## Podcast Transcript

Gravity is the weakest of the fundamental forces of nature, yet, if you have enough of it, it can create the most powerful thing in the known universe: a black hole.

The very idea of a black hole didn’t really exist until the early 20th century, and now they are regularly found by the world’s most powerful telescopes.

As much as we know about them, there is, even more we don’t know and probably will never know.

Learn more about black holes, what they are, and how they work on this episode of Everything Everywhere Daily.

When I say that gravity is the weakest of the fundamental forces in nature, it might surprise some of you. After all, gravity is what causes us to fall down, and it is why the planets revolve around the sun.

However, consider this, if you pick up an object, you can counter the entire gravitational force of planet Earth with just your arm.

Technically, there is a gravitational attraction between any two objects, however, for the most part, it is very weak. The gravitational attraction between two people, for example, is so weak that it can barely be measured.

Gravity is a fundamental property of mass, and the thing is, you can just keep piling up mass to get more and more gravity.

The gravity we experience on Earth is defined as one g or one gravitational equivalent.

Let’s say that you landed on a planet with twice the gravitational force of Earth or 2-gs. You would definitely notice that something was different. If you weighed 150 pounds or 68 kilograms, it would be like walking around with that weight on your shoulders all the time.

Walking would be difficult and simple falls would potentially break bones.

At 3g, even simple movement would be difficult for all but elite athletes.

The Icelandic strongman Hafþór Björnsson once set a world record by taking five steps with a 1,430-pound log on his shoulders. That would be the equivalent of walking in a 4.6 g environment.

A 5-Gs, it would be difficult for any human to stand up from a seated position, and breathing would be extremely difficult.

At 10-Gs, taking even a single step would break bones, assuming you could even take a step.

At 90-G, your bones would be crushed by gravity alone.

10-Gs, however, is nothing. It might be beyond the ability of humans to withstand, but cosmically speaking, it is nothing.

Stars can accumulate an enormous amount of mass. Our star is huge compared to Earth, but there are other stars in our galaxy that have a mass 250x that of our sun.

When a star reaches the end of its life and can no longer produce fusion, the heat which expanded the star outward disappears, and its gravity causes it to collapse.

Depending on the mass of the star, it might collapse down to what is known as a white dwarf. A white dwarf can have the mass of our sun but be the size of the Earth. A teaspoon of matter from a white dwarf star would weigh 15 tons or three elephants.

The only thing which stops a white dwarf from collapsing further is something known as electron degeneracy pressure. This is when quantum effects in the individual atoms are the only thing fighting against gravity.

This has a limit, in particular, it is known as the Chandrasekhar limit. At 1.4 solar masses, electron degeneracy pressure can no longer continue to withstand gravity.  Gravity will cause all the individual atoms to collapse, and the electrons to merge with protons. The result is called a neutron star.

A neutron star is like one massive atomic nucleus. It is the forces within the atomic nucleus which is now the only thing holding back against gravity.

Whereas a white dwarf is something the mass of our sun in the size of a planet, a neutron star could be of similar mass but only a few kilometers across.

But what if we keep piling more mass onto a neutron star? Then what?

Eventually, it reaches a point where nothing we know of can withstand the gravity. It becomes a black hole.

When I mean nothing can withstand the gravity, I do mean nothing can withstand it. There are no physical forces, and no objects that can escape the gravity of a black hole. Light can’t even escape, which is why they are known as black holes.

In a black hole, most of what we know about reality simply falls apart. A planet or star has a radius, a size. A black hole doesn’t have a size. There is no size, no matter how small, that it could be because gravity would always crush it even smaller.

Black holes are often called singularities for this reason. Their size would be shrunk down to a mathematical point.

Instead, a black hole would have what is known as an event horizon. Anything within the event horizon can never escape. Moreover, it can’t communicate with anything outside of the event horizon.

In many episodes, I’ll talk about something which was at least considered centuries before it came into being. In the case of black holes, the first person who considered such a body was the English clergyman John Michell in 1784.

It wasn’t until the theory of relativity developed by Albert Einstein did people take the implications of such extreme gravity seriously.

However, a black hole was simply a theoretical object for decades. No one was sure if they really existed or if it was something that just went weird with the equations once the mass reached a particular point.

The debate ended in 1971 with the discovery of Cygnus X-1, the first black hole which was discovered.

You might be wondering, if a black hole doesn’t emit or even reflect any light, then how can it be detected?

While you can’t see a black hole directly, you observe the stuff around it. Because of their high gravity, black holes will often have an accretion disk around them that spins quite rapidly. This disk is usually the source of X-rays or other wavelengths of light that are emitted.

Since the discovery of Cygnus X-1, there has been a steady stream of black holes which have been discovered. One of the biggest findings is that there is a supermassive black hole at the center of most galaxies.

Known as an active galactic nucleus or AGN, these can sometimes produce an incredible amount of energy around their accretion discs, and they are the source of energy for quasars.

