Water,
plain and simple water, is an intriguing substance. Its chemical formula is H2O.
A water molecule is composed of one oxygen atom and two hydrogen atoms. A
simple chemical structure indeed. But the spatial structure of water is not as
simple as we might imagine. The most ‘straightforward’ structure is the most
symmetrical one, with the two hydrogen atoms opposite of each other and the
oxygen atom in the middle, the three atoms nicely on a straight line. Like the
three atoms in carbon dioxide, CO2, for example. No, in water the
three atoms form a triangle. The angle at the vertex where the oxygen sits is 104.5
degrees. And this is of vital importance. Literally.
It
makes water a polar molecule, with the negative charge (the electrons) closer
to the oxygen than to the hydrogen. Its bent structure and the charge
distribution determine the properties of water as a solvent. They also make
water a liquid at standard temperature and pressure, which is quite exceptional
for such a light molecule. (Even H2S, which has a similar structure
but a smaller dipole moment, is a gas, even if it is heavier than water.)
Without water and its properties, largely determined by its bent shape, there
would be no life.
The
structure of water can be understood by describing its electron orbits as
determined by quantum mechanics. The radii of these orbits (the distance from
the relatively heavy and immobile oxygen and hydrogen nuclei) and the velocity
of the electrons are such that their product is of the order of one (in so
called ‘natural units’). Through the ‘uncertainty relations’ we know that in
this case we enter the ‘quantum domain’ and quantum mechanics is the theory we
need to describe the physical reality.
It
would go too far to call quantum mechanics a straightforward theory. It is quite
an amazing theory, but once you have accepted it, its use is straightforward.
This does not mean it is easy: water consists of three nuclei and ten electrons
and even if you simplify the nuclei to pointlike particles it is impossible,
for now and always, to exactly solve the quantum mechanical equation (the
Schrödinger equation) for this many body system. It is, however, possible to
make acceptable approximations and to resort to the use of computers and the
result is that the structure of water, vital for life, follows compellingly
from quantum mechanics, a perhaps counter-intuitive but profound construct of
the human mind and an accurate description of the physical world.
All
this was known when I went to university in 1967 and it was exciting, very
exciting, to ‘rediscover’ this during the initial years of university training.
There was more to be discovered and personally I was (and I am) attracted by
the challenges of elementary particle physics. In recent years a connection
between particle physics and another field really catching the imagination,
cosmology, has been made. In the beginning there was particle physics... Our
understanding of the beginning of the universe, of the very initial phases of
the Big Bang, is hampered by our lack of understanding of gravity. It must have
played an important role at the quantum level, initially, when the energies
involved were very high. A lack of understanding of quantum-gravity does not
mean a lack of candidate theories for including gravity in the description of
the elementary world. Based on the principles of the theories of relativity and
quantum mechanics (a ‘minimal’ condition for candidate theories) and with the
notion of ‘elementary particle’ replaced by ‘elementary string’ in fact a
practically infinite number of theories can be chosen from. Even if we assume,
perhaps somewhat naively, that the right ‘theory of everything’ has been found
in principle, no one is capable of singling it out from the huge number of
possible theories.
I
will now tread on slippery ground. Cosmologists have invoked the ‘anthropic
principle’ to explain, or rather to be absolved of explaining why the universe
and the laws that govern it are as they are. According to the ‘anthropic
principle’, the universe we observe and the laws we find must be consistent
with ‘us’, with conscious life – if not, we would not be there to make
observations and find explanations in the first place. So only universes
compatible with conscious life (is this well-defined?) can ‘exist’. The ‘anthropic
principle’ may be interesting philosophically, but, as far as I can see, it is
not useful to help us find the right ‘theory of everything’ amidst the huge number
of possible theories. Neither is it able to explain the structure of water, but
quantum mechanics is, and was so long before anyone felt the need for an anthropic
principle!
Jos
Engelen
August
17, 2012