Physics Problems for the Next Millennium

In 1900 the world-renowned mathematician David Hilbert presented twenty-three problems at the International Congress of Mathematicians in Paris. These problems have inspired mathematicians throughout the last century. Indeed, Hilbert's address has had a profound impact on the direction of mathematics, reaching far beyond the original twenty-three problems themselves.
As a piece of millennial madness, all participants of the Strings 2000 Conference were invited to help formulate the ten most important unsolved problems in fundamental physics. Each participant was allowed to submit one candidate problem for consideration. To qualify, the problem must not only have been important but also well-defined and stated in a clear way.

The best 10 problems were selected at the end of the conference by a selection panel consisting of:
Michael Duff (University of Michigan)
David Gross (Institute for Theoretical Physics, Santa Barbara)
Edward Witten (Caltech & Institute for Advanced Studies)

Here are the problems:

Are all the (measurable) dimensionless parameters that characterize the physical universe calculable in principle or are some merely determined by historical or quantum mechanical accident and uncalculable?

David Gross, Institute for Theoretical Physics, University of California, Santa Barbara

How can quantum gravity help explain the origin of the universe?

Edward Witten, California Institute of Technology and Institute for Advanced Study, Princeton

What is the lifetime of the proton and how do we understand it?

Steve Gubser, Princeton University and California Institute of Technology

Is Nature supersymmetric, and if so, how is supersymmetry broken?

Sergio Ferrara, CERN (European Laboratory of Particle Physics) Gordon Kane, University of Michigan

Why does the universe appear to have one time and three space dimensions?

Shamit Kachru, University of California, Berkeley Sunil Mukhi, Tata Institute of Fundamental Research Hiroshi Ooguri, California Institute of Technology

Why does the cosmological constant have the value that it has, is it zero and is it really constant? Andrew Chamblin, Massachusetts Institute of Technology Renata Kallosh, Stanford University

What are the fundamental degrees of freedom of M-theory (the theory whose low-energy limit is eleven-dimensional supergravity and which subsumes the five consistent superstring theories) and does the theory describe Nature?

Louise Dolan, University of North Carolina, Chapel Hill Annamaria Sinkovics, Spinoza Institute Billy & Linda Rose, San Antonio College

What is the resolution of the black hole information paradox?

Tibra Ali, Department of Applied Mathematics and Theoretical Physics, Cambridge Samir Mathur, Ohio State University

What physics explains the enormous disparity between the gravitational scale and the typical mass scale of the elementary particles?

Matt Strassler, Institute for Advanced Study, Princeton

Can we quantitatively understand quark and gluon confinement in Quantum Chromodynamics and the existence of a mass gap?

Igor Klebanov, Princeton University Oyvind Tafjord, McGill University

13 things that do not make sense

Unsolved problems in physics

This is a list of some of the unsolved problems in physics. Some of these problems are theoretical, meaning that existing theories seem incapable of explaining some observed phenomenon or experimental result. Others are experimental, meaning that there is a difficulty in creating an experiment to test a proposed theory or investigate a phenomenon in greater detail. Lastly, some even border on the pseudo-sciences, e.g. the fringes of science which are widely discredited by today's community of physicists, but may some day show promise or conclusive evidence.

Phenomena lacking clear scientific explanation:

Accretion disc jets
Why do the accretion discs surrounding certain astronomical objects, such as the nuclei of active galaxies, emit relativistic jets along their polar axes?
Accelerating universe
Why is the expansion of the universe accelerating, as we have observed? What is the nature of the dark energy driving this acceleration? If it is due to a cosmological constant, why is the constant so small, yet non-zero? Why isn't it huge, as predicted by most quantum field theories, or zero for some yet unknown symmetry reason? What is the ultimate fate of the universe?
Amorphous solids
What is the nature of the transition between a fluid or regular solid and a glassy phase? What are the physical processes giving rise to the general properties of glasses?
Arrow of time
Why did the universe have such low entropy in the past, resulting in the distinction between past and future and the second law of thermodynamics? Why does time flow in one direction at all, on macroscopic scales, when there does not seem to be an arrow of time on the scale of fundamental interactions?
Ball lightning
Are these glowing, floating objects real? How can they be explained?
Baryon asymmetry
Why is there far more matter than antimatter in the universe?
Charge
What gives quarks charge, why do different particles have different denominations of it, and why do opposite charges attract?
Cold fusion
What is the theoretical explanation for the apparent production of excess heat and helium in palladium metal when it is saturated with deuterium?
Corona heating problem
Why is the Sun's Corona (atmosphere layer) so much hotter than the Sun's surface?
Cosmological constant
Why doesn't the zero-point energy of vacuum cause a large cosmological constant? What cancels it out?
Why does the cosmological constant have the value that it has, is it zero and is it really constant?
Dark matter
What is dark matter? How is it generated? Is it related to supersymmetry? Do the phenomena attributed to dark matter really flow from some kind of matter at all, or are those phenomena in fact an indication of flawed physical laws?
Electroweak symmetry breaking
What is the mechanism responsible for breaking the electroweak gauge symmetry, giving mass to the W and Z? Is it the simple Higgs mechanism of the Standard Model?
Fundamental physical constants
Why do we observe these values, and not others? Have the values of the so called "fundamental physical constants" varied at all over time?
Gamma ray bursts (short duration)
What is the nature of these extraordinarily energetic astronomical objects that last less than two seconds?
High-temperature superconductors
Why do certain materials exhibit superconductivity at temperatures much higher than around 50 kelvins?
Mass
What causes anything to have mass?
Neutrino mass
What is the mechanism responsible for generating neutrino masses? Is the neutrino its own antiparticle?
Pioneer anomaly
What causes the apparent residual sunward acceleration of the Pioneer spacecraft?
Sonoluminescence
What causes the emission of short bursts of light from imploding bubbles in a liquid when excited by sound?
Turbulence
Is it possible to make a theoretical model to describe the behavior of a turbulent flow (in particular, its internal structures)?
Ultra-high-energy cosmic ray
Why is it that some cosmic rays appear to possess energies that are impossibly high (the so called Oh-My-God particle), given that there are no sufficiently energetic cosmic ray sources near the Earth? Why is it that (apparently) some cosmic rays emitted by distant sources have energies above the Greisen-Zatsepin-Kuzmin limit?
Universe asymmetry
What are the origins of asymmetries in general in the Universe?

