### 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 ...

### 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 ...

### Special relativity / Elementary Tour: Conclusion

As this brief tour of special relativity has shown, we have to re-think our notions of space and time in Einstein's world. Moving clocks tick at a slower rate, light speed is the same for all (inertial) observers, and lengths and distances depend on who ...

### Special relativity / Elementary Tour part 5: Spacetime

"All the world's a stage." - that's how we're used to viewing space: As a stage on which objects are located and where the dramas of their movements and evolution take place. In special relativity, as was mentioned briefly, simultaneity is relative, and so ...

### Special relativity / Elementary Tour part 4: The speed of light

On the previous pages, relativity reigned supreme. Although we usually think of lengths and times as absolute, they turned out to be observer-dependent. On this page, the shoe is on the other foot. Ordinarily, we think of velocities as relative, but one of ...

### Special relativity / Elementary Tour part 3: The relativity of space and time

One of the most surprising features of special relativity is that a number of statements and results which we usually think to be absolute turn out to be observer-dependent. In particular, statements about space and time, distances and duration turn out to be ...

### 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, ...