# Questions tagged [spacetime]

As a consequence of the Lorentz transformations, time and space transform into each other when changing reference frame. This calls for a unified description: Minkowski spacetime.

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### Ultrastatic spacetime and cosmological constant

A spacetime $(\mathcal{M},g)$ is called "ultrastatic", if it admits a set of coordinates such that $$g=-\mathrm{d}t^{2}+h$$ where $h$ is a Riemannian metric, which does not depend on time. ...
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### What is the Minkowski vacuum state?

I am reading about the Unruh effect in General Relativity. I encounter very often the term Minkowski vacuum state. A typical phrase may be, "observer accelerating in the Minkowski vacuum". ...
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### The temporal extent of matter

In most branches of physics an object such as a particle is assumed to follow a trajectory in spacetime (or in space and time in the case of non-relativistic physics), and therefore at any instant it ...
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### What is the formal criteria that the spacetime is curved?

We suppose we have three scenarios. We are far away from mass and energy in a spot in the universe. We put in free movement a small object $m$, for example, an apple. At the same time, we send a ...
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### How does a field $\phi(x,t)$ have infinite degrees of freedom and why are its inputs labels not variables?

Consider a classical Lagrangian $L(q, \dot{q})$, which by definition has a discrete number of degrees of freedom. Now suppose we have a quantum field (or any field) which we denote by $\phi(x,t)$. It ...
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When you are solving the Einstein field equations (EFE), you basically have to input a stress–energy tensor and solve for the metric. $$R_{\mu \nu} - \frac{1}{2}R g_{\mu \nu} = 8 \pi T_{\mu \nu}$$ ...