1921 ,] 
Farr.—Relativity. 
241 
the ether, the velocity of light is the same. The lengths of distances 
depend upon.the speed we may assign, and the lengths of intervals of time 
also so depend ; but this all-important and most fundamental quantity— 
the velocity of transmission of a disturbance through it—is unaffected by 
the velocity we may give to the source from which the disturbance springs. 
This is a fact, and a remarkable one, that is well worth cogitating upon 
by those who have more than a passing interest in relativity. 
Thus we may perhaps sum up the situation by imagining two motorists 
meeting in direct collision in interstellar space. Each would accuse the 
other of “ road-hogging ” along the celestial roads, and assert that he 
himself was not moving at all. The one would say, “ I exhibited a square 
to show you which side to pass on ”; the other would contradict him, 
and say it was a rectangle ; while a third motorist, flashing past at the 
moment, would call out that both were wrong, as he saw the signal, and 
it was an oblong. The one would say, “ You have had plenty of time 
to avoid me, as I have been at this spot for years, and saw you coming all 
the time ”; the other would say that the first man was a-, and that 
it was he who suddenly, the very instant before the collision, appeared 
on the scene ; whilst the third would shout out, as he whisked awav to 
avoid further trouble, that they were both wrong again, as he had seen 
them both for the last hour. The passengers in each car would strongly 
and emphatically support their own drivers, and we can well imagine that 
the arguments might be very heated, for each car would see a world of 
his own which would totally disagree with that of the other man. 
And now, if you have followed me thus far, it will be quite easy to 
take the next step, which we owe to Minkowski. These expandings and 
shrinkings of lengths, these lengthenings and shortenings of time, as we 
give to the earth’s motion through the ether one speed or another—what 
do they mean ? Are they the products of a disordered imagination, or 
is the world of reality a vastly different thing from that of appearance ? 
Do we, that is to say, seem to our unaided senses to live in a world of one 
sort, whereas in reality we are living in one of quite another kind ? In 
an address which I had the honour of giving to you last year I pointed out 
that what seems to be solid matter is in its ultimate structure diaphanous 
and gauzy—so much so that with one of the “ eyes ” which modern science 
has been able to provide itself with it has been possible to “ see ” through 
as much as 11 in. of such a seemingly opaque, and certainly dense, body 
as lead. So that we must be prepared to find perhaps that our unaided 
senses are telling us only half-truths, and possibly in some cases deliberate 
untruths. 
Guided largely by these unaided senses—for the “ eyes ” of science are 
but a recent development—man has, through past generations, gradually 
developed the idea that we live in a world of three dimensions : that is to 
say, in more usual language, we are used to the three directions, north- 
south, east-west, and up-down ; or, put another way, length, breadth, 
and height. To these we certainly add, perhaps unconsciously, the idea 
of time ; but it is, as it were, of a thing thrown in as a side-issue—a by¬ 
product, so to speak, for us to make “ ducks and drakes ” of—as some 
of us, 1 fear, occasionally do—but not at all a thing of the same sort as 
the three already mentioned. Let us suppose, however, that we do not 
live, as we think we do, in a world of three dimensions, but in one of 
four—or, rather, while we live in a world of three dimensions, the universe 
is constructed of four dimensions. How should we—nan we at all, indeed 
—become aware of the fact ? I remember, many years ago, reading a 
