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Experiments Reveal That Deformed Rubber Sheet Is Not Like Spacetime
KentuckyFC writes "General relativity is mathematically challenging and yet widely appreciated by the public. This state of affairs is almost entirely the result of one the most famous analogies in science: that the warping of spacetime to produce gravity is like the deformation of a rubber sheet by a central mass. Now physicists have tested this idea theoretically and experimentally and say it doesn't hold water. It turns out that a marble rolling on deformed rubber sheet does not follow the same trajectory as a planet orbiting a star and that the marble's equations of motion lead to a strangely twisted version of Kepler's third law of planetary motion. And experiments with a real marble rolling on a spandex sheet show that the mass of the sheet itself creates a distortion that further complicates matters. Indeed, the physicists say that a rubber sheet deformed by a central mass can never produce the same motion of planet orbiting a star in spacetime. So the analogy is fundamentally flawed. Shame!"
like a car looking for a parking place.
Space time bends you!
I'm not sure the analogy was ever meant to be a rigorous and exact model, but more of a kind of way of visualizing space-time. All analogies break down if you try to map them exactly to the phenomenon you're trying to explain. After all, it's an analogy, not a model.
The world's burning. Moped Jesus spotted on I50. Details at 11.
This was figured out more than 100 years ago. A rubber sheet can be mapped to a scalar theory of gravity. If you made it past the first two lectures of a class on General Relativity, you would know that Relativity is a tensor theory. That is why it is so horrendously complicated.
It's probably the closest and most easily understood real-world analogy you're going to get.
I thought that would have been known by now.
Call me a troll if you like, but this is not exactly a revelation or news in any form to me.
for shame!
Thought experiments using analogues like the rubber sheet are often useful for visualization, organizing your thoughts, or providing a template to work on, but that doesn't mean that they necessarily provide a picture that is correct in all respects. The fact that they aren't accurate in all respects doesn't mean that they aren't useful representations.
much of left-wing thought is a kind of playing with fire by people who don't even know that fire is hot - George Orwell
Let's come up with a real complicated illustration for laymen to explain how space time might work. Perhaps a ladybug in a drinking fountain or something.
Maybe it's just a general analogy for general relativity that's easy to understand, and not to be taken so so literally. Did they bother to come up with another analogy? Didn't think so. What dicks.
Politics; n. : A religion whereby man is god.
This was figured out more than 100 years ago. A rubber sheet can be mapped to a scalar theory. However, as anyone who has made it past the first two lectures of a course on General Relativity would know, Relativity is a tensor theory. That is why it is so horrendously complicated.
... that anyone who had a grasp of high school phsyics, and who understood the analogy - of 3D matter flattened to represent a 2D metaphor for our real 3D world, which lives in 4D spacetime - or who understood that gravity attracted mass towards mass and not towards the "down" direction perpendicular to the sheet - would think for a second that such a demonstration would create the same exact trajectory as actual interaction between 3D objects in 4D spacetime.
No analogy is perfect. However relativity is sufficiently complex that I don't know if any analogy would be perfect at all. This analogy at least provides a general sense of the way it works, it may not be 100% accurate but's relatively (see what I did there) close enough to provide a general understanding.
Who seriously expected the physics of a marble rolling on a rubber sheet to exactly match the physics of a planetary body in orbit? Who thought the analogy was ever meant to make that statement? It's still a pretty good analogy for giving a layperson the gist of how gravity works, and I seriously doubt it was ever meant to do any more than just that.
All analogies are flawed in some way. They're analogies. They're not the actual thing. If the rubber sheet's characteristics match that of spacetime exactly, it probably is spacetime.
But even if it's not exact, I think it's still a useful way to illustrate to the general relativity-illiterate (yours truly being among them) what the theory is all about, and why it's significant. General relativistic effects are not something that can be demonstrated (easily) in the classroom. Putting a marble on a rubber sheet is.
"If a nation expects to be ignorant and free in a state of civilization, it expects what never was and never will be."
