Summary

In 2019 and 2022 the Event Horizon Telescope released images of two supermassive black-hole candidates. Both showed bright, asymmetric rings enclosing central brightness depressions of the scale expected for black holes. That agreement — at the level of a few percent in the measured ring diameter — is what makes them scientifically powerful. The ring is not merely beautiful; it tests a model for near-horizon emission and spacetime geometry.

The more interesting question, the one I want to spend time on here, is: what would a wormhole look like? For particular traversable-wormhole metrics, radically different. The images are thus evidence against some alternatives, but not a geometry-independent ruling-out of every wormhole.

The Images

The Event Horizon Telescope is a planet-scale interferometer: radio dishes from Hawaii to the South Pole, phase-locked to atomic clocks, synthesizing an effective aperture the diameter of Earth. At millimetre wavelengths, this gives an angular resolution of around 20 microarcseconds — enough to resolve a grapefruit on the Moon.

In April 2019 the collaboration published six simultaneous papers on M87*, the supermassive black hole at the centre of the Virgo A galaxy (EHT Collaboration et al., 2019a, 2019b). The ring had an angular diameter of \(42 \pm 3\) μas, consistent with a black hole of mass \(M = (6.5 \pm 0.7) \times 10^9 \, M_\odot\) at a distance of 16.8 Mpc. The southern arc of the ring was brighter — I will return to why.

In May 2022 the same team published results on Sagittarius A*, the Milky Way’s central black hole (EHT Collaboration et al., 2022). The ring diameter: \(51.8 \pm 2.3\) μas, corresponding to a mass of \(\sim 4 \times 10^6 \, M_\odot\) at 8.18 kpc. M87* is roughly 1500 times more massive than Sgr A* and roughly 2000 times farther away — so the two apparent ring sizes are within 25% of each other. The universe arranged the coincidence; the EHT exploited it.

The Physics of the Ring

The ring is not the black hole itself. You cannot image an event horizon: by definition, no information escapes from it. The EHT reconstructs lensed emission from hot plasma around the compact object, including light associated with the strong-lensing region. Its central brightness depression is consistent with the predicted shadow, but the observed ring is not a resolved image of a single photon orbit.

For a non-rotating (Schwarzschild) black hole, the photon sphere sits at:

$$ r_\text{ph} = \frac{3GM}{c^2} = \frac{3}{2} R_S $$

where \(R_S = 2GM/c^2\) is the Schwarzschild radius. Light orbiting here is in unstable equilibrium: a small perturbation sends it either spiralling inward or escaping to infinity. The critical impact parameter — the perpendicular distance from the optical axis at which an incoming photon just grazes the photon sphere — is:

$$ b_c = \frac{3\sqrt{3} \, GM}{c^2} \approx 5.196 \, \frac{GM}{c^2} $$

The angular diameter of the shadow as seen by a distant observer is therefore:

$$ \theta_\text{shadow} = \frac{2 b_c}{D} = \frac{6\sqrt{3} \, GM}{c^2 D} $$

Plugging in the EHT numbers for M87* (\(M = 6.5 \times 10^9 \, M_\odot\), \(D = 16.8\) Mpc):

$$ \theta \approx \frac{6 \times 1.732 \times 6.5 \times 10^9 \times 1477 \, \text{m}}{16.8 \times 3.086 \times 10^{22} \, \text{m}} \approx 40 \, \mu\text{as} $$

The EHT measured a \(42 \pm 3\) μas emission-ring diameter. The Schwarzschild scale estimate is therefore close, but it is not a one-parameter prediction: the measured ring also depends on the emission model, observing resolution, spin and viewing geometry. The EHT’s modelling analyses nevertheless found consistency with a Kerr black hole and its independently constrained mass.

The first numerical simulation of this image was done by Jean-Pierre Luminet in 1979, using punch cards and an IBM 7040 (Luminet, 1979). He computed the geodesics, rendered the result by hand on photographic paper, and produced an image that looks startlingly like the 2019 photograph — forty years before the telescope existed.

The Brightness Asymmetry

The southern arc of the M87* ring is brighter. Relativistic Doppler beaming in an orbiting, magnetised plasma is a natural contribution: emission from material moving towards the observer is boosted and blueshifted, while that from receding material is deboosted (EHT Collaboration et al., 2019b). The image morphology is also shaped by the plasma and lensing, so it is not a direct map of a thin accretion disk.

The southern brightness is consistent with the orientation inferred from M87’s large-scale jet. It does not, by itself, measure the black hole spin: the 2017 EHT image could not determine spin quantitatively without stronger assumptions about the emitting plasma.

Interstellar Did It Right

In 2014, the visual effects company Double Negative rendered the black hole Gargantua for Christopher Nolan’s Interstellar. They did this by integrating the actual geodesic equations for a rapidly spinning (near-extremal Kerr) black hole. Kip Thorne, one of the producers, collaborated on two companion papers with the visual effects team (James et al., 2015a).

The resulting image showed the accretion disk wrapping both above and below the black hole, producing a characteristic double-arc structure — direct emission at the equator plus a secondary image of the disk mirrored by gravitational lensing. This was not artistic licence. It was the first photorealistic render of a black hole produced from first principles, and the physicists found new results in the process: features of the lens map that had not previously been worked out analytically.

The same team published a companion paper on the wormhole in Interstellar (James et al., 2015b). That paper is where things get interesting.

