# ELi5: is there a gradual shift in Earth’s gravity field and how consistent is it?

19 views
0

From what I understand, air gets thinner the further up you go. This I assume is because Earth’s gravity pulls all the heaviest particles down to its centre. I’m wondering about gravity itself though, is there a gradual shift in the strength of gravity as there is in the consistency of the air, or does gravity apply to the whole field surrounding the Earth up to the very perimeter and then quickly fall off?

In: 9

Earth’s gravity reduces the further up you go until it becomes insignificant, but it never falls to zero. So technically Earth’s gravity (or the gravity of the matter that Earth has formed from) is felt everywhere in the observable universe.

Air gets thicker closer to the Earth’s surface mostly because it’s being pushed down and compressed by the air above it (air, unlike water, is very compressible).

Gravity falls at square of distance. (This is from center of mass not surface)

Double your distance is 1/4 gravity.

&#x200B;

The strength of gravity falls off with distance squared. This is the distance to the center of mass of the object though. So there’s about 6.3k km to center of the earth. If we were to double that distance by going 6000 km up, you would only feel 1/4 of the gravity, so about 2m/s^2 acceleration

The force of gravity follows the inverse square law. That is, the strength of gravity falls off with the square of the distance to the mass. Double your distance from a source of mass and the force isn’t halved, it’s divided by X^2. That means it falls off pretty quickly.

However, it never actually gets to zero. The force of gravity is infinite. No matter how far away from the Earth you go, you will still feel the pull of the Earth. Local sources of gravity will almost certainly overwhelm it, though. Like, if you’re right next to Jupiter, the force of gravity from the Earth isn’t zero, but you’re *very* far away and the force is pretty tiny, while the force from Jupiter is much *much* stronger, so you will fall towards Jupiter.

The Earth doesn’t have a smooth transition from “not space” to “space.” There are various definitions used for convenience, but there’s never a line where if you’re coming to Earth from space suddenly you *are* in the atmosphere but you weren’t before. The International Space Station, for example, has to periodically boost itself back up because it does experience just a teeny tiny bit of drag from the very very thin wisps of atmosphere at that altitude above the Earth. The atmosphere sticks to Earth because, mostly, the force of gravity is stronger than other forces that would push or pull it away. But some atmosphere is lost to solar winds, or just wanders off into deep space. But other particles come in from space and get captured by Earth’s gravity. So, overall, the atmosphere stays *more or less* the same.

Some other things to consider: The Earth is not a *point*, it’s a big ball with volume. You’re already pretty far away from the center of Earth’s gravity. But you’re also right next to *part* of the Earth – the ground underneath you. But you’re also quite far away from the opposite side of the Earth. The point is that although gravity falls off very quickly with distances, you’re already at a distance, not right next to *all* of the mass. [This graph](https://upload.wikimedia.org/wikipedia/commons/thumb/b/be/Grav_field_sphere.svg/524px-Grav_field_sphere.svg.png) shows how the force changes over the distance. Notice that at first the line goes up: if you’re at the center of a sphere of mass – at the center of the Earth – there’s no mass below you, only mass above you. And, there’s mass in all directions canceling it all out so that the *overall* force of gravity is zero. As you move away from the center of the Earth, there is more mass below you and more mass above you, so the force increases until you reach the surface. After that, all of the mass is below you (except for the atmosphere, which is negligible) and the force will decrease as you move away.

the strength of gravity depends on mass (direct variation) and distance (inverse square variation). This means if you double the mass, you double the strength, but if you double the distance of separation, the strength decreases by 4 (2×2; 2 squared) times.

Pressure is simply the weight of the overlying material that depends on its mass and distance from the center of gravity (really gets pulled in all directions but the pulls sum out to be as if the pull was at the center of the mass; less mass pulling up is weaker than more mass pulling down so the net (total) pull is down).

Gravity does vary because density varies. Density varies because different materials respond to pressure differently, some compress a lot (increase density a lot with pressure) and some do not at all. On top of all that, there is even a matter of the different masses of each element that comes into play and affects density (the same number of atoms per volume of a heavy element will have more mass, more weight, higher density than that of a lighter element).

Higher density has more mass so pulls harder than lower density. In the earth, the inside is made of (mostly) solids, but these solids have quite different densities depending on composition, pressure, temperature. So gravitational pull is slightly different depending on where you are and how those densities might differ from location to location. There are zones of “High” gravity and zones of “low” gravity if you measure total gravity to a very precise level.

The result is that sea level (the elevation above the center of the earth where gravity is constant and the same everywhere does change a bit, by a few meters to tens of meters, but the changes are gradual and cannot be seen by the naked eye (a few meters relative to the 6 million meter (6000 kilometer) distance to the center of the earth is not much at all), but it is there just the same.

Does gravity change? As already explained, yes it does over space (location), but it also does change over time, but only very slowly and only on the very tiny levels of some parts of a million over fairly large distances. These temporal (time-related) changes happen because the materials inside the earth do move around a bit and do cause density to change location. It is very slow.

Gravity is thought to be something that exists forever in space. It just becomes very very very weak with distance of separation. To cause a gravitational attraction a long distance away requires the existence of a huge mass.

It isn’t exactly a matter of heaviness so much as a matter of density (amount of mass per unit volume). Density is mostly a response to pressure (pressure makes the atoms tend to get closer together, so amount of mass per volume increases even when the individual atoms in the lower density and higher density zones are identical in composition and proportion). Temperature works in the opposite direction, making atoms try to move apart (keep bouncing into each other). Density is the concentration that results when the pushing together caused by pressure is balanced with the pushing apart caused by temperature.