The first Law of Thermodynamics says that the energy of the universe is constant; it can be neither created nor destroyed. The electricity that powers all things, including a pitcher’s strong left arm, is borrowed from a greater whole. Eventually, that electricity is going to be reclaimed and redistributed, leaving behind only residual traces of itself.

When Randy Johnson’s April went into the record books, it suggested better things to come in May. True, his record was only 2-2 with an unsightly (for him) ERA of 3.74. He also allowed six home runs, a large number by his standards, and his strikeout rate was a low – again, for him – 8.9 per nine innings, as compared to his career rate of 11.12.

Still, the 41-year-old had allowed just 34 hits in 43.1 innings, or 7.1 hits per nine, right in line with his career rate. Given the reluctance Yankees fielders had shown to get in the way of live baseballs (the Bombers trail the universe in turning balls in play into outs), the low hit rate seemed to suggest vintage Johnsonian dominance to come. If he could keep runners off base in defiance of the shoddy defense, surely he was doing something right. The old man was only awaiting the arrival of warm weather to limber up and throw a perfect game or two.

But it has been an unusually cold May in the Northeast, and Johnson has been colder. Despite his 4-3 record, the five-time Cy Young award winner has been noticeably less effective than in 2004, when he put up a 2.60 ERA with 290 strikeouts and just 44 walks in 245 2/3 innings. At times in 2005, it has looked like a different pitcher has taken over the Big Unit’s body.

Against the Mariners on May 9, Johnson was reminiscent of himself, striking out seven in eight innings and allowing three runs on seven hits. Tellingly, though, for the fifth time in seven starts, he allowed more grounders than fly balls. As with most power pitchers, Johnson has been a fly-ball pitcher throughout his career, getting 1.21 grounders for every fly he’s allowed.

Johnson’s new relationship with grounders intensified in his next two starts, both unsuccessful. On May 15, Johnson was credited with a win against the A’s despite allowing four runs (three earned) in six innings and striking out none. It was the longest appearance in his career without a strikeout. He induced 14 grounders and four flies, roughly three times his career rate. In his May 21 start, a loss to the Mets, a few strikeouts returned (five in 6 2/3 innings), but the Unit was pounded for 12 hits. He was again in double-figures for grounders: The Mets put 15 balls on the ground as opposed to six in the air. Johnson’s groundball/fly ball ratio is 1.61 and climbing; the major league median G/F ratio is 1.31.

An increasing number of grounders would, at the very least, suggest that Johnson had changed his approach. The question that arises naturally is, “Why?” It has been suggested that his groin injury has caused more trouble than either pitcher or team has been willing to admit, meaning that Johnson’s legs have been cut out of his delivery. Jim Kaat of YES noted that Johnson has been shocked that spring has bypassed New York this year, and that his arm is still defrosting. Will his struggles be as transient as the weather? We don’t know – yet.

What we do is that power pitchers age well as a group. It’s counterintuitive, but the act of firing a straightforward 95 mph fastball takes less out of the arm than serving up years worth of junk. Because of this, the pitchers at the top of the line seem like they can go on forever. In practice, they last longer, but their decline phase is abbreviated. There’s no soft landing. They go from having a demonstrated ability to pitch 250 innings a year at a high level, only to vanish suddenly.

Nolan Ryan, the most extreme strikeout pitcher of all-time, was literally here one day, gone the next. At 45 he threw 157 innings with above-average effect; at 46 his elbow went “twang” from his ulnar collateral ligament and he was gone. Dazzy Vance, the great, ragged-sleeve strikeout pitcher of the 1920s (6.2 strikeouts per nine as compared to a league average of 2.9), had a season for the ages at 39, a mediocre year at 40, and thereafter was no fun anymore. Tom Seaver had a final terrific season at 40 in 1985, going 16-11 with a 3.17 ERA in 238 innings; his body gave out in 1986. Steve Carlton pitched a strong 283 innings at age 38 in 1983, was a league average pitcher the next year, and was a punching bag until he mercifully retired in 1988.

Most great pitchers fall out of baseball in their mid-30s. With those that don’t, there’s a temptation to think, “Well, he made it past the retirement nexus, so maybe he can just keep doing this forever.” The Yankees are not alone in rolling the dice on an ancient power pitcher; the Red Sox have done the same with 38-year-old Curt Schilling, getting great results in 2004 and an injured, diminished pitcher this year. This same truth will be socked home to the Astros and Roger Clemens sooner than later – which is why it would behoove owner Drayton McLane to listen carefully to any trade entreaties that come his way.

In Johnson’s case, the shift from strikeouts to grounders may suggest a desperate act of on-the-job reinvention. He may still prove that knowledge, experience, and a low-90s fastball can suffice where pure velocity has failed. Tonight in the Bronx, Johnson will ask the potent Red Sox lineup and the defenders behind him to help test the theory.

Mr. Goldman is the author of “Forging Genius,” a biography of Casey Stengel, released this year.