Bearings in space

[Hakubak]

Originally Posted by Agemegos

Every point in the Universe is equally the centre of the Universe (because the Universe is unbounded).

I honestly don't know. My understanding is that time-space is expanding outward from where the big bang took place. Again, as I understand it, it isn't just stars and matter flying away from that origin point, but space itself - a tricky concept that I only claim to partially comprehend.

If this is true, then there is an outer edge, outside of which there is no space. I haven't even tried to wrap my head around that one.

For most discussions, I would agree that no point in our known universe is any better than any other for being the origin. If the big bang is science fact, then I think it would make a good origin for a navigation system. On the other hand, the center of one's own galaxy might be better. The scales at which space travel would occur would make the center of the universe too remote. The differences at our distance from it would be too minute to be practical. (Please excuse the stream-of-semi-consciousness here.)

[Anthony]

Originally Posted by Archangel Beth

How about Galactic North?

It might not be highly detailed, but knowing which way is "north" is probably reasonable enough till you get closer to your goal, yes?

I think it's only 'north' from earth. It's a large globular cluster, but it's not very bright and globular clusters aren't very distinctive.

[Anthony]

Originally Posted by Hakubak

I honestly don't know. My understanding is that time-space is expanding outward from where the big bang took place. Again, as I understand it, it isn't just stars and matter flying away from that origin point, but space itself - a tricky concept that I only claim to partially comprehend.

Your understanding is incorrect. The big bang took place at every point in space, and the big bang is actually space itself expanding, carrying all the matter with it. There is no center, and there is no outer edge.

[transmetahuman]

My understanding (big, big disclaimers in bright neon here) is that it goes something like this: If you map the "location" of the Big Bang in that early Big-Bang-sized universe to the current universe, the "location of the Big Bang" maps to everywhere. So everything is flying away from everywhere, equally, and there is no reference point, and no space-outside-of-space.

Kind of like those "small parallel universes" where great distances in ours map to casual walks in the other universe (a common rationale for FTL travel), only the "small universe" in this case is our early universe, and it was point-sized.

I could be totally wrong, but that's the model in my head.

[William]

Originally Posted by transmetahuman

My understanding (big, big disclaimers in bright neon here) is that it goes something like this: If you map the "location" of the Big Bang in that early Big-Bang-sized universe to the current universe, the "location of the Big Bang" maps to everywhere. So everything is flying away from everywhere, equally, and there is no reference point, and no space-outside-of-space.

Essentially correct. The usual model used is a balloon that's blowing up, with the galaxies drawn in marker on the balloon: all the galaxies are moving away from the Milky Way galaxy, and likewise all the galaxies are moving away from the Andromeda galaxy, even though each galaxy is sitting in place on the balloon as it expands.

The Universe, then, forms a 3-dimensional "balloon." When the Big Bang happened, the Universe was extremely small, and the Big Bang happened everywhere; there simply wasn't much everywhere. It didn't have a boundary, though, no more than a circle has a boundary (you can walk indefinitely in the circle's one dimension) or a sphere has a boundary (you can walk indefinitely in its two dimensions). Both are finite, but unbounded.

That circle does have a center. It's just not anywhere on the circle, and if the circle is all of space, then that center isn't anywhere.

[Mysterious Dark Lord v3.2]

To navigate anywhere you need bearings from two or more fixed known reference points. On Earth, that's the Royal Observatory in Greenwich, England and the Equator.

In space, the logical points would be the Galactic Core (the radio signature of Saggitarius A* is easy to locate) and S Doradus (The brightest and most prominent star in the Large Magellanic Cloud, with a very distinctive spectrum). The relative positions of those two would give a rough general notion of where one is located, as well as distance, and could be designated "main markers".

Purely "local" prominent phenomena (pulsars, supernova remnants, and anomalous objects) would help fix the position to greater degrees of accuracy. In our galactic neighborhood, Alpha Cygni (also called Deneb) would be one such local marker, as white supergiant stars are rare and it has an anomalous spectrum (heavy iron concentration).

