Speculations...

[rbubany]

Suppose that we have developed the capacity to travel among the stars within our galaxy.

Magnetic north won't work. What would be our point of reference?

Latitude and longitude won't work because they are two dimensional. x, y, z coordinates won't work because during the time it takes to, say, cross the galaxy our destination would have moved thousands of miles, even though we have the hypothetical capability to make the trip in under one day. I guess we would have to operate under a four dimensional system - x, y, z, time.

If our destination is a planet across the galaxy, how do we know at the time we leave that the planet's star has not just gone nova? This would seem to be of importance to know before we leave.

I can't sleep.

[Jon Hickman]

A straight line trajectory between where we started and where the star we're shooting for will be in the amount of time it takes to travel there - some modifications as neccessary to avoid passing through black holes, other stars, and other cosmic "debris"...

[Dave Venne]

Suppose that we have developed the capacity to travel among the stars within our galaxy.

Magnetic north won't work. What would be our point of reference?

Latitude and longitude won't work because they are two dimensional. x, y, z coordinates won't work because during the time it takes to, say, cross the galaxy our destination would have moved thousands of miles, even though we have the hypothetical capability to make the trip in under one day. I guess we would have to operate under a four dimensional system - x, y, z, time.

The situation isn't all that different from navigation within the Solar System. We use a coordinate system that is fixed, even though everything within it is moving. Time is a coordinate because journeys aren't instantaneous, whether we're going to the Moon or the galactic center.

A galactic coordinate system could be simply latitude, longitude and radius (measured from the galactic center). We'd have to recognize that everything is moving around it, but that's the same as we do within the Solar System. Add to that the idea that most things move slowly in our galaxy; in a million years, the Sun moves through less than 0.5% of its orbit around the galaxy.

If our destination is a planet across the galaxy, how do we know at the time we leave that the planet's star has not just gone nova? This would seem to be of importance to know before we leave.

This depends on the star you're talking about. A one solar mass star can't go nova until it's over ten billion years old. Compare that to the amount of time you're traveling to it to figure the odds of arriving at a cinder. A trip of even a million years is so short a time in comparison that there's almost no chance of a nova while in transit.

A more massive star would go nova sooner, but even then your flight time is far shorter. And I would guess when you said "capacity to travel" you meant something that might allow for much shorter trip times than a million years.

Which makes me wonder if they'd serve snacks on these flights? A million years of snack boxes at $10 a pop would add up.

[rbubany]

...some modifications as neccessary to avoid passing through black holes, other stars, and other cosmic "debris"...

How about pot holes, other cars and other construction debris? My Garmin doesn't handle that very well.

[rbubany]

A galactic coordinate system could be simply latitude, longitude and radius (measured from the galactic center). We'd have to recognize that everything is moving around it, but that's the same as we do within the Solar System. Add to that the idea that most things move slowly in our galaxy; in a million years, the Sun moves through less than 0.5% of its orbit around the galaxy.

So, our point of reference is the galactic center then. Is that something we can find? As the galaxy wobbles around, does the galactic center shift positions? These movements are so slow they probably don't matter...

But, how does our navigation device detect the galactic center?

I still can't sleep.

[Dave Venne]

So, our point of reference is the galactic center then. Is that something we can find? As the galaxy wobbles around, does the galactic center shift positions? These movements are so slow they probably don't matter...

But, how does our navigation device detect the galactic center?

Think of it in terms of how a go-to telescope "navigates" the sky. It doesn't need to know where the north celestial pole is, nor does it need to find the point where the ecliptic crosses the celestial equator. All it needs is to find a few reference stars whose positions are known and it's good to go (or go-to).

The same thing could apply to interstellar navigators. They will navigate by stars whose positions are known in the coordinate system. It's more complicated than for go-to telescopes because of the finite speed of light and the long flight times, but that's something a computer can handle.

We don't need to know where the exact center of the galaxy is in order to define our coordinate system. We only need to pick a point that's convenient and build the coordinate system around it. Once we determine the positions and motions of navigation stars, we can "see" the coordinate system no matter where we are.

At that point we can set sail, navigating by the stars.

[Bill Bynum]

Seems like you could pick the supermassive black hole in the center of the galaxy as 0,0,0,0 point.

Where's 0 longitude though? Through Sol's center of mass?

[Dave Venne]

Seems like you could pick the supermassive black hole in the center of the galaxy as 0,0,0,0 point.

Better watch out selecting the time origin at a black hole. Things are confusing enough out here in "normal" space.

Where's 0 longitude though? Through Sol's center of mass?

