Fig. 4.

Measured group velocities for space-time (ST) wave packets. By changing the tilt angle θ of the spectral hyperplane $\mathcal{P}\left(\theta \right)$, we control v_g of the synthesized ST wave packets in the positive subluminal and superluminal regimes (corresponding to 0 < θ < 90°). We plot v_g on a logarithmic scale. Measurements of ST wave packets with negative-v_g (corresponding to θ > 90°) are given as points in the inset on a linear scale. The data in the main panel and in the inset are represented by points and both curves are the theoretical expectation v_g = ctanθ. The error bars are obtained from the standard error in the slope resulting from linear regression fits (see Methods and Supplementary Fig. 4). The error bars for the measurements are too small to appear, and are provided in Supplementary Table 1