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DNA — a new twist on lifeThe determination in 1953 of the structure
of deoxyribonucleic acid (DNA), with its two entwined helices and paired organic
bases, was a tour de force in X-ray crystallography. But more significantly,
it also opened the way for a deeper understanding of perhaps the most important
biological process. In the words of Watson and Crick: "It has not escaped our
notice that the specific pairing that we have postulated immediately suggests
a possible copying mechanism for the genetic material." * Click here for an obituary of Francis Crick. * Click here for an obituary of Maurice Wilkins. From here to there — quantum teleportationQuantum teleportation is the transmission
and reconstruction of the state of a quantum system — an idea that was demonstrated
experimentally by Dik Bouwmeester and colleagues in 1997. As their principal teleportation
resource, the team used a pair of entangled photons; to effect the teleportation,
they initiated a measurement involving one photon of the pair and a third photon.
As a consequence of this interaction, the state of polarization of the third photon
was transferred perfectly to the second photon of the entangled pair. In principle,
this process should work even if the teleportation takes place over an arbitrarily
large distance. Kathode raysIn 1896, Jean-Baptiste Perrin reported the results of experiments
on the mysterious 'kathode rays' that emanate from a cathode within an evacuated
tube. Hertz had proposed that these rays were a form of light, but Perrin's experiments
showed that they were charged with negative electricity, and that positive charge
flowed towards the cathode even as the negative rays came from it. In March 1897,
J. J. Thomson reported corroborating observations, setting the stage for his announcement
one month later that the cathode rays were comprised of corpuscles with mass 1,000
times smaller than that of the hydrogen atom. V-particles — stranger still!In 1947, Rochester and Butler twice observed
that cosmic rays produced peculiar V-shaped tracks in a cloud chamber. These events,
they suggested, reflected the decay of unknown particles with masses roughly 1,000
times that of an electron. In 1949 and 1951, Brown et al. and Armenteros
et al. offered more extensive corroborating evidence for these 'V-particles',
showing that there were at least two different kinds, which produced protons and
pions when they decayed. These were the first observations of strange particles
— now known as kaons, lambdas, cascades and sigmas — which are produced
by the strong interactions, but can only decay by the weak interaction. See also: K meson — now that is strange! Alfv�n wavesAny movement
within a conducting fluid that is in the presence of a magnetic field will generate
electrical currents. These currents will then interact with the field to produce
mechanical forces which act back on the fluid. In 1942, Hannes Alfv�n noted that
in this scenario "a kind of combined electromagnetic-hydrodynamic wave is produced
which� so far as I know, has as yet attracted no attention". Alfv�n calculated
the properties of such waves, suggesting that they could be important in solar
physics. Today, Alfv�n waves and other related magnetohydrodynamic waves take
centre stage in the study of laboratory, space and astrophysical plasmas. Gravitational microlensingIn 1986, Bohdan Paczynski suggested that dark, compact
objects in the halo of our Galaxy — the local cousins of similar objects
conjectured to contribute dark matter to the haloes of other galaxies — could
be detected on Earth, as their gravitation might occasionally amplify the light
arriving from more distant stars. In 1993, in surveys of several million stars
in the Large Magellanic Cloud, C. Alcock et al. and
E. Aubourg et al. observed several candidate events.
In total, the two groups detected three stars for which the brightness increased
by two magnitudes over an interval of roughly one month. Gravitational lensingIn 1979, Walsh, Carswell and Weymann reported the observation
of two quasistellar objects that looked suspiciously similar. Known as 0957 +
561 A and B, and separated in the sky by only 5.7 arc seconds, the two sources
had nearly identical magnitudes, redshifts and detailed spectra. "Difficulties
arise in describing them as two distinct objects," the researchers pointed out.
Gravitational lensing, they suggested, offered a more likely explanation: light
from a single source, after travelling along distinct, bending paths through the
gravitational field of some large intervening object, was arriving at the Earth
from two different directions. Neutron-induced radioactivityIn September 1934, Leo Szilard and T. H. Chalmers let
gamma rays fall onto a beryllium target, noting that emissions from the target
induced radioactivity in iodine. "We conclude," they wrote, "that
neutrons are liberated from beryllium by gamma rays." Two months later, A.