The prime meridian is completely arbitrary, so why not? Kind of puts a stamp of "humans started here" on the whole Galactic Empire thing, too.

[Bill Bynum]

Whoah! Totally blew the physics on that one Even calling it the 0,0,0 point is a little iffy.

Sure would hate to get a "Divide by 0, please reboot galaxy" error!

[Dale Smith]

Alpha Centauri is close enough to Earth that most constellations, etc. would be in approximately the same position as on Earth. Only a few of the very closest stars would require major correction for parallax. Guide stars and general celestial navigation should work well for the immediate stellar neighborhood.

As we go farther across the galaxy individual guide stars would get fainter and harder to find. I suggest bigger markers. For example, the Andromeda Galaxy is visible from much of the Milky Way Galaxy. It is about 2,500,000 light years away. The Milky Way Galaxy is about 100,000 light years across. Applying a little trigonometry two observers on opposite sides of the Milky Way viewing the Andromeda Galaxy would be looking within 8° or less of the same direction.

Closer to home are the 150 - 200 globular clusters that orbit the Milky Way at distances ranging from about 10,000 to 130,000 light years. These would be useful for triangulation.

[rbubany]

Anything within the galaxy, or even nearby, would be a problem so far as being a reliable reference point. Stars are bad because you might not be able to see them from anywhere in the galaxy, worse yet, how would you locate them in a background of millions of stars.

Clusters might work assuming there are some that are so unique in appearance you can identify them. But even so, I suspect they are moving around and that it would be difficult to pinpoint the location of a far away target due to this motion.

Basically, it would seem to me that one must find a source that is well outside the galaxy - perhaps Andromeda and a couple other galaxies as suggested earlier. They move relative to our galaxy, but perhaps this motion is negligible? Too much math for me.

If you accept the premise that your reference points need to be outside of your traveling environment in order to work well, then imagine the difficulties if we expand this discussion to travel within the entire universe.

We would have to find reference points outside of our universe? Can this be done?

I'm beginning to think speed is only the first problem in navigating the universe. Should we find a way to "warp" our way around, we would still have huge problems to solve in terms of navigating.

I think.

Maybe I should get new blankets to help me sleep.

[Ben Mullin]

Ron,

I think your sleeping problem has to do with when you are trying to sleep. 1:30 pm is when I usually have the most trouble staying awake.

Why do you need to be able to unique identify your reference points? Are you assuming that you are going to show up somewhere not knowing where you are and have to solve for your position? All along your way you can track where things are. If they go out of view you should be able to keep track of where they should be. They should have fairly well known trajectories.

I don't think the premise that the reference points need to be outside the travel environment need be accepted. I navigate with local references all the time. Granted mine are not also in motion relative to my destination, but if they did move in predictable fashions they could still be used.

Ben

[Dave Venne]

I don't think the premise that the reference points need to be outside the travel environment need be accepted. I navigate with local references all the time. Granted mine are not also in motion relative to my destination, but if they did move in predictable fashions they could still be used.

I agree -- when I was mentioning guide stars before, I was assuming there would be a lot of them, and that they would be within the galaxy. There's no need for them to be exceptionally bright or external. All they have to be is in sufficient density where you want to travel.

[Dale Smith]

The reference points do not necessarily have to be outside the travelling environment, but they do need to be bright enough to observe and have positions that are known with sufficient accuracy. The fact that they are in motion is not a problem as long as we can calculate where that motion will take it. The sun, moon and planets are all in motion, but make acceptable celestial navigation targets because their positions for any given time and date can be accurately calculated.

Let us take the bright stars that define the outline of Orion as an example. Distances to the belt stars Alnitak, Alnilam and Mintaka, the shoulder stars Betelguese and Bellatrix and the leg stars Rigel and Saiph range from a few hundred to 1400 light years. Travelling 4 light years to Alpha Centauri would not change their apparent positions very much. The outline would still be recognizable as Orion.

However, as we go further afield two problems arise for the stars in Orion. First, some of these distances have error bars as big as ±50%. Second, the far side of the Milky Way is about 70X further than the furthest of the stars I just listed. Since light intensity drops off as the square of the distance. this means the intensity would be 1/70² or 1/4900 of what we see here on Earth.

That is why I suggested galaxies and globular clusters. They are bright enough to be visible from most places in the Milky Way and far enough away that their apparent positions will not change much on the time scale of human travel.

If, as Ben suggests, we are travelling in Star Trek fashion and can continuously monitor our position it should not be a problem. But perhaps Ron is envisioning either awakening from decades long suspended animation or travel through a Star Gate like worm hole in which reference points cannot be clearly seen. Hopefully in the case of suspended animation the onboard computer is intelligent enough to do the continuous monitoring. However, the worm hole scenario could indeed pose the problem that Ron has mentioned.

