Gravity and the rubber membrane analogy

Miriam English

I was reading a theory about space itself being quantized. Very interesting. It has some cool ramifications. But the author made reference to gravity curving space and mentioned the rubber membrane analogy. This is one of my pet peeves.

Physicists often explain gravity as the bending of space, but it doesn't make sense. Einstein used the analogy of a rubber membrane being curved by the weight of objects as a way to visualise the effect of gravity on things rolling across the membrane in the vicinity of the weights. It is often cited as a way to understand gravity itself and the curving of space. But it does neither of those things. It is only useful as a way to understand the effect of gravity on motion. It can't help to understand gravity itself because the rubber membrane analogy uses gravity in its explanation. The membrane is deformed by the weight of objects under gravity and the projectiles crossing the membrane take curved courses because gravity affects them. You can't use something to describe itself -- the description becomes circular and worthless.

Describing gravity as curved space doesn't even fit the facts. If gravity curves space, or curved space is what gravity is, then a straight line that gets bent by gravity would be the path followed by fast and slow objects alike, but in fact the high speed paths are curved less than low speed ones. It isn't space that gravity is affecting, it is the objects that have mass.

In any case saying that gravity is the curving of space is useless -- how does space come to be curved by an object? You might as well say that nearby objects are attracted by gravity -- this is a simpler, easier statement to investigate.. Saying that gravity curves space uses a common religious trick: the explanation is a nonsense, which blocks further questions by confounding the inquirer.


2004-08-27