A high point of my career as a science journalist was a cosmology workshop I bulled my way into in 1990. Thirty luminaries of physics gathered in a rustic resort in northern Sweden to swap ideas about how our universe was born. Stephen Hawking, although almost entirely paralyzed, was the id of the meeting, a joker with a Mick Jagger smirk. Martin Rees, cool and elegant, was the superego, as was befitting for a future president of the Royal Society, one of science’s most venerable institutions.

Personalities aside, Hawking and Mr. Rees had much in common. Born in 1942, both became professors at the University of Cambridge, where Newton once taught. Both contributed to our modern understanding of the big bang, black holes, galaxies and other cosmic matters. Both were committed to telling the public about science’s astonishing revelations.

One afternoon everyone piled into a bus and drove to a local church to hear a concert. As the scientists proceeded down the center aisle of the packed church, led by Hawking in his wheelchair, parishioners stood and applauded. These churchgoers seemed to be acknowledging that science was displacing religion as the source of answers to the deepest mysteries, like why we exist.

That scene came to mind as I read two new books, “Brief Answers to the Big Questions,” by Hawking and “On the Future: Prospects for Humanity” by Mr. Rees. The authors’ styles differ—Hawking cocky, Mr. Rees sober—but the substance of their books overlaps. They offer brisk, lucid peeks into the future of science and of humanity. They evince a profound faith in science’s power to demystify nature and bend it to our ends.

Yet reading these books was a bittersweet experience, and not only because Hawking died last March, at 76. (His book was completed by colleagues and family members.) The works resemble relics from a long-gone golden age: The high priests of science no longer enjoy the prestige they did just a few decades ago.

Hawking in this book is less brash than he once was. In 1980 he proclaimed that, by the end of the 20th century, physicists would discover an “ultimate theory” that would solve the riddle of existence. It would tell us what reality is made of, where it came from and why it takes the form that it does. In “Brief Answers” Hawking concedes that “we are not there yet,” and he pushes back his prediction for a “theory of everything” to the end of thiscentury. But he continues to promote the same ideas that he has for decades. String theory remains his favorite “theory of everything.” Also called M-theory, it conjectures that reality is made of infinitesimal strings, loops or membranes wriggling in a hyperspace of 10 dimensions.

Noting that, according to quantum mechanics, empty space seethes with particles popping into and out of existence, Hawking suggests that the entire universe began as one of these virtual particles. The universe is “the ultimate free lunch,” he says. Our universe may also be just one of many. M-theory, quantum mechanics and inflation—a theory of cosmic creation—all suggest our cosmos is just a minuscule bubble in an infinite ocean, or “multiverse.”

To explain why we live in this universe rather than one with radically different laws, Hawking invokes the “anthropic principle”: If our universe were not as we observe it to be, we would not be here to observe it. Our scientific picture of the cosmos, Hawking proposes, is already so complete that it eliminates the need for God. “No one created the universe,” he declares, “and no one directs our fate.”

Science can save us, too, Hawking states. It gives us the means to establish colonies on Mars and elsewhere in case the Earth becomes unlivable—whether because of nuclear war, runaway warming, pandemics or an asteroid collision. “If humanity is to continue for another million years,” he states, “our future lies in boldly going where no one else has gone before.”

Mr. Rees’s worldview differs in a few respects from Hawking’s. He describes himself as a “practising but unbelieving Christian.” He respects believers, with whom he shares “a sense of wonder and mystery.” As for space-colonization, Mr. Rees asserts that it is “a dangerous delusion to think that space offers an escape from Earth’s problems.” He dwells more than Hawking on threats posed by climate change, nuclear weapons, bioterrorism, asteroid collisions and even economic inequality. He urges redistribution of the “enormous wealth” generated by the “digital revolution.”

Yet the Cambridge colleagues agree on major issues. That machines will inevitably become super-intelligent, capable of learning without human guidance and pursuing their own goals. That we can nonetheless harness these machines for our own ends, or even merge with them. That we need more science and technology to help us overcome challenges to our peace and prosperity. That science will eventually explain the origin of this universe and even confirm the existence of other universes.

“It’s highly speculative,” Mr. Rees says of multiverse theory. “But it’s exciting science. And it may be true.” Mr. Rees also shares Hawking’s vision of “post-human” cyborgs fanning out through the universe to colonize other star systems. Our bionic descendants might be smart enough to invent warp-drive spaceships and time machines, Mr. Rees suggests. They might even solve what many scientists and philosophers consider the greatest mystery of all, the mind-body problem. This puzzle asks, as Mr. Rees puts it, “how atoms can assemble into ‘grey matter’ that can become aware of itself and ponder its origins.”

Hawking and Mr. Rees recognize science’s declining status. They call for better science education to lure more young people into science and to counter public ignorance about vaccines, genetically modified foods, climate change, nuclear power, and evolution. “The low esteem in which science and scientists are held is having serious consequences,” Hawking complains.

Both authors fail to mention that science’s wounds are at least partially self-inflicted. In 2005 statistician John Ioannidis presented evidence that “most published research findings are wrong.” That is, the findings cannot be replicated by follow-up research. Many other scholars have now confirmed the work of Mr. Ioannidis. The so-called replication crisis is especially severe in fields with high financial stakes, such as oncology and psychopharmacology.

But physics, which should serve as the bedrock of science, is in some respects the most troubled field of all. Over the last few decades, physics in the grand mode practiced by Hawking and Mr. Rees has become increasingly disconnected from empirical evidence. Proponents of string and multiverse models tout their mathematical elegance, but strings are too small and multiverses too distant to be detected by any conceivable experiment.

In her new book “Lost in Math,” German physicist Sabine Hossenfelder offers a far more candid and compelling assessment of modern physics than her English elders. She fears that physicists working on strings and multiverses are not really practicing physics. “I’m not sure anymore that what we do here, in the foundations of physics, is science,” she confesses.

As I finished “Brief Answers to the Big Questions” and “On the Future,” a few questions of my own came to mind. Will science regain its luster? Will it earn back the public’s trust, or will its authority be permanently diminished? And what outcome should we prefer? I’m glad I witnessed science’s high priests at the height of their glory. But perhaps we are better off doubting all authorities, including scientific ones.