Black holes all look the same, regardless of their size

Releasing a remarkable image of the black hole at the center of our galaxy isn’t just an incredible scientific achievement — it matches exactly with predictions about what black holes are and how these strange objects are formed by the force of gravity.

The black hole, called Sagittarius A*, is a type called a supermassive black hole, which is at the center of almost all galaxies. Ours is on the small side for such giants: At 4.3 million times the mass of the Sun, it is much smaller than other samples such as Messier 87 photographed in 2019 and which is 6.5 billion times the mass of the sun is.

The EHT collaboration created a series of images of Sagittarius A*, using ray tracing, a technique that visualizes the properties of the black hole based on data collected with the radio telescope array and predictions made by Einstein’s general theory of relativity. The images shown here were created by UArizona’s Chi-kwan Chan. Ben Prather/EHT Theory Working Group/Chi-Kwan Chan

However, the images of these two black holes are strikingly similar, both with a distinctive donut shape. And that matches exactly with the predictions of physicists, who said that black holes would look the same regardless of their size.

“The fact that the light looks like a ring, with the black shadow in it, tells you it’s pure gravity,” black hole researcher Dimitrios Psaltis of the University of Arizona explained in a statement. “It’s all predicted by Einstein’s general theory of relativity, the only theory in the cosmos that doesn’t care about scale.”

This scale is unusual in that most things that exist on different scales look very different – Psaltis gives the example of an ant and an elephant, which look very different because of, among other things, the way their masses are supported. But black holes aren’t like that, it seems, because they’re the same whether they’re big or small. Messier 87 is 1,500 times more massive than Sagittarius A* and is also much larger, as seen in a comparison image from the European Southern Observatory. But the two are very similar.

Size comparison of the two black holes captured by the Event Horizon Telescope (EHT) Collaboration: M87*, at the heart of the galaxy Messier 87, and Sagittarius A* (Sgr A*), at the center of the Milky Way . EHT Collaboration (Courtesy: Lia Medeiros, xkcd)

That means that even very small black holes, if we could image them, would look like the images of Sagittarius A* and Messier 87. They would all have the same donut shape.

“Wherever we look, we should see donuts, and they should all look more or less the same,” Psaltis said, “and the reason this is important — in addition to confirming our prediction — is that no one In physics, we tend to dislike a world where things don’t have an anchor point, a defined scale.”

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