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

Hello! We are Dr. Roman Berens, Prof. Alex Lupsasca, and Trevor Gravely (PhD Candidate) and we are physicists working at Vanderbilt University. We are excited to share Black Hole Vision: https://apps.apple.com/us/app/black-hole-vision/id6737292448.

Black Hole Vision simulates the gravitational lensing effects of a black hole and applies these effects to the video feeds from an iPhone's cameras. The application implements the lensing equations derived from general relativity (see https://arxiv.org/abs/1910.12881 if you are interested in the details) to create a physically accurate effect.

The app can either put a black hole in front of the main camera to show your environment as lensed by a black hole, or it can be used in "selfie" mode with the black hole in front of the front-facing camera to show you a lensed version of yourself.

There are several additional options you can select when using the app. The first lensing option you can select is "Static black hole". In this mode, we simulate a non-rotating (Schwarzschild) black hole. There are two submodes that change the simulated field-of-view (FOV): "Realistic FOV" and "Full FOV". The realistic FOV mode takes into account the finite FOV of the iPhone cameras, leading to a multi-lobed dark patch in the center of the screen. This patch includes both the "black hole shadow" (light rays that end up falling into the black hole) and "blind spots" (directions that lie outside the FOV of both the front-and-rear-facing cameras). The full FOV mode acts as if the cameras have an infinite FOV such that they cover all angles. The result is a single, circular black hole shadow at the center of the screen.

Next, you can select the "Kerr black hole" mode, which adds rotation (spin) to the black hole. Additionally, you can augment the rotational speed of the black hole (its spin, labeled "a" and given as a percentage of the maximal spin).

In a nutshell, the app computes a map from texture coordinate to texture coordinate. This map is itself stored as a texture --- to obtain the value of the map on texture coordinates (x,y), one samples the texture at (x,y) and the resulting float4 contains the outputs (x',y') as well as a status code.

When the user selects the "Static black hole" mode, this texture is computed on the GPU and cached. The "Kerr black hole" textures, however, have been precomputed in Mathematica, due to the need for double precision floating point math, which is not natively available in Apple's Metal shading language.

The source code, including the Mathematica notebook, can be found here https://github.com/graveltr/BlackHoleVision.

We hope you enjoy watching the world with Black Hole Vision and welcome any questions or feedback. If you like the app, please share it with your friends!

The code was written at Vanderbilt University by Trevor Gravely with input from Dr. Roman Berens and Prof. Alex Lupsasca. This project was supported by CAREER award PHY-2340457 and grant AST-2307888 from the National Science Foundation.

License: This app includes a port of the GNU Scientific Library's (GSL) implementation of Jacobi elliptic functions and the elliptic integrals to Metal. It is licensed under the GNU General Public License v3.0 (GPL-3.0). You can view the full license and obtain a copy of the source code at: https://github.com/graveltr/BlackHoleVision.

By any chance, was Andrew Strominger involved in this at all? He gave the Andrew Chamblin Memorial Lecture in Cambridge last month and demoed something that looked similar.

Yes, Andy has been very involved in the story of the photon ring and was one of the lead authors on the original paper that started it all: https://www.science.org/doi/10.1126/sciadv.aaz1310

(And he was also my PhD advisor.)

I think what he showed you was likely a version of this that was coded up by Harvard graduate student Dominic Chang: https://dominic-chang.com/bhi-filter/

It works very well (and in a browser!) but is limited to a non-rotating (Schwarzschild) black hole---we really wanted to include black hole spin (the Kerr case). As we write on the github, talking with Dominic about his implementation was very useful and we are hoping to get a paper explaining both codes out before the end of the year.

I’m confused by what I see.

It looks like nothing actually disappears. I expected a black hole to not just affect what an area looked like, but also to “disappear” some part of what was there.

I feel like this app could also be an app clip to make it so that you don’t have to outright install the app to use it: https://developer.apple.com/app-clips/

I think that’s why this demonstration is interesting. It’s showing how the light can be bent around the black hole. Anything that crosses the event horizon won’t be coming back, but because of the lensing of the light you can “see” behind a black hole.

So if I’m understanding correctly, the black hole is supposed to be between me and what I’m looking at, not in what I’m looking at?

If so, then my question is wouldn’t some light be lost to the black hole? Shouldn’t a substantial portion of the light coming at me from the other side of the black hole disappear into the black hole, making what does lens around dimmer?

