According to
Einstein's Theory of
Special Relativity, moving objects appear shorter (that is,
compressed along their direction of
motion) than they do
at rest. This is only visible for objects travelling at near
relativistic speeds.
Consider a space ship travelling at 0.8c (approximately 2.4 * 108
m/s) between Earth and Venus (approximately 5 * 1012 m) (to the
astronauts, it appears that Earth is receeding at 0.8c and Venus is approaching at
0.8c). A stationary observer (on Earth, for example) would estimate that the ship
would take roughly 5.8 hours (time = distance/velocity) to complete the trip. Because of time dilation, however, people on the space ship would measure an approximate elapsed time of only 3.5 hours.
Because the astronauts measured their velocity to be the same as that measured from Earth, they
would calculate the distance between Earth and Venus to be only 3 * 1012 m
(again, from time = distance/velocity).
The equation for length contraction is written:
L = L0 √(1 - v2/c2)
Where:
L = measured length of the object
L0 = length of the object at rest
v = velocity of the object
c = speed of light
Consider an
18-wheeler pulling a standard 52-foot (
15.85 m) long
trailer. According to the Theory of
Special Relativity that
trailer will appear compressed, depending on its
velocity:
100 km/h (5.56 * 10-6c) -- 99.9999999985% of normal (15.85 m)
10000 km/h (5.56 * 10-4c) -- 99.9999845465% of normal (15.85
m)
1000000 km/h (0.0556c) -- 99.8453456802% of normal (15.83 m)
10000000 km/h (0.556c) -- 83.1222316823% of normal (13.17 m)
15000000 km/h (0.834c) -- 55.1900095096% of normal (8.74 m)
17807672 km/h (0.99c) -- 14.1067359797% of normal (2.24 m)
17987367 km/h (0.99999c) -- 0.447212477465% of normal (0.07 m)
PhasedWeasel has contributed an interesting writeup in
The Barn And The Pole: A Relativity Paradox that discusses loss of
simultaneity.