What would a dyson sphere look like . . .

[Albertus]

Haven't read the whole thread, but had to comment. "lucid visualization"............ waking dreams? Unslept images? Too many pints? Get a grip.

 

[Shaw]

Haven't read the whole thread, but had to comment. "lucid visualization"............ waking dreams? Unslept images? Too many pints? Get a grip.

Apparently (to prometheuspan) it is similar to math.

 

[Forbin]

Thanks Christopher & Timo. I guess I'm not as stupid as I thought.

Some of the travel in Ringworld would probably boggle if thought of too closely. The ring is already doing 770mps, and in Ringworld Engineers our heroes travel, within the air envelope, at up to 20,000 mph in various directions in their lander, spinward, antispinward and crossways. But they travel high enough to exit the air several times, which makes one wonder how/if the spin-generated gravity is acting on the ship at any given time.

Plus, their mode of levitation is a scrith-repulsor! Once they levitate off the ground and above the air, the lander is still repulsing the scrith, but without being in the influence of the spin, what opposite force is keeping them from flying off into space?

Maybe Niven was hoping we didn't think too hard on it.

 

[Jimmy_C]

Thanks Christopher & Timo. I guess I'm not as stupid as I thought.

Some of the travel in Ringworld would probably boggle if thought of too closely. The ring is already doing 770mps, and in Ringworld Engineers our heroes travel, within the air envelope, at up to 20,000 mph in various directions in their lander, spinward, antispinward and crossways. But they travel high enough to exit the air several times, which makes one wonder how/if the spin-generated gravity is acting on the ship at any given time.

Plus, their mode of levitation is a scrith-repulsor! Once they levitate off the ground and above the air, the lander is still repulsing the scrith, but without being in the influence of the spin, what opposite force is keeping them from flying off into space?

Maybe Niven was hoping we didn't think too hard on it.

When he jumps off the ringworld, the person is still moving at 770 mps. Just at a slightly different direction.

 

[Christopher]

^^That's right. You retain your sideways momentum even if you push upward from the surface (i.e. inward toward the center). And since the surface is curving toward you, a straight-line sideways path would eventually converge with the surface and essentially be "falling down," so you'd need more centripetal thrust (i.e. a push away from the inner surface) to keep you from falling. Conversely, if you thrust fast enough to cancel out the Ringworld's tangential velocity and accelerate in the opposite direction, you'd again be on a straight vector that would take you "down" into the curved surface unless you have a centripetal thrust component. Although the Ringworld is huge enough that it would take a long time for that curvature to be felt, so the scrith repulsion drive might only need to be used intermittently.

However, it would not be necessary if you just flew above the atmosphere, cancelled your momentum, and hovered while you let the surface rotate beneath you. On the contrary, in that case you'd need a thrust in the opposite direction to keep you from drifting in toward the star, since you wouldn't be orbiting it. In fact, in the second case I mentioned in the last paragraph, I think I was not entirely correct, because if you thrust opposite the direction of motion, then you're slowing your orbital velocity, which would cause you to fall out of orbit toward the star, effectively "falling up" from the Ringworld surface. It's only if you accelerate to a speed equivalent to or greater than orbital velocity in the opposite direction that the physics I proposed for the second case above would apply.

So what about flying breadthwise, perpendicular to the direction of the Ringworld's rotation? In the Sun's reference frame, that's just increasing the inclination of your orbit; your orbital velocity would remain unchanged. So presumably the same basic physics as in the first case I mentioned would apply, with the breadthwise vector making no significant difference.

 

[Jimmy_C]

Well the orbital velocity of the Earth is 18.5 mi/s (=2*pi*1au/1yr). This is much less than the 770 mi/s of Ringworld and its inhabitants. So I would think anything on its inner surface would be trying to escape outward and isn't in danger of falling towards the star, right? Or would it be trying to spiral inward? Orbital Mechanics is hard!

 

[Christopher]

Okay, you're right. I forgot that the Ringworld was spinning much faster than orbital velocity in order to achieve Earthlike centrifugal gravity.

 

[Forbin]

Aye. In Ringworld Engineers, they occasionally mention bringing the Hot Needle of Inquirey down to "merely orbital velocity" below the scrith and let the Ringworld zip by over their heads to move to a new location.

 

[Meredith]

If "gravity" is generated by spin, as in the Ringworld or an O'Neil colony like Babylon 5, is something in the air, such as an aircraft, affected? My brian is telling me that you're only held to the surface by the spin, but once you've separated from the surface you're no longer affected by the spin.