Because, for an external observer, time infinitely slows down near the event horizon. In other words, during one hour by the clock of the far-away observer, the time that passes by the clock of the falling observer approaches zero as he approaches the event horizon. So, when you look from the outside, objects get 'frozen' as they approach the event horizon. For the falling observer, nothing special happens at the event horizon, and he just falls through.

If you happen to approach the event horizon closely and come back again far away to where you started, you will see that a lot of time passed at your origin, while by your clock, the trip might have been short.

Yes some light would be lost the black hole, but also some light you would not have normally seen is now coming your way due to space time warping.

Here's Veritasium video on Gravitational Lensing effect: https://www.youtube.com/watch?v=zUyH3XhpLTo

As far as I can tell, the black hole's you're generating don't look especially correct in the preview: they should have a circular shadow like this https://i.imgur.com/zeShgrx.jpeg

What the black hole looks like depends on how you define your field of view. And if the black hole is spinning, then you don't expect a circular shadow at all. But in our app, if you pick the "Static black hole" (the non-rotating, Schwarzschild case) and select the "Full FOV" option, then you will see the circular shadow that you expect.

The preview shows that you have a static black hole selected

This shape of the shadow is also wrong for kerr though, this is what kerr looks like:

https://i.imgur.com/3cS1fNI.png

"static" is "Shwarzchild without rotation"?

Do black holes have hair?

Where is the Hawking radiation in these models? Does it diffuse through the boundary and the outer system?

What about black hole jets?

What about vortices? With Gross-Pitaevskii and SQR Superfluid Quantum Relativity

https://westurner.github.io/hnlog/ Ctrl-F Fedi , Bernoulli, Gross-Pitaevskii:

> "Gravity as a fluid dynamic phenomenon in a superfluid quantum space. Fluid quantum gravity and relativity." (2015) https://hal.science/hal-01248015/ :

> FWIU: also rejects a hard singularity boundary, describes curl and vorticity in fluids (with Gross-Pitaevskii,), and rejects antimatter.

Actual observations of black holes;

"This image shows the observed image of M87's black hole (left) the simulation obtained with a General Relativistic Magnetohydrodynamics model, blurred to the resolution of the Event Horizon Telescope [...]" https://www.reddit.com/r/space/comments/bd59mp/this_image_sh...

"Stars orbiting the black hole at the heart of the Milky Way" ESO. https://youtube.com/watch?v=TF8THY5spmo&

"Motion of stars around Sagittarius A*" Keck/UCLA. https://youtube.com/shorts/A2jcVusR54E

/? M87a time lapse

/? Sagittarius A time lapse

/? black hole vortex dynamics

Yes but the preview has 'realistic FOV' selected not 'full'. And the rotating black hole does have the same shadow as your image if you turn the rotation speed up.

Hey, thanks for not collecting personal data for no good reason!

When you say "normally," do you mean all else being equal except no black hole? Or substitute an opaque mass for the black hole?

"Cosmic Simulation Reveals How Black Holes Grow and Evolve" (2024) https://www.caltech.edu/about/news/cosmic-simulation-reveals...

"FORGE’d in FIRE: Resolving the End of Star Formation and Structure of AGN Accretion Disks from Cosmological Initial Conditions" (2024) https://astro.theoj.org/article/94757-forge-d-in-fire-resolv...

STARFORGE

GIZMO: http://www.tapir.caltech.edu/~phopkins/Site/GIZMO.html .. MPI+OpenMP .. Src: https://github.com/pfhopkins/gizmo-public :

> This is GIZMO: a flexible, multi-method multi-physics code. The code solves the fluid using Lagrangian mesh-free finite-volume Godunov methods (or SPH, or fixed-grid Eulerian methods), and self-gravity with fast hybrid PM-Tree methods and fully-adaptive resolution. Other physics include: magnetic fields (ideal and non-ideal), radiation-hydrodynamics, anisotropic conduction and viscosity, sub-grid turbulent diffusion, radiative cooling, cosmological integration, sink particles, dust-gas mixtures, cosmic rays, degenerate equations of state, galaxy/star/black hole formation and feedback, self-interacting and scalar-field dark matter, on-the-fly structure finding, and more.

A lot of light would be absorbed by the black hole. A lot of light paths would be bent and miss or nearly miss the black hole, making edges of the black hole quite bright. The dimming effect would be much larger than the (brighter) immediate periphery.