My problem with the relativity representation on gravity.

Join the discussion
Ask a follow-up here, or get your own question answered by working scientists, mathematicians and engineers — people, not an autocomplete.
Real named experts · corrections over time · the nuance an AI answer skips
Sign up Log in
70 replies · 10K views
A.T. said:
It isn't. The rubber sheet represent purely spatial geometry.

Really? Then all it can be showing is the effect of a gravitational potential well, albeit with the wrong slope. How can it represent anything about GR if it doesn't include time?
I can't believe it is much use at all if all it does is to show, roughly, how a star will affect the trajectory of a passing planet.
 
Physics news on Phys.org
sophiecentaur said:
Really?
Well, which of the two sheet dimensions is supposed to be time?
sophiecentaur said:
How can it represent anything about GR if it doesn't include time?
Exactly.
 
Generally a dotted line is drawn to indicate a 2nd object's path, or else it is seen moving in an animation.
 
ikjyotsingh said:
Be careful, here. You can't explain gravity using GR. In fact, I assume you're talking about the Schwarzschild vacuum metric still, in that case, the solution is static, so embeddings of the rt plane would look the same in every slice, so this embedding wouldn't tell you anything.

I don't quite understand why you say that. While any embedding is ultimately a visual aid or tool, you can learn a lot from this one - see the original paper by Marolf for details. There are simpler illustrations out there, though, which I would continue to reocmmend - the feature that makes Marolf's embedding particularly interesting is that it does model the entire Kruskal geometry (including the Schwarzschild geoemtry as one part), and that it does include time.
 
Last edited:
A.T. said:
It isn't. The rubber sheet represent purely spatial geometry.

I am confused. The rubber sheet model is frequently used in conjunction with discussions about GR, in which it's used to show the concept of "space/time" being distorted in the presence of mass. Which bit of the model represents time and how does the model portray anything other than a 'simple', classical 2D potential well? If time is involved then an animation could be misleading (we don't normally animate simple xy graphs). Perhaps I am just having a problem with interpretation.
 
sophiecentaur said:
I am confused. The rubber sheet model is frequently used in conjunction with discussions about GR, in which it's used to show the concept of "space/time" being distorted in the presence of mass. Which bit of the model represents time and how does the model portray anything other than a 'simple', classical 2D potential well? If time is involved then an animation could be misleading (we don't normally animate simple xy graphs). Perhaps I am just having a problem with interpretation.

lundyjb said:
I think this is a poor representation.

DaleSpam said:
So do most people here.

Add me to the list of people here (many of which are science advisors, and/or PF mentors) who think the diagram is unusually poor.

The reasons for the poorness have mostly been discussed, and a few better alternatives have been mentioned.

So while you may or may not be misinterpreting it, even if you did interpret it correctly, it would, unfortunately, do little to actually aid you in understanding general relativity.

Amongst diagrams I can somewhat recommend are the "parable of the apple", which you can find in http://www.eftaylor.com/pub/chapter2.pdf, which is a publically downloadable chapter from "Exploring Black Holes" by E F Taylor. You can find a similar diagram in Misner, Thorne, and Wheeler's textbook "Gravitation". In fact, you can see it on the front cover :-).

https://www.amazon.com/dp/0716703440/?tag=pfamazon01-20

minus the explanatory text - and unfortunately it doesn't make that much sense without the explanation.
 
Last edited by a moderator:
sophiecentaur said:
The rubber sheet model is frequently used in conjunction with discussions about GR, in which it's used to show the concept of "space/time" being distorted in the presence of mass. Which bit of the model represents time
None.

sophiecentaur said:
how does the model portray anything other than a 'simple', classical 2D potential well?
It doesn't.
 
Once again, the rubber sheet analogy is ALWAYS used to represent the Schwarzschild solution. It is not used for any other solution in G.R. You can only have an isolated mass bending spacetime in a static, spherically symmetric setting, this spacetime is then asymptotically flat. If you take the Schwarzschild vacuum metric, and take constant time slices and embed that into cylindrical coordinates, you get the equation of a surface, which is a paraboloid, which represents the "dimple" in spacetime that is caused by an isolated mass. Any particle moving in the vicinity of this mass moves along a geodesic assuming there are no non-gravitational forces.

