Spacetime Before Pulleys: An Interview with Scott MacDonald

“I doubt most students go into physics because they really want to study pulleys,” said Scott MacDonald, a professor of physics at USC, in an interview with The Lab Report. MacDonald has spent the last few years rethinking the introductory sequence for physics majors: teaching special relativity before Newton's laws. Through this teaching tactic, students learn about spacetime, right out of the gate.

Why Flip the Script

Undergraduate introductory physics has followed the same sequence since textbooks like Halliday and Resnick established it in the 1960s: kinematics, then Newton's laws, building upward brick by brick. MacDonald followed this route himself for eleven years and still respects it. “It's very logical, it gives a very coherent story,” he said.

Teaching the 160 series — USC’s set of three introductory physics classes for physics majors — he kept noticing a disconnect. Many of these students arrive having taken AP Physics C, which gives students a solid introductory background, and choose to major in physics because of black holes and quantum mechanics. 

MacDonald describes this difference from expectation: “Welcome to university, you're officially getting your degree in physics, but we're still gonna do Newton's laws for the first 15 weeks.”

Relativity, he realized, sits in a sweet spot. The math is familiar and tractable. While the concepts unsettle everything you thought you knew, they correct your existing intuition rather than replacing it wholesale. 

But, there's a physics argument too: in the traditional sequence, conservation laws like that of energy and momentum arise as afterthoughts of Newtonian mechanics. From a modern perspective, however, conservation laws and symmetries are fundamental, and Newton's laws follow from them. “I'm not hiding anything from you,” MacDonald tells his students. “I'm telling you as it is, right away.”

To make this new approach a reality in his 161 class, MacDonald found a handful of textbooks that attempted similar approaches, and they gave him courage. He asked his students to be guinea pigs … and they agreed.

What Relativity Does to Your Brain

MacDonald borrows from Zen Buddhist teachings to describe what happens when students encounter relativity. He points to a saying attributed to the 13th-century Zen master Dōgen: “Before one studies Zen, mountains are mountains and waters are waters; after a first glimpse into the truth of Zen, mountains are no longer mountains and waters are no longer waters; after enlightenment, mountains are once again mountains and waters once again waters.”

The arc maps onto learning relativity almost too neatly. When you first study physics, length is what’s measured with a meter stick and time is what’s measured by a clock. Relativity then dismantles that: different observers get different measurements, space and time blur into one fabric, and you start questioning whether any of these concepts actually mean what you once assumed. Then, after enough wrestling, you circle back. Length really is what’s measured with a meter stick. The statement is the same, but now has a deeper, heightened meaning.

The profound insight underneath all of this is that cause and effect in the universe is finite. There is no instantaneous influence. This tenet forces different observers to perceive space and time differently, and it's what makes spacetime one thing rather than two. “Not everybody agrees on this,” MacDonald said, “which is pretty crazy.”

What surprised him is where this leads students emotionally. Relativity teaches that two observers can get different numbers for the same measurement and both be correct. He offers an example: “I may have gotten 2.8, maybe you got 3.2, and I don't mean because you made a mistake on the calculator.”

This is precisely what rewires students' brains. From grade school onward, they have been trained to chase the one correct answer — the value at the back of the textbook, the number the teacher is waiting for. Relativity asks them to give that up. What matters is no longer landing on a single value, but comparing values across frames of reference and communicating between perspectives that are both equally valid. Physics, it turns out, asks for a kind of open-mindedness that goes well beyond equations.

From Confidence to Capability

MacDonald's hope is straightforward: if 18-year-olds can successfully work with four-vectors and Lorentz transformations in their first semester, they'll believe they can tackle harder material sooner. “You were able to do these kinds of things that Einstein did, right off the bat,” he said. “These ideas are not out of the reach of 18, 20-year-olds.”

Still, he's careful to distinguish accessibility from mastery. “You may not be able to quite master it yet, but they're certainly accessible, and you can get your hands dirty, your feet wet, and you can tinker with these things and get something out of it right away,” MacDonald said.

The goal is a cascading confidence: a student who has survived relativity may decide to pick up quantum mechanics or electromagnetism earlier than they otherwise would have, simply because they feel capable.

Beyond the Physics Department

MacDonald has encountered a common reaction almost every time he tells someone he teaches physics. “Oh, wow, you're one of those smarty pants, I couldn't do physics.” He is blunt about this: “I despise that response.” He sees the idea that physics requires innate genius rather than willingness to sit with difficulty as a falsehood, and a failure of early education in the sciences.

Relativity, he thinks, exposes what really makes physics hard. It forces sustained, vertical thinking about a single idea, the kind of depth that modern culture increasingly avoids. “You get afraid of failure, you get afraid of not understanding. Your ego comes into the story,” he said. 

The fear is real. But when a non-major sees their roommate grappling with spacetime and succeeding, MacDonald hopes the takeaway is simple: maybe I could do that too.

He is currently writing a textbook based on this relativity-first approach, with the ambition of offering it to other institutions. The best outcome, as he sees it, is that the approach passes through the hands of many great teachers, gets refined far beyond what he could do alone, and eventually becomes a new standard. “We'll see future physicists being that much more well-prepared and doing that much more excellent work,” he said.

MacDonald's experiment is still young, and his students are still, by his own admission, guinea pigs. But the underlying bet is an important one: that the ideas which drew young people to physics in the first place are the same ideas they're ready to learn. The traditional sequence protects students from difficulty. MacDonald would rather trust them with it.