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Black Holes

What is a black hole?

On National Geographic, in Black Holes, explained, Maya Wei-Haas writes

Black holes are objects in space, usually created from collapsing stars, that are so dense they create deep gravity sinks.

Beyond a certain region, not even light can escape the powerful tug of a black hole’s gravity.

Beyond this region, any matter that enters is seemingly compressed to a singularity – a point of infinite density.

Videos on black holes

Black holes create infinite gravity wells

What’s a gravity well? Think about lifting something up off the ground, and putting it into orbit. Takes a lot of energy, right?

If you’re on an asteroid, it only has a little gravity, so it takes less energy to lift something up and out into orbit. If you’re on the moon, it takes a lot more energy. If you’re on something as massive as the Earth, it takes much more energy.

A gravity well refers to how much energy you need to lift something “up out of the well”

Or the other way around, the well tells us how much energy a falling object will have once it falls from orbit down to the ground.

Gravity well analogy

How deep does a gravity well get? It depends on the mass (how much stuff there is) and the volume (how much three-dimensional space it takes up)

If you take the same mass and compress it into a smaller volume, it’s density increases.

So what happens if you take all the mass of an object, and compress it into zero volume?

As the volume becomes closer to zero, the density increases towards infinity.

This is the famous “you can’t divide by zero” issue, turned into something real and physical.

Well, according to Einstein’s theory of General Relativity (more on this later), in some cases a star can collapse all the way down to zero volume, and thus to a point of infinite density.

Let’s see what happens to the gravity well:

Black hole sun white dwarf

Wowza – the graph of that last gravity well one keeps on going down – infinitely down.

If the predictions of General Relativity are totally correct (read on, we’ll get to that) then the well is infinitely deep! Nothing that gets in can ever get out!

Blach hole gravity well singularity

H. K. Wimmer’s rendition of a black hole modified by Attractor321, for Wikipedia. “Black-hole continuum and its gravity well”


Types of black homes, by mass

stellar – A black hole with a mass of up to 20 or so Suns, fitting inside a ball no larger than a diameter of ten miles.  Dozens may exist in our galaxy.

supermassive – “Have masses greater than 1 million suns combined. Would fit inside a ball with a diameter about the size of the solar system. Scientific evidence suggests that every large galaxy contains a supermassive black hole at its center. The supermassive black hole at the center of the Milky Way galaxy is called Sagittarius A. It has a mass equal to about 4 million suns and would fit inside a ball with a diameter about the size of the sun.” (NASA)

primordial black holes – possibly as small as a single atom – yet with the mass of a large mountain. Thought to have formed in the early universe, soon after the big bang.


How We Know Black Holes Are There

This section from NASA Knows!

A black hole can not be seen because of the strong gravity that is pulling all of the light into the black hole’s center.  Yet we can see the effects of its strong gravity on the stars and gases around it.

If a star is orbiting a certain point in space then we can study the star’s motion to find out if it is orbiting a black hole.

When a black hole and a star are orbiting close together, high-energy light – X-rays and gamma rays – is produced. We can see this high-energy light.

A black hole’s gravity can sometimes be strong enough to pull off the outer gases of the star and grow a disk around itself called the accretion disk. As gas from the accretion disk spirals into the black hole, the gas heats to very high temperatures and releases X-ray light in all directions.

Formation of Black holes

On National Geographic, in Black Holes, explained, Maya Wei-Haas writes

Most form by stellar death. As stars reach the ends of their lives, most will inflate, lose mass, and then cool to form white dwarfs. But the largest of these fiery bodies, those at least 10 to 20 times as massive as our own sun, are destined to become either super-dense neutron stars or so-called stellar-mass black holes.

Here we see two pathways towards forming black holes.

How we predicted that black holes exist

They were first considered in the 18th century – John Michell and Pierre-Simon Laplace speculated that there could be objects whose gravity was so strong, that not even light could escape.

They used the classical laws of physics, before Einstein’s theory of general relativity, so these hypothetical black holes are called “classical black holes.” They don’t have all the same properties as the black holes later predicted by Einstein.

1916 – Karl Schwarzschild discovers that Einstein’s theory of general relativity had solutions with unusual mathematical and physical properties. He “discovered” them in the math, but didn’t propose that they actually exist in our universe. He just thought that math solution was bizarre and interesting.

1958 – David Finkelstein created a revolution in physics – he decided to interpret Schwarzschild ‘s math as something that is physically real. There really was a region of space from which nothing can escape.

