Relativity and gravitation . gether one above another in theirproper order, we shall build up a solid block of filmwhich will be a model of our simplified world ofspace-time and in which there will be a series ofdots representing the motion of the fly over thepaper. Just as I can state the exact position of anobject in my room by defining its height above thefloor, its distance from the north wall and its dis-tance from the east wall, so we can reduce the posi-tions of the dots to figures for use in calculations bymeasuring their distances from the three faces inter-secting in the lines OX, OY
Relativity and gravitation . gether one above another in theirproper order, we shall build up a solid block of filmwhich will be a model of our simplified world ofspace-time and in which there will be a series ofdots representing the motion of the fly over thepaper. Just as I can state the exact position of anobject in my room by defining its height above thefloor, its distance from the north wall and its dis-tance from the east wall, so we can reduce the posi-tions of the dots to figures for use in calculations bymeasuring their distances from the three faces inter-secting in the lines OX, OY, and OT, where21 322 RELATIVITY AND GRAVITATION OXAYTBCD represents the block of film. Themathematician would call the three lines OX, OY,OT the coordinate axes. Measuring all the dots inthis way we shall obtain the motion of the fly relativeto the coordinate axes OX, OY, OT. If we add ablock OTDYEFGH of plain film we can us EX,EH, EF as coordinate axes and again obtain themotion of the fly relative to these new axes; or we. can add block after block so as to keep the axesmoving. We can conceive of other changes ofaxes. The operator making the movie record mighthave taken the fly for the hero of the piece andmoved the camera about so as to keep the fly moreor less central in the picture; or he might, by turningthe handle first fast and then slow and by movingthe camera, have made the fly appear to be doingstunts. Moving the camera would change the axespf x and y, and turning the handle at different speeds Einsteins theory of relativity 323 would change the axis of time. Again, we mightchange the axes by pushing the block out of shape orby distorting it into a state of strain. Whateverchange of axes we make, any dot in the block offilm will signify a coincidence of the fly with a certainpoint of the paper at a certain time, and the seriesof dots will, in every case, be a representation ofthe motion of the fly. Maybe the representationwill be a distorted one, but who is to say w
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