tidal gravity

anonymous asked:

Would dwarves be naturally more resistant to long exposure to low-grav, due to higher bone density? What's the gravity like in their habs?

(With reference to this post here.)

I’m going to tackle this one as a separate post because it’s long and technical and the sort of thing that would be first on the cutting room floor if I was writing an actual sourcebook.

Anyway, the issue of bone density loss tends not to come up because most dwarven orbital habs have residential and recreational sections that can be spun up to provide artificial gravity. Oddly enough, this is another area where dwarves’ biology gives them an advantage.

In a nutshell, there are two major drawbacks associated with using rotating habitats to provide artificial gravity: tidal gradients, and Coriolis forces.

A tidal gradient is basically when gravity acts with different force on different parts of the same object. The most familiar example is (of course) Earth’s tides, where the position of the Moon causes its gravity to exert uneven force on the Earth’s oceans, raising and lowering the water level accordingly.

You see the same effect in rotating habitats: specifically, the artificial gravity is stronger the closer you are to the habitat’s rim, and weaker the closer you are to the habitat’s hub. If the habitat’s diameter is very large, this gradient will be difficult to notice on a human scale - but if it’s small, it may be discernible.

In fact, if the diameter of the rotating habitat is small enough, you can end up with a situation where the force of artificial gravity on your feet is significantly stronger than the force of artificial gravity on your head (provided that you’re standing up). This can cause blood to pool in your legs, inducing circulatory distress, oxygen and nutrient deprivation to the brain, and other nasty effects.

Coriolis forces, meanwhile, are virtual forces that act on moving objects in rotating reference frames. That’s really technical - the plain English version is that if the thing you’re standing on is spinning, you’re constantly experiencing a slight acceleration in order to keep you in sync with it, and that acceleration can do funky things to fluids and trajectories.

The most familiar example is, again, meteorological: hurricanes and other pressure systems consistently rotate in different directions depending on which hemisphere you’re in - counter-clockwise in the Northern hemisphere and clockwise in the Southern hemisphere - because the deviation induced by Coriolis forces is enough for them to favour one direction over the other.

If you’re into scientific trivia, you’re probably wondering why I used hurricanes instead of toilet bowls as my “familiar example” - after all, we’ve all seen far more of the latter than we have of the former. That’s actually a common misconception: the direction that water rotates in a toilet bowl is determined by the geometry of the bowl, not Coriolis forces. Counterintuitively, even though the Earth is spinning at breakneck speed, its rotational velocity relative to its diameter is small. Since the strength of the Coriolis forces associated with a given system is in proportion with that system’s rotational velocity, the Earth’s Coriolis forces are quite weak - far too weak to mess with localised systems like the water in a toilet bowl.

Some of you may have guessed where I’m going with this: the smaller your habitat, the faster it needs to rotate relative to its own diameter in order to produce useful artificial gravity. In practice, this means that for a given level of artificial gravity, the smaller the diameter of the habitat, the stronger the Coriolis forces upon everything inside it will be. If the habitat is small enough, those forces can be strong enough to screw with the fluids in your inner ear, which are responsible for your sense of balance. This essentially induces a permanent case of motion sickness - not a fun time for anyone!

The solution to both of these problems is the same: go big. For humans, a habitat that’s been spun up for artificial gravity needs a diameter on the order of hundreds of meters in order to be comfortably habitable; needless to say, this poses non-trivial engineering challenges.

For dwarves, it’s a different picture. Their short stature and robust circulatory systems help to moderate the effects of tidal gradients, both by making them better able to pump blood against gravity, and simply by reducing the distance between their feet and their heads. Meanwhile, the dwarven inner ear is relatively insensitive, perhaps because falling over is less dangerous for them; this insensitivity is usually a disadvantage, but in this specific situation it’s advantageous, because it renders them largely immune to motion sickness, including Coriolis-force-induced vertigo.

