Author: James Riordon
_James Riordon_
Reading time: 22 minutes
Synopsis
Crush (2025) takes you into a world. It shows how everyday things – like water swirling in a sink – can lead to big ideas. These ideas are about black holes, how space and time bend, and the strange rules of the universe. The book talks about our fear of heights and the future of the cosmos. It’s an interesting look at gravity. We think we know gravity well, but we actually understand very little about it.
What’s in it for me? Learn about gravity, and how our ideas about it have changed over many years.
It’s easy to forget about gravity, but we shouldn’t. People have been very interested in gravity for a long time. Thinkers, creators, spiritual people, and scientists have all tried to understand it. They wanted to know why this unseen power affects our bodies, our thoughts, planets, and the whole universe. Almost everything is touched by gravity. But there is still much we don’t fully understand. Gravity is beautiful, confusing, personal, and huge at the same time.
Sir Isaac Newton started our understanding of gravity in the 1600s. Albert Einstein improved on it about 100 years ago. But gravity is still the force we know least about. Einstein even said we shouldn’t call it a force! So, to solve some of these puzzles, let’s explore James Riordon’s book, Crush. This summary has five parts.
Blink 1 – Freefalling, and feeling gravity
What is gravity? Even if you can’t explain it, we all know what it feels like. There’s that strange fear you get when you stand at the edge of something high. Maybe it’s a building, a cliff, or a half-pipe for snowboarding. Gravity makes you fall fast, about 60 kilometers an hour. That feeling is strong enough to make you nervous and step back.
Acrophobia, or the fear of heights, feels like an old instinct. It seems built into our genes for survival. But is it really? In the 1950s, scientists did famous “visual cliff” tests. They put young animals on a glass platform. Part of the platform looked like a deep drop. Rats, kittens, goats, turtles, and even baby humans and chicks were tested. Most of them stopped or froze. Baby humans’ hearts beat faster, and they held onto their parents when moved towards the “cliff.” For many years, people thought this proved that fear of heights was natural. But later studies showed something different. Baby humans are actually interested in cliffs. Their hearts beat fast mostly from excitement. It seems we learn the fear of heights, like other fears such as snakes, spiders, or strangers. We learn it from bad experiences and warnings. This is important because learned fears can be treated.
Now, let’s talk about the physics of gravity’s “pull.” On Earth, we feel about one “g” of acceleration. This is 9.8 meters per second per second. This constant pull has a big effect. It keeps our bones strong and our muscles working. It also decides how fast we speed up when we fall. When we are on a swing or a roller coaster, the “g” forces change for a short time. These are quick, fun changes. Astronauts in space orbit are always falling. Their blood moves from their legs to their chest and head. Their leg muscles get smaller. Their bodies get a bit puffy. Many feel sick, have headaches, and feel confused at first. Later, they can lose bone and muscle, have vision problems, and feel sad or anxious.
People want to prepare for a future where we might spend more time in space or places with less gravity. So, they have tried to stop these problems. But so far, they have found only partial solutions. These include special treadmills with bungee cords, exercise machines, and “vacuum pants” that pull blood to the legs. These things help, but they cannot fully replace the need for Earth-like gravity.
This problem has led to some wild ideas. In a few billion years, the Sun will get hotter and make Earth too hot to live on. One idea to solve this, and also problems with gravity and resources, is to turn Earth into a “rogue planet.” With careful timing, gravity could slowly pull Earth away when the Sun gets bigger and becomes a red giant. Our planet would then be like a slow-moving, fully ready spaceship. This dream of “Spaceship Earth” leads to the next question: what makes any world a good home?
Blink 2 – The just-right planet
You might have heard of the “Goldilocks principle” for finding planets where life could exist. Usually, this means a planet is at the right distance from its star. It needs to be warm enough for ice to melt, but not so hot that oceans disappear. It gets harder because these “right zones” move as stars get older.
But being in the right place isn’t enough. In this case, size also matters. For example, the Moon is in the same good zone as Earth, but nothing can live there. It is simply too small to hold a thick atmosphere or keep liquid water. There is a minimum size for a planet – a bit less than 3 percent of Earth’s mass. This size is needed to stop air and oceans from going into space. A magnetic field is also needed. Charged particles constantly flow through space. Without a magnetic field to protect a planet, these particles hit its atmosphere hard. This makes the surface very dangerous, like being exposed to radiation.
Earth’s shield comes from deep inside. Gravity helped pack our planet tightly and heated it when it formed. Hot elements inside keep heating it. This creates a still-liquid metal core that moves and churns, making a kind of power generator. The field it creates is weak in everyday terms; it only moves a compass needle. But it reaches far and stops many incoming particles. Smaller worlds cool and become solid faster. Their inner generators stop, and their fields disappear.
