.

To be accurate, g changes, but G doesn't. g is the local gravity, G is the universal gravitational constant.

But there's no need to do things like changing the definition of c or time. Seconds are by definition absolute. A minute is 60 seconds, an hour is 60 minutes. A day is one full rotation period of the planet (with different definitions depending on whether you're using the sun or the distant stars returning to the same point in the sky as a reference).

A week is, I think, fairly arbitrary - it could be any number of days. A month is based on the orbital period of the moon around the Earth (about 28 days). A year is the orbital period of the planet around its sun. These three units would be the most variable on different planets. In some cases, you might not need months or weeks in a year if the year is too short or long, for example.

As for c, it's quite enough to assume it's equal to 300,000 km/s - you just do not need to redefine everything else because for all intents and purposes it's so close to that anyway that it may as well be right.

 

[Gruffty]

Spinning off a little from the current theme (but still quite closely related), I was under the impression that a certain number of lengths of oscillation of pure white light (travelling in a vacuum at RL speed) equals 1 mm, and thus the length of (say) 1 km is directly related to that.

Or I could, of course, be completely wrong.....

 

[TheDS]

The Second is just as arbitrary. The Day was defined in ancient times as the time it takes for the sun to go all the way around the earth and return to the same place. This was later divided into 24 parts, called hours.

Then some ancient civilization which used base-60 to count divided hours into 60 minutes, and minutes into 60 seconds.

I believe that 24 hours was used because of the importance of the number 12 to certain cultures; 12 hours of day, 12 hours of night followed.

In a metric world, probably France would have said that there are 100,000 seconds in a day. They may have done that when they invented the meter and other things, but if they did, it never caught on. Eventually, the French metric system caught on and has replaced measurement systems almost everywhere in the world.

We will not be at odds with the meter or the gram or the gee when we go to other worlds, but we will be at odds with the day and possibly the year and hour.

Considering that the meter and gram can be used anywhere without having to change the way you think, it would be logical for time measurements to follow. The second itself is quite acceptable, but anything bigger is going to be confusing. I would suggest that the length of a second should be based on the rest of the metric system.

The meter was arbitrary, but everything that derives from it is a logical extension and has a basis in nature. Why is time not treated the same, and based on the meter?

 

[Anthony]

Originally posted by Gruffty:
Spinning off a little from the current theme (but still quite closely related), I was under the impression that a certain number of lengths of oscillation of pure white light (travelling in a vacuum at RL speed) equals 1 mm, and thus the length of (say) 1 km is directly related to that.

Or I could, of course, be completely wrong.....

There's no such thing as pure white light.

From 1960 to 1983, a meter was defined as 1,650,763.73 wavelengths of Kr-86. Currently, it's defined as the distance light travels in 1/299,792,458 of a second.

One second is 9,192,631,770 vibrations of Cs-133

 

[Malenfant]

metres, seconds, and kilograms are the cornerstone of the metric system. They're defined using parameters that are the same across at least our part of the universe. Like it or not, the imperial, US and other systems are not going to be the ones that are used for science and engineering when we get offworld - they're impractical and unscientific. This is one reason why so many people complain about scifi RPGs that give measurements in non-metric units.

That said, there's currently no good reason to change the cornerstones of the metric system in the future. A second can still be a second whether you're on Earth, Mars, Regina, or even near a black hole, because no matter where you are, it's still 9,192,631,770 vibrations of a Cs-133 atom. (now, why they chose that number of vibrations, I dunno. Probably because it's the same length of time as the traditional definition of a second, however that was defined).

 

[veltyen]

The babylonians used base 12.

It is where the current defninition of time units comes from. Once you have a working system it is really hard to dispose of it. There is an enormous barrier to change.

Anyway, a system designed on multiplying small numbers together is a pretty good way to go. 60 is 3*4*5. 24 is 2*3*4.

 

[aramis]

Mal, according to SI, g is grams... 1g is the mass of 1cm^3 of pure h20 at maximum density (4 degrees C).

G is mean gravity of earth at mean surface level.

(accroding to Perry's ChemEngr Handbook...)

