Crush

Close Encounters with Gravity

By James Riordon

Category: Science | Reading Duration: 22 min | Rating: 4.3/5 (32 ratings)

About the Book

Crush (2025) pulls you into a world where everyday experiences – like watching water swirl in a sink – open doors to black holes, bending spacetime, and the strange physics that shape the universe. From our fear of heights to the fate of the cosmos, it’s a fascinating look at gravity, the force we think we know best, but barely understand at all.

Who Should Read This?

Curious science fans who love learning how the universe works
Space enthusiasts drawn to the strange physics behind cosmic extremes
People who enjoy big ideas about our origins

What’s in it for me? Get the low down on gravity, and how our understanding of it has evolved over the centuries.

It may be tempting to take gravity for granted, but we shouldn’t. In fact, people have been captivated by gravity for as long as we’ve had words for wonder. Philosophers, inventors, mystics, and scientists have all tried to pin down why this invisible influence shapes our bodies, thoughts, planets, and the universe itself. Pretty much everything is influenced by gravity in one way or another.

And yet, there’s a lot we still don’t quite understand. Gravity is elegant, maddening, intimate, and cosmic all at once. Even though Sir Isaac Newton laid the groundwork for our understanding of gravity back in the seventeenth century, and Albert Einstein spruced it up about a hundred years ago, gravity remains the force we understand least. As Einstein would suggest, we shouldn’t even call it a force at all! So, to clear up some of these mysteries, let’s cross the event horizon and dip into this five-section Blink on James Riordon’s Crush.

Chapter 1: Freefalling, and feeling gravity

What is gravity? Even if you can’t define it, we all have a strong relationship with it. There’s that queasy sense of fear you get when you’re standing at the edge of a drop. Whether it’s a rooftop, a cliff, or the lip of a snowboarding superpipe, gravity is what gets you moving at sixty kilometers an hour during a typical freefall, and that’s strong enough to make your nerves jangle and take a step back.

Acrophobia, or a fear of heights, feels like ancient wiring, a survival instinct baked into our DNA. But is it really? Psychologists in the 1950s built the famous “visual cliff” experiments. They put young animals on a glass platform that suddenly revealed what looked like a sheer drop: rats, kittens, goats, turtles, even baby goats and chicks. Most balked or froze. Human infants showed racing heartbeats and clung to caregivers when carried toward the “cliff.

” For years, that was taken as proof that fear of heights was innate. But later work showed a twist: human infants are drawn to cliffs. Their hearts pound mostly from excitement. Acrophobia, like many phobias – be it snakes, spiders, or strangers – seems to be something we learn through bad experiences and warnings. That matters, because learned fears can be treated. Now, as for the physics of gravity’s “pull”.

At Earth’s surface we live under about one g of acceleration, 9. 8 meters per second per second. That steady tug has a lot of influence. It’s what keeps our bones dense and our muscles engaged, along with deciding how quickly we speed up when we fall. Whenever we’re on a swing or a rollercoaster, we flirt with changes in g, but those are short, playful jolts. As for astronauts in orbit, they live in near-continuous freefall.

Blood shifts from legs to chest and head, calves shrink, torsos puff slightly, and many battle nausea, headaches, and disorientation before longer-term issues like bone loss, muscle wasting, vision changes, and mood disorders set in. Since there’s plenty of interest in preparing for a future when people might spend more time in space or lower gravity environments, efforts have been made to combat these side-effects. But so far, we’ve only found partial fixes, like bungee-tethered treadmills, resistance machines, and vacuum “pants” that pull blood toward the legs. They help but can‘t fully substitute our need for Earth-like gravity. It’s a problem that’s led to some wild ideas. At some point in the next few billion years, the Sun’s rising heat will make the Earth uninhabitable – at least in its current position.

One possible solution – one that solves some of the issues around gravity and resources – would be to turn Earth into a rogue planet. Carefully timed gravitational slingshots could, in principle, tug Earth itself outward when the Sun swells into a red giant, turning our planet into a slow-moving, fully furnished ark. That dream of “Spaceship Earth” sets up the next question: what makes any world a good home in the first place?

Chapter 2: The just-right planet

You may have heard of the “Goldilocks principle” when it comes to finding hospitable planets elsewhere in the universe. The usual starting point is somewhere around a star where it’s warm enough for ice to melt but not so hot that oceans evaporate. Making it trickier is the fact that as stars age, those zones drift. Yet location alone doesn’t do the job.

In this case, size also matters. For example, the Moon shares our place in the Sun’s habitable zone but is utterly inhospitable. It’s simply too small to hang on to a thick atmosphere or retain liquid water. There’s a minimum planetary mass – a little under 3 percent of Earth’s mass – needed to keep air and oceans from escaping to space. A magnetic field is another necessity. Charged particles stream constantly through space, and without the protection of a magnetic field they hammer a world’s atmosphere, making the surface essentially radioactive.

