For this week’s post, I’ve decided to talk about the thing that keeps us all grounded, makes us fall, and keeps us from venturing too high. It’s a very weighty subject, something I hope will draw you in, and it’s apparently a great source of puns. It is gravity.
Writers have gone to great lengths to circumvent this fundamental law of nature. When gravity can be eliminated or overcome, new and astounding opportunities arise. Our characters can strap themselves into rockets, dirigibles, and aircrafts to view our world from amazing heights, or visit entirely different worlds.
Unfortunately, many writers think their grasp of gravity is sufficient enough to excuse them from any research. In most cases this is true, but when glaring mistakes prevent readers from being immersed in the story, a little research would have been invaluable.
Gravity is many magnitudes weaker than electromagnetism at the microscopic level, allowing even a weak fridge magnet to resist the pull of our entire planet. What gives? Why isn’t it pulling its own weight? That was the last pun, I swear. The truth is, while electromagnetism predominates over the small, on the planetary scale, gravity always prevails. So for such a strong force, why do we have no idea how it works?
Unveiling the mysteries of gravity is the sole priority of many research labs around the world, and many theories have been proposed, but only a few have been found scientifically and mathematically sound (though they make many assumptions). The problem with all these theories is that they offer few testable hypotheses, and are based principally on math. Some, like Einstein’s theory of General Relativity and Loop Quantum Gravity (LQG), believe that gravity isn’t a force at all, but instead a warping or changing of the geometry of space-time (4 dimensions). While other theories, like Quantum Field Theory (QFT) and M-theory (string theory), believe the force of gravity is mediated by particles, the graviton, which propagate out like any other particle, not just in our 4 dimensions, but into other dimensions. There are pros and cons to each theory, and some go to extraordinary lengths to justify the strength of gravity relative to electromagnetism or the method by which it propagates. Some theories don’t try to explain everything at all (create a unifying theory) because it simply won’t work. Recently LIGO detected the previously hypothetical gravitational wave from two colliding black holes, giving researchers some clues as to how gravity propagates through space. This casts some doubts on those theories that suggest gravity has everything to do with space-time geometry, unless you somehow justify that ‘ripples in space’ are somehow to blame.
If you are creating a story line where gravity manipulation is a major plot point, some of these theories might make a good starting point for ‘anti-gravity’ technologies. Since no theory has been proven, there is a lot of room for creative license. I myself have absolutely no formal training in physics. In fact, I probably got away with taking fewer physics courses than the average science graduate. I also am pretty terrible at math. I have been looking into theories of gravity for several months now, and most of them gave me headaches. I applaud anyone who can make sense of them. I, with my limited understanding of physics, think gravity might be a combination of several theories. I theorize that perhaps gravity is a distortion of space-time, but that this distortion is the result of a particle, the graviton. If the Higgs boson (spin 0) is unable to move freely through space (or the Higgs field, whatever that is), imparting mass to objects, and the photon (spin 1) is able to move across space freely, what if the graviton (hypothetical spin 2), had the same speed as a photon, but also affected space, not by resistance, but repulsion. If the graviton were responsible for distorting space, this could explain its weakness (displacing space at the plank scale), its ability to affect time, and its ability to propagate like other particles. I have no idea if this theory has already been suggested or has already been debunked, but it’s what I am going with until someone decides to educate me. Seriously, if you are a physicist, we should chat.
Whether or not you came up with your own theory, gravity is still one of the most studied and characterized ‘forces.’ It follows certain rules. These rules are so defined, that we can take Newton’s equations from the 16th century, and use them to launch a tiny ship about 240,000 miles to land exactly where we want on the moon with zero to minimal course adjustment. So here are some considerations when writing your novel:
It is important to note that just because your characters are in space does not mean they will be weightless. If you take a balloon to the very edge of space, you will still feel the gravity pulling you down. It is only when you achieve angular momentum, momentum away from gravity, that weightlessness occurs. This is an orbit.
While orbits are relative, we tend to say one thing is orbiting another thing, when that second thing is the more massive of the two. Orbits are pretty simple to understand, and once understood, writers can avoid making some simple mistakes. A stable orbit is when a body has an angular momentum (outward force) that is equal to the inward force supplied by gravity. Because the force of gravity decreases according to the inverse square law, the further something is from the center of mass, the slower it has to travel to remain in orbit. For example, the International Space Station has to travel at nearly 17,500 miles an hour to remain in a low earth orbit of about 200 miles (orbits every 90 min), whereas a satellite in geosynchronous orbit, or about 22,000 miles from earth, only has to travel at about 7,000 miles an hour (orbits once 24 hours). At about 5 trillion miles from the sun, the objects in the Oort cloud barely need to move at all and only need to be nudged in order for them to come careening toward us as a comet. If you need a planet to revolve around another, but close enough to fill a quarter of the sky, they need to be revolving pretty fast around each other in order to counterbalance the pull of gravity, which will also cause some extreme tidal forces (to the core and oceans).
Like orbits, directions are relative. North on our planet is simply the magnetic north, where the magnetic field lines converge back on our planet (the place in the north pole where the compass starts to act a little erratic). On other planets, this may not be the case. Venus for example, has an extremely weak magnetic field, perhaps due to its very slow spin, or the loss of convective forces due to a thick crust.
Gravity is also the only thing that differentiates up from down. The saccule and utricle of the inner ear contain grains of calcium carbonate that respond to gravity and momentum, tugging on hair cells (mechanoreceptors). This as well as visual stimuli, help you orient yourself to gravity and keep you from falling over.
