World like a doughnut... and more

Cobalt Jade · Sep 3, 2017

I have an idea in mind for a SF story, about an unusual planet shaped like a doughnut. That is, with a big hole in its center, that theoretically a spaceship could fly through, or contain an atmosphere so creatures could fly through. This planet would rotate its star with its axis of rotation pointing up, like a record rotating on a phonograph. So far, so good. I've found a few sites giving information about this.

Here's where it gets complicated. Imagine this thin doughnut is invisible. Coiling around that invisible doughnut is a solid, cylindrical tube that makes seven twists. That is the actual world -- artificial of course. An endless coiled one.

What would be the physics of this? Would characters be able to walk around all sides? What kind of day and night would there be?

blacbird · Sep 3, 2017

Cobalt Jade said:
I have an idea in mind for a SF story, about an unusual planet shaped like a doughnut. That is, with a big hole in its center, that theoretically a spaceship could fly through, or contain an atmosphere so creatures could fly through. This planet would rotate its star with its axis of rotation pointing up, like a record rotating on a phonograph. So far, so good. I've found a few sites giving information about this.

Here's where it gets complicated. Imagine this thin doughnut is invisible. Coiling around that invisible doughnut is a solid, cylindrical tube that makes seven twists. That is the actual world -- artificial of course. An endless coiled one.

What would be the physics of this?

In compete violation of everything we know about physics. Kind of like the floating mountains in "Avatar".

caw

AW Admin · Sep 3, 2017

Larry Niven. Ringworld.

Dennis E. Taylor · Sep 3, 2017

A world can (briefly) be doughnut-shaped. It has to be mostly molten, or at least hot enough to be very malleable, and rotating very fast. As you increase rotation speed, a spinning body will go from sphere, to oblate spheroid, to something that resembles a red blood cell, and finally to a torus.

WeaselFire · Sep 4, 2017

Cobalt Jade said:
What would be the physics of this? Would characters be able to walk around all sides? What kind of day and night would there be?

All these questions have no answers until you write them. This type of world does not exist in real life and cannot be described in terms of known physics.

And Larry Niven's Ringworld is artificial. As is a Dyson sphere. Though you might research Koreshan beliefs about heaven existing on the inside of the Earth.

Jeff

DrDoc · Sep 4, 2017

The gravity would be differential depending on where you were on the 'spiral'. Those on the outside would weigh more than those on the inside. I recall reading a story about life on a neutron star and the problems they had with differences in centripetal force near the equator: walking east/west was more difficult nearer the equator. It was an interesting story once you accept the conceit that life could exist on the surface of a neutron star.

blacbird · Sep 4, 2017

I think it would be more like a bagel.

But, as a reader, I would have a bigger problem: What is the point of such a world, in terms of affecting your story?. To me, this concept sounds like any number of "world-building" ideas that are entirely disconnected from any story. So, as it stands right now, I coudn't care less what the shape of the world is. What is going on there? And why should I read about it?

caw

Cath · Sep 5, 2017

Cath said:
I'm seeing a lot of replies that either:

Comment on the story rather than answering the question.

Suggest the OP 'makes it up' because no-one expects fiction to be true to life (especially if the OP is asking about Science Fiction or Fantasy).

This won't fly here. Posters should expect fact-based or experience-based responses. And this is NOT a place for brainstorming stories ideas without a fact-seeking element to the question, there's a whole other area of the forum for that.

Posts that suggest making up the facts to fit the story or otherwise commenting on the story rather than answering the question may either be punted to another forum or deleted entirely. You have been warned.

Posts have been removed.

Read the Forum Guidelines before posting again.

Al X. · Sep 5, 2017

The dynamics will largely be dependent on the size of the toroid planet, and its speed of rotation. With regards to gravity, pull will be a composite effect. Exactly how it will be distributed at any given point on the toroid or surrounding space would be fairly difficult to describe but it could actually be modeled using FEA analysis. It is fairly safe to say that, at the exact center of the hole it will all cancel out.

With respect to rotation, there will be centripetal acceleration which, depending on the rotational speed and size of the donut, may or may not be significant. That by the way is an effect on the surface of the earth. It is called the Eötvös effect. It is the apparent reduction of gravity when travelling in a westerly direction and an apparent increase in gravity when traveling in an easterly direction, which is strongest at the equator and zero at the rotational polar axes. It isn't significant in practical terms but it is measurable.

The other effect which will be at play will be some variant of the Coriolis effect. On earth, if you say, fire a gun on the equator facing north, the projectile will continue on a straight horizontal line but the earth will continue to rotate under it, displacing the impact point laterally of the geographically straight line to the target. This is taken in account by artillery ballistic computers. On a rotating toroid, the effect would be interesting, and would affect travel dynamics through space, and again could be modeled.