[rbubany]

I didn't explain my assumptions when I started this thread. Let me clarify.

In the latest "adult" version of Battlestar Galactica, the ships are able to "jump" in an instantaneous fashion to somewhere else. You merely notify the ships in the fleet the coordinates of the destination and then flip the jump switch. Poof, you disappear and poof again and you reappear somewhere else.

Of course that can't happen, but assuming it could, the issue becomes one of what would these coordinates that they use would actually be.

They obviously don't have a decent map of the galaxy, since they have no idea where earth is. Yet, they can all hit the same target location within sight of each other.

Early navigation across the ocean must have been similiar to this. As our ancestors traveled in this two dimensional plane they used a reference in the third dimension to establish latitude and longitude.

Anyhow, we ponder galactic travel as being a problem of figuring out how to solve the "speed of light" dilemna. But if we overcame that problem via some miracle, we still couldn't travel to the other side of the galaxy and back because we would be unable to precisely pick a set of coordinates. Even if we could make an end to end jump of the galaxy in as short a time as one day, we would miss our destination by thousands of miles - at least one would think so because all of our astronomy books talk about stars moving at thousands of kilometers per second...

I am up really early this morning.

[Dave Venne]

In the latest "adult" version of Battlestar Galactica, the ships are able to "jump" in an instantaneous fashion to somewhere else. You merely notify the ships in the fleet the coordinates of the destination and then flip the jump switch. Poof, you disappear and poof again and you reappear somewhere else...

...They obviously don't have a decent map of the galaxy, since they have no idea where earth is. Yet, they can all hit the same target location within sight of each other.

In the Olde Days of sci-fi when jumps like this were used, travel across the galaxy was done in a series of short hops. Traveling from point A to point Z you would first jump to B, have the ship compute its new position from local guide stars, and line up the next jump. Then you would jump to C, and repeat until you reached your destination. That process could be explained in a couple of sentences, and in a written story the entire trip from A to Z might take only a few lines.
On TV, it's far more dramatic (and much less time-consuming) to jump right from A to Z. This requires them to have the capability to do this, but not to explain how it's done to the viewer -- so long as they can make it seem plausible. It's like the transporter on Star Trek, which is a convenient replacement for boring shuttle flights. They make the transporter plausible, even if they have yet to explain how it manages to work.

Early navigation across the ocean must have been similiar to this. As our ancestors traveled in this two dimensional plane they used a reference in the third dimension to establish latitude and longitude.

The third dimension you're talking about here is that of time -- whether determined by the lunar distance method or a timepiece. Perhaps the navigators of Galactica will have access to higher dimensions that are useful for pinpointing jumps? Maybe they travel through those dimensions when jumping?

I am up really early this morning.

You and me both. I've got to have some more clear and warm nights to re-establish my proper lack-of-sleep cycle.

[merle]

Interesting thread Ron... I'm going to throw a couple of pennies worth of thought in...

In the latest "adult" version of Battlestar Galactica, the ships are able to "jump" in an instantaneous fashion to somewhere else. You merely notify the ships in the fleet the coordinates of the destination and then flip the jump switch. Poof, you disappear and poof again and you reappear somewhere else.

Do you need to do anything more than "point your spaceship in that direction", to travel in space. Just like a ship on the ocean, you can just sail away. Now, if you need to go to some place exactly (which will probably be the case), you'll need to navigate to x,y,z - relative to your ships orientation. In the Battlestar Galactica world, "jumping" to x,y,z and having all the ships end up in the same place would require all the ships to be orientated the same way or at least think they are. Using the coordinates of one ship, say Galactica, would make sure everyone is headed in the same direction and ends up in the same place.

I don't think we'll be able to travel through space like we "normally" think of travel. Some sort of space warp has to occur where you, more or less, instantly arrive at your destination. I think it might be something like more like Monsters Inc. as opposed to Star Trek. Wouldn't it be nice if the coordinates of a region of space we wanted to visit were entered in a computer and we stepped through a doorway that took us there... Hmmm, come to think about it, Star Trek (TOS) had a few episodes like that didn't they.

[Dick Jacobson]

The main question seems to be, do we anchor our coordinate system to objects within the galaxy, or outside of the galaxy?

Within the galaxy, the central black hole seems to be an ideal "origin" point for the coordinate system. With something like 3 million solar masses in one infinitesimal point, nothing short of a galactic collision could move it very much. As for the x, y, z axis directions, one obvious choice is the direction perpendicular to the spiral disk. Since the Milky Way is a barred spiral, a second direction could be along the length of the bar. The third axis would be perpendicular to these two.

