http://www.ck12.org Chapter 22. The Special Theory of Relativity
FIGURE 22.8
FIGURE 22.9
Classical Newtonian view
The reason that an observer on the train moving with velocityvsees the lightning arrive fromBsooner than fromA
is because the velocity of the light fromBrelative to the observer isc+v. Meanwhile, the velocity of light fromA
relative to the observer isc−v.
A Newtonian, or classical, physicist would claim that the events were simultaneous in both inertial frames (the
embankment and the train), and assert that time is an absolute quantity unchanged by motion.
Modern view
According to a modern physicist the (actual) reason the observer on the train sees the light from pointBsooner than
the light from pointAis because the distance the light must travel fromBto reach the observer is smaller than the
distance the light must travel fromA. (The train is moving toward the light fromBand away from the light atA.)
The velocity of the light relative to the observer isc, regardless if the observer is moving toward or away from the
light. This is the second postulate of the theory of special relativity, as counterintuitive as it may seem to be!
The twin paradox
Let’s say that one of two twins decides to take a relativistic trip, which we define as a trip where the effects of time
dilation cannot be ignored. If the traveling twin leaves when she is 20 years old, traveling at about 0. 87 c, she will
age half as slowly as the twin who remains on Earth. If, after 10 years, as measured by a clock aboard her ship, she
returns, she’ll find that 20 years will have passed on Earth. When the two twins meet again, the traveling twin will
be 30 years old and the stay-at-home twin will be 40 years old.
Does this seem reasonable? The objection is sometimes raised that since all inertial frames are equivalent, the
stay-at-home twin could make the same argument, and thus she should be younger than the traveling twin.