A Black Hole's Core Could be a Strange "Planck Star"

Yes, black holes are weird, but could there actually be something besides a singularity in their core?
Jaime Trosper
Artist rendering of a supermassive black holeNASA/JPL-Cal-tech

There's a lot of interesting theories about black holes, and what happens inside of them. The notion that once matter enters the event horizon around the singularity, nothing, not even photons themselves, can escape is one idea.

Traditionally, we have thought that within black holes, all matter the black hole consumes is compressed into a tiny, infinitely dense point - a singularity. Because the curvature of spacetime within this infinitely dense point also becomes infinite, we cannot know what happens inside of a singularity. Some ways to explain what happens even seem to contradict some universal laws.

In fact, when we try to model a singularity, the math tends to break down, which can lead to some wild results. However, although it may seem like black holes should not be able to exist (I mean, it's sort of illogical that matter can collapse down to an infinitely tiny point), they are mathematically compatible with certain theoretical models of the universe. 

Many physicists through the years have come up with different theories that argue with the idea that the math is consistent, or that a singularity could exist at all. One such theory replaces a singularity with something beyond strange - known as a Planck star (or sometimes called a dark star).

What is Planck length?

To understand a Planck star, we must first understand something called Planck length. In the most basic terms, Planck length is the smallest possible unit of measurement. How small is it? Well, it's approximately equal to 1.6 x 10-35 m, in other words, it's about 10-20 times the size of a proton (or about a trillion times smaller than a proton - one of the smallest particles in existence). So, it's very, very tiny.

Since those numbers are hard to wrap your mind around, let's compare it to something we can perceive with our own eyes - such as a single strand of hair. That strand of hair is more comparable in size to the observable universe than it is to a single Planck length.

Planck Scale
A comparison between known things and Planck Scale. Source: UNSW School of Physics

Researcher Joe Wolfe, from the University of New South Wales, explains further, "To give you an idea, let's compare it with the size of an atom, which is already about 100,000 times smaller than anything you can see with your unaided eye [an atom's size is about 0.0000000001 meters]. Suppose that you measured the diameter of an atom in Planck lengths and that you counted off one Planck length per second. To simply attempt to measure the atomic diameter in Planck lengths, it would take you 10,000,000 times the current age of the universe."

What is a Planck Star?

In one theory, a Planck star is theorized as a compact, exotic star that exists within a black hole's event horizon, and is created when the energy density of a collapsing star reaches the Planck density. Planck density is the Planck mass (believed to be the smallest black hole possible) divided by the Planck volume (this is a Planck length cubed, 4.22 x 10-105 cubic meters), or approximately 5.1 x 1096 g/cm3.

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In a Planck star, the matter is compressed down to the smallest possible scale - the Planck length. If a black hole harbored a Planck star in its core, the matter would not be compressed infinitely, but would actually have an infinitesimally tiny amount of volume - therefore eliminating the tricky singularity. 

Because a Planck star is not a singularity, a black hole containing a Planck star would not have an event horizon, as the gravitational pull would not exceed the speed of light. However, to outside observers, the gravitational pull would still be so strong that it would look and act like an event horizon. So, how can we tell whether or not there is a Planck star at the center of the black hole?

The Black Hole Information Paradox

Renowned physicist Stephen Hawking came up with a solution to this conundrum. He proposed that thermal radiation spontaneously leaks from black holes - purporting the existence of something called Hawking Radiation.

Hawking proposed that pairs of subatomic particles with negative and positive energy will arise naturally near the event horizon and that the positive particle will escape the vicinity of the black hole, thereby emitting a quantum of Hawking radiation. At the same time, the particles with negative energy disappear into the black hole - reducing its mass until it vanishes completely in a final burst of radiation.

To put it simply, Hawking Radiation is the manner in which black holes eventually, and slowly, evaporate, re-emitting the lost particles at the end of their long, long lifespans (predicted to be around 14 billion years).

Turns out, Hawking may have been right. In papers published last year, physicists revealed that they are close to understanding the Black Hole Information paradox, which deals with this purported loss of information, and ultimately circles back around to our Planck star. We'll get there... 

A Black Hole's Core Could be a Strange "Planck Star"
Source: NASA/CXC/M.Weiss

Hawking's theory leads to the conclusion that a black hole will fully evaporate in some finite time in the far future. In this case, it will only emit a finite amount of information encoded within this Hawking radiation. If we assume that at the time, more than half of the information had already been emitted, then any outgoing particle emitted must be entangled with all the Hawking radiation the black hole has previously emitted. This creates a paradox because a principle called "monogamy of entanglement" requires that the outgoing particle cannot be fully entangled with two independent systems at the same time.