Theoretical ideas in search of experimental evidence:

Axions
Is the Peccei-Quinn theory (i.e. mechanism) the solution to the strong CP problem? What are the properties of the predicted axion?
Cosmic inflation
Is the theory of cosmic inflation correct, and if so, what are the details of this epoch? What is the hypothetical inflaton field giving rise to inflation?
Extra dimensions
Does nature have more than four spacetime dimensions?
Faster-than-light
Is it possible to go faster than the speed of light? Is it possible to transmit information faster than the speed of light?
Gravity
Is our universe filled with gravitational radiation from the big bang? From astrophysical sources, such as inspiralling neutron stars? What can this tell us about quantum gravity and general relativity? Does gravity behave as predicted at very small distance scales?
Physical information
Do physical phenomena irrevocably destroy information about their prior states?
Magnetic monopoles
Are there any particles that carry "magnetic charge", and if so, why are they so difficult to detect?
Matter
Why is there matter at all? What was here before the Big Bang? How did the four fundamental forces appear?
Proton decay
Many theories beyond the Standard Model predict proton decay. Do protons decay? If so, then what is their half-life?
Quantum chromodynamics (QCD) in the non-perturbative regime
The equations of QCD remain unsolved at energy scales relevant for describing atomic nuclei. How does QCD give rise to the physics of nuclei and nuclear constituents?
Quantum gravity
How can the theory of quantum mechanics be merged with the theory of general relativity to produce a so-called "theory of everything"? Is string theory the correct step on the road to quantum gravity, or a blind alley? Is there any way to extract experimental information about the nature of physics at the Planck scale?
Quantum mechanics in the correspondence limit
Is there a preferred interpretation of quantum mechanics? How does the quantum description of reality, which includes elements such as the superposition of states and wavefunction collapse, give rise to the reality we perceive?
Size of the universe
Is there anything beyond the limits of the theoretical boundaries of the observable universe?
Standard Model Higgs mechanism
Does the Standard Model Higgs particle exist? More generally, does inertial mass have a basis or mechanism separate from gravitational mass?
Supersymmetry
Is supersymmetry a symmetry of nature? If so, what is the mechanism of supersymmetry breaking?
Technicolor
Does nature make use of strong dynamics in breaking electroweak symmetry?

Problems solved recently:

Long duration gamma ray bursts (2003)
Long-duration bursts are associated with the deaths of massive stars in a specific kind of supernova-like event commonly referred to as a collapsar.
Solar neutrino problem (2002)
Resolved by a new understanding of neutrino physics, requiring a modification of the Standard Model of particle physics - specifically, neutrino oscillation.
Quasars (1980s)
The nature of quasars was not understood for decades. They are now accepted as a type of active galaxy where the enormous energy output results from matter falling into a massive black hole in the center of the galaxy.

Timeline of Fundamental Physics Discoveries

Nobel Prize in Physics

The Nobel Prize in Physics has been awarded to 178 persons since 1901.

1901
Wilhelm Conrad Röntgen (Prussia, afterwards Germany)
"in recognition of the extraordinary services he has rendered by the discovery of the remarkable rays (or x-rays)"

1902
Hendrik Antoon Lorentz (The Netherlands) and Pieter Zeeman (The Netherlands)
"in recognition of the extraordinary service they rendered by their researches into the influence of magnetism upon radiation phenomena"

1903
Antoine Henri Becquerel (France)
"in recognition of the extraordinary services he has rendered by his discovery of spontaneous radioactivity"

Pierre (France) and Marie Curie (Poland/France)
"in recognition of the extraordinary services they have rendered by their joint researches on the radiation phenomena discovered by Professor Henri Becquerel"

1904
John William Strutt, 3rd Baron Rayleigh (UK)
"for his investigations of the densities of the most important gases and for his discovery of argon in connection with these studies"

1905
Philipp Eduard Anton von Lenard (Germany)
"for his work on cathode rays"

1906
Sir Joseph John Thomson (UK)
"in recognition of the great merits of his theoretical and experimental investigations on the conduction of electricity by gases"

1907
Albert Abraham Michelson (Poland/US)
"for his optical precision instruments and the spectroscopic and metrological investigations carried out with their aid".