Uh, that whole 'spacetime is like a rubber sheet' thing was just a bad metaphor used to attempt to explain the (relativistic) effect of mass on the 'shape' of space. I don't recall ever seeing studies on the effects of vulcanizing on the elasticity of space (cue the Star Trek jokes). Think about it - using gravity to explain gravity? That sounds like teching the tech to me.
No. A rubber sheet is a flawed *model* for the shape of spacetime; as an *analogy*, it's still reasonable...
Analogies help to understand something... up to a certain point.
It only illustrates the basic concepts. After that, one has to go beyond the analogy and do the math.
I remember a poster on a door at the math department of my university (parafrazing from memory): "Do not try to visualize a space with more than 3 dimensions. Nobody can do that, trying will just twist your mind. Just use the formulas with the correct number of variables and leave it at that."
/. refugees on Usenet: news:comp.misc
So that lay people can wrap their head around a very hard concept. One would think that this would be clearer to the person who wrote the paper. And if they offered an analogy that was as easy to use as an entry point to this very complex set of concepts, they would come off less as d-bags.
meep!
Let's hope the researchers now understand the difference between an analogy and a model...
Next they are gonna to tell me my Fisher Price bath boats are not sufficient for planning naval invasions.
Table-ized A.I.
But the truth is that this work cannot diminish the extraordinary utility of this analogy. And so the public love affair with general relativity is safe. Long may it continue!
So what's the damn point?
Politics; n. : A religion whereby man is god.
Suppose that you had a big rubber sheet stretched out, and onto that sheet you place a ball. Now suppose that there's a force that pulls the ball down, creating a depression in the sheet. Well, gravity is a lot like that force. Really a lot like it.
A deformed rubber sheet is a two dimensional simplification. Of course it's inadequate.
Obviously it's not like space-time.
You forgot to add the Dark Matter.
It's in the third shelf from the left.
-- Tigger warning: This post may contain tiggers! --
Attempting to use anything flat will fail. Space isn't flat.
So the analogy is fundamentally flawed. Shame!
The analogy is not fundamentally flawed. The Slashdot summary is. There is nothing wrong with doing this kind of test, it's kind of "mythbusters" semi-science. It's kinda nifty. The problem, as usual, is the over-reporting of science in an attempt to create pithy quotable summaries.
In other news, experts pointed out that rubber sheets provide a two-dimensional surface, while the real spacetime continuum provides three spatial dimensions and one of time. Experts also pointed out that rubber sheets have nonzero friction with rolling marbles, while empty space has zero friction; and that the rubber sheets do not provide the time dilation effects that gravity provides.
Experts also pointed out that the whole rubber sheet thing is what is known as an "analogy" and pretty much by definition is inexact.
Personally, I found the article interesting, but the tongue-in-cheek "Shame!" of the summary a bit over the top.
P.S. From TFA:
lf(1): it's like ls(1) but sorts filenames by extension, tersely
... but I'm not sure how it's a "Relativity Shock" as the second links suggests.
Upon thinking about a marble rolling on a rubber sheet I immediately see two big differences between it and a planet moving in space: (i) the marble.. well.. rolls along the sheet, which planets don't do while moving through space; and (ii) the rubber sheet doesn't propagate disturbances at the speed of light (or anywhere close to it).
I think most of us grasped this intuitively on some level. If nothing else, a ball rolling on a sheet is always going to experience friction. It doesn't orbit. It spirals in. It's "like" relativity. Then if you get into serious physics you learn the equations that are not merely "like" but *are* relativity.
For all intensive purposes, "whom" is no longer a word. That begs the question, "who cares"?
1. You have to first assume that the rubber sheet has no mass.
2. Then you have to assume some other things
3. ...
4. Profit!!!
Know what else? The water circuit analogy to electrical theory falls apart under scrutiny. And the easter bunny isn't real.
Where is he when you need him??
Obligatory XKCD
http://xkcd.com/895/
didn't you ever do those silly physics questions.
"Bobby is in a charged glider over an infinte ground plane.."
The rubber sheet analogy isn't using gravity to explain gravity. The marble goes into the divot in the sheet not because it's lower and therefore pulled in there by gravity, but because the surface of the sheet is curved.
