Best Evidence Yet That Black Holes Exist

A team of astronomers has found indirect evidence of a supermassive black hole’s event horizon, providing further proof that these wacky objects actually exist in nature.

An artist's conception of a supermassive black hole. NASA/JPL-Caltech

An artist's conception of a supermassive black hole.
NASA/JPL-Caltech

There are many wild ideas in science. But the theory of black holes — those objects so massive and compact that nothing, not even light, can escape — definitely makes the top of the list in my book.

Although the evidence for these grandiose objects is convincing, it isn’t 100% conclusive. So in a field that’s skeptical by nature, some astronomers are still working to prove that black holes are not just the concoction of theorists whose mathematics got away from them. When it comes to “something this extraordinary, we have to work harder before we’re willing to accept it,” says Ramesh Narayan (Harvard-Smithsonian Center for Astrophysics). It’s the carefully balanced dichotomy of wild ideas and obstinate suspicion that enables science to slowly zigzag toward the truth.

Now, a team of scientists including Avery Broderick (Perimeter Institute for Theoretical Physics, Canada), Narayan, and others has found further evidence of these elusive objects. The team peered into the very heart of M87, an elliptical galaxy whose supermassive black hole is 6 billion times heavier than the Sun. There, the astronomers saw indirect evidence for the monster’s event horizon: the point of no return that outlines the black hole itself.

The team used three radio telescopes in California, Arizona, and Hawai‘i to make the observations, which are part of the still-developing Event Horizon Telescope project. Their goal was to distinguish whether this object truly has an event horizon, which is characteristic of only a black hole, or if it actually has a surface.

As material streams onto any massive object, it heats up and emits torrents of high-energy photons. So the gas around the massive object should be extremely bright. But there will be a noticeable difference in that brightness depending on whether the object has a surface or an event horizon.

“If there’s a surface, then the stuff that falls in will splash onto the surface,” says co-author John Kormendy (University of Texas at Austin). And this splash will cause the material to glow even hotter and brighter. But if there isn’t a surface — like in the case of an event horizon — then from our perspective, the material eventually vanishes and therefore can’t keep boosting the glow.

So an object with an event horizon should appear darker than an equally massive object with a surface.

Under this assumption, the next few steps are relatively straightforward (but far from simple). First, the team measured the amount of material falling onto M87’s supermassive black hole. Second, the team calculated the expected brightness for an object with a surface. That number came in at roughly 100 billion times brighter than the Sun. Third, the team checked to see if their observations matched the expected brightness. If so, then the object has a surface. If not, then the object likely has an event horizon.

The team found an object that appeared darker than expected (by at least an order of magnitude) for an object with a surface. “So we conclude that the stuff is falling through an event horizon and giving out a lot less light than it would if it was bashing into a surface,” says Kormendy.

Black Holes Proven? Not Quite Yet.

Although very few astronomers doubt the existence of black holes, this result adds one more thread to the long spool of evidence that suggests black holes exist in nature. Previous studies looked at stellar mass black holes and the Milky Way’s own supermassive black hole, Sgr A*, and also found no evidence of a surface. The new M87 result expands the list of missing surfaces to heavier black holes beyond our galaxy.

It might not be the final nail in the coffin; the team is its own critic. “I will be the first to agree that this is not totally foolproof,” says Narayan. “It’s kind of proving something to be true, by proving that the opposite is not true. We’re proving that there is no surface so we’re saying there must be a horizon. That logical step is not 100% safe.”

The next nail in the coffin will come once the Event Horizon Telescope project is completed. In the next few years, the telescope will include antennas that span from the South Pole to North America, essentially creating a single Earth-size telescope that will cut through the gas and dust enshrouding these supermassive black holes and image the silhouette of the event horizon itself.

Reference:

Avery Broderick et al. “The Event Horizon of M87.” Posted to arXiv.org April 5, 2015.


For more information on the Event Horizon Telescope project check out our February 2012 digital issue of Sky & Telescope magazine.

11 thoughts on “Best Evidence Yet That Black Holes Exist

  1. Peter WilsonPeter Wilson

    When it comes to “something this extraordinary, we have to work harder before we’re willing to accept it,” says Ramesh Narayan.

    I would say the reverse is true; that the idea of a “surface” is actually more extraordinary than a “horizon.” For example, the heliocentric model of the solar system is only “extraordinary” with respect to our intuition; the Earth “feels” stationary. But an intellectual investigation into the matter reveals the geocentric model to be the “extraordinary” one. The geocentric model is convoluted; the heliocentric is straight-forward.

