2013-12-18 08:30:33 8 Comments

I've seen many science popularisation documentaries and read few books (obviously not being scientist myself). I am able to *process and understand* basic ideas behind most of these. However for general relativity there is this one illustration, which is being used over and over (image from Wikipedia):

I always thought that general relativity gives another way how you can describe gravity. However for this illustration to work, there needs to be another force, pulling the object down (referring to a direction in the attached image). If I put two non-moving objects in the image, what force will pull them together?

So where is my understanding incorrect? Or is general relativity not about explaining gravity and just describes how heavy objects bends spacetime (in that case the analogy is being used not correctly in my opinion)?

**UPDATE** Thank you for the answers and comments. Namely the XKCD comics is a spot on. I understand that the analogy with bent sheet of fabric pretty bad, but it seems that it can be fixed if you don't bent the fabric, but just distort the drawn grid.

Would you be so kind and answer the second part of the question as well - whether general relativity is explaining gravitational force. To me it seems that it is not (bending of spacetime simply can not affect two non-moving objects). However most of the time it is being presented that it does.

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## 3 comments

## @Abhimanyu Pallavi Sudhir 2018-05-26 15:27:35

There is no problem with the analogy when stated correctly, and I haven't seen a single general relativity textbook that pretends the curvature is similar to a physical sheet getting bent due to a weight, or that stuff moving in geodesics is akin to stuff rolling down such a physical sheet. Perhaps there are popular science books that say this, I don't remember.

The reason the analogy is good is because curvature is the distortion of distances, so something can be bigger on the inside. The best way to show this in a 2d sheet short of drawing squiggly axes is to bend it externally, and the exact same mathematics -- stuff like area being greater than $\frac1{4\pi}$ the square of the circumference, or the angles in a triangle not equalling 180 applies to it. The analogy is then a visualisation of the geodesic equation, which is that stuff follows the path of the greatest spacetime interval (space-like convention).

The only nitpick I have about traditional representations of this analogy is that they only show curvature of

space, and call it spacetime curvature. To show spacetime curvature, you should show the entire worldline of the gravitating object, and the curvature around it. This is not much use, though, since curvature in 1 spatial dimension is lame, so you should just call it the curvature of a space-like slice/cross-section of spacetime.## @Pavel Horal 2018-09-06 13:37:01

I asked this question after watching pretty popular video youtube.com/watch?v=MTY1Kje0yLg . Real gravity plays quite an important role in that demo and that confused me a lot... also the fact that so many people were amazed by that (a bit misleading) demonstration.

## @BMS 2015-01-21 20:55:22

I happened upon this excellent source while browsing a related Physics SE question.

The analogy presented therein is somewhat similar to the rubber sheet one, but does away with the weight in the center.

Imagine a 2D spherical shell embedded in 3D space. Two people are located side by side on the equator of this sphere. They both begin walking parallel to each other, "northward" toward the one of the poles. It will appear initially that they are walking parallel, but eventually they meet up at the north pole. That is, their paths cross. The two people might interpret their closeness as a result of some "force," when in fact the geometry of the space in which they live caused them to move closer to each other.

## @Arturo don Juan 2015-11-04 14:40:58

This is a wonderful example and should be popularized much more!

## @Abhimanyu Pallavi Sudhir 2018-05-26 15:29:00

This is good as an analogy, but the rubber sheet analogy is better than an analogy -- it's a mathematically consistent interpretation. You just need to throw out bogus oversimplifications like "the Earth weighs spacetime down!"

## @John Rennie 2013-12-18 09:35:12

You're quite correct that the metaphor is misleading, and indeed you'll find professional relativists tend to be rather scornful of it. There are a number of problems with it, of which the problem you mention is just one. For example the diagram implies only space is bent, while the bending is of

spacetimeso time is bent as well. The diagram also implies there is a third dimension out of the plane in which the bending occurs. Applied to our 4D spacetime this would mean there would have to be a fifth dimension for spacetime to bend in, but this isn't the case and the type of bending that occurs is calledintrinsic curvatureand needs no extra dimensions.The problem is that GR is really, really unintuitive. If you want to know more than the hints suggested by the rubber sheet metaphor the only course is to roll up your sleeves and start learning the maths.

It would be nice if there were some intermediate course between the misleading but simple rubber sheet metaphor and the maths, but I don't know of anything. I think the problem is that you won't get anywhere without first understanding coordinate invariance and this is a really tough idea to understand. If you really want to learn more I'd start with special relativity as this contains the seeds of the ideas you'll need.

Response to comment:In your edit you say

bending of spacetime simply can not affect two non-moving objects. I'm guessing that you're thinking about objects rolling around on a curved surface as shown in the common metaphors for GR. The question is then why objects that aren't rolling around should experience a force.The reason for this is that an apparantly stationary object

ismoving because it's moving in time. For the usual 3-D velocities we see around us we describe velocity as a 3-vector $\vec{v} = (v_x, v_y, v_z)$. But remember that spacetime is four dimensional, and the velocity for objects in relativity is a 4-vector called the 4-velocity that includes change in the time coordinate. The reason a stationary object experiences a force is that the time coordinate is curved just like the space coordinates. This brings me back to one of my criticisms of the rubber sheet analogy i.e. that it cannot show that the time coordinate is curved just like the spatial coordinates.At the risk of getting repetitive, it's hard to explain why curvature in time causes the force without getting into the maths. The simplest explanation I've seen is in twistor59's answer to What is the weight equation through general relativity?. This shows, with the bare minimum of algebra, why a stationary object in a gravitational field experiences a force.

## @Stan Liou 2013-12-18 10:06:28

Einstein on coordinate invariance as a tough idea to understand: "Why were another seven years required for the construction of the general theory of relativity? The main reason lies in the fact that it is not so easy to free oneself from the idea that coordinates must have a an immediate metrical meaning."

## @John Rennie 2013-12-18 10:07:32

@StanLiou: I spent ages looking for that quote to illustrate my answer! Thanks :-)

## @Pavel Horal 2013-12-19 08:13:08

Thank you for the answer and comments! Can you please just add a short answer on the second part of the question (see updated question)? I will accept this answer after that. Thank you very much again.

## @John Rennie 2013-12-19 08:48:23

@PavelHoral: I've added a shortish answer to the second part of your question. I hope this helps, though I fear even this short step into GR already raises some conceptual hurdles.

## @Pavel Horal 2013-12-19 23:21:35

@JohnRennie it will surely take me some time to process this (mainly the linked answer :)). But I think I understand, that even stationary objects are always moving through the time dimension. And curvature of spacetime can transform some of this "time-velocity" into movement inside spatial dimensions.