Below, James Riordon shares five key insights from his new book, Crush: Close Encounters with Gravity.
James is a science journalist who has written for Science News, Scientific American, New Scientist, Popular Science, and The Washington Post, to name a few. He was formerly President of the DC Science Writers Association and is Co-founder of the Southwest Science Writers Association.
What’s the big idea?
Gravity is the most mysterious force in the universe, despite being the one humanity has been aware of the longest. Although its an everyday presence, gravity is weird, puzzling, and still far from fully understood.
1. Life, death, and gravity.
I have a pretty good idea about how you’re going to die. If you live long enough, gravity is probably going to kill you. Falling is the leading cause of death for people over 65. It’s the massive planet beneath you, and the gravity it creates, that makes falling so hazardous for aging bones.
Gravity, however, is also largely the reason we’re all here. It takes the gravity from trillions of trillions of tons of hot gas to compress atoms enough to drive the nuclear fusion that lights up stars. If gravity were a tiny bit weaker, stars couldn’t shine, and the universe would be cold, dark, and lifeless. If gravity were a bit stronger, the universe would have collapsed soon after the Big Bang, leaving no time for life to emerge and evolve.
It’s not yet clear why our current force of gravity is just right for life as we know it. The fortunate strength of gravity is a puzzle that scientists and philosophers are actively debating. It could be a lucky accident. Or it might be that ours is one of infinitely many universes. If so, most of them are sterile places with less-than-perfect gravity. For a tiny fraction, just-right gravity means life is inevitable.
It could be that our bias toward life of the as-we-know-it variety is misguided. Maybe very different kinds of life emerge in universes where the forces are different from ours. Testing these ideas won’t be easy. In the meantime, enjoy your Goldilocks gravity that’s not too weak and not too strong, at least until it kills you.
2. Gravity is the weirdest force.
We’ve known about gravity longer than any of the other forces. Aristotle was already pondering gravity’s effects in ancient Greece. The three other fundamental forces—electromagnetism and two nuclear forces that hold atoms together—would have to wait more than two thousand years to make it into science textbooks.
And yet, the oldest known force remains the most puzzling. Gravity is trillions upon trillions of times weaker than the other forces. But for anyone who has taken a tumble or climbed a long flight of stairs, gravity doesn’t feel weak. That’s because gravity is weird.
“The oldest known force remains the most puzzling.”
The other forces tend to cancel themselves out. Positive electrical charges attract negative electrical charges in a near-perfect balance. That ensures that we often don’t notice electrical forces. Gravity, on the other hand, doesn’t come in offsetting charges. It only adds up. That makes it the one force that we deal with all the time, even though it’s comparatively weak.
The weakness of gravitational forces may be because gravity isn’t really a force at all. According to Einstein, gravity emerges from the geometry of space and time. Things fall simply because they follow curves of warped spacetime.
If gravity isn’t a force, then comparing it to the other forces doesn’t make much sense. But some scientists have suggested that gravity is a side effect of yet another fundamental force. If so, that force might be comparable to the other three known forces.
When we put it all together, gravity is a force . . . that isn’t really a force . . . but might be caused by a force . . . that we haven’t discovered yet. Gravity is just plain weird.
3. There’s a black hole in your kitchen sink.
No one can see inside a black hole. They’re black because they have gravity that’s so intense that anything falling in can never come back out, including light.
Still, you can get a pretty good idea—in the comfort of your own home—what it might look like inside a black hole. A surprisingly accurate image of the interior of a black hole is only as far away as your kitchen sink.
If you clear a space in your sink and turn the water on as a thin, steady stream, it will form a distinctive pattern. Around the point where the stream hits the sink basin, it spreads into a circular pattern of smoothly flowing water. At the outer edge of the circle is a ridge-like ring. Beyond that, the water ripples and burbles like the surface of a small, choppy sea.
“A surprisingly accurate image of the interior of a black hole is only as far away as your kitchen sink.”
In 2011, physicists in France realized that the smooth, rapidly flowing circle of water in your sink is mathematically identical to the inside of a black hole. Outside the ring-shaped ridge, the burbling water is mathematically like open space. As for the ridge itself, that’s comparable to the event horizon—the point of no return surrounding a black hole.
If you could slice a black hole in half like an onion and you had the magical ability to see warped spacetime with your naked eye, it would look just like the pattern you can make with water in your kitchen sink.
4. It’s a big, dark universe.
No one has any idea what most of the universe is made of. Ironically, our ignorance was born of astronomical advances over the last 50 years.
In recent centuries, we’ve learned more about the cosmos with each new and improved telescope. Technological advances in astronomy have historically nibbled away at our scientific ignorance. Beginning in the 1970s, though, astronomers started making discoveries that suggest we know a lot less about the universe than we thought—something like 95 percent less.
The first clue to our ignorance came when new telescopes and instruments revealed that most galaxies are far heavier than they should be. The simplest reason anyone has thought of for this is that stars in galaxies are outweighed by an abundance of mysterious, invisible dark matter.
More recently, astrophysicists discovered that the universe is expanding at an ever-increasing rate, and that we’re a tiny bit of shrapnel in a cosmic explosion. But an explosion takes energy. We don’t know where the energy comes from or why it exists. Whatever it is, scientists call it dark energy, and there’s a lot of it.
“Fully 95 percent of the universe is utterly inscrutable.”
In terms of a cosmic cocktail, the recipe for the universe is one part matter, five parts dark matter, and a whopping 14 parts dark energy. Fully 95 percent of the universe is utterly inscrutable.
We’re at least getting closer to solving the dark energy and dark matter puzzle by figuring out what they aren’t. In the words of Sherlock Holmes, “When you have eliminated the impossible, whatever remains, however improbable, must be the truth.” We still have a lot of eliminating left to go.
5. Gravitational oddities.
Einstein didn’t believe in black holes. His theory of relativity describes them, but Einstein didn’t buy it. A one-way hole in the universe, where gravity becomes infinitely strong and matter falls in faster than the speed of light? Impossible.
On that point, Einstein was wrong. Not only do we know that black holes are real, but scientists are finding more of them every year. One way we know black holes exist is through the catastrophic crashes that occur when pairs of black holes collide. The cataclysms cause ripples in space called gravitational waves that register as faint chirps in observatories around the world.
Einstein didn’t believe in gravitational waves either. When he ultimately conceded that they might exist, he argued that the waves would be so feeble that there would be no way to detect them. For decades, he was right.
These days, astronomers have found that most of the trillions of galaxies in the universe harbor central black holes. And gravitational wave observers hear dozens of black hole pairs colliding each year. Black holes and gravitational waves are just two of the hard-to-believe oddities that Einstein’s version of gravity allows. Other theoretical possibilities abound, including:
- Wormholes that are shortcuts through space and time, or routes to other universes.
- White holes, which are black holes running in reverse (spewing out matter and energy).
- Faster-than-light warp drive-powered rockets.
- Even time machines.
Recently, the most far-seeing observatory ever built, the James Webb Space Telescope, caught a glimpse of what may be ancient, monstrous, ultra-bright stars powered by dark matter instead of fusion. Einstein would likely have had doubts about all of these oddities, even though his own theory allows them. Maybe we’ll prove him wrong again.
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