Gravity also sets an upper limit. If a planet has too much mass, it turns into a gas giant. Then it becomes a “brown dwarf” where complex chemicals are burned away. Even big rocky planets – “super-Earths” up to ten times our mass – might not be good homes. Their gravity could pull in thick layers of hydrogen and helium. These gases would cover the delicate mix of oxygen, nitrogen, and carbon dioxide that life enjoys here.
Studies suggest the best size is for planets a bit bigger than ours: between one and three times Earth’s mass. They would be heavy enough to keep a strong magnetic field and thick, stable air. They would be light enough for moving tectonic plates and rich chemistry on the surface. Worlds about one and a half times Earth’s mass might be “superhabitable.” This means they are even more suitable for life than our own planet.
In a far part of our solar system, in the outer Kuiper Belt, scientists think there might be Planet 9. Data suggests there could be a frozen super-Earth hiding far beyond Neptune. If it exists, it would be safe from the swollen Sun billions of years from now. Its surface would be harsh. But its gravity could feel familiar. And its warm inside might power hidden seas or warm places underground. In that far future, “Goldilocks gravity” could make a distant, dark world an unexpected safe place. This naturally leads to the theories that explain how gravity works at all sizes.
Blink 3 – Newton vs. Einstein
To understand why some planets can support life and others become extreme, it’s good to step back. Let’s ask what gravity really is. Newton’s answer was simple and right: things with mass pull each other. This pull depends on how heavy they are and how far apart they are. Later, scientists added the idea of “fields.” These are like mathematical maps that show how strongly gravity would act at every point in space. So, Earth creates a gravitational field, and the Moon reacts to it.
But then, Einstein came along and basically changed everything. The old three-dimensional idea wasn’t enough. We are all moving through spacetime. This is like four dimensions – space and time joined together. So, according to his theory of general relativity, gravity isn’t a force at all. Big objects bend spacetime. Anything moving through this bent fabric follows the straightest path it can. This path is called a geodesic. If nothing stops you, you keep falling freely. When the ground stops you from following your natural path in spacetime, you feel weight. That is gravity.
Recently, people have started to see space and gravity more like a liquid. Imagine you are in an inner tube, floating through spacetime like a calm river. As you float closer to Earth, spacetime bends or “falls” inward. This means you will speed up as you get near the surface. Now, if you are standing on the ground, it’s like standing on a pier sticking out into the river. This is because space, like water, is flowing around you toward the center of the planet at 40,260 kilometers an hour. So, if you stopped your inner-tube trip by grabbing a rope tied to that pier, the water would be pushing you downstream compared to the pier. But compared to the water, the pier would be pulling you upstream.
So, gravity isn’t really pulling us down. Einstein showed that it’s the constant movement compared to space that causes the effect. This movement determines our weight on a scale. The bigger the planet, the faster that “dip” will be. And the more the planet’s surface will push against you to stop this movement.
Newton also found a number for gravity, called G. This number sets the overall strength of gravity. Scientists first tried to find G in the 1700s. Today, we are still trying with lasers, super-cold atoms, and vacuum chambers. Despite all this work, experiments have not shown consistent results. So, G is still not known as accurately as other numbers in physics.
It’s strange, but the more we try to understand gravity, the more questions appear. This is especially true when we try to connect what we know about gravity with quantum mechanics. Quantum mechanics explains how things work at the smallest sizes. We also have questions about how gravity behaves in its most extreme places – black holes.
Blink 4 – Sinks, singularities, and cosmic waves
Of course, there are places where gravity is extreme: black holes. The idea of a star so heavy that light cannot escape was another strange idea from the 1700s. Since then, we have learned that black holes are areas where spacetime is twisted so much that all paths lead inward. If you cross the event horizon – the point of no return around a black hole – any matter or light is doomed. It will fall toward a central point called a singularity. In our current equations, the density there becomes infinite, and known physics stops working.
To get a sense of this without leaving your kitchen, turn on the tap. A thin stream of water hitting the bottom of your sink will create a circle. Inside that circle, the water is smooth. But at the edges, it suddenly forms a raised, round ridge. Here, the water gets thicker and starts to ripple. Scientists who study liquids call this ridge a “hydraulic jump.” Inside it, the water moves faster than surface waves can travel. So, disturbances cannot move outwards. This pattern is like a black hole: the fast-flowing inner part is like space rushing inward faster than light can escape. The jump is the event horizon. The outer rippling area is where information can still spread out.
Pretty cool, right?
