How Do We Know Black Holes Exist if We Cannot See Them?

A black hole is a region where gravity is so strong that nothing, not even light, can get out. That raises an obvious problem: if it emits no light, how can we be sure it is there? The answer is that black holes are not invisible in their effects. They pull on stars, heat gas until it glows in X-rays, shake spacetime itself and cast shadows against bright surroundings. Our Relativity & Cosmos tees celebrate Einstein’s 1915 general relativity, the theory that predicts them. This post extends our explainer Do Black Holes Evaporate?

What size is a black hole?

For a non-rotating black hole, general relativity gives the radius of the event horizon, the point of no return, as the Schwarzschild radius:

R_S = 2GM / c²

where R_S is in metres, G = 6.674 × 10⁻¹¹ N·m²/kg² is the gravitational constant, M is the mass in kilograms and c = 2.998 × 10⁸ m/s is the speed of light. For one solar mass, R_S is about 2.95 km (our calculation). It scales directly with mass, so a black hole of 21 solar masses has a horizon radius of about 62 km. That is tiny on astronomical scales, which is why we need indirect methods.

The idea that collapse to a black hole really happens in nature got its theoretical backing from Roger Penrose, who shared the 2020 Nobel Prize in Physics for showing that black hole formation is a robust prediction of general relativity (Nobel Prize 2020^[1]).

Evidence 1: Why was Cygnus X-1 the first black hole?

In 1972 two independent teams looked at a bright X-ray source in the constellation Cygnus. B. Louise Webster and Paul Murdin measured the orbit of a star there and reported in Nature that it was a spectroscopic binary with a heavy companion (Webster & Murdin, 1972^[2]). Charles Bolton identified the X-ray source with the same star, HDE 226868 (Bolton, 1972^[3]).

OpenStax Astronomy explains the reasoning. The visible star is a hot spectral type O star. Doppler shifts in its spectral lines show it is orbiting an unseen companion, and X-rays flickering from the system indicate gas falling onto a very small object (OpenStax Astronomy, §24.6^[4]). OpenStax’s table lists Cygnus X-1 with an O supergiant companion, an orbital period of 5.6 days and a black hole mass estimate of 21 solar masses. A dead star that heavy is too massive to be a white dwarf or a neutron star, which leaves a black hole as the explanation.

Evidence 2: What is at the centre of the Milky Way?

Stars obey the same law of orbits as planets. Kepler’s third law, which NASA describes as a precise mathematical relationship between an orbit’s size and its period (NASA Science^[5]), lets you weigh whatever a star is orbiting. In Newton’s form it reads

M = 4π² a³ / (G T²)

where M is the central mass in kilograms, a is the orbit’s semi-major axis in metres and T is the orbital period in seconds.

For decades, teams led by Reinhard Genzel and Andrea Ghez tracked stars orbiting the centre of our galaxy. The Nobel Prize press release summarises the result: around four million solar masses packed into a region no larger than our solar system (Nobel press release, 2020^[6]). Gillessen and colleagues give 4.28 million solar masses from their 2017 update (Gillessen et al., 2017^[7]). The star S2 completes its orbit in 16 years, and the GRAVITY Collaboration used it to measure our distance to the Galactic Centre as about 8,178 parsecs (GRAVITY Collaboration, 2019^[8]).

Our worked number: putting M = 4.28 million solar masses and T = 16 years into Kepler’s law gives a semi-major axis of about 1,030 astronomical units. That is a large orbit, but the mass inside it is enormous and dark. Its Schwarzschild radius is about 12.6 million km, roughly 0.085 AU. No cluster of ordinary stars could be that heavy, that dark and that compact for long, which is why the object, called Sagittarius A*, is accepted as a supermassive black hole.

Evidence 3: Can we hear black holes merge?

Yes. On 14 September 2015 the two LIGO detectors in the United States recorded a gravitational-wave signal, now called GW150914. According to the discovery paper, its frequency swept up from 35 to 250 hertz and it reached a peak strain of 1.0 × 10⁻²¹. The waveform matched the prediction of general relativity for two black holes of about 36 and 29 solar masses spiralling together into one of about 62 solar masses, with about 3 solar masses’ worth of energy radiated as gravitational waves, at a distance of about 410 megaparsecs (LIGO & Virgo, 2016^[9]). The paper calls it the first direct detection of gravitational waves and the first observation of a binary black hole merger. The 2017 Nobel Prize in Physics recognised the detection (Nobel Prize 2017^[10]).

