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TH!: PROBLEM OF SPA-CE. ETHER. AND THE FIELD IN PHYSICS 281
Since the interaction of bodies was supposed to be accomplished
through fields, there had also to be a gravitational
field in the ether, whose field-law had, however, assumed no
clear form at that time. The ether was only supposed to be the
seat of all forces acting across space. Since it had been realized
that electrical masses in motion produce a magnetic field, whose
energy provided a model for inertia, inertia also appeared as a
field-action localized in the ether.
The mechanical properties of the ether were at first a mystery.
Then came H. A. Lorentz's great discovery. All the phenomena
of electromagnetism then known could be explained on the
basis of two assumptions: that the ether is firmly fixed in spacethat
is to say, unable to move at all, and that electricity is firmly
lodged in the mobile elementary particles. Today his discovery
may be expressed as follows: physical space and the ether are
only different terms for the same thing; fields are physical states
of space. For if no particular state of motion can be ascribed
to the ether, there does not seem to be any ground for introducing
it as an entity of a special sort alongside of space. But the
physicists were still far removed from such a way of thinking;
space was still, for them. a rigid, homogeneous something, incapable
of changing or assuming various states. Only the genius
of Riemann, solitary and uncomprehended, had already won its
way by the middle of the last century to a new conception of
space, in which space was deprived of its rigidity, and the possibility
of its partaking in physical events was recognized. This
intellectual achievement commands our admiration all the more
for having preceded Faraday's and Maxwell's field theory of
electricity. Then came the special theory of relativity with its
recognition of the physical equivalence of all inertial systems.
The inseparability of time and space emerged in connection
with electrodynamics, or the law of the propagation of light.
Hitherto it had been silently assumed that the four-dimensional
continuum of events could be split up into time and space in
an objective manner-i.e., that an absolute significance attached
to the "now" in the world of events. With the discovery of
the relativity of simultaneity, space and time were merged in a
single continuum in a way similar to that in which the three