If you had a "sticky marble" and took the rubber sheet into orbit and deformed it, the marble would still curve toward the divot. Not because gravity pulls it there but because the sheet is curved. That's the whole point of the analogy.
To think that the "real" gravity acting from outside is what causes the marble to curve its path is to misunderstand the analogy.
... when we use analogies we're dealing with the most relevant information to communicate an idea. There is certainly nothing wrong with the analogy because it was never intended to communicate the whole complexity of the phenomenon, it was always meant as a starting point.
When we use analogy we're talking in ratio's and proportion, we're saying the phenomena is a X percent like this macroscopic phenomenon that everyone knows, but of course it is more complex then that, but that's a rough approximation of what is happening.
Nobody of any intelligence should get pedantic about it because that was never the intent of the communicator to begin with. I hate these ignorant pedants that stupidly misunderstand the intent of the person who originally made the analogy. You're not being smart, you've proven you don't get anything at all about communication of complex ideas.
We 'layer' people in to understanding by giving them basic models to get across general rough approximations and then we ease them into the deeper complexities, contradictions and unknowns.
I suppose they are going to tell my rubber girlfriend is not representative of.......naaaaah, who cares!
Table-ized A.I.
I hope these guys get a good grade proving something which should be a no-brainer for an 8 year old. I also hope that no tax money was used in these efforts!!!
My karma is bad. Don't get too close!!!
Funding for tenured physics professors.
So what if the sheet is curved? Banking on a race track corner works due to gravity, not because of the shape of the curve.
I never envisioned that the universe was literally like a rubber sheet or any object creating an indentation in a material due to its weight. But I still get the basic idea. That's the point, it communicates the basic idea in a way the average people can comprehend. Besides, most people are now exposed to this example through a computer rendered simulation, not a rubber sheet.
Brought to you by Carl's Junior.
The point of the rubber sheet analogy is to discuss the SHAPE of the surface which is a stand in for forces of gravitation.
As such, even if following the analogy you shouldn't use an actual rubber sheet because it will be distorted by the marble itself. Rather, use a hard surface modeled on what a rubber sheet would do with that deformation.
Will that be perfect? Probably not but will be less of a failure then this spandex idea.
I've decided to stop wasting my time responding to AC trolls/sockpuppets... so if you want a response from me... login.
A marble rolling on rubber is affected by a great deal more than just gravity. It's only a model, not Camelot. Next on the news: scientists find out that one dimension isn't equal to a dot and two dimensions isn't equal to a line.
No shit they're different. It's an analogy, not the same thing. Of it were perfectly the same, it wouldn't be an analogy, it would be the same thing. The analogy still serves its purpose.
What a bunch of monkeys that don't know the meaning of 'analogy'.
It was used only as a visual aid to be able to teach mere mortals about gravity.
Will someone please smack these researchers, they obviously have zero social skills or understand you have to find a simple way of explaining things to the normals without their eyes glossing over and wandering away.
Do not look at laser with remaining good eye.
So no other approximations will be allowed?
And what's next? People modeling the Sun's gravity as a point?
I'm pleasantly surprised that everyone seems to have thought like me...
And that's
"Huh? It was never supposed to be an accurate model, just a way to visualize it in a rough sense. Nobody ever suggested the math would actually work out"
In my experience, rubber sheets are actually very good at holding water. I'm guessing one of these guys has a wife who, due to the number of years her husband spent on his Ph.D., is feeling the old biological clock ticking and she's taken a needle to every rubber she can find.
Wrong, kind of. Yes a bit and no a bit.
Think about a 90 degree bank. Or a loop. Or a motorbike in a sphere of death (with a bike with magnetic tyres in free fall with no gravity) . They work in spite of gravity or the lack of. More to do with Centripetal acceleration than gravity for example.
Which is kind of what space time is talking about, its the shape of space that bends the path.
