Nearly two decades after its proposal, the String Theory Landscape remains divisive among physicists. “In the beginning there were people who hated it. Some hate it even now, and more strongly than before,” Andrei Linde said.

Many view the Landscape as a kind of Faustian bargain: It elegantly explains why the universe appears to be so eerily fine-tuned for life – there are myriad universes and we just happen to live in one that’s tuned for us – but it does so by dashing Einstein’s dream of one day uncovering a “theory of everything” from which the precise values of nature’s laws and constants logically and inevitably arise.

The idea that our universe must have laws suitable for life is called the anthropic principle, and it’s a notion many physicists despise. One U.S. Nobel laureate called it “defeatist” and “dangerous” and said it “smells of religion and intelligent design.”

Even Landscape proponents accept anthropic selection only with a measure of resignation and ambivalence. “Anthropics is distasteful to most string theorists. You’d rather have a nice equation which you can solve to predict the mass of the electron, but that seems very unlikely to me,” Shamit Kachru said. “On the other hand, the Landscape offers in compensation a different kind of elegance: Over vast cosmological scales many solutions are realized; so in that sense, nothing is wasted.”

Renata Kallosh conceded that the anthropic principle “would not be my first choice” for explaining why the universe is the way it is. “In science, the preference will always be given to non-anthropic explanations – unless, however, there is nothing better,” she said.

And there is nothing better at present, according to Leonard Susskind . “It’s not enough to say, ‘I hate the idea.’ You have to say, ‘Here’s a better idea.’ Every month or so somebody will come out with some screwball theory of why the cosmological constant is close to zero, but it won’t last for more than a week,” he said. “A legitimate controversy is when there are two more or less equally good ideas which are in conflict with each other. The simple fact is there is no competition.”

Susskind also thinks that the reports of the death of Einstein’s dream of a unified field theory are greatly exaggerated, and he offers an analogy. “Imagine you live in a world where you only know about one or two different animal species,” he said. “If you were of a scientific bent, you might say, ‘I need to explain why those two species are exactly the way they are.’ Then it’s discovered that there are a lot more different kinds of species, zillions of them. Does that mean that the search for a grand unified theory of life is to be abandoned? No, it means that whatever the fundamental principles are, they shouldn’t be expected to only give rise to a very small number of possibilities. Whatever the ultimate theory for physics is, it should not lead to a conclusion that there’s only one universe. It should lead to the conclusion that there are lots of them.”

After nearly 20 years, particle physicist Savas Dimopoulos is tired of the debate. “Nature doesn’t care about our wishes and hopes,” said Dimopoulos. “Our job is to find out what is true. The important thing is not whether we like these ideas or not, but how to test them. Because in the end, physics has two legs: theory and experiment. To find the truth, you need both.”