The upshot is that orbital habitats designed for dwarves can get away with much smaller diameters for their rotating sections, making them both simpler and cheaper to build.


7 Ways Earth Would Change If Our Moon Were Destroyed

6.) Our axial tilt would be unstable. This is an unfortunate one. Earth spins on its axis, tilted at 23.4° with respect to our orbital plane around the Sun. (This is known as our obliquity.) You might not think the Moon has much to do with that, but over tens of thousands of years, that tilt changes: from as little as 22.1° to as much as 24.5°. The Moon is a stabilizing force, as worlds without big moons – like Mars – see their axial tilt change by ten times as much over time. On Earth, without a Moon, its estimated that our tilt would possibly even exceed 45° at times, making us a world that spun on our sides. Poles wouldn’t always be cold; the equator might not always be warm. Without our Moon to stabilize us, ice ages would preferentially hit different parts of our world every few thousand years.”

Our Moon is pretty unusual as far as the Solar System goes: of all the planets, our Moon has the largest mass and radius ratios when compared to its parent planet. It’s enough to not only illuminate our night sky quite fiercely – the full Moon is 14,000 times brighter than the next brightest object in the night sky – but it has some significant gravitational effects on our world. You might think this is restricted to the tides, but that’s not the full story. The day lengthens, our axial tilt is stabilized, and even without a Moon, we’d still have tides after all! Yet destroying the Moon, even in our imaginations, gives us a way to help envision all the things it actually brings to Earth, for better and for worse.

Come learn all seven ways our Earth would change if our Moon were destroyed. Did you know all seven?

Black Holes: A Summary

I got asked this lovely question yesterday afternoon and instead of just answering it, I wanted to write a comprehensive post about black holes and their many intricacies.  So, here we go: let’s talk about black holes!


We’re going to work with General Relativity (mostly) because it simplifies these concepts down into something a lot more understandable.  General Relativity is the perception of gravity as not an inherent force, but instead caused by the curvature of spacetime, a two-dimensional interpretation of the four dimensions of Minkowski spacetime (space in x, y and z directions and time).  The extent of the curvature of spacetime is directly related to the mass of the object.  Quantum theory will come up briefly, but not in the creation of black holes nor in the analysis of their properties.  

We’re also going to assume that the black holes discussed are gravitational, static and eternal.  This means that the black holes have gravity generated by their mass, do not spin and do not deteriorate over time.  I will discuss black hole deterioration in a separate section, but that concept won’t be relevant in the earlier sections.  

Keep reading


Surface cracks on Europa, one of Jupiter’s many moons.

Europa is full of water and water ice - the cracks are thought to be a result of tidal forces. Jupiter’s gravity is incredibly strong, as it swings around Jupiter, the gravity pulls the surface ice apart and it refreezes, creating these brilliant surface scars. The tidal forces also keep the interior nice and toasty, creating liquid water under the surface. Let’s hope there’s some sort of space shark down there!


Rather neat - tide rapidly coming in and flooding a tidal flat, Kenya

In Honor of Black History Month,

Death By Black Hole
A poem by Neil deGrasse Tyson 🌚☄✨💫

In a feet first dive
to this cosmic abyss
You will not survive
because you surely will not miss
The tidal forces of gravity will create quite a calamity when you’re stretched head to toe
Are you sure you wanna go?
Your body’s atoms - you’ll see them - will enter one by one
The singularity will eat ‘em, and of course, you won’t be having fun..

kira-darf  asked:

On the post about V404 Cyngi, you said that it emitted bright flashes of light from material it could not swallow. Can you explain how that works? How can a black hole not be able to swallow something?

  Excellent question!  I’m gonna take this opportunity to talk about the awesomeness of this binary. 