Brasch and colleagues, including Szilard and Chalmers, reported a similar effect
using X-rays rather than gamma rays. More ominously, the existence of neutron-induced
radioactivity also suggested the possibility of neutron chain-reactions —
using the neutrons emitted by radioactive elements to induce radioactivity, and
liberate further neutrons, from other nuclei. The first demonstration came four
years later, following the discovery of nuclear fission in uranium (see looking
back: "Breaking up is easy — nuclear fission discovered"). An astrophysical water maserIn 1963, Harold Weaver and colleagues observed
unusual emissions from the Orion nebula. The radio frequency suggested the presence
of hydroxyl (OH) groups, but the line intensities were wrong. The explanation?
Weaver et al. suggested that these were not ordinary thermal emissions,
but that clouds of OH gas were emitting by maser action, being pumped by infrared
light from nearby star-forming clouds. Six years later in Nature, Cheung
et al. reported the discovery of another class of astrophysical maser —
spectacularly bright 22-GHz water masers — observations of which are now
widely used to probe the dynamics of their environments. Quantum correlations in lightClassical interferometry works by detecting correlations
in the phases of two waves. In Nature in 1956, R. Hanbury-Brown and R.
Q. Twiss demonstrated another technique that probes quantum-mechanical correlations
in the electromagnetic field. Splitting an incoherent light beam, they found that
photon detections in the two daughter beams were correlated: the photons were
bunching together. This corresponds to a correlation in the intensity of light
in the two beams, which Hanbury-Brown and Twiss suggested could be used to infer
the angular size of distant stars. Physicists now rely on the effect to probe
the quantum character of complex light sources. * Click here for an obituary of Robert Hanbury Brown. Spiral growth of crystalsMicroscopic spirals often appear on the surfaces
of solids grown slowly from solution. In 1949, F. C. Frank proposed an explanation,
suggesting that crystal growth could lead to screw dislocations — linear
defects oriented normally to the growing surface and forming the core of a lattice
structure locally akin to a spiral staircase. In Nature, four years later,
Ajit Ram Verma and S. Amelinckx offered experimental support for the idea. Photos
of a solid forming on a surface revealed a growth front spiralling outward around
a central point. Measurements confirmed the height of the growing layer as a single
unit cell. Superconductivity defeats gravitySuperconductors have no electrical resistance and are
strongly diamagnetic. In the Meissner effect, a superconductor expels a magnetic
field. In 1947, in a letter to Nature, Russian physicist V. Arkadiev demonstrated
a striking consequence of such diamagnetism. Using a steel magnet and a superconducting
lead disk resting in liquid helium, Arkadiev revealed in a photograph how the
magnet was "repelled from the horizontal surface with such force" that
it hovered in the air with no other support. Today, with liquid nitrogen and modern
high-temperature superconductors, Arkadiev's levitation is a common trick in the
physics classroom. Neutrinos and neutrino mass from a supernovaIn 1987, a supernova exploded in
the nearby Large Magellanic Cloud. Bahcall, Dar and Piran were quick to point
out in Nature that a neutrino burst from the collapsing star should have
reached Earth and that, if detected, it would provide "a unique opportunity
to test the theory of neutron star formation in Type II supernova explosions".
A note added in proof to their 'Scientific Correspondence' confirmed that the
Kamiokande neutrino detector in Japan had indeed picked up a signal. The Kamiokande
data also afforded the first opportunity to determine the mass of the electron
neutrino from astronomical data. A few weeks later, in a letter to Nature,
Bahcall and Glashow had set an upper limit on the electron neutrino mass at 11
eV. Rayleigh (almost) discovers argon"I am much puzzled by some recent results
as to the density of nitrogen," admitted Lord Rayleigh in a Nature
issue of 1892. In his experiments, samples of nitrogen purified by different methods
gave different values for the density of the gas. Rayleigh's persistence in tracking
down this anomaly led to the discovery, with William Ramsay, of the first noble
gas — argon — in 1895*. By the end of the century, helium, neon, krypton
and xenon had all been isolated, and in 1904 Rayleigh and Ramsay won Nobel prizes
for their work — Rayleigh in physics and Ramsay in chemistry. The existence of a neutral pion is proposedIn 1935, Hideki Yukawa proposed
that the force holding the atomic nucleus together resulted from the exchange
