A CHAT ON SCIENCE.

Hokitika Guardian, 26 January 1925, Page 4

A CHAT ON SCIENCE.

THE SPEED OE LIGHT. Light is the swiftest thing in the world, speaking either practically or theoretically, l’ratically, because we do not know of anything faster; theoretically, because we cannot conceive anything faster. For according to Einstein, a body gains in mass as it gains in speed. To push the smallest particle faster than light would require infinite force, and infinite forces do not exist outside of hooks. The velocity of light through empty space seems to ho one of the inexorable limitations of nature, like absolute zero. It is a constant quantity that turns up in all sorts of calculations. ft is only the speed of the visible rays of light, but also of the X-rays that are some ten thousand times shorter, and of the radio rays that are more than a million times longer. The flash of a firefly and the ravs of the sun travel at this same velocity. It is the yard-stick by which the starry heavens are measured, and molecular magnitudes as well, ft gives us a fixed standard for time. Consequently it is important that its value should be known as exactly as possible, But the reason given by Prof. A. A. Miclielsoii, of the University of Chicago. for undertaking a new determination of this constant was different 1 rom any of those. "M lion asked by a member of the U. S. Coast and Geodetic Survey why lie was going to put in the summer repeating the experiments, since previous observers had alrudy got remarkably close figures, he answered: “Because it is such good fun.”

Then, seeing that this reply did not satisfy the interlocutor, lie added another and more practical reason, that if the velocity of light were known to one part in 200,000 it would he possible to set up a flashlight on one peak and a mirror on another as far away as could be seen, and so got the distance between them in two weeks as aceura tely as it can now bo measured In- the chain in two years. This method would also he of use in determining distances where direct measurements are impossible. Timing the speed of light lias been a hobby of Professor Miclielsoii from boyhood. When lie was a midshipman at Annapolis ho set us a new form of apparatus with a range of 2000 ft along the sen wall of the Naval Academy, and got hotter results than had been obtained before. Later lie assisted Prof. Simon Newcomb in a s'wies of determinations.

In 102:5 lie took up the work again in California on a longer range, and with 'more delicate apparatus than ever, but the forest fires made the air too hazy. But last summer lie succeeded in getting excellent determinations, which lie lias reported at the centenary celebrations of the Franklin Institute.

The method used is so simple that anyone can understand it, although the difficulty of carrying out a determination with such unapproached precision call hardly he appreciated. It consisted essentially in sending a beam of light from one mountain peak to another at a known distance, reflecting it back from a mirror there, and timing tbe round trip. The sending station was located at Mount Wflsori, not far from the hundred-inch telescope, tbe largest in the world. The receiving and reflecting station was on tbe top of .Mount Sail Antonio, 22 miles away. The distance was measured by tbe U.S. Coast, and Geodetic Survey with an accuracy of two parts in a million. Tbe source of tbe rav was a powerful electric arc lamp, giving a light almost as bright as the sun. Passing through a minute bole in front of tbe lamp tbe ray was caught on a revolving octagonal mirror, sent to -Mount San Antonio, retlccted back from there, and received on the original mirror, which is revolved at such a rate as to eateb the returned ray on the succeeding face of tbe octagon. Tbe mirror was rotated by a blast ot air playing on a little windmill, and made 5.10 revolutions a second, its speed being regulated by a tuning fork of known pitch. The making of the mirror was one of the most delicate parts of the apparatus, for if angles ot the octagon were not exact the results would lie erroneous. When this was com■pleted. find its angles tested, they were found to be equal with an uiK-cr-lainitv of only one part in a million.

In this simplified .apparatus enlv two measurements are necessary: (1) The distance between the two stations which is known by direct measurements, and (2)’ the time of the round trip, which is given by the speed of the rotation of the mirror. The average results of eight observations gives the velocity of light in a vacuum as 186,300 miles per second. This cannot be wrong hv more than 20 miles.

But Professor Hiehelson is not yet satisfied. He will try it again next summer, and hopes to get it right within a mile by steadying the speed „f the mirror, lie thinks it possible that the distance may he extended to a hundred miles, which would enable him to get the figure accurate to within one part of a million. Let’s hope, for other reasons as well, that there will be no ‘forest fires in California next year.