V404 Cygni is a variable microquasar, a low-mass X-ray binary that consists of a stellar-mass black hole and a companion star. The companion star is slightly smaller than our sun and it orbits a black hole 10 times its mass. The star’s orbital period is just 6.5 days, which indicates that it’s a close binary. The close orbit and the black hole’s powerful gravity produce tidal forces that pull a stream of gas (accretion stream) from its companion. The gas slowly forms a rotating disk around the black hole, known as an accretion disk.  Gravity and the interaction of particles in the disk will cause material to compress and spiral in towards the black hole, and release energy in different X-ray spectral states (low/hard state). However, there is a much larger X-ray outburst that can cause the binary to shine hundreds of times brighter than normal.

Side note: the entire disk emits light in the infrared, the UV light comes from the disk’s inner regions. Gas closer to the black hole is hotter and emits more energetic radiation, X-rays or even gamma rays.

Most of the time, the turbulent flow inside the spinning disk is steady, although, it’s clumpy enough that the binary system can appear to flicker a little, and emit short bursts of low-energy X-rays, which is one reason why it’s designated as a variable and a soft X-ray transient. The stability of the flow within the disk depends on the rate of matter flowing into it from the stellar companion to that falling into the black hole. But, there’s a glitch in V404 Cygni’s case; the disk fails to maintain that steady internal flow as the gas continues to build up around the black hole, like water behind a dam. The disk becomes progressively hotter as it reaches a critical density and when the temperature reaches the ionization level, the dam breaks, and V404 Cygni becomes an X-ray nova.

It is important to keep in mind that the accretion disk is a complex hydrodynamic place, it is subject to instabilities, in this case; the viscous/thermal disk instability model can broadly explain the sudden X-ray outbursts. This thermal-viscous instability triggers cooling and heating fronts that propagate throughout the disk, alternating between low viscosity state –  a cooler, less ionized state where gas simply collects in the outer regions of the disk, and a high viscosity state – a hotter, more ionized state that sends a tidal waves toward the black hole (illustrated below).

In the cool state the disk accumulates mass from the companion star, this can take several decades, and in the hot state the disk loses mass, at an increased rate, the heating waves propagate through the disk bringing it to a bright hot state at which the X-ray luminosity reaches its maximum. Hence the name X-ray nova. An X-ray nova is a short-lived X-ray flare that appears suddenly, and then fades out over a period of weeks or months. Now the interesting part. The powerful outbursts generate beams of plasma (hot gas) ejected at great speeds (relativistic jets) along the polar axis of the disk. So, if nothing can escape a black hole, not even light, then why do some black holes have these bipolar jets?  

The origin of these jets remains elusive, the exact process is not well understood, however, strong magnetic fields are suspected to play a role; spinning black holes that are devouring interstellar gas also expel some of it in twin collimated jets, because magnetic forces can be as strong as gravity near black holes. The black hole itself is not directly involved in the jet launching, the powerful jets of plasma emerge from the inner parts of accretion disk and travel along the open lines of the poloidal magnetic field, which extend to large distances above the disk surface.

Here’s a visual aid to give you an idea of what’s happening. This is a computer simulation of the formation of jets from a rotating accreting black hole. The accretion disk is the yellow, doughnut shaped object, the outer disk and the wind is in green/orange, the plasma beams are blue/red and the magnetic field lines are bright green.

Image credit: NASA/Goddard Space Flight Center, 3D simulation; McKinney and Blandford

Io in true color

The strangest moon in the Solar System is bright yellow. This picture, an attempt to show how Io would appear in the “true colors” perceptible to the average human eye, was taken in 1999 July by the Galileo spacecraft that orbited Jupiter from 1995 to 2003. Io’s colors derive from sulfur and molten silicate rock. The unusual surface of Io is kept very young by its system of active volcanoes. The intense tidal gravity of Jupiter stretches Io and damps wobbles caused by Jupiter’s other Galilean moons. The resulting friction greatly heats Io’s interior, causing molten rock to explode through the surface. Io’s volcanoes are so active that they are effectively turning the whole moon inside out. Some of Io’s volcanic lava is so hot it glows in the dark.

Image credit: Galileo Project, JPL, NASA