What Do You Mean, Time Slows Down? 69
Why does this
haen?
Haven't we covered this
before? Reca that the
special theory of relativity
can only be used for an
inertial frame with uniform
linear motion (a constant
velocity).
now that you
mention it...
you had said that
the thing that was
"special" about
special relativity...
was the special
seing in which "the
observer is not
subject to gravity or
aeleration."
Special theory of relativity
To return to Earth,
the rocket must
turn around.
When it turns
around, it caot
procd at a fixed
spd but must
decelerate and
aelerate.
Observer
screch
RELATIVITY_03.indb 69 3/15/2011 3:26:47 PM
70 Chapter 2
Ah...
It's coming
back!
In other words, when the
rocket turns around, the
crdinate system of the
older sister who is riding
in it no longer is an inertial
frame, and special relativity
caot be alied.
If special relativity
caot be alied to
the older sister's
crdinate system,
what haens?
In that case, the
general theory
of relativity wi
be used.
I briefly mentioned
at first that with
general relativity,
time slows down if
strong gravity is
alied, right?
Gravity
Time
That rings
a be...
Why did you
do that...?
I just
like to.
RELATIVITY_03.indb 70 3/15/2011 3:26:48 PM
To make it easier
to understand, let's
prepare this kind of
rocket.
It's perfectly
round and has
engines facing
both forward
and backward.
Crdinate system in which Earth is at rest
Velocity
rocket moves at a fixed velocity
Aeleration
Velocity
decreases
rocket blasts its engines to
decelerate and reverse direction.
Velocity
becomes
zero
Velocity briefly becomes zero as
rocket reverses direction.
Now velocity
increases in
the return
direction.
rocket stops blasting its engines
when it reaches a fixed velocity
backtoward earth.
When the rocket
turns around to
return to Earth...
The older sister inside fls
as if she is being subjected
to strong gravity rather
than thinking that she is
being decelerated and
aelerated.
But The rocket
actuay decelerates
and aelerates to
turn around.
The older sister
observes that Earth
changes direction
because of that gravity.
In other words, she
observes it faing in
her direction.
From the older
sister's viewpoint,
it's Earth that is
faing.
G
r
a
v
i
t
y
G
r
a
v
i
t
y
RELATIVITY_03.indb 71 3/15/2011 3:26:49 PM
72 Chapter 2
She observes Earth's clock
advancing faster than her own
clock at this time.
Slow
Sluish
Fast
tick
The "spding up" of Earth's
clock is greater than the
"slowing down" that she
observed while Earth was
advancing at a fixed spd.
Welcome
back!
The rocket's
aeleration is also
gravity?
Although I'm going
to omit a diicult
explanation about
that now...
...the equivalence principle
states that the aarent
force that a person in the
rocket is subjected to due
to the rocket's aeleration
is considered to be the
same as gravity.
That was a foundational
principle Einstein used
when he created the
general theory of
relativity.
RELATIVITY_03.indb 72 3/15/2011 3:26:50 PM

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