of a new particle several hundred times heavier than an electron. In consecutive
Letters to Nature in 1938, Kemmer and Bhabha developed Yukawa's theory
further and proposed that the Yukawa particle, in Kemmer's words, "has a
charged and an uncharged state"; the latter, said Bhabha, could "explain
the close-range proton-proton interaction". Kemmer added that "its relation
to experiment is admittedly quite uncertain" — in 1937 the newly discovered
muon had been misidentified as the Yukawa particle. But later the situation became
clear: Yukawa's particle was in fact the p meson,
or pion. The charged members of the pion family were discovered in 1947 (see The
discovery of the pion); Bhabha and Kemmer were proved right when, at Berkeley
in 1950, the neutral pion became the first unstable particle to be discovered
using an accelerator. Polymer LEDsBy 1990, the development of solid-state light-emitting diodes
(LEDs) had come a long way. Efficient LEDs based on inorganic semiconductors had
already found widespread application. Molecular organic semiconductors were also
coming to the fore — not only were they available in a range of colours but,
unlike their inorganic counterparts, they could be readily made into flexible,
large-area displays. But physicists were encountering problems with the long-term
stability of the organic films. Then Jeremy Burroughes and colleagues produced
the first polymer LED: moving from molecular to macromolecular materials solved
the stability problem and meant that high-quality films could be made easily. London's concept of superconductivityIn 1937, Fritz London introduced his 'New
Conception of Supraconductivity' to the readers of Nature. He proposed
"representing all supracurrents realizable in a simply connected supraconductor
by even one single electronic state". Some 20 years later, Bardeen, Cooper
and Schrieffer built on this idea to produce the 'BCS theory' of superconductivity,
which is based on a correlated, 'single-state' system of electron pairs. In 1938,
London himself applied a similar idea to the phenomenon of superfluidity, suggesting
that it may be a manifestation of bosonic condensation of atoms (see "Superfluidity
III - the l-transition explained"). The spin of the photonChandrasekhara Venkata Raman is probably most famous
for the discovery of the effect that bears his name — the Raman effect describes
the change in frequency and phase of light as it is scattered in a medium. In
1930, Raman won the Nobel Prize in Physics for this work, and two years later
he was still concerned with the passage of photons through materials. With his
colleague S. Bhagavantam, he performed a careful study of the degree to which
light becomes depolarized as it Rayleigh-scatters through gaseous oxygen, carbon
dioxide and nitrous oxide. Their conclusion was clear and fundamental —"the
light quantum possesses an intrinsic spin equal to one Bohr unit of angular momentum". The first transatlantic wireless telegraphThis week's feature is not a
scientific paper but nevertheless is a text of interest: one hundred years ago
Nature's news section, then known as "Notes.", reported that, on
12 December 1901, "Mr. Marconi" had succeeded in sending a wireless telegraph
from Cornwall to Newfoundland — the first transatlantic wireless transmission.
In 1909, Marconi shared the Nobel Prize in Physics with Carl Ferdinand Braun "in
recognition of their contributions to the development of wireless telegraphy". Isotopes and protonsIn 1913, only the barest outlines of the structure of the
atom had been drawn. Frederick Soddy, although struggling to understand how an
electron could be emitted from the nucleus during beta-decay, supported the conclusions
of A. van de Broeck — that an element's atomic number, not its atomic weight,
is the fundamental parameter determining chemical properties. Soddy introduced
the word 'isotope' for elements that occupy the same place in the periodic table
and hence have identical properties, though different mass. He also contested
"Rutherford's tentative theory" that the nucleus has only positive charge.
One week later, a rather indignant Ernest Rutherford responded: the nucleus has
"resultant" positive charge, he said, and as he elaborated, Rutherford
came tantalizingly close to postulating the proton. Flux quantization in high-Tc superconductorsIn copper
oxides, the transition temperature to superconductivity (Tc)
is unusually high. A year after the discovery of this phenomenon, C. E. Gough
and colleagues measured the quantization of magnetic flux in a superconducting
copper oxide and got a value of h/2e (where h is Planck's
constant and e is the electron charge). According to Gough et al.,
their results imply that, in high-Tc materials as in conventional
superconductors, "the charge carriers of superconductivity are electron pairs".
