Physical world and mathematics

Irreducible representation

An irreducible representation, or irrep, is a representation of a group or algebra with no smaller nontrivial representation; irreps are the building blocks of representation theory.

An irregular galaxy is a galaxy lacking a recognizable symmetric structure like spiral arms or an elliptical profile, and is typically small, blue, and gas-rich; the Magellanic Clouds are familiar examples.

Irregular variable

An irregular variable is a variable star with no detectable period in its brightness changes. The GCVS classes LB and LC cover irregular red giants and supergiants.

Irresistible force paradox

The irresistible force paradox, also called the unstoppable force paradox, is the contradiction that an unstoppable force meeting an immovable object must move and not move it.

Irreversible process

An irreversible process, also called an irreversible change, is a thermodynamic process that cannot be reversed without a permanent change, because reversing it would violate the second law.

Oct 21, 2026

Irruputuncu

Irruputuncu (Irruputunco, Iruputuncu) is a small active Chile-Bolivia border stratovolcano, 5,163 m, Central Andean arc, fumarolic, whose only confirmed eruption was a minor VEI 2 phreatic event in 1995.

Irvin Cohen

Irvin Sol Cohen (1917–1955) was an American mathematician at MIT who worked in commutative algebra and is known for his 1946 structure theorem for complete local rings and the rings named after him, Cohen rings.

Irving Kaplansky

Irving Kaplansky (1917–2006) was a Canadian mathematician who made major contributions to ring theory, group theory, and operator algebras, spending most of his career at the University of Chicago.

Irving Langmuir

Irving Langmuir (1881–1957) was an American physical chemist who spent his career at General Electric in Schenectady and won the 1932 Nobel Prize in Chemistry for surface chemistry.

Irving Segal

Irving Segal (1918–1998) was an American mathematician and mathematical physicist, professor at MIT and a National Academy of Sciences member, known for the C-algebra, the GNS construction, and the controversial chronometric cosmology.

Irwin I. Shapiro

Irwin I. Shapiro is an astrophysicist, Timken University Professor Emeritus at the Center for Astrophysics | Harvard & Smithsonian, known for predicting the Shapiro time delay in 1964.

Isaac F. Silvera

Isaac F. Silvera is a physicist and Thomas D. Cabot Professor at Harvard, known for stabilizing the first cold-atom gas and reporting the contested 2017 observation of metallic hydrogen.

Isaac Held

Isaac Held (Isaac M. Held) is an American atmospheric scientist who spent four decades at NOAA's Geophysical Fluid Dynamics Laboratory in Princeton, known for the Held–Suarez model test.

Isaac Khalatnikov

Isaac Khalatnikov, or Isaak Markovich Khalatnikov (1919–2021), was a Soviet and Russian theoretical physicist, a student of Lev Landau, founding director of the Landau Institute, and co-discoverer of the Landau pole and the BKL singularity.

Isaac Newton

Sir Isaac Newton (1642–1727) was an English mathematician and physicist who formulated the laws of motion and universal gravitation in his Principia Mathematica of 1687.

Isaac Newton Institute

The Isaac Newton Institute for Mathematical Sciences is a national research institute in Cambridge, England, established in 1992 and built around intensive themed visitor programs.

Isaac Newton Medal

The Isaac Newton Medal is the most prestigious annual physics prize of the Institute of Physics, open to physicists of any nationality worldwide since 2008.

Isaac Roberts

Isaac Roberts (1829–1904) was a Welsh-born builder and engineer who became a pioneer of long-exposure astrophotography, taking the first photograph to show the Andromeda Nebula as a spiral.

Isaak Bacharach

Isaak Bacharach (1854–1942) was a German mathematician and secondary-school teacher in Erlangen and Nürnberg whose 1886 correction of Cayley's intersection theorem became known as the Cayley–Bacharach theorem.

Isaak Khalatnikov

Isaak Markovich Khalatnikov (1919–2021) was a Soviet and Russian theoretical physicist who co-discovered the chaotic BKL singularity and founded and directed the Landau Institute for Theoretical Physics.

Isaak Pomeranchuk

Isaak Yakovlevich Pomeranchuk (Исаа́к Я́ковлевич Померанчу́к) was a Soviet theoretical physicist, born in Warsaw in 1913, known for the Pomeranchuk effect, the Pomeranchuk theorem, and the pomeron.

Isabel P. Montañez

Isabel P. Montañez is a sedimentary geochemist and paleoclimatologist at UC Davis who reconstructs late Paleozoic climates, studying Earth's transition from ice age to greenhouse world.

Isabella Karle

Isabella Karle was an American X-ray crystallographer who spent 63 years at the US Naval Research Laboratory, turning direct methods into a working procedure for solving molecular structures.

Isabelle (proof assistant)

Isabelle is an interactive theorem prover, introduced by Lawrence C. Paulson at Cambridge in 1986 and developed with Tobias Nipkow in Munich; its main instance, Isabelle/HOL, uses classical higher-order logic.

Isabelle Boutron

Isabelle Boutron (born 1971) is a French epidemiologist and professor at Université Paris Cité who directs Cochrane France and co-led the 2025 CONSORT and SPIRIT reporting guidelines.

ISAC (TRIUMF)

ISAC (Isotope Separator and Accelerator) is a rare-isotope beam facility at TRIUMF in Vancouver, Canada, producing radioactive isotopes by the ISOL method for nuclear physics research since 1999.

Isadore Perlman

Isadore Perlman (1915–1991) was an American nuclear chemist who developed plutonium extraction for the Manhattan Project, first isolated curium, and later pioneered archaeological provenance studies.

Isadore Rudnick

Isadore Rudnick was an American physicist who spent 39 years on the UCLA faculty, discovered third sound in superfluid helium films, and won the Acoustical Society's Gold Medal.

Isadore Singer

Isadore Singer (1924–2021) was an American mathematician at MIT best known for the Atiyah–Singer index theorem, proved with Michael Atiyah, which won the 2004 Abel Prize.