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The Foundational Questions

The following is a selection of foundational questions that were raised at the Iceland conference in 2007 and triggered spirited debate.

Questions Include:

  • Are there reasons to believe that standard QM is insufficient?
    • Can quantum mechanics be falsified? In other words, when experiments agree with QM, what other consistent theories do they rule out?
    • Can we devise further interesting tests of quantum mechanics by embedding it in a larger parametrized theory?
    • Is it possible to characterize/parametrize the complete set of hidden variable theories? Local and nonlocal ones?
    • Are there interesting incompatibilities between some versions of quantum theory and (special) relativity?
  • Can we apply QM to the entire universe?
    • Are some QM interpretations worthless for cosmology, or are they all suitable in some way?
    • is the Universe like any closed system, or might it have qualitative differences (e.g. being spatially infinite)?
  • What is quantum physics fundamentally about?
    • About reality?
    • About information?
    • About something else?
    • What is the real role of the observer?
  • How much information is really there in a quantum state?
    • Does a qubit have 1 bit, or 2 bits, or infinitely many?
    • Is it a cheat to say that QM, with qubits, really turns the information content finite, when the description of a qubit requires two real numbers?
    • If we consider an entangled state of N qubits, is the amount of information exponential or polynomial in N?
  • Can one define probabilities in an eternally inflating spacetime?
    • If two sets are countably infinite (or in general have the same cardinality), is there a meaningful way in which they can be said to have different sizes?
    • Do measures exist which are sensible and avoid both the ‘youngness’ and ‘oldness’ (Boltzmann’s Brain) paradoxes?
    • Is counting observers inherently problematic because it gives rise, e.g. to the Doomsday, Boltzmann’s Brain, and Simulation paradoxes?
  • On what side of the borderline between science and philosophy are parallel universes?
    • In what senses are the inflationary many universes equivalent to or different from the Everett many-worlds?
    • What, if any, observational signatures might exist of other inflationary universes?
    • in many-world QM, what does it mean for the other worlds to be ‘real’? What does it mean for one world to be, e.g., 1% as real as another, or that the ratio depends on the basis employed?
    • Can we consistently employ the Copernican/Mediocrity principle (frequentist statistics over observers) and also accept many-worlds QM?
  • What is dark energy?
    • Are we in a true vacuum or false vacuum?
    • Are early inflation and late inflation (dark energy) related?
  • Should we expect the “constants of nature” to be constant?
    • Are observable changes in fundamental parameters ruled out by near-constancy of vacuum energy?
    • If fundamental constants oscillate, how well can we constrain this?
    • How exactly do fundamental constants couple to vacuum transition in something like the string-theory landscape?
  • Why did our universe begin in a low-entropy state?
    • Are the relations between all of the ‘arrows of time’ understood?
    • What role, if any, does the vacuum energy play in cosmic initial conditions?
    • What is the nature of time in quantum physics?
    • To what extent can quantum physics retrodict earlier states?
    • Can one define Boltzmann entropy in quantum mechanics?
  • Is there any hope of experimentally testing quantum gravity?
    • Are there fundamental limits to experimentally determining the theory of quantum gravity (e.g. scattering high-energy particle may just form black holes)?
    • Can we devise a gedanken experiment to interfere different causal structures?
  • What will the ultimate theory (or at least the next one) be like?
    • Will it actually use the current form of QM?
    • Will we construct it by starting with a classical theory and quantizing it?
    • Will it involve tensor category theory?
    • As we go up in energy, should we expect to find more or less degrees of freedom?
    • Will the next/final theory be simple or complicated?
    • Beautiful or ugly?
    • Funny or boring?
    • What does it mean to understand something? Does ability to compute all answers mean that we really understand something? Or are ’emergent’ phenomena just a real (and call for just as much explanation) as the processes underlying them?
  • Is nature fundamentally analog or digital (continuous or discrete)? – Is that a well-posed question?
    • Is there a continuum between discreteness and continuity?
    • Are there experiments in physics that really require one or the other?
  • Is nature completely mathematical?
    • If not, what would the extra ingredient(s) be?
    • Is ‘being observed’ in QM such a non-mathematical property?
  • Do any of our capabilities and experiences inform us that we are not in a computer simulation?
    • is there a Measure catastrophe (the Simulation argument)?
    • Does Penrose’s argument have anything to say? Is there a variant that might?
    • Can subjectivity exist in a simulation (the Hard AI problem)?
    • Can temporal duration or ‘now’ have meaning in a simulation?

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