以下為個人讀書筆記，屬節錄摘要類。為避免涉及版權問題，特註明出處，包括作者、出版社、章節。

[The Void] by Frank Close, Oxford University Press

"Field"

‘Warp fields in the space-time continuum’. The jargon suggests that there is a lot going on out there in the supposed void.

A map of air pressure is an example of what mathematicians know as a field.

If all that is needed to define the field is a collection of numbers, it is known as a scalar field.

A map of wind speed is an example of what is known as a vector field. This involves both number and direction at each point.

In the case of the atmospheric pressure and the winds we have a physical medium, the air, whose varying density determines the fields.

The concept of ‘field’ applies even when there is no material medium. This is the idea behind the gravitional and electric fields, which give magnitude and direction of the respective forces throughtout space.

The loss of potential energy is compensated by the rise in kinetic energy. This is a universal law.

The same is true for electric charge and electric field. High voltage equates to high potential.

In SLAC electrons are accelerated to 300,000 km a second. They pass through 30 billion volts in 3 km, which equates to electric fields of 10 million volts per meter.

At SLAC the electric field is ten volts in each millionth (0.000001) of a meter; inside an atom of hydrogen, the electric field is ten volts in each tenth of a billionth (0.0000000001) of a meter. The fields within atoms are over ten thousand times greater than we can achieve in macroscopic technology.

It is within the confines of the atom that the full power of unshielded opposite charges is realized.

Electric charges in motion give rise to magnetic forces whose effects can spread over vast distances as in the case of the magnetic field of the Earth. The molten iron core of our planet swirls as we rotate, the heat disrupting its atoms so that their electrons flow freely. The resulting electric currents make the Earth into a huge magnet with north and south poles, and with magnetic arms that stretch out into space.

Cosmic rays and the solar wind of electrically charged particles from the Sun are deflected by these magnetic forces. This is a crucial protective shield, as exposure to these radiations would destroy our DNA.  Were the magnetic field to disappear, as is the case on Mars, it could be terminal to our species.