Researchers now believe they've finally cracked the code and have proven that if you were to swan dive into a black hole, sure, you'd get spaghettified and totally torn apart, but the atoms that once made you, and the information contained within those atoms, will eventually return to space. This puts an end to the violations of the so-called unitarity theory, wherein quantum physics tells us that the information from the present and past of the universe must be preserved at all times.

According to Quantum Magazine, "Physicists thought they resolved the paradox in 2004 with the notion of black hole complementarity. According to this proposal, information that crosses the event horizon of a black hole both reflects back out and passes inside, never to escape. Because no single observer can ever be both inside and outside the black hole’s horizon, no one can witness both situations simultaneously, and no contradiction arises."

This brings us to...

The Firewall Paradox:

In 2012, the Firewall Paradox got a reimagining. The firewall is a hypothetical phenomenon where an observer falling into a black hole encounters high-energy quanta at the event horizon.

No physicist was able to expand on what exactly happens when a black hole consumes subatomic particles, or how information might leak out of a black hole in the form of quanta. The traditional understanding of physics has always said that all matter consumed by a black hole would immediately be lost to outside observers once it is sucked into the event horizon and the singularity. However, an astronomer named Joseph Polchinkski and several of his coworkers surmised that the entanglement between the infalling particle and the outgoing particle is somehow immediately broken. This would release large amounts of energy, creating a "black hole firewall" at the black hole event horizon and preventing us from observing the information.

Other physicists have proposed that the outgoing and infalling particles are somehow connected by wormholes. Another idea, propounded by string theorists, reimagines black holes as “fuzzballs,” with no singularity and no event horizon. Rather, the entire region within what was envisioned as the event horizon is a tangled ball of strings — those fundamental units of energy that string theory says vibrate in various complicated ways to give rise to space-time and all the forces and particles therein. Instead of an event horizon, a fuzzball has a “fuzzy” surface, more akin to that of a star or a planet.

Samir Mathur, a string theorist at Ohio State University, believes fuzzballs are the true quantum description of a black hole and has become a vocal champion of his own self-described “fuzzball conjecture” expanding on the concept.

These so-called fuzzballs would provide a resolution to the disconnect between classical and quantum mechanics, at least as far as our view of black holes is concerned. However, this theory does come with its own unique set of problems - namely, we would have to reimagine the structure of black holes themselves, replacing the event horizon and singularity with something completely new. 

Regardless of whether the traditional view of black holes is right or wrong, or something in between, there's still much work that needs to be done to reconcile their existence with modern physics. A Planck Star could be the start of a new understanding, or not..... only time will tell.

Are you confused still? A quick summarization 

A Black Hole's Core Could be a Strange "Planck Star"
Source: draganab/iStock


You might be wondering how Planck stars fit into the paradoxes we've looked at here. Let's connect everything together, and help simplify the definition of Planck stars.

Planck Stars are theoretical objects in which a massive star breaks down and what would typically become a singularity - where the density of spacetime is infinite, meaning nothing can escape - instead becomes a Planck Star, where the collapse is stopped by a form of repulsion created by energy density arising from the Heisenberg's uncertainty principlebefore the object reaches an infinitely dense point. The result is an object not much larger than the Planck length, which happens to be the smallest unit of measurement.

Being that this replaces an inescapable singularity with something extremely small, but not infinitely dense (bigger than the Planck scale, but not by many magnitudes), this would allow the resolution of a host of paradoxes around the way that matter and energy work in and around black holes  - although not without creating some new questions along the way.

The biggest shift is that the existence of a Planck star at the center of a black hole would completely do away with the information paradox, as there's enough volume and density in a Planck star to ensure that information about matter and the quantum states of matter engulfed by a black hole are not destroyed - thus also eliminating a contradiction between black hole theory and general relativity. If you're confused about what is meant by information, try to frame it like this: Where are you located right now, are you in motion, what (quantum) state are you in, etc? That is all information.

Interestingly, it's believed that the Planck star in the center of a black hole would continue to grow as more matter falls in. Eventually, the Planck star would consume so much matter and information, it would intersect with the event horizon - causing all of this information to be expelled in a flash. 

The Planck star also does away with the firewall paradox, as there are some interesting implications for how the universe might bounce, and crunch, but those will be the subject of a new article in the next couple of days.

Overall, Planck stars are interesting but complicated ideas about the ways in which black holes may work. I'll leave it up to astronomers to decide whether they are viable replacements to the singularity theorem or not. 

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