### Relativity and the Quantum / Elementary Tour part 4: Loop quantum gravity

From the point of view of Einstein´s theory, it comes as no surprise that all attempts to treat gravity simply like one more quantum force (on par with electromagnetism and the nuclear forces) have failed. According to Einstein, gravity is not a force – it ...

### Relativity and the Quantum / Elementary Tour part 3: The need for quantum gravity

So far in the pages of Elementary Einstein, we have encountered two examples in which the limits of general relativity were reached. Both cases involved space-time-singularities. The first example lurked in the interior of a black hole. As briefly described ...

### Cosmology / Elementary Tour part 3: The early universe

An expanding universe, with the distances between galaxy increasing all the time, must have been much more dense, and the galaxies much closer together, in the past. The details follow from Einstein's equations which connect the way expansions runs its course ...

### General relativity / Elementary Tour part 1: Einstein’s geometric gravity

The key idea of Einstein's theory of general relativity is that gravity is not an ordinary force, but rather a property of spacetime geometry. The following simplified analogy, which substitutes a two-dimensional surface for four-dimensional spacetime, serves ...

### General relativity / Elementary Tour part 2: The cosmic dance

In this new picture, there is no gravitational force that masses exert on other masses. Instead, there are spacetime distortions. Spacetime in the presence of a mass is curved. In flat, empty spacetime, small test particles follow straight lines. However, ...