THE SPEED OF LIGHT

Evening Post, Volume CXII, Issue 107, 2 November 1926, Page 9

THE SPEED OF LIGHT

FASTEST KNOWN VELOCITY

NEW ATTEMPT AT MEASURE- : MENT

!A. SCIENTIST'S IDEA OJT REAL FUN.

Nothing is known to travel as fast hs"light—except "light" in its various forms which range from cye-experi-eneed illumination to X-rays at one end of the scalo to the long waves used in radio communication at the other. &11 these forms of "light" are alike Jn.-being manifestations of electrical jtoree. Not only is the velocity of light the highest known; it is, in the view of physicists, the highest imaginable; ami, indeed, the theory of relativity declares that no material thing can possibly travel at such a velocity. But the exact speed at which light travels, though it is a ''constant" of extreme importance, is actually knows .only approximately. .The following interesting account of the latest attempt to determine the exact value of tho velocity of light is ■written by Henry Norris Russell, Ph.D., in tho "Scientific American." The experimenter, Professor Michelson, is the inventor of the well-known interferometer method of making very exact measurements, and in association with Morloy conducted the famous experiment which was largely responsible for Einstein's work on .relativity.

The stars are "beginning to fade in the earliest light of a California dawn and the observers at the grc t telescopes on Mount Wilson end their last exposures, and close the long night's work. Tho summits of the Sierra Madre begin to stand out dark against the slowly brightening sky. But, while most of tho work is over, a light shines in one small building on a spur of tho mountain, and the attention of tho passer-by is attracted by a strange sound which breaks the quiet of the early morning—a shrill note, rising at first in pitch and intensity, and then, settling into a sustained scream, assembling that of the familiar warning- siren, but more piercing. The casual visitor —were such passjag at this uncrowded hour—might well be perplexed, but the observers of the a-egular staff, walking back to their (sleeping-quarters, say only, "Michelson. is having a good morning," for they know that in this unpretentious ■ temporary building, the dean, of Ame•rican physicists is at work on tho determination of one of. the most fundamental constants of Nature, with hopes of attaining accuracy far surpassing $vy previous knowledge. VISUALISING THE SPEED OF LIGHT. The measurement of the velocity of SighV is nothing new. The world has -^knovrn for generations that it is 186,----«00 miles a second—to the nearest round 'jthousand. But science is never contented with round numbers —she desires the faiost precise determination that it is at £ll practicable to attain. In Professor Miehelson's present j'tvork —which the writer had the pleasure of hearing explained by his own fsips a month ago—a beam of light, Reflected from a rapidly rotating miriSror, is. sent to another mirror at a distant station, and returns after a minuto Infraction of a secon" to the spinning 'jjnirror again, only to find that this mirror has turned in the interval, so that £the reflected ray is not sent back to tho jyource as it would be if the mirror were [Stationary, but in a. different direction. fJThis rotating mirror method is a very powerful one for measuring exgtremely short intervals of time. jSupposej ■ fov example, that the mirror were a mile [feway, light would travel thero and "back in 1-93,000 of a second, hopelessly (jtoo short a, time, apparently, to measijire. But suppose that the mirror (SWere turning at the rate of GOO revoluHrcms a second—a quite attainable speed. jSn this short time it would have turned through 1-186 of a revolution, or jltearJy two degrees. The reflected ray deviated twice as much, or almost gfoul- degrees. Bun the mirror back-! |wards and the deviation is four degrees ! jjui the opposite direction. The differ-1 fence of nearly eight degrees between I |Jhe. two results can be measured to jinuch less than a thousandth of a degree—which amounts to saying that the !**light-time" of l-93,00(V0f a second r*aa be measured to less than a tenfyhousandth of its own amount.

■ It is evident that for a complete determination of the velocity of light we fflnusfc measure three quantitie,: the jgistance of the remote station, the rate Sfc which the mirror is.turning, and the pangle through which the reflected beam ks deviated. And, of course, to measure any one of them with the highest is hard work. If, however, [we could get along by measuring two Quantities instead of three, the problem jwtmldbe much simplified; and ProfcsPjor Michelson has done this in a very Characteristic fashion, by a device as fimple as it is effective.

