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There are at least two seemingly fundamental energy scales in nature, the
electroweak scale and the Planck scale
GeV, where gravity becomes as strong as the gauge interactions. Over the
last two decades, explaining the smallness and radiative stability of the hierarchy has been one of the greatest driving forces behind
the construction of theories beyond the Standard Model (SM) . While many
di.erent specific proposals for weak and Planck scale physics have been made,
there is a commonly held picture of the basic structure of physics beyond the
SM. A new e.ective field theory (e.g. a softly broken supersymmetric theory
or technicolor) is revealed at the weak scale, stabilizing and perhaps explaining the origin of the hierarchy. On the other hand, the physics responsible for
making a sensible quantum theory of gravity is revealed only at the Planck
scale. The desert between the weak and Planck scales could itself be populated with towers of new effective field theories which can play a number
of roles, such as triggering dynamical symmetry breakings or explaining the
pattern of fermion masses and mixings.
In this picture, the experimental investigation of weak scale energies is
quite exciting, as it is guaranteed to reveal the true mechanism of electroweak
symmetry breaking and stabilization of the hierarchy. One can also hope
that a detailed measurement of low energy parameters can give valuable
clues to the structure of e.ective field theories at higher energies, perhaps
even approaching the Planck scale. Nevertheless, it is fair to say that in this
paradigm, the thorough exploration of the weak scale will never give a direct
experimental handle on strong gravitational physics.
It is remarkable that such rich theoretical structures have been built on
the assumption of the existence of two disparate fundamental energy scales,
and . However, there is an important di.erence between these
scales. While electroweak interactions have been probed at distances ,
gravitational forces have not remotely been probed at distances : gravity has only been accurately measured in the ~1cm range. Our interpretation of as a fundamental energy scale (where gravitational interactions
become strong) is then based on the assumption that gravity is unmodified
over the 33 orders of magnitude between where it is measured at a ~1 cm
down to the Planck length cm. Given the crucial way in which
the fundamental role attributed to affects our current thinking, it is
worthwhile questioning this extrapolation and seeking new alternatives to
the standard picture of physics beyond the SM.
In fact, given that the fundamental nature of the weak scale is an ex-
perimental certainty, we wish to take the philosophy that mEW is the only
fundamental short distance scale in nature, even setting the scale for the
strength of the gravitational interaction.
The major part of the content is taken from:
N. Arkani-Hamed, S. Dimopoulos, and G. Dvali, "The Hierarchy Problem and New Dimensions at a Millimeter," Phys. Lett. B 429, 263 (1998)
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