Or is it that the air itself is being held down by the spin, and, being in that envelope of air, so is the flying object? Would that then mean that if the habitat contained a vacuum, a floating object would not be affected by the "gravity?"

That's essentially correct. It also depends on how the flying object is moving. Let's look at a couple of situations:

1) You're hanging from a high ledge on a building in an O'Neill cylinder. You lose your grip and fall. What happens?

While you're hanging, you're following a circular path around the center. Let go, and it's like cutting the cord on a whirling tetherball -- suddenly you're travelling in a straight line, tangent to the previous circle. This takes you outward, your momentum carrying you toward the perimeter of the cylinder, which brings you closer to the "ground." Basically you're flying sideways, but the ground curves up to meet you. To an observer within the rotating frame of reference, it looks like you're falling down on a curved path. The fictitious force that seems to be pushing you sideways is the Coriolis force.

2) You're in an aircraft hovering at the axis of the O'Neill cylinder. At this point, you're in free fall. What happens if you jump out?

If you stay close to the axis, you'd continue to hover. However, let's assume you push off the side of the aircraft and drift away from it. From the axis, any direction is "down." In a vacuum, you'd just keep drifting slowly until you converged with the surface -- although it would be rotating pretty fast relative to you and the landing could be messy as a result. An observer on the "ground" would see you spiraling outward, the Coriolis force "pushing" you sideways ever faster.

With atmosphere, however, things are different. This is where the transfer of force that Timo was referring to comes into play, though not in the way he implies. The air itself is rotating because of its mutual friction -- the rotation of the ground pulls the air in contact with it, which pulls the air higher up, and so on and so on. It doesn't all rotate as a solid mass; an observer on the surface would feel a steady, gentle Coriolis wind. Still, all the air is rotating at some velocity. So let's assume you push off from the plane in atmosphere. As you drift away from the axis, air resistance slows you, so you don't follow the same path you would in vacuum. But as you move partway out from the axis, the air is now rotating, and that tends to push you sideways. Again, because it's a circular habitat, going sideways (tangentially) takes you outward. So that pushes you into an area where the air is spinning a bit faster and pushes you farther sideways/outward. And so you're accelerated toward the surface, more gently at first than by actual gravity, but it adds up. To the observer on the ground, your path is different than it would've been in vacuum, still curved but not as severely, and with your acceleration being outward as well as sideways. The observer would perceive your fall in terms of the interaction of an outward gravitational/centrifugal force and a sideways Coriolis force, whereas in vacuum it would be perceived entirely in terms of the Coriolis force.

At least, I'm pretty sure that's how it works. Note that if your aircraft is not at the axis, but is flying opposite the habitat's direction of spin at a velocity equal to the rotational speed, then it's essentially standing still and its interior would be more or less in free fall. Even if you're in a car driving on the surface, your weight would increase if you drive fast in the direction of spin and decrease if you drive opposite to it. So aircraft would be advised to take off in an antispinward direction, since that would make it easier to leave the ground.

Reminds me of that episode of Babylon 5 where Cheridan jumped off an exploding monorail running the center length of an O'neill cylinder and he was falling slowly and kept accellerating.

 

[Christopher]

^^Yeah, but I think they explained it wrong there, in terms of the centrifugal gravity increasing the further you got from the axis. Which is true as far as it goes; if you build a really tall skyscraper in an O'Neill cylinder, the gravity will get lower the higher you are. But that's because you're travelling in a curve as the floor beneath you rotates. It's more complicated when it comes to falling through the air.

By the way, something neat would happen in an elevator in that tall skyscraper. The farther out from the center/closer to the ground you are, the faster your rotational velocity, since you have to go faster to cover a larger circumference in the same amount of time. So if you're in an elevator going upward, as you ascend, your momentum is suddenly carrying you too fast for your altitude and you'll feel a Coriolis push sideways into the spinward wall. Conversely, in an elevator going downward, you'll start out with a slower sideways velocity and descend into portions with a faster one, so the "back" (antispinward) wall will push against you. Fast elevators would probably have to be spherical or cylindrical and able to rotate to compensate for this effect. At least they'd need padded walls.

And for rock climbers, or catburglars, this is a decided advantage; so long as you're ascending a spinward wall or slope, the faster you climb, the less steep it will seem to become for you. Same if you're descending an antispinward wall or slope. In theory, if you got up enough speed in proportion to the size and rotation of the habitat, it would be possible to run up a vertical wall.

