*Below is an excerpt *The greatest ideas in the universe: space, time and motion* by Sean Carroll.*

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The greatest ideas in the universe: space, time and motion

My dream is to live in a world where most people have informed views and passionate opinions about modern physics. Where you unwind after a hard day’s work, hit the pub with friends and argue about your favorite dark matter candidate or competing interpretations of quantum mechanics. A world where, while kids are running around at a birthday party, one parent says, “I don’t understand why anyone would think there should be new particles near the electroweak scale,” and another immediately replies, “How on earth in the world are you going to address the problem of hierarchy?” People end up having opinions about supply-side economics or critical race theory. Why not inflationary cosmology and superstring theory?

This isn’t quite the world we live in. Even more than most other scientific disciplines, physics is a field of and for specialists. Practitioners speak to one another in highly specialized jargon dominated by mathematical concepts that most people have never heard of, let alone mastered. There are reasonable reasons for this, but it need not be so. The situation is due in large part to how physicists tend to share their knowledge with the rest of the world.

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## Immerse yourself in the greatest ideas in the universe

If you’re a non-expert interested in learning about modern physics, you basically have two options. One is to stick to a popular level of explanation where you can learn some of the relevant concepts without delving into the technical or mathematical details. You can read books, attend lectures, watch videos, listen to podcasts. The good news is that we have a vibrant ecosystem of such resources, and it’s possible to learn some, albeit in a somewhat haphazard way. But in the end you know you’re not getting the real stuff. What you get are images and metaphors, rough translations of the underlying mathematical essence into ordinary language. You can cover an impressive distance on this route, but something important will always be missing.

The other way is to become a physics student. That can be literally at a university or by putting together the right textbooks and online resources. Along the way, you’ll need to have a pretty good grasp of math: primarily calculus and differential equations, but also aspects of vector analysis, complex numbers, linear algebra, and more. The journey will be rewarding but frustratingly slow. It usually takes at least a year of introductory courses before a student ever hears about relativity or quantum mechanics. And most physics students can get a bachelor’s degree — or even go all the way to a PhD — without learning anything about particle physics, black holes, or cosmology. These delicacies are only reserved for specialists in certain sub-areas.

The gulf between learning physics as an interested amateur, relying on metaphors and grim translations, and becoming a recognized expert skilled at pushing around equations of daunting complexity is wide, but not insurmountable. Just because I don’t want to be a professional racer doesn’t mean I can’t race at all. Surely there is a way to grapple with some of the authentic essences of modern physics—even if that means looking at a few equations—without sipping through years of a standard curriculum.

That’s exactly what I set out to do in The Biggest Ideas in the Universe, a series dedicated to the idea that it’s possible to actually learn about modern physics, equations and all, even if you’re more of an amateur than a professional and have every intention of staying that way. It is intended for people who have no more math experience than high school algebra but are willing to look at an equation and think about what it means. If you are willing to think this little bit, a new world will open up.

Here’s the thing about equations: They’re not that scary. They are just a way of summarizing a relationship between different quantities in a compact way. And while an equation can contain Greek letters, learning to understand an equation doesn’t mean you have to learn how to speak and write Greek.

I think we need a middle ground—but first, let me elaborate on this distinction between solving and understanding, because it’s key to my dream of talking about physics like a baseball game. Einstein’s equation doesn’t just relate a specific collection of mass and energy to the curvature of a specific spacetime. It’s a perfectly general relationship of the form, “You give me a distribution of mass and energy, and I’ll tell you how spacetime warps in response.” Fulfilling that promise is what we call “solving the equation.” to understand.

Sometimes solving an equation is easy: if the equation is x=y2 and we are told that y=2, the solution is x=4. Not that hard. But real physical equations are more complicated than that and involve ideas from calculus (the mathematics of continuous change) and other advanced concepts. Solving such equations can become a full-time job for working physicists. So, sensibly, a large part of their training consists of learning to solve equations.

But what if we decided that there was value for lay physicists in not solving the equations that explain the world around us, but in understanding them, even those considered relatively advanced by the standards of physics textbooks? This turns out to be much tastier and more exciting. My goal with The Biggest Ideas in the Universe is to make the ideas of modern physics – the real, non-watered down metaphorical versions – accessible to anyone willing to think just a little about the equations and what they mean.

Out of *The greatest ideas in the universe: space, time and motion* by Sean Carroll with permission from Dutton, an imprint of Penguin Publishing Group, a division of Penguin Random House, LLC. Copyright © 2022 Sean Carroll.

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## Meet the writer

Sean Caroll

### About Sean Carroll

dr Sean Carroll is the author of *The greatest ideas in the universe: space, time and motion*and is the Homewood Professor of Natural Philosophy at Johns Hopkins University in Baltimore, Maryland.