    So it is with black-hole vs. surface. The event horizon is a natural endpoint to massive gravitational systems; the alternative, the solid surface, requires some convoluted, unknown physics to maintain. The burden-of-proof should be on the black-hole skeptics, who propose new physics to prevent collapse, not the other way around.

    1. BernieGoetz

      The contents of a star that contained light would be expected to be ultra-relativistic. The salient equation for ultra-relativistic matter is the pressure P = (density)(c^2)/3. That describes a gas, not a solid.

      Using the virial equation would indicate a star radius of about 0.82 SR (SR = Schwarzschild radius). This star model can not be proved by observing accreting matter as described in the above article, but it could be proved by observing the merger of 2 orbiting black holes that are not terribly dissimilar in size. The contents of the smaller star would massively eject at the location of the merger.

  2. Nick-DePiano

    So an object with an event horizon should appear darker than an equally massive object with a surface.

    What super massive objects (on the order of billions of sun masses) have been observed that have a surface? Im just an amateur astronomer, but it seems unlikely that an object as massive as the super massive black holes being observed would have a surface to observe and compare with.

  3. Bill Gaede

    There is no object in Physics known as ‘black hole’. The proponent who believes in such nonsense has the burden of DEFINING the term ‘black hole’ before he introduces this supernatural entity into Science…
    .
    https://www.youtube.com/watch?v=W1UPkjRIKYo
    .
    And it doesn’t matter if Einstein, Hawking and all the Nobel Prize winners since 1900 declare this surrealistic concoction to be a fact. Authority has no authority in Science. People should learn to think instead of gawking at celebrities.

    1. hellometeor

      Utter nonsense.

      You have no idea what you are talking about. Have you read even a single book for the lay person on astrophysics?

      A black hole is a loose term. It can be used to describe a region. The event horizon is a boundary to a region. But the boundary doesn’t have substance. It’s not an object. It’s only significance is that it’s where the escape velocity is the speed of light.

      But since it’s such a clearly defined boundary, it’s still reasonable to thonk of that sphereical boundary as an object, even if in a strict senss it isn’t one.

      And you refer back to the planet Vulcan, whi disparaging “relativists.” You do know it was a relativist who disproved Vulcan right?

    2. Normand-RivardNormand-Rivard

      Nothing supernatural here, Mr. Gaede. A black hole is a region of space at the center of which a tiny object called a singularity resides. This singularity is the dead body of a star, collapsed into a minuscule point by its own gravity. The black hole is the area around it where anything, not even light, can escape its gravitational pull. By themselves, both are undetecable, but the matter pulled into the black hole must be detectable because at the tremendous speed the matter is attracted, it must shine brightly in X-rays. Hope it clarifies

  4. ROBERT STENTON

    Many scientists have equated a Black Hole to a quantum particle since a Black Hole like a quantum particle can be fully described by just a few numbers (spin, mass and net charge). Missing is the quantum concept of “Uncertainty”. If a Black Hole is a quantum particle, then the Event Horizon as now visualized would represent that particle’s “Uncertainty”. Since nothing can escape the Event Horizon, proving the existence of the Event Horizon solves the riddle of the existence of a Singularity and its infinite density that make Black Holes so unimaginable to people like Bill as well as many scientist. If the findings of this article are correct, then the visualized “Uncertainty” means that we can not state that the Singularity inside a Black Hole is infinitely small and dense but only that the so call Singularity has a size equal to or less than its Event Horizon.

    1. Peter WilsonPeter Wilson

      Agreed.
      We can’t say what happens before birth or after death, but that doesn’t mean birth and death don’t happen. Likewise, we can’t say what happens beyond the event horizon, but that doesn’t mean event horizons don’t happen.

  5. Faye_Kane_girl_brainFaye_Kane_girl_brain

    > an object with an event horizon should appear darker than an equally massive object with a surface.

    That’s REAL tenuous, man. It relies on us knowing how bright stuff is when it splashes onto a compact 6 billion M☉ object with a surface. Since no such object can exist, we have no way to calibrate our belief about its brightness.

    This truck is rolling down the road of “finding exactly what you want to find,” just like a the evidence for dark energy.

    —Faye Kane ♀ girl brain

All comments must follow the Sky & Telescope Terms of Use and will be moderated prior to posting. Please be civil in your comments. Sky & Telescope reserves the right to use the comments we receive, in whole or in part, and to use the commenter’s username, in any medium. See also the Terms of Use and Privacy Policy.

COMMENT