OBSERVING A BLACK HOLE

What does WebGR show?

WebGR simulates what an observer would see when flying around in a rocketship, close to a black hole. You can fly around freely, unaffected by gravity, look at the black hole from any angle, and learn about how its powerful gravity bends the path of light rays, thus acting as a gravitational lens. WebGR produces images using calculations based on Einstein’s general theory of relativity (GR).

The image projected on the celestial sphere may be chosen by the user; the default image is an all-sky stellar map made using GAIA data.

ESA/Gaia/DPAC, CC BY-SA 3.0 IGO

How can you observe a black hole?

When we speak of `observing’ black holes, we generally mean that we indirectly observe the black hole through its effects on its surroundings. The black hole itself is not directly visible, except through the action of gravitational lensing (light paths bending in the black hole’s powerful gravitational field). The black hole casts a `shadow’ against and distorts the image of, any light-emitting material whose emission passes close to the black hole.

FIGURE 1:
The EHT image of the nucleus of M87. The first direct image of an accreting black hole was presented by the Event Horizon Telescope collaboration in 2019. In this image, where mm-wave emission is pictured as visible colours, we see a radiating plasma that orbits a black hole. We are, therefore, looking at a gravitationally lensed image of a `local emitter’, a surrounding torus of accreting gas

Event Horizon Telescope Collaboration

On this figure, we can clearly see the black hole’s shadow, contrasted against the light emitted by gas falling (accreting) onto the black hole. In contrast to Fig. 1, the current version of WebGR shows a non-rotating (Schwarzschild) black hole in a locally empty region of space – a vacuum, meaning there is no gas or other material nearby to interact with the black hole (it is not accreting any material). Thus, in WebGR, we do not see a luminous disk of accreting gas with a black-hole shadow in the middle. Rather, we only see distant stars and other objects on the celestial sphere, like the stars we see at night. The black hole’s shadow is only visible when contrasted against a bright background (see Fig. 2).

FIGURE 2:
A screenshot from WebGR, showing the distorting effect of the black hole’s gravitational lens on the stellar background. Unlike Fig. 1, there is no light-emitting material near the black hole; rather, the light is emitted by the stars, infinitely far away.

Can we see the gravitational-lens effects shown in WebGR in real astrophysical images?

Figure 3 shows an image of a gravitational lens taken by the Hubble Space Telescope. Much like the black hole in WebGR, this gravitational lens distorts the image of the celestial sphere – which consists of light from distant stars.

FIGURE 3:
An image of gravitational lensing (a distorted image of the celestial sphere) made by the Hubble space telescope. A luminous red galaxy (LRG 3-757), the bright yellow spot at the center of the image, distorts the image of a background galaxy (the blue circular arc) through the action of its gravitational field. In other words, we see a gravitationally lensed image of a `distant emitter’. Although this is a galaxy acting as a lens rather than a black hole, the principle (and shape of the gravitational field, at this scale) is similar.

NASA/ESA

FIGURE 4:
WebGR Einstein ring. A black hole distorts the image of a background galaxy, namely the Large Magellanic Cloud (LMC, the white circle). We see a gravitationally lensed image of a distant emitter.

Figure 4 is an attempt to reproduce the lensing seen in Fig. 3 in WebGR. Here, we place our observer so that the black hole sits directly between ourselves and the lensed object. An ‘Einstein ring’ can then be observed, as the gravitational lens creates a circular set of curved paths between observer and object.

Traveling deeper into the black hole’s gravitational field

Astrophysical black holes are always observed from (near) Earth, so that the geometry is always that of a very distant observer. In WebGR, on the other hand, we can choose to place our observer at any distance from the black hole. Let’s illustrate two special distances.

FIGURE 5:
Observer view from photon sphere. This image was taken at r=3, which is the black hole’s `photon sphere’. Photons may orbit around the black hole indefinitely, but only when their orbits lie on (are great circles of) this sphere. The practical implication of this is that for observers on the photon sphere, half of the directions in which they can look lead to orbits that plunge into the black hole (bottom half of image), while the other half leads to orbits that escape (top half); in other words, half of the celestial sky is taken up by the black hole’s event horizon!

FIGURE 6:
Observer view from just outside the event horizon, looking outwards from the black hole. This image was taken very close to r=2, which is the black hole’s event horizon. At this extreme location, virtually all look directions correspond to light orbits that plunge into the black hole, so the event horizon takes up almost all of our celestial sphere, and the outside universe appears to us as a shrinking circle of light directly ‘above’ us.

How is WebGR related to the EHT’s research?

WebGR is based on the technique of general-relativistic ray tracing (GRRT). GRRT is a technique that is used extensively in the EHT, in order to produce simulated observations of accreting black holes. Such simulated observations may then be compared to actual observations made using the EHT network, and in this indirect way, we can learn about which kinds of models are a good fit to the observations. See Fig. 6 for an example of such a simulated observation.

FIGURE 6:
EHT simulation. This image shows a simulated observation of an accreting black hole made with the RAPTOR code. Unrestrained by WebGR’s limitations, it shows a spinning black hole with local emitter geometry (and NO distant emitters, which are negligible in this case). The x- and y-axes are calibrated in units of the black hole’s gravitational radius. The dashed line indicates the black-hole shadow.

Unlike the scientific codes used in the EHT, WebGR was specifically developed for realtime rendering and outreach applications. In order to achieve this goal, it has (so far) been necessary to leave out the ability to render local emitters and rotating black holes, although its output has otherwise not suffered in accuracy. It is possible that techniques like the ones used in WebGR may prove useful in research in the future, leading to much faster methods useful for creating simulated observations.