S E C T I O N 3 9 . 4 • Consequences of the Special Theory of Relativity
1257
The Twin Paradox
An intriguing consequence of time dilation is the so-called twin paradox (Fig. 39.10).
Consider an experiment involving a set of twins named Speedo and Goslo. When they
are 20 yr old, Speedo, the more adventuresome of the two, sets out on an epic journey
to Planet X, located 20 ly from the Earth. (Note that 1 lightyear (ly) is the distance
light travels through free space in 1 year.) Furthermore, Speedo’s spacecraft is capable
of reaching a speed of 0.95c relative to the inertial frame of his twin brother
back home. After reaching Planet X, Speedo becomes homesick and immediately
returns to the Earth at the same speed 0.95c. Upon his return, Speedo is shocked to
discover that Goslo has aged 42 yr and is now 62 yr old. Speedo, on the other hand,
has aged only 13 yr.
At this point, it is fair to raise the following question—which twin is the traveler and
which is really younger as a result of this experiment? From Goslo’s frame of reference,
he was at rest while his brother traveled at a high speed away from him and then came
back. According to Speedo, however, he himself remained stationary while Goslo and
the Earth raced away from him and then headed back. This leads to an apparent
Suppose you are driving your car on a business trip and
are traveling at 30 m/s. Your boss, who is waiting at your
destination, expects the trip to take 5.0 h. When you
arrive late, your excuse is that your car clock registered
the passage of 5.0 h but that you were driving fast and so
your clock ran more slowly than your boss’s clock. If your
car clock actually did indicate a 5.0-h trip, how much time
passed on your boss’s clock, which was at rest on the
Earth?
Solution We begin by calculating * from Equation 39.8:
!
1
√
1 # 10
#
14
* !
1
√
1 #
v
2
c
2
!
1
√
1 #
(3 $ 10
1
m/s)
2
(3 $ 10
8
m/s)
2
If you try to determine this value on your calculator, you
will probably obtain * ! 1. However, if we perform a
binomial expansion, we can more precisely determine the
value as
This result indicates that at typical automobile speeds, * is
not much different from 1.
Applying Equation 39.7, we find 't, the time interval
measured by your boss, to be
'
t ! * 't
p
!
(1 & 5.0 $ 10
#
15
)(5.0 h)
!
5.0 h & 2.5 $ 10
#
14
h !
Your boss’s clock would be only 0.09 ns ahead of your car
clock. You might want to think of another excuse!
5.0 h & 0.09 ns
* !
(1 # 10
#
14
)
#
1/2
% 1 &
1
2
(10
#
14
) ! 1 & 5.0 $ 10
#
15
Example 39.2 How Long Was Your Trip?
(a)
(b)
Speedo
Goslo
Speedo
Goslo
Figure 39.10 (a) As one twin leaves his brother on the Earth, both are the same age.
(b) When Speedo returns from his journey to Planet X, he is younger than his twin Goslo.