 

[Forbin]

Reminds me of that episode of Babylon 5 where Cheridan jumped off an exploding monorail running the center length of an O'neill cylinder and he was falling slowly and kept accellerating.

Which is why I mentioned that scene in another post.

Cheridan?

 

[Jimmy_C]

If you were to make a particle accelerator with magnets as powerful as the LHC that went around the entire Ringworld, how much energy will the collision of two protons going around in both ways have?

 

[Christopher]

Forty-two.

 

[The Inquisitor]

Going back to the original question of what would the view be like at ground level.

Presuming the sphere had an Earth like atmosphere and was the same distance away from the Sun I should imagine (with the Earth like atmosphere) there would be blue skies and clouds, given that during daytime the atmosphere only lets through the blue light (very technical I know). I know you can just about see the moon sometimes during daylight hours but anything too much further away (bar very bright stars) tends to just disappear in a haze of blue.

Also, in reference to the potential size of a dyson sphere it pretty much depends on the size and type of the star its being constructed around. The luminosity and output of a host star would dictate the distance of the 'goldilocks zone'. Sizes could range anywhere from a Red Dwarf to a Hyper Giant like VY Canis Majoris (Hyper Giants would be pretty useless due to their life expectancy).

Either way, a very interesting thread that I was about 2 years late to pitch in on.

 

[Christopher]

Presuming the sphere had an Earth like atmosphere and was the same distance away from the Sun I should imagine (with the Earth like atmosphere) there would be blue skies and clouds, given that during daytime the atmosphere only lets through the blue light (very technical I know). I know you can just about see the moon sometimes during daylight hours but anything too much further away (bar very bright stars) tends to just disappear in a haze of blue.

You're probably right -- with one exception. Assuming there are large enough oceans on the far side, they would reflect the sun's light rather brightly -- far more so than, say, Venus (which is visible in the daytime sky when the angle permits). So one would see a blue sky, the sun at zenith, and various bright points at fixed positions in the sky.

Of course, by "large enough," we're probably talking oceans that are substantially bigger than the entire planet Earth; but there's certainly precedent for that in existing fiction involving megastructures (particularly Ringworld).

 

[The Inquisitor]

Good point Christopher, I forgot that planets and the odd star can be visible during the daytime.

One thing that would be sorely missed should you live in a sphere is sunrise and sunset, seeing as the sun would be directly overhead at all times wherever you were stood.

In regards to the day/night cycle (if there was one at all) I suggest some form of atmospheric field/layer that could be programmed to let specific amounts of light through at certain points in the day, a bit like 'smart glass'. Given the technological capability of any species or civilisation capable of constructing a sphere I doubt there would be much trouble in creating a 'night switch'. Who knows, maybe during night time hours the sun at the centre of the system would look a lot like the moon should the atmospheric filters be programmed that way.

 

[Ronald Held]

Should we assume any inner planets, as they may have been used for the sphere? I would expect an atmosphere relatively close to the inner surface.

 

[RAMA]

Does anyone have Probert's original rendering of the Dyson Sphere at ground level?

 

[The Inquisitor]

I should imagine any atmosphere, were the sphere be designed to harbour human(oid) life, would have a similar consistency to that of earth. Somewhere between 10 and 20 kilometres perhaps? Any more might create a pressure issue on the surface.

As for inner planets I should imagine it would be possible. Gravitationally lumped together masses of sphere construction machinery could well be drifting about which could well be in the region of planetary scale. Then again, maybe inner planets would interfere with any force field based stabilisation the sphere might require to stay stable/intact.

 

[Timo]

We might easily have missed inner planets in the "visual inspection" we got - but if the sphere had "shadow squares" similar to those used in Ringworld for the day/night cycle, those ought to have been visible, because their whole point is being visible.

Since all we ever saw of the inner surface of the sphere was uniform daylight, we'd probably do well to think that the sphere had an all-encompassing dimming system (a forcefield around the star, say) that either would have come online in a few hours, or had broken down due to the instability of the star. Or then there had been shadow plates orbiting the star, but those had been lost already, perhaps falling into the star if they were held aloft by radiation pressure and that in turn grew unstable.

As for the nature of the atmosphere, it could be exotic thanks to the gravity holding it being exotic. The gravity of starship decks doesn't seem to follow the inverse square law; different laws would stack the atmosphere differently, and there could be some trickery there such as a gravity field that increases in strength as one moves from the surface towards the star. That'd e.g. help in keeping the gas from escaping into the zero-gee interior of the sphere from the upper fringes of the atmosphere.

Timo Saloniemi