Earth’s shield comes from deep within. Gravity helped pack our planet tightly and heat it during formation. Radioactive elements inside keep topping up that internal heat. The result is a still-molten metallic core that churns and convects, acting as a dynamo. The field it generates is weak in everyday terms, just enough to twist a compass needle, but huge in reach, deflecting much of the incoming particle radiation. Smaller worlds cool and solidify more quickly, their dynamos sputter out, and their fields fade.

Gravity also creates an upper limit. Add too much mass and a planet morphs into a gas giant, then into a brown dwarf where complex chemistry is roasted away. Even big rocky planets – super-Earths up to ten times our mass – might be poor hosts if their gravity grabs thick blankets of hydrogen and helium that swamp the delicate mix of oxygen, nitrogen, and carbon dioxide life enjoys here. Simulations suggest the sweet spot lies with planets modestly bigger than ours: between one to three Earth masses. Heavy enough to keep a strong magnetic field and dense, climate-smoothing air; light enough for mobile tectonic plates and rich surface chemistry. Worlds around one-and-a-half Earth masses may be “superhabitable” – more forgiving than even our own.

In a distant part of our solar system, in the farthest section of the Kupier Belt region, is where scientists are hypothesizing about Planet 9. Data suggests that there may be a frozen super-Earth lurking far beyond Neptune. If it exists, it would sit safely beyond the swollen Sun billions of years from now. Its surface would be brutal, but its gravity could feel familiar, and its warm interior might power buried seas or geothermal refuges. In that far future, Goldilocks gravity could make a distant, dim world an unlikely sanctuary – and that leads naturally to the theories that tell us how gravity behaves on every scale.

Chapter 3: Newton vs. Einstein

To understand why some planets can cradle life while others spin off into extremes, now’s a good time to step back and ask what gravity actually is. Newton’s answer was simple and spot on: things with mass attract each other, with a strength that depends on how heavy they are and how far apart they sit. Physicists later added the idea of fields – mathematical maps that say how strongly gravity would act at every point in space – so the Earth shapes a gravitational field, and the Moon responds to that. But then, along came Einstein who basically rewrote the script.

The old three dimensional model wasn’t enough. We are all moving through spacetime, a four-dimensional continuum. And so, according to general relativity, gravity isn’t a force at all. Massive objects curve spacetime, and anything moving through that curved fabric continues to follow the straightest path it can – called a geodesic. If nothing gets in the way, you continue to freefall. When the ground stops you from following your natural spacetime path, you feel weight.

That’s gravity. Recently, it’s become more common for people to look at space and gravity with a sort of fluid perspective. So imagine being in an innertube, floating through spacetime as if it were a calm river. As you float closer to Earth, spacetime curves or “falls” inward, which means you’ll speed up as you approach the surface. Now, if you’re standing on the ground, it’s like you’re standing on a pier sticking out into the river, because space, like water, is flowing around you toward the center of the planet at 40,260 kilometers an hour. So, if you decided to stop your inner-tube trip by grabbing hold of a rope tethered to that pier, relative to the pier, the water would be pushing you downstream, but relative to the water, the pier would be dragging you upstream.

So gravity isn’t really pulling us down. Einstein revealed that it’s the continual acceleration relative to space that is doing the work. That acceleration determines our weight on a bathroom scale. The bigger the planet, the faster that dip is going to be, and more the planet’s surface will push against you to resist the acceleration. Now, Newton also came up with a gravitational constant, known as G – the number that sets the overall strength of gravity. Scientists first tried to pin down G in the eighteenth century.

Nowadays, we’re still trying with lasers, ultra-cold atoms, and vacuum chambers. Despite the continued effort, experiments have failed to show consistent results and G remains poorly known compared with other constants. Oddly enough, the more we try to pin down gravity the more questions emerge. Especially when we try to square what we know about gravity with what we know about quantum mechanics and how things unfold in gravity’s wildest playgrounds – black holes.

Chapter 4: Sinks, singularities, and cosmic waves

There are, of course, places where gravity goes wild: black holes. The idea of a star so dense that light cannot escape was another wild twist of eighteenth century physics. Since then, we’ve come to find out that black holes are regions where spacetime has been twisted so hard that all paths lead inward. Cross the event horizon – the one-way boundary that surrounds a black hole – and any matter or light is doomed to fall toward a central singularity where, in our current equations, density blows up and known physics fails.

To get a feel for this without leaving your kitchen, turn on the tap. A thin stream of water striking the bottom of your sink will create a circle. Within that circle is a smooth sheet. But on the edges it abruptly forms a raised, circular ridge where the flow thickens and begins to ripple. Fluid physicists call that ridge a hydraulic jump. Inside it, the water races faster than surface waves can travel, so disturbances cannot move outward.