Supermassive black holes are hundreds of thousands to billions of times the mass of our sun. The largest black hole which has been discovered to date is in the center of the galaxy Holm 15A, 700 million light-years from Earth. It is believed to have a mass equivalent to 40 billion times that of our sun.

Something this massive is believed to have been created through multiple collisions with other galaxies and mergers with other black holes.

When black holes merge, it is an infrequent event but the most gravitationally impactful event in the universe. In 2015, the LIGO gravitational observatory, on which I’ve done a previous episode, for the first time measured the gravitational waves from a black hole merger here on Earth.

While very large astronomical black holes are what capture the attention of astronomers, in theory, a black hole can be of any mass. If you condense the mass of any object enough, you can, in theory, crease mini or micro-black holes.

To create this type of black hole, you would need an enormous amount of energy. Such conditions might have existed just before the big bang, or they might also exist inside a particle accelerator.

When the Large Hadron Collider opened in Europe, there was a small group who didn’t want it to open because they were afraid it might create a micro black hole that could destroy the Earth.

Their fears, it turned out, were unwarranted for several reasons. First, the Large Hadron Collider would need to be about 35 times more powerful to even theoretically create a micro black hole. Secondly, even in theory, micro black holes would only last for a tiny fraction of a second before they disappeared.

Both micro black holes and the method by which they would evaporate were first proposed by the greatest black hole theorist of all time, Steven Hawking.

Hawking proposed the idea that black holes could disappear over time due to something called Hawking Radiation.

According to quantum mechanics, particles and antiparticles can be created spontaneously from vacuum fluctuations. Normally, these particles and antiparticles quickly annihilate each other, returning their energy to the vacuum. However, if this process occurs near the event horizon of a black hole, one of the particles can be drawn into the black hole while the other escapes. This creates a net energy loss from the black hole, and the escaping particle is observed as Hawking radiation.

The rate of Hawking radiation emission is inversely proportional to the mass of the black hole, meaning that smaller black holes emit more radiation than larger ones.

In the case of micro black holes, they would vanish almost instantly.

The threat of micro black holes also has to do with a misunderstanding of how black holes work.

Black holes do not suck things into them. A black hole created by a few sub-atomic particles still would have the gravitational attraction of a few sub-atomic particles, and the event horizon of such a tiny black hole would be even smaller.

Black holes are just large sources of gravity. They are not omnidirectional space vacuum cleaners.

For example, let’s assume that our sun was instantly turned into a black hole with the same mass as our sun. What would happen to the Earth and other planets in the solar system?

The answer is, for the most part, nothing. They could continue to orbit the new black hole just as they do the sun because it would have the same mass. It would be a whole lot darker, but the orbit of the planet wouldn’t change.

Albert Einstein famously showed that mass and energy were equivalent in his famous equation E=mc2.  One of the implications of this is that it is theoretically possible to create a black hole just by using energy.

If enough energy could be concentrated in one spot, it could form what is known as a kugelblitz.  A kugelblitz would have an event horizon and would be indistinguishable from a regular black hole.

I’ll end with one of the most challenging questions regarding black holes. The black hole information paradox.

According to the principles of quantum mechanics, information is never truly lost but rather is encoded in the state of a system. However, according to general relativity, any matter that falls into a black hole is considered irretrievably lost, as it is trapped behind the black hole’s event horizon and cannot be observed from the outside.

This creates a paradox, as it suggests that information can be lost in violation of the principles of quantum mechanics.

Resolving this paradox has been a major question in theoretical physics, and the best guesses as to an answer lie in the previously mentioned Hawking Radiation.

Black holes are at the forefront of theoretical physics because, in them, everything we know about the universe ceases to make sense. We can never peer into a black hole or send a probe into one to gather data.

Black holes have an important part to play in the creation of galaxies and might even be part of the solution to the riddles of dark matter or dark energy.

Nothing that we know of, or at this point can even theoretically think of, can beat a black hole. That is why black holes are, and will always be, the most powerful thing in the universe.

The Executive Producer of Everything Everywhere Daily is Charles Daniel.

The associate producers are Thor Thomsen and Peter Bennett.

The number of reviews incoming has surpassed my ability to read them, so I’m going to start reading two.

The first review comes from listener Ava over on Apple Podcasts in the United States. They write:

G o a t

I listened to the Ramadan episode because I’m Muslim, and I love the show ? Honestly, it’s hard to fast.

Ava age 10

Thanks Ava! If you are listening at age 10, then you are off to a great start.

The next review comes from bdawghoya also from Apple Podcasts in the United States. They write:

Amazing podcast

Proud recent joiner of the completionist club (and possibly the only member to join listening solely on airplanes for work trips). Anyway, the bite-size format, varied topics, and Gary’s great research combine to make an amazing podcast. This should be mandatory listening for everyone.

Thanks, bdawg! Your completionist club card will now come with the rare aviation badge unlocked, something which you can now showoff to all the other members.

Remember, if you leave a review or send me a boostagram, you too can have it read on the show,