But you should explain this while you are using your flawed analogy. Explain why its flawed. Teaching physics is less like teaching and telling someone facts as it is hypnotising them and talking about zen like questions to use confusion to further understanding of a concept or principle. Here look at this tennis ball go near to the bowling ball on the rubber mat. Lets see if we can make a gravitational slingshot happen and make the tennis ball travel faster. How is that possible? Do photons exist if they aren't observed? Cat is in a box is it alive/dead? What's the biggest thing that can be alive dead? If photons have no mass why does gravity affect them? What is mass? What is gravity?
Then you guide them on their journey of introspective self discovery. They should end up with a Feynman like state of uncertainty where they aren't really sure about anything and know they know nothing about a lot of things. They should have a blank look of deep thought and a 1000 mile stare on their faces. Then you explain they only have read the first letter of a 1000 km thick book about physics, and that first character is a word that tries to start of explaining an approximation of what physics is.
I hate having people tell me how to teach. In my state we have a dot point that says "use the water analogy to teach electricity". Most teachers say water = electricity and thats that. They don't understand themselves the limitation of the water analogy. Then they write a test to see if the student has memorised a fact which is wrong.
Physics isn't like chemistry or Biology or history or any other stamp collecting subject. Its not even like mathematics. Memorising physics facts will get you nothing.
Physics you should come out a bit confused and not sure of anything, because really, you can't be sure of anything, just a level of confidence of what you are observing.
so even though the formula for gravity is the same as the formula for light intensity, we just turn a blind eye and say it is more likely that gravity is caused by a bending of an inperceivable dimension rather than there simply being gravitrons that are emitted by objects with mass?
Banking on a race track corner works due to gravity, not because of the shape of the curve.
Gravity is pretty much the same everywhere on Earth. Racetracks have different curve shapes. Your hypothesis seems flawed. Inertia is probably a better answer as to why banking works.
I've seen models created out of hardened plastic that more realistically reflect the curvature of space. It still isn't perfect since you still have friction, but it does a decent job of demonstrating orbits.
The true advantage of stretched rubber sheets is that it is a cheap and easy demonstration to create. That was especially important in the days before computer simulations or even television.
Creationism!
All science is wrong!
Feel free to add other crap. Seriously, how many analogies are exact? An object makes a dent in time/space, and another object reacts to that dent.
Remember, the analogy is there to give an idea on how to understand space/time, not for people who already understand space /time to figure out how rubber sheets work. That's for kinky people and kids with bedwetting problems to figure out.
The shepherds did so well protecting the flock that the sheep no longer believed that wolves existed.
That was the part of the whole thing that I found funny.
If it's not like the rubber sheet and marble, is there a better model?
"mass of the sheet itself creates a distortion that further complicates matters"
Sort of like... dark matter.
Are you saying that electrons and planets don't behave like billiard balls ?!?!
So I guessed they proved that spandex is not spacetime. What will they think of next?
right...when I used to explain this, I introduce the sheet/marble analogy, then describe, as you did above why it's limited.
i then would tell them (students) to imagine an infinite number of sheets surrounding the ball in all directions stretching to the limits of the universe to give them a possible hint as to how it actually behaves
Thank you Dave Raggett
A flawed analogy can still be very useful. Very instructive. Even true.
An old planetary clockwork can still give you a very good idea of the movement of the planets, even though it's way off and in some ways misleading.
Anyway, the rubber sheet with the marble still beats "heaven above, hell below and Earth in the center of the universe", or "the stars represent the light of angels". Analogies are either better, or worse. More useful or less useful.
You are welcome on my lawn.
So what if the sheet is curved? Banking on a race track corner works due to gravity, not because of the shape of the curve.
Fail. Banked curves are all about the shape causing centrifugal force. They work great with no gravity at all.
Source: I've failed many times before.
No, the shape of the bank exerts a sideways force on the car. This causes it to turn.
When our name is on the back of your car, we're behind you all the way!
FORD:
Alright imagine this: you get a large round bath made of ebony.
ARTHUR:
Where from? Harrod’s was destroyed by the Vogons.
FORD:
Well it doesn’t matter -
ARTHUR:
So you keep saying!
