In
Modern Times historian Paul Johnson writes: ‘The modern world began on
29 May 1919 when photographs of a solar eclipse, taken on the island of Principe
off West Africa and at Sobral in Brazil, confirmed the truth of a new theory of
the universe “
The
new theory of the universe was Einstein’s General Theory of Relativity which
was submitted to The Royal Prussian Academy of Sciences in
November of 1915, one hundred years ago.
I have never learned tensors so I will not write about
General Relativity which due to my lack of math knowledge I never fully grasped.
But here is a quote from Feynman’s
Lectures on Physics on a phenomenon from the Einstein’s Special Theory of
Relativity which is easier.
16–2 The twin paradox
To continue our discussion of the Lorentz transformation and
relativistic effects, we consider a famous so-called “paradox” of Peter and
Paul, who are supposed to be twins, born at the same time. When they are old
enough to drive a space ship, Paul flies away at very high speed. Because
Peter, who is left on the ground, sees Paul going so fast, all of Paul’s clocks
appear to go slower, his heart beats go slower, his thoughts go slower,
everything goes slower, from Peter’s point of view. Of course, Paul notices
nothing unusual, but if he travels around and about for a while and then comes
back, he will be younger than Peter, the man on the ground! That is actually
right; it is one of the consequences of the theory of relativity which has been
clearly demonstrated. Just as the mu-mesons last longer when they are moving,
so also will Paul last longer when he is moving. This is called a
“paradox” only by the people who believe that the principle of relativity means
that all motion is relative; they say, “Heh, heh, heh,
from the point of view of Paul, can’t we say that Peter was moving and should therefore appear to
age more slowly? By symmetry, the only possible result is that both should be the
same age when they meet.” But in order for them to come back together and make
the comparison, Paul must either stop at the end of the trip and make a
comparison of clocks or, more simply, he has to come back, and the one who
comes back must be the man who was moving, and he knows this, because he had to
turn around. When he turned around, all kinds of unusual things happened in his
space ship—the rockets went off, things jammed up against one wall, and so
on—while Peter felt nothing.
So the way to state the rule is to say that the man who has felt the accelerations, who has
seen things fall against the walls, and so on, is the one who would be the
younger; that is the difference between them in an “absolute” sense,
and it is certainly correct. When we discussed the fact that moving mu-mesons
live longer, we used as an example their straight-line motion in the
atmosphere. But we can also make mu-mesons in a laboratory and cause them to go
in a curve with a magnet, and even under this accelerated motion, they last
exactly as much longer as they do when they are moving in a straight line.
Although no one has arranged an experiment explicitly so that we can get rid of
the paradox, one could compare a mu-meson which is left standing with one that
had gone around a complete circle, and it would surely be found that the one
that went around the circle lasted longer. Although we have not actually
carried out an experiment using a complete circle, it is really not necessary,
of course, because everything fits together all right. This may not satisfy
those who insist that every single fact be demonstrated directly, but we
confidently predict the result of the experiment in which Paul goes in a
complete circle.
