User:Dan Gluck/sandbox

Speculations into why the universe is quantum mechanical
Though this question is usually thought to dwell beyond the scope of physics, several prominent physicists have speculated regarding the reason for which our universe is quantum mechanical, rather than being classical, which seems much more intuitive.

Most speculations have been anthropic, trying to show that quantum mechanics is essential or at least tightly linked to our existence as sentient beings able to ask such questions.

Is quantum mechanics intimately related to consciousness?
John von Neumann and later Eugene Wigner suggested an interpretation to quantum mechanics that ties wavefunction collapse to consciousness, suggesting that a quantum mechanical universe allows for a special role for consciousness. However, consciousness is in fact not needed to explain the features of quantum mechanics, as wavefunction collapse by any large object would suffice to explain them. The emerging understanding of decoherence processes have further made any possible role of consciousness in wavefunction collapse (and arguably the collapse itself) a redundant one.

A different line of thought was proposed by Roger Penrose, who have suggested quantum gravity (quantum mechanics combined with general relativity) is somehow needed for consciousness to arise. This hypothesis is not supported by modern brain research, however.

Is quantum mechanics required to create our universe?
Quantum gravity plays a central role in almost every speculation related to the anthropic principle. It seems that the specific details and numbers entering the fundamental theories we have - the standard model of particle physics and the standard model of cosmology - are finely tuned to allow for the creation of galaxies, stars and chemistry, and thus also life. In quantum gravity, this can be explained in scenarios where there are many different options for the physical laws and constants, and we live in just one of this options, one that allows life to emerge. It is then no mystery that the universe we live in allows life.

Stephen Hawking and Leonard Mlodinow have further suggested that quantum gravity is required to create the universe "out of nothing". Indeed, quantum mechanics combined with general relativity allows the universe to be created from an extremely small early and simple universe: According to the standard model of particle physics, quantum field theory allowed the creation of matter in the early universe; according to the standard model of cosmology, general relativity predicts the "creation" of space-time from an extremely small early universe, in a process called the Big Bang. These are well-established and widely tested theories.

The theory of cosmic inflation further suggests that quantum effects in the cosmological level created random patterns in the very early universe, from which structures such as galaxies emerged. This theory is testable and gained some supportive evidence. Several hypotheses regarding quantum gravity, such as the Hartle–Hawking theory, even suggest the Big Bang may have began with the spontaneous emergence of the tiny early universe. It is not clear, however, how such hypotheses can be tested.

Hawking and Mlodinow go even further, and suggest that under the many-worlds interpretation, multiple types of universes with different outcome are all deterministically produced in parallel. In that way, the peculiar properties of quantum mechanics allows our universe to be deterministically created with no initial information needed - no "creator" per their interpretation.

While Hawking and Mlodinow do suggest some directions at making such bold claims testable, it is currently contended that they are indeed testable, or even meaningful at all. Additionally, claiming that quantum gravity processes might indeed have created the universe "out of nothing" can be within the realm of physics and might be testable; but not claiming that this is the reason why our universe is quantum.