GeorgeK · Sep 6, 2017

My understanding is that planets and even things as small as Earth's moon are mostly spheroids because of their mass which would suggest that a torus would have to be a rather small planet and that would seem to also mean a low gravity planet. I'm assuming a rocky planet and not some artificial construct.

Layla Nahar · Sep 6, 2017

You could read Hal Clement's Mission of Gravity. That takes place on a lentil-shaped world, and it's nicely treated in a hard-SF kind of way. It would give you an idea of how one person successfully approached something similar.

ETA: Doh! I just saw that yours is a manufactured world. Adding to the voice that suggests you read Ringworld. & I'd read the Clement book, too, to get some other ideas of how to play out a strange shaped world.

Mark HJ · Sep 7, 2017

The first thing that occurs to me (other than the excellent advice above) is that your scenario might not be stable. If the 'central axis' of your world is 'pointing up', so the world's torus is like a donut lying flat on the plane of the ecliptic, all those wacky mass distributions you just surely get will eventually tip it over so the the 'axis' points towards the star, with some degree of precession about the planet/star line.

That 7-twist coil of cylinder is going to create quite an unevenness in the mass distribution, which means the centre of mass with respect to the gravitational pull between planet and star is going to change as it turns on its axis. I'm trying to do this in my head, but I think it needs a serious bit of computer modelling to be sure - I have a suspicion your world will follow a very erratic orbit, with its 'centre' moving towards and away from the star, and suffer all sorts of cyclic gravitational stresses.

Al X. · Sep 7, 2017

I think it is fair to say that any structure that is able to maintain its shape without collapsing in to a sphere doesn't have enough mass to have a significant gravitational pull.

PeteMC · Sep 8, 2017

I *think* that for this to be really viable your "doughnut" would have to function like the Ringworld, that is to be so vast that it has its star at the centre of the hole. Otherwise, as AI X says, it's eventually going to collapse into a sphere whatever it's made of.

I have to say though, this is a very cool idea.

WeaselFire · Sep 8, 2017

According to commonly accepted physics, a doughnut-shaped world can't form because matter won't naturally condense into a doughnut. If you can plausibly define why the center is missing, you could possibly make this a realistic natural world. The first problem though is that, as far as common theories hold, solid worlds have a molten core while a gaseous world will at least have a small solid core. Without those cores, I'm not sure that any believable physics can account for the planet still existing.

So, at least as far as I can see, you can only create this world with a manufactured, or at least altered, world.

Jeff

Dennis E. Taylor · Sep 8, 2017

http://mentalfloss.com/article/501332/astronomers-propose-new-donut-shaped-celestial-object

http://www.scienceworldreport.com/articles/59580/20170524/earth-once-donut-shaped-study-suggests.htm

https://curiosity.com/topics/yes-a-donut-shaped-planet-is-technically-possible-curiosity/

writbeyondmeasure · Sep 9, 2017

From a climate/weather point of view: it would be a very stormy around the outside and inside of the planet because of the high temperature gradients between the regions that are in the sun and those that aren't. Depending on the angle the planet makes with the sun, the top and bottom of the doughnut will be in perpetual day/night. The people/creatures should be able to walk around on the planet however the gravity will vary by location (stronger closer to the centre of mass). Also, gravity and climate affect evolution. So the people who evolved closer to the centre of the planet would be smaller or have stronger bones/muscles due to the extra force of gravity. People who evolved on the top or bottom (again depending on angle between planet and sun) will be either super pale (if it's always night) or super dark skinned (if it's always day)

neandermagnon · Sep 9, 2017

writbeyondmeasure said:
From a climate/weather point of view: it would be a very stormy around the outside and inside of the planet because of the high temperature gradients between the regions that are in the sun and those that aren't. Depending on the angle the planet makes with the sun, the top and bottom of the doughnut will be in perpetual day/night. The people/creatures should be able to walk around on the planet however the gravity will vary by location (stronger closer to the centre of mass). Also, gravity and climate affect evolution. So the people who evolved closer to the centre of the planet would be smaller or have stronger bones/muscles due to the extra force of gravity. People who evolved on the top or bottom (again depending on angle between planet and sun) will be either super pale (if it's always night) or super dark skinned (if it's always day)

Stupid question from a biology nerd: wouldn't everything be drawn to the centre inside the hole in the middle of the donut because isn't that where the centre of gravity would be*? I don't get how anyone on the inside of the donut would be stuck to the donut. On the outside, sure. Drawn towards the centre of gravity. But what really would happen to the people on the inside of the donut? (this is a genuine question that I want to know the answer to, not a rhetorical question)

*further thought - is this why planetary scale donut shapes wouldn't stay donuts as all the matter would be drawn to the centre of gravity and clump together to form a sphere?

