I often hear that gravity is not a force, but simply the curvature of spacetime. A common example given is having a heavy ball placed on a rubber sheet. The ball sinks, distorting the sheet. Therefore, other “orbiting” objects will also tend to roll towards the sunken part.
What I can’t understand is, what causes the objects to “fall” anyway? On earth, that’s just the planet’s gravity. If you did the same experiment on space the objects wouldn’t roll down. So how is this an explanation of gravity as a curvature, when it requires a *force* to work? Is there a better explanation? Am I just missing something?
In: Physics
Everything in the universe pulls everything else in the universe towards it, the magnitude of which is determined by the mass of the objects and their distance from each other.
The rubber sheet analogy is somewhat useful but not super accurate because, as you said, it only makes sense in the presence of gravity.
Think of it this way. All things in the universe travel in a straight line, unless acted upon by an outside force. This makes sense but then how do you explain the orbits of planets, right? They certainly seem to be going around in circles and not in a straight line.
The explanation is that they ARE going in a straight line, but spacetime itself is curved. The objects in the gravity well are following a geodesic. So, the straight line ends up curving when the spacetime it’s moving through is curved by gravity.
Gravity is just a force in classical physics. If you start to venture down General Theory’s perspective of gravity, the 5-year-old could get lost. And you might be ready for a book on general relativity.
The ball rolling in the sheet is 4-dimensional model of physics where we only see the universe in 3-D but the sheet is actually a 4-D surface and each mass’s gravity distorts space away from a 4-D plane. However 4-D space / time is just a model that provides a solution to general relativity.
The simplest explanation of gravity is that it IS a force of attraction between two objects with mass. This explanation is pretty much good enough for anything other than things moving at very high speeds, very long distances, very small particles, and for massive objects like stars.
Any other explanation is not so easy without resorting to reasonably advanced mathematical models.
Everything that has mass wants to get closer to each other. Smaller objects move to larger objects faster. The earth is way bigger than you, so when you jump, you fall back to the earth fast.
The gravity of earth extends beyond just the objects on earth. It keeps the moon in orbit, for example. These together also act on the sun and other planets, which in turn act on other solar systems and galaxies etc. Space itself doesn’t have gravity, just objects with mass. So any mass has a gravitational force, and they all influence one another.
Instead of imagining a trampoline with one object in the center, you might imagine space as a really large stretchy sheet with many objects. Any place there is a planet it will make a dimple in the fabric. As the objects move around their dimples cause the other objects to be affected.
Let’s say you have an object like a bowling ball, it will have a large dimple. Now imagine rolling a marble towards it. (The marble creates its own small dimple). If the marble isn’t going very fast, it will fall into the bowling ball dimple and stay there. But if you roll it fast enough along the edge of the bowling ball it will change direction and come out the other side and keep going (slingshot).
As to why objects distort this space time? It’s not entirely clear. We don’t know yet. Like you said, the trampoline model works because of earths gravity. It’s just an illustration that makes it easy for us to visualize. You are intended to ignore the gravity of earth on the trampoline. The object itself is what distorts the fabric.
Mass slows time.
The closer you are to a mass, and the larger the mass, the more it slows down time.
This means there is a time gradient. Your head is being slowed less by the mass (the Earth) than your feet.
Every atom, every particle in every atom also experiences this gradient.
Time moving faster on the far side of a particle than the near side causes that particle to be deflected towards the Earth.
Literally, its future is on the ground.
> On earth, that’s just the planet’s gravity. If you did the same experiment on space the objects wouldn’t roll down.
Except they would. Things in space fall. The Moon is falling towards the Earth (the Earth’s gravity is pulling it down), satellites and space stations are falling towards the Earth. The Sun is falling towards the Earth (as is the Earth towards the Sun).
The pull of gravity extends to infinity (with some qualifiers). Stars pull each other towards themselves to form spiralling galaxies, galaxies are pulled together to form clusters, gravity is everywhere. Anything with mass (and so energy, as the two are equivalent) pulls anything else with mass (or energy) towards it.
Gravity is just very weak. So it takes really huge things (like planets) for it to be noticeable, and then accelerations it creates can be fairly small (think how easy it is to create 1g of acceleration or “ge-force” by other means).
> So how is this an explanation of gravity as a curvature, when it requires a force to work?
Gravity can be modelled as curvature of spacetime. It isn’t the only way; the force model mostly works as well. And no one has quite found a solid way to combine the curvature model with quantum mechanics, so there may be some stuff missing.
The thing you may be missing with the rubber sheet analogy (and remember that analogies, like cars, tend to break down eventually) is that the “sinking” in the sheet extends forever (in this model). Spacetime isn’t a small sheet with defined boundaries that are held in place. So placing a small weight on one part of the sheet – distorting it there – will still create a distortion all the way to infinity. Even if that distortion will get infinitely small, and will quickly be drowned out by the distortions caused by other objects.
The weakness of the analogy is exactly what you’ve spotted. The rubber sheet thing only works *because of gravity* pulling the ball down (and so the sheet with it), and causing other objects to “stick” to the sheet (because they want to fall through). With actual gravity there is no “force” pulling things down, and spacetime isn’t pulled in some extra dimension. Instead spacetime is squished together.
If it helps, instead of thinking of spacetime as being distorted or curved through some extra direction, think of it as being bunched up; imagine two points in space that are 10m apart from the outside. There should be 10m of space between them. But if you try to walk from one point to the other (in a ‘straight’ line) you end up walking 16m. Because there is more space per space. Space is bunched up so that on the inside the points are 16m apart, even if from the outside they should be 10m apart. In this case, if the bunching is local, you might find that going around the distortion is a shorter distance than going through (so if you went around in a circle, you would walk 15.7m assuming no distortions happened there).
And it turns out this happens. Objects by default travel in “straight lines”; except a “straight line” in this context means the shortest distance between two points. So moving objects will curve around distortions in spacetime (parts where spacetime is bunched up).
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