The Dyson Sphere (The Dyson Shell)

[Adam Dray]

A lot of this is based on the Jump Drive theory of 100D being wrong...but what if it is right?

Yeah, I was trying to apply known-universe general relativity rules to jump-space, which is full of logical flaws, right?

Jump-space obviously isn't something that has to follow general relativity, so maybe matter in real-space perturbs jump-space in a way that isn't related to gravity at all.

That is, I'm agreeing with you.

So if the 100D rule depends on matter and energy perturbance and not gravitational curvature, then a 1-AU Dyson Sphere would have a 100-AU jump shadow and you couldn't jump inside of it.

Or you could say that the 100D rule doesn't apply to objects filled with empty space, so Dyson Spheres have a different jump shadow formula. Maybe there's just a maximum distance that applies and overrides the 100D rule. I'd suggest that it would involve the Schwarzschild radius, as I think Werner is saying.

 

[Werner]

Yeah, I was trying to apply known-universe general relativity rules to jump-space, which is full of logical flaws, right?

Jump-space obviously isn't something that has to follow general relativity, so maybe matter in real-space perturbs jump-space in a way that isn't related to gravity at all.

That is, I'm agreeing with you.

So if the 100D rule depends on matter and energy perturbance and not gravitational curvature, then a 1-AU Dyson Sphere would have a 100-AU jump shadow and you couldn't jump inside of it.

Or you could say that the 100D rule doesn't apply to objects filled with empty space, so Dyson Spheres have a different jump shadow formula. Maybe there's just a maximum distance that applies and overrides the 100D rule. I'd suggest that it would involve the Schwarzschild radius, as I think Werner is saying.

Most important thing is that it serve the game, and it doesn't serve any purpose to have pc starship appear 200 au out, 1 au is good enough.

 

[whulorigan]

So if the 100D rule depends on matter and energy perturbance and not gravitational curvature, then a 1-AU Dyson Sphere would have a 100-AU jump shadow and you couldn't jump inside of it.

Actually, the interior surface of the sphere is 1 AU from the star (this is a radius measurement). The diameter of the sphere would be 2 AU, so the Jump shadow would be 200 AU out from the sphere in the case you describe.

 

[whulorigan]

I interpret the 100-diameter limit as a nod to either gravitational frame dragging or static mass increase due to general relativity.

In the case of frame dragging, a starship in the vicinity of a larger, spinning object experiences small torsional gravity forces from the spinning object. These could throw off very sensitive jump-drive components.

In the case of static mass increase, a starship gains inertia while in the vicinity of a larger object. The change is very, very small -- even hard to measure -- so it's hard to believe that it'd throw off a jump drive, but maybe it's that sensitive of an operation.

Both of these effects are tiny. Furthermore, there is no gravitational formula applying to an object in space that cares about the diameter of the star or planet (all roughly spherical masses can be reduced to a point mass).

Rationalizing the 100-diameter rules with real-universe physics in going to require a lot of handwaving. A real jump-distance formula would need to be calculated based on the mass of the nearby object, not its diameter.

This is one of the reasons why (if you are basing it ultimately on mass, and diameter is merely a rule-of-thumb) I like the tidal-force approximation (tidal force going as an inverse-cube (i.e. g's per meter) instead of inverse-square). It gives g/meter results for both stars and planets that are not too divergent from each other. So figure a good g/meter value based on ~ 100 diameters (I would use a rocky-planet as Werner suggests), and figure out the "cut-off" tidal stress at 100 dia, and use that value for all bodies as the "real" number. This would also mean that "frame-dragging" and torsional stress would also potentially become an issue (any change in g's/meter across the dimensions of the vessel).

The problem with using straight g's in a General Relativity model is that accelerations and g-fields are indistinguishable under GR. So an accelerating body at 1g in free space is identical to a stationary one in a g-field of 1g. So a ship would have to be not accelerating/maneuvering when it jumped. Further, an object in free-fall is identical to an object in gravitationally unperturbed free space -- so would that mean as long as my ship is "weightless" (even in freefall within a g-field), would I be able to safely jump? With a tidal stress across the dimensions of the hull, the issue become more understandable (the g-gradient cannot be different along different parts of the hull (in fact it needs to be very close to zero in all directions), otherwise the jump equations give different results for different sections of the ship).

 

[whulorigan]

...
iii) Either (a) The event-horizon is an optical surface: Just not for the same reasons as for stars and planets or (b) it isn't: and black holes become real dangers in jump-space.