Regarding skin colour - if there's no sunlight at all then there's no evolutionary advantage to having light skin, because the evolutionary advantage is the ability of the skin to make vitamin D at lower light levels. If there's no light at all, then there's no way to make vitamin D at all so skin colour wouldn't make any difference. Though if intelligent life evolved on another planet they wouldn't be enough like humans for the question of how much vitamin D human skin can make to be an issue. An equivalent of fur/feathers could protect from sunburn, maybe.

Additionally, if parts of the planet are in permanent light and permanent dark, they'd be way too hot and cold (respectively) for life to evolve at all. You need the regular changing from night to day to keep the temperature stable enough for life to be able to evolve. However, it has been theorised that planets that have a hemisphere in permanent darkness and a hemsiphere in permanent light will have "goldilocks" zones around the borders between the dark and light that could harbour life, and these planets AFAIK are considered as possible planets that may have life, along with those that are in the "goldilocks" zone (i.e. just the right distance from their star).

Al X. · Sep 12, 2017

neandermagnon said:
Stupid question from a biology nerd: wouldn't everything be drawn to the centre inside the hole in the middle of the donut because isn't that where the centre of gravity would be*? I don't get how anyone on the inside of the donut would be stuck to the donut. On the outside, sure. Drawn towards the centre of gravity. But what really would happen to the people on the inside of the donut? (this is a genuine question that I want to know the answer to, not a rhetorical question)

*further thought - is this why planetary scale donut shapes wouldn't stay donuts as all the matter would be drawn to the centre of gravity and clump together to form a sphere?

Regarding skin colour - if there's no sunlight at all then there's no evolutionary advantage to having light skin, because the evolutionary advantage is the ability of the skin to make vitamin D at lower light levels. If there's no light at all, then there's no way to make vitamin D at all so skin colour wouldn't make any difference. Though if intelligent life evolved on another planet they wouldn't be enough like humans for the question of how much vitamin D human skin can make to be an issue. An equivalent of fur/feathers could protect from sunburn, maybe.

Additionally, if parts of the planet are in permanent light and permanent dark, they'd be way too hot and cold (respectively) for life to evolve at all. You need the regular changing from night to day to keep the temperature stable enough for life to be able to evolve. However, it has been theorised that planets that have a hemisphere in permanent darkness and a hemsiphere in permanent light will have "goldilocks" zones around the borders between the dark and light that could harbour life, and these planets AFAIK are considered as possible planets that may have life, along with those that are in the "goldilocks" zone (i.e. just the right distance from their star).

Regarding your first question, gravitational attraction does not act exactly like a 'center of gravity' (center of mass, rather) but would more closely resemble magnetism. It is true that the composite center of mass will in fact be in the center of the toroid hole, but objects will be attracted to the closest part of the toroid where they are located. In the exact center of the hole, gravitational attraction will be effectively cancelled out. This would not be a stable condition, and any deviation of the object from the exact center would send it to some part of the toroid.

That said, I suppose if you were to take an object and locate it on a line perpendicular to the plane of the toroid (think two halves of a bagel with plate glass sandwiched in between) then yes an object would be pulled to the center of the hole. Although more likely, it will veer off slightly and attach itself to some part of the toroid.

But yes, if it has enough mass that gravity is actually an issue, it will most likely collapse in to a ball.

blacbird · Sep 12, 2017

Al X. said:
But yes, if it has enough mass that gravity is actually an issue, it will most likely collapse in to a ball.

And that doesn't need to be a huge amount of mass. Many of the smaller planetary moons in our solar system are essentially spherical. And that minimum of mass will not produce a gravitational attraction nearly strong enough to hold a gaseous atmosphere (e.g., our own moon).

caw

World like a doughnut... and more

Cobalt Jade

blacbird

AW Admin

Dennis E. Taylor

Get it off! It burns!

WeaselFire

Benefactor Member

DrDoc

Ex everything; trying something new

blacbird

Cath

The mean one

Al X.

GeorgeK

ever seeking

Layla Nahar

Seashell Seller

Mark HJ

Cat whisperer

Al X.

PeteMC

@PeteMC666

WeaselFire

Benefactor Member

Dennis E. Taylor

Get it off! It burns!

writbeyondmeasure

neandermagnon

Nolite timere, consilium callidum habeo!

Al X.

blacbird