This seems to be a fairly common representation of why planets/satellites stay in orbit around other massive bodies.
My question: doesn’t the trajectory of the orbiting body as depicted in this sort of representation actually depend on a gravity pushing it down onto the curved surface to actually work. And doesn’t that make it kind of, well wrong? Intuitive, sure. But wrong.
It’s an analogy though. It’s a mostly two-dimensional representation of a three (four?) dimensional scenario. Forget about the gravity holding the thing down - that’s just because they’re using stuff we can get our heads round to illustrate stuff we can’t. So yeah, it’s wrong, but it’s still useful.
Can’t watch the video here at work, but I assume it’s some variation of the typical “bowling ball on a mattress” explanation. And yes, it doesn’t really make sense, for the reason you give. But it’s hard to come up with metaphors for the curvature of 4-dimensional spacetime, so we take what we can get.
A better explanation in my opinion is to forget about things rolling on a slope, and just think of what things do in the absence of force – they move in straight lines. One perspective on gravity is that it bends space, and thus changes the definition of a straight line. So objects acting under gravity have a different trajectory than they otherwise would, because the “straight line” they’re following is now actually a curve.
I’m a scientific illiterate, but…
My favorite ways of visualizing a gravitational well is thinking of those devices that roll a coin down a 3-foot wide funnel:
You put the coin on a small ramp and release it, whereupon it corkscrews around the inside of the funnel, slowly descending and seeming to move faster and faster as it moves toward the narrow hole in the middle.
It’s not a perfect comparison. To make it better you’d have to change the funnel to a flat disk and put it around the equator of a planet (or maybe use a cute little black hole.)
Although the analogy isn’t exact, it is something most people are familiar with that clearly demonstrates gravity grabbing an object that’s moving in a straight line and curving its path.
If the coin was launched down the ramp fast enough, it could curve around the funnel briefly and then zip right out of it and continue on its straight line path. Like a comet, or a photon that nears the sun.
I think…
In a way, it isn’t all that wrong. The same forces are still coming out of it if you vector everything out.
In the real world, the planet stays in orbit because gravity from the star pulls it toward the star. It has a stable orbit because its velocity is high enough to keep it from being pulled into the star and yet low enough that it doesn’t just escape off into space. We know there’s a force inward toward the star because we observe it in real life, but we don’t have any explanation for why that force is there or how it is transmitted.
In the analogy shown in the video, we’re assuming that gravity is pointing downward rather than being related to the star at all. We’re also assuming that a single plane of space-time is curved in a funnel-like shape. In this case, we have vectors of “gravity” - partly the planet is being pulled down and partly it is being pushed toward the star in the center of the funnel. But the analogy also has our sheet of space-time pushing up on the planet, which means it counteracts the downward force. Thus, the net force left over is always pointed in toward the star at the center of the funnel shape.
So, we have different assumptions and different scenarios, but the objects are in the same positions, at the same speeds, experiencing the same accelerations. In both theories we have a force that we can’t fully explain.
kindof a hijack:
I was reading A Short History of Nearly Everything by Bill Bryson, and he mentions that space is unimaginably empty (that is, the average distance between particles in space is gigantic, like millions of miles).
If you flew a spaceship out to some part of space where gravity is negligible, and then used various thrusters to completely stop your momentum, would the spaceship just… float there?
Pretty much, yes.
You’d still feel some gravitational effect, but it would take very good instruments or a very long life span to notice it. The sun is orbiting the center of the galaxy, which is 60,000 light years away. There are galactic clusters where they clearly attract each other even over hundreds of thousands or millions of light years.
Why, what else would it do?
Nothing, I guess. Just seemed counter-intuitive and wanted to confirm my conclusions.
Hey
It makes more sense than masses exchanging virtual graviton particles back and forth and that somehow creates an attactive force…
I’ve often thought that the rubber sheet analogy was self-referential and therefore of limited worth (an analogy is supposed to be where you illustrate something by showing something else - not the same thing).
It makes a tiny bit more sense if you imagine the sheet being printed with a grid of squares when it was flat - after the weight is added and the sheet is warped, an object following the straight lines of that grid will no longer actually move in a proper straight line.
I think it’s possible to conceive a valid frame of reference where gravity does that - rather than pulling things, it warps spacetime so that things can follow a straight path in the warped spacetime and end up looking like they’re following a curved path.
Not only is it possible, that’s exactly how general relativity explains gravity (given suitable definitions for things like “a straight path”).