LUCA and LECA: the origins of the eukaryotes. The point of fusion (marked "?") below LECA is the FECA, the first
eukaryotic common ancestor, some 2.2 billion years ago. Much earlier,
some 4 billion years ago, the LUCA gave rise to the two domains of prokaryotes, the bacteria and the archaea.
After the LECA, some 2 billion years ago, the eukaryotes diversified
into a crown group, which gave rise to animals, plants, fungi, and
protists.
Eukaryogenesis, the process which created the eukaryotic
cell and lineage, is a milestone in the evolution of life, since
eukaryotes include all complex cells and almost all multicellular
organisms. The process is widely agreed to have involved symbiogenesis, in which an archaeon and one or more bacteria came together to create the first eukaryotic common ancestor (FECA). This cell had a new level of complexity and capability, with a nucleus, at least one centriole and cilium, facultatively aerobic mitochondria, sex (meiosis and syngamy), a dormant cyst with a cell wall of chitin and/or cellulose and peroxisomes. It evolved into a population of single-celled organisms that included the last eukaryotic common ancestor (LECA),
gaining capabilities along the way, though the sequence of steps
involved has been disputed, and may not have started with symbiogenesis.
In turn, the LECA gave rise to the eukaryotes' crown group, containing the ancestors of animals, fungi, plants, and a diverse range of single-celled organisms.
Life arose on Earth once it had cooled enough for oceans to form. That developed into the last universal common ancestor (LUCA), an organism which had ribosomes and the genetic code, some 4 billion years ago. It gave rise to two main branches of prokaryotic
life, the Bacteria and the Archaea. From among these small-celled,
rapidly-dividing ancestors arose the Eukaryotes, with much larger cells,
nuclei, and distinctive biochemistry. The eukaryotes form a domain that contains all complex cells and most types of multicellular organism, including the animals, plants, and fungi.
In the theory of symbiogenesis, a merger of an archaean and an aerobic bacterium created the eukaryotes, with aerobic mitochondria, some 2.2 billion years ago. A second merger, 1.6 billion years ago, added chloroplasts, creating the green plants.
According to the theory of symbiogenesis (the endosymbiotic theory) championed by Lynn Margulis, a member of the archaea gained a bacterial cell as a component. The archaeal cell was a member of the Promethearchaeatikingdom. The bacterium was one of the alphaproteobacteria,
which had the ability to use oxygen in its respiration. This enabled it
– and the archaeal cells that included it – to survive in the presence
of oxygen, which was poisonous to other organisms adapted to reducing conditions. The endosymbiotic bacteria became the eukaryotic cell's mitochondria, providing most of the energy of the cell. Lynn Margulis and colleagues have suggested that the cell also acquired a Spirochaete bacterium as a symbiont, providing the cell skeleton of microtubules and the ability to move, including the ability to pull chromosomes into two sets during mitosis, cell division. More recently, the archaean has been identified as belonging to the unranked taxonHeimdallarchaeia of the phylumPromethearchaeota.
Last eukaryotic common ancestor (LECA)
The last eukaryotic common ancestor (LECA) is the hypothetical last common ancestor of all living eukaryotes, around 2 billion years ago, and was most likely a biological population. It is believed to have been a protist with a nucleus, at least one centriole and cilium, facultatively aerobic mitochondria, sex (meiosis and syngamy), a dormant cyst with a cell wall of chitin and/or cellulose, and peroxisomes.
It had been proposed that the LECA fed by phagocytosis, engulfing other organisms. However, in 2022, Nico Bremer and colleagues confirmed that the LECA
had mitochondria, and stated that it had multiple nuclei, but disputed
that it was phagotrophic. This would mean that the ability found in many
eukaryotes to engulf materials developed later, rather than being
acquired first and then used to engulf the alphaproteobacteria that
became mitochondria.
