The bottom of things
Empty space and information, then the smallest length, black holes and time, where quantum theory and gravity meet and no tested theory joins them, and last, why there is anything at all.
6 stopsShallows to unknown
The order, and why it is this one
Join anywhere. Each stop carries the route with it, so the foot of every question here offers the next one and the way back to this page.
- Is empty space empty?In quantum field theory the vacuum is the lowest energy state of a set of fields, not an absence. It has measurable consequences, including a force between nearby plates. It also has an energy density, and the value that theory predicts for it is wrong by some fifty orders of magnitude on a careful estimate and 120 on the simplest.Deepdepth, meaning: Major unanswered questions.Uncertain. Meaningful scientific, historical or philosophical disagreement remains.Read
- Does information weigh anything?Information is not an abstraction floating above physics. Erasing it has an unavoidable thermodynamic cost, which sets a floor under any computation that throws information away and is the standard answer to a paradox about the second law that had stood for more than a century.Shallowsdepth, meaning: Interesting questions with substantial evidence and real uncertainty remaining.Known. Strongly supported by evidence and broadly established.Read
- Is there a smallest length?Combining gravity with quantum mechanics produces a length below which the usual ideas of distance appear to fail. Models inspired by the candidate theories predict observable consequences, and astronomical observations have already bounded the simplest of them past the Planck scale itself.Abyssdepth, meaning: Fundamental questions where our understanding is extremely limited.Unknown. Humanity does not currently have a satisfactory answer.Read
- Where does information go in a black hole?By Hawking’s calculation, a black hole radiates and eventually disappears. If the radiation carries no trace of what fell in, quantum mechanics is violated. If it does, there is no agreed account of how. Recent calculations suggest the information comes out, by a route that is not yet understood.Abyssdepth, meaning: Fundamental questions where our understanding is extremely limited.Unknown. Humanity does not currently have a satisfactory answer.Read
- Is time fundamental?Quantum mechanics treats time as an external parameter. General relativity treats it as part of a dynamical geometry. Combining them produces a description in which time has no separate role, and whether that is a deep truth or a technical artefact is unresolved.Abyssdepth, meaning: Fundamental questions where our understanding is extremely limited.Unknown. Humanity does not currently have a satisfactory answer.Read
- Why is there anything at all?Physics can describe the early universe in remarkable detail and has nothing to say about why there is a universe to describe. It is not clear that any possible evidence would settle it.Unknowndepth, meaning: Questions where we may not even know what an answer would look like if we found one.Unknown. Humanity does not currently have a satisfactory answer.Read