The rubber sheet analogy (which is a mathematically correct analogy), only applies to the Schwarzschild metric.
 
Rubber sheet model

sophiecentaur said:
I am confused. The rubber sheet model is frequently used in conjunction with discussions about GR, in which it's used to show the concept of "space/time" being distorted in the presence of mass. Which bit of the model represents time and how does the model portray anything other than a 'simple', classical 2D potential well? If time is involved then an animation could be misleading (we don't normally animate simple xy graphs). Perhaps I am just having a problem with interpretation.

Hello. As I explained in my other post, the rubber sheet model is not a general model in G.R., and is a visualization of only ONE solution in G.R., a solution that is static and spherically symmetric, that is, it has one time-like and 3 space-like Killing vectors, and by Birkhoff's theorem, this can only be the Schwarzschild metric.

Time plays no role in THIS class of solutions, as the solution is static, and asymptotically flat, more precisely, it is time-symmetric.

In general, you cannot show the concept of spacetime, as spacetime is a 4D pseudo-Riemannina manifold, these are impossible to visualize in the way we are used to visualizing geometry. You must embed these manifolds in Euclidean geometry to get a good visualization of them.

Also, more generally, the idea of evolving a spacetime in time is not trivial, the whole field of numerical relativity and 3+1 dynamical relativity is built upon this. One must essentially break apart the space-time symmetry to consider a foliation of spatial slices moving in time. It is how the curvature of those spacelike slices evolve in "time" that determine the dynamics of the model. In the Schwarzschild/rubber sheet visualization, each spatial slice is constant in time, which is what you would expect.

Hope this helps.
Ikjyot Singh Kohli
 
It is about embedding diagrams

DaleSpam said:
Yes, that is my point. Even if they show a correct graphic the description is not correct.

You seem to think that this is a discussion about embedding diagrams. It is not. It is a discussion about the rubber sheet analogy. Even if the rubber sheet analogy is presented with an accurate graphic of an embedding diagram, it remains a deeply flawed analogy, and the embedding diagram is irrelevant to the analogy.

This is a discussion about embedding diagrams, no one seems to realize it, that's why there is so much confusion! The rubber sheet analogy which is an Euclidean embedding of the Schwarzschild vacuum metric only applies in this metric. You cannot have the rubber sheet analogy in any other context in G.R. It only applies for a static, spherically symmetric, vacuum, and asymptotically flat metric. Embed the S-metric into Euclidean space and you get the rubber sheet, there is simply no other context in G.R. which can produce the rubber sheet analogy.

The Schwarzschild solution, perhaps due to the prediction of black holes is the most popular solution in G.R. (other than FLRW), that's why these rubber sheet diagrams have taken on so much momentum, but they only apply in the S-metric case.

The idea then, is not flawed at all, and the embedding diagram is where the analogy comes from!
 
ikjyotsingh said:
This is a discussion about embedding diagrams, no one seems to realize it, that's why there is so much confusion! The rubber sheet analogy which is an Euclidean embedding of the Schwarzschild vacuum metric only applies in this metric. You cannot have the rubber sheet analogy in any other context in G.R. It only applies for a static, spherically symmetric, vacuum, and asymptotically flat metric. Embed the S-metric into Euclidean space and you get the rubber sheet, there is simply no other context in G.R. which can produce the rubber sheet analogy.

The Schwarzschild solution, perhaps due to the prediction of black holes is the most popular solution in G.R. (other than FLRW), that's why these rubber sheet diagrams have taken on so much momentum, but they only apply in the S-metric case.

The idea then, is not flawed at all, and the embedding diagram is where the analogy comes from!
A rubber sheet supporting a weight doesn't deform into Flamm's paraboloid. It deforms approximately into a gravity well. The rubber sheet analogy is simply not an embedding diagram of any spacetime, even the Schwarzschild spacetime.
 
Rubber shet

DaleSpam said:
A rubber sheet supporting a weight doesn't deform into Flamm's paraboloid. It deforms approximately into a gravity well. The rubber sheet analogy is simply not an embedding diagram of any spacetime, even the Schwarzschild spacetime.

