24. Quantum electrodynamics – the origin of virtual reality
The central concept the quantum theory introduces in the description of nature is
he idea of virtual particles. Virtual particles are short-lived particles; they owe their
existence exclusively to the quantum of action. Because of the quantum of action, they do
not need to follow the energy-mass relation that special relativity requires of normal, real
particles. Virtual particles can move faster than light and can move backward in time. Des-
pite these strange properties, they have many observable effects.
Ships, mirrors and the Casimir effect
When two parallel ships roll in a big swell, without even the slightest wind blowing, they
will attract each other. This effect was well known up to the 19th century, when many
places still lacked harbours. Shipping manuals advised captains to let the ships be pulled
apart using a well-manned rowing boat.
Figure will be added in the future
Figure 270 Ships in a swell
Wave induce oscillations of ships because a ship absorbs energy from the waves. When
oscillating, the ship also emits waves. This happens mainly towards the two sides of the
ship. As a result, for a single ship, the wave emission has no net effect on its position. Now
imagine that two parallel ships oscillate in a long swell, with a wavelength much larger than
the distance between the ships. Due to the long wavelength, the two ships will oscillate in
phase. The ships will thus not be able to absorb energy from each other. As a result, the
energy they radiate towards the outside will push them towards each other.
The effect is not difficult to calculate. The energy of a rolling ship is
E = mgh α2/2
where α is the roll angle amplitude, m the mass of the ship and g = 9,8m/s2 the acceleration
due to gravity. The metacentric height h is the main parameter characterizing a ship, espe-
cially a sailing ship; it tells with what torque the ship returns to the vertical when inclined
by an angle α . Typically, one has h =1.5 m.
When a ship is