Top: simulated ALMA images of the disk after 150 inner orbits (18 900 yr) at a wavelength of 871 μm (left) and 1.3 μm (right). The position, the disk inclination, distance, and stellar luminosity of HL Tau were used for these simulation. At both wavelengths, a gap is visible. The gap in the images is emphasized and we note that its origin lies in the magnetic field in the disk, and not a planet. Bottom left: spatially resolved map of the spectral index calculated from the simulated observation on the top. The map is rotated clockwise by 40° in comparison to the maps at the top. Bottom right: profile of the spectral index along the cut, indicated by a dashed line on the bottom left figure.