The Special Theory of Relativity, Part 2 – Mass, Energy & Space / Time – Richard Jones

Richard’s presentation on Special Relativity Part 2 began where Part 1 ended, looking at the experience of a person travelling at relativistic speeds. The visual effects are aberration of light from stars around a spacecraft, the Doppler shift and the so-called searchlight effect. The twin paradox of unequal ageing of travellers was briefly mentioned but this is properly in the domain of General Relativity.

Building on ideas of Length Contraction and Time Dilation from Part 1, we considered how Einstein looked at the Principle of Conservation of Momentum. This physical law must apply to all observers, and the only way to ensure this happens is to postulate that as the speed of an object moving relative to an observer increases, so must its mass, as reckoned by that observer. As speed increases, momentum and energy given to an object tend ultimately to increase the mass and not the speed, hence the impossibility of light speed travel – the spacecraft would have to possess infinite mass!

From here, it is possible to see how the famous equation E=mc2 could be developed. Since light, although massless, can transfer momentum, the idea that mass and energy are in some way interchangeable arises. Nuclear fission, initially of the isotope uranium-235, was discovered by Otto Hahn and Lise Meitner in 1939. The total masses of fission products are measurably less that those of a U-235 atom and the initiating neutron. Einstein foresaw that this process, carried out in a chain reaction, would deliver vast amounts of energy, far greater than that supplied by fossil fuel in a power station or by a chemical high explosive in a bomb. He wrote a now famous letter to President Roosevelt, warning him of Hitler’s interest in fission and urging him to begin research on fission in the USA.

Relativistic mass increase has importance in particle accelerators, such as the Diamond Light Source. It also has an important bearing on the possibility of space travel at relativistic speeds, partly because of vast energy/fuel implications and also because of the huge amounts of energy released in collisions in interstellar gas with overweight hydrogen atoms. This is compounded by excessively blue Doppler shifted Cosmic Background Radiation commuted to lethal gamma rays.

The talk concluded with an examination of geometry in four dimensions and Hermann Minkowski’s important work in proposing that four-dimensional Spacetime would permit a comprehensive understanding of Einstein’s ideas. His equations give a Spacetime Interval, which is the same for all observers when their measurements of distance and time vary. Spacetime diagrams in two dimensions or two dimensional representations of a three dimensional picture show the path of light as 45 degree lines or light cones. There is a time-like future, limiting the possibility of travel to and the influencing of events in the future. We have a light cone of the past, which most believe is inaccessible if backwards time-travel is not allowed. Within these regions alone all World Lines must lie and the Principle of Causality applies. The space-like region of Elsewhere on these diagrams is inaccessible since Cause would have to communicate with Effect faster than the speed of light.

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