FORD:
No, No listen. Just imagine that you’ve got this ebony bath, right? And it’s conical.
ARTHUR:
Conical? What kind of bath is -
FORD:
No, no, shh, shhh, it’s, it’s, it’s conical okay? So what you do, you fill it with fine white sand right? Or sugar, or anything like that. And when it’s full, you pull the plug out and it all just twirls down out of the plug hole but the thing is
ARTHUR:
Why?
FORD:
No, the clever thing is that you film it happening. You get a movie camera from somewhere and actually film it. But then you thread the film in the projector backwards.
ARTHUR:
Backwards?
FORD:
Yeah, neat you see. So what happens is you sit and you watch it and then everything appears to swirl upwards, out of the plug hole and fill the bath amazing.
ARTHUR:
And that’s how the universe began?
FORD:
No. But it’s a marvellous way to relax.
Anyone who hasn't seen the spandex analogy have a look at this YouTube Video.
No it doesn't. The reason the bank exerts a sideways force on the car is because gravity pushes downwards, there's an equal and opposite resistive force from the road (Newton's Third Law), and since the road is not perpendicular to the gravitational force this has a vertical and horizontal component. Without gravity, there would be no horizontal component of the resistive force.
"If you had a "sticky marble" and took the rubber sheet into orbit and deformed it, the marble would still curve toward the divot. Not because gravity pulls it there but because the sheet is curved. That's the whole point of the analogy."
What's the "stickiness"? You've simply introduced another mysterious force to replace the gravity pulling the marble down towards the curve. Why should there be a "stickiness"? What is the stickiness? Where does it come from? Did someone apply glue to the marble, or the sheet? You aren't answering the fundamental questions any better by using a "sticky marble".
The only fundamental flaw is with the physicists level of seriousness in documenting this test.
The rubber sheet is used as an analogy to describe the quintessential elements of the space-time theory to people uncomfortable with mathematics. It's not intended to be directly equivalent to an astronomical system! Obviously other effects (like friction and fabric warping) are more dominant on the experiment scales than at astronomical scales.
Reading the paper, there are something like five other references on marbles and spandex to simulate space-time warping. I mean, really? This is probably a good teaching tool for graduate students, but we must have too many underfunded physicists in the world if they are wasting actual research time with spandex and marbles. There are more useful projects that can be investigated cheaply and experimentally.
But if it helps, yes, just like that.
.
Prisencolinensinainciusol. Ol Rait!
Shape your Hotwheels track into a loop.
Roll a car through it real fast. Notice that the car stays on the track when it's upside down - when gravity is trying to pull the car AWAY from the track.
Notice also when the car is vertical gravity is pulling the car backwards, neither toward the track or away from the track. You now see that gravity has nothing to do with it
How is this news. It's a fucking rubber sheet analogy to get the basic idea across. Anyone's free to come up with a better one...
King; "Look! General Relativity!"
Knights; "General Relativity!"
Minion; "It's only a model."
King; "Shh!"
rubber is full of sheet.
Table-ized A.I.
The rubber sheet (simple thing that 'uneducated' people understand) is a way of explaining the curvature of 4d space/time by mass (complicated thing that really requires a graduate level math degree to do anything meaningful with) by dropping down to 2d space. For what it is intended to do, it is a wonderful tool.
I always imagined 3d space with fluctuating 'density' gradients when I think of relativistic effects. Imagine being in a pool, where some of the water was was really dense or thick, and took great effort to swim through.
HA! I just wasted some of your bandwidth with a frivolous sig!
That an analogy is not reality? What are they going to test next? Plato's cave? Whether life really is like a box of chocolates?
so you want to explain space curving by a mass. And to do so you put a mass into a rubber sheet which is curved by earth gravity. Oh, nice, so to replace the gravity in your experiment you use... gravity. Means the curvature of spacetime is shown by using spacetime effects on the model.
For me the thought experiment in itself is flawed. It's basically shifting dimensions, so our 3D becomes 2D and the spacetime becomes the 3rd dimension. Explaining what gravity is, it can't.