Anything supporting a mass in G.R. necessarily deforms into Flamms' paraboloid. I have a challenge for you then, in this regard. Find a solution of the Einstein Field equations that supports a mass other than the S-metric!
 
ikjyotsingh said:
Anything supporting a mass in G.R. necessarily deforms into Flamms' paraboloid.
No, that is not true. A spring supporting a mass certainly doesn't even remotely deform into a Flamms paraboloid. On the surface of the Earth where the weak field limit is appropriate a rubber sheet deforms to first order into a Newtonian potential well, not a Flamm's paraboloid. Here is an explanation:

http://en.wikipedia.org/wiki/Gravity_well#The_rubber-sheet_model

ikjyotsingh said:
I have a challenge for you then, in this regard. Find a solution of the Einstein Field equations that supports a mass other than the S-metric!
The point is that the deformation in a rubber sheet is not a solution to the EFE.
 
Rubber Sheet model

DaleSpam said:
No, that is not true. A spring supporting a mass certainly doesn't even remotely deform into a Flamms paraboloid. On the surface of the Earth where the weak field limit is appropriate a rubber sheet deforms to first order into a Newtonian potential well, not a Flamm's paraboloid. Here is an explanation:

http://en.wikipedia.org/wiki/Gravity_well#The_rubber-sheet_model

The point is that the deformation in a rubber sheet is not a solution to the EFE.


How would you even model a spring supporting a mass in T_{ab}? I don't know what that has to do with anything.

So, according to your wikipedia article, spacetime is being modeled by a physical rubber sheet, of course that's nonsense. In all my years of studying G.R., I have never considered "rubber sheet" to mean an actual rubber sheet. This won't work in general, but not for the reasons listed in this forum thus far. The deformations in the rubber sheet are given by the Poisson's equation which is a second-order ELLIPTIC partial differential equation. Such PDEs are acausual and thus would not exist in a pseudo-Riemannian manifold. This is actually why the rubber sheet analogy considering an actual, physical rubber sheet would fail.
 
ikjyotsingh said:
So, according to your wikipedia article, spacetime is being modeled by a physical rubber sheet, of course that's nonsense. In all my years of studying G.R., I have never considered "rubber sheet" to mean an actual rubber sheet.
Many people get fooled by that nonsense. I am glad that you weren't, but that is why we get so many questions here on this topic.
 
MikeGomez said:
Here is a youtube video from someone who took the effort to do a more reasonable job. We could poke holes in this one also, but at least he saw the problems with the standard explanations and tried to do a better job. I think he succeeded.



I like what he did.

I've been wanting to see an interactive program that would allow you to draw such an array. Ideally, it would allow you to alter the "mass" and the size of the center attractor, and to see the effect on the gridlines due to the changes.

Even better would be the ability to add another, small mass, and see it go into orbit (or not) depending on its velocity and direction. I'd love to model black holes using software like that.
 
Last edited by a moderator:
Isn't it a bit misleading for all [not all but most] of the video's captions to refer to "space" as being curved by gravity? After all, it is not objects which are distorted or bent by the presence of the planet, but space-TIME which is curved, inclining freely *moving* objects to veer toward the Earth unless held aloft.


"...space all around an object is bent...space is bent toward the objects..."
 
Last edited by a moderator:
sophiecentaur said:
I am confused. The rubber sheet model is frequently used in conjunction with discussions about GR, in which it's used to show the concept of "space/time" being distorted in the presence of mass.

The rubber sheet with dents in conjunction with GR shows just the spatial distortion described by the Schwarzshild metric. It looks similar to potential wells:

http://en.wikipedia.org/wiki/Gravity_well#Gravity_wells_and_general_relativity

But it has nothing to do with them. The spatial curvature is irrelevant to objects at rest in space, like an apple that starts to fall.
 
MikeGomez said:
I agree with you 100% They are doing exactly what you say. They are explaining gravity in term of gravity. It’s stupid, annoying, and does not explain anything.

