Special Relativity

By Caitlin French

Introduction to Special Relativity

Special relativity, developed by Albert Einstein in 1905, completely transformed our ideas about space and time. Although Newtonian mechanics provides good approximations at low speeds, Einstein corrected mechanics in order to handle situations involving motion near the speed of light. Einstein replaced the Galilean transformations of Newtonian mechanics with the Lorentz transformations in his theory of Special Relativity.

Einstein’s theory is based on two main postulates, from which many interesting things follow. But the main idea of Special Relativity is that, if you move fast enough through space, the observations you make about space and time differ from the observations of other people who are moving at different speeds. 

The Two Postulates of Special Relativity

Special Relativity is based on two postulates:

1. The laws of physics are invariant (identical) in all inertial systems (non-accelerating frames of reference moving at a constant speed).

2. The speed of light (c) in a vacuum is the same in all frames of reference, regardless of motion relative to the light source. This is required for the laws of electrodynamics to apply equally for all frames.

Thermodynamics, Entropy and the Arrow of Time

by Caitlin French

The Arrow of Time

Why does a pane of glass smash but it doesn’t piece itself back together again? Why can an egg scramble but not unscramble? Why does an ice cube melt but it doesn’t spontaneously become solid again? In our everyday macroscopic world, we experience time asymmetry. Time only flows in one, forwards direction, creating the arrow of time. However, physical laws at the microscopic level have time reversal symmetry – it is theoretically possible for events to run both forwards and backwards. If you played a video of a swinging pendulum in a vacuum, you would not be able to tell whether it was running forwards or backwards.

Imagine you filmed a particle falling towards the ground, accelerating downwards due to gravity. If you then watched the film in reverse, the particle would decelerate upwards, which would be possible, provided the particle was given an initial velocity. By giving it an initial velocity, momentum is conserved. This initial velocity in the time-reversed scenario would be provided by the vibrations of atoms as the particle hit the ground in the initial falling scenario. The fact that these vibrating particles have kinetic energy means that energy is conserved in both scenarios as well. This is all theoretically possible. However, we don’t see these time-reversal effects in everyday life. Evidently, there is a conflict between time-reversible microstates and the one-way time of macrostates.