In particle physics, the desert refers to a theorized gap in energy scales, between the TeV scale and the GUT scale, in which no new physics appears. According to this theory, above the scale of approximately 1012 eV, below which all the Standard Model particles were discovered, there are no new particles to be discovered, until reaching the scale of approximately 1025 eV. It can also be described as a gap in the lengths involved, with no new physics below 10−18 m (the currently probed length scale) and above 10−31 m (the GUT length scale).
The idea of the desert was motivated by the observation of approximate, order of magnitude, gauge coupling unification at the GUT scale. Adding additional new physics at an intermediate scale generically disrupts the gauge coupling unification. With the Minimal Supersymmetric Standard Model particle content, adjustment of parameters can make this unification exact. This unification is not unique, since alternative scenarios like the Katoptron model can also lead to exact unification after a similar energetic desert. If neutrino masses are due to a seesaw mechanism, the seesaw scale should lie within the desert.
The desert theory is attractive because, in such a scenario, measurements of TeV scale physics at the LHC and the near-future ILC will allow extrapolation all the way up to the GUT scale. The obvious downside of the particle desert is that experimental physics will simply have nothing more fundamental to discover, over a very long period of time, which, depending on the rate of increase in experiment energies, might span centuries, millennia or more. Presumably, even if the energy achieved in the LHC, ~ 1013 eV, would be increased by up to 12 orders of magnitude, this would only result in producing more copious amounts of the particles known today, with no underlying structure being probed. The aforementioned timespan might be shortened by observing the GUT scale through a radical development in accelerator physics, or by a non-accelerator observational technology, such as examining tremendously high energy cosmic ray events, or another, yet undeveloped technology.
The alternative to the desert is one or more new physical theories, with new particles, fields or other phenomena unfolding with every few orders of magnitude increase in the energy scale.