Is String Theory in Trouble?
"So even if you accept the multiverse and the idea that certain local physical laws are anthropically determined, you still need a unique mega-theory to describe the whole multiverse? Surely it just pushs the question back?
Yes, absolutely. The bottom line is that we need to describe the whole thing, the whole universe or multiverse. It's a scientific question: is the universe on the largest scales big and diverse or is it homogeneous? We can hope to get an answer from string theory and we can hope to get some information from cosmology.
There is a philosophical objection called Popperism that people raise against the landscape idea. Popperism [after the philosopher Karl Popper] is the assertion that a scientific hypothesis has to be falsifiable, otherwise it's just metaphysics. Other worlds, alternative universes, things we can't see because they are beyond horizons, are in principle unfalsifiable and therefore metaphysical - that's the objection. But the belief that the universe beyond our causal horizon is homogeneous is just as speculative and just as susceptible to the Popperazzi...
Is it premature to invoke anthropic arguments - which assume that the conditions for life are extremely improbable - when we don't know how to define life?
The logic of the anthropic principle requires the strong assumption that our kind of life is the only kind possible. Why should we presume that all life is like us - carbon-based, needs water, and so forth? How do we know that life cannot exist in radically different environments? If life could exist without galaxies, the argument that the cosmological constant seems improbably fine-tuned for life would lose all of its force. And we don't know that life of all kinds can't exist in a wide variety of circumstances, maybe in all circumstances. It a valid objection...
If we do not accept the landscape idea are we stuck with intelligent design?
I doubt that physicists will see it that way. If, for some unforeseen reason, the landscape turns out to be inconsistent - maybe for mathematical reasons, or because it disagrees with observation - I am pretty sure that physicists will go on searching for natural explanations of the world. But I have to say that if that happens, as things stand now we will be in a very awkward position. Without any explanation of nature's fine-tunings we will be hard pressed to answer the ID critics. One might argue that the hope that a mathematically unique solution will emerge is as faith-based as ID."
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"So if this planet is made of earth, why not other planet made of fire? What is the scientific reason to deny it? Because I cannot live in the fire, it does not mean other living entities cannot live there. There are different kinds of living entities. Just like you cannot live within the water, within the ocean, but there are other living entities... Just like fish. They live very comfortably within the water. So why should we conclude that there is no life in the sun planet or moon planet?"
Srimad-Bhagavatam 1.2.16 -- Vrndavana, October 27, 1972