Biomedical Engineering Reference
In-Depth Information
We now turn our attention to the volume energy densities of
biological systems compared with free space. It is claimed that
THz frequencies are needed to break the DNA bond. But these
cellular structures reside in biological systems and not in free
space. The volume densities within biological systems given by
Eq. (1.1f) can be far greater than that of free space. The energy
density of photons in free space is magnified enormously by the
permittivityandpermeabilityofbiologicaltissues.Henceafarlower
exogenous field in free space than THz is needed to perturb the
DNA from its normal role of replication. In 1986 Morgan used an
axisymmetric inhomogeneous lossy dielectric model of the human
cranial structure for both an 'adult' and an 'infant' to calculate the
interior spatial distribution of the absorbed power density. These
computations foundsignificant'hotspots' at both1 GHz and3 GHz.
There is a very interesting cosmological analogue of this
situation. It appears that the energy densities in the early universe
were far higher than they are today. In the 1920s Hubble found
the distance between galaxies was red-shifted; the further the
galaxy, the bigger the red shift. This led to the big bang theory
and its relationship to general relativity discovered by Friedmann
and Lemaˆıtre. The 20th century saw an explosion of cosmological
knowledge in general. For instance, it was found that electrons and
protons recombined to form hydrogen atoms when the universe
was roughly 380,000 years old, a red shift of
z
=
1,100. At
this time photons are considered to have decoupled from other
matter, travelling through the universe without further interaction
to appear today as the cosmic microwave background radiation.
While these photons got through as a coherent signal, many others
did not. Among these are those that travel between galaxies. In
recent years there have been reports to indicate that the universal
expansion is accelerating. Dark energy is one accepted theory as to
the reason for this acceleration. It is also known that if a medium
is expanding then the speed of a particle will increase if a collision
processisinvolvedduringtheexpansion.Thisappearstobetruefor
all particles, including bosons such as the photon. Like all matter,
the photon appears to have been getting faster as cosmological
evolution has proceeded. As discussed in Chapter 3 a red shift also
has an important relationship to biological evolution. It has been
