I need help with an analogy here because I simply don’t know/can’t visualise the true physics here.
Many people use an analogy for gravity acting on space-time as a sheet of material stretched across a plane and a heavy object in its center acting as a celestial body.
This is great for envisioning orbits, the curvature of spacetime and so on.
Now this is a “2D” sheet/plane that deforms “downward” in the 3rd dimension, I get it… But how does it translate to the actual universe? The universe is always 3D in all directions, isn’t it?
I’m stuck here guys, science help me!
In: 81
**TL;DR** – Pinching, ants, and beachballs.
The go to analogy to explain gravity is the ball on a rubber sheet analogy which, while helpful at illustrating that the fabric of spacetime is “bendy”, is dogshit in pretty much every other way. It explains gravity using gravity and it doesn’t give people an intuitive sense about why time plays a role in gravity. All they see is a marble rolling down a slope like a marble in a bowl because of… gravity.
The reality is that spacetime **curves** which you can think of like “pinching” when being represented in 2D space. The Super Mcduper important part is to pay attention to what happens when we “pinch”.
Imagine a series of gridlines which run along the sheet and when you set something heavy on the sheet (or more aptly *into* the sheet) instead of the sheet “bending down” it instead begins pinching the gridlines together (which you don’t need “up” or “down” to do). You’ll notice that two lines – which were formerly straight and parallel to one another when the grid was “flat” – now bend towards each other once we introduce curvature. Another analogy would be to stretch a rubber band between your fingers so that the bands are parallel to one another. Pinch the center together and you’ll see that they converge so that the bands now intersect. It’s that effect of two straight parallel lines eventually intersecting that’s the defining feature of the type of curvature we see in spacetime. We call that **curvature** gravity.
If you’re struggling to make sense of this no worries. Think of it like this:
Imagine two ants at the equator of a beachball, parallel to one another but spaced several inches apart. At the same exact moment both ants begin marching towards the North Pole at the exact same time – always putting one foot in front of the other and so always moving “straight ahead” – never turning. As they walk up the ball we’d notice that they begin moving closer and closer together, until they eventually intersect with one another at the North Pole.
How is it that the ants started parallel to one another, both only moved straight ahead so never turned, and yet still managed to run into each other? Was it some magical force that pulled them together? No, it was just the curvature of the beach ball and the effect that curvature has on straight parallel lines. If the ants made the same journey on a flat piece of paper their paths would never intersect, because the straight lines would remain parallel with one another forever. But because of the curved geometry of the ball their straight line paths curve towards one another until eventually they do intersect. The ants move closer and closer together – as if seemingly pulled towards one another.
The big leap in this analogy is understanding that when we talk about curvature in spacetime we’re talking about curvature in space **and time**. So if you ask yourself the ants come together if they were never moving along the line in the first place remember that they’re always moving along the line because *they’re always moving in time from their past to their future*. Away from their yesterday and towards their tomorrow. In this analogy “past” is where they started on the equator and “future” is where they ended on the North Pole. They move along their straight line axis of time, and it’s the geometry, the curvature, that brings them closer and closer together towards one another. Curved spacetime. Replace one ant with the Earth and another ant with an apple and you can visualize how the apple “falls” to the ground.
Now keep in mind that this shit is legit Einstein levels of complex and so any ELI5 analogy we use is going to be imprecise to what is really going on, but if you don’t know jack shit about general relativity then this should hopefully give you a better intuition about the role that geometry and time play which I think the rubber sheet isn’t very good at.
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