Of course, since the galaxy is rotating the entire coordinate system will also rotate. It will get totally messed up in a few billion years when we collide with the Andromeda Galaxy!

Maybe it's better, then, to go with an extragalactic system for the axis directions. One choice that is already used by radio astronomers is to use distant quasars for extremely precise, non-moving directions. Another possibility is to use the ripples in the cosmic microwave background. Although this "background wallpaper" is fuzzy, it would look virtually identical to anyone anywhere in the galaxy so I think would be a good choice for a permanent galactic coordinate system.

Great topic Ron, I look forward to your future bouts of insomnia!

[rbubany]

OK. Let's carry this further - velocity.

In our ship we are traveling at, for all practical purposes, an infinite number of velocities. They are all relative, of course - speed with respect to the sun, with respect to Betelgeuse, with respect to Andromeda, and so on.

Once we make our jump, what is our set of relative velocities and direction?

Now when we jump to our destination we most likely would set the coordinates to be "near" the planet we want to visit. We will use impulse power and line of sight to finish the trip.

Possibly, the relative velocity of the planet vs our ship ends up being greater than the velocity we can achieve (in a reasonable time frame) with our impulse engines. What now? Another jump?

Seems we need that external fixed point again, so that we can measure all relative velocities against it. Without it we have velocity chaos?

I'm so tired.

[Dave Venne]

Of course, since the galaxy is rotating the entire coordinate system will also rotate. It will get totally messed up in a few billion years when we collide with the Andromeda Galaxy!

Maybe it's better, then, to go with an extragalactic system for the axis directions. One choice that is already used by radio astronomers is to use distant quasars for extremely precise, non-moving directions. Another possibility is to use the ripples in the cosmic microwave background. Although this "background wallpaper" is fuzzy, it would look virtually identical to anyone anywhere in the galaxy so I think would be a good choice for a permanent galactic coordinate system.

If the reason for going with external references is the fact that our galaxy will be seriously distorted in a few billion years, I think that precludes using quasars and the cosmic microwave background. In a few billion years those quasars will have aged and may not even appear as quasars. And in a few billion years, won't the microwave pattern will look different?

With instantaneous jump technology there's no reason to be concerned with what happens in a few billion years -- or even a few thousand. Which means that the galaxy we should concern ourselves with is pretty much an unchanging "snapshot" to which we must apply corrections to account for target velocities. Precision targeting relies more on the details of how jumps are performed, and we don't know those.

Ron's question about the relative velocity between a ship and its target is an old one that's usually not mentioned. I first came across it in a sci-fi story where transporters allowed people to move from point to point on Earth. Imagine a person stepping into a transportation booth in Minneapolis and stepping out of a booth on the equator at the same longitude. Because of the Earth's spin, there's a relative speed difference of about 300 mph. That first step out of the booth would be your last. The author solved this by saying that the booth system absorbed the kinetic energy of all velocity differences.

So all we need to do is specify that this damping technology is built into the jump drive. Make it so, Scotty!

[Dale Smith]

First off, I must report an error in my previous calculations. Two observers on opposite sides of the Milky Way will report a 2.3° difference in the direction of the Andromeda Galaxy, not 8° as I previously stated. I did the calculations on an Excel spreadsheet and momentarily forgot that Excel does angle calculations in radians rather than degrees. I think there may have also been a typo. (Mea culpa. Mea cupla. Mea maxima cupla.)

Closely allied with navigation is the problem of mapping. Imagine that we were suddenly presented with a device capable of delivering us instantaneously to anywhere on earth to the nearest inch, but the only available map of destinations was a cartoon map on a McDonald's place mat. What good is incredibly accurate navigation if there is huge uncertainty in specifying the destination? Hopefully travel to other star systems will occur at a pace that allows the accuracy of our stellar cartography to keep up.

