s the same thing as space
when we take the earth as at rest, we ought to find that the velocity of
light relatively to the earth varies according to the direction from
which it comes.

These observations on earth constitute the basic principle of the famous
experiments designed to detect the motion of the earth through the
ether. You all know that, quite unexpectedly, they gave a null result.
This is completely explained by the fact that, the space-system and the
time-system which we are using are in certain minute ways different from
the space and the time relatively to the sun or relatively to any other
body with respect to which it is moving.

All this discussion as to the nature of time and space has lifted above
our horizon a great difficulty which affects the formulation of all the
ultimate laws of physics--for example, the laws of the electromagnetic
field, and the law of gravitation. Let us take the law of gravitation
as an example. Its formulation is as follows: Two material bodies
attract each other with a force proportional to the product of their
masses and inversely proportional to the square of their distances.
In this statement the bodies are supposed to be small enough to be
treated as material particles in relation to their distances; and we
need not bother further about that minor point. The difficulty to which
I want to draw your attention is this: In the formulation of the law one
definite time and one definite space are presupposed. The two masses are
assumed to be in simultaneous positions.

But what is simultaneous in one time-system may not be simultaneous in
another time-system. So according to our new views the law is in this
respect not formulated so as to have any exact meaning. Furthermore an
analogous difficulty arises over the question of distance. The distance
between two instantaneous positions, _i.e._ between two event-particles,
is different in different space-systems. What space is to be chosen?
Thus again the law lacks precise formulation, if relativity is accepted.
Our problem is to seek a fresh interpretation of the law of gravity in
which these difficulties are evaded. In the first place we must avoid
the abstractions of space and time in the formulation of our fundamental
ideas and must recur to the ultimate facts of nature, namely to events.
Also in order to find the ideal simplicity of expressions of the
relations between events, we restrict ourselves to event-particles. Thus