"How do we know that...." *
PHYS 444 web page
Examples of original papers (mostly experimental) which discovered, confirmed,
or established some 'Physics Phacts' which we now take for granted. The list is
very selective, focusing more on gravitational, particle, and atomic physics,
and quantum mechanics.
Classical gravity and
relativity (special and general):
- "How do we know that inertial mass (mI) and gravitational mass (mG) are the same
- "How do we know that the force in Newton's law of
gravitation is proportional to 1/r2? -- and over what distances
do we know that?"
- "How do we know that there is a gravitational red
- "How do we know that the relativistic connection
between kinetic energy and speed is valid? - Does E = γmc2 really work?
- "How do we know that moving clocks run
slower?" (Using real clocks!)
- Theoretical prediction: 'Around-the world atomic clocks: Predicted
relativistic time gains',
J. C. Hafele and R. E. Keating, Science,
Vol. 177, No. 4044, 166-168 (1972).
- Experimental verification: 'Around-the world atomic clocks: Observed relativistic time
gains', J. C. Hafele
and R. E. Keating, Science, Vol. 177, No. 4044, 168-170
- "How do we know that particles experience time
- "How do we know that massive objects (like the
Sun) can bend light?"
- “What did Einstein say about gravitational waves?”
- If you can read German, the
original paper by Albert Einstein about gravitational waves is “Uber Gravitatationalswellen” and you can find a
- "How do we know that binary stars lose energy by
radiating gravity waves?"
- "How do we know that gravitational radiation is
produced in merger of two black holes?"
do we know that there is no ‘preferred’ frame of reference?” – that there is no ‘luminiferous
do we know that Lorenz
invariance holds”? – at least at the
level of 10-17!
- ‘Laboratory test of the isotropy of light propagation
at the 10-17 level’, Ch. Eisele, A. Yu. Nevsky, and S. Schiller, Phys. Rev. Lett. 103 090401
- ‘Rotating optical cavity experiment testing Lorentz
invariance at the 10-17 level’, S. Herrmann, A. Senger, K.
Möhle, E. V. Kovalchuk,
and A. Peters, Phys. Rev. D 80, 10511 (2009)
- ‘Lorentz Symmetry Violations from Matter-Gravity
Couplings with Lunar Laser Ranging’, A. Bourgoin et al., Phys.
Rev. Lett. 119, 201102 (2017).
- ‘Superconducting-Gravimeter Tests of Local Lorentz
N. A. Flowers, C. Goodge, and J. D. Tasson,
Phys. Rev. Lett. 119, 201101 (2017)
- "How do we know the value of Planck's
constant, using the photoelectric effect"?
- “How do we know how Schrödinger came up with his
- "How do we know that the energy levels of a
'bouncing neutron' are quantized?"
- "How do we know that the phase of a neutron's wavefunction is effected by gravity?"
- "How do we know that particles exhibit wave-like
properties, e.g. diffract?" Examples of increasingly complex systems with larger
particle mass (given in atomic mass units, a.m.u.)
- Electrons (5 x 10-4): 'Diffraction of
electrons by a crystal of nickel' , C. Davisson and L. H. Germer, Phys. Rev. 30, 705 (1927).
- Neutrons (1): 'Wave-optical
experiments with very cold neutrons', R. Gahler and A. Zeilinger,
Am. J. Phys. 59, 316 (1991).
- Helium atoms (4): 'Young's
double-slit experiment with atoms: A simple atom interferometer' , O. Carnal and J. Mlynek, Phys. Rev. Lett. 66, 2689 (1991).
- Helium molecules (2 x 4 =
8): 'Nondestructive mass selection of
small van der Waals clusters', W. Schollkopf
and J. P. Toennies, Science 266,
- Sodium atoms (23): 'An
interferometer for atoms', D. W. Keith, C. Ekstrom, Q. A. Turchette,
and D. E. Pritchard, Phys. Rev. Lett. 66, 2693 (1991).
- Sodium molecules (2 x 23 =
46): 'Optics and interferometry with Na2 molecules', M. S. Chapman et
al., Phys. Rev. Lett. 74, 4783 (1995).
- Buckeyballs (C60) molecules
(60 x 12 = 720): 'Quantum interference experiments
with large molecules', O. Nairz, M.
Arndt, and A. Zeilinger, Am. J. Phys. 71,
- Large molecules – phthalocyanine and derivatives thereof (514 and
1298): ‘Real-time single-molecule imaging of quantum
interference’, T. Juffmann et
al., Nature Nanotechnology, 7,
- "How do we know that |ψ(x)|2 really
is a probability density?”
- "How do we know that the Pauli principle
- "How do we know that the electromagnetic potential
can have an effect on the quantum phase of a particle, even in a region
where there are no EM fields?"
