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An alternative to the “ball on rubber sheet” model of gravity is “twisting a lump out of a sheet of silly putty.” You get the same curvature without relying on

An alternative to the “ball on rubber sheet” model of gravity is “twisting a lump out of a sheet of silly putty.” You get the same curvature without relying on gravity to serve as a model of gravity (which always bothered me a bit) For clarity, here’s what I mean: if you flatten out some silly putty (or pizza dough should work) then pinch and twist together some of the sheet into a lump, that pulls along the surrounding putty. So, if you drew lines on the putty then pulled it into lumps, you’d see the distortion to the lines.

Levitating 2y ago
That's a pretty clever model. Do you know any videos demonstrating it?
- passion__desire 2y ago
  A new way to visualize General Relativity https://www.youtube.com/watch?v=wrwgIjBUYVc https://www.youtube.com/watch?v=wrwgIjBUYVc This uses Adobe tools to visualize and take advantage of video format.
tambourine_man 2y ago
The recursive model always bothered me too. That’s nice, though harder to explain in words.
- IgorPartola 2y ago
  You can compromise: take a rubber net. Now bunch up a number of squares together and hold them with your hand. What happens to the bigger squares? Now explain that mass likes to move from smaller squares to larger squares and towards the bunched up areas.
dullcrisp 2y ago
> without relying on gravity to serve as a model of gravity (which always bothered me a bit) Why though? Would it help if the sheet were in a centrifuge?
- carapace 2y ago
  > Why though? It bothers me too, why? because it's circular reasoning: acceleration can't explain acceleration. > Would it help if the sheet were in a centrifuge? No, for the same reason. (You're imagining a tube-shaped membrane?) To make it worse, it's developing a wrong idea that hides the right and deeply strange idea: when an object passes a mass and its path seems to deflect what's really happening is that the object is moving in a straight line the whole time and space itself is curved. (The situation is actually a little stranger than that, but I'm no physicist so I won't try to explain any further.)
  - photonthug 2y ago
    As a sibling points out, it’s not explanatory as much as a visualization. Or if it explains something it’s not gravity itself but a less familiar kind of geometry and a new concept of straightness where sometimes the shortest path is what we’d usually think of as curved. Seems fine. Little chance of understanding gravity until you can grasp the prerequisite concepts you’re going to describe it with. And anyway explaining gravity v2 in terms of v1’s first approximation isn’t that strange when recursive definitions are going to play a part in lots of higher education anyway. When you’re a kid, 5 is just a concept useful to describe every instance of 5 apples, but later, a number N is perhaps best understood as the successor of N-1.
    - carapace 2y ago
      Gravity is one of the fundamental forces, there's no recursion in it's definition, indeed we don't "define" it we describe it. The actual description of gravity isn't hard to understand, it's just really strange. There is no good reason to introduce confusing and intrinsically incorrect models.
      - photonthug 2y ago
        > There is no good reason to introduce confusing and intrinsically incorrect models. Well, teaching is the usual reason, otherwise physics with trig would all just wait for calculus, and calculus wouldn't get covered before real analysis, etc. Incorrect, or more charitably "useful, prerequisite, & intentionally not rigorous" models are a pretty standard tool for walking students closer to real understanding. Clearly no one is proposing trampolines and bowling balls models because they are expecting to win a Nobel. > The actual description of gravity isn't hard to understand, it's just really strange. Hmm, Einstein thought it was pretty hard, and I didn't know that you and Witten had finished quantum gravity or the other latest and greatest. > Gravity is one of the fundamental forces, there's no recursion in it's definition, indeed we don't "define" it we describe it. Unpacking this to a first approximation it actually sounds like you're looking for The Truth instead of useful models, that you always know Truth when you see it, that you don't believe that apparently different descriptions can result in formally equivalent stuff. I'm also guessing that you might be strongly inside a "only cares about math because physics" camp whereas I sympathize more with "only cares about physics because math". Lots of interesting conversations in there, but it'll all just be philosophy at that point anyway and you probably wouldn't like it.
raattgift 2y ago
