else may be, they are bodies, bodily force and bodily matter. As units of electricity, they are units of electrical force and matter; and as units of electrical matter they are perhaps the elementary matter of chemical atoms, but throughout bodily matter

because resistant.

It does not follow that electrical particles are absolute units of all matter. It may very well be that, as a chemical atom is a minimum of chemical force yet divisible into electrical particles, so in its turn an electrical particle is a minimum of electrical force divisible into other material particles. A particle charged with electricity seems too complicated to be primary matter; it fails also to explain all forcegravitation for example; and it possesses a resistance which has no known minimum. There may be still subtler particles with resistance but without electricity. Probably there are, and if so they are bodies -bodies the matter of bodies-the only matter ever yet discovered by science. So far then from units of electricity being simpler than units of matter, as Mr. Balfour supposes, rather the probability is that units of matter are simpler than units of electricity. We have not got to the bottom of bodily matter; we can only say with Bacon, subtilitas naturae subtilitatem sensus et intellectus multis partibus superat '.

My reason, Sir, for addressing this letter to you is a hope of helping to stem the tide of paradox. Speculations of this kind are usually confined to the few; but when they are publicly proclaimed by the Prime Minister they bewilder the public mind, which, taught that science is infallible, does not know enough to distinguish the point at which science becomes nescience. It is then that a comprehensive view of things becomes a national necessity. Now, this is exactly what is not furnished by natural science, with all its discoveries. Nay, as it becomes

more independent of other studies without, and more specialized within, yet at the same time more general in its own problems, natural science begins to run wild, lose judgement, and trifle with metaphysical paradoxes, such as that the world is not what men think, and matter consists of non-matter, and what is the real is the ideal.

The only way to combine the partial truths of natural science with a comprehensive view of things is to insist on the inseparable connexion between ancient Greek and modern Science in the highest education. It is curious how on this very point of resolving matter into electricity its authors would have been saved from paradox by the Greeks. To the Greeks we owe the very conception of body, force and matter. They did not all define it in the same way; but those who took the palm as physicists, the Atomists, define body as that which is extended in three dimensions and has resistance (Antitypia). If our electricians had only considered them as well as themselves, they would have seen at once that electrical particles, having resistance, are bodies and bodily matter. That is not all; to another Greek, to Aristotle, we owe the theory of attributes and their abstraction, and of the fallacy of turning the abstract into the concrete. Into this very fallacy our electricians, careless enough of Aristotle, have sometimes fallen. They suppose something simply electrical, turn electricity into an independent thing, and then construct matter out of an abstraction. But there is no such thing as electricity by itself, because it involves resistance, and resistance involves extension in three dimensions. In short, the things which are really known to be electrical are bodies, geometrically solid, mechanically resistant, and therefore physically material, like other bodies.

I i

VIII

LETTERS ON THE THEORY OF EINSTEIN

I. NEWTON AND EINSTEIN

THE ABSOLUTE AND THE RELATIVE

Albert Einstein was born in 1879 at Ulm in Würtemberg. He bolds a Professorship at Berlin. This great physicist and mathematician is best known as the founder of the theory of Relativity.

FROM The Times, NOVEMBER 22, 1919.

In spite of the abstruseness of the subject, the reputation of so great a man as Sir Isaac Newton can hardly be left in doubt. As reported in your issue of November 7, a joint session of the Royal and Astronomical Societies discussed the results obtained by British observers of the total solar eclipse of May 29. The facts obtained were that at the moment of totality light transmitted from certain stars, as it passed the sphere of influence of the sun, was deflected, and deflected to an extent twice what would follow according to Sir Isaac Newton, but closely agreeing with what had been predicted by Professor Albert Einstein, and in accordance with his theory of relative space. The scientific men present appear to have accepted the facts, and concluded that the cause is some influence or other of the sun; and in a letter published by you on Nov. 8, Sir Oliver Lodge said that the eclipse result is a triumph for Einstein; the quantitative agreement is too close to allow much room for doubt. It may be taken then that the prediction of Einstein, verified by the observations of the deflection of light, has corrected Newton in an important detail.

So far so good; but at the discussion in the rooms of the Royal Society it became a question what value is to be attached to Einstein's theory of rela

tive space, according to which not only the light but also the relative space is deflected, and when deflected causes effects on light. In this case the cause of the observed deflection of light would be not merely some influence of the sun, but a dislocation of the relative space. But in his prediction his theory contained only a causa cognoscendi, not a causa fiendi, of the deflection of light. Moreover, as pointed out in your columns (page 8) of this morning, by Mr. E. Nevill, F.R.S., there are other causes which would produce such a deflection. Perhaps it is a pity that, when it was clear that the facts occurred under the influence of the sun, the discussion was not adjourned for further inquiry concerning things far more obvious to observation than relative space, before bringing into question the reputation of Newton, who was President of the Royal Society for the last twenty-five years of his life and raised the Society to the acme of its fame. However, the Royal Society, having once broached the question, has challenged the learned world to decide whether Newton or Einstein is to be taken as the authority on space, time, motion, and the spatial universe.

According to Newton's theory of space, stated in the Scholium appended to the definition with which he starts his Principia, absolute true and mathematical space without relation to anything external always remains similar and immovable; whereas relative space is any movable measure of that space, which is defined by our senses by means of its situation towards bodies-e. g. the measure of a space by means of its situation towards the earth; and this relative space is vulgarly taken for immutable space, but is absolutely mutable. According to Einstein, on the other hand, no space is absolute and immovable, but all space is relative and movable.

The two theories differ little about relative space. Newton certainly would not have denied that, when

light is deflected, the relative space of that light is deflected with it. Indeed, it is universally true that everything spatial which moves carries its space with it, except so far as its extension is changed during its passage. The earth, for example, as it revolves round the sun, carries its space with it. Similarly, the light transmitted from a star carries its space with it; if by the influence of the sun it is deflected it has its space also deflected, and if it arrives at the earth deflected it brings its space to the earth also deflected. Newton and Einstein agree then in the commonplace that the relative space of anything movable is movable; and the only difference is that Newton would certainly not have agreed that light could be deflected by the deflection of its relative space, because he knew that space is not a force.

So far, then, Einstein has not overthrown Newton; and Newton has advanced the doctrine of relative and movable space in a rather better way than Einstein. But the real issue begins when we ask ourselves whether Einstein is right in rejecting the absolute, true, and mathematical space which Newton regarded as immovable. Is space nothing but relative and movable? The answer is that, whenever anything spatial is moved from place to place, it moves in a containing space which does not move with the thing moving, but remains unmoved and indifferent to the things moving about within it. For example, if I go in a train from Weymouth to Oxford, carrying my own relative space with me, the whole space containing those two places and the interval between them is unaffected by my journey, indifferent to the rest or movement of all the bodies on the way, and so far unmoved; otherwise the journey might start from Weymouth, but would never end at Oxford. Similarly, when light moves from a star, is deflected under the influence of the sun and finally arrives at the earth, then,