If you strike a free rigid rod with a sharp blow at one point, why does a particular other point on the rod feel almost no jolt at all, while striking closer to that point makes your hand sting badly, as if the rod 'chooses' where to react?
Physics
Post a problemCan a closed loop of buoyant floats, rigged to pass continuously up through a column of water and back down outside it, be made to turn forever and deliver free work, since Archimedes' principle gives every submerged float an upward buoyant force?
How far can a stack of identical bricks, each balanced on the one below it, be made to overhang the edge of a table before it topples — is there any theoretical limit to how far the top brick can stick out?
Can a block of 'ice' actually catch fire and burn, and can a lump of ordinary tin be made to melt just by dropping it into boiling water?
Is it possible for a physical system to have negative heat capacity — that is, to actually cool down when energy is added to it, or heat up when energy is removed?
Why can burning the same amount of fuel to give a rocket the same boost in speed release far more kinetic energy when the rocket is already moving fast than when it starts from rest — seemingly getting extra energy for free?
The Gomboc: does a self-righting convex body with just one stable and one unstable point exist
Research level
Does a convex, uniformly dense three-dimensional shape exist that always rights itself to the same single stable resting position, like a roly-poly toy but with no internal weighting?
Ball lightning
Research level
What physical mechanism, if any, produces the glowing floating spheres of light during thunderstorms reported as 'ball lightning'?
Sonoluminescence: why do collapsing bubbles flash light
Research level
Why does a tiny gas bubble trapped in liquid and driven by sound waves emit a flash of light each time it violently collapses?
Boiling crisis / critical heat flux
Research level
What sets the maximum heat flux a boiling liquid can carry away before the surface suddenly overheats and burns out (the boiling crisis), a limit engineers have used empirically for decades?
Why trees cannot grow taller than about 130 meters
Research level
What physical limit stops trees from growing indefinitely taller, and what is the actual maximum height set by water transport physics?
Von Neumann's law generalized to 3D foam coarsening
Research level
The 1952 law for how 2D bubbles in a foam grow or shrink based only on their number of sides was long unmatched in 3D -- does an analogous simple law exist for real three-dimensional foams and grain structures?
Does deeply supercooled water actually separate into two distinct liquid phases (a long-hypothesized liquid-liquid critical point), rather than remaining just a single metastable liquid?
Why can the same pile of sand or grain flow like a liquid under some conditions and lock into a rigid, solid-like jammed state under others?
Diffuse interstellar bands
Research level
What molecules cause the hundreds of unexplained absorption lines (diffuse interstellar bands) seen in starlight passing through interstellar space, and why have only a handful been identified after a century of study?
Non-Abelian anyons at quantum Hall filling 5/2
Research level
Does the fractional quantum Hall state observed at filling factor 5/2 host exotic quasiparticles (non-Abelian anyons) whose braiding statistics could form the basis of topological quantum computers, and can this be conclusively demonstrated?
Proton radius puzzle
Research level
Why did early 2010s measurements of the proton's charge radius using muonic hydrogen come out about 4% smaller than the previously accepted value from electron-based measurements, and which value is correct?
Internal structure of pentaquarks and exotic hadrons
Research level
Now that pentaquark particles (five-quark states) have been confirmed to exist, are they tightly bound compact five-quark objects, or loosely bound 'molecules' of a meson and a baryon stuck together, and which quark combinations are actually possible?
Origin mechanism of fast radio bursts
Research level
What physical process produces fast radio bursts — millisecond-long, extremely bright pulses of radio waves arriving from distant galaxies — and why do some sources repeat unpredictably while the majority appear to burst only once?
Black hole information paradox
Research level
When a black hole evaporates via Hawking radiation, does the information about what fell into it get destroyed forever (violating quantum mechanics), or is it somehow preserved and encoded in the outgoing radiation?