curved space strikes again
Sunday, 7 March 2010 06:48 am![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
miriam_eMost of us tend to think in visual metaphors. It is a very useful thing to do. The visual processing areas of our brains are some of the most complex. They are capable of constructing very sophisticated models. It doesn't mean that they are infallible though. Being able to look to "experts" for information is useful too. It lets us use a kind of "division of labor" for knowledge. Collectively we can know more if each person tends to learn about different things than those learned by others. Unfortunately experts are just people and are prey to the same kinds of mistakes that the rest of us are.
I was reading something recently in which a physicist mentioned "curved space" as explaining gravity. This seems to be a very common thing for physicists to do. It comes from Einstein's visual metaphor for gravity as altering the movement of moving objects the way heavy spheres will roll across a large rubber membrane on which other heavy spheres sit. Each sphere pulls the membrane down, distorting it and changing the way objects roll across it. The membrane is supposed to represent space under the action of gravity. This is a very powerful image. It sticks in the mind. Unfortunately people have confused "metaphor" with "explanation".
Saying that gravity is the curving of space explains nothing, because in Einstein's metaphor gravity curves the rubber membrane and acts on the objects rolling across it. The membrane pushes back against these objects. This coupled with gravity deflects their motion.
When applying this metaphor to the real world the membrane is used to represent space being distorted under the effect of gravity -- space is said to be curved. But this is a nonsense. It explains nothing. Not only does it not explain gravity (gravity is still needed to affect the objects and space) it adds another peculiarity to be explained: curvature of space! Simple stretching of space can't explain gravity because objects with different masses and/or speeds curve through space to different degrees. If it was simply that space was curved then they would all follow the same curved path because each would feel it was following a straight line. Softly thrown balls would curve to the ground the same way strongly thrown balls do. But they don't. In common language we explain that by saying that gravity pulls the object, while inertia wants it to maintain a straight line. Nobody really believes that space is curved because the idea breaks down in ordinary life. Belief that gravity is explained by curved space requires us to hold two contradictory ideas simultaneously, and unfortunately we humans are notorious for our ability to do that.
In the membrane metaphor the rubber pushes back against gravity pulling down. If we really want something to be curved as an explanation for gravity then we must say that the universe is filled with two things that push against all objects in opposing directions at right angles to the three dimensions we know. In free space those two forces cancel out, but near to massive objects one of them gets distorted causing motion to be deflected, in our three ordinary dimensions, toward the mass. Okay. This fits all the parts of the rubber membrane metaphor so that it is closer to being an actual explanation. Unfortunately, far from explaining gravity away, gravity remains as mysterious a force as ever, but now it acts through another dimension and we need a whole other mysterious force to complete things. Even then space itself is not curved, unless you are talking about curvature through another dimension of the action of one of these two mysterious forces.
To say that gravity is curved space explains nothing. It is the merest beginning of a complicated way of explaining an unknown by turning it into three other unknowns. And it still doesn't explain why one of those would alter near to masses. Sounds like epicycles to me.*
I've never heard anybody think this through to its conclusions. Yet it is very common to hear undeniably smart people simply say that space is curved and that this explains gravity. It is far too easy to defer to experts, when all history shows us how terribly wrong they can be on occasion. This is one of those occasions. "Curved space" is used to confound thought, not enhance it.
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* epicycles -- the planets appear to follow complicated movements over the sky, like the movements of those reflectors in the spokes of bicycles at night. This confounded astronomers for a long time when the Earth was thought to lie at the center of the universe. To make the planets' movements fit calculations they kept having to add epicycles. Of course when it was realised that we live on an orbiting planet just like all the other planets, the planetary motions became simple, smooth orbits. Using two mysterious forces acting in another mysterious dimension to account for one mysterious force sounds a little like adding epicycles to planetary motion. The people who believed in epicycles were not stupid; they were simply trapped in a mistake. We humans do it all the time.
I was reading something recently in which a physicist mentioned "curved space" as explaining gravity. This seems to be a very common thing for physicists to do. It comes from Einstein's visual metaphor for gravity as altering the movement of moving objects the way heavy spheres will roll across a large rubber membrane on which other heavy spheres sit. Each sphere pulls the membrane down, distorting it and changing the way objects roll across it. The membrane is supposed to represent space under the action of gravity. This is a very powerful image. It sticks in the mind. Unfortunately people have confused "metaphor" with "explanation".
Saying that gravity is the curving of space explains nothing, because in Einstein's metaphor gravity curves the rubber membrane and acts on the objects rolling across it. The membrane pushes back against these objects. This coupled with gravity deflects their motion.
When applying this metaphor to the real world the membrane is used to represent space being distorted under the effect of gravity -- space is said to be curved. But this is a nonsense. It explains nothing. Not only does it not explain gravity (gravity is still needed to affect the objects and space) it adds another peculiarity to be explained: curvature of space! Simple stretching of space can't explain gravity because objects with different masses and/or speeds curve through space to different degrees. If it was simply that space was curved then they would all follow the same curved path because each would feel it was following a straight line. Softly thrown balls would curve to the ground the same way strongly thrown balls do. But they don't. In common language we explain that by saying that gravity pulls the object, while inertia wants it to maintain a straight line. Nobody really believes that space is curved because the idea breaks down in ordinary life. Belief that gravity is explained by curved space requires us to hold two contradictory ideas simultaneously, and unfortunately we humans are notorious for our ability to do that.
In the membrane metaphor the rubber pushes back against gravity pulling down. If we really want something to be curved as an explanation for gravity then we must say that the universe is filled with two things that push against all objects in opposing directions at right angles to the three dimensions we know. In free space those two forces cancel out, but near to massive objects one of them gets distorted causing motion to be deflected, in our three ordinary dimensions, toward the mass. Okay. This fits all the parts of the rubber membrane metaphor so that it is closer to being an actual explanation. Unfortunately, far from explaining gravity away, gravity remains as mysterious a force as ever, but now it acts through another dimension and we need a whole other mysterious force to complete things. Even then space itself is not curved, unless you are talking about curvature through another dimension of the action of one of these two mysterious forces.
To say that gravity is curved space explains nothing. It is the merest beginning of a complicated way of explaining an unknown by turning it into three other unknowns. And it still doesn't explain why one of those would alter near to masses. Sounds like epicycles to me.*
I've never heard anybody think this through to its conclusions. Yet it is very common to hear undeniably smart people simply say that space is curved and that this explains gravity. It is far too easy to defer to experts, when all history shows us how terribly wrong they can be on occasion. This is one of those occasions. "Curved space" is used to confound thought, not enhance it.
----
* epicycles -- the planets appear to follow complicated movements over the sky, like the movements of those reflectors in the spokes of bicycles at night. This confounded astronomers for a long time when the Earth was thought to lie at the center of the universe. To make the planets' movements fit calculations they kept having to add epicycles. Of course when it was realised that we live on an orbiting planet just like all the other planets, the planetary motions became simple, smooth orbits. Using two mysterious forces acting in another mysterious dimension to account for one mysterious force sounds a little like adding epicycles to planetary motion. The people who believed in epicycles were not stupid; they were simply trapped in a mistake. We humans do it all the time.