My coworker told me gravity isn’t a force it’s an effect mass has on space time, like falling into a hole or something. We’re not physicists, I don’t understand.
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Imagine you have a deep, round hole in the ground, and you want to put a ball at the center, without gluing it in place. If you roll the ball down the side of the hole, from really far away, it’ll start to fall into the hole, getting closer and closer to the center as time goes on. Mathematicians say it’s “rolling along a geodesic”; basically, it’s moving in the straightest possible line it can, given that it’s confined to the surface of this hole. What just happened when the ball fell in? If the ball is a laser beam, instead, it’ll follow the same path it did before! If you were on the surface of the ball, and you didn’t see it fall, you wouldn’t even be able to notice that it did, except that it gets a little cooler when the digging guy sticks it in the ground.
There’s really no sensible way to say that gravity “is” a force in this analogy. If anything, in this scenario, the “force” of gravity comes from the curvature of the hole, not from the ball at the bottom!
Heres an answer that wont help: the four fundemental forces of nature are strong interaction (binds matter together), weak interaction (particle decay), electro magnetism (magnets woo) and gravity (apples). There are particles (bosons) that are associated with each on of these forces. Well there are for all but gravity. The Graviton is theoretical, but not discovered or proven to exist.
In physics we build models to help us understand the world around us, explain things, and predict things.
But they are models.
When we use models we tend to simplify things, and how much we simplify them depends on what we need the model for. Imagine constructing a model of a house. If we’re trying to plan out a neighbourhood we could model it by drawing a rectangle on a page. Probably good enough. If we’re trying to figure out how it will fit in a plot of land we might need to draw an accurate scale diagram of the house’s footprint. If we’re trying to figure out how to decorate and furnish all the insides, we might need fairly detailed floor-plans and possibly a scaled 3D model. Maybe even showing where things like power sockets are. If we’re trying to wire up the house maybe we only need a rough plan of the house (not to scale) but with lines marking where all the wires are.
Each model is perfectly valid. In the right circumstances. Eventually they will all break down, and they will break down at different points. But that doesn’t mean they aren’t useful, or “correct.”
Gravity is a word we use to describe a series of effects we observe. There are various ways to model gravity.
The traditional (1600s-1900s) way of modelling gravity is by a force; a pull by things with mass on other things. This is Newtonian Gravity. Things accelerate downwards because their is a force pulling them downwards.
The more modern way (1920s onwards) way of modelling gravity is by a curvature to spacetime caused by the presence and distribution of energy. This is General Relativity. Things accelerate downwards because their local “forward through time” direction is twisted a bit into the global “downwards” direction.
The “force” or “curvature of spacetime” models are different ways of looking at or understanding gravity. They each have situations where they are useful.
The Newtonian/force model breaks down if we poke it too hard (really accurate measurements, dealing with light, dealing with large scales or very steep gravitational gradients). The GR model also breaks down in some situations (at r = 0, and in some corners when trying to get it to work with quantum mechanics). But they are good enough for what we need them for.
Gravity is like an invisible force that holds us down to earth. It is as if the earth is pulling us towards it. But it is also true that some physicists say that gravity is more like an effect. They say that things with mass bend and stretch the spacetime around them, and that causes things to move towards each other. It may seem a little confusing, but it’s just one way of explaining how gravity works. So, in short, gravity is like a force that keeps us on the ground, but some say it’s more like an effect on spacetime
This is a FANTASTIC video about gravity. It is a professor teaching people with real visual examples. It is very easy to follow and even if you do not understand the talk, you will be able to appreciate the demonstrations.
It’s very much a force in that a mass can distort space-time around it. And another mass passing through that space is affected by that distortion. So the force isn’t exerted between the masses directly (although that can be a handy shortcut way to look at it), but indirectly through space-time.
I’d suggest helping your friend expand their definition of a “force.” Try this definition: A force is any external effect that influences an object’s intertial (“rest”) state.
For gravity, it provides an external acceleration towards the center of the Earth. It also provides an acceleration towards the sun… and the moon.. and.. etc.
A ball rolling downhill is accelerating and being pulled along by a force. Even if gravity is caused by the curvature of spacetime, it still acts as a force on the surrounding universe.
Your coworker is correct.
Gravity is really the result of the curvature of spacetime, which is a fancy way of saying the dimentions of the universe. We’re aware of 3 of them, you can go forward and backwards, up and down, and left and right. Any movement in our universe can be described by a combination of those three dimentions.
General Relativity (the [theory](https://en.wikipedia.org/wiki/Scientific_theory) that describes all of this through some complicated equations) states that there is a fourth dimension, Time, that we’re constantly moving through. This makes sense because really, if you want to describe ***where*** an object is, ***when*** it was where you’re describing is important, especially if your system is dynamic (moving and changing).
Massive objects (“massive” meaning they have a lot of mass, but dont necissarily have to be big) curve spacetime around it (yes this means that time acts differently too). Things in our universe like to follow straightline paths. You can demonstrate this by rolling a ball, assuming the surface is flat it will move in a straight line in the direction you pushed it.
Something falling does the same thing. If you’ve ever fell a considerable distance, like bungey jumping or skydiving, you notice that you don’t ***feel*** the force of gravity while you’re falling. Its only when you hit the ground that you feel anything. Objects are following straight line paths through spacetime, but since its curved, those paths eventally collide with the earth, and the earth getting in the way, is what you feel as the force of gravity.
Hopefully im correct in my summary of General Relativity but if I missed anything please point that out.
I love how this is a ELI5 post, yet while I thought I had a decent basic understanding of gravity before from an adult layperson perspective, not even a 5 year olds, reading this post of physicist particles, gravitons, spacetime, and phlogistons has my mind swirling into nothing but confusion.
Imagine 2 ants on a sphere, parallel on the globe climbing up to the north pole. eventually they would reach the same spot, due to nothing more than the geometry of the globe. If the ants didnt know they were on a sphere they might think that there is just a mysterious force acting on them that forces them together as they climb up. But that is wrong. The analogy is the same for spacetime. We are on a funny 4 dimensional curved surface and due to the curvature in certain places, we get closer to massive objects over time.
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