But an event horizon is an entirely mathematical surface, it is not physical (though there are physical effects that are observable close to that calculated mathematical distance *IF* there is material that is close to it and falling toward the singularity).

 

[Werner]

Actually, the interior surface of the sphere is 1 AU from the star (this is a radius measurement). The diameter of the sphere would be 2 AU, so the Jump shadow would be 200 AU out from the sphere in the case you describe.

That's just ridiculous a Dyson sphere is so thinly spread out that it doesn't add much to the gravity of the star. A Dyson sphere has about the mass of Jupiter which has a mass one thousandth of the Sun. There should be no difference between building a dyson sphere around the Sun and just dropping Jupiter into the Sun, I don't think either of those things should push the jump limit out to 200 au from 89 million kilometers.

How about simply have ships precipitate out of jump space at the jump limit of the star inside the Dyson Sphere, not outside of of it? Jump space operates through a higher dimension does it not? Since it is traveling through 4d space, it doesn't have to pass through the surface of the Dyson sphere to get inside of it, after all it's not a warp drive.

 

[whulorigan]

That's just ridiculous a Dyson sphere is so thinly spread out that it doesn't add much to the gravity of the star. A Dyson sphere has about the mass of Jupiter which has a mass one thousandth of the Sun. There should be no difference between building a dyson sphere around the Sun and just dropping Jupiter into the Sun, I don't think either of those things should push the jump limit out to 200 au from 89 million kilometers.

How about simply have ships precipitate out of jump space at the jump limit of the star inside the Dyson Sphere, not outside of of it? Jump space operates through a higher dimension does it not? Since it is traveling through 4d space, it doesn't have to pass through the surface of the Dyson sphere to get inside of it, after all it's not a warp drive.

You'll note that my post was correcting a calculational error made by Adam Dray in a partcular non-gravitational case among several options that he was addressing (see posts above).

 

[Werner]

You probably still don't want to come out of jump space inside a star even if it were possible to do so. Some stars like Betelgeuse are so bloated and spread out that it is possible for a planet to orbit inside of them for some time before getting destroyed.

 

[whulorigan]

You probably still don't want to come out of jump space inside a star even if it were possible to do so. Some stars like Betelgeuse are so bloated and spread out that it is possible for a planet to orbit inside of them for some time before getting destroyed.

Of course. That is why a pure gravity/tidal model is not the entirety of the story. Gravity gradient and physical displacement/distance will likely both play a part, hence the typical astrogator's rules-of thumb. You wouldn't likely want to come out at 100 diameters OR the typical tidal-stress cut-off at a neutron star, for example, due to the intense EM-radiation.

All of this is presupposing that the "100 diameter limit" is based on our presumptions above, of course, and not something completely different. How individual referees choose to interpret it for their games is of course, their business.

 

[Davik]

Where did this come from?

[/INDENT]If the ship's jump-line directly intersected the 100 diameter limit of the shell, it would cause the ship to precipitate out of jump at the 100 diameter limit of the shell after 1 week in jump.

Curious which version of Traveller introduced this idea?

 

[aramis]

Curious which version of Traveller introduced this idea?

CT. In JTAS.

 

[Werner]

Curious which version of Traveller introduced this idea?

I think the main purpose is to require a Starship to do a bit of flying before it can make an escape to Jump Space.

 

[Davik]

Furthermore, there is no gravitational formula applying to an object in space that cares about the diameter of the star or planet (all roughly spherical masses can be reduced to a point mass).

Sure there is! The gravitational pull between any two objects falls as the square of their distance apart. F=Gm1m2/r2. Going with 100 diameters (200 radii) is a nod to this to keep the game simple.

We could, instead, solve for F using G, m1 as the mass of Vland, and r as 100 diameters of Vland. The Vilani probably had a lot of interplanetary traffic long before they discovered Jump. As a wag to what they might've been using and to their cultural conservatism, we might assume 10K ton displacement ships as their norm, and that the "100 d" was calculated not based on a theory, but on practical experience with a built in margin of error. No idea what the average density of a ship would be... but lets say it's .5, so a ship with a displacement of 10K ton, would have a mass of 5K tons. Plug that in for m2.