The "balls on a rubber sheet" is a pain because nothing is in free-fall: there are dissipative contact forces between the balls and the rubber sheet. Consequently realistic initial [position, velocity] values for the test ball cannot give you a stable circular orbit around the central mass ball. Venus isn't about to fall into the sun. Now try setting up Earth-Moon or the Jovian-Gallilean systems on the rubber sheet. It's fun to try to peel away defects in the "ball on a rubber sheet" in an effort to arrive at Newtonian limit equations of motion for balls in placed around it. (First advanced question: how do we adapt Einstein-Hilbert to reflect trapping onto the sheet? Does dimensional reduction work?) What sort of membrane in constant acceleration could generate scaled 2+1 timelike geodesics for model solar system objects placed on it? Can one scale this to a model of the solar system with all orbits flattened onto the membrane? For instance, what do the IVs look like for a ~ 1050:1 mass ratio between a model sun and a model Jupiter that is shrunk down to classroom size and retains a good match to Jupiter's real orbital parameters (or if you prefer, lengths and angles) across many orbits? Without destroying this scale model orbit, can we add the inner solar system? Can we get sensible orbits of scale model Galilean moons? (And of course all of the above has completely neglected the rotations of these bodies about their axes, which is clearly always very wrong with a typical in-classroom rubber sheet + balls demonstration. We can blame friction for that.) I'm not sure what you're representing on silly putty: are the drawn lines solutions to geodesic equations? What would the twists-pulls of the putty in a scale model ~kg:g Sun and Jupiter system look like? Or are you thinking about a relativistic regime somewhere in the right half of this diagram : <https://en.wikipedia.org/wiki/Post-Newtonian_expansion#/media/File:GR2bodyparameterspace.png https://en.wikipedia.org/wiki/Post-Newtonian_expansion#/medi...> ?
- cyberax 2y ago
  > The "balls on a rubber sheet" is a pain because nothing is in free-fall The balls on a rubber sheet model is actually really great, but not in the way it's typically presented (rolling a ball down the curvature). Instead, just use a pen to draw lines to show the concept of geodesics. Start like this: 1. Imagine that you can move without friction if you stay at the same vertical level. 2. Draw a line on a flat rubber sheet, that's a line in free space. It's just a straight line that can go on to infinity. 3. Now put a ball onto the rubber sheet, so you get some curvature. Now the lines near the ball that stay on the same level are not straight lines, but circles.
smusamashah 2y ago
Balls on sheet is a wrong model imo. It needs gravity to work and therefore doesn't explain gravity. I like to imagine a sponge. If you could somehow make dense lumps inside the sponge (may be apply heat in its center somewhere using microwaves?) everything around that lump will be feel a tension/attraction towards that lump. That's my mental model.
- the__alchemist 2y ago
  I think the balls on sheet is an OK compromise. The reason for me is, it's tough (but not impossible as you point out) to visualize functions over 3D space. Your sponge does that and I like it! (You could also use color-coding, vector-gradients etc) The ball and sheet uses a spacial dimension as the function value, and that's the dimension the needs gravity to work acts on. So, if you accept that as a compromise, it's OK; we are saying that dimension is a convenience.
  - IgorPartola 2y ago
    I like to explain it all as graph paper where the squares get bigger or smaller depending on what’s near them.
    - xscott 2y ago
      That helps explain curvature of space, but it doesn't explain any of the motion. When you see those stretched rubber sheets, they rarely/never show the grid lines moving. So let's say you place a small *stationary* object at a specific grid point (coordinate), and there's a large object that deforms the grid itself. Nothing about that visual provides any new intuition about why the small object would leave it's current grid position and move towards the larger one. The visual tries to explain gravity by appealing to your existing notion of gravity.
- jayd16 2y ago
  It just needs force, no? You could use magnetism or even intertia if you accelerated the system, right?
jacknews 2y ago
Or ripples in a table-cloth - the ripples gather the surrounding cloth, just like mass deforms spacetime. These models are also an intuitive way to illustrate why the speed of light is a limit. A ball rolling on a rubber sheet, or a boat on a lake, etc, can travel faster than waves in the rubber or water. So why can't matter travel faster than light? With ball-on-sheet type models, you need to resort to abstract relativity arguments about mass going to infinity, time slowing, causality, etc. But if particles are actually just waves or knots or whirlpools or whatever, they clearly can't possibly travel faster than the speed of waves in the medium.

