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).

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’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.

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

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

Commenting to reinforce this idea: I'd love an AR approach where I can pin a black hole with a given radius into my living room, and walk around it!

The geosharing augmented reality thing mentioned by the parent comment is very very cool too, I'd pay a few bucks for that! Maybe make it social by letting black holes that people drop somewhere IRL merge, etc...

Reach out to me if you eventually would like to spin up a cheap bit of infrastructure to host the data of where people dropped their black holes, and need some help with that!

That's an excellent idea! And indeed, part of the reason we started with the iPhone is because we've been thinking from the get-go about an eventual extension to Apple Vision Pro. As I wrote in my other comment, this is part of an outreach effort to get the public (and students) excited about black hole physics, so we will always keep the code free and open source.

You need a full 3D scan of the environment of everything the black hole can "see" from the position you want to put it in, not just the traditional "augmented reality" that sits on top of a current camera feed, because black holes are also essentially 360 degree cameras that from some angle will let you see anything around them. Not impossible, but harder than "just" taking an augmented reality feed.

Not everything needs to generate cash :)

The rotating black hole version that we implemented requires GPU code, and porting that to Android is nontrivial---though we'd love it if someone took our open-source code and ported it!

In the meantime, check out this code developed by Dominic Chang (grad student at Harvard) that implements lensing by a non-rotating (Schwarzschild) black hole in your browser: https://dominic-chang.com/bhi-filter/

Glad it worked on second boot! We used to have some bugs in the elliptic integral implementation that led to the app crashing, but we think we've eliminated those, so hopefully this is just a fluke... Anyone else with this issue?

Something to rejoice about, no? ;)

Glad to hear that! You'll probably also enjoy reading about the Black Hole Explorer (BHEX): a proposed space mission that will take the sharpest images in the history of astronomy and resolve the "photon ring" of orbiting light around a black hole. https://www.blackholeexplorer.org/

Good point, the minimum version should be an earlier version of iOS, we don’t use any APIs that are only available in 17.5 or later.

Thanks for pointing that out.

It does

Adding options to drop the multi-camera view and to capture a screenshot is relatively straightforward, and I think we can implement that in the next update. Pinning the black hole to a specific place is a whole other undertaking...

Yes I was wondering if they used shaders to implement this.

1. Division by zero

2. Neutron stars I think

I haven't looked into the equations but I expect the effect of varying the electric charge would be similar to (but less dramatic than) changing the spin of the black hole, which as you can see in our app is not that big of a change.

Good point. In a future update, we can add a button to show / hide the camera views.

There are other apps out there for this kind of black hole vision...

Yes, but one issue is that the amount of redshift depends on the motion of the emitter, so we would have to artificially assign some four-velocity to your surroundings in order to give them some redshift. There doesn't seem to be a "natural" choice for how to do this.

TLDR: redshift depends not only on the position of the source, but also its velocity.

As Project Scientist for BHEX, I am of course excited about the project and eager to spread the word about it! But as I wrote in my other comment, what this is really trying to "sell" is gravitational physics to students interested in black holes, and this effort is supported in part by the National Science Foundation.

I don’t think anybody really knows what‘s inside a black hole. That’s kind of their defining property.

Well technically it approaches infinite gravity. It's a gravitational asymptote. But like the other commenter said, no way to know what it actually is in reality, as we only have mathematical concepts that may or may not match reality.

We're thinking about it! Likely 2025 though.

We wanted the app to work on an iPhone and that required the use of Apple Metal code. This could of course be ported to a desktop but we're not sure there would be much interest in that?

I was able to install it on my M1 Mac, fwiw.

It looks needlessly complicated and messy because the visually interesting region when rotation is turned on is blocked out by the FOV cutouts. We felt it was best to only allow the user to select the full FOV in this mode.

Thanks for the question!

Not currently. As I mentioned elsewhere, the rotating black hole version that we implemented requires GPU code, and porting that to Android is nontrivial---though we'd love it if someone took our open-source code and ported it!

In the meantime, check out this code developed by Dominic Chang (grad student at Harvard) that implements lensing by a non-rotating (Schwarzschild) black hole in your browser: https://dominic-chang.com/bhi-filter/

That would be cool, but then you wouldn't be seeing the world around you anymore. In other words, at that point it becomes a GRMHD simulation, and there is no point in using cameras since the user's environment is obscured, no? Or did you have something else in mind?

Yes, agreed. We thought it would be fair to call it a "simulation" of what your surroundings would look like if a black hole were within your FOV, but as you say we do not take into account all effects (time delays in particular would require a lot of buffering and we decided this would be impractical to implement, and not that illuminating).