Droplet-shaped wave

In physics, droplet-shaped waves are casual localized solutions of the wave equation closely related to the X-shaped waves, but, in contrast, possessing a finite support.

A family of the droplet-shaped waves was obtained by extension of the "toy model" of X-wave generation by a superluminal point electric charge (tachyon) at infinite rectilinear motion ^[1] to the case of a line source pulse started at time t = 0. The pulse front is supposed to propagate with a constant superluminal velocity v = βc (here c is the speed of light, so β > 1).

In the cylindrical spacetime coordinate system τ=ct, ρ, φ, z, originated in the point of pulse generation and oriented along the (given) line of source propagation (direction z), the general expression for such a source pulse takes the form

$s(\tau ,\rho ,z) = \frac{\delta (\rho )} {2\pi \rho} J(\tau ,z) H(\beta \tau -z) H(z),$

where δ(•) and H(•) are, correspondingly, the Dirac delta and Heaviside step functions while J(τ, z) is an arbitrary continuous function representing the pulse shape. Notably, H (βτ − z) H (z) = 0 for τ < 0, so s (τ, ρ, z) = 0 for τ < 0 as well.

As far as the wave source does not exist prior to the moment τ = 0, a one-time application of the causality principle implies zero wavefunction ψ (τ, ρ, z) for negative values of time.

As a consequence, ψ is uniquely defined by the problem for the wave equation with the time-asymmetric homogeneous initial condition

$\begin{align} & \left[ \partial _\tau ^2 - \rho^{-1} \partial_\rho (\rho \partial_\rho) - \partial _z^2 \right] \psi(\tau,\rho,z) = s(\tau,\rho,z) \\ & \psi(\tau,\rho,z) = 0 \quad \text{for} \quad \tau < 0 \end{align}$

The general integral solution for the resulting waves and the analytical description of their finite, droplet-shaped support can be obtained from the above problem using the STTD technique.^[2][3][4]

See also

• X-wave

References

1. E. Recami, M. Zamboni-Rached and C. Dartora, Localized X-shaped field generated by a superluminal electric charge. Physical Review E 69(2), 027602 (2004), doi:10.1103/PhysRevE.69.027602
2. A.B. Utkin, Droplet-shaped waves: casual finite-support analogs of X-shaped waves. arxiv.org 1110.3494 [physics.optics] (2011).
3. A.B. Utkin, Droplet-shaped waves: casual finite-support analogs of X-shaped waves. J. Opt. Soc. Am. A 29(4), 457-462 (2012), doi:10.1364/JOSAA.29.000457
4. A.B. Utkin, Localized Waves Emanated by Pulsed Sources: The Riemann-Volterra Approach. In: Hugo E. Hernández-Figueroa, Erasmo Recami, and Michel Zamboni-Rached (eds.) Non-diffracting Waves. Wiley-VCH: Berlin, ISBN 978-3-527-41195-5, pp. 287-306 (2013)