Show HN: Physically accurate black hole simulation using your iPhone camera

Original Article ↗ Hacker News Discussion ↗

App concept and physics approach

  • iOS app applies general-relativistic gravitational lensing of a black hole to live iPhone camera feeds (front and rear).
  • Two main modes:
    • Static black hole: non-rotating (Schwarzschild), with “Realistic FOV” (limited by actual camera view, producing lobed dark regions and blind spots) and “Full FOV” (idealized infinite field of view, giving a single circular shadow).
    • Kerr black hole: rotating, with adjustable spin parameter; textures for this mode are precomputed in higher precision and used as a lookup map on the GPU.
  • Developers emphasize that this is a visual simulation of lensing, not a full black-hole simulation (no time delays, redshift, jets, accretion disks, etc.).

Visual behavior and realism questions

  • Some users expect objects to “disappear” or get much dimmer; others explain that strong lensing lets you see behind the black hole while some light is still absorbed.
  • Discussion around shadow shape:
    • Non-rotating case with full FOV should produce a circular shadow.
    • Rotating (Kerr) case is expected to deviate from a circle; some commenters question whether the preview screenshots match theoretical shapes.
  • Redshift is discussed: in the chosen setup (sources effectively at infinity and at rest relative to the camera) gravitational redshift at the camera vanishes, so it is not shown.

AR/VR and feature ideas

  • Strong interest in AR features: “pin” a black hole in a fixed spot and walk around it; geoshared singularities in real locations; stats based on mass, Hawking radiation, and local gravity.
  • Others note this would require full 3D environment mapping or VR, since a black hole acts like a 360° lens.
  • Requested features: capture button for warped images, option to hide multi-camera thumbnails, ability to fix orientation of the black hole.

Platforms, implementation, and performance

  • Implemented with Apple’s Metal on iPhone; authors note surprising GPU performance enabling real-time high-res lensing.
  • Porting to Android or desktop/WebGPU is seen as possible but nontrivial; a browser-based non-rotating version by another researcher is referenced as an alternative.
  • One user reports an initial crash related to camera permissions; minimum iOS version requirement (17.5) is acknowledged as unnecessarily high and slated for correction.
  • App can be installed on Apple Silicon Macs and works there for at least one user.

Scope, terminology, and expectations

  • Some argue that “simulation” overstates it since it’s essentially an image filter for one effect; developers agree but defend the term for “what your surroundings would look like.”
  • Suggestions arise for simulating accretion disks or Kerr–Newman black holes; devs respond that doing so would move toward full GRMHD simulations and obscure the live camera view.
  • There is meta-discussion on monetization vs. pure educational/outreach value; several commenters appreciate that it’s free, open source, and collects no data.