The foundations of Einstein's theory of gravitation
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| Einstein |
From the introduction
The Universe is limited by the properties of light. Until half a century ago it was strictly true that we depended upon our eyes for all our knowledge of the universe, which extended no further than we could see. Even the invention of the telescope did not disturb this proposition, but it is otherwise with the invention of the photographic plate.
It is now conceivable that a blind man, by taking photographs and rendering their records in some way decipherable by his fingers, could investigate the universe. However, still, it would remain true, that all his knowledge of anything outside the earth would be derived from the use of light and would therefore be limited by its properties.
On this little earth there is, indeed, a tiny corner of the universe accessible to other senses: but feeling and taste act only at those minute distances which separate particles of matter when " in contact: " smell ranges over, at the utmost, a mile or two; and the greatest distance which sound is ever known to have travelled (when Krakatoa exploded in 1883) is but a few thousand miles a mere fraction of the earth's girdle.
The scale of phenomena manifested through agencies other than light is so small that we are unlikely to reach any noteworthy precision by their study. Few people who are not astronomers have spent much thought on the limitations introduced by the news agency to which we are so profoundly indebted. Light comes speedily but has far to travel, and some of the news is thousands of years old before we get it. Hence our universe is not coexistent: the part close around us belongs to the peaceful present, but the nearest star is still in the midst of the late War, for our news of him is three years old; other stars are Elizabethan, others belong to the time of the Pharaohs, and we have alongside our modern civilization yet others of prehistoric date.
The electric telegraph has accustomed us to a world in which the news is approximate of even date: but our forefathers must have been better able, from their daily experience of getting news many months old, to realize the unequal age of the universe we know. Nowadays inequality is almost entirely the concern of the astronomer, and even he often neglects or forgets it. But when fundamental issues are at stake, the time taken by the messenger is an essential part of the discussion, and we must be careful to take account of it, with the utmost precision.
Our knowledge that light had a finite velocity followed the invention of the telescope and the discovery of Jupiter's satellites: the news of their eclipses came late at times and these times were identified as those when Jupiter was unusually far away from us. But the full consequences of the discovery were not realized at first.
One such consequence is that the stars are not seen in their true places, that is in the places which they truly held when the light left them (for what may have happened to them since we do not know at all they may have gone out or exploded).
Our earth is only moving slowly compared with the great haste of light: but still, she is moving, and consequently, there is " aberration " a displacement due to the ratio of the two velocities, easy enough to recognize now, but so difficult to apprehend for the first time that Bradley spent two years in worrying over the conundrum presented by his observations before he thought of the solution.
It came to him unexpectedly, as often happens in such cases. In his own words " at last when he despaired of being able to account for the phenomena which he had ob- served, a satisfactory explanation of them occurred to him all at once when he was not in search of it." He accompanied a pleasure party in a sail upon the river Thames.
The boat in which they were was provided with a mast which had a vane at the top of it. It blew a moderate wind, and the party sailed up and down the river for a considerable time. Dr Bradley remarked that every time the boat put about, the vane at the top of the boat's mast shifted a little as if there had been a slight change in the direction of the wind.
The sailors told him that this was due to the change in the boat, not the wind: and at once the solution to his problem was suggested. The earth running hither and thither round the sun resembles the boat sailing up and down the river: and the apparent changes of wind correspond to the apparent changes in the direction of the light of a star. But now comes a point of detail does the vane itself affect the wind just around it?
And, similarly, does the earth by its movement affect the ether just round it, or the apparent direction of the light waves? This question suggested the famous Michelson and Morley experiment (Phil. Mag., Dec. 1887). It is curious to think that in the little corner of the universe represented by the space available in a laboratory an experiment should be possible that alters our whole conceptions of what happens in the profoundest depths of space known to us^but x so it is.
The laboratory experiment of Michelson and Morley was the first step in the great advance recently made. It discredited the existence of the virtual stream of ether which is the natural antithesis to the earth's actual motion.
