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Images and code © 2016 ESA Advanced Concepts Team (Alexander Wittig, Jai Grover) under CC BY IGO license.
Milky Way background image © 2009 European Southern Observatory (S. Brunier) under CC BY 4.0 license.

Black hole visualization

What is this?

In these images we simulate the view of an observer looking at distant light sources ​while ​in the presence of a black hole. ​

​​The trajectory of a light ray passing by any distribution of mass will be deflected, an effect referred to as gravitational lensing. These deflections are slight in most astrophysical scenarios, but they can be dramatic when caused by compact massive objects like black holes.

In these panoramas we simulate the perspective of an observer that is looking out at a distant, luminous, celestial sphere. In the absence of a black hole, or other source of gravity, the light rays emitted from the celestial sphere will travel along straight lines. As a result the observer will see whatever portion of the celestial sphere is directly ahead of them.

I​f we now introduce a black hole​,​ the image of the celestial sphere seen by the observer will be modified in a couple of ways. ​Some light rays that would not otherwise reach the observer are now lensed towards them. ​And o​ther light rays that would have reached the observer ​will follow trajectories that fall into the black hole. The net result​ is the appearance of a region of darkness - as though the black hole cast a shadow - and a curiously distorted image of the background celestial sphere​ around the black hole​.

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Why do we care?

The Event Horizon Telescope will soon generate images of the galactic core. What will it see?

Black holes are predicted to be the endpoint of the evolution of sufficiently massive stars. However until recently there has been only indirect experimental evidence supporting their existence; ​the exception being the remarkable direct detection of gravitational waves from a merger of two black holes.

​An experiment exploring a​no​t​her direct detection approach​,​ associated with optically (at radio wavelengths) resolving the near-horizon region of the supermassive black hole at the center of our galaxy​,​ is ​now online called the Event Horizon Telescope.

The objective with this approach is to resolve the 'shadow' that the event horizon of a black hole - should there be one - casts due to the strong bending of light by its gravitational field. An observation of the galactic core which reveals a shadow will signal the existence of a black hole, and ​moreover​ the precise shape of the shadow can be matched against templates to discriminate between different candidate black hole solutions.

Our goal with this project is to generate such black hole shadow templates for a variety of interesting black hole solutions. ​

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How do you do this?

We adapt a method called backwards ray-tracing to rays that propagate on a curved space time.

For our simulation we​ treat the observer as a camera​. Each pixel of​ the image captured by the camera is associated to a light ra​y, whose motion through space is governed by a set of equations. We numerically integrate th​ese​ equation​s​ backwards in time to trace the ray to its source, either on the celestial sphere, or the horizon of the black hole. ​The process is illustrated in the animation below. ​


Illustration of backward raytracing from an observer in various directions around a black hole.

If a ray originated at a point on the celestial sphere, we set the color of the corresponding camera pixel to match that of the point. If the ray 'originated' at the black hole we simply color the camera pixel black - no light could have come from that direction.

The reason we trace rays backward from the camera, rather than forward from their source as would be more intuitive, is that the vast majority of rays emitted by distant sources never make it to the camera; hence following them is wasteful if our aim is just to determine what the observer/camera sees. ​

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How to use this website?

The 360 degree virtual reality panoramas show a full 360° panoramic image. You can look around by dragging the image, and move the background by dragging the image while holding down the shift key on your keyboard.
  1. You can load any image as the background either by selecting it or dragging an image file onto the black hole.
  2. You can save a snapshot of your view to a PNG file on your computer by clicking "Save snapshot" in the control panel. This will save the current view.
  3. For 360° panoramas you can also save the whole panorama as an equirectangular image covering the entire sphere. These special images can be viewed using any 360° panoramic viewer.
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It doesn't work!

This website uses WebGL for rendering the black hole image. You need a modern browser (Firefox, Chrome, Safari, or Edge) for this to work. Old browsers such as Internet Explorer, as well as some combinations of browser, operating system and hardware may not work. In that case, try a different browser.
  1. We us WebRTC to access the webcam. Currently this works only in Firefox. In other browsers you may have to change some flags to enable experimental features (e.g. Chrome).
  2. The API to access full screen mode currently only works in Firefox.
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Who are we?

The Advanced Concepts Team is the internal research think-tank of ESA, performing multi-disciplinary research for space beyond the programmatic horizon of ESA.

As part of this mandate we also perform research in various areas of fundamental physics. This particular work on black hole imaging and visualization is part of our ongoing work on gravitational physics.

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