Atari rules... ermm... ruled.
Don't worry, you'll be grateful for them when you're older.
A marble rolling on a rubber sheet will create a gyroscopic effect due to the strict alignment of the plane of rotation with direction of motion.
A planet spinning in space will have it's rotation in a completely different plane, and apparantly aligned (although often observably skewed) with the plane of orbit.
No wonder it's not comparable.
not using four dimensional rubber.
The fact that the deformation of the rubber sheet is caused by gravity acting on a mass is just a matter of convenience. You could do the demonstration in outer space using a magnetic field and metal balls, or attach pieces of elastic of various lengths between points on the sheet and the floor, or use some other force as your stand-in for grav-... for whatever it is that fundamentally causes spacetime to deform in the presence of mass. The trouble is we don't really have that many forces to play with, and gravity is the most convenient one.
systemd is Roko's Basilisk.
could it be something similar to electro-static buildup/charge? all of the planets are moving through a field of charged particles emitted by the sun. is there anyway to tell how much gravity a rogue planet possesses?
Obviously they didn't use enough cars.
Damn, now i have to transfer my stocks in rubber sheet companies from my "Technology" portfolio to my "Recreational" portfolio.
You never know. Maybe some form of plastic will actually perfectly model the distortion of space time.
Better get on that. There are a lot of materials left to test.
Does this mean the proof by analogy is no longer valid? Ahhhhhhhhhhhhhhhhh.........
... shoot, I'm drawing a blank. Any help?
"Happy families are all alike; every unhappy family is unhappy in its own way." -- Anna Karenina by Leo Tolstoy
It doesn't matter what force is moving the rubber ball, whether it be earth's gravity or someone moving the thing with their hand.
You might as well be complaining that "movies aren't good analogies to real life because the people on the screen aren't real people they're just little LED lights on a 2d plane".
SMH
The analogy is not "flawed" except in that it is an analogy.
analogy
n noun (plural analogies) a comparison between one thing and another made for the purpose of explanation or clarification. Øthe process of making such a comparison. Øa thing regarded as analogous to another; an analogue.
In an analogy, you suggest that one thing is LIKE another - not identical. If they were identical, they would be THE SAME THING.
The marble/rubber sheet analogy is helpful in some ways; I have always found it so, at any rate. It never occurred to me for a moment to test its exact physical behaviour, because I wouldn't expect it to be identical. The purpose of the analogy is to guide one's imagination and help it to grasp the kind of phenomenon being described. One might as well condemn Bohr's model of the atom because electrons don't behave exactly like planets.
I am sure that there are many other solipsists out there.
In my physics classes I use the rubber-sheet analogy to discuss electric fields as well as gravitational fields: it offers an alternative to the notion that the Earth is directly pulling on each of us individually or a positive charge is directly pushing on another positive charge, even when they're not touching. If you place a bowling ball in the middle of the rubber sheet (or blanket, which is what I use in class) and then roll a tennis ball by it, it's obvious that the bowling ball is not reaching out and pulling on the tennis ball, but the attraction of the two is a two-step process: the bowling ball distorts the sheet, and then the tennis ball reacts to that distortion.
The field description of forces works the same way: the Earth first creates its gravitational field (whether there's anything there to feel it or not), which is a distortion in spacetime, and then our bodies react to that field by feeling a force. The electric field can also be thought of in a similar way: a distortion of spacetime that causes positive charges to feel a force in one direction, and negative charges in the opposite direction.
These results, while interesting, don't ruin the analogy for that purpose.
Of course it's not a sheet.
It's more like a big ball. Of wibbly-wobbly...timey-wimey...stuff....
Why did they even need to test this? It is quite obvious that it is just an analogy and that it doesn't actually work that way. For one, the marble rolling on the rubber sheet is pulled by our real gravity below the sheet, and not by some mysterious rubber sheet warping the pulls the marble in towards the mass. And the talk in the article about the sheet not forming a cone shape like gravity space warping does. For the correct orbiting motion using a surface under our gravity field you don't even want a cone. Most people have probably seen those curved funnel things where you roll a coin into it and it orbits quite well all the way down into the center part. The profile of that device is not linear like a cone is, but starts out shallow at the outer edge and gets steeper in towards the center. It could be derived from some parabola or something, but it definitely isn't a cone. So the analogy is just an analogy and things don't really work like that. Big deal! If it help your mind grasp the underlying math involved in a more concrete way then it did it's job!
