# Tag Info

### Where does gravity come from?

Physics is complicated and has produced a large number of concepts in a short time. This has led to a lot of reuse of words where the same word is given substantially different meanings in different ...
• 64.2k

### Where does gravity come from?

Dale has already contributed a good answer, but I'd like to supplement it. It's important not to confuse the terrain for the map. At a fundamental level, "gravity" is the word we use to ...
• 5,619

### Why doesn’t horizon distance move exactly proportional to the height of the observer?

Consider the following image showing the earth (with radius $R$) and an observer at height $h$ above the ground. The distance from the observer to the horizon is $s$. The theorem of Pythagoras ...
• 25.6k

### Why doesn’t horizon distance move exactly proportional to the height of the observer?

On earth, the distance to the horizon, say $d_h$ and the height of an observer, say $h_o$ cannot have a linear relationship $$d_h=\text{constant}\cdot h_o$$ or proportional relationship you speak of, ...
• 24.8k

### How do we figure out what is the right geometry of space?

Don't we we already have a geometry of space time as soon as we write down the Minkowski metric? Yes, but that's putting the cart before the horse. Einstein's equations are a system of differential ...
• 50.6k

### Does the spacetime curvature in the vicinity of a massive body increase, decrease or remain unchanged with the increasing velocity of an observer?

The curvature of the spacetime geometry most often is expressed in terms of curvature invariant, examples are: the Kretschmann scalar, the Chern–Pontryagin scalar and the Euler scala. Look at: https://...
• 103
1 vote

### Where does gravity come from?

Gravity is a mathematical construct used to describe objects falling down. Newton wanted to be able to make a mathematical description of the motion of free falling objects, and this particular ...
• 101
1 vote
Accepted

### Do Einstein field equations only relate local spacetime curvature to local energy-momentum of matter?

Yes, they cannot be extended to relate global curvature to global energy-momentum, not in general at least. You can see this by noting that the Einstein Field Equations can be derived by demanding ...
• 1,964
1 vote

### What is the significance of commutator relationships in physics, e.g. $qp-pq = i \hbar$, $R(X, Y)Z = \nabla_X\nabla_Y Z - \nabla_Y\nabla_X Z$, etc?

Differential forms For the Riemann tensor and the thermodynamics quantities, the common point is that those object are differential forms on a manifold (for the Riemann tensor, this is not explicit in ...
• 3,424
1 vote
Accepted

### Why is the flatness problem called the "flatness" problem? What is its connection to geometry?

Relation between curvature $k$ and density parameter $\Omega$ can be described with 1st Friedmann equation. $$(\frac{\dot{a}}{a})^2 +\frac{kc^2}{a^2} = \frac{ 8\pi G }{3}\rho$$ Define Hubble parameter ...
• 193

Only top scored, non community-wiki answers of a minimum length are eligible