Does Einstein's general theory of relativity correctly describe gravity in every regime tested so far, including the strongest fields near black holes and at cosmological scales?
#partly-solved
Post a problemDo black holes really exist exactly as general relativity predicts, with event horizons as described, and do hypothetical thread-like 'cosmic strings' left over from the early universe exist?
What cataclysmic events power gamma-ray bursts, the most luminous explosions in the universe, and are long and short bursts produced by different mechanisms?
Why does a pile of dry sand (or any granular material) poured from a point always come to rest at roughly the same characteristic slope, the 'angle of repose,' regardless of how much sand is poured?
Is it true that the Earth's rotation (the Coriolis effect) makes bathtubs and sinks drain counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere, and if so, why don't ordinary home bathtubs reliably show this?
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?
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?
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?
Why do electrons confined to two dimensions in a strong magnetic field condense into exotic liquid states carrying fractionally charged, sometimes non-abelian, quasiparticles?
Gamma-ray lasers (grasers) and superhigh-power lasers
Research level
Can a laser-like device be built that emits coherent gamma rays (a 'graser'), using nuclear rather than atomic transitions, and how far can achievable laser intensities be pushed?
Superheavy elements and the island of stability
Research level
How far up the periodic table can atomic nuclei be pushed by fusing lighter nuclei together, and does a predicted 'island of stability' of unusually long-lived superheavy isotopes actually exist?
Hadron mass spectrum, QCD, and the quark-gluon plasma
Research level
Can the masses of protons, neutrons and other hadrons be computed from the underlying quark-gluon theory (QCD), and can the free quark-gluon plasma believed to have filled the early universe be created and studied in the laboratory?
CP violation and the matter-antimatter asymmetry
Research level
Where and how strongly does nature violate the combined charge-parity (CP) symmetry, and can the observed amount explain why the universe is made of matter rather than antimatter?
The cosmological constant, dark energy, and inflation
Research level
What is causing the expansion of the universe to accelerate (dark energy, or a cosmological constant), and did the universe undergo a brief phase of exponential inflationary expansion in its first fraction of a second?
Neutron star interiors and supernova explosion mechanisms
Research level
What is the exact state of matter inside a neutron star (its equation of state), and what physical mechanism actually revives a massive star's collapsing core into a supernova explosion?