. His rotating mirror is many-sided— < fits cross-section, at right angles to the 1 Jaxis about which it spins, being a iegu- ' war polygon which, ia the apparatus now ] m use, has twelve sides. All the faces . fere accurately figured, polished, and sil- > Jeered, so that in one turn of the axis, ; Stwclve successive mirror surfaces come ■ pnto the path of the light. If the mir- ] fcor can be spun fast enough, it is pos- : Bible to catch the returning beam of i iignt, not on the surface from which ift was originally reflected, but on the ' hext. For example, the present distant '■ station on Mount San Antonio is some '< 432 miles from Mount Wilson. Light : bakes about 1-4200 of a second (i i round wnmbers) to make the return journey. M the mirror is spun at 850 revolutions ' feer second (a possible xate) it will taake just 1-12 of a turn in this interS*C The returning beam will find ttie |ext successive face of the mirror exjcWy where the preceding one was W.a , i st? rted ' a"d will therefore be : X/] ? ]™8 ■")' USfc tho same Path as gnirror had been at rest For faster W Blower S peed oE revolution, howpoU.™ 11 be "evkted *. taS thlS arran Bomeiit,8omeiit, therefore, it |ack ie just the same direction's from &- stationary mirror, and we can then SVTf «,V h? lighUimc » «»S B-12 of that ot a revolution. The Brord "exactly" is here used advisedly. #ti%Jf nf SH CCeSS- ive a"gles between Rpe taces of the mirror are made as Nearly equal as instrumental skill can produce them to the theoretical value j££ IdO.degrees, but since the reflected Jimage which the eye can see is made -Jup of thousands of successive flashes Reflected from all the mirror faces, its Apparent deviation depends on the average of the twelve angles—and this jnust be exactly 150 degrees. To get a precise determination, (therefore, only two things need to be Pleasured—the distance of the mirror |n the remote mountain and the rate of dotation of the spinning part of the fHparatus. I ffHE CAUSE OF THE SCREAM? .The first: of these lias been 'found, Jjfitoe for all, by Hip co-operation of Hie jCjited States Coast aad Geodetic SSunv-ej?, .which /executed a. special, ands ;•■

very precise triangulation for the express purpose. The measurement of the second is accomplished with the aid of a tuning-fork driven by electrical means at a very uniform rate. At every vibration of the fork, a beam of light from a little mirror, attached to one of the prongs, is reflected on to the revolving mirror, and thence to an auxiliary eyepiece. If the periods of vibration of the fork and of rotation of the mirror are exactly tho same, the successive reflected flashes will fuse into an apparently stationary image, but if one is going faster than the other by even tho minutest fraction, this image will appear to move. (This slroboscopic method is, of course, familiar.) The rotating mirror is driven by a little compressed-air turbine (which when running gives out the car-piercing shriek which was mentioned at tho start). An assistant, with his hand on the throttle of this turbine, and looking through the eyepiece of the stroboscopic system, can adjust the speed of turbine and mirror so that the images are stationary—that is, so that the period of the mirror is exactly that of the fork. Such an adjustment can be kept satisfactory for only :i few seconds at a time, but this sufiiees; for at such moments—indicated by a suitable signal, Professor Michelaon, at the eyepiece in which is seen the imago produced by the light which has been to the distant station and back, can make his settings which measure where tho reflected ray has •been sent.

If the rate of the fork is precisely "right" the mirror would have turned exactly 1/12 of a revolution in the light-time, and this reflected image would be in just the same place as that given by a stationary mirror. Actually the interval defined by the fork will b.e slightly too short or too long, and the rjosition of the reflected image a little to the right or to the left. By running the mirror backward (which can be "clone within half a 'minute or so) the image is now deviated by an equal amount on the other side of its ideal position. The combination of the two settings suffice to determine with extreme .precision just how much longer or shorter the light-time is than the time of one beat of tho fork. It remains to find this difference —which varies slightly from day to day with changes in temperature. This is again done by a stroboscopic method by which the fork is compared with a. standard pendulum, itself very carefully calibrated.

"SUCH GOOD FUN."

The original beam of light comes from a military searchlight of the highest power. After passing through a

narrow slot, and being reflected from the rotating mirror, it is rendered parallel by a two-foot concave mirror, and sent to the distant station —there to be returned precisely along its course by a beautifully simple optical device. When the rotating mirror is held fixed in the proper position, the return beam is visible to the eye, looking like ■ a brilliant star on the mountain side. By an ingenious arrangement of the optical train, the faint returning beam of light is caught, not on the same face of the mirror as the intense original beam, but on the opposite one—thus avoiding the stray light which would otherwise drown it out.

It is already known that tho results of observation on many nights agree so well that we may hope for a final value which will be accurate well within ton miles per second and perhaps a good deal better. But why. should months and years of labour bo spent in seeking such great precision 1 Professor Michelson, speaking recently to a group of students of science, gave two answers. ' One is that such, accuracy may be of ijraetical value in precise surveying. If we want to know the distance of a mountain a hundred miles away within a foot or two, it can be found by a trigonometric survey with the expenditure of great labour and cost, provided that this mountain can be sighted on from at least two others in different ' directions. When once the ■velocity of light is known as accurately as Professor Mielvslson hopes to find it, the distance would be jnoasured in a few nights' work— measuring fhc light-time—with the same precision. And even in the case of an island visible from but one peak on the main land, his method would succeed when the ordinary one would not work at all.

"But," said Professor Michelson, *'I will confess that this is not my main motive. My real reason for wanting to do this is that it is such good fun." Cold type cannot convey the impression which his hearers gained from the flash in the eyes of this veteran of science as he made this confession. But anyone who knows the deep fascination of investigation, and the joy of successfully overcoming obstacles, may understand.