We must note carefully that the possibility of this mode of
interpretation rests on the fundamental property of the gravitational
field of giving all bodies the same acceleration, or, what comes to
the same thing, on the law of the equality of inertial and
gravitational mass. If this natural law did not exist, the man in the
accelerated chest would not be able to interpret the behaviour of the
bodies around him on the supposition of a gravitational field, and he
would not be justified on the grounds of experience in supposing his
reference-body to be " at rest."
Suppose that the man in the chest fixes a rope to the inner side of
the lid, and that he attaches a body to the free end of the rope. The
result of this will be to strech the rope so that it will hang "
vertically " downwards. If we ask for an opinion of the cause of
tension in the rope, the man in the chest will say: "The suspended
body experiences a downward force in the gravitational field, and this
is neutralised by the tension of the rope ; what determines the
magnitude of the tension of the rope is the gravitational mass of the
suspended body." On the other hand, an observer who is poised freely
in space will interpret the condition of things thus : " The rope must
perforce take part in the accelerated motion of the chest, and it
transmits this motion to the body attached to it.
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