That pattern mirrors a black hole: the inner fast-flowing region is like space plunging inward faster than light can escape; the jump is the event horizon; the outer rippling area is where information can still propagate. Pretty cool, huh? Now, let’s get back on our spacetime innertube and imagine what you’d see if you floated toward a black hole instead of Earth. From afar, you’d see light from background stars bent into arcs and rings by the object’s intense gravity. Closer in, you’d drop through a whirl of infalling gas heated to thousands of times the Sun’s surface temperature, outshining entire galaxies. Once you cross the event horizon of a large hole, for a moment, nothing would seem special.

But then tidal forces would ramp up, stretching your body until it tears it apart molecule by molecule in a process affectionately called spaghettification. This may be gravity at its most dramatic. But for the most part gravity is so weak that it was debated whether or not you could pick up its signals, known as gravitational waves. These are the ripples created in spacetime by massive bodies in motion, like colliding black holes. These ripples travel at light speed, and after decades of theory and false starts, the Laser Interferometer Gravitational-Wave Observatory, or LIGO for short, finally caught one in 2015: two black holes spiraling together 1. 3 billion light-years away.

The cool thing is, we can now use microwave telescopes to pick up on the ancient ripples that might even carry news from the universe’s first fractions of a second. But even though we’ve gotten better at being able to pick up the tiniest details about our universe, in some cases this has led to more questions. In the final blink we’ll look at how we’ve tried to reconcile all of these big-picture rules with the quantum rules running the show at the smallest scales.

Chapter 5: Loops, strings, and the unfinished story

One of Einstein’s driving goals was to find a unifying theory – a theory of everything. Long story short, we’re still looking for it. Right now, general relativity explains how planets orbit and light bends, and quantum theory explains atomic spectra, chemical bonds, and the chips in your phone. On their own, their rules make sense, but trouble arrives in regions where both should agree – like deep inside black holes or at the universe’s birth.

While relativity predicts singularities, quantum mechanics objects to the kind of infinite densities that singularities represent. String theory was the first major attempt to heal this rift. It imagines elementary particles as tiny vibrating strings, their different vibrational patterns giving rise to electrons, quarks, and gravitons that can gravity at the quantum level. But buying into string theory requires buying into extra dimensions that are curled up beyond our perception and, so far, experiments have turned up with no supporting evidence. Another option is loop quantum gravity. In this theory, spacetime itself is woven from discrete loops, like a kind of microscopic foam.

The smallest volume is called Planck volume, and in a collapsing star, the granularity of Planck volume would halt the implosion before a singularity can form, instead creating what theorists call a Planck star. Over cosmic time, that rebound could blow apart the surrounding black hole from within. If loop quantum gravity proves true, old black holes should someday detonate as sudden, intense bursts of radiation. None have been seen yet, but the prediction makes loop quantum gravity at least potentially testable. The same idea also suggests a specific origin story: a cosmic, recurring big “bounce” instead of a singular Big Bang. A prior universe would have collapsed to near-Planck density, rebounded, and gave rise to ours.

This is a cycle that may have happened many times. A chain of universes expanding and recollapsing. It’s an appealing idea, yet it’s also one entangled with thermodynamic worries. In our universe, things tend to run down, not reset forever. For now, a one-shot Big Bang, with some quantum refinements, remains the default story, though better observations of the early universe could still reveal twists to this story. The renowned physicist Stephen Hawking was happy to still have room for speculation – that there were still discoveries yet to be made.

Newton, watching an apple fall, asked what law governs its motion. Einstein imagined being the falling worker and asked what the world feels like along the way. One perspective anchors rockets, bridges, and daily life. The other lets us reason about black hole interiors, gravitational waves, and the fate of all things. Somewhere in between, in the unfinished overlap of quantum foam, dark matter, and warped spacetime, the story of gravity is still being written.

Final summary

In this Blink to Crush by James Riordon, you’ve learned that gravity is the quiet architect of entire universes. It not only shapes our fears and our bodies, it sets the rules for which planets can hold air and water, power magnetic shields and become possible homes for human life. It forges black holes that crush matter and bend light into rings, sends gravitational waves humming across the cosmos, and governs the fates of galaxies. Perhaps, most fascinating of all, is that despite how interwoven gravity is in every facet of life, it still holds mysteries.

When the laws of gravity run headlong into quantum mechanics, the lack of a unifying theory has led researchers into string theory, loop quantum gravity, gravitons, extra dimensions, Planck stars, and bouncing cosmologies, but they all fall short of fully explaining our universe. The constants that permit stars, chemistry, and observers like us seem finely tuned, yet may never be fully explained. Gravity is both the most familiar force in our lives and the least understood in its depths, and the real joy lies in using it as a guide to keep asking bigger, stranger questions about how reality holds together. Okay, that’s it for this Blink.

We hope you enjoyed it. If you can, please take the time to leave us a rating – we always appreciate your feedback. See you in the next Blink.

About the Author

James Riordon is a veteran science journalist whose work has appeared in outlets such as Scientific American, Popular Science, The Washington Post, and Physics Today. He has served as president of the DC Science Writers Association and co-founded the Southwest Science Writers Association, solidifying his esteemed stature in the science-writing community.