Radio Astronomy Question.

[BigDon]

Stemming from a Science fiction story, if nobody minds.

So in ancient locations on Earth we find a gate/portal system, dating back to the Cretaceous. One of those always on, you can look through and see another sky type set ups. Just step across.

A small by-line was after poking around and finding the other side liviable, they try to find out *where* in the Universe they are. And they dragged across a "small radio telescope" in sections, re-assembled it and took two and a half months to determine that they where still in the Milky Way, about 25% further spinward.

Now my ignorance of radio astronomy is rather vast.

Assuming you had all the resources such an event would get you and you were tapped you solve where you were, and given unlimited supercomputer time and exculsive use of the telescope.

I assume not knowing if you were even in the same galaxy would be very influential on the search pattern.

Would that be enough time without being very lucky?

What would be the first markers to look for? Distant quasars or local clouds? I assume they would be distant ones.

About how far from home would you think even a radio telescope wouldn't help you, without at least Earth decades of observation? I would "guess" once out of the Local Group you may as well not bother. (but you wouldn't know that until you looked. )

Or would that still depend on the view? Would you still be more lost if you were a full 50% around the Milky Way, with the full galaxy core in the way over being in another galaxy outward from our side, on the same side, with the same outward view? (I know galaxies don't align up normally)

I'll reword that last one if I have too.

[antoniseb]

It would probably not take long to recognize the LMC & SMC and from there be able to find M31. These would triangulate you pretty fast even without a radio telescope. From there you could look for pulsars, but they would have tens of thousands of years apparent difference in spin-down time, so who knows how they'd look, and if pulsars are directional, whether you'd see them from there.

[BigDon]

It would probably not take long to recognize the LMC & SMC and from there be able to find M31. These would triangulate you pretty fast even without a radio telescope. From there you could look for pulsars, but they would have tens of thousands of years apparent difference in spin-down time, so who knows how they'd look, and if pulsars are directional, whether you'd see them from there.

I hadn't thought about that at all!

Okay I had a couple more questions stemming from that statement alone, but trying to articulate them is making me dizzy.

[IsaacKuo]

What about looking for supernovas, or flare stars, or other omni-directional events? If you can find these, then you have an exact time delay difference between when they were observed on Earth and when they were observed on "gateworld".

If you can identify at least four of these events, then that gives you a rough idea where in space-time you are relative to Earth. This depends on how accurate our estimates of how far away those events took place, of course.

Generally, you would be able to identify events that are only a few light decades off. If the stargate doesn't move us in time very much, then that means we'll be able to identify events along the plane perpendicular to the midpoint between Earth and gateworld.

Essentially, this is the multi-lateration technique used by GPS receivers. Except instead of using artificially generated GPS signals, we're using naturally occuring omni-directional events.

[parejkoj]

I'd first do what antoniseb said: look for things like the LMC, SMC, Orion nebula complex, etc. Big things that are easy to spot.

To determine where I was in the Galaxy to within a few dozen light years, I'd map out the positions of radio loud extra-galactic sources using long baseline interferometry (needing a few radio telescopes spaced a thousand kilometers apart or so), and compare it to the ICRF. This wouldn't be affected much by intervening dust, and there are plenty of sources out of the Galactic plane. Since the current version is accurate to a milliarcsecond or better, a similarly-accurate measurement on the new planet (mind you: this would probably take at least a year once the telescopes were set up!) should pin down our location to within ~10 parsecs.

(To derive this yourself, compute the baseline needed to achieve a parallax angle of 1 milliarcsecond for a source at a redshift of 1, which should be about right for the ICRF. It gets even more accurate if the sources are closer than that.)

Now, antoniseb's comment about the time difference probably doesn't matter too much here: most of the quasars in the ICRF are going to be radio-loud for a million years or more. But some of them would be brighter or dimmer, which would complicate matters, but it's doable.

To get a more precise measurement, I'd use pulsar positions and spindown(up) timing. Because they are closer, the parallax angle for pulsars would be much larger resulting in much better positional precision. And one could use the timing measurements to improve the precision, once we had a rough position from extra-galactic sources. Though the small beam angle might make that difficult (I hadn't thought of that at first, antoniseb: good catch!). Hmmm... I'm not sure exactly what the limits on this would be.

Now, if the planet was more than a few million light years away (say, in outside the local group), we'd be in trouble. Many (most?) of the ICRF sources would have very different luminosities if observed at a time more than a few million years removed from our own, so we might not be able to compare with that. One could conceivably make a map of the large scale structure out to a redshift of ~0.1, and compare it with our own local maps, but that would take an optical telescope, and several years. And it would at best give you a position within a million light years or so.

It's an interesting question. I think the biggest problem is not the map, but how to identify objects that don't change much over timescales greater than a few million years.

[IsaacKuo]

You don't need objects that don't change much over long timescales. Unless the endpoints of the stargate are inside each other's light cones, there will always be objects that are the same distance from both. These are objects along a particular hyperboloid. If the stargate endpoints are at the same time relative to each other, then this hyperboloid will actually be the flat plane exactly in between them.

[BigDon]

Thank you guys for humoring me.

Anton, the difference in the age of the sky was something I hadn't thought of at all and parejkoj expanded nicely on the same subject.

Par, I loved those links and I had no clue as to the existance of the ICRF.

Isaac, could you reword your last post? I'm interested in what you are trying to tell me but you lost me at light cones.

Guys, consider your daily quota for educating your fellow man fulfilled.

Collect your gold stars from Anton or Toseek.

[IsaacKuo]

Basically, if you travel forward or backward in time, then you travel within your own light cone.

For example, suppose you travel forward in time to Earth a million years from now. Then there will not be any objects in the universe which will look the same age as when you left. All objects will look older.

However, if the stargate brings you to a point outside your light cone, then there will be some objects in the universe which will look exactly the same age from this point in space-time as from where/when you left.

If the stargate moves you in space but not in time, then those objects will be the ones which are exactly the same distance from both stargates. Draw the line segment between the two stargates. The midpoint is exactly the same distance between the stargates, as well as all points along the plane perpendicular to the line.

If the stargate moves you in time as well as space, then you need objects which are further away from the "future" point by the time difference in light years.

For example, suppose the stargate sends you to the Tadpole Galaxy, fifty million years into the past. Then you need to look for objects which are fifty million light years closer to the Tadpole Galaxy than Earth. This will define a hyperboloid curving away from Earth rather than a flat plane.

This only works when the time difference is less than the spacial distance in light years.

[astromark]

I can only add the obvious. That mapping the near by universe will take some time from a relatively different place. Time is an important tool in our quest for defining the locations and distances across this galaxy.

A relative map by radio astronomy paints a familiar view when a direct visual spectrum image is compared. Its like looking in infa-red.. Its very different. But a sameness can be established. I would think given telescope time I could find a familiar pattern within a month. The local group of galaxies would soon reveal there position. The advantage with radio astronomy is just that the frequency of energies received is not adversely effected by any interferences. Some of the telescope aria's can be very accurate... and big.