« AnkstesnisTęsti »
Experiments corresponding to these were repeatedly tried with Roentgen rays in the hope of establishing their vibratory character. But the results were so indecisive that they carried but little weight. The structures used were too coarse to give undeniable effects until a little over a year ago Laue, Friedrich and Knipping bethought them to see if the structure of the atoms in the regular arrangement found in crystals would show diffraction effects with X-rays. A parallel beam of rays was allowed to pass through a section of a crystal of zinc blende, just as the light from the arc lamp passed through the handkerchief and the resulting pattern was caught, not in the eye as in our experiment, but on a photographic plate. A distinct diffraction pattern was obtained proving in the first place that the X-rays contained waves similar to light waves but of a minuteness hitherto unmeasured. We know the mass of the different kinds of atom in the crystal molecule and from a knowledge of the total mass of a unit volume of the crystal we can find how far each atomic group is from its neighbors and from this again we find that the waves we have been dealing with have a length of about 10-9 centimetre or the ten thousandth part of the hundred thousandth part of an inch. In other words the wave length is about one-tenth of the “diameter of a molecule.” We can go farther even than this. We cannot alter the spaces between the molecules as we pulled the threads of the handkerchief but we can examine slices cut at different angles through the crystal and we can use crystals of different substances. It must be remembered that the connection between the diffraction pattern and the spaces in the diffracting structure bear definite mathematical relationships so that we may, as it were, find what our crystal looks like from different points of view simply by examining the different patterns obtained. The cases of diamond and of zinc blende have been worked out fully. In the diamond every carbon atom has four neighbors at equal distances from it in directions parallel to those which join the centre of a regular tetrahedron to its corners and the distance from atom to atom is 1.52 x 10-8 centimetre. In zinc blende the arrangement is the same but the atoms are alternately zinc and sulphur. Further light is thrown on the structure
. both of the rays and of the crystals by an analysis of the beams reflected from crystal surfaces. Measurements of this kind show that in the rays so far examined we have groups of wave trains of different wave length.
All of this work has been published in the past year and we have every indication that a new lead into the wonders of Nature has been vouchsafed us. It is to be remembered that the “living garment of God” is as it always was, it is we who develop.
W. C. B.
When Newton's theory of light as consisting of streams of particles gave way to the wave theory by Huyghens backed by the overwhelming evidence of Fresnel's experiments, early in the nineteenth century, it became necessary to give some account of the transmission of these light waves through the apparently empty space separating us from the sun and the stars. The wave theory demanded at the outset a medium which could transmit the waves; and in accepting the theory, scientists had to admit the existence of an invisible, impalpable medium filling all space, so rare as to permit the passage of heavenly bodies without appreciable friction and yet so highly elastic as to account for the transmission of light-waves at the enormous speed of one hundred and eightysix thousand miles per second. This hypothetical medium is called the ether. The eighty years that have elapsed since the acceptance of this theory have witnessed most conspicuous advances in physical science. To light we have added other forms of radiation, which differ from light only in the length of the waves: the waves of wireless telegraphy, the waves emitted by bodies heated below the glowing temperature, and the photographically active waves beyond the violet end of the spectrum. Besides these we have the Roentgen rays which are believed to differ from light in much the same way that the pattering of hail on a roof differs from the note of an organ. The ether has been found indispensable in explaining the nature of each of these radiations, yet we are today as far from any experimental knowledge of the ether as in the beginning. Throughout this period of progress the ether has remained an unconfirmed hypothesis.
Not, however, through any lack of effort. Numerous experiments have been performed to obtain direct evidence of the existence of the ether. One of the most hopeful methods attempt to detect the ether by the effect of the earth's motion on the speed of light on the earth. if it is true that the earth in its course around the sun is passing through the stationary ether in which light is transmitted at a definite speed, then the speed of light measured in the direction of the earth's motion should be less than the speed of light measured in the opposite direction since in the first case the earth's motion causes the destination of the ray to recede, while in the second the destination of the ray advances. The effect to be detected was small, since even the speed of the earth in its orbit is but an insignificant fraction of the speed of light; but modern optical instruments have been brought to such perfection that the effect would certainly have been detected had it existed. The complete failure of every such experiment brings us face to face with an inconsistency in our physical theory, which can be removed only by some radical alteration of our fundamental ideas.
