The following is a list of unsolved problems grouped into broad areas of physics. Is there a single possible past? What links the quantum arrow of time to the thermodynamic arrow? What constitutes principles of physics 10th edition pdf “measurement” which apparently causes the wave function to collapse into a definite state?

3 spatial dimensions and 1 temporal dimension? Do “fundamental physical constants” vary over time? Are any of the fundamental particles in the standard model of particle physics actually composite particles too tightly bound to observe as such at current experimental energies? Are there fundamental particles that have not yet been observed, and, if so, which ones are they and what are their properties? What is the minimum number of dimensionless physical constants from which all other dimensionless physical constants can be derived?

Are dimensional physical constants necessary at all? The values of the fundamental physical constants are in a narrow range necessary to support carbon-based life. In quantum mechanics time is a classical background parameter and the flow of time is universal and absolute. How can these two concepts of time be reconciled? Is the theory of cosmic inflation in the very early universe correct, and, if so, what are the details of this epoch?

Why is there something rather than nothing? How did the conditions for anything to exist arise? 93 billion light-years, but what is the size of the whole universe? What is the identity of dark matter? Is a non-spherically symmetric gravitational pull from outside the observable universe responsible for some of the observed motion of large objects such as galactic clusters in the universe?

Some large features of the microwave sky at distances of over 13 billion light years appear to be aligned with both the motion and orientation of the solar system. Neither the curvature nor the topology is presently known, though the curvature is known to be “close” to zero on observable scales. Is spacetime fundamentally continuous or discrete? Are there deviations from the predictions of general relativity at very small or very large scales or in other extreme circumstances that flow from a quantum gravity theory? Do black holes produce thermal radiation, as expected on theoretical grounds? Or does the radiation stop at some point leaving black hole remnants?

If so, what is their size? Are dimensions a fundamental property of the universe or an emergent result of other physical laws? Can we experimentally observe evidence of higher spatial dimensions? Are there non-local phenomena in quantum physics? 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? How does this elucidate the proper interpretation of the fundamental nature of quantum physics? Why are these scales so different from each other? The Planck mass plays an important role in parts of mathematical physics.

A series of researchers have suggested the existence of a fundamental particle with mass equal to or close to that of the Planck mass. The Planck mass is however enormous compared to any detected particle even compared to the Higgs particle. It is still an unsolved problem if there exist or even have existed a particle with close to the Planck mass. This is indirectly related to the hierarchy problem. Did particles that carry “magnetic charge” exist in some past, higher-energy epoch?

If so, do any remain today? Is the proton fundamentally stable? Or does it decay with a finite lifetime as predicted by some extensions to the standard model? How do the quarks and gluons carry the spin of protons? Is spacetime supersymmetry realized at TeV scale? If so, what is the mechanism of supersymmetry breaking? Does supersymmetry stabilize the electroweak scale, preventing high quantum corrections?