 

[Malenfant]

I think it depends... there's the c/g/s system, which used to be used more in the 70s and 80s and in some parts of engineering and astronomy I think, and there's the m/kg/s system which is the "proper" SI system. I'm fairly sure that the kg is the mass that is actually defined (I thought it was the mass of a certain lump of platinum in Paris, or something?), and gram is derived from that, but I could be wrong. Maybe they've updated things...

cgs is bloody awful IMO. All that faffing around with weird units like 'ergs' for energy instead of joules. Yuk.

Again though... g (note the lower case) is the symbol used for surface gravity (i.e. 9.80665 m/s2 on Earth). G (upper case) is the universal gravitational constant (6.672e-11 m3/kg/s2).

 

[RandyT0001]

And all this time I thought that G was a spot on a woman that when physically stimulated would cause her to become very excited. Guess I should give up finding it.

 

[veltyen]

And all this time I thought that G was a spot on a woman that when physically stimulated would cause her to become very excited. Guess I should give up finding it.

Never give up, never surrender, ... and practice your finger strength.

 

[Anthony]

Originally posted by Malenfant:
(now, why they chose that number of vibrations, I dunno. Probably because it's the same length of time as the traditional definition of a second, however that was defined).

Every time a unit is redefined, it's correct to the previous definition, within the limits of accuracy of the previous definition.

 

[BetterThanLife]

Metric is just as stuck with Traditional time units as Imperial measurements. (Or however you want to define the backwards system we Yanks use.

) BTW a Pound, Mile, Yard, Gallon, etc is just as precise as the metric equivalent. As far as time goes, I like David Weber's Honorverse way of dealing with time. A Standard year is 365 days, a Standard day is 24 hours and a Standard hour is 60 minutes or 3600 seconds. A Local day is divided into standard hours, though the last hour of the day is a shortened hour called compensate. (And there can be however many hours in a day as it takes for the world to rotate on its axis.) Though how that would work for a tidally locked planet, a Ringworld or a Dyson Sphere is beyond me. In Space, with no frame of reference time probably reverts to "Standard days."

Since the Metric system is supposed to be universal then KPH would be the same value no matter what planet we stand on, or don't stand on.

Just my Cr0.02

 

[Malenfant]

Sure, other units work for everyday use. I don't doubt that if the Yanks colonise another planet they'll be using feet and miles on it while they're driving around on it in their SUVs

. But that doesn't change the fact that the definition of the basic measurements are defined in metric terms. I'm sure you can come up with an equivalent for non-SI, but you never hear that one foot is defined as 495,229.119 wavelengths of Kr-86 - it's always defined to give you the length in metres. In other words, the non-SI units are always going to be derivative in nature.

In practical terms, that might not matter so much. But if you want accuracy, then you can always fall back on the definitions used for the metric terms, and you know they're always going to be the same wherever you are.

 

[mike wightman]

At least the Imperial system gives us a proper unit for weight.

How often do you buy groceries in newtons?

And if the Brits or Yanks ever colonize another world, I'll bet their ATVs still have a speedometer in mph

 

[Malenfant]

Um, Newtons wouldn't be pratical to use on other worlds anyway.

1 kg of food on Earth has a weight of about 10 N. On the Moon, 1 kg of food would have a weight of about 1.7 N. It's therefore much more useful to refer to the mass of the object, rather than the weight of it. And for that, kg works just fine.

Besides, the unit of mass in the Imperial system is the "slug", which is 32 lbs. And I have no clue how much 32 lbs is in real units

. There's some info on it (you'll have to copy/paste the link, just remove the url= bit):
url=http://en.wikipedia.org/wiki/Force_(physics)

Personally, I think the kg is much more practical than the slug

.

 

[mike wightman]

But we don't buy stuff by mass.
How many people ask "what's the mass of that beef joint?" - not very many.

I know that the kg can be used for weighing things on Earth, it's just the casual missuse of the unit that bugs me.

For example, the weights and measures act forbids traders in the UK to sell groceries in ounces/pounds only, but requires them to have the "weight" diplayed in kg
It shold be weight in N

And now I know why cowboys and gangsters in old films get drunk so quickly when they ask at the bar for a slug of whiskey