### Special relativity / Elementary Tour part 6: E=mc²

Now that the new features of space and time are sufficiently explored, it's time to examine how this affects the way objects move, either freely or when influenced by forces. In other words: How does relativistic mechanics work? This question has led to the ...

### 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 ...

### Special relativity / Elementary Tour part 2: The principle of relativity

If you think about space stations drifting along in empty space, some statements that are surely relative spring to mind right away: statements about velocities. Imagine that, from the point of view of observer A sitting on the upper deck of his or own space ...

### Gravitational waves / Elementary tour part 1: The rhythm of geometry

Distortions of geometry: what does that mean? First of all, distances shrink and expand in a certain coordinated way. That's the main mechanism by which gravitational waves act on the rest of the world: they rhythmically distort distances between freely ...

### Relativity and the Quantum / Elementary Tour part 1: Relativity in the micro-world

Quantum theory and relativity theory are two products of the early 20th century, developed in parallel. Not surprisingly, physicists early on began thinking about the possibility of combining the two. Could one, for instance, formulate a relativistic theory ...

### 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 ...