-- ssoorrrryy,, dduupplleexx sswwiittcchh oonn.. -Quote found on actual fortune cookie.
They want you to understand one of the most difficult theories in one go. ...
That is silly. Of course spandex is not time-space but it is okay to get a"foot in the door"
to get on your way to understand general relativity methinks.
Also instead of bashing the "spandex-is-space-time" analog they should rather
suggest a better analog
Just make everything really really difficult, wrap it ina veil as to keep the laymen out of the temple
and the priests "of high knowledge" EMPLOYED .
Complification secures jobs eh?
Whoever said it was a good analogy? Why is this news? OMG people, the sky is blue!
not an accurate example of how it works.
The Kruger Dunning explains most post on
Until we invent a meta material that allows us to manipulate gravity, OF COURSE it's 'shifting' the dimensions. G'ah.
The Kruger Dunning explains most post on
...sometimes saves a ton of explanation.
-Saki
Analogies are like Microsoft programs, there is always something wrong with them.
Yes, there's a lot wrong with that old rubber sheet analogy. As a zillion previous /.ers have stated, it's just a simple crude means of making an intuitive point, that in some general way curvature affects objects moving about in the arena they inhabit.
Of course the biggest bit of silliness is that the analogy relies on gravity in order to explain gravity. Duh.
Some have mentioned that marbles rolling about on a curved rubber sheet are _rolling_, and that has no analogy with reality. But we can ignore that.
Rubber, or whatever more or less solid type of "fabric of space-time" one uses, allows longitudianal motions, stretching and jiggling tangentially along the surface. This has no analog in reality. We can just ignore that too. A better way to think of curved space is as liquid, in an analogy with soapy water films and bubbles. There's no stretching, as liquid readily flows to fill in areas being pulled apart. There's only curvature and boundary conditions as the films of liquid find minimal surface area.
But then, even that fails, as gravity as we deal with it in the real world is almost entirely explained by the time-time component of the metric. Curvature of space is secondary, hard to measure, and a matter for precision experiments involve spacecraft and lasers. It's the differential rate of progress of time between nearby points that "explains" gravity. But to explain it intuitively to the layman, that I'm not so sure about.
Honestly, this may be a good thing. Maybe I'm being a bit naive, but the number (and ridiculousness) of analogies and/or metaphor in science television programming lately has been getting crazy. Its hard to fight against the anti-intellectual mindset when every time the general populace sees a scientist on TV they're explaining something using yet another ridiculous metaphor.
Just explain the concept. A CGI model as a visual aid works well. If the viewer doesn't understand they likely wouldn't do any better with a metaphor.
"People who think they know everything are very annoying to those of us who do."-Mark Twain
posit "what if the rubber mat was massless"?
At least this is the first question that would need answering in my mind if we are to get closer to understanding "how stuff moves and how gravity affects said aforementioned stuff".
- Zav - Imagine a Beowulf cluster of insensitive clods...
I'm not an expert in relativity, but I get it well enough. When someone asks how the heck a ball on a rubber mat relates to spacetime curvature, I reference Eddington's experiment (http://en.wikipedia.org/wiki/Arthur_Eddington#Relativity) and note that the light traveling beside the sun is like a penny rolling on the rubber mat between point A and B tangent to the ball in two dimensions. Then relate that to three dimensions using the tesseract/hypercube-to-cube/cube-to-2D projection shadow analogy that Sagan used in Cosmos (http://en.wikipedia.org/wiki/Tesseract).
"Who are you?" "No one of consequence." "I must know." "Get used to disappointment."
So do we have a plausible alterative theory? Or are we just being negative nancies?
rubber sheet? in the original destructions, i mean...