For the moment let's limit the discussion to travel within the Milky Way. Thoedolites (used in surveying) can measure angles to about the nearest arcsecond (1/3600 of a degree). This is a little better than 0.0001% of a circle. A sighting on the Andromeda Galaxy (M31) from anywhere else in the Milky Way would put us within less than 2° of the same direction as observed here on earth. Comparing the angular distances between, say, M31 and the comparatively nearby galaxies M32 and M33 or between M31 and an even more distant galaxy it should be possible to calculate a position. Assuming angular error can be held to 0.001° (≈4 arcseconds) the resulting error in position is roughly constant at about ±45 light years regardless of the distance traveled within our galaxy. (For observers within 1000 light years of earth the angular difference is too small to measure. Guide stars can be used at distances less than this. At greater distances the increase in angular accuracy due to a wider baseline roughly offsets the loss of accuracy due to the distance.) Also, uncertainty in our measurement of the distance to Andromeda will produce a proportional error in calculated position (e.g. a 10% error in our knowledge of the distance to Andromeda will produce a 10% error in calculated position). From there it should be possible to estimate approximate angles of some globular clusters (GC). Since GC’s are 20 – 250X closer than M31/M32/M33 it should then be possible to use the GC’s to recalculate the position to within a few to a few dozen light years. That should be plenty close enough to start consulting local star charts for a more exact fix. If such local charts do not yet exist it probably means we are new to that region of space and probably had only a crude idea of the destination’s position when the journey began. This points up the need for having reference markers at all different scales along the way. Local markers are only good locally. Long distance markers are good for approximate positions, but can only get you so close before losing local resolution.

Another potential factor is the Heisenberg Uncertainty Principle (ΔxΔp ≥ 5.276 E-35 J-sec or the uncertainty in the position times the uncertainty in the momentum is greater than or equal to Planck’s Constant divided by 4π). Normally it is relegated to calculations at atomic and subatomic scales. At the scale of everyday objects the necessary uncertainties in position and momentum are too small for our measuring devices to measure. It may rear its ugly head again as we move up the scale to truly enormous distances coupled with relativistic velocities. I estimate that it probably would not be significant for travel within the Milky Way, but might be significant for intergalactic travel.

[Dave Venne]

Closely allied with navigation is the problem of mapping. Imagine that we were suddenly presented with a device capable of delivering us instantaneously to anywhere on earth to the nearest inch, but the only available map of destinations was a cartoon map on a McDonald's place mat. What good is incredibly accurate navigation if there is huge uncertainty in specifying the destination? Hopefully travel to other star systems will occur at a pace that allows the accuracy of our stellar cartography to keep up.

I think you have to understand what the capability of instantaneous travel would mean and how not having a highly accurate map would be dealt with almost immediately.

It would be simple to flood the galaxy with navigational, automated probes that would provide the map and suitable guide stars. These would simply hop from place to place, bootstrapping the map, with Sol as the final destination. How fast the galaxy could be charted would depend on navigation requirements, the time it takes to make jumps, and how long it takes to chart the local guide stars. This would do away with any need for external navigational guides.

I'm speculating that the probes could be very, very efficient and a complete map could be made in far less than a human lifetime. It would all depend on the resources committed to the endeavor.

Local markers are only good locally.

Why would this be true? If you know the position and velocity of a star at a given time thanks to it being charted, why can't you do a jump across the galaxy to it? This assumes the jump technology has that kind of accuracy, but Ron's posts suggests this is the case.

[rbubany]

As I understand it now, there are indeed objects within (and also without) the galaxy that we could use as a fixed reference for navigation.

But I still have a problem.

All of our potential reference points are far away. Their light takes time to get to our space ship. Since we are in relative motion, our reference object seems not where it is, but rather where it was.

I guess if the light tells us where it was, and somehow we know where it's been going, and we know how long the light took to get to us, we could figure out where it IS right now. Then we can use that as a base for our desired set of coordinates.

I think from earth we know where are reference point is moving to based on our local observations. But, our space ship is nowhere near the earth, so how do we know where our reference point has been moving to? What standard reference point did we use to figure that out?

This is worse than having a pea under one's mattress.

[Matt.Merrell]

Good idea with sending something that could be sacrificed should we pop out into the star or blackhole . That would make the journey a little more interesting.

How do we map out the Milky Way? We should have a working 3d model of it all by now. Well at least the parts we can see right now. Put it in motion check the atomic clock to get the local time when you are leaving so you know where your destination would be when you get there.

[Dave Venne]

All of our potential reference points are far away. Their light takes time to get to our space ship. Since we are in relative motion, our reference object seems not where it is, but rather where it was.

I guess if the light tells us where it was, and somehow we know where it's been going, and we know how long the light took to get to us, we could figure out where it IS right now. Then we can use that as a base for our desired set of coordinates.

We know what the speed of light is, and we would know (from the scouting probes) the positions and velocities of stars relative to the probes. The positions would not be simultaneous, as you point out -- but I think they would be reasonably accurate if care was taken in planning the survey probes' jumps. If you know the distance to the stars, you can compensate for the light travel time and coordinate the positions to a reference time. Once you've done that, you can plan jumps.

If the probes are carrying Sol's motion with them as they jump, then I'd guess that can be compensated for when you define your "fixed" coordinate origin and the motions of stars relative to that.