getSymmetric(particle) {
    const tha2 = particle.v.getGradient();
    const tha1 = this.getGradient(particle);

    const tha = 2 * (tha1 - tha2);
    const oldx = particle.v.x;
    const oldy = particle.v.y;

    particle.v.x = oldx * Math.cos(tha) - oldy * Math.sin(tha);
    particle.v.y = oldx * Math.sin(tha) + oldy * Math.cos(tha);
  }
Calculates the symmetric position of a particle.
@param {Object} particle - The particle to calculate symmetry for.