[March 14, 19 18] 



NATURE 



o:) 



years, and find a new law of gravitation which 

 fall in with our requirements. 

 This amended law nas been found by Einstein. It 

 ;ars to be the only possible law that meets our 

 juirements, and in the limited applications which 

 16 under practical observation is sufficiently close to 

 old law that has served so well. In practical appli- 

 ions the two laws are indistinguishable, except for 

 or two crucial phenomena to which reference will 

 .made later. But in gravitational fields far stronger 

 any of which we have experience, and for bodies 

 nng with velocities much greater than those of the 

 lets, the difference would be considerable, 

 lis idea of the distortion of space as the modus 

 randi of gravitation has led to a practical result — 

 law of gravitation. It is not brought in as a 

 Jthetical explanation of gravitation; if Einstein's 

 irv is true, it is simply of the nature of an experi- 

 ital fact, 

 we draw a circle on a sheet of paper and measure 

 ratio of the circumference to the diameter, the 

 lit gives, if the experiment is performed accurately 

 jfugh, the well-known number -, which has been 

 "culated to 707 places of decimals. Now place a 

 fy particle at or near the centre and repeat the ex- 

 iment ; the ratio will be not exactly equal to tt, but 

 Ittle less. The experiment has not been performed, 

 is not likely to be performed, because the 

 lerence to be looked for is so small ; but, 

 if Einstein's theory is correct, that must be the result. 

 The space around the heavy particle does not obey 

 ordinary geometry ; it is non-Euclidean. The change 

 in its properties is not metaphysical, but something 

 which, with sufficient care, could be measured. You 

 (in keep to Euclidean space if you like, and say that 

 he measuring-rod has contracted or expanded accord- 

 ^ as it is placed radially or transversely to the gravi- 

 ional force. That is all very well if the effect is 

 all, but in a very intense gravitational field it would 

 :d to ridiculous results like those we noticed in con- 

 otion with the Michelson-Morley experiment — every- 

 liing expanding or contracting as it changed position, 

 and no one aware of any change going on. I think 

 \\r have learnt our lesson that it is better to bo con- 

 it with the space of experience, whether it turns out 

 be Euclidean or not, and to leave to the mathe- 

 iiician the transformation of the phenomena into a 

 ice with more ideal properties. 



Ihis consequence of the new law of gravitation, 



: hough theoretically observable, is not likely to be put 



ro any practical test either now or in the immediate 



iMture. But there are other consequences which just 



me within the range of refined observation, and so 



\e an immediate practical importance to the new 



ory, which has indeed scored one very striking suc- 



-s. If we could isolate the sun and a single planet, 



n under~the Newtonian law of gravitation the planet 



v.ould revolve in an ellipse, repeating the same orbit 



iidefinitely. Under the new law this is not quite true; 



!■ orbit is nearlv an ellipse, but it does not exactly 



ise up, and in the next revolution the planet describes 



new ellipse in a slightly advanced position. In other 



words, the elliptic orbit slowly turns round in the same 



direction in which the planet is moving, so that after 



the lapse of many centuries the orbit will point in a 



(iifferent direction. The rate at which the orbit turns 



depends on the speed of motion of the planet in its 



orbit, so we naturally turn to the fastest moving 



planets, Mercury, Venus, and the earth, to see if the 



effect can be detected. Mercury moves at thirtv miles 



a second, Venus at twenty-two, the earth at eighteen 



and a half. But there is a difficulty about Venus and 



the earth. Their orbits are nearly circular, and you 



cannot tell in which direction a circle is pointing. 



NO. 2524, VOL. lOl] 



Mercury combines the favouiable conditions of a high 

 speed and a satisfactorily elongated orbit the direction 

 of which at any time can be measured with considerable 

 precision. It is found by observation that the orbit of 

 Mercury is advancing at the rate of 574 seconds of arc 

 a century. This is in great measure due to the attrac- 

 tion of the other planets, which are pulling the orbii 

 out of shape and changing its position. The amount 

 i of this influence can be calculated very accurately, and 

 ! amounts to 532 seconds per century. There is thus a 

 ! difference of forty-two seconds a century unaccounted 

 '' for; and this has for long been known as one of the 

 j most celebrated discordances between observatFon and 

 gravitational theory in astronomy. It is thirty times 

 greater than the probable error which we should expect 

 from uncertainties in the observations and theor>'. 

 There are other puzzling discordances, especially in 

 connection with the motion of the moon; but the con- 

 ditions in that case are more complicated, and I 

 scarcely think they offer so direct a challenge to gravi- 

 tational theory. Now Einstein's theory predicts that 

 there will be a rotation of the orbit of Mercury addi- 

 tional to that produced bv the action of the planets ; 

 I and it predicts the exact amount — namely, that in one 

 revolution of the planet the orbit will advance by a 

 j fraction of a revolution equal to three times the square 

 ! of the ratio of the velocity of the planet to the velocity 

 ' of light. We can work that out, and we find that the 

 I advance should be forty-three seconds a century — just 

 i about the amount required. Thus, whilst the New- 

 I tonian law leaves a discordance of more than forty 

 i seconds, Einstein's law agrees with observation to 

 i within a second or so. 



Of course this superiority would be discounted if we 

 could find some other application where the old New- 

 tonian law had proved the better. But that has not 

 happened. In all other cases the two laws agree so 

 ' nearly that it has not been possible to discriminate 

 between them by observation. The new law corrects 

 I the old where the old failed, and refrains from spoiling 

 any agreement that already exists. The next best 

 chance of applying the new theory is in the advance of 

 the orbit of Mars; here Einstein's new law "gilds 

 refined gold " by slightly improving an agreement 

 which was already sufficiently good — a "wasteful and 

 ridiculous excess," which is at any rate not unfavour- 

 able to the new theory. 

 1 There is another possibility of testing Einstein's 

 j theory, which it is hoped to carry out at the first oppor- 

 j tunity. This relates to the action of gravitation on a 

 ray of light. It is now known that electromagnetic 

 j energy possesses the property of inertia or mass, and 

 I probably the whole of the mass of ordinary matter is 

 I due to the electromagnetic energy which it contains.. 

 Light is a form of electromagnetic energy, and there- 

 ' fore must have mass — a conclusion which has been 

 found true experimentally, because light falling on any 

 object exerts a pressure just as a jet of water would. 

 We ordinarily measure mass in pounds, and it is quite 

 proper to speak of "a pound of light," just as we speak 

 of a pound of tobacco. In case anyone should be 

 i thinking of going^ to an electric light company to buy 

 1 a pound of light, I had better warn you that it is a 

 I rather expensive commodity. They usually prefer to 

 I sell it by a mysterious measure of their own, called the 

 Board of Trade unit, and charge at least 3d. a unit. 

 .'\t that rate T calculate that they would let you haw 

 a pound of light for 141,615,000?. Fortunately, we get 

 most of our light free of charge, and the sun showers 

 down on the earth 160 tons daily. It is just as well 

 we are not asked to pay for it. 



But although light has mass, it does not follow that 

 light has weight. Ordinarily, mass and weight are 

 associated in a constant proportion, but whether this. 