1908
Gabriel Lippmann (Luxembourg)
"for his method of reproducing colours photographically based on the phenomenon of interference"

1909
Guglielmo Marconi (Italy) and Karl Ferdinand Braun (Germany) (Germany)
"in recognition of their contributions to the development of wireless telegraphy"

1910
Johannes Diderik van der Waals (The Netherlands)
"For his work on the equation of state for gases and liquids."

1911
Wilhelm Wien (Germany)
"For his discoveries regarding the laws governing the radiation of heat."

1912
Nils Gustaf Dalén (Sweden)
"For his invention of automatic regulators for use in conjunction with gas accumulators for illuminating lighthouses and buoys."

1913
Heike Kamerlingh-Onnes (The Netherlands)
"For his investigations on the properties of matter at low temperatures which led, inter alia, to the production of liquid helium"

1914
Max von Laue (Germany)
"For his discovery of the diffraction of X-rays by crystals."

1915
Sir William Henry Bragg (England, UK) and William Lawrence Bragg (South Australia, afterwards Australia)
"For their services in the analysis of crystal structure by means of X-rays."

1916
(The prize money was allocated to the Special Fund of this prize section.)

1917
Charles Glover Barkla (England, UK)
"For his discovery of the characteristic Röntgen radiation of the elements."

1918
Max Planck (Germany)
"In recognition of the services he rendered to the advancement of Physics by his discovery of energy quanta."

1919
Johannes Stark (Germany)
"For his discovery of the Doppler effect in canal rays and the splitting of spectral lines in electric fields."

1920
Charles Edouard Guillaume (Switzerland/France)
"in recognition of the service he has rendered to precision measurements in Physics by his discovery of anomalies in nickel steel alloys"

1921
Albert Einstein (Germany)
"for his services to Theoretical Physics, and especially for his explanation of the photoelectric effect"

1922
Niels Henrik David Bohr (Denmark)
"for his services in the investigation of the structure of atoms and of the radiation emanating from them"

1923
Robert Andrews Millikan (USA)
"for his work on the elementary charge of electricity and on the photoelectric effect"

1924
Karl Manne Georg Siegbahn (Sweden)
"for his discoveries and research in the field of X-ray spectroscopy"

1925
James Franck (Germany) and Gustav Ludwig Hertz (Germany)
"for their discovery of the laws governing the impact of an electron upon an atom"

1926
Jean Baptiste Perrin (France)
"for his work on the discontinuous structure of matter, and especially for his discovery of sedimentation equilibrium"

1927
Arthur Holly Compton (USA)
"for his discovery of the effect named after him".
Charles Thomson Rees Wilson (Scotland, UK)
"for his method of making the paths of electrically charged particles visible by condensation of vapour".

1928
Owen Willans Richardson (England, UK)
"for his work on the thermionic phenomenon and especially for the discovery of the law named after him"

1929
Prince Louis-Victor Pierre Raymond de Broglie (France)
"for his discovery of the wave nature of electrons".

1930
Sir Chandrasekhara Venkata Raman (India)
"for his work on the scattering of light and for the discovery of the effect named after him"

1931
(The prize money was allocated to the Special Fund of this prize section.)

1932
Werner Karl Heisenberg (Germany)
"for the creation of quantum mechanics, the application of which has, inter alia, led to the discovery of the allotropic forms of hydrogen"

1933
Erwin Schrödinger (Austria) and Paul Adrien Maurice Dirac (England, UK)
"for the discovery of new productive forms of atomic theory"

1934
(The prize money was with ⅓ allocated to the Main Fund and with ⅔ to the Special Fund of this prize section.)

1935
James Chadwick (England, UK)
"for the discovery of the neutron"

1936
Victor Franz Hess (Austria)
"for his discovery of cosmic radiation"

Carl David Anderson (USA)
"for his discovery of the positron"

1937
Clinton Joseph Davisson (USA) and George Paget Thomson (England, UK)
"for their experimental discovery of the diffraction of electrons by crystals".

1938
Enrico Fermi (Italy)
"for his demonstrations of the existence of new radioactive elements produced by neutron irradiation, and for his related discovery of nuclear reactions brought about by slow neutrons"

1939
Ernest Orlando Lawrence (USA)
"for the invention and development of the cyclotron and for results obtained with it, especially with regard to artificial radioactive elements"

1940, 1941, 1942
The prize money was with ⅓ allocated to the Main Fund and with ⅔ to the Special Fund of this prize section.