Solve for F, and we have a tiny number representing the "safe" force of gravity on a ship from the nearest gravity well... NOW the annoying part! Rearrange the formula to be r=sqrt(Gm1m2/F). Use that "safe" F dictated by the bureaucrats of Vland, G, the mass of whatever planet or star we're talking about as m1, and your current ship's mass (again, wagging at .5 x displacement) as m2, and you can find the actual separation distance for each and every "safe" jump point.

I'd say 100d is probably a much easier route!

 

[Davik]

I think the main purpose is to require a Starship to do a bit of flying before it can make an escape to Jump Space.

Sorry, I wasn't referring to the distance to the Jump Point for a ship entering Jump Space, but to the idea that gravity wells (other than the destination) in normal space can effect a ship in Jump Space.

 

[Davik]

CT. In JTAS.

Thx... would you know which one?

 

[whulorigan]

Thx... would you know which one?

I believe it is JTAS #24, but Aramis may have been thinking about an earlier issue.

 

[whartung]

I believe it is JTAS #24, but Aramis may have been thinking about an earlier issue.

JTAS #24 doesn't talk about 100D breaking "line of jump", just how it affects entry and exit.

From my post on JTAS #24 from *cough* almost 10 years ago:

One, is the affect of the 100D limit. Don't start jump within a 100D limit, but if you try to exit jump within a 100D limit, the nature of the system simply spits you out at the 100D limit. This implies limited "jump masking". Notably if you're within the 100D limit of a star, you can not jump, nor can you enter within the 100D limit of a star. However nothing here suggests that the 100D sphere around a body "blocks line of sight". There's nothing to suggest that you will be pulled out of jump by a rogue gravity well. Rather, only when you actually leave jump space does the 100D limit apply, not within jump space. So, if you want to jump to the other side of a star in the same system, the star can't stop you, it's not "in the way". You can't jump in or out of it within 100D, but once you're in, it's not yanking you out. You're in the wrong dimension.

 

[Condottiere]

1. As I interpret the current edition treatment of jumping, bad things are more likely to happen if you try it too close to a gravitational well.

2. But you cannot say that gravitational wells have no effect during transition, if they act as speed bumps while you're still in transition, because regardless if the starship is in final approach mode, you're still in transition when you hit the hundred diameter limit of a gravitational well.

3. I rather doubt you can plot a jump through the heart of a star.

4. My theory is that jump technology is very much linked to the understanding of gravity, and anti gravitational drives.

 

[whartung]

1. As I interpret the current edition treatment of jumping, bad things are more likely to happen if you try it too close to a gravitational well.

2. But you cannot say that gravitational wells have no effect during transition, if they act as speed bumps while you're still in transition, because regardless if the starship is in final approach mode, you're still in transition when you hit the hundred diameter limit of a gravitational well.

Nothing so far disputes this. The premise being that there is a "poorly understood" "thing" called jump space. Jump drives transfer the ship in and out of jump space. And that those transitions are influenced by local gravity wells.

3. I rather doubt you can plot a jump through the heart of a star.

4. My theory is that jump technology is very much linked to the understanding of gravity, and anti gravitational drives.

But there was nothing in the #24 article that suggested ONCE IN JUMP SPACE, that "normal space" has any affect whatsoever (including being "yanked out of jump" because you encounter another gravity well).

You don't plot a jump "through" a star, you enter jump space away from said star, travel in jump space around said star, and arrive at the other side. You never were near the heart of it at anytime.

 

[Davik]

Nothing so far disputes this. The premise being that there is a "poorly understood" "thing" called jump space. Jump drives transfer the ship in and out of jump space. And that those transitions are influenced by local gravity wells.



But there was nothing in the #24 article that suggested ONCE IN JUMP SPACE, that "normal space" has any affect whatsoever (including being "yanked out of jump" because you encounter another gravity well).

You don't plot a jump "through" a star, you enter jump space away from said star, travel in jump space around said star, and arrive at the other side. You never were near the heart of it at anytime.

Thanks! I read it all the way you do... so I'm guessing one of the later versions of Traveller added that idea. I have the LBBs from 1977 and from 1981, and T5.

T5 DOES talk about "Straight Courselines" (pg 112 of BBB2): "Gravity Sources in Real Space affect Jumpline: a straight line course cannot pass through a bubble surrounding a mass of any appreciable size..." which to me completely contradicts page 118 BBB2, which says "A ship in Jump Space is totally isolated from Real Space," and "is undetectable." If the former is true, the latter cannot be... and in my mind (and in MTU) the latter is true, NOT the former.