Since 1958 – the math made many, very specific predictions, for what we would see with visible light, X-rays, gamma rays, radio waves, and even gravitational waves.

Every one of these predictions has since been actually verified with observations, giving us a very high degree of certainty that these black holes are real.

How much mass is in a typical black hole?

Twenty million Sols compacted into a volume smaller than the Earth.

Black Hole vs Stars – Mass & Size Comparison – With Stunning Animation by: Morn1415

Mass of Supermassive Black Hole in Sun masses

What would a black hole look like?

“Simulated view of a black hole in front of the Large Magellanic Cloud. Note the gravitational lensing effect, which produces two enlarged but highly distorted views of the Cloud. Across the top, the Milky Way disk appears distorted into an arc.”


How would a black hole interact with a nearby star?


Black hole eating a star GIF


As objects fall towards a black hole, tidal forces develop.  Most folks know of “tides” as the way that the oceans move inland, and then out, over the course of a day, creating “high tide” and “low tide” conditions.  But what causes this?

Combining gravity with geometry leads to the inevitable result – gravity has a differential pull on objects of any volume.  This differential pull is called a tidal force.

Learn more about this in our lesson on tides.  Tidal forces lead to the spaghettification of anything falls towards a black hole.

Consider what would happen to four objects near a black hole. Hold them in place, then let go and see what happens.

“Each follows a slightly different course, because the gravity of the planet falls off with distance and acts from the center. The net result is elongation in one direction and squashing in the other. Note: this effect has been massively exaggerated in this animation for the sake of clarity.” (wikipedia)

Spaghettification GIF Wikimedia

Image by Theresa Knott for Wikipedia/Wikimedia,

What would happen if we put a moon or planet near a black hole, and let it fall in? We can see the result by placing a ring of particles far from a black hole, set in a circle, near a black hole, and then letting go.

The red circles here represent parts of a planet or moon. The blue vectors represent the pull of gravity. Note how the circle is distorted and elongated. As the particles get closer to the black hole, they are also pulled closer together laterally (sideways), again leading to spaghettification.

Spaghettification GIF another

Breakthroughs! Directly imaging black holes

We have strong evidence that there is a supermassive black hole at the center of our galaxy. We haven’t been able to image it directly yet, but there have been a lot of breakthroughs.

Here we are looking at the center of our galactic home, the Milky Way, as imaged by 64 radio telescopes in the South African wilderness.

South African MeerKAT radio telescope

black hole center of the Milky Way By SARAO

This image shows filaments of particles, structures that seem to exist in alignment with the galaxy’s central black hole. It’s unclear what causes these filaments. Maybe they are particles ejected by the spinning black hole; maybe they are hypothesized “cosmic strings;” and maybe they’re not unique, and there are other, similar structures waiting to be found, according to a 2017 release from Harvard-Smithsonian Center for Astrophysics.

“This image from MeerKAT is awesome to me because the fine filaments seen in the radio image are excellent tracers of the galactic magnetic field, something we don’t get to see in most optical and infrared data,” Erin Ryan, research space scientist at the NASA Goddard Space Flight Center.
– Ryan F. Mandelbaum, “New South African Telescope Releases Epic Image of the Galactic Center” Gizmodo, 7/13/2018

Unprecedented 16 year long study tracks stars orbiting Milky Way Black Hole

Science Daily, 12/10/2008. With info from the ESO (European Southern Observatory)

“In a 16-year long study, using several of ESO’s flagship telescopes, a team of German astronomers has produced the most detailed view ever of the surroundings of the monster lurking at our Galaxy’s heart — a supermassive black hole. The research has unravelled the hidden secrets of this tumultuous region by mapping the orbits of almost 30 stars, a five-fold increase over previous studies. One of the stars has now completed a full orbit around the black hole.

By watching the motions of 28 stars orbiting the Milky Way’s most central region with admirable patience and amazing precision, astronomers have been able to study the supermassive black hole lurking there. It is known as “Sagittarius A*” (pronounced “Sagittarius A star”). The new research marks the first time that the orbits of so many of these central stars have been calculated precisely and reveals information about the enigmatic formation of these stars — and about the black hole to which they are bound.”
– ESO/ S. Gillessen, R. Genzel

Radio-telescope imaging of a black hole

The black hole itself is invisible, but as material is pulled inside, some of that material emits light that escapes. Also, not all material near a black hole actually enters it. Some is pulled around it, compressed and heated, and then flung out – all the time radiating various forms of radiation.