1943
Otto Stern (Germany)
"for his contribution to the development of the molecular ray method and his discovery of the magnetic moment of the proton"

1944
Isidor Isaac Rabi (Galicia, Austria-Hungary, now Poland)
"for his resonance method for recording the magnetic properties of atomic nuclei"

1945
Wolfgang Pauli (Austria)
"for the discovery of the Exclusion Principle, also called the Pauli principle"

1946
Percy Williams Bridgman (USA)
"for the invention of an apparatus to produce extremely high pressures, and for the discoveries he made there within the field of high pressure physics"

1947
Sir Edward Victor Appleton (England, UK)
"for his investigations of the physics of the upper atmosphere especially for the discovery of the so-called Appleton layer"

1948
Patrick Maynard Stuart Blackett (England, UK)
"for his development of the Wilson cloud chamber method, and his discoveries therewith in the fields of nuclear physics and cosmic radiation"

1949
Hideki Yukawa (Japan)
"for his prediction of the existence of mesons on the basis of theoretical work on nuclear forces".

1950
Cecil Frank Powell (England, UK)
"for his development of the photographic method of studying nuclear processes and his discoveries regarding mesons made with this method"

1951
Sir John Douglas Cockcroft (England, UK) and Ernest Thomas Sinton Walton (Republic of Ireland)
"for their pioneer work on the transmutation of atomic nuclei by artificially accelerated atomic particles"

1952
Felix Bloch (Switzerland/USA) and Edward Mills Purcell (USA)
"for their development of new methods for nuclear magnetic precision measurements and discoveries in connection therewith"

1953
Frits (Frederik) Zernike (The Netherlands)
"for his demonstration of the phase contrast method, especially for his invention of the phase contrast microscope"

1954
Max Born (Germany)
"for his fundamental research in quantum mechanics, especially for his statistical interpretation of the wavefunction"
Walther Bothe (West Germany)
"for the coincidence method and his discoveries made therewith"

1955
Willis Eugene Lamb (USA)
"for his discoveries concerning the fine structure of the hydrogen spectrum". See: Lamb shift
Polykarp Kusch (Germany/USA)
"for his precision determination of the magnetic moment of the electron"

1956
William Bradford Shockley (England, UK/USA), John Bardeen (USA), and Walter Houser Brattain (USA)
"for their researches on semiconductors and their discovery of the transistor effect"

1957
Chen Ning Yang (China/USA) and Tsung-Dao Lee (China/USA)
"for their penetrating investigation of the so-called parity laws which has led to important discoveries regarding the elementary particles"

1958
Pavel Alekseyevich Cherenkov (Soviet Union), Il'ia Frank (Soviet Union), and Igor Yevgenyevich Tamm (Soviet Union)
"for the discovery and the interpretation of the Cherenkov-Vavilov effect"

1959
Emilio Gino Segre (USA) and Owen Chamberlain (USA)
"for their discovery of the antiproton"

1960
Donald Arthur Glaser (USA)
"for the invention of the bubble chamber"

1961
Robert Hofstadter (USA)
"for his pioneering studies of electron scattering in atomic nuclei and for his thereby achieved discoveries concerning the structure of the nucleons"
Rudolf Ludwig Mössbauer (Germany)
"for his researches concerning the resonance absorption of gamma radiation and his discovery in this connection of the effect which bears his name".

1962
Lev Davidovich Landau (Soviet Union)
"for his pioneering theories for condensed matter, especially liquid helium"

1963
Eugene Paul Wigner (Hungary)
"for his contributions to the theory of the atomic nucleus and the elementary particles, particularly through the discovery and application of fundamental symmetry principles"
Maria Goeppert-Mayer (Katowice, then in Germany, now Poland) and J. Hans D. Jensen (Germany)
"for their discoveries concerning nuclear shell structure"

1964
Charles Hard Townes (USA), Nicolay Gennadiyevich Basov (Soviet Union), and Aleksandr Prokhorov (Australia/Soviet Union)
"for fundamental work in the field of quantum electronics, which has led to the construction of oscillators and amplifiers based on the maser-laser principle"

1965
Sin-Itiro Tomonaga (Japan), Julian Schwinger (USA), and Richard Phillips Feynman (USA)
"for their fundamental work in quantum electrodynamics, with deep-ploughing consequences for the physics of elementary particles"

1966
Alfred Kastler (Guebwiller, then in Germany, now France)
"for the discovery and development of optical methods for studying Hertzian resonances in atoms"

1967
Hans Albrecht Bethe (Strasbourg, then in Germany, now France)
"for his contributions to the theory of nuclear reactions, especially his discoveries concerning the energy production in stars"

1968
Luis Walter Alvarez (USA)
"for his decisive contributions to elementary particle physics, in particular the discovery of a large number of resonance states, made possible through his development of the technique of using hydrogen bubble chamber and data analysis"

1969
Murray Gell-Mann (USA)
"for his contributions and discoveries concerning the classification of elementary particles and their interactions".