In space, without gravity, this sensation is lost and many astronauts have to deal with a bit a vertigo and nausea as their eyes tell them something their ears are not. In space, people need to orient themselves to something besides gravity, like a feature of the galaxy (quadrant), the orientation of equipment or text on the spaceship, or they can learn to ignore directions all together. In my current work in progress, it is a struggle to describe motion and actions when lacking a directional cue. Are you really reaching up to flip a switch if you are upside down relative to everyone else? It is important to orient your reader to the character’s POV and sense of direction in order to prevent confusion.
Other effects of gravity.
As mentioned earlier, a world orbiting closely to a gas giant will likely have tremendous geological activity and a molten interior from tidal forces. A small world about the size of our moon that is not orbiting a gas giant will likely have very little atmosphere and no molten interior, since gravity is responsible for both.
Buoyancy is another ‘force’ that exists because of gravity. As Archimedes’ principle states, whether it is water or air, if something weighs less than the stuff around it, it will rise above it until it finds an equilibrium. It will sink when the object weighs more than the medium. This upward force is caused by the pressure differential in the medium, i.e. the pressure of the medium against the bottom of the object will be slightly greater than the pressure of the medium against the top of the object, causing it to rise. Neutral buoyancy occurs when the medium it displaces weighs the same as the object itself, and when the pressure difference between the top and bottom equals zero. The larger the volume of the object, the more medium it will displace, but it will also tend to weigh more. That is why ‘density,’ the weight of a certain volume of an object, is commonly used to estimate buoyancy. In orbit, the effect of gravity is canceled out and everything will be weightless. Thus there is no buoyancy in space. You can inject a drop of air into a sphere of water, and it will stay in place and not rise to the surface.
There are also many health effects associated with a long exposure to weightlessness, including muscle and bone loss as well as some neurological and visual problems. Rather than go into all of this, I will encourage you to read this post from Amber, a fellow sci-fi blogger and science nerd.
Difference between mass and weight.
This is a relatively minor point, but that should make it easy to remember. Mass is measured in grams, and weight is measured in newtons (gravitational force multiplied by mass). Most of the time weight and mass can be used interchangeably, unless there is space travel involved. Your character’s mass will be the same on earth as it is on mars, however, their weight will have changed. Weight is the measure of an objects gravitational attraction to another object, whereas mass is a physical property of the matter the object is made of. Mass is caused by the Higgs boson, but weight is caused by gravity (perhaps the graviton). While the incorrect use of this terminology will probably not dissuade many of your readers, it might cast doubt on your knowledge of the subject. Better safe than sorry.
This encompasses any technique that is used to mimic the effect of gravity, and it is used in nearly every hard sci-fi story where astronauts are able to walk instead of float around on their space-ships. Creating an artificial gravity will be required for prolonged periods of weightlessness to prevent many of the adverse health effects. It it important to note that artificial gravity is not gravity at all.
Centrifugal ‘force’ is a method used to generate artificial gravity, whereby a torus, or another type of structure, is rotating around a central point. This has the effect of making all objects within the structure want to fly outward, but the structure itself is preventing that (centripetal force), thus allowing all the objects to be forced to the inside of the structure, with up being the center of rotation, and down being out into empty space. There are some pretty simple equations that will allow you to estimate the amount of rotations/min needed for a torus of a certain radius, to generate a certain amount of force (equivalent Gs).
Other continuous forms of acceleration can also apply a constant force, however, rockets will run out of thrust eventually, and when the people inside the rocket catch up to the rockets velocity, they will become weightless again.
It is up to the writer whether or not they want to address how gravity is simulated in their ship. I personally prefer there to be some mention of it to avoid logical inconsistencies. For example, if there is no torus or rocket used to apply this continuous force, then I will assume gravity has been mastered and replicated. If that is true, why would you need propulsion at all? What happens when the ship loses power (assuming generating gravity consumes power)? Where is the device that creates it? Where are all the floating cities, flying people, and gravity weapons? Conquering of gravity would of course result in all these and many other amazing things.
It does not exist (yet). Sadly, most contraptions that claim to be working by anti-gravity are in fact operating by buoyancy or propulsion. In order from something to be anti-gravity, it must ignore gravity, or perhaps reverse it, not just compensate for it. This probably won’t happen until we find out what gravity really is. Is it the curvature of space time by Mass? A change in the geometry of space-time?
But when we finally do unravel the mysteries of gravity, we may be able to redirect it, amplify it, or turn it off altogether. Exciting times are ahead, but as sci-fi writers, we don’t have to wait, we can bring that excitement to the here and now.
5 thoughts on “The science of gravity”
Ahh all sorts of cool fodder for stories. Playing with orbits and solar systems, gravity weapons and floating cities, oh dear why can’t I write novels faster!
I generally go on the “Star Trek” assumption… if gravity isn’t mentioned, I assume they have some unnamed artificial gravity device that works like the Enterprise. I always liked in B5 how the races had varying levels of technology, and all the human ships still needed rotational gravity while their allies did not. I went with a similar thing for my WIP, because, well, I stole half of everything from B5 anyway 😛 But I did have a great deal of fun with a new scene I added where a saboteur disrupts a ship’s engine/computer controls and the POVC is trying to survive a firefight and get to escape pods as gravity gradually dissipates… terribly inaccurate I’m sure, but we’ll see what makes the final cut!
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