equations:geodesic_equation
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equations:geodesic_equation [2018/04/14 11:08] aresmarrero [Why is it interesting?] |
equations:geodesic_equation [2018/05/05 21:59] jakobadmin [Concrete] |
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| <tabbox Intuitive> | <tabbox Intuitive> | ||
| + | Solutions of the geodesic equations are called geodesics. | ||
| - | <note tip> | + | Geodesics are the "shortest" paths between two points in a flat spacetime and the straightest path between two points in a curved spacetime. |
| - | Explanations in this section should contain no formulas, but instead colloquial things like you would hear them during a coffee break or at a cocktail party. | + | |
| - | </note> | + | |
| | | ||
| + | On a sphere the geodesics are "great circles". | ||
| <tabbox Concrete> | <tabbox Concrete> | ||
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| The Lagrangian for a free point particle in a spacetime $Q$ is | The Lagrangian for a free point particle in a spacetime $Q$ is | ||
| - | \begin{align*} | + | \begin{align} |
| L(q,\dot{q}) &= m\sqrt{g(q)(\dot{q},\dot{q})} \\ | L(q,\dot{q}) &= m\sqrt{g(q)(\dot{q},\dot{q})} \\ | ||
| &= m\sqrt{g_{ij}\dot{q}^i\dot{q}^j} | &= m\sqrt{g_{ij}\dot{q}^i\dot{q}^j} | ||
| - | \end{align*} | + | \end{align} |
| - | just like in [[models:special_relativity|special relativity]] but instead of the [[advanced_tools:minkowski_metric|Minkowski metric]] $\eta_{ij}$, we now have a general metric $g_{ij}$. Alternatively we can use | + | just like in [[theories:special_relativity|special relativity]] but instead of the [[advanced_tools:minkowski_metric|Minkowski metric]] $\eta_{ij}$, we now have a general metric $g_{ij}$. Alternatively we can use |
| \begin{align*} | \begin{align*} | ||
| L(q,\dot{q}) &= \tfrac{1}{2}m g(q)(\dot{q},\dot{q}) \\ | L(q,\dot{q}) &= \tfrac{1}{2}m g(q)(\dot{q},\dot{q}) \\ | ||
| Line 49: | Line 49: | ||
| \end{align*} | \end{align*} | ||
| where the last line follows since $g_{ik}=g_{ki}$. | where the last line follows since $g_{ik}=g_{ki}$. | ||
| - | Now we define the so-called [[advanced_notions:general_relativity:christoffel_symbols|Christoffel symbols]] | + | Now we define the so-called [[advanced_tools:connections:levi_civita_connection|Christoffel symbols]] |
| \[ | \[ | ||
| \Gamma_{ijk} \equiv -\bigl(\partial_{i}g_{jk}-\partial_{k}g_{ij}-\partial_{j}g_{ki}\bigr) | \Gamma_{ijk} \equiv -\bigl(\partial_{i}g_{jk}-\partial_{k}g_{ij}-\partial_{j}g_{ki}\bigr) | ||
| Line 69: | Line 69: | ||
| This action is exactly the proper time when $(Q,g)$ is a Lorentzian manifold, or arclength when $(Q,g)$ is a Riemannian manifold. | This action is exactly the proper time when $(Q,g)$ is a Lorentzian manifold, or arclength when $(Q,g)$ is a Riemannian manifold. | ||
| <tabbox Why is it interesting?> | <tabbox Why is it interesting?> | ||
| - | | + | Almost every problem in classical mechanics can be regarded as geodesic motion. |
| - | Almost every problem in classical mechanics can be regarded as geodesic motion. | + | In the geometric optics approximation light acts like particles tracing out geodesics, i.e. the shortest paths. |
| - | Especially in [[models:general_relativity|general relativity]] particles always follow geodesics. To be precise, a free particle in general relativity traces out a geodesic on the Lorentzian manifold, i.e. spacetime. | + | Especially in [[theories:general_relativity|general relativity]] particles always follow geodesics. To be precise, a free particle in general relativity traces out a geodesic on the Lorentzian manifold, i.e. spacetime. |
equations/geodesic_equation.txt · Last modified: 2018/12/19 11:01 by jakobadmin
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