Now, let’s get back on our spacetime inner tube. Imagine what you’d see if you floated towards a black hole instead of Earth. From far away, you’d see light from distant stars bent into arcs and rings by the black hole’s strong gravity. Closer in, you’d fall through a swirling cloud of gas. This gas would be heated to thousands of times the Sun’s surface temperature. It would shine brighter than whole galaxies. Once you cross the event horizon of a large black hole, for a moment, nothing would seem different. But then, the pulling forces would get stronger. They would stretch your body until it breaks apart molecule by molecule. This process is playfully called spaghettification.
This might be gravity at its most dramatic. But mostly, gravity is so weak that people argued if we could even detect its signals, called gravitational waves. These are ripples created in spacetime by huge objects moving, like black holes crashing into each other. These ripples travel at the speed of light. After decades of theories and failed attempts, the Laser Interferometer Gravitational-Wave Observatory, or LIGO, finally found one in 2015. It was from two black holes spiraling together 1.3 billion light-years away.
The exciting thing is, we can now use special telescopes to find ancient ripples. These might even carry news from the very first moments of the universe. But even though we’ve gotten better at finding the smallest details about our universe, sometimes this has led to more questions. In the last part, we will look at how we have tried to connect these big-picture rules with the quantum rules that work at the smallest sizes.
Blink 5 – Loops, strings, and the unfinished story
One of Einstein’s main goals was to find one theory that explains everything. To keep it short, we are still looking for it. Right now, general relativity explains how planets move and how light bends. Quantum theory explains atoms, how chemicals connect, and the chips in your phone.
On their own, their rules make sense. But problems appear in places where both theories should agree. For example, deep inside black holes or at the start of the universe. Relativity predicts “singularities” – points of infinite density. But quantum mechanics does not allow such infinite densities.
String theory was the first big try to fix this difference. It imagines very small particles as tiny vibrating strings. Their different ways of vibrating create electrons, quarks, and gravitons. Gravitons are particles that carry gravity at the quantum level. But to believe in string theory, you must believe in extra dimensions that are curled up and too small for us to see. So far, experiments have found no proof for this theory.
Another idea is loop quantum gravity. In this theory, spacetime itself is made of small loops, like a tiny foam. The smallest possible volume is called Planck volume. In a collapsing star, the tiny size of Planck volume would stop the star from collapsing fully before a singularity can form. Instead, it would create what scientists call a Planck star. Over a very long time, that bounce could blow apart the black hole from the inside.
If loop quantum gravity is proven true, old black holes should someday explode with sudden, strong bursts of radiation. None have been seen yet. But this prediction means loop quantum gravity could possibly be tested.
This same idea also suggests a specific story for how the universe began: a repeating big “bounce” instead of a single Big Bang. A universe before ours would have collapsed to almost Planck density. Then it would have bounced back and created our universe. This cycle may have happened many times. A chain of universes expanding and collapsing. It’s an appealing idea, but it also has problems with energy laws. In our universe, things tend to run down, not reset forever. For now, the idea of a single Big Bang, with some quantum changes, is still the most accepted story. But better observations of the early universe could still change this story.
The famous physicist Stephen Hawking was happy that there was still room for new ideas. He believed there were still discoveries to be made. Newton, watching an apple fall, asked what rule made it move. Einstein imagined being a falling worker and asked what the world felt like during the fall. One view helps us build rockets, bridges, and understand daily life. The other lets us think about the inside of black holes, gravitational waves, and the future of everything. Somewhere in between, in the unfinished mix of quantum foam, dark matter, and bent spacetime, the story of gravity is still being written.
Final summary
In this summary of Crush by James Riordon, you have learned that gravity is the quiet architect of whole universes. It not only shapes our fears and our bodies. It also sets the rules for which planets can hold air and water. It powers magnetic shields and makes planets possible homes for human life. It creates black holes that crush matter and bend light into rings. It sends gravitational waves humming across the cosmos and controls the future of galaxies. Perhaps the most interesting thing is that even though gravity is part of every aspect of life, it still holds many secrets. When the rules of gravity meet quantum mechanics, we lack one theory that explains both. This has led scientists to ideas like string theory, loop quantum gravity, gravitons, extra dimensions, Planck stars, and bouncing universes. But none of them fully explain our universe. The rules that allow stars, chemicals, and people like us to exist seem perfectly set. Yet, they may never be fully explained. Gravity is both the most common force in our lives and the least understood in its deepest parts. The real joy is using it as a guide to keep asking bigger, stranger questions about how reality works.
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Source: https://www.blinkist.com/https://www.blinkist.com/en/books/crush-en