Strain is the fractional change in length that a passing wave produces, so it has no units. A strain of 10⁻²¹ stretches a 1 km length by 10⁻¹⁸ m, about a thousandth of the width of a proton. That rising “chirp” is the sound our Chirpa character is named after.

Evidence 4: Have we actually photographed a black hole?

We have photographed its shadow. The Event Horizon Telescope links radio telescopes around the world into an Earth-sized virtual telescope. On 10 April 2019 it released the first image of a black hole, in the galaxy Messier 87, 55 million light-years away, with a mass 6.5 billion times that of the Sun (ESO, 2019^[11]). On 12 May 2022 it released an image of Sagittarius A*, the black hole at the centre of our own galaxy, about four million solar masses and about 27,000 light-years away (ESO, 2022^[12]).

What the images show is a ring of glowing hot gas around a dark centre. Light passing close to the hole is bent around it, and some can even orbit it, which is the idea on our Photon Sphere hoodie. The same bending of light by gravity also produces the arcs and rings in our post Einstein Rings and Gravitational Lensing.

So are black holes proven?

Four very different methods, using X-rays, starlight, gravitational waves and radio images, all point to the same kind of object: masses so heavy packed into regions so small that black holes are the explanation the evidence keeps pointing to. What remains open is what happens deep inside the horizon, where general relativity is expected to need a quantum theory of gravity to finish the story.

Quick FAQ

Can you see a black hole?

Not directly, since no light escapes. You can see its effects, and its shadow against hot gas around it.

What was the first black hole discovered?

Cygnus X-1, identified in 1972 as a heavy, unseen companion to a supergiant star.

How heavy is the black hole in our galaxy?

About four million times the mass of the Sun. Gillessen and colleagues give 4.28 million.

How do gravitational waves prove black holes exist?

The measured waveform of GW150914 matched general relativity’s prediction for two merging black holes, and the discovery paper says the observations demonstrate that binary black hole systems exist.

References

1. NobelPrize.org, The Nobel Prize in Physics 2020 (Penrose, Genzel, Ghez). https://www.nobelprize.org/prizes/physics/2020/summary/
2. B. L. Webster & P. Murdin (1972), “Cygnus X-1—a Spectroscopic Binary with a Heavy Companion?,” Nature 235, 37–38. https://www.nature.com/articles/235037a0
3. C. T. Bolton (1972), “Identification of Cygnus X-1 with HDE 226868,” Nature 235, 271–273. https://www.nature.com/articles/235271b0
4. A. Fraknoi, D. Morrison & S. C. Wolff, Astronomy 2e (OpenStax, Rice University), §24.6 “Evidence for Black Holes”. https://openstax.org/books/astronomy-2e/pages/24-6-evidence-for-black-holes
5. NASA Science, “Orbits and Kepler’s Laws”. https://science.nasa.gov/solar-system/orbits-and-keplers-laws/
6. NobelPrize.org, Press release: The Nobel Prize in Physics 2020 (Penrose, Genzel, Ghez). https://www.nobelprize.org/prizes/physics/2020/press-release/
7. S. Gillessen et al. (2017), “An Update on Monitoring Stellar Orbits in the Galactic Center,” Astrophys. J. 837, 30 (arXiv:1611.09144). https://arxiv.org/abs/1611.09144
8. GRAVITY Collaboration (R. Abuter et al.) (2019), “A geometric distance measurement to the Galactic Center black hole with 0.3% uncertainty,” Astron. Astrophys. 625, L10 (arXiv:1904.05721). https://arxiv.org/abs/1904.05721
9. LIGO Scientific & Virgo Collaborations (B. P. Abbott et al.) (2016), “Observation of Gravitational Waves from a Binary Black Hole Merger,” Phys. Rev. Lett. 116, 061102 (arXiv:1602.03837). https://arxiv.org/abs/1602.03837
10. NobelPrize.org, The Nobel Prize in Physics 2017 (gravitational waves). https://www.nobelprize.org/prizes/physics/2017/summary/
11. ESO (10 April 2019), eso1907: “Astronomers Capture First Image of a Black Hole”. https://www.eso.org/public/news/eso1907/
12. ESO (12 May 2022), eso2208: “Astronomers reveal first image of the black hole at the heart of our galaxy”. https://www.eso.org/public/news/eso2208-eht-mw/

Written by Pixelated Physics. Every factual claim is linked to the source we checked; points that are still debated are labelled open.