Does The Force From an Extra Dimension Contradict Physics in 4D?

We examine the question of whether violation of 4D physics is an inevitable consequence of existence of an extra non-compactified dimension. Recent investigations in membrane and Kaluza-Klein theory indicate that when the metric of the spacetime is allowed to depend on the extra coordinate, i.e., the cilindricity condition is dropped, the equation describing the trajectory of a particle in one lower dimension has an extra force with some abnormal properties. Among them, a force term parallel to the four-velocity of the particle and, what is perhaps more surprising, $u_{\mu}f^{\mu} \neq u^{\mu}f_{\mu}$. These properties violate basic concepts in 4D physics. In this paper we argue that these abnormal properties are {\em not} consequence of the extra dimension, but result from the formalism used. We propose a new definition for the force, from the extra dimension, which is free of any contradictions and consistent with usual 4D physics. We show, using warp-products metrics, that this new definition is also more consistent with our physical intuition. The effects of this force could be detected observing objects moving with high speed, near black holes and/or in cosmological situations.Comment: Minor addition in the Introduction to match the version to appear in Phys. Lett.

Brane-world models emerging from collisions of plane waves in 5D

We consider brane-world models embedded in a five-dimensional bulk spacetime with a large extra dimension and a cosmological constant. The cosmology in $5D$ possesses "wave-like" character in the sense that the metric coefficients in the bulk are assumed to have the form of plane waves propagating in the fifth dimension. We model the brane as the "plane" of collision of waves propagating in opposite directions along the extra dimension. This plane is a jump discontinuity which presents the usual ${\bf Z}_2$ symmetry of brane models. The model reproduces the {\em generalized} Friedmann equation for the evolution on the brane, regardless of the specific details in $5D$. Model solutions with spacelike extra coordinate show the usual {\em big-bang} behavior, while those with timelike extra dimension present a {\em big bounce}. This bounce is an genuine effect of a timelike extra dimension. We argue that, based on our current knowledge, models having a large timelike extra dimension cannot be dismissed as mathematical curiosities in non-physical solutions. The size of the extra dimension is small today, but it is {\em increasing} if the universe is expanding with acceleration. Also, the expansion rate of the fifth dimension can be expressed in a simple way through the four-dimensional "deceleration" and Hubble parameters as $- q H$. These predictions could have important observational implications, notably for the time variation of rest mass, electric charge and the gravitational "constant". They hold for the three $(k = 0, + 1, - 1)$ models with arbitrary cosmological constant, and are independent of the signature of the extra dimension.Comment: In V2 the signature of the extra dimension is discussed and new references are added. In V3 typos are correcte