Imaged via a project from the Event Horizon Telescope (EHT), a planet-scale array of eight ground-based radio telescopes

The EHT team decided to target two of the closest supermassive black holes to us:

1. One at the center of Messier 87 (M87), a massive elliptical galaxy in the nearby Virgo galaxy cluster. This black hole resides 55 million light-years from Earth. It has a mass 6.5 billion times that of the Sun.

NASA image galaxy Messier 87 (M87) with black hole

Chandra X-ray Observatory close-up of the core of the M87 galaxy. Credits: NASA/CXC/Villanova University/J. Neilsen


Event Horizon Telescope Collaboration BLACK HOLE

2. They also looked at a black hole in Sagittarius A*, the one at the center of our Milky Way. They haven’t produced an image of this yet.


Why does the photo of the black hole look the way that it does?


Event Horison Telescope image black hole 2019

Guardian (UK newspaper) graphic. Source: Nature

Why haven’t they yet photographed the black hole at the center of our own galaxy,?

By Mike Wehner

It would make sense to capture a photo of the closest black hole to Earth, especially if we want to see it in great detail. Unfortunately, Earth — and the vast majority of the planets in the galaxy — just aren’t in the right position to see our galaxy’s black hole with optical technology.

The Milky Way is a spiral galaxy with long arms filled with hundreds of billions of stars, and it’s arranged like a flat disc. If you were to look at the entire galaxy from its face, you’d quickly see our dilemma:

The dot labeled “Sun” is where our solar system resides in the galaxy, riding the edge of one of the Milky Way’s long, curved arms. From our vantage point, gazing in the direction of the center of the galaxy looks something like this:

[Image shows a wide swath of stars… and dark areas? That’s interstellar dust within our own galaxy, blocking out a lot of the light.]


Milky Way Galaxy dust

Trying to see our galaxy’s own black hole is like trying to see the center of a vast forest while standing along its fringe. There’s just too much stuff in the way, including stars, planets, gas, and dust.

To have any hope of seeing our own black hole we’d have to send a spacecraft tens, or even hundreds of thousands of light years towards the center of it, allowing it to view the Milky Way from its face rather than its side, at least when talking about the optical spectrum. [And we can’t do that. At all. That’s Star Trek warp drive technology.]

Radio telescopes are capable of cutting through a lot of the cloudy debris and light that obscures our view. An array of such telescopes, spread across the globe, is exactly what the Event Horizon Telescope is, making it possible to glimpse Sagittarius A*, but first the black hole needs to cooperate.

The Milky Way’s black hole is significantly more difficult to capture in images due to how much its signal changes, and how rapidly those changes occur. Researchers with the EHT project still hope to capture a suitable image of Sagittarius A*, but they’re not quite there yet.

Why didn’t scientists photograph the black hole at the center of the Milky Way? BGR.com


Limits of our understanding of black holes

We first really began to understand how black hole with Albert Einstein’s theory of GR (general relativity.)

GR is amazingly accurate, but it isn’t – by itself – compatible with QM (quantum mechanics.)

A GR description of a black hole’s interior yields solutions with mathematical pathologies, infinities, at a certain point.

GR implies that the volume of a black hole is actually zero!?  However, this result contradicts quantum mechanics.  So at the present, we do not know the ultimate final volume of a black hole, nor the nature of matter itself within it.

How must we proceed in order to better understand black holes?

We need a model that unites GR with QM. This may be an exceedingly difficult task that may take mankind centuries more to complete.

At the present time there are some fascinating models that partially offer ways to reconcile relativity and QM. These let us learn more about black holes.

Loop quantum gravity- See the-origins-of-space-and-time


AdS/CFT correspondence – anti-de Sitter/conformal field theory correspondence, sometimes called Maldacena duality.


So do they really contain a singularity?

A black hole, almost by definition, is an object defined by Einstein’s theory of general relativity (GR) that has a singularity at the center. Yet we have also known all along that this theory is only part of the story. GR is an only an approximation of the ultimate, higher level, unified understanding of how the universe works, which we expect to be a unification of GR and quantum mechanics.

While we don’t have a unified theory of everything yet, we do have mathematical models of such theories which show, in principle, how GR and QM are two aspects of one greater reality. Looking at these mathematical models we see that when a black hole forms, the center wouldn’t actually become a singularity. Therefore here are a number of proposed models of black holes that don’t have singularities. Or, if we are strict about definitions, then we can say that these are “black hole alternatives.”