But although the Bardeen–Cooper–Schrieffer theory has successfully described
conventional superconductivity, the exact mechanism for high-Tc
superconductivity remains a mystery. Johnson and 1/f noiseWhile trying to perfect the design and manufacture
of the vacuum valve, the pioneers of electronic engineering uncovered a fundamental
problem — noise. Walter Schottky first postulated the existence of thermal
noise and shot noise in 1918. In a letter to Nature in 1927, J. B. Johnson
commented on voltage fluctuations that appear "to be the result of thermal
agitation of the electric charges in the material of the conductor". Johnson
would later become associated with thermal noise — now also known as Johnson
noise — after he published a definitive experiment on noise in 1928, alongside
Harry Nyquist's theoretical explanation*. But his letter of 1927 was intended
to bring "a similar phenomenon" to the attention of Nature readers.
In this case the fluctuations depend not on temperature but inversely on frequency
— Johnson had discovered '1/f noise'. * Johnson, J. B. Phys. Rev. 32, 97–109 (1928); Nyquist, H. Phys. Rev. 32, 110–113 (1928). The random walkIn the warm summer months of 1905, Karl Pearson was perplexed
by the problem of the random walk. He appealed to the readers of Nature
for a solution as the problem was — as it still is — "of considerable
interest". The random walk, also known as the drunkard's walk, is central
to probability theory and still occupies the mathematical mind today*. Among Pearson's
respondents was Lord Rayleigh, whose assistance led Pearson to conclude that "the
most probable place to find a drunken man who is at all capable of keeping on
his feet is somewhere near his starting point!". Synthesis of carbon nanotubesOn 7 November 1991, Sumio Iijima announced in Nature
the preparation of nanometre-size, needle-like tubes of carbon — now familiar
as 'nanotubes'. Used in microelectronic circuitry and microscopy, and as a tool
to test quantum mechanics and model biological systems, nanotubes seem to have
unlimited potential. Ten years on, new research with nanotubes appears regularly
in the pages of Nature and other journals. The discovery of an exoplanetIn 1995, Michel Mayor and Didier Queloz made
the first discovery of a planet outside our Solar System, in orbit around a Sun-like
star in the constellation of Pegasus. Despite controversy over similar, earlier
claims, Mayor and Queloz's discovery has withstood the test of time. Their Jupiter-sized
planet completes its orbit every 4.2 days — placing it at a distance from
its star, 51 Pegasi, that is much less than the Sun–Mercury distance. The neutrino — the mystery and the discoveryIn the 1920s, physicists
were confused: the phenomenon of b decay (in which
an electron is emitted from the atomic nucleus) seemed to violate conservation
laws. The energy spectrum of the electrons, or b-rays,
is continuous: if energy is conserved, another, variable, amount of energy must
somehow leave the system. In 1927, Ellis and Wooster1 tried —
and failed — to capture and measure that missing energy. By 1933, Pauli had
devised an explanation in terms of another, undetected, particle being emitted
by the nucleus; Fermi called it 'the neutrino'. Only in 1956 was the existence
of the neutrino proved: Reines and Cowan2 sent Pauli a telegram to
inform him of their discovery. X-ray crystallography — the first image of myoglobin To understand
how a protein performs its individual biological function, it is essential to
know its three-dimensional structure. As early as 1934, J.D. Bernal and Dorothy
Hodgkin (then Dorothy Crowfoot) showed* that proteins, when crystallized, would
diffract X-rays to produce a complex pattern of spots. They knew that these patterns
contained all the information needed to determine a protein's structure but, frustratingly,
that information could not be deciphered. By comparing patterns from crystals
containing different heavy-metal atoms, Max Perutz and colleagues devised the
approach that was to solve this riddle. In 1958, J. C. Kendrew et al. applied
Perutz's technique to produce the first three-dimensional images of any protein
— myoglobin, the protein used by muscles to store oxygen. *Bernal, J. D. & Crowfoot, D. Nature 133, 794–795 (1934). The neutron hypothesis Iwanenko�s tentative suggestion that the neutron
might be a constituent of the nucleus is certainly one of the more curious contributions
to this feature. It was published in 1932, just two months after �Dr. J. Chadwick�s
explanation of the mysterious beryllium radiation� that marked the discovery of
the neutron (see Chadwick�s
discovery of the neutron). Although he was wrong about �nuclei electrons being
all packed in a-particles or neutrons�, Iwanenko hit
the target as he mused that the neutron may be an elementary particle �something
like protons and electrons� with �a moment ��. Black-hole evaporation It is often said that nothing can escape from a black
hole. But in 1974, Stephen Hawking realized that, owing to quantum effects, black
holes should emit particles with a thermal distribution of energies — as
if the black hole had a temperature inversely proportional to its mass. In addition
to putting black-hole thermodynamics on a firmer footing, this discovery led Hawking
to postulate 'black hole explosions', as primordial black holes end their lives
in an accelerating release of energy. The physics of golfIt is interesting to note which of the outstanding
questions in physics was occupying J. J. Thomson in 1910, 13 years after his discovery
of the electron: the dynamics of the golf ball. "There are so many dynamical
problems connected with golf", he said, that in his Royal Institution lecture
he would consider only the flight of the ball. Luckily for Nature readers,
P. G. Tait had commented* on some other aspects of the physics of golf a couple
of decades earlier. The mass of the neutronFollowing the discovery of the neutron in 1932
(see Chadwick's
discovery of the neutron), there was some debate as to whether the new particle
was really a fundamental building block, or a composite of the proton and the
electron (as Rutherford had predicted). In 1934, Chadwick and Maurice Goldhaber
made the first determination of the neutron mass that was accurate enough to decide
the question. By determining the g-ray energy required
to disintegrate the deuteron, Chadwick and Goldhaber were able to constrain the
binding energy of the deuteron, and hence the neutron mass. The ponderomotive forceIn 1957, Boot and Harvie reported the observation
of a force on charged particles in an inhomogeneous electric field, which originated
from second-order terms of the equation for the Lorentz force on the particles.
