WHY IS WATER SO GOOD AT NEUTRALIZING ELECTRIC FIELDS
Compared to other liquids made of polar molecules water has a far
greater power
to dissolve ionic substances. This is a consequence of the network of
hydrogen bond links
between pairs of neighboring molecules, which greatly enhances
the ability of water to neutralize electric fields. This is because
(a) the hydrogen bonds
effectively increase the average dipole moment of each molecule in
the liquid with respect
to the vapor phase, and (b) they enable the molecules to react in
unison to an applied field.
The actual magnitude of these two effects has been an open and
frequently debated question
for more than 60 years. This paper provides a precise and
indisputable answer from a computer
simulation based on fundamental quantum theory, i.e. a simulation in
which the molecular
dynamics derives from the dynamics of the nuclei and the electrons
with no need for empirical input.
The results are in good agreement with experiment not only for liquid
water but also for solid ice.
In the simulation the effects that are by far the most important
occur at short range,
between a molecule and its first shell of neighbors. Thus, even the
minuscule quantities of water
that can be found in protein pockets should be capable to
substantially shield electrostatic
interactions which play an essential role in protein structure and
function.
***
New Superstring Predictions for Gravitational Scattering
It has been known for over 20 years that superstring theory predicts
deviations from Einstein's general relativity for the scattering of
gravitational waves at high-energies.
In this letter, these high-energy deviations are determined with much more
accuracy using a new more powerful formalism for computing superstring
amplitudes.
***
Drunk cars
If you drive a car in a huge empty park at constant speed while randomly
turning the wheel, you perform a well-known type of random motion called
persistent Brownian motion. If now instead of keeping the speed constant
you randomly press the gas and break pedals while still randomly turning
the direction wheel, the resulting movement is no longer well described
as plain persistent Brownian motion, and the statistics of the movement
change. In this work the new features introduced by the fluctuations in
the speed are exposed. In particular, it is shown that there is a complex
transient, together with changes in the asymptotic properties of the
movement due to the speed fluctuations. Our results could be relevant to
understand the motion of small self-propelled particles subject to large
fluctuations in their motion. It is suggested that some crawling cells,
which exhibited fluctuations in the speed and the direction of motion,
could be described as such "drunk cars".
