麻豆传媒

Einstein鈥檚 Mirror by Tony Hey and Patrick Walters,
Cambridge University Press, 拢50 hbk, 拢16.95 pbk, ISBN 0 521
43504 8

THE paradox of Einstein鈥檚 theory of special relativity is that it is so
simple鈥攁t least compared to the ether-pervaded notions that predate
it鈥攜et it nevertheless defies our intuition. For this we can blame God.
After all, he was the one who sentenced us to a life in the slow lane while
knowing full well that the essential unity of space and time, momentum and
energy, and so on, can be appreciated only from the express lane.

It is the view from the express lane, where the effects of the finite and
constant speed of light become blatantly obvious, that is described by Tony Hey
and Patrick Walters in Einstein鈥檚 Mirror. Here you can learn why moving
clocks run slow, why bodies shrink in the direction of their motion, why heating
up an object makes it heavier鈥攁ll the mind-bending consequences of special
relativity.

Einstein鈥檚 Mirror is the companion volume to Hey and Walters鈥檚
The Quantum Universe, which highlights quantum theory, that other great
revolution in 20th-century physics. The new volume uses the same highly
successful recipe, blending solid explanation, historical colour and copious
ilustrations into a highly attractive package.

Relativity, as Hey and Walters point out, was always an inappropriate name
for Einstein鈥檚 theory of space and time. For the truth is that there does exist
an 鈥渁bsolute鈥� canvas against which the cosmic drama is played out. Instead of
being Newtonian and three-dimensional, it鈥檚 Minkowskian and four-dimensional,
better known as space-time.

The concept of space-time was the crucial bridge from special relativity to
Einstein鈥檚 pi猫ce de r茅sistance, the general theory of
relativity, since it is by warping space-time that matter (or, more precisely,
energy-momentum) generates gravity. The great irony here is that the man who
built the bridge, Einstein鈥檚 former mathematics professor Hermann Minkowski, had
believed that his student was a 鈥渓azy dog鈥� who would amount to nothing. This was
the physics equivalent of turning down the Beatles. Fortunately, Minkowski
swallowed his pride and rectified his terrible mistake.

One pillar of general relativity is the idea that matter warps space-time.
The other, which Einstein referred to as 鈥渢he happiest thought of my life鈥�, is
the idea that matter responds to gravity by free-falling down geodesics, the
curved-space generalisation of straight lines. On these twin foundations
Einstein built the great edifice of general relativity, with its predictions of
light bending by gravity, the precession of the perihelion of Mercury and the
gravitational red shift. Hey and Walters skilfully present all these effects and
more, such as the time delay on signals from space probes passing near the Sun,
and the gravitational lensing of quasars.

Here, Hey and Walters emphasise the crucial differences between gravitational
lenses and conventional converging lenses, something which few authors bother to
do. It is because of the nonlinear effect of light-bending by gravity, they say,
that gravitational lenses produce multiple, not single, images of distant
quasars. Another difference that occurs to me is that a glass lens bends light
most markedly far from the optic axis, whereas the opposite is true for a
gravitational lens.

Hey and Walters may be clear in their treatment of gravitational lensing, but
they are wrong about the phenomenon鈥檚 discovery. They attribute it to Oliver
Lodge and Arthur Eddington in 1919, but the details were worked out by
Einstein鈥攕even years earlier. Einstein even discussed the possibility of
observing the effect with astronomer Erwin Freundlich in Berlin in 1912 (This
Week, 18 January, p 7).

I should emphasise that Hey and Walters are innocent on this point鈥擨 am
the one who is guilty of mischief-making. For Einstein鈥檚 early calculations of
gravitational lensing have come to light only recently, and Hey and Walters
could not possibly have known about them. The correction will have to wait for a
second edition of their delightful book, for a second edition there will surely
be.