Classical bremsstrahlung of a massless scalar field Φ is studied in gravity mediated ultra-relativistic collisions with impact parameter b of two massive point particles in the presence of d non-compact or toroidal extra dimensions. The spectral and angular distribution of the scalar radiation are analyzed, while the total emitted Φ−energy is found to be strongly enhanced by a d−dependent power of the Lorentz factor γ. The direct radiation amplitude from the accelerated particles is shown to interfere destructively (in the first two leading ultra-relativistic orders) with the one due to the Φ − Φ − graviton interaction in the frequency regime γ/b ω γ 2 /b in all dimensions.

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We examine the evolution of the ground state of a Bose-Einstein condensate in a two-dimensional circular box, the wall of which is initially at rest and then recedes with large and constant speed. The final state of the condensate depends on the rapidity of the expansion of the box. If the number of atoms in the condensate is small compared to the dimensionless speed of the wall, then the condensate becomes a mixture of excitations and follows the expansion of the box, leaving empty though an increasingly larger region between the condensate boundary and the wall. If, on the other hand, the number of atoms is large compared to the dimensionless speed of the wall, then the condensate is always in the ground state and spreads uniformly in all of the expanding box, the condensate boundary always coinciding with the receding wall. Approximate analytic expressions are found for the evolving wave function.

A classical computation of vector bremsstrahlung in ultrarelativistic gravitational-force collisions of massive point particles is presented in an arbitrary number d of extra dimensions. Our method adapts the post-linear formalism of General Relativity to the multidimensional case. The total emitted energy, as well as its angular and frequency distribution and characteristic values, are discussed in detail.For an electromagnetic mediation propagated in the bulk, the emitted energy E em of scattering with impact parameter b has magnitude E em ∼ e 4 e ′ 2 γ d+2 /(m 2 b 3d+3 ), with dominant frequency ω em ∼ γ 2 /b. For the gravitational force the charge emits via vector field, propagated in the bulk, energy Finally, for the ADD model, including four dimensions, the electromagnetic field living on 3-brane, loses on emission the energyThe contribution of the low frequency part of the radiation (soft photons) to the total radiated energy is shown to be negligible for all values of d. The domain of validity of the classical result is discussed. The result is analyzed from the viewpoint of the de WittBrehme-Hobbs equation (and corresponding equations in higher dimensions). The different frequency domains and their competition mentioned above, may be explained as coming from different terms in this equation. Thus the whole emission process may be naturally split in two sub-processes with drastically different spectral and temporal characteristics.

The collision of two massless, gravitationally interacting, point-like massless particles, boosted to the speed of light, colliding with an impact parameter b is being investigated. The collision takes place in four space-time flat dimensional background. A perturbative scheme is employed and the corrections to the energy momentum tensor and to the metric are computed and closed form formulas are provided. This includes the back-reaction on the metric after the collision. Including such corrections suggests that the tracelessness of the initial stress tensors of the colliding particles is preserved during and after the collision. The necessity for introducing an impact parameter in the perturbative treatment is highlighted and the breaking of the underlying perturbative approach at b = 0 is motivated. In addition, the energy radiated in the form of gravitational bremsstrahlung radiation is discussed while an example from gravitational-waves collision is being studied.