What a Wormhole Would Actually Look Like

A traversable Morris-Thorne wormhole connects two regions of spacetime through a throat. An observer near the throat would see both connected universes simultaneously — one on each side of the throat boundary. The key visual feature, worked out in detail by Thomas Müller (Müller, 2004), is this:

  • Looking through the throat, you see the far-side universe compressed into a disk, bounded by a bright Einstein ring at the throat.
  • Outside the ring, you see the near-side universe, heavily distorted by the wormhole’s gravitational field.
  • There is no shadow in the sense a black hole has — no region from which light cannot escape. Instead, the ring acts as a portal: all light that reaches the throat passes through rather than being absorbed.

The James et al. (2015b) wormhole paper shows this explicitly. The Interstellar wormhole was rendered as a spherical lens with a celestial hemisphere visible through it. The visual signature is a double celestial sphere: your own sky distorted around the outside, and a compressed view of a distant universe through the middle.

This looks nothing like the EHT images.

The EHT sees a central brightness depression surrounded by a bright ring. A simple, symmetrically illuminated Morris–Thorne wormhole would instead transmit light through its throat, so its centre need not be dark. But an image depends on the wormhole metric, the illumination on both sides and the plasma distribution; “a second universe in the centre” is a useful rendering of one setup, not a universal observational prediction.

The Images Rule Something Out

This is the point I find underappreciated. The EHT results are usually discussed as confirming that M87* and Sgr A* are black holes consistent with GR. That framing is correct. But the images are also falsifying evidence against alternatives.

Several exotic compact-object proposals — gravastars, boson stars and some wormhole metrics — can produce shadow-like features. Horizon-scale images can constrain parameters of selected alternatives, including wormhole spacetimes, but their present resolution and the uncertain emission model leave degeneracies. The EHT morphology is consistent with Kerr-based models; it is not a unique image-level identification of an event horizon.

The constraint is not absolute. Wormhole geometries can have photon spheres and can mimic a black-hole shadow at current resolution. The clean portal picture belongs to a specified, idealised Morris–Thorne geometry; neither M87* nor Sgr A* has established that geometry or excluded every traversable shortcut through spacetime by morphology alone.

For more on wormhole theory — ER bridges as time-reversal symmetry, the Einstein-Rosen caterpillar, and Euclidean wormholes in single theories — see a later post. The physics is rich and ongoing. But a picture of a wormhole, if one were ever imaged, would not look like what the EHT published. It would look like a portal.

The Astonishing Thing Is That It Worked

I want to end on this. The ring around M87* was predicted in 1916 from a theory written down without any observation of a black hole, by people who were not sure black holes existed, using mathematics developed for entirely different purposes. Luminet computed the image in 1979 on punch cards, and it matched a photograph taken in 2019 with a planet-scale interferometer.

The agreement in this scale estimate is a few percent. That is impressive, but it is not an exactness claim: the uncertainty budget includes the source model as well as the measurement.

The images are astonishing not because they surprised physicists — they confirmed what general relativity predicted. They are astonishing because general relativity is apparently the kind of theory that earns the right to be trusted at microarcsecond precision, at distances of 16.8 megaparsecs, around objects whose entire interiors are, by construction, hidden from us.

Peer review welcome. If you have a wormhole geometry whose shadow is indistinguishable from a Kerr black hole at current EHT resolution, I would genuinely like to read the paper.

Literature checked through 2026-07-11.

References

  • Event Horizon Telescope Collaboration et al. (2019). First M87 Event Horizon Telescope results. I. The shadow of the supermassive black hole. The Astrophysical Journal Letters, 875, L1. DOI: 10.3847/2041-8213/ab0ec7
  • Event Horizon Telescope Collaboration et al. (2019). First M87 Event Horizon Telescope results. V. Physical origin of the asymmetric ring. The Astrophysical Journal Letters, 875, L5. DOI: 10.3847/2041-8213/ab0f43
  • Event Horizon Telescope Collaboration et al. (2022). First Sagittarius A* Event Horizon Telescope results. I. The shadow of the supermassive black hole in the center of the Milky Way. The Astrophysical Journal Letters, 930, L12. DOI: 10.3847/2041-8213/ac6674
  • Luminet, J.-P. (1979). Image of a spherical black hole with thin accretion disk. Astronomy & Astrophysics, 75, 228–235.
  • James, O., von Tunzelmann, E., Franklin, P., & Thorne, K. S. (2015). Gravitational lensing by spinning black holes in astrophysics, and in the movie Interstellar. Classical and Quantum Gravity, 32, 065001. DOI: 10.1088/0264-9381/32/6/065001
  • James, O., von Tunzelmann, E., Franklin, P., & Thorne, K. S. (2015). Visualizing Interstellar’s wormhole. American Journal of Physics, 83(6), 486–499. DOI: 10.1119/1.4916949
  • Müller, T. (2004). Visual appearance of a Morris-Thorne wormhole. American Journal of Physics, 72(8), 1045–1050. DOI: 10.1119/1.1758220
  • Vagnozzi, S., Roy, R., Tsai, Y.-D., & Visinelli, L. (2023). Horizon-scale tests of gravity theories and fundamental physics from the Event Horizon Telescope image of Sagittarius A*. Classical and Quantum Gravity, 40, 165007. DOI: 10.1088/1361-6382/acd97b

Changelog

  • 2026-07-11: Clarified that EHT images reconstruct lensed plasma emission and a central brightness depression rather than directly resolving a photon sphere or measuring spin; narrowed the wormhole discussion to specified metrics and illumination assumptions.