The LECA has been described as having "spectacular cellular complexity". Its cell was divided into compartments. It appears to have inherited a set of endosomal sorting complex proteins that enable membranes to be remodelled, including pinching off vesicles to form endosomes. Its apparatuses for transcribing DNA into RNA, and then for translating the RNA into proteins, were separated, permitting extensive RNA processing and allowing the expression of genes to become more complex. It had mechanisms for reshuffling its genetic material, and possibly for manipulating its own evolvability. All of these gave the LECA "a compelling cohort of selective advantages".
Eukaryotic sex
Sex in eukaryotes is a composite process, consisting of meiosis and fertilisation, which can be coupled to reproduction. Dacks and Roger proposed on the basis of a phylogenetic
analysis that facultative sex was likely present in the common ancestor
of all eukaryotes. Early in eukaryotic evolution, about 2 billion years
ago, organisms needed a solution to the major problem that oxidative
metabolism releases reactive oxygen species that damage the genetic material, DNA. Eukaryotic sex provides a process, homologous recombination during meiosis, for using informational redundancy to repair such DNA damage.
Scenarios
Competing sequences of mitochondria, membranes, and nucleus
Biologists
have proposed multiple scenarios for the creation of the eukaryotes.
While there is broad agreement that the LECA must have had a nucleus,
mitochondria, and internal membranes, the order in which these were
acquired has been disputed. In the syntrophic model, the first eukaryotic common ancestor (FECA, around 2.2 gya) gained mitochondria, then membranes, then a nucleus. In the phagotrophic model, it gained a nucleus, then membranes, then mitochondria. In a more complex process, it gained all three in short order, then
other capabilities. Other models have been proposed. Whatever happened,
many lineages must have been created, but the LECA either out-competed
or came together with the other lineages to form a single point of
origin for the eukaryotes.
Nick Lane and William Martin
have argued that mitochondria came first, on the grounds that energy
had been the limiting factor on the size of the prokaryotic cell. Enrique M. Muro et al. have argued, however, that the genetic system
needed to reach a critical point that led to a new regulatory system
(with introns and the spliceosome), which enabled coordination between genetic networks. The phagotrophic model presupposes the ability to engulf food, enabling
the cell to engulf the aerobic bacterium that became the mitochondrion.
Eugene Koonin and others, noting that the archaea share many features with eukaryotes, argue that rudimentary eukaryotic traits such as membrane-lined compartments were acquired before endosymbiosis added mitochondria to the early eukaryotic cell, while the cell wall
was lost. In the same way, mitochondrial acquisition must not be
regarded as the end of the process, for still new complex families of
genes had to be developed after or during the endosymbiotic exchange. In
this way, from FECA to LECA, the organisms can be considered as
proto-eukaryotes. At the end of the process, LECA was already a complex
organism with protein families involved in cellular
compartmentalization.
Viral eukaryogenesis
Another scenario is viral eukaryogenesis,
which proposes that the eukaryotes arose as an emergent superorganism,
with the nucleus deriving from a "viral factory" alongside the
alphaproteobacterium mitochondrion, hosted by an archaeal cell. In this
scenario, eukaryogenesis began when a virus colonised an archaeal cell,
making it support the production of viruses. The virus may later have
assisted the bacterium's entry into the reprogrammed cell. Eukaryotes share genes for several DNA synthesis and transcription enzymes with DNA viruses (Nucleocytoviricota). Those viruses may thus be older than the LECA and may have exchanged DNA with proto-eukaryotes.
Diversification: crown eukaryotes
In turn, the LECA gave rise to the eukaryotes' crown group, containing the ancestors of animals, fungi, plants, and a diverse range of single-celled organisms with the new capabilities and complexity of the eukaryotic cell. Single cells without cell walls are fragile and have a low probability of being fossilised.
If fossilised, they have few features to distinguish them clearly from
prokaryotes: size, morphological complexity, and (eventually) multicellularity. Early eukaryote fossils, from the late Paleoproterozoic, include acritarch microfossils with relatively robust ornate carbonaceous vesicles of Tappania from 1.63 gya and Shuiyousphaeridium from 1.8 gya.