1970
Hannes Olof Gösta Alfvén (Sweden)
"for fundamental work and discoveries in magneto-hydrodynamics with fruitful applications in different parts of plasma physics"
Louis Eugene Félix Néel (France)
"for fundamental work and discoveries concerning antiferromagnetism and ferrimagnetism which have led to important applications in solid state physics"

1971
Dennis Gabor (Hungary)
"for his invention and development of the holographic method"

1972
John Bardeen (USA), Leon Neil Cooper (USA), and John Robert Schrieffer (USA)
"for their jointly developed theory of superconductivity, usually called the BCS-theory"

1973
Leo Esaki (Japan/USA) and Ivar Giaever (Norway)
"for their experimental discoveries regarding tunneling phenomena in semiconductors and superconductors, respectively"
Brian David Josephson (Wales, UK)
"for his theoretical predictions of the properties of a supercurrent through a tunnel barrier, in particular those phenomena which are generally known as the Josephson effect"

1974
Sir Martin Ryle (England, UK) and Antony Hewish (England, UK)
"for their pioneering research in radio astrophysics: Ryle for his observations and inventions, in particular of the aperture synthesis technique, and Hewish for his decisive role in the discovery of pulsars"

1975
Aage Niels Bohr (Denmark), Ben Roy Mottelson (USA), and Leo James Rainwater (USA)
"for the discovery of the connection between collective motion and particle motion in atomic nuclei and the development of the theory of the structure of the atomic nucleus based on this connection"

1976
Burton Richter (USA) and Samuel Chao Chung Ting (USA)
"for their pioneering work in the discovery of a heavy elementary particle of a new kind". In other words: for discovery of the J/Ψ particle as it confirmed the idea that baryonic matter (such as the nuclei of atoms) is made out of quarks.

1977
Philip Warren Anderson (USA), Sir Nevill Francis Mott (England, UK), and John Hasbrouck van Vleck (USA)
"for their fundamental theoretical investigations of the electronic structure of magnetic and disordered systems"

1978
Pyotr Leonidovich Kapitsa (Пётр Леонидович Капица) (Russia)
"for his basic inventions and discoveries in the area of low-temperature physics"
Arno Allan Penzias (Germany/USA) and Robert Woodrow Wilson (USA)
"for their discovery of cosmic microwave background radiation"

1979
Sheldon Lee Glashow (USA), Abdus Salam (Pakistan), and Steven Weinberg (USA)
"for their contributions to the theory of the unified weak and electromagnetic interaction between elementary particles, including, inter alia, the prediction of the weak neutral current"

1980
James Watson Cronin (USA) and Val Logsdon Fitch (USA)
"for the discovery of violations of fundamental symmetry principles in the decay of neutral K-mesons".

1981
Nicolaas Bloembergen (The Netherlands) and Arthur Leonard Schawlow (USA)
"for their contribution to the development of laser spectroscopy"
Kai Manne Börje Siegbahn (Sweden)
"for his contribution to the development of high-resolution electron spectroscopy"

1982
Kenneth G. Wilson (USA)
"for his theory for critical phenomena in connection with phase transitions"

1983
Subrahmanyan Chandrasekhar (India)
"for his theoretical studies of the physical processes of importance to the structure and evolution of the stars".
William Alfred Fowler (USA)
"for his theoretical and experimental studies of the nuclear reactions of importance in the formation of the chemical elements in the universe"

1984
Carlo Rubbia (Italy) and Simon van der Meer (The Netherlands)
"for their decisive contributions to the large project, which led to the discovery of the field particles W and Z, communicators of weak interaction"

1985
Klaus von Klitzing (Poland/Germany)
"for the discovery of the quantized Hall effect"

1986
Ernst Ruska (Germany)
"for his fundamental work in electron optics, and for the design of the first electron microscope"
Gerd Binnig (Germany) and Heinrich Rohrer(Switzerland)
"for their design of the scanning tunneling microscope"

1987
Johannes Georg Bednorz (Germany) and Karl Alexander Müller (Switzerland)
"for their important break-through in the discovery of superconductivity in ceramic materials"
1988
Leon Max Lederman (USA), Melvin Schwartz (USA), and Jack Steinberger (Germany/USA)
"for the neutrino beam method and the demonstration of the doublet structure of the leptons through the discovery of the muon neutrino"

1989
Norman Foster Ramsey (USA)
"for the invention of the separated oscillatory fields method and its use in the hydrogen maser and other atomic clocks"
Hans Georg Dehmelt (Germany/USA) and Wolfgang Paul (Germany)
"for the development of the ion trap technique"

1990
Jerome Isaac Friedman (USA), Henry Way Kendall (USA), and Richard Edward Taylor (Canada/USA)
"for their pioneering investigations concerning deep inelastic scattering of electrons on protons and bound neutrons, which have been of essential importance for the development of the quark model in particle physics"

1991
Pierre-Gilles de Gennes (France)
"for discovering that methods developed for studying order phenomena in simple systems can be generalized to more complex forms of matter, in particular to liquid crystals and polymers"

1992
Georges Charpak (France)
"for his invention and development of particle detectors, in particular the multiwire proportional chamber"

1993
Russell Alan Hulse (USA) and Joseph Hooton Taylor Jr. (USA)
"for the discovery of a new type of pulsar, a discovery that has opened up new possibilities for the study of gravitation"

1994
Both
"for pioneering contributions to the development of neutron scattering techniques for studies of condensed matter"
Bertram Neville Brockhouse (Canada)
"for the development of neutron spectroscopy"
Clifford Glenwood Shull (USA)
"for the development of the neutron diffraction technique"

1995
Both
"for pioneering experimental contributions to lepton physics"
Martin Lewis Perl (USA)
"for the discovery of the tau lepton"
Frederick Reines (USA)
"for the detection of the neutrino"

1996
David Morris Lee (USA), Douglas Dean Osheroff (USA), and Robert Coleman Richardson (USA)
"for their discovery of superfluidity in helium-3"

1997
Steven Chu(USA), Claude Cohen-Tannoudji (France), and William Daniel Phillips (USA)
"for development of methods to cool and trap atoms with laser light"

1998
Robert B. Laughlin (USA), Horst Ludwig Störmer (Germany), and Daniel Chee Tsui (China/USA)
"for their discovery of a new form of quantum fluid with fractionally charged excitations".

