View Full Version : rubber sheet

2007-Jan-11, 01:19 PM
Is there anyone, like me, who has a problem with the rubber sheet analogy for gravity?

You know the one: the universe is envisaged as a flat rubber sheet, and the way mass produces gravity is analogious to the way a weight placed on the sheet produces a depression around itself.

The problem I have, is this very model needs a force to make it work, i.e. gravity, the very thing it is trying to explain. If the rubber sheet was in a weightless environment, masses placed on it would not produce depressions.

The only way the analogy works is to imagine there is some force which is trying to suck mass out of the universe and into "somewhere else". But I've never been aware of any such force being mentioned.

What do you think? Am I missing something?

2007-Jan-11, 02:26 PM
I agree that it still needs gravity to make the rubber sheet. I'd say the only thing you're missing is that it's just an analogy. It's helpful sometimes to visualize the working of gravity but it's far from perfect. When you look in more detail, mass actually warps space around it and what causes light to bend is that it follows straight lines through this warped space. The warping and following of straight lines in the analogy requires gravity but the accurate model does not work the same way and, I believe, is fully consistent on its own.

2007-Jan-11, 04:23 PM
As mentioned, the rubber sheet thing is just a way to *visualize* this. they are far from suggesting the universe is a flat sheet with bodies of mass pressing down upon it.

If you really want to fry your brain, take the analogy one step further: a body of mass (lets say earth) on the rubber sheet, but gravity is not one-dimensional. So there would have to be a rubber sheet for each possible angle. all intersecting. but all being bent as if the earth was pushing perpendicular to that particular sheet.

In CGI terms, an artist/modler familiar with 3DStudio (and perhaps other programs) can better visualize the GR gravity theory as an FFD Warp. But unless you are familiar with that particular animation program, this analogy probably doesn't help you much. :-P sorry.

Jeff Root
2007-Jan-11, 05:31 PM
I realized the irony of that several years ago, but it is no more
than a mildly-humorous irony. A bigger problem for me is that
the rubber sheet would sag inappropriately, especially when
more than one major mass is placed on it. I can imagine what
it should look like ideally, but that isn't how a real rubber sheet
would actually deform.

-- Jeff, in Minneapolis

2007-Jan-11, 06:34 PM
<<mass actually warps space around it >>

But WHY does it do this?

2007-Jan-11, 06:45 PM
I was just thinking about a good analogy for the relation of matter to the vacuum. Reading the Wikipedia article about virtual particles reinforced to me the idea that matter shouldn't be considering wholly distinct from space itself; as the article says, it's an "excited state" of the vacuum.

So, I thought of a metaphor to make sense of this (at least to myself): imagine the vacuum as a thread or rope, and any kind of particle is a knot in the rope. The particle is part of the vacuum itself. And extending the analogy further, you can create larger structures out of multiple knots, just like you can crochet a Christmas stocking by knotting yarn together, without the need for an independent base like you'd have in needlepoint.

Anyway, this metaphor helps me, maybe it'll help somebody else.

2007-Jan-11, 09:45 PM
<<mass actually warps space around it >>

But WHY does it do this?

LoL if any of us knew the answer to that, well...then....hmm... this is going nowhere. let me put it another way.

If, for instance, I knew the answer, in 50 years all those einstine posters would have my ugly face on them instead :)

2007-Jan-13, 03:41 AM
LoL if any of us knew the answer to that, well...then....hmm... this is going nowhere.

Actually, if you did know the answer to this, you would be going somewhere, Sweden. ;)