D O C . 1 3 D I A L O G U E A B O U T R E L A T I V I T Y T H E O R Y 6 9

1. Clock gets accelerated by

an external force along the positive -

axis until it attains the velocity v.

remains at rest.

1. There arises a gravitational field in

the direction of the negative x-axis, in

which the clock falls accelerated

until it attains the velocity v. An exter-

nal force along the positive x-axis acts

upon the clock and prevents it

from moving. The gravitational field

vanishes again as soon as clock

has attained the velocity v.

2. moves with constant veloc-

ity v until it reaches point on the

positive -axis. remains at rest.

2. moves with constant veloc-

ity v until it reaches point on the

negative -axis. remains at rest.

3. Clock gets accelerated by

an external force acting along the neg-

ative -axis until it attains the veloc-

ity v in the negative -direction.

remains at rest.

3. There arises a homogeneous

gravitational field in the direction of

the positive x-axis, in which the clock

gets accelerated in the direction of

the positive x-axis until it has attained

the velocity v, whereupon the gravita-

tional field again vanishes. An exter-

nal force along the negative x-axis

prevents the clock from making

any movement in this gravitational

field.

4. moves back with constant

velocity v in the direction of the nega-

tive -axis close to remains at

rest.

4. moves with constant veloc-

ity v in the direction of the positive -

axis close to . remains at rest.

5. An external force brings to

rest.

5. There arises a gravitational

field in the direction of the negative

-axis which brings clock to rest,

whereupon the gravitational field van-

ishes again. During this time is

kept at rest by an external force.

U

2

x

U

1

U1

U2

U1

U2

B

x

U1

U1

B′

x

U2

U2

x

x

U1 U1

U2

U2

[p. 699]

x

U1. U1

U1

x

[7] {1}

U2 U2

U2

x

U1

U2