Relativity is a new general theory of physics put forward to explain this inconsistency. Like every physical theory, it assumes as generally true certain statements called postulates, which have been invariably confirmed by experiment. The two postulates upon which relativity rests are, first, that it is in the nature of things impossible to detect any unaccelerated motion of the earth by any experiments made on the earth; and, second, that the speed of light is unaltered by any motion of the source or of the observer. Put in other words, the first postulate means that the only sort of motion we can ever know is motion determined by reference to some outside body, that is, relative motion; hence the name relativity. The second postulate has always been accepted without question. The superstructure of the relativity theory, erected on these two postulates, consists of conclusions as to the different aspects which given phenomena present to an observer, according as he is at rest or in motion with respect to the system in which the phenomena occur.
One of the most striking of these conclusions has to do with the passage of time on a given system as it appears to an observer moving with the system and to one who is watching the system pass before him. If we suppose the two observers to be provided with watches identically alike, each observer will conclude that his own watch ticks faster than the other. Every natural process—such, for example, as the growth of grain from the seed to maturity—appears to occur more rapidly on the observer's own system than on the other; and the more so as the speed of the relative motion increases. If this speed could be made equal to the speed of light, then each observer would conclude that time on the other system had been annihilated, and that the other observer was in eternity. It appears from this, that in the theory the passage of time is only relative, and that our fixed idea of absolute time must be abandoned.
In the measurement of length, as of time, the two observers are at variance when the length is in the direction of the relative motion. To each observer, the effect of the motion of the other system past him is to shorten all such lengths on that system in comparison with corresponding lengths on his own. Spheres on the other system appear flattened to ellipsoids, and if the relative speed approach that of light, each observer sees the other and all his appurtenances shrunken to silhouettes lying at right angles to the motion.
These two conclusions lead readily to another involving the question of the simultaneity of two events at different places. When we wish to set clocks in two distant places so that they shall strike noon simultaneously, we establish a central station provided with a standard clock. At noon by this clock a telegraphic signal is dispatched to both places, and upon receipt of it the clocks are set at noon. All three clocks are then, for all practical purposes, indicating the same hour at any subsequent instant, so far as any person on the earth is concerned. But to an observer in outer space, watching the earth pass him, the indication of any particular hour by the different clocks is not simultaneous, but occurs later at the clock which is forward in point of motion, the difference depending on the distance between the clocks and on the speed of the motion. Simultaneity of events at different places, then, is not an absolute thing, but is dependent on the relative motion or rest of the observer. In certain cases, the order in which two events occur may be reversed for the observer in outer space. Suppose, for instance, that a man standing at some forward point of the earth, as it moves, should fire a gun, and that immediately afterwards a man at some point toward the rear, previously in good health, should suddenly die. To people on the earth it would look like a case of what used to be called in Arizona, acute lead poisoning. To the outside observer, however, it would appear that the deceased, though probably an unpopular citizen, had died from natural causes, and the shot was fired afterward to celebrate the event. Thus relativity might explain the miscarriages of formal justice which have occurred in the past, on the south-western frontier of this continent.
The speed of light in relativity corresponds to "infinity" in mathematics. If the motion of a body be made up of two parts, (as in the case of a man walking forward on the deck of a moving ship) each of which is less than the speed of light, the total speed of the body must always be less than the speed of light, even though the arithmetic sum of the two parts exceed that speed. Again, if one of the parts be equal to the speed of the light and the other less, the total speed will be equal to the speed of light. In other words, the speed of light cannot be attained by the combination of any smaller speeds; and, once attained, it cannot be increased nor diminished by combination with smaller speeds.
The application of these results to dynamics requires a complete revision of the Newtonian view of that subject. Newton taught that a body's mass (that is, the inertia by which it resists a change in its motior.) is constant under all circumstances. In relativity, a gain of speed is accompanied by an increase of mass, and at the speed of light, the mass increases to an infinite value, which of course implies that its energy also is infinite and therefore that it is practically impossible to move any material thing with the speed of light. This line of thought leads to a complete identification of mass and energy, so that either may be expressed in terms of the other.
Each of these conclusions affords a theoretical basis for experimental verification or disproof of relativity, but the effects predicted would be appreciable only when the speed