Generic object of dark energy (GEODE and GEODEs) refers to a class of non-singular theoretical objects that mimic black holes, but with dark energy interiors instead.

Fuzzball (string theory) – Fuzzball theory replaces the singularity at the heart of a black hole by positing that the entire region within the black hole’s event horizon is actually a ball of strings, which are advanced as the ultimate building blocks of matter and energy.

Gravastars – gravitational vacuum condensate stars

Gravastars could, in theory, from a collapsing star – except that early on in that collapse, the star’s regular matter turns into “exotic matter” that keeps it from fully caving in. (That’s the same theoretical stuff that’s supposed to keep wormholes open, by the way). The final product would be nearly as compact as a black hole, but not quite enough to form that tricky event horizon.

MECOs – Magnetospheric Eternally Collapsing Objects

The idea here is that as a collapsing object gets super dense and super hot, the radiation it produces creates outward pressure that prevents its collapse, leaving it as a hot ball of plasma rather than a black hole.

Boston stars

While the matter particles (fermions) that make up normal stars have to follow particular rules about how they arrange themselves, bosons (force carrying particles) can huddle together so tightly that they become one big collective particle …If this mysterious stuff formed a star, it’d be a transparent, donut-shaped blob that would emit no light but possess an intense gravitational pull. You know, kind of like what black holes are supposed to do. “Boson stars could mimic black holes,” theorist Steve Liebling tells New Scientist. “And it is possible that we are getting tricked.”

Black Holes Might Not Exist, So Here Are 5 Weirder Alternatives


Physicists Still Aren’t Sure Whether Singularities Actually Exist. Forbes/Quora

Q: What are singularities? Do they exist in nature? Ask a mathematician physicist

Singularities and Black Holes, Stanford Encyclopedia of Philosophy

What really happens inside black holes?

These articles will take you to the frontier of what is known.

Black Hole Cores May Not Be Infinitely Dense: Inside Science

Falling Into a Black Hole

Inside Black Holes

The mass of a supermassive black hole relative to the Sun explained in one crazy GIF

The Fuzzball Fix for a Black Hole Paradox, Quanta magazine

Where gravity is weak and naked singularities are forbidden: Gravity conjectures! Quanta magazine

Related articles

Detecting gravitational waves

Newfound Wormhole Allows Info to Escape Black Holes

External links

How We Discovered the Black Hole at the Center of Our Galaxy, By Erika K. Carlson

Watch Stars Orbit The Milky Way’s Supermassive Black Hole

Learning Standards

Next Gen Science Standards

HS-ESS1-3 – Communicate scientific ideas about the way stars, over their life cycle, produce elements.

Science and engineering practices: Obtaining, evaluating, and communicating information in 9–12 builds on K–8 experiences and progresses to evaluating the validity and reliability of the claims, methods, and designs. Communicate scientific ideas (e.g. about phenomena and/or the process of development and the design and performance of a proposed process or system) in multiple formats (including orally, graphically, textually, and mathematically).

Common Core Connections: ELA/Literacy

WHST.9-12.2 Write informative/explanatory texts, including the narration of historical events, scientific procedures/ experiments, or technical processes. (HS-ESS1-3)

WHST.9-12.1 Write arguments focused on discipline-specific content. (HS-ESS1-6)

SL.11-12.4 Present claims and findings, emphasizing salient points in a focused, coherent manner with relevant evidence, sound valid reasoning, and well-chosen details; use appropriate eye contact, adequate volume, and clear pronunciation. (HS-ESS1-3)

Common Core Connections Mathematics

MP.2 Reason abstractly and quantitatively. (HS-ESS1-3)

MP.4 Model with mathematics. (5-ESS1-1),(5-ESS1-2)

AAAS Benchmarks

C. Relating Matter & Energy and Time & Space

Students will be very interested in the “gee whiz” aspects of relativity—the speed of light limit, time slowing down, nuclear energy release, black holes. This interest can be drawn upon to make the more important points that under extreme conditions the world may work in ways very different from our ordinary experience, and that the test of a scientific theory is not how nearly it matches common sense, but how well it accounts for known observations and predicts new ones that hadn’t been expected…. As another consequence of disproportional change of properties, some “laws” of science (such as how friction depends on speed) are valid only within a certain range of circumstances. New and sometimes surprising kinds of phenomena can appear at extremely large or small values of a variable. For example, a star many times more massive than the sun can eventually collapse under its own gravity to become a black hole from which not even light can escape.

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