1999
Gerardus 't Hooft (The Netherlands) and Martinus J.G. Veltman (The Netherlands)
"for elucidating the quantum structure of electroweak interactions in physics"

2000
Zhores Ivanovich Alferov (Belarus/Soviet Union) and Herbert Kroemer (USA)
"for developing semiconductor heterostructures used in high-speed- and optoelectronics"
Jack St. Clair Kilby (USA)
"for his part in the invention of the integrated circuit"

2001
Eric Allin Cornell (USA), Wolfgang Ketterle (Germany), and Carl Edwin Wieman (USA)
"for the achievement of Bose-Einstein condensation in dilute gases of alkali atoms, and for early fundamental studies of the properties of the condensates"

2002
Raymond Davis Jr. (USA) and Masatoshi Koshiba (Japan)
"for pioneering contributions to astrophysics, in particular for the detection of cosmic neutrinos"
Riccardo Giacconi (Italy/USA)
"for pioneering contributions to astrophysics, which have led to the discovery of cosmic X-ray sources"

2003
Alexei Alexeevich Abrikosov (Russia), Vitaly Lazarevich Ginzburg (Russia) and Anthony James Leggett (England, UK)
"for pioneering contributions to the theory of superconductors and superfluids"

2004
David J. Gross (USA/Israel), H. David Politzer (USA) and Frank Wilczek (USA)
"for the discovery of asymptotic freedom in the theory of the strong interaction"

2005
Roy J. Glauber (USA)
"for his contribution to the quantum theory of optical coherence"
John L. Hall (USA) and Theodor W. Hänsch (Germany)
"for their contributions to the development of laser-based precision spectroscopy, including the optical frequency comb technique"

2006
John C. Mather (USA) and George F. Smoot (USA)
"for their discovery of the blackbody form and anisotropy of the cosmic microwave background radiation"

Top 10 Experiments: 10. Foucault's pendulum

Last year when scientists mounted a pendulum above the South Pole and watched it swing, they were replicating a celebrated demonstration performed in Paris in 1851. Using a steel wire 220 feet long, the French scientist Jean-Bernard-Léon Foucault suspended a 62-pound iron ball from the dome of the Panthéon and set it in motion, rocking back and forth. To mark its progress he attached a stylus to the ball and placed a ring of damp sand on the floor below.

The audience watched in awe as the pendulum inexplicably appeared to rotate, leaving a slightly different trace with each swing. Actually it was the floor of the Panthéon that was slowly moving, and Foucault had shown, more convincingly than ever, that the earth revolves on its axis. At the latitude of Paris, the pendulum's path would complete a full clockwise rotation every 30 hours; on the Southern Hemisphere it would rotate counterclockwise, and on the Equator it wouldn't revolve at all. At the South Pole, as the modern-day scientists confirmed, the period of rotation is 24 hours.

Top 10 Experiments: 9. Rutherford's discovery of the nucleus

When Ernest Rutherford was experimenting with radioactivity at the University of Manchester in 1911, atoms were generally believed to consist of large mushy blobs of positive electrical charge with electrons embedded inside — the "plum pudding" model. But when he and his assistants fired tiny positively charged projectiles, called alpha particles, at a thin foil of gold, they were surprised that a tiny percentage of them came bouncing back. It was as though bullets had ricocheted off Jell-O. Rutherford calculated that actually atoms were not so mushy after all. Most of the mass must be concentrated in a tiny core, now called the nucleus, with the electrons hovering around it. With amendments from quantum theory, this image of the atom persists today.

Top 10 Experiments: 8. Galileo's experiments with rolling balls down inclined planes

Galileo continued to refine his ideas about objects in motion. He took a board 12 cubits long and half a cubit wide (about 20 feet by 10 inches) and cut a groove, as straight and smooth as possible, down the center. He inclined the plane and rolled brass balls down it, timing their descent with a water clock — a large vessel that emptied through a thin tube into a glass. After each run he would weigh the water that had flowed out — his measurement of elapsed time — and compare it with the distance the ball had travelled.

Aristotle would have predicted that the velocity of a rolling ball was constant: double its time in transit and you would double the distance it traversed. Galileo was able to show that the distance is actually proportional to the square of the time: Double it and the ball would go four times as far. The reason is that it is being constantly accelerated by gravity.

