In "It's My Birthday, Too" Harry uses gravity magic to kill two Blampires. The description in the text is as follows:
In technical terms, I didn’t actually increase the gravity of the earth beneath it. I only concentrated it a little. In a circle fifty yards across, for just a fraction of a second, gravity vanished. The cars all surged up against their shock absorbers and settled again. The thin coat of snow leapt several inches off the parking lot and fell back. In that same fraction of a second, all of that gravity from all of that area concentrated itself into a circle, maybe eighteen inches across, directly at the vampire’s feet. There was no explosion, no flash of light—and no scream. The vampire just went down, slammed to the earth as suddenly and violently as if I’d dropped an anvil on him.
There are a number of points I'd like to make:
1) Note that the fact that the text is talking about circles in two dimensions seems to imply that Harry didn't "turn off" gravity in general, but "merely" concentrated the gravity coming from the center of the Earth. Otherwise, the effect should have more spherical -- all the concentrated gravity would have turned the center of the blampire into a nice, dense ball of matter. Similarly in the second example, there would have been no reason for the second floor to fall
down (rather than up/in/ whatever).
----> We have established that Harry's spell is pretty likely earth-centric.
2) The description of what would happen if Earth's gravity vanished for a split-second is pretty much spot on. Consider that we're on a rotating planet, and the thing that keeps us "anchored" is gravity. If gravity stopped for a given object, it would continue in a straight line (modulo secondary gravitational effects like the sun) rather than curve around the earth. Since the Earth is rotating at about 1/3 of a mile an hour (at the equator), if you let something go in a straight line instead of curving around the earth, it would
appear to float upward (probably wobbly -- a whole bunch of secondary effects come into play) for nearly a minute (as the earth dropped away), and then move "west" in increasing speed. Since Harry describes the process of the spell to take a "fraction of a second", the description appears to be spot on.
3) Power -- So how much gravity did the poor blampire experience? Well, it's kinda hard to say since you can't really concentrate gravity in that way -- certainly not in a 2D form. Still, let's assume that it works in the same way a magnifying glass works when you use it to light a fire.
In a magnifying glass, all the energy of the light rays passing through the glass get concentrated at one point. The effective temperature at that point is essentially determined by how much energy is being concentrated. The better the lens (i.e. better parabola), the smaller the area in which the energy is concentrated and hence the hotter it gets. Ditto for size -- make the lens ten times bigger and you get ten times the energy (think Tavi at the bridge).
Fortunately, Harry tells us the measurements: he concentrates the gravity from a circle 50 yards across to a circle 18 inches across -- a ratio of 100:1. Thing is, the number that counts is the
area of those circles, something that goes as the ratio squared. Thus, we get an energy ratio of 10000:1.
Now the "energy" of gravity is pretty much linear in scale (GM/R is the Newtonian function that comes to mind), so this gives us the whopping number of 10000G, or localized gravity 10,000 times what we normally feel on planet earth.
4) Effect:
So what does this feel like? A LOT. Just for comparison, did you know that gravity on the surface of the sun is only 28G (which makes the whole ending of the Green Lantern movie
really stupid)? 10,000G is what you get on the surface of a neutron star.
From wikipedia:
The neutron star's compactness gives it a surface gravity of up to 7×1012 m/s² with typical values of a few ×1012 m/s² (that is more than 1011 or makes the gravity roughly 10,240 times that of Earth). One measure of such immense gravity is the fact that neutron stars have an escape velocity of around 100,000 km/s, about a third of the speed of light.
No wonder the vampire got flattened. With that kind of gravitational force, it's interesting that we didn't see all kinds of weird-science side effects. Light that happened to pass through those 18 inches during the spell would have been distorted up the wazoo with Doppler effects that would have done cool things to it. Air particles would have been smashed down causing a temporary vacuum that would have sucked outside air in, causing an implosion effect. Under that kind of pressure, the air inside would actually solidify on the bottom of the cylinder of concentrated gravity. Heck, the speed in which those air molecules would collide would resemble the inside of a Super-collider. I'm surprised Harry didn't discover the Higgs Boson on the spot.
5) Scaling up:
Finally, consider what would happen if Harry had made the "zero gravity" circle just 10 times larger. Going by the radius squared law, we'd get a 100-fold increase in gravity at the center which would easily take us into the black-hole range.
(Note that the surface gravity of a black hole is very hard to measure because of a lack of a "surface". You can use the Schwarzschild radius, but then you get the weird effect where the larger the black-hole is, the
weaker amount of surface gravity it has. Nevertheless 1,000,000G is very comfortably a black hole).
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Not much more to say here, except that wizards are dang powerful.