So a galactic-scale campaigh would require a eight-place coordinate system - the first two indicating the relative position between Sagittarius A* and S Doradus (flat and inclined plane). The third, fourth, and fifth numbers would be the catalog identification of a local marker and it's relative position from the closer main marker; sixth, seventh, and eighth being the same for another marker relative to the further main marker.

With all these numbers, there could only be one location where these objects would be seen at these exact angles. Thus coordinates are fixed.

In a "local galactic" campaign, the Core and S Doradus might be too large-scale. In which case only six coordinates need be used; the identity of two markers and bearing from them.

In a "local space" campaign (covering only a couple of dozen parsecs), three coordinates showing the bearing and distance from an arbitrary "fixed" zero location is all that is necessary. The small number of stars allow them to be identified by spectrum and relative position. The system used in GURPS SPace 3e is adequate for that.

(A coordinate does NOT mean a one-digit number! It can be as many digits as necessary. Or even have letters, in the case of identifying markers.)

Hope this helps.

[William]

To navigate anywhere you need bearings from two or more fixed known reference points. On Earth, that's the Royal Observatory in Greenwich, England and the Equator.

The "known reference points" you refer to on Earth are actually two coordinates -- longitude and latitude respectively -- which you need because the surface of Earth is a two-dimensional object. If you do not know where you are on Earth and need to determine your position, you need to find bearings to three markers of known location. The Equator is not a point!

In three-dimensional space, you need bearings to four markers to find your position, which can be marked in three coordinates. In a relatively round, disc-like galaxy, a number of systems can be applied, with the most likely candidates being spherical (two angles from fixed reference lines -- probably convenient bright stars -- and a distance from an agreed central object) or cylindrical (height from a reference disc -- the "galactic equatorial plane" -- radius out projected on that disc, and angle from a reference line on that disc, again probably from a convenient star).

[lwcamp]

Originally Posted by William

The Universe, then, forms a 3-dimensional "balloon." When the Big Bang happened, the Universe was extremely small, and the Big Bang happened everywhere; there simply wasn't much everywhere. It didn't have a boundary, though, no more than a circle has a boundary (you can walk indefinitely in the circle's one dimension) or a sphere has a boundary (you can walk indefinitely in its two dimensions). Both are finite, but unbounded.

There is no evidence, one way or the other, that the universe "wraps around" on itself like a circle or sphere. It is possible that the volume of the universe is infinite (unlike the surface of a 4-sphere, which would have finite volume). We cannot see all that volume, of course, because for anything beyond roughly 15 billion light years away it would take longer than the lifetime of the universe for light to reach us. In this sense there is an edge to the observable universe, even if the universe were infinite in extent.

Other possibilities involve various ways for the universe to wrap back around on itself (although, it would do this without any curvature), or that the obervable universe sits inside a finite "bubble," outside of which different physics apply (possibly being the hot, dense inflationary "stuff" that prevailed during the first instant of the big bang, or possibly separated from our universe by a boundary known as a "domain wall," which is thought to cut off regions with one type of physics from those with another. Note that when I say that physics is different, gravitation is probably the same but there exist a different combination of particles and the forces that act on them than our everyday up and down quarks, electrons, electromagnetism, weak force, and strong nuclear force).

Despite amazing advances in comsology and lots of very clever ideas and measurements, there is still a lot we do not know about the universe.

Luke

[lwcamp]

Originally Posted by Anthony

Within the milky way, locating some combination of Andromeda, the Magellenic Clouds, and Cygnus X-1 should do the trick.

Is Cygnus X-1 really observable all over the galaxy? Or are you thinking of Sag A?

Luke

[Anthony]

Originally Posted by lwcamp

Is Cygnus X-1 really observable all over the galaxy? Or are you thinking of Sag A?

Luke

I was thinking of Sag A*, though Cygnus X-1 is probably visible from most of the galaxy as well, at a distance of 2500 parsecs it's the brightest hard X-ray source in the sky. There are also a few hypergiants in the milky way (such as Eta Carinae and the Pistol Star) which should be identifiable.