I'm becoming a moralistic prig in my dotage. Someone dear to me just proudly told me that her son, a freshly minted Harvard grad, is training to be an investment banker. This privileged young man, I grumbled, should try to make the world a better place rather than playing in a rigged, high-stakes gambling racket.

I apologized later—and vowed privately to be less self-righteous in my judgments of others' career choices. After all, I ain't exactly Gandhi. But then I read Brian Greene's new book, The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos (Knopf, 2011), and my moral hackles got all quivery again. (Weird coincidence alert: In 2006, the publisher RiskDoctor, Inc., released a book titled Options Trading: The Hidden Reality.) A physicist at Columbia University, Greene is an immensely talented science explicator who has brought physics to the masses through his smart, witty bestsellers, The Elegant Universe (turned into a television series narrated by Greene) and The Fabric of the Cosmos.

My beef with Greene is this: He has become a cheerleader for the descent of theoretical physics into increasingly fantastical speculation, disconnected from the reality that we can access empirically. Greene has argued eloquently for the plausibility of string theory, which (as I pointed out in a previous post) postulates the existence of particles that are far too small to be detected in any conceivable experiment.

In his new book Greene takes us even further away from reality, asking us to consider not just hypothetical particles but entire universes that lie beyond the reach of our instruments. Multiverses are old hat, of course. In a 1990 article for Scientific American on cosmology I included a sidebar, "Here a universe, there a universe…," about speculation that our universe "is only one in an infinitude of cosmos."

My tone was lightly mocking, because cosmologists themselves seemed to be kidding—even embarrassed—when they talked about all these alternate universes. But now Greene—as well as Stephen Hawking, Leonard Susskind, Sean M. Carroll and other prominent physicist/popularizers—want us to take multiverses seriously.

In Hidden Reality Greene notes that different theories of modern physics yield many different multiverse theories. One of the oldest is the many-worlds theory, which conjectures that all of the possible histories of our world allowed for by quantum mechanics are realized in other universes. Greene also touts the inflationary multiverse, which holds that new universes are constantly springing into existence via a mysterious antigravity force called inflation. String theory yields the brane multiverse; strings plus inflation produces the landscape multiverse; and that still leaves us with the quilted, cyclic, holographic and simulated multiverses, all of which Greene cheerfully elucidates.

These multiverse theories all share the same fundamental defect: They can be neither confirmed nor falsified. Hence, they don't deserve to be called scientific, according to the well-known criterion proposed by the philosopher Karl Popper. Some defenders of multiverses and strings mock skeptics who raise the issue of falsification as "Popperazis"—which is cute but not a counterargument. Multiverse theories aren't theories—they're science fictions, theologies, works of the imagination unconstrained by evidence.

At their best, science fiction and theology can leave us awestruck before the unutterable strangeness and vastness of the cosmos. Multiverse theories used to arouse these emotions in me. When the Russian physicist Andrei Linde—one of the inventors of the inflation theory of cosmic creation—first explained his chaotic, self-reproducing, fractal, inflationary multiverse theory to me 20 years ago, my reaction was, "Wow! That's so cool!"

Multiverse theories don't turn me on anymore. Perhaps it's because of 9/11 and all its bloody consequences, especially the U.S. wars in Afghanistan and Iraq. Also, I have two teenage kids, and I'm worried about the enormous problems they're inheriting from my generation. Not only wars overseas but also global warming, species extinction, pollution, poverty, pandemics and so on.

Now, multiverse theories strike me as not only unscientific but also immoral, for two basic reasons: First, at a time when we desperately need science to help us solve our problems, it's irresponsible for scientists as prominent as Greene to show such a blithe disregard for basic standards of evidence. Second, like religious visions of paradise, multiverses represent an escapist distraction from our world.

I find two multiverse concepts especially loathsome. One is the idea that an infinite universe contains infinite copies of our world. Greene writes that in another cosmos "your doppelganger is now reading this sentence, along with you. In others…he or she has, well, a less than felicitous disposition and is someone you'd rather not meet in a dark alley." Even worse is the proposition that our world is artificial, a simulation being run on a computer designed by an alien civilization. This sort of adolescent claptrap devalues our reality even more than heaven, Valhalla, nirvana and other ancient fantasies do.

Is theorizing about parallel universes as immoral as betting on derivatives based on subprime mortgages? I wouldn't go that far. Nor do I think all scientists should be seeking cures for cancer, more efficient solar cells or other potential boons to humanity. But scientists should, at the very least, investigate the world in which we live rather than worlds that exist—as far as we will ever know—only in their imaginations.

P.S.: Earlier this week, I learned that JR Minkel, a former writer for Scientific American, died. Even before we met five years ago, I was a fan of JR's work. He was not only a talented young journalist with an offbeat sensibility; he was also a thoughtful, gentle soul. I'll miss him.