Anatomy of a Black Hole

About this exhibit

How the picture is made

Nothing here is a painted effect. For every pixel, the program follows a ray of light backward from the camera through the curved spacetime of a spinning (Kerr) black hole, solving Einstein's equations of motion for light step by step, about forty times per pixel per frame. Where the ray meets the disk, it reads the gas's temperature and motion; where it escapes, it reads the star field. Lensing, the shadow, the photon ring and the lifted far side of the disk all come out of that calculation on their own.

Physics included

• Exact light paths around a spinning black hole, including frame dragging (Kerr metric, Kerr–Schild coordinates, 4th-order Runge–Kutta).
• A thin disk whose inner edge sits at the innermost stable circular orbit, with the Novikov–Thorne temperature profile.
• Doppler shift and beaming, gravitational redshift and light-travel delay for every ray.
• The falling clock follows a true free-fall path. What you see of it is computed from the moment its light left it.
• The photon lab integrates each photon forward in time with the same equations.

Simplifications

• The disk is infinitely thin and glows like a blackbody. Real disks are puffy, magnetised and partly transparent, and many (including M87* and Sagittarius A*) are too faint to look like this.
• In realistic mode, brightness counts all wavelengths. Hue is the blackbody colour of the shifted temperature, scaled so the hottest gas looks white. Real disks are hundreds of thousands to millions of kelvin and would look blue-white to the eye.
• The camera hovers in place. No jets, no light reflected back onto the disk, no quantum effects.

Keys

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