I don't think it's that bad. The trampoline model works like this: You have a bowling ball sitting on a trampoline, causing the trampoline's surface to become warped, so it's no longer flat. Then you roll a marble across the surface of the trampoline. Its path isn't straight, but is curved.

General Relativity can be thought of as describing two different effects: (1) How does matter and energy affect spacetime curvature, and (2) How does spacetime curvature affect the motion of particles (and more generally, other non-gravitational physical phenomena)? The trampoline model helps to understand effect number (2), but not effect number (1). As far as I know, there is no help for understanding effect number (1).

The part that too me is the hardest to understand from the trampoline model is that in GR, the warping is to spacetime, not space alone.
 
1977ub said:
Isn't it a bit misleading for all of the video's captions to refer to "space" as being curved by gravity? After all, it is not objects which are distorted or bent by the presence of the planet, but space-TIME which is curved, inclining freely *moving* objects to veer toward the Earth unless held aloft.


"...space all around an object is bent...space is bent toward the objects..."


The captions are correct. And that's why this pictures are just as useless as the rubber sheet: it shows ony spatial distortion. Making it 3D doesn't make it better. Eventually worse. I doubt that those distorted 3D grids have anything to do with the spatial Schwarshild metric. Probably just some artists vague idea of distortion.

But the main problem is, they omit the time dimension.
 
Last edited by a moderator:
A.T. said:
The captions are correct. And that's why this pictures are just as useless as the rubber sheet: it shows ony spatial distortion. Making it 3D doesn't make it better. Eventually worse. I doubt that those distorted 3D grids have anything to do with the spatial Schwarshild metric. Probably just some artists vague idea of distortion.

But the main problem is, they omit the time dimension.

Perhaps a two dimensional projection of a 2D space plus time? Can't get my brain around what it could look like but we can handle 2D presentation of 3D images fairly well.
 
sophiecentaur said:
Perhaps a two dimensional projection of a 2D space plus time? Can't get my brain around what it could look like but we can handle 2D presentation of 3D images fairly well.

For radial fall you just need 1 spatial dimension. That is what is used in the links i posted before. If you want 2+1 curved space time, you need two diagrams.
 
I just want to know why everyone here is interested in visualizing spacetime?
 
Umm yeah

DaleSpam said:
Spacetime is fairly important in GR.

DaleSpam said:
Spacetime is fairly important in GR.

Umm yeah, but visualizing it is such a feeble task. You can only visualize it where an appropriate embedding exists, which according to my memory only exists for Bianchi I, V, and IX models, in addition to Schwarzschild family of metrics.

Why don't you talk about visualizing it in terms of how a pseudo-Riemannian manifold is constructed as a Hausdorff atlas of charts rather than all this diagram nonsense. At least the atlas of charts is correct both mathematically and physically, and provides an interpretation for which there can be no confusion.
 

Attachments

  • image.jpg
    image.jpg
    9.9 KB · Views: 418
A.T. said:
The captions are correct.

Are you saying that "space is bent" near the planet?
 
ikjyotsingh said:
Why don't you talk about visualizing it in terms of how a pseudo-Riemannian manifold is constructed as a Hausdorff atlas of charts rather than all this diagram nonsense. At least the atlas of charts is correct both mathematically and physically, and provides an interpretation for which there can be no confusion.
This doesn't help you visualize curvature, it is a standard textbook depiction of the requirement of diffeomorphic transitions between overlapping coordinate charts of a maximal atlas. It doesn't help one visualize the physics of relativity in curved space - time in any way. Note however that I'm not saying one MUST have a way of visualizing the physics in GR; it is silly to ask for such visualizations in full generality because we are talking about pseudo - riemannian 4 - manifolds and of course we can't visualize such things innately.
 
The best visualisation of curved space-time I came across are the three papers in

https://www.physicsforums.com/showthread.php?t=381683

see post #7 (from A.T.)

they are mathematically sound (at least for me) and it gave me a bit of intuition.
I think one can't hope for more than what's presented in those papers.
 

Similar threads

Graduate Nonsensical representation of gravity
  • #Thomas#
  • · Replies 58 ·
  • Other Physics Topics
2
Replies
58