Almost immediately it was realized that this 'ponderomotive force' could be used
to trap and control electrons. But the force is weak: only with the development
of modern laser technology is the ponderomotive force being exploited in new particle-acceleration
techniques and inertial confinement fusion. A proposal for television The first all-electronic scheme for television
was outlined by A. A. Campbell Swinton in 1908. Campbell Swinton imagined a receiving
apparatus comprising an electron beam, deflected by electromagnets, scanning across
a "sensitive fluorescent screen". He suggested that the camera might
also incorporate a scanning electron beam, but anticipated that a new photoelectric
phenomenon would need to be discovered, to make such a camera a reality. The hydrogen 21-cm line September 1st marks the fiftieth anniversary of
Ewen and Purcell's report in Nature of the discovery of the hydrogen 21-cm
line. In fact, they had seen the signal months before, but waited for corroboration
by Dutch and Australian astronomers before publishing: Muller and Oort's paper*
in the pages following Ewen and Purcell's report includes the text of a cable
sent by Pawsey from Australia. Oort had already realized the significance of the
discovery — that detection of this spectral line, produced by transitions
between hyperfine levels of the ground-state hydrogen atom, would permit measurements
of velocities by the Doppler effect. The 21-cm line put radioastronomy on the
map, and brought about a revolution in the study of galactic structure. *Muller,
C. A. & Oort, J. H. Nature 168, 357–358 (1951). Integer masses of the elements In 1919, Francis Aston built the first mass
spectrograph capable of measuring the masses of the elements with useful precision.
By December of that year, he had obtained enough data to propose the 'whole-number
rule': that the elements are mixtures of isotopes, each of which has a mass that
is an integer multiple of one-twelfth the mass of carbon, or one-sixteenth the
mass of oxygen. Aston's prescient conclusion: "Should this integer relation
prove general it should do much to elucidate the ultimate structure of matter." The quasar enigma solved In the early 1960s, astronomers were puzzled
by quasars — sources of intense radio emission that seemed to be stars, but
had unintelligible optical spectra. In 1963, Maarten Schmidt solved the puzzle
by recognizing the Balmer lines of hydrogen, strongly redshifted, in the spectrum
of the quasar 3C 273. Schmidt reached the "most direct and least objectionable"
conclusion, that 3C 273 was no star, but the enormously bright nucleus
of a distant galaxy. The discovery of the pion1947 was the year of the pion — flick through
Nature volumes 159 and 160 and watch the story unfold. In February of that
year, Cecil Powell and Giuseppe Occhialini reported the observation of six star-like
patterns in emulsions exposed to cosmic rays. Powell's group had finally found
the Yukawa particle, predicted in 1935 to be the carrier of strong force inside
the atomic nucleus. In fact, Don Perkins pipped them to the post with the publication
of a single, similar star-like event just two weeks earlier*. Later in the year,
another paper from Powell's group announced the first observation of pion decay
to a muon — the particle picture was beginning to take shape. *Perkins, D. H. Nature 159, 126–127 (1947) Tunnel vision — a-decay explainedThe half-lives
of a-emitters span an enormous range — from less
than a microsecond to more than 1015 years — although the emitted a-particles
vary in energy by less than an order of magnitude. This extreme sensitivity of
the escape probability to the particle's energy was explained in 1928 by Gurney
and Condon (and, independently, by George Gamow*) by invoking the recently discovered
phenomenon of quantum-mechanical barrier penetration, or tunnelling. Until this
time, a-decay had been envisaged as a violent process;
by contrast, Gurney and Condon suggested that the tunnelling a-particle
"slips away almost unnoticed". *Gamow, G. Z. Phys. 51, 204–212 (1928). The Zeeman effect The last experiment performed by Michael Faraday was
an unsuccessful attempt to observe the influence of a magnetic field on the spectral
lines of sodium. More than 30 years later, Pieter Zeeman took up the challenge