- "How do we know that spinor wavefunctions
(for spin ½ particles) change sign under a 2π rotation?"
- Theory prediction: 'Observability
of the sign change of spinors under 2π rotation', Y. Aharonov and L.
Susskind, Phys Rev. 158, 1237 (1967).
- Theory prediction: 'Spin precession
during interferometry of fermions and the phase factor associated with
rotations through 2π radians', H. Bernstein, Phys. Rev.
Lett. 18, 1102 (1967).
- Experimental verification: 'Verification of
coherent spinor rotation of fermions', H. Rauch, A. Zeilinger, G. Badurek, A.
Wilfing, W. Bauspiess, and U. Bonse, Phys. Lett. 54A, 425 (1975). This
experiment used slow neutrons.
- "How do we know that Quantum Electrodynamics (QED) works so well? - Allowing theory and
experiment to agree to 12 decimal places, at least for a very few special
- "How do we know that the proton is
'stable'? or at least what limits there are on its lifetime?"
- "How do we know that the electron is
'stable'? or at least what limits there are on its lifetime?"
- “How do we know the muon (μ) or
- “How do we know that the tau (τ)
- "How do we know that the charm quark exists?"
- "How do we know that the bottom quark
- "How do we know that the top quark
- “How do we know that the gluon exists?” Are there direct
tests that produce gluons?
- ‘Discovery of three-jet events and a test of
quantum chromodynamics at PETRA’, D. P. Barber et al.
(MARK-J Collaboration), Phys. Rev. Lett. 43, 830 (1979).
- ‘Evidence for planar events in e+e- annihilation at high
energies’, R. Brandelik et al. (TASSO
Collaboration) Phys. Lett. 86B,
- ‘Evidence for gluon bremsstrahlung in e+e- annihilations at high
energies’, Ch. Berger et al. (PLUTO Collaboration), Phys. Lett. 86B, 418 (1979).
- ‘Observation of planar three-jet events in e+e- annihilation and evidence
for gluon bremsstrahlung’, W. Bartel
et al. (JADE Collaboration), Phys. Lett. 91B, 142 (1980).
- "How do we know that the Higgs boson
- "How did they confirm the prediction
of a particle consisting of three strange (or s) quarks"? ('strangeness
minus three' as it was called)
- "How do we know that neutrinos
neutrino: 'Detection of
the free neutrino: A confirmation', C. L. Cowan, F. Reines, F. B. Harrison, H. W. Kruse, and A. D.
McClure, Science 124, 103 (1956).
- Muon neutrino: 'Observation of
high-energy neutrino reactions and the existence of two kinds of
neutrinos', G. Danby et al., Phys. Rev.
Lett. 9, 36 (1962).
- Tau neutrino: 'Observations of
tau neutrino interactions' K. Kodama et al. (DONUT
Collaboration), Phys. Lett. B 504, 218 (2001).
- "How do we know that there are only
three (light) neutrinos?"
- “How do we know that neutrinos oscillate
(change their ‘flavor’)?”
- “How do we know that there are heavy bosons
which mediate the weak interactions or at least how were they discovered?’
- Discovery of
the W boson:
- Discovery of
the Z boson:
- Amongst the many physicists
who contributed to the construction of the CERN accelerator complex and
the UA1/UA2 experiments which led to these discoveries, two were cited
for the Nobel Prize in Physics in
C. Rubbia and S. van der Meer.
- “How do we know what the charge of the
- "How do we know that the photon is
massless? or what are the limits on its mass?"
experimental test of Coulomb's law: A laboratory upper limit on the
photon rest mass', E. R. Williams, J. E. Faller,
and H. A. Hill, Phys. Rev. Lett. 26, 721 (1971).
- Using plasma
physics/astrophysical observations: 'Using plasma physics to weigh the
photon', D. D. Ryutov, Plasma Phys. Control. Fusion 49,
- Review of many different
types experimental bounds: 'Terrestrial and extraterrestrial
limits on the photon mass', A. S. Goldhaber
and M. M. Nieto, Rev. Mod. Phys. 43, 277 (1971).
- "How do we know that the photon is
electrically neutral? or what are the limits on its charge?
- "How do we know that the charge of the
electron and proton are equal and opposite? or to what extent do we know
that matter is electrically neutral?"
- “How do we know that there is a neutron?”
- "How do we know that the electron has
- "How do we know that the proton has an
- "How do we know that that
- "How do we know what the
momentum-space wavefunction, φ(p), of the
hydrogen atom look like?"
- "How do we know that unstable particles can form
atoms the same way electrons do?"
- "How do we know that electronic wavepackets in atoms can exhibit classical periodic
- "How do we know that electronic wavepackets in atoms also exhibit non-classical
quantum mechanical revival behavior? Spreading out but eventually reforming into
something like their original state.
