# Space and Earth's (and other planet's) movement through it

1. Jul 14, 2013

### Menaus

My question stems from the idea from GR, that space is a physical quantity bended by gravity. The question is this:

If space be a tangible, physical, thing, in which gravity effects it, among other things, then shouldn't the Earth's movement(and the movement of other bodies) be dragged by this space? Additionally, we have dark matter and dark energy, both of which are masses which should impede Earth's movement along its orbit.

Are space, and dark matter and energy, so insignificant that they are essentially moving through the Earth? Or do they drag as the Earth moves through all of it? Or a combination of both?

2. Jul 14, 2013

### Staff: Mentor

3. Jul 14, 2013

### Bill_K

The idea that space is a "thing" is one of those intuitively appealing analogies which only serves to mislead. Space does not behave like something tangible, and there is nothing to be gained from imagining that it does. It does not have any of the properties of a material thing.

If you say that space is "bent and stretched" by gravitational fields, then what are its elastic coefficients?

Likewise the terminology "frame dragging" does not imply that space is viscous! If the analogy held, one could imagine that frame dragging would cause the Kerr solution to slow down and eventually come to a stop.

4. Jul 15, 2013

### Menaus

Well, often we try to apply properties to space, this tends to indicate that indeed it is a 'thing', because otherwise we would be applying properties to that which doesn't exist.

Read Hume's treatise on Human Knowledge. Within, it is said that space has properties which can be measured, therefor space is physical. If it wasn't a 'thing' we wouldn't be able to measure it. I don't see what bars us from the idea that space is some sort of physical entity, as much in the Standard Model seems to imply it.

5. Jul 15, 2013

### The_Duck

The curvature of spacetime influences the motion of objects, but you shouldn't imagine that space exerts any sort of "drag" on objects. For one thing, this would violate the principle of relativity, which tells us that objects in uniform motion through space are allowed to regard themselves as being at rest, with the rest of the universe moving around them. Objects at rest certainly shouldn't feel any drag from space, so objects in motion shouldn't either.

Dark matter shouldn't offer any appreciable resistance to the Earth's motion because it is supposed to pass right through regular matter most of the time.

Dark energy is much more mysterious. It may just be an extra term in the equations of GR that influences the evolution of spacetime. Such a term would not exert any sort of drag on Earth's motion through spacetime.

If you want to call space a "physical entity," that's fine. Just don't get misled by the phrase "physical entity" into thinking that space should cause some drag on motion, or something like that. It doesn't. By your definition, a "physical entity" is just "something with measurable properties" and that does not imply that a "physical entity" has to have *familiar* properties like drag or friction.

Spacetime has the familiar property of "curvature" but none of the other properties of material things, like friction, viscosity, elasticity, hardness, density, inertia, mass, speed, momentum, color, or opacity. This is why Bill_K wants to discourage you from thinking of spacetime as a material thing.

Last edited: Jul 15, 2013
6. Jul 15, 2013

### Bill_K

Menaus, This forum is for the discussion of physics, not philosophy. If you want to expound on David Hume and his opinions about the properties of space, please do it elsewhere.