and observed a broadening of the lines, which was soon recognized to be the splitting
that we know as the Zeeman effect. Zeeman's account of the discovery, translated
for Nature from the Proceedings of the Physical Society of Berlin,
includes an interpretation based on Hendrik Lorentz's idea of "small molecular
elements charged with electricity", and a rough calculation of the charge
to mass ratio of these "ions". The first stored-program computer The modern computer was born on 21 June
1948, when the University of Manchester's Small-Scale Experimental Machine, nicknamed
the 'Baby', successfully executed its first program. Designed and built by F.
C. Williams and Tom Kilburn*, the Baby kept only 1,024 bits in its main store,
but it was the first computer to store a changeable user program in electronic
memory and process it at electronic speed. *Click here for an obituary of Tom Kilburn. A new form of carbonFor five years after its discovery in 1985 (see
Curious
carbon — the 'buckyball'), the buckminsterfullerene molecule, C60,
remained something of a curiosity. The development by Krätschmer et al.
of a technique for synthesizing C60 as a bulk solid brought fullerenes
into the realm of materials science and condensed-matter physics, with rewards
that are still being reaped today. Hard pressed — the first synthetic diamondsClaims of the conversion
of carbon to diamond date back to 1880, but it was not until 1955 that the first
reproducible synthesis was reported. Bundy et al. describe the high-pressure,
high-temperature apparatus that enabled them to reach the stability field of diamond,
and prove that the material obtained was indeed diamond. Ironically, some of the
same authors discovered 38 years later that the very first diamond grown by their
technique was not synthetic after all, but a fragment of a natural diamond that
got into the experiment. Fortunately, however, the technique was sound, and marked
the beginning of the present synthetic-diamond industry. Einstein on general relativityThe general theory of relativity has been one
of the most successful developments in twentieth-century physics, providing the
foundation for our concepts of space, time and gravity. In 1921, Albert Einstein
described the sequence of ideas that led to this theory, before concluding with
some prescient remarks on the questions that remained. Pulsar planetsThe utility of the clock-like pulses emitted by pulsars (see
The
first pulsar) did not end with the insights they have yielded into the properties
of neutron stars, and hence of high-density matter. In 1992, Wolszczan and Frail
reported precise pulsar timing measurements which exhibited periodic variations.
Unlike similar observations reported in the previous year (cited as ref. 3 in
the paper), these variations were not an artefact, but revealed the presence of
two or more planet-sized objects orbiting the neutron star — the first such
objects to be detected outside the Solar System. The invention of holographyThe spherical aberration of electron lenses
has long been the bane of electron microscopy. Enter Dennis Gabor in 1948 with
a proposal for an 'electron interference microscope', which did not rely on traditional
optical principles. Instead, interference between the illuminating and scattered
electron wavefronts was used to record a three-dimensional representation of the
object under investigation. This principle is now known as holography. Superfluidity III — the l-transition explainedThe
rapid pace of discovery that characterized early experimental work in superfluidity
(see Superfluidity
II — the fountain effect) was equalled by theorists. In one of the great
conceptual leaps in the history of physics, Fritz London proposed in April 1938
that the l-transition in liquid helium was analogous
to Bose-Einstein condensation, predicted by Einstein to occur in dilute gases.
Just one month later, Laszlo Tisza extended London's proposal by invoking a two-fluid
model for helium II, which could qualitatively explain the observed transport
phenomena, including the fountain effect.
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Superfluidity
II — the fountain effect
Beam
time — the Cockcroft-Walton accelerator
MRI
— a new way of seeing
See-through
science — the discovery of X-rays