Levitating 2y ago

That's a pretty clever model. Do you know any videos demonstrating it?

passion__desire 2y ago
A new way to visualize General Relativity https://www.youtube.com/watch?v=wrwgIjBUYVc https://www.youtube.com/watch?v=wrwgIjBUYVc This uses Adobe tools to visualize and take advantage of video format.

tambourine_man 2y ago

The recursive model always bothered me too. That’s nice, though harder to explain in words.

IgorPartola 2y ago
You can compromise: take a rubber net. Now bunch up a number of squares together and hold them with your hand. What happens to the bigger squares? Now explain that mass likes to move from smaller squares to larger squares and towards the bunched up areas.

dullcrisp 2y ago

> without relying on gravity to serve as a model of gravity (which always bothered me a bit) Why though? Would it help if the sheet were in a centrifuge?

carapace 2y ago
> Why though? It bothers me too, why? because it's circular reasoning: acceleration can't explain acceleration. > Would it help if the sheet were in a centrifuge? No, for the same reason. (You're imagining a tube-shaped membrane?) To make it worse, it's developing a wrong idea that hides the right and deeply strange idea: when an object passes a mass and its path seems to deflect what's really happening is that the object is moving in a straight line the whole time and space itself is curved. (The situation is actually a little stranger than that, but I'm no physicist so I won't try to explain any further.)
- photonthug 2y ago
  As a sibling points out, it’s not explanatory as much as a visualization. Or if it explains something it’s not gravity itself but a less familiar kind of geometry and a new concept of straightness where sometimes the shortest path is what we’d usually think of as curved. Seems fine. Little chance of understanding gravity until you can grasp the prerequisite concepts you’re going to describe it with. And anyway explaining gravity v2 in terms of v1’s first approximation isn’t that strange when recursive definitions are going to play a part in lots of higher education anyway. When you’re a kid, 5 is just a concept useful to describe every instance of 5 apples, but later, a number N is perhaps best understood as the successor of N-1.
  - carapace 2y ago
    Gravity is one of the fundamental forces, there's no recursion in it's definition, indeed we don't "define" it we describe it. The actual description of gravity isn't hard to understand, it's just really strange. There is no good reason to introduce confusing and intrinsically incorrect models.
    - photonthug 2y ago
      > There is no good reason to introduce confusing and intrinsically incorrect models. Well, teaching is the usual reason, otherwise physics with trig would all just wait for calculus, and calculus wouldn't get covered before real analysis, etc. Incorrect, or more charitably "useful, prerequisite, & intentionally not rigorous" models are a pretty standard tool for walking students closer to real understanding. Clearly no one is proposing trampolines and bowling balls models because they are expecting to win a Nobel. > The actual description of gravity isn't hard to understand, it's just really strange. Hmm, Einstein thought it was pretty hard, and I didn't know that you and Witten had finished quantum gravity or the other latest and greatest. > Gravity is one of the fundamental forces, there's no recursion in it's definition, indeed we don't "define" it we describe it. Unpacking this to a first approximation it actually sounds like you're looking for The Truth instead of useful models, that you always know Truth when you see it, that you don't believe that apparently different descriptions can result in formally equivalent stuff. I'm also guessing that you might be strongly inside a "only cares about math because physics" camp whereas I sympathize more with "only cares about physics because math". Lots of interesting conversations in there, but it'll all just be philosophy at that point anyway and you probably wouldn't like it.