- "How do we know that integral spin objects undergo
Bose-Einstein condensation (so-called BEC) at low temperatures?"
- "How do we know that Bose-Einstein condensates
have a common quantum phase?" - Hint: Do two condensates interfere with each
other the way other coherent waves can?
- "How do we know that fundamental constants of
nature (for example the fine structure constant, α) don't vary with
- “How do we know about the history of
- “How do we know that high temperature
- "How do we know that resistance is
quantized in certain materials, independent of sample size?"
- "How do we know that the circulation in superfluid
helium is quantized?"
- "How do we know that the magnetic flux in a
superconductor is quantized?"
- “How do we know how scanning tunneling microscopy
- "How do we know that some materials exhibit giant
magnetoresistance?" - also known as GMR
magnetoresistance of (001)Fe/(001)Cr magnetic superlattices', M. N. Baibich et
al., Phys. Rev. Lett. 61, 2472 (1988)
magnetoresistance in layered magnetic structures with antiferromagnetic
interlayer exchange', G. Binasch, P. Grünberg, F. Saurenbach,
and W. Zinn, Phys. Rev. B 39, 4828 (1989).
- Note that Albert Fert and Peter Grünberg shared the Nobel Prize Physics in 2007 for this discovery.
- Wikipedia describes GMR as
follows: "The main application of GMR is magnetic field
sensors, which are used to read data in hard disk drives, biosensors,
microelectromechanical systems (MEMS) and other devices. GMR multilayer
structures are also used in magnetoresistive
random-access memory (MRAM) as cells that store one bit of
- “What were the first
papers which described Density Functional Theory (DFT)?”
- “How do we know that stars (including our
sun) burn hydrogen into helium, generating neutrinos?” – These papers
describe the theoretical background and experimental realization of a 30
year long experiment (pioneered by Ray Davis) to detect neutrinos from the
neutrinos. I. Theoretical’, J. N. Bahcall, Phys. Rev.
Lett. 12, 300 (1964).
neutrinos. II. Experimental’,
R. Davis Jr., Phys. Rev. Lett. 12,
of the solar electron neutrino flux with the Homestake
chlorine detector’, B.
Cleveland, T. Daily, R. Davis Jr., J. R. Distel,
K. Lande, C. K. Lee, and P. S Wildenhain, Ap. J. 496, 505 (1998).
- R. Davis shared the Nobel Prize in Physics in 2002 for his long-term work on solar neutrino detection.
- “How do we know that the collapse of stars
into supernovae generate neutrinos?”
- ‘Observation of a neutrino burst from the
supernova SN1987A’, K. Hirata et al. (Kamiokande II Collaboration), Phys. Rev. Lett. 58,
- The observation
of these neutrinos almost immediately led to stringent bounds on the mass
of the electron neutrino. If neutrinos had too much mass, neutrinos of
different energy would have take different
lengths of time to arrive on earth, but a short (in time) pulse was
observed. Two papers which derived such bounds (including one by my Ph.D.
adviser!) are listed below.
- ‘Upper limit on the mass of the electron
neutrino’, J. N. Bahcall and S. I. Glashow,
Nature, 326, 476 (1987).
- ‘Neutrino mass limits from SN1987A’, W. D. Arnett
and J. L. Rosner, Phys. Rev. Lett. 58, 1906 (1987).
- "How do we know that there are
- "How do we know how the light elements were made
during the evolution of the early universe?"
- 'The origin of the chemical
elements', R. A. Alpher, H. Bethe, and G. Gamow,
Phys. Rev. 73, 803 (1948).
- Note the (very
intentional!) pun with the authors names --
first three letters of the Greek alphabet, α, β, and γ.
- "How do we know that there is a cosmic
microwave background (CMB) in the universe, and what its temperature
- "How do we know that the CMB is the
best black-body spectrum ever discovered"? - like ever!
- “Why do we think that there is a maximum
energy to cosmic rays?” – due to
their interaction with the cosmic microwave background (CMB).
mechanics and electricity/magnetism:
- “How do we know that there is deterministic
- ‘Deterministic non-periodic flow’, E. Lorenz, J.
Atmos. Sci. 20, 160 (1963).
- This was one of
the first papers to demonstrate that many physical and mathematical
dynamical systems can exhibit ‘sensitive dependence on initial
conditions’, one of the hallmarks of chaos.
- "How do we now that terrestrial
gravity works on things as small as neutrons?"
- "How do we know that Coulomb's law behaves as 1/r2?"
* - "To understand the future, we have to go back in
From 'Back in Time' - written by Armandano
Christian Perez (a.k.a. Pitbull), Adrian Trejo, Urales Vargas, Sylvia
Robinson, Mickey Baker, Ellas Mcdaniel,
and March Kinchen.
† - R. W. Robinett
Last updated 02/28/2018