Top 10 Experiments: 7. Eratosthenes' measurement of the Earth's circumference

At Syene (now Aswan), some 800 km (500 miles) southeast of Alexandria in Egypt, the Sun's rays fall vertically at noon at the summer solstice. Eratosthenes, who was born in c. 276 BC, noted that at Alexandria, at the same date and time, sunlight fell at an angle of about 7° from the vertical. He correctly assumed the Sun's distance to be very great; its rays therefore are practically parallel when they reach the Earth. Given estimates of the distance between the two cities, he was able to calculate the circumference of the Earth. The exact length of the units (stadia) he used is doubtful, and the accuracy of his result is therefore uncertain; it may have varied by 0.5 to 17 percent from the value accepted by modern astronomers.

Top 10 Experiments: 6. Cavendish's torsion-bar experiment

The experiment was performed in 1797–98 by the English scientist Henry Cavendish. He followed a method prescribed and used apparatus built by his countryman, the geologist John Michell, who had died in 1793. The apparatus employed was a torsion balance, essentially a stretched wire supporting spherical weights. Attraction between pairs of weights caused the wire to twist slightly, which thus allowed the first calculation of the value of the gravitational constant G. The experiment was popularly known as weighing the Earth because determination of G permitted calculation of the Earth's mass.

Top 10 Experiments: 5. Young's light-interference experiment

Newton wasn't always right. Through various arguments, he had moved the scientific mainstream toward the conviction that light consists exclusively of particles rather than waves. In 1803, Thomas Young, an English physician and physicist, put the idea to a test. He cut a hole in a window shutter, covered it with a thick piece of paper punctured with a tiny pinhole and used a mirror to divert the thin beam that came shining through. Then he took "a slip of a card, about one-thirtieth of an inch in breadth" and held it edgewise in the path of the beam, dividing it in two. The result was a shadow of alternating light and dark bands — a phenomenon that could be explained if the two beams were interacting like waves. Bright bands appeared where two crests overlapped, reinforcing each other; dark bands marked where a crest lined up with a trough, neutralizing each other.

The demonstration was often repeated over the years using a card with two holes to divide the beam. These so-called double-slit experiments became the standard for determining wavelike motion — a fact that was to become especially important a century later when quantum theory began.

Top 10 Experiments: 4. Newton's decomposition of sunlight with a prism

Isaac Newton was born the year Galileo died. He graduated from Trinity College, Cambridge, in 1665, then holed up at home for a couple of years waiting out the plague. He had no trouble keeping himself occupied.

The common wisdom held that white light is the purest form (Aristotle again) and that colored light must therefore have been altered somehow. To test this hypothesis, Newton shined a beam of sunlight through a glass prism and showed that it decomposed into a spectrum cast on the wall. People already knew about rainbows, of course, but they were considered to be little more than pretty aberrations. Actually, Newton concluded, it was these colors — red, orange, yellow, green, blue, indigo, violet and the gradations in between — that were fundamental. What seemed simple on the surface, a beam of white light, was, if one looked deeper, beautifully complex.

Top 10 Experiments: 3. Millikan's oil-drop experiment

Oil-drop experiment was the first direct and compelling measurement of the electric charge of a single electron. It was performed originally in 1909 by the American physicist Robert A. Millikan. Using a perfume atomizer, he sprayed tiny drops of oil into a transparent chamber. At the top and bottom were metal plates hooked to a battery, making one positive (red in animation) and the other negative (blue in animation). Since each droplet picked up a slight charge of static electricity as it traveled through the air, the speed of its motion could be controlled by altering the voltage on the plates. When the space between the metal plates is ionized by radiation (e.g., X rays), electrons from the air attach themselves to oil droplets, causing them to acquire a negative charge. Millikan observed one drop after another, varying the voltage and noting the effect. After many repetitions he concluded that charge could only assume certain fixed values. The smallest of these portions was none other than the charge of a single electron.

Top 10 Experiments: 2. Galileo's experiment on falling objects

In the late 1500's, everyone knew that heavy objects fall faster than lighter ones. After all, Aristotle had said so. That an ancient Greek scholar still held such sway was a sign of how far science had declined during the dark ages.

Galileo Galilei, who held a chair in mathematics at the University of Pisa, was impudent enough to question the common knowledge. The story has become part of the folklore of science: he is reputed to have dropped two different weights from the town's Leaning Tower showing that they landed at the same time. His challenges to Aristotle may have cost Galileo his job, but he had demonstrated the importance of taking nature, not human authority, as the final arbiter in matters of science.

Top 10 Experiments: 1. Double-Slit Electron Diffraction

The French physicist Louis de Broglie proposed in 1924 that electrons and other discrete bits of matter, which until then had been conceived only as material particles, also have wave properties such as wavelength and frequency. Later (1927) the wave nature of electrons was experimentally established by C.J. Davisson and L.H. Germer in New York and by G.P. Thomson in Aberdeen, Scot.

To explain the idea, to others and themselves, physicists often used a thought experiment, in which Young's double-slit demonstration is repeated with a beam of electrons instead of light. Obeying the laws of quantum mechanics, the stream of particles would split in two, and the smaller streams would interfere with each other, leaving the same kind of light- and dark-striped pattern as was cast by light. Particles would act like waves. According to an accompanying article in Physics World, by the magazine's editor, Peter Rodgers, it wasn't until 1961 that someone (Claus Jönsson of Tübingen) carried out the experiment in the real world.

Top 10 Experiments

The most beautiful experiment in physics, according to a poll of Physics World readers, is the interference of single electrons in a Young's double slit.