raattgift 2y ago

The "balls on a rubber sheet" is a pain because nothing is in free-fall: there are dissipative contact forces between the balls and the rubber sheet. Consequently realistic initial [position, velocity] values for the test ball cannot give you a stable circular orbit around the central mass ball. Venus isn't about to fall into the sun. Now try setting up Earth-Moon or the Jovian-Gallilean systems on the rubber sheet. It's fun to try to peel away defects in the "ball on a rubber sheet" in an effort to arrive at Newtonian limit equations of motion for balls in placed around it. (First advanced question: how do we adapt Einstein-Hilbert to reflect trapping onto the sheet? Does dimensional reduction work?) What sort of membrane in constant acceleration could generate scaled 2+1 timelike geodesics for model solar system objects placed on it? Can one scale this to a model of the solar system with all orbits flattened onto the membrane? For instance, what do the IVs look like for a ~ 1050:1 mass ratio between a model sun and a model Jupiter that is shrunk down to classroom size and retains a good match to Jupiter's real orbital parameters (or if you prefer, lengths and angles) across many orbits? Without destroying this scale model orbit, can we add the inner solar system? Can we get sensible orbits of scale model Galilean moons? (And of course all of the above has completely neglected the rotations of these bodies about their axes, which is clearly always very wrong with a typical in-classroom rubber sheet + balls demonstration. We can blame friction for that.) I'm not sure what you're representing on silly putty: are the drawn lines solutions to geodesic equations? What would the twists-pulls of the putty in a scale model ~kg:g Sun and Jupiter system look like? Or are you thinking about a relativistic regime somewhere in the right half of this diagram : <https://en.wikipedia.org/wiki/Post-Newtonian_expansion#/media/File:GR2bodyparameterspace.png https://en.wikipedia.org/wiki/Post-Newtonian_expansion#/medi...> ?

cyberax 2y ago
> The "balls on a rubber sheet" is a pain because nothing is in free-fall The balls on a rubber sheet model is actually really great, but not in the way it's typically presented (rolling a ball down the curvature). Instead, just use a pen to draw lines to show the concept of geodesics. Start like this: 1. Imagine that you can move without friction if you stay at the same vertical level. 2. Draw a line on a flat rubber sheet, that's a line in free space. It's just a straight line that can go on to infinity. 3. Now put a ball onto the rubber sheet, so you get some curvature. Now the lines near the ball that stay on the same level are not straight lines, but circles.

smusamashah 2y ago

Balls on sheet is a wrong model imo. It needs gravity to work and therefore doesn't explain gravity. I like to imagine a sponge. If you could somehow make dense lumps inside the sponge (may be apply heat in its center somewhere using microwaves?) everything around that lump will be feel a tension/attraction towards that lump. That's my mental model.

the__alchemist 2y ago
I think the balls on sheet is an OK compromise. The reason for me is, it's tough (but not impossible as you point out) to visualize functions over 3D space. Your sponge does that and I like it! (You could also use color-coding, vector-gradients etc) The ball and sheet uses a spacial dimension as the function value, and that's the dimension the needs gravity to work acts on. So, if you accept that as a compromise, it's OK; we are saying that dimension is a convenience.
- IgorPartola 2y ago
  I like to explain it all as graph paper where the squares get bigger or smaller depending on what’s near them.
  - xscott 2y ago
    That helps explain curvature of space, but it doesn't explain any of the motion. When you see those stretched rubber sheets, they rarely/never show the grid lines moving. So let's say you place a small *stationary* object at a specific grid point (coordinate), and there's a large object that deforms the grid itself. Nothing about that visual provides any new intuition about why the small object would leave it's current grid position and move towards the larger one. The visual tries to explain gravity by appealing to your existing notion of gravity.
jayd16 2y ago
It just needs force, no? You could use magnetism or even intertia if you accelerated the system, right?

jacknews 2y ago

Or ripples in a table-cloth - the ripples gather the surrounding cloth, just like mass deforms spacetime. These models are also an intuitive way to illustrate why the speed of light is a limit. A ball rolling on a rubber sheet, or a boat on a lake, etc, can travel faster than waves in the rubber or water. So why can't matter travel faster than light? With ball-on-sheet type models, you need to resort to abstract relativity arguments about mass going to infinity, time slowing, causality, etc. But if particles are actually just waves or knots or whirlpools or whatever, they clearly can't possibly travel faster than the speed of waves in the medium.