I’m covering the trial with my colleague Mike Isaac, but the court gave The New York Times only one press pass. I take it one day; he gets it the next. Even with the pass, I have to show up at least an hour before the 8 a.m. start time. The idea is that if you have a pass, you can skip the line. But for this blockbuster trial, even the express entrance to the courthouse is painfully slow. It gets clogged with all the lawyers hired by Mr. Musk and the two companies he’s suing: OpenAI and Microsoft, the lab’s chief financial and technological partner.

(The New York Times sued OpenAI and Microsoft in 2023 for copyright infringement of news content related to A.I. systems. The two companies have denied those claims.)

For security reasons, Mr. Musk and Mr. Altman enter from the basement. But they still have to walk through the metal detectors just inside the building’s front doors. When they arrive, the many photographers standing outside the building suddenly rush toward those doors, trying to snap pictures of the tech moguls through the glass. (Photography is not allowed inside the federal courthouse.)

Once we enter the courtroom, Mike and I sit on wooden benches in the gallery alongside about 40 other reporters from various outlets. One of the perks of covering a trial near Silicon Valley is that we can use laptops and phones inside the courtroom. In places like Washington, D.C., courts often ban the use of electronic devices. That means we can type out our stories and blog posts and send them to editors right away. But we’re still forbidden from taking photos, and we’re not allowed to record audio or video.

For two days running, Judge Yvonne Gonzalez Rogers, who is presiding over the trial, admonished people for recording court proceedings and snapping pictures from the overflow room, which holds dozens of people who did not get a spot in the courtroom. If they continued, she said, she would confiscate their devices and shut down the overflow room.

The figure that most eyes focus on is Mr. Musk, the world’s richest person. We all recognize him from photos, but his mannerisms are quirkier than you might think. He often purses his lips, which you can see in one of the photos captured by Jason Henry, the freelance photographer hired by The Times to stand outside the courthouse.And as Mr. Musk marched out of the courtroom on Tuesday afternoon, he was clutching what appeared to be a small, soft ball, squeezing it over and over again.

Mr. Altman is less conspicuous. This week, a fellow reporter who had never seen him in person made a point of saying he was much shorter than she had thought. On Monday, before jury selection, Mr. Altman approached me and said, “I hope you enjoy this.” Later, someone who seemed to be one of Mr. Musk’s lawyers replied to another Times reporter who had posted that quote on social media, saying, “What can we do about that?”

When Mr. Musk took the stand on Tuesday and Wednesday, Mike and I got a pretty good taste of the two very different sides of his personality — though our view was often blocked by lawyers and giant computer displays used to show court evidence. When Mr. Musk was questioned by his own lawyer, he calmly explained that he and his many tech companies were on a mission to save the world. Under cross-examination, he grew combative, occasionally raising his voice and tossing sly insults at OpenAI’s lead counsel.

Mr. Musk left OpenAI less than three years after he founded the lab with Mr. Altman. Now, they don’t exactly like each other. On social media, Mr. Musk recently called his old co-founder “Scam Altman.”

When Mr. Musk arrived in court for the first time on Tuesday, the judge lit into him for posting about the trial on X, the social media platform he owns. He said he was just responding to things that OpenAI had posted online. So the judge told both Mr. Musk and Mr. Altman to start with a “clean slate” and to “keep things to a minimum” on social media.

They agreed. We’ll see if that sticks.

The shareholders sold their Twitter stock after Mr. Musk said he would put his deal to buy the company on hold, but the billionaire later paid $44 billion for the social platform in October 2022. The shareholders sued Mr. Musk that same year, claiming he had tried to drive down the share price of the company, now X, to force a renegotiation of his offer.

Investors pointed to Mr. Musk’s posts about the deal, in which he said that Twitter was riddled with fake accounts and that the purchase was “temporarily on hold.” Those posts prompted some shareholders to unload their stakes in the company at significantly lower prices than they would have gotten when the deal closed, according to the lawsuit.

The jury’s decision in March was a rare loss for Mr. Musk, who successfully battled several previous shareholder lawsuits. In 2023, he won a case brought by investors against his electric carmaker, Tesla, over their losses in 2018 after he falsely posted on social media that he had “funding secured” to take the company private. He also won Tesla shareholder lawsuits over an acquisition of a solar company and his lucrative pay package.

Lawyers for Mr. Musk said in a written statement at the time that they viewed the jury’s decision “as a bump in the road,” adding that “we look forward to vindication on appeal.”

Mark C. Molumphy, a lawyer who represented Twitter shareholders, said, “The jury’s verdict sends a strong message that just because you’re a rich and powerful person, you still have to obey the law and no man is above the law.”

Mr. Musk testified in the trial, in U.S. District Court for the Northern District of California, that his concerns about fake accounts, known as bots, were sincere and that he had not intended to harm Twitter’s stock price.

“If this was a trial about whether I made stupid tweets, I would say I’m guilty,” Mr. Musk testified. Still, he said, he did not think the posts would jolt markets.

“I did not even have the time to attend board meetings,” Mr. Musk said on Wednesday, the second day of his testimony in a trial that pits him against OpenAI, two of its co-founders and the company’s giant partner, Microsoft.

That year, the seeds of today’s court fight were sown. Executives at OpenAI, which was founded as a nonprofit, were considering attaching a for-profit company to its structure. Mr. Musk did not have a problem with that, he said, but he wanted to make sure the nonprofit was still in charge. Now he regrets that he did not play a more active role before he left the lab in early 2018 and that he gave it money to get off the ground in the first place.

“I was a fool who provided them free funding to create a start-up,” Mr. Musk said in response to a question from his lawyer, Steven Molo. “I gave them $38 million of essentially free funding to create what would become an $800 billion company.”

