Re: limits for the EM spectrum?

Tiago,

This is a excellent question!  Understanding electromagnetic radiation can
be really hard because (amoung other things) sometimes you need to think
about it as a simple wave (classical EM), sometimes as a particle (high
energy photons), and sometimes as a field (quantum field theory).  I think
that most physicists will tell you that one of the hardest courses is
Classical Electrodynamics (sometimes referred to as "Jackson", named after
the author of a somewhat famous textbook on the subject).

I believe that, according to classical electromagnetic theory, there are no
theoretical limitations on the size of EM wavelengths, either short or
long.  There are, however, some practical limitations to the classical view
and, in light of modern quantum mechanics, some limitations to how short
the wavelengths can be.  The question about an "upper wavelength limit" was
already asked on MadSci
( http://www.madsci.org/posts/archives/oct98/909713014.Ph.q.html)
and the short answer is that you probably can't have a wavelength longer than
the size of the universe.  Its also unlikely that there are any practical ways
to create EM radiation with wavelengths larger than the size of the solar
system or other such astronomical scales.

The "shorter" wavelength end of the spectrum is very different.  Quantum
mechanics seems to indicate that there is a likely minimum length scale in
the universe called the "Planck length" (described in this response
http://www.madsci.org/posts/archives/dec2001/1008210525.As.q.html).

Accordingly, it should not be possible to have a wavelength any shorter
than about 10^(-35) meters.  However, even before you reach this short
length scale, there are other issues that alter the way EM radiation works.

Because of the short distances involved, the standard classical view of EM
radiation as a simple wave doesn't work.  Instead, one needs to have a
quantum mechanical understanding of EM radiation.  As wavelengths, and thus
distances, get shorter, there is a point where the distinct Electromagnetic
force merges with another distinct force called the "weak" force to form a
single entity, called the "electroweak" force, which occurs around 10^(-17)
meters.  The electroweak force then merges (in theory) with the "strong"
force into a "grand unified theory"
( http://www.madsci.org/posts/archives/aug2000/966270604.As.r.html)
at around 10^(-30) meters.  Though photons still exist and still have
wave-like properties, once you reach these unification scales, photons
start to become indistinguishable from the other force particles.  Not that
photons with these short wavelengths can't exist, its more that the physics
used to describe them are no long simple waves.

With these two limits in mind, are there any forbidden
frequencies/wavelengths?  No, there are no forbidden frequencies, though it
might be extremely difficult or impractical to create certain ones.  To see
this, assume that there is a particular frequency that you can create, say
red light from a laser.  The light looks red to you because you are not
moving relative to the device that made the laser.  If you were moving, the
frequency would look different to you - either "red shifted" if you were
moving away from it or "blue shifted" if you were moving towards it
( http://www.madsci.org/posts/archives/may97/860507695.Ph.r.html).
 This is one of the predictions of Einstein's relativity theory and, since it
must hold of all photons, there really can't be a forbidden frequencies.

What about red shifts at the "Planck" scale?  I already said that there is
a lower limit to a photon's wavelength, so couldn't you just "red shift"
that?  I believe the answer to that question is that we don't really know.
 The Planck scale is the "place" where our current understanding of physics
starts to fall apart, so there are all sorts of contradictions between what
Quantum Mechanics might say (i.e. there is a minimum distance) and what
General Relativity would say (i.e. space is continuous).

Eric Gauthier