Page 107 Discussion

[Grayhome]

Have you tried Space Engine Icekatze?

[Siber]

Space Engine is pretty great, but as far as I'm aware it doesn't let you get information about relative stellar velocities. I think stars are all static unless they're orbiting a barycenter currently, in fact.

[Arioch]

I don't have recent experience with any of these packages.

[Zakharra]

Counting binary and trinary star-systems? I was under the(probably false) idea that the 400ish closest star systems to us were within a 150 LY radius.

The RECONS 25 database of known stars within 25 parsecs (81.5 light years) lists 3,069 objects in 2,167 systems. The actual number is probably much larger, as the majority of stars are dim red and brown dwarfs that are difficult to detect at greater distances. The estimated density of stars within 5 parsecs is about 0.14 stars per cubic parsec, so the expected number of stars within 25 parsecs is about 9,100.

For a sphere with 150 light year radius, using this density we would expect about 57,000 stars. For a box 250 light years on a side, we would expect about 63,000 stars.

Here's a nice visualization of the RECONS 25 database:



Note how the stars become less red as distance increases, because they're missing from the database.

That is very neat to watch and listen too. Science fucking rocks!

[Auspice]

Hi

Minor nitpick from page 107, where Alex is giving info to Beryl about measurements and stuff. It's unlikely that Alex (or Humaniti in general) would use oscillations of a cesium-133 atom as the basis for his measurements. It's an oddly random thing to use when he could just use water.

- A mol of water is 6.022e23 molecules of water (a mol of anything is 6.022e23 atoms or molecules of it).
- A mol of water is approximately 18 mililiters of water (if you want extra precision you can explain it's based on the atomic mass of water, so Beryl can figure out the exact number).
- A mililiter of water has a mass of one gram.
- A mililiter of water occupies a cube one centimeter on each side.
- Then obviously you explain metric prefixes (centi = /100, milli = /1000, etc.)
- Then you measure the speed of light in meters per second (an integer: 299,792,458 m/s, or 300 megameters/s if he wasn't Alex and he didn't memorize the exact number) and you're basically done.

Once he teaches Beryl how to count in base-10 (which he'd need to do), the rest is much easier than knowing the number of oscillations in a cesium-133 atom. Sure, cesium clocks are pretty stable, but a mol is a much easier number to remember with only 4 significant digits and is no less precise.

edited because I'm stupid

[icekatze]

hi hi

There is nothing unlikely, odd or random about it. The second is defined by humanity by the oscillations of the cesium 133 atom, today.

The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.

- The International System of Units

A major flaw with using the mol as a method of explaining a unit of measurement is that Avogadro's constant ((6.02214078 +/- 0.00000018) x 10 ^23) requires an already precise definition of a gram, and is only an estimation.

[Razor One]

Water is actually a terrible measure because its physical properties vary with temperature and pressure, far moreso than the oscillations of a cesium 133 atom. Water expands when heated, not greatly, but enough to throw off the necessarily precise measurements you'd need. Pressure will alter the boiling point. You'd need to explain standard temperature and pressure to the Loroi in addition to everything else, most of which still requires a temporal component.

Cesium oscillations on the other hand are measured at 0K and can be considered a measure of time objective from the vagaries of temperature and pressure. It's also a somewhat recognisable as a measure of time to the average reader. Optical lattice clocks would probably be more likely by 2160 (if not something more advanced), but aren't quite as recognisable as "A really accurate way of measuring time" to the average reader.

[Michael]

OMG UPDATES

[Eluvatar]

hi hi

There is nothing unlikely, odd or random about it. The second is defined by humanity by the oscillations of the cesium 133 atom, today.

The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.

- The International System of Units

A major flaw with using the mol as a method of explaining a unit of measurement is that Avogadro's constant ((6.02214078 +/- 0.00000018) x 10 ^23) requires an already precise definition of a gram, and is only an estimation.

I believe there are serious plans to redefine the kilogram in terms of Avogadro's constant.

[icekatze]

hi hi

They're really going to go with the Avogadro's constant route? Last I heard, they were going to try to revise the kilogram using Planck's constant.

Proposed definition: The kilogram, kg, is the unit of mass; its magnitude is set by fixing the numerical value of the Planck constant to be equal to exactly 6.62606X×10−34 when it is expressed in the unit s−1·m2·kg, which is equal to J·s.

- National Institute of Standards and Technology

[Eluvatar]

hi hi

They're really going to go with the Avogadro's constant route? Last I heard, they were going to try to revise the kilogram using Planck's constant.

Proposed definition: The kilogram, kg, is the unit of mass; its magnitude is set by fixing the numerical value of the Planck constant to be equal to exactly 6.62606X×10−34 when it is expressed in the unit s−1·m2·kg, which is equal to J·s.

- National Institute of Standards and Technology

Looks like that proposal is indeed preferred to defining the Kilogram in terms of Avogadro's constant. However, Avogadro's constant is nevertheless going to be fixed, it seems, in order to define the mol (independently of the kilogram). Presumably experiments are being run to make sure that the resulting independent definitions of mol and kg match up with the original definition of mol in terms of kg.

[icekatze]

hi hi

To be honest, I find that disconnecting the mol from a known weight to defeat the purpose in the first place. I mean, if one knows the amount of elemental entities in a measure of substance, can't one just use that number instead? It's like making a "dozen" a standard unit, when scientists could just write 12.