Chapter 6.1 Killing grandmothers

Using faster-than-light velocities we could telegraph into the past. (Einstein, quoted by Sommerfeld, 1908) If you could travel faster than light, you could kill your grandmother before she gave birth to your mother. Since this is impossible, we have a second argument for the celestial speed limit. The strategy of the argument is a reductio … Read more

Chapter 5.4 The block universe argument

Some physicists believe that relativity theory has proved that the past and future exist in a giant four-dimensional block universe. Although his views changed during his career, Einstein, for example, made the following statement in 1952, a few years before he died. He argued that the relativity of simultaneity implies a block universe: The four-dimensional … Read more

Chapter 5.3 Spacetime

Upon giving up the hypothesis of the invariant and absolute character of time, particularly that of simultaneity, the four- dimensionality of the time–space concept was immediately recognised. It is neither the point in space, nor the instant in time, at which something happens that has physical reality, but only the event itself. There is no … Read more

Chapter 4.4 Chain reactions

Atomic bombs are dangerous because they are so simple. As far as we know, every nation that has attempted to build and explode them has succeeded. Relatively crude technology will do. The first bombs were built in the early 1940s before transistors and computers were invented. Think of an old radio or automobile from that … Read more