A Brief History of Time

by Stephen Hawking

A Merry-Go Round Buffet of the World of Physics

Hawking will take you at pace through all of physics most important ideas, through classical, atomic/nuclear, relativity & quantum theories, culminating in explorations of the time’s interesting ideas, of blackholes etc. All the way up to the exciting pursuit of a general, unifying theory, which Hawking prophesises to be close.

It is concise, and sometimes leaves plenty unsaid, but never inaccessible or hard to follow. You could pick this up with little to know scientific knowledge and do just fine, with Hawking’s guidance.

Interference, Waves, & Feynman’s Stopwatch

In QED by Richard Feynman the idea of the stopwatch, later the rotating “amplitude” are provided as the way to determine the direction of the amplitudes which combine to make the final probability of an event, the movement of an electron or photon (and other particles). The principle is that when the amplitudes point in the same direction, they add to create a larger final amplitude, a larger probability. Feynman demonstrates how this happens when events take roughly equally lengths of time to happen, because the rotation is almost the same in those cases. Conversely, drastically different amounts of time will result in arrows pointing in opposing directions, adding up to a much smaller final arrow.

Hawking explains how this is a visualisation, a way of thinking about “interference”, which is the cancelling out of waves. When the trough of one wave meets the peak of another, they can cancel out. This is exactly what Feynman’s stopwatch is showing: waves peaking vs trough-ing is caused when they’ve taken different amounts of time to come from the source, one slightly more of a wavelength than the other. The stopwatch would point in the opposite direction.

An example Hawking gives is when light reflects back and forward through a bubble. As the waves go back and forward, a peak might meet a trough passing one another, as they aren’t “in sync” with the different amounts of “bouncing” they’ve done. They might cancel out in such a case. This is why we see colours on the film of a bubble that light is shining through.

This is the principle of interference in quantum physics.

Occasionally I send out an idea & ask for your thoughts.