The list below shows the top 10 most frequently mentioned experiments by readers of Physics World.

Top 10 beautiful experiments:

1 Young's double-slit experiment applied to the interference of single electrons
2 Galileo's experiment on falling bodies (1600s)
3 Millikan's oil-drop experiment (1910s)
4 Newton's decomposition of sunlight with a prism (1665-1666)
5 Young's light-interference experiment (1801)
6 Cavendish's torsion-bar experiment (1798)
7 Eratosthenes' measurement of the Earth's circumference (3rd century BC)
8 Galileo's experiments with rolling balls down inclined planes (1600s)
9 Rutherford's discovery of the nucleus (1911)
10 Foucault's pendulum (1851)

Others experiments that were cited included:

Archimedes' experiment on hydrostatics
Roemer's observations of the speed of light
Joule's paddle-wheel heat experiments
Reynolds's pipe flow experiment
Mach & Salcher's acoustic shock wave
Michelson-Morley measurement of the null effect of the ether
Röntgen's detection of Maxwell's displacement current
Oersted's discovery of electromagnetism
The Braggs' X-ray diffraction of salt crystals
Eddington's measurement of the bending of starlight
Stern-Gerlach demonstration of space quantization
Schrödinger's cat thought experiment
Trinity test of nuclear chain reaction
Wu et al.'s measurement of parity violation
Goldhaber's study of neutrino helicity
Feynman dipping an O-ring in water

Chapter 2. Space & Time

Galileo rolled two different weight balls on a smooth surface to prove that bodies of different weight fall at different speed.

This means that effect of force is to change the speed and not just to set the body moving. This was Newton's first law which came in 1687.

In 1676, Christensen Roemer measured the speed of light by measuring the motion of Jupiter's moons. He calculated the speed as 140,000 miles per second. Actual speed is 186,000 miles per second.

James Clerk Maxwell's exeriments, in 1865, unified electricity and magnetism. According to him, light waves travel at fixed speed and they travel relative to 'ether'.

In 1887, Michelson and Morley compared light's speed in the direction of earth's motion with that at 90 degree to earth's motion. Result showed that 'c' was same. Michelson became the first American to get the Noble prize.

Between 1887-1905, Lorentz's experiments proved that objects contract and clocks slow down when they move through ether.

Then in 1905, Albert Einstein gave his theory of 'no absolute time' and so he said that idea of ether is unnecessary. According to his 'theory of realtivity', 'all laws of science are same for all freely moving observers, whatever be their speed'. Einstein also gave the world famous formula E=mc^2 which prove that nothing travels faster than light. Also, the energy which an objects has due to its motion, add to its mass. For example,
if v = 10% of c, then m = m + 0.5% of m, and
if v = 90% of c, then m = more than 2m.

Now let's see some point of differences between Newton's theory and the theory of relativity.
Newton's theory:
Suppose there are two points A and B. A light pulse starts from A for B. Different observers agree on the time that the journey of light pulse took, since time is absolute. But they'll not agree on how far light travelled, since space is not absolute. So different observers measure different speeds for light.
Einstein's theory:

Galileo

Chapter 1. Our Picture Of The Universe

It was in 340 BC when Aristotle first came with his idea that earth is actually spherical in shape. Before that people used to think that earth is plane and due to this they even scared to travel in seas so they don't fall out from the point where the plane ends.

In 2 AD Ptolemy proposed that earth is the centre point of the universe.

Copernicus gave another concept in 1514. He was the first to claim that not earth but the sun is the centre and the planets like earth revolve around it.

This Copernican theory was proved by Galileo in 1609.

During those years only, Kepler proposed that planets move in an elliptical path around the sun.

Then it was time for Newton who proved Kepler's elliptical orbit theory. He also gave his laws of motion and the law of universal gravitation. His theories were published in his book 'Philosophiae Naturalis Principia Mathematica' in 1687. In 1691 he also proved that there is no central point for the stars to fall.

One interesting point to note is that before 20th century, no one even thought on the fact whether our universe is expanding or contracting.

Immanuel Kant, in 1781, came with his 'Critique of Pure Reason' in which he gave his thesis as well as antithesis. The thesis claimed that there is no beginning and there is infinite time before any event. This actually was absurd. The antithesis claimed that if the universe had a beginning and infinite time before it, then why there is a particular time for beginning.

In 1823 a German scientist Heinrich Olbers gave his theory of an infinite static universe.

Then in 1929, Edwin Hubble proved that galaxies are moving away from each other, i.e., the universe is expanding. This made everyone think about the starting of the universe when it was infinitesimally small and infinitely dense. That was the Big Bang.

Presently, two basic partial theories exist:
(1) General theory of relativity, which deals with the force of gravity and the large-scale structure of universe.
(2) Quantum mechanics, which deals with the phenomena on small scales.
Both these theories, when combined, gives the 'Quantum Theory of Gravity'.

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Behind it all
Is surely an idea so simple,
So beautiful,
So compelling that when-
In a decade' a century,
Or a millenium-
We grasp it,
We will all say to each other,
How could it have been otherwise?
How could we have been so stupid
For so long?