Author and physics professor Brian Greene is host of “The Elegant Universe” on “NOVA” (7 p.m., PBS). For those in the audience who aren’t up on their physics, string theory embraces the new and exciting notion that the entire universe is made up of infinitesimally tiny strings of energy. The strings are too tiny to be seen or measured, so belief in the theory borders on faith. But a growing number of scientists think that string theory may be a variation on the unified theory of, well, everything, that Albert Einstein was working on during his final decades.

Parts one and two of “The Elegant Universe” take viewers through a survey course in the history of physics, from Newton’s theory of gravity through 20th century theories of relativity, electromagnetism, atomic physics and quantum mechanics. Don’t worry, it’s all presented in an entertaining fashion that even a TV critic can comprehend. Part three airs next week and attempts to explain a string theory view of the universe that is comprised of 11 (count ’em) dimensions. It’s truly the stuff of science fiction, and great, thought-provoking, even mind-blowing, television.

“24” (8 p.m., Fox) returns for a third season, set three years after last year’s cliff-hanger. In the interim, Jack spent more than a year undercover tailing and capturing a drug lord. Along the way, he picked up a rather nasty habit.

Hour one opens with a fast-spreading virus threatening Los Angeles and beyond. It’s also introduces some new key players. President Palmer has recovered from the assassination attempt, but now relies on his chief of staff, who happens to be his control-freak brother (D.B. Woodside).

Kim Bauer has changed from Jack’s trouble-prone daughter to a full-fledged, techno-babbling member of the Counter Terror Unit. She’s also dating her father’s new right-hand man.

Matt learns the truth about his stint on the fake reality show “Lap of Luxury” on the two-hour series finale of “Joe Schmo” (8 p.m., Spike).

“Newlyweds” fans facing Nick and Jessica withdrawal may have to console themselves with “Rich Girls” (9:30 p.m., MTV), an eight-episode series following two young heiresses as they shop, party and name-drop in Manhattan.

Tonight’s other highlights

• Lorelai is smitten with her dad’s new partner on “Gilmore Girls” (7 p.m., WB).
• Dr. Crane snitches on Martin’s new love interest (Wendie Malick) on “Frasier” (8 p.m., NBC).
• Snoop Dogg guest stars on the NFL soap opera “Playmakers” (8 p.m., ESPN).
• Beverly D’Angelo guest stars on “Law & Order: Special Victims Unit” (9 p.m., NBC).
• A serial killer strikes again at a fancy watering hole on “NYPD Blue” (9 p.m., ABC).

Series notes

A sailor’s overdose on “Navy NCIS” (7 p.m., CBS) … Poker night on “Whoopi” (7 p.m., NBC) … A whole continent to offend on “Joe Millionaire” (7 p.m., Fox).

Tim’s birthday proves to be eventful on “Happy Family” (7:30 p.m., NBC) … A scary blast from the past on “I’m with Her” (7:30 p.m., ABC) … A dad makes peace between his ex and his new squeeze on “All of Us” (7:30 p.m., UPN).

Dylan’s ex-husband haunts Jake on “Good Morning, Miami” (8:30 p.m., NBC) … Maya moves out on “Girlfriends” (8:30 p.m., UPN) … Inappropriate comments on “Judging Amy” (9 p.m., CBS).

Late night

Courteney Cox and Doris Roberts visit “Late Show with David Letterman” (10:35 p.m., CBS) … Jay Leno welcomes Jada Pinkett Smith and R.E.M. on “The Tonight Show” (10:35 p.m., NBC) … Norm MacDonald and Triumph the Insult Comic Dog are booked on “Late Night with Conan O’Brien” (11:35 p.m., NBC).