why observed spacetime has 3 spatial dimensions and 1 temporal dimension?
Why does time have a direction?
Is there a single possible past?
Is the present moment physically distinct from the past and future, or is it merely an emergent property of consciousness?
How does the quantum description of reality, which includes elements such as the superposition of states and wavefunction collapse or quantum decoherence, give rise to the reality we perceive? What constitutes a “measurement” which apparently causes the wave function to collapse into a definite state?
Some quantum mechanical processes (such as quantum teleportation arising from quantum entanglement) cannot be simultaneously “local”, “causal”, and “real”, but it is not obvious which of these properties must be sacrificed,[or if an attempt to describe quantum mechanical processes in these senses is a category error such that a proper understanding of quantum mechanics would render the question meaningless. Can a multiverse resolve it?
The values of the fundamental physical constants are in a narrow range necessary to support carbon-based life.Is this because there exist other universes with different constants, or are our universe’s constants the result of chance, or some other factor or process?
Are there non-local phenomena in quantum physics?[If they exist, are non-local phenomena limited to the entanglement revealed in the violations of the Bell inequalities, or can information and conserved quantities also move in a non-local way? Under what circumstances are non-local phenomena observed? What does the existence or absence of non-local phenomena imply about the fundamental structure of spacetime?
In quantum mechanics time is a classical background parameter and the flow of time is universal and absolute. In general relativity time is one component of four-dimensional spacetime, and the flow of time changes depending on the curvature of spacetime and the spacetime trajectory of the observer. How can these two concepts of time be reconciled?
How did the conditions for anything to exist arise?
Is the universe heading towards a Big Freeze, a Big Rip, a Big Crunch, or a Big Bounce, or is it part of an infinitely recurring cyclic model?
The diameter of the observable universe is about 93 billion light-years, but what is the size of the whole universe?
Why is there far more matter than antimatter in the observable universe?
What is the identity of dark matter? Is it a particle, the lightest superpartner (LSP), or do the phenomena attributed to dark matter point not to some form of matter but actually to an extension of gravity?
Do black holes produce thermal radiation? Does this radiation contain information about their inner structure? If not, and black holes can evaporate away, what happens to the information stored in them (since quantum mechanics does not provide for the destruction of information)? Or does the radiation stop at some point leaving black hole remnants? Is there another way to probe their internal structure somehow, if such a structure even exists?
Fusion energy may potentially provide power from abundant resource (e.g. hydrogen) without the type of radioactive waste that fission energy currently produces. However, can ionized gases (plasma) be confined long enough and at a high enough temperature to create fusion power?
How is long-term memory stored on a biological substrate undergoing constant turnover?
What are the basic building blocks of immune system networks?