In what is expected to be a monthlong trial in federal court in Oakland, Calif., Mr. Musk is arguing that OpenAI breached its founding contract when it took on Microsoft as an investor and started creating commercial products. He is asking for $150 billion in damages and for OpenAI to unwind the for-profit company it created last year.

Today, OpenAI is one of the tech industry’s most influential companies. Mr. Musk, of course, emerged from that rough year to become the wealthiest man in the world. But the relationships he had at the time with Sam Altman, OpenAI’s chief executive, and Greg Brockman, the company’s president, have long since been severed.

The hard feelings between the tech tycoons have been difficult to miss in the trial. Mr. Musk and Mr. Altman have known each other for years and hobnobbed in the same elite Silicon Valley circles. But mutual admiration — which led to their creation of OpenAI in 2015 — has soured to unapologetic animosity.

She was appointed to the U.S. District Court for the Northern District of California in 2011 by former President Barack Obama, and has a strong reputation in the courtroom. She often interrupts questioning with her own queries or accusations, some of which seem designed to catch a witness off guard.

In 2021, Judge Gonzalez Rogers oversaw the trial of Epic Games, the maker of Fortnite, which sued Apple over claims that it violated antitrust laws by forcing developers to use its App Store payment system.

During the trial, she told Apple’s chief executive, Tim Cook, that he didn’t seem to feel much competitive pressure. She also prodded Tim Sweeney, the chief executive of Epic Games, for ignoring a prior lawsuit from a different company over the same issue and forging ahead with his own. She even shared her personal views about technology, noting that she’d forbidden her son from playing video games as a child.

The trial ended with Judge Gonzalez Rogers ordering Apple to loosen its grip on the App Store and to allow app developers to collect sales directly from users. Four years later, she presided over a follow-up hearing after Epic complained that Apple had defied her ruling by creating new feeds and rules for developers.

Throughout that hearing, Judge Gonzalez Rogers questioned whether Apple was telling the truth. She called the thousands of documents that Apple shared to support its case a “sham.” She rebuked Apple for disobeying the court, accused executives of having “outright lied under oath” and openly criticized Mr. Cook.

“Apple knew exactly what it was doing and at every turn chose the most anticompetitive option,” Judge Gonzalez Rogers wrote.

Elon Musk’s lawsuit against Sam Altman involves onetime colleagues and buddies who became peevish enemies. Now they would like to take each other down. Happens all the time. These guys just have more lawyers.

Ignoring this conflict would be a mistake, however. The rancorous dispute between Mr. Musk and Mr. Altman, which went to trial this week with opening statements in an Oakland, Calif., federal courtroom on Tuesday, goes to the heart of Silicon Valley, a place that has always cloaked itself in virtue.

Mr. Altman and Mr. Musk started working on what was supposed to be a different sort of tech lab in 2015. OpenAI was a Manhattan Project for artificial intelligence, a nonprofit venture that would act as a shield against rapacious behavior by less benevolent outfits. The goal was to shift the dialogue “toward being about humanity winning rather than any particular group or company,” according to a document in the case.

Mr. Musk, the chief executive of Tesla, provided the initial funding. Mr. Altman was OpenAI’s leader and spokesman. But Mr. Musk says their interests quickly diverged when it became clear just how much money was up for grabs. OpenAI converted to a for-profit company last year. “A textbook tale of altruism versus greed,” Mr. Musk asserted in his suit’s opening salvo.

The fact that the person calling himself an altruist here is likely to become the world’s first trillionaire doesn’t necessarily make it untrue. In his lawsuit, filed in 2024, Mr. Musk said Mr. Altman, OpenAI president Greg Brockman and others “unjustly enriched” themselves in the development of OpenAI “to the tune of billions of dollars.”

OpenAI, whose value is approaching $1 trillion, had the inevitable response: No, you’re the one who is greedy. The company argued that Mr. Musk walked away when he could not take over the entire enterprise.

“This case has always been about Elon generating more power and more money for what he wants,” OpenAI said in a statement.

When Mr. Musk founded OpenAI with Mr. Altman and several young A.I. researchers, he saw the research lab as a necessary counterweight to the A.I. work underway at Google. He believed that Google and Larry Page, one of its founders, did not understand the dangers of A.I.

OpenAI’s founders — backed largely by donations from Mr. Musk — vowed to freely share their technology with the public as open source software. They argued that A.I. would be too powerful and too dangerous to be controlled by a single company.

But by late 2017, many inside OpenAI were arguing that open sourcing may be more dangerous than keeping the technology closed. And they worried that if the lab remained a nonprofit, it might not raise the money needed to reach its lofty goal of building artificial general intelligence, or A.G.I., a machine that can do anything the human brain can do.

That included Mr. Musk. In February 2018, he forwarded an email to the lab’s other founders suggesting that OpenAI attach itself to Tesla, his electric car company, and build its A.I. using the supercomputers that Tesla was developing.

“Tesla is the only path that could even hope to hold a candle to Google,” he wrote. “Even then, the probability of being a counterweight to Google is small. It just isn’t zero.”

After Mr. Altman and others refused to give Mr. Musk control, he quit. Later that month, he announced his departure to OpenAI’s staff on the top floor of the lab’s office in San Francisco. He withdrew his financial support for the lab.

Forced to find other sources of funding, Mr. Altman bolted a for-profit company onto the original nonprofit and eventually raised $13 billion from Microsoft. The lab also curtailed its efforts to open-source its technologies.

OpenAI has since emerged as one of the most important tech companies in the world, worth an estimated $730 billion as a for-profit company overseen by the original nonprofit. The start-up is heading toward one of the biggest initial public offerings in history, which could come as soon as this year.

The company has expanded to more than 4,000 employees working in offices around the world, and is pursuing a data center expansion plan that could cost hundreds of billions of dollars.