The Manga Guide to Relativity

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

seing 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

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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

actuay 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 aarent

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.

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