How can the basilisk ('Jesus Christ') lizard run across the surface of open water without sinking?
Problems by archive
Post a problemWhy does an arrow that appears aimed to the side of the target (because it must flex around the bow at release) actually fly straight to the target?
Why can a water droplet levitating on its own vapor cushion above a very hot surface spontaneously start oscillating into a rotating star-like polygonal shape?
Why do some smooth, spinning balls at intermediate speeds curve in the direction opposite to what the ordinary Magnus effect predicts?
Why does a thinning thread of a polymer or viscoelastic fluid form a regular string of beads instead of smoothly pinching off into a single drop?
Why can small insects and water striders walk on water using surface tension, while this same strategy becomes impossible for animals much larger than about a human hand?
Why does the pressure at the bottom of a tall container of grain or sand not keep increasing with depth like a liquid would, but instead level off?
Why can pathogen-laden droplets from a sneeze or cough travel much farther than the classical 1930s evaporation-falling-curve models predicted?
Is there a theoretical model that can predict the statistical structure of turbulent flow (e.g. energy distribution across eddy sizes) directly from the Navier-Stokes equations, without ad hoc empirical closures?
What is the physical nature of the glass transition — why does a liquid cooled fast enough become a rigid amorphous solid with a huge (often 10+ orders of magnitude) slowdown in molecular motion, without any obvious change in structure or a sharp thermodynamic phase transition?
Does a fundamental scalar field (the Higgs field) permeate all of space and give elementary particles their mass through their interaction with it, as predicted by electroweak theory in the 1960s?
Do isolated magnetic north or south poles (magnetic monopoles) exist as fundamental particles, as predicted by some grand unified theories, or is magnetic charge always found in inseparable north-south pairs as observed so far?
Why did every experiment from the 1960s through the 1990s detect only about one-third to one-half of the electron neutrinos from the Sun's fusion reactions that Standard Solar Model calculations predicted?
Why do measurements of the universe's current expansion rate (the Hubble constant) using the early universe (cosmic microwave background plus the standard cosmological model) consistently give a lower value (~67-68 km/s/Mpc) than measurements using nearby stars and supernovae (~73 km/s/Mpc)?
Do ripples in spacetime predicted by Einstein's general relativity — gravitational waves produced by cataclysmic events like merging black holes — actually exist and can they be directly measured on Earth?
Why did NASA's Pioneer 10 and 11 spacecraft, as they left the solar system in the 1980s-90s, experience a tiny unexplained extra deceleration (about 8.7 x 10^-10 m/s^2) toward the Sun that didn't match standard gravitational predictions?
Why did several NASA and ESA spacecraft (including Galileo, NEAR, Cassini, and Rosetta) show small unexplained changes in speed (a few mm/s) during close Earth gravity-assist flybys in the 1990s-2000s, beyond what standard orbital mechanics predicts?
What is dark matter actually made of — is it a new fundamental particle (like a WIMP or axion) that barely interacts with ordinary matter, or does the evidence for it instead point to gravity itself working differently than Einstein and Newton predicted on galactic scales?
What is causing the expansion of the universe to accelerate — is it a constant vacuum energy density (the cosmological constant), a dynamical field (quintessence) that changes over time, or a sign that general relativity itself needs modification on cosmic scales?
Why does the observable universe consist almost entirely of matter with essentially no primordial antimatter, when the Big Bang should have produced matter and antimatter in equal amounts that would have mutually annihilated?