On January 28th, after the San Francisco 49ers won the N.F.C. Championship game, against the Detroit Lions, the 49ers’ tight end George Kittle came to the postgame press conference with something he wanted to say: “Why do analytics people say that momentum isn’t a real thing?” Kittle mentioned a conversation he’d recently had with the ESPN personality Pat McAfee. “He was like, ‘All these people are telling me that momentum’s not real.’ And that’s just the biggest load of horse crap I’ve heard of in my entire life.”
For evidence, Kittle could point to the third quarter of that night’s game. When the quarter began, the Lions were up 24–7. The 49ers kicked a field goal on the opening drive, then the ball went back to Detroit, who, in the first half, had been moving downfield almost at will. The Lions got into 49ers territory and then, on fourth-and-two, tried for a first down instead of attempting a field goal. They failed.
On the next drive, the 49ers’ quarterback Brock Purdy threw a long pass that hit a Lions player, Kindle Vildor, in the facemask and bounced into the hands of the 49ers’ receiver Brandon Aiyuk, for a fifty-one-yard gain. Purdy found Aiyuk for a touchdown three plays later. The sound of the crowd swelled through the stadium. The 49ers could sense it. The Lions could sense it. Fans watching the drama unfold through television screens, even thousands of miles away, could sense it. Momentum.
“You could just kind of feel an energy,” Kittle went on. “We go down and score. ‘All right, this is huge.’ Turnover. I was, like, ‘Ah, man, all bets are off now.’ Bang bang.” The final score was 34–31, San Francisco.
Afterward, the 49ers’ head coach, Kyle Shanahan, described the players’ mind-set at halftime. The vibe, he said, was “We’re not going out like this. Guys didn’t want today to be the last day.” In the second half, Shanahan said, “We were able to get the ball to bounce the right way.” A headline in the San Francisco Chronicle declared, “49ers Are Going to the Super Bowl Thanks to a Force That Can’t Be Measured.”
“George Kittle is one of my favorite players, so I’m going to be a little sad if he just dunks all over me,” the sportswriter Bill Barnwell told me this week when I asked him about Kittle’s comments. Barnwell is the analytics person most closely associated with the argument against momentum. Ten years ago, he wrote a two-part essay, “Nomentum,” for Grantland, in which he laid out philosophical objections to the word’s overuse and presented a series of studies that questioned the significance of momentum.
Momentum, Barnwell found, didn’t reliably carry over from the regular season to the postseason. Teams that surged at the end of the season were not reliably more likely to win the Super Bowl than teams that had got off to a hot start. Momentum also didn’t appear to carry over from week to week or minute to minute. Looking at data from games in the National Hockey League, he found that teams which scored late to force overtime were not more likely to win that game. In football, he found that stopping a team on downs—as the 49ers had done, in the third quarter, against the Lions—did not create any measurable momentum. The team that then received the ball was not more likely to score on the following drive than if the opposing team had punted to them instead.
Barnwell dug deep. He read academic papers from decades before. He conducted more studies. He scrutinized the ways that commentators, coaches, and fans used the word “momentum” to see whether he could find any consistent patterns. He learned that a Cincinnati Bengals quarterback named Virgil Carter had used computers to analyze reams of data and had come to some of the same conclusions that Barnwell did—in 1970.
The problem with momentum, in Barnwell’s view, was not that it doesn’t exist. He actually had no idea whether it existed—and, these days, he’s willing to concede that it probably does. (“I’ve heard very few players who say, Yeah it doesn’t exist,” Barnwell told me. “I can’t think of one.”) The problem was that it isn’t well defined. In physics, momentum is a measure of mass in motion: the product of a particle’s mass and velocity. In the analysis of football games, though, the concept seems to abide by no rules. It is “an arbitrary, abstract idea that you can mold into just about anything you want to tell the story you’re looking to tell,” Barnwell wrote in “Nomentum.” That makes it a poor basis for play-calling—such as whether to kick a field goal on fourth-and-two instead of trying to get a first down—and a poor basis for criticizing the plays a coach decides to call. Teams generally win or lose football games because of some combination of sound decision-making, good execution, and luck, not because they are carried by a mystical force unleashed by a dropped pass or a goal-line stop.
Barnwell sounded right to me. But so did Kittle, to be honest. It was a load of horse crap: not only does momentum exist, it is fundamental to the game. Statistical analysis can tell you what has happened, and probabilities can tell you what is likely to happen, but momentum describes the experience of what is happening, both for players and for the crowd. It’s a shared feeling, a sense of being swept into events as they unfold. It’s connected to confidence, with its attendant rush of dopamine, and to joy. And everyone knows what demoralization feels like, on the other side.
The tricky thing, the thrilling thing, is that this feeling is unstable, and its effects are unpredictable, and likely unmeasurable. Some people seem to wilt under pressure. Others—probably most of us—have good moments and bad, rising and falling in unsteady fashion. And some people are Patrick Mahomes.
Mahomes, the Kansas City Chiefs’ quarterback, didn’t have the best regular season, by his standards. But, when the playoffs arrived, he became his dominant self again. Mahomes has yet to throw an interception since the regular season. He’s been completing more passes, throwing more touchdowns, and running more effectively. In the A.F.C. Championship game, against the Ravens, in Baltimore, he comfortably connected on thirty of his thirty-nine passes for two hundred and forty-one yards and a touchdown. The game never seemed in doubt. The Chiefs didn’t need a force that can’t be measured. They just needed Mahomes.
But the Chiefs tell their story as one of momentum, too. Their story starts not in Baltimore, nor during their previous playoff-game win against the Miami Dolphins. Their tale begins on Christmas Day, shortly after a loss to the Las Vegas Raiders. It seemed demoralizing at the time, but now the Chiefs say it inspired them. They won the next two games, the final games of the regular season. “We carried that momentum in the playoffs,” Mahomes said a few days ago. He inspires so much confidence—and so much respect on the part of his opponents—that he seems to be momentum itself.
How’s this for a swing? Mahomes has led eighteen potential game-tying or go-ahead drives in the fourth quarter or overtime and converted twelve of them. He’s 4–2 in the playoffs in games when the Chiefs have trailed by ten points or more. The last time the Chiefs faced the 49ers in the Super Bowl, in 2020, he led the Chiefs back from a 20–10 third-quarter deficit; the Chiefs scored twenty-one points in under seven minutes, as Mahomes, after a miserable start, made a series of spectacular throws. If I had to place a bet on Mahomes vs. momentum, I would bet on Mahomes. ♦
