"Falsifiability or refutability is the logical possibility that an assertion could be shown false by a particular observation or physical experiment. That something is "falsifiable" does not mean it is false; rather, it means that if the statement were false, then its falsehood could be demonstrated.

The claim "No human lives forever" is not falsifiable since it does not seem possible to prove wrong. In theory, one would have to observe a human living forever to falsify that claim. On the other hand, "All humans live forever" is falsifiable since the presentation of just one dead human could prove the statement wrong (excluding metaphysical assertions about souls, which are more difficult to falsify). Moreover, a claim may be true and still be falsifiable; "All humans have red blood" is a true statement but remains falsifiable because we can at least imagine finding a human with blood of another color to prove this statement wrong.

Some statements are only falsifiable in theory, while others are even falsifiable in practice (i.e. testable). For example, "it will be raining here in one billion years" is theoretically falsifiable, but not practically so.

Falsifiability, particularly testability, is an important concept in science and the philosophy of science. The concept was made popular by Karl Popper in his philosophical analysis of the scientific method. Popper concluded that a hypothesis, proposition, or theory is "scientific" only if it is, among other things, falsifiable. That is, falsifiability is a necessary (but not sufficient) criterion for scientific ideas. Popper asserted that unfalsifiable statements are non-scientific, although not without relevance. For example, meta-physical or religious propositions have cultural or spiritual meaning, and the ancient metaphysical and unfalsifiable idea of the existence of atoms has led to corresponding falsifiable modern theories. A falsifiable theory that has withstood severe scientific testing is said to be corroborated by past experience, though in Popper's view this is not equivalent with confirmation and does not guarantee that the theory is true or even partially true.

Popper invented the notion of metaphysical research programs to name such ideas. In contrast to positivism, which held that statements are senseless if they cannot be verified or falsified, Popper claimed that falsifiability is merely a special case of the more general notion of criticizability. Still, he admitted that tests and refutation is one of the most effective methods by which theories can be criticized."

"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."