The position of the LECA on the eukaryotic tree of life remains
controversial. Some studies believe that the first split after the LECA
happened between the Unikonta and the Bikonta (Stechmann and Cavalier-Smith 2003), or between Amorphea and all other eukaryotes (Adl et al. 2012; Derelle and Lang 2012). Some believe that the first split happened within Excavata (al Jewari and Baldauf 2023). Yet others believe in a first split between the Opisthokonta and all others (Cerón-Romero et al. 2024).
The diversity of species and genes in ecological communities affects the functioning of these communities. These ecological effects of biodiversity in turn are affected by both climate change through enhanced greenhouse gases, aerosols and loss of land cover, and biological diversity, causing a rapid loss of biodiversity and extinctions of species and local populations. The current rate of extinction is sometimes considered a mass extinction, with current species extinction rates on the order of 100 to 1000 times as high as in the past.
The two main areas where the effect of biodiversity on ecosystem
function have been studied are the relationship between diversity and
productivity, and the relationship between diversity and community
stability. More biologically diverse communities appear to be more productive (in terms of biomass production) than are less diverse communities, and they appear to be more stable in the face of perturbations.
Also animals that inhabit an area may alter the surviving conditions by factors assimilated by climate.
Definitions
In order to understand the effects that changes in biodiversity
will have on ecosystem functioning, it is important to define some
terms. Biodiversity is not easily defined, but may be thought of as the
number and/or evenness of genes, species, and ecosystems in a region. This definition includes genetic diversity, or the diversity of genes within a species, species diversity, or the diversity of species within a habitat or region, and ecosystem diversity, or the diversity of habitats within a region.
Two things commonly measured in relation to changes in diversity are productivity and stability. Productivity is a measure of ecosystem function. It is generally measured by taking the total aboveground biomass
of all plants in an area. Many assume that it can be used as a general
indicator of ecosystem function and that total resource use and other
indicators of ecosystem function are correlated with productivity.
Stability is much more difficult to define, but can be generally
thought of in two ways. General stability of a population is a measure
that assumes stability is higher if there is less of a chance of
extinction. This kind of stability is generally measured by measuring
the variability of aggregate community properties, like total biomass, over time. The other definition of stability is a measure of resilience and resistance, where an ecosystem that returns quickly to an equilibrium after a perturbation or resists invasion is thought of as more stable than one that does not.
The
importance of stability in community ecology is clear. An unstable
ecosystem will be more likely to lose species. Thus, if there is indeed a
link between diversity and stability, it is likely that losses of
diversity could feedback on themselves, causing even more losses of
species. Productivity, on the other hand, has a less clear importance in
community ecology. In managed areas like cropland, and in areas where animals are grown or caught, increasing productivity increases the economic success of the area and implies that the area has become more efficient, leading to possible long term resource sustainability. It is more difficult to find the importance of productivity in natural ecosystems.
Beyond the value biodiversity has in regulating and stabilizing
ecosystem processes, there are direct economic consequences of losing
diversity in certain ecosystems and in the world as a whole. Losing
species means losing potential foods, medicines, industrial products, and tourism, all of which have a direct economic effect on peoples lives.
Effects on community productivity
Complementarity Plant species coexistence is thought to be the result of niche
partitioning, or differences in resource requirements among species. By
complementarity, a more diverse plant community should be able to use
resources more completely, and thus be more productive. Also called niche differentiation, this mechanism is a central principle in the functional group approach, which breaks species diversity down into functional components.
Facilitation Facilitation
is a mechanism whereby certain species help or allow other species to
grow by modifying the environment in a way that is favorable to a
co-occurring species. Plants can interact through an intermediary like nitrogen, water,
temperature, space, or interactions with weeds or herbivores among
others. Some examples of facilitation include large desert perennials
acting as nurse plants, aiding the establishment of young neighbors of
other species by alleviating water and temperature stress, and nutrient enrichment by nitrogen-fixers such as legumes.
The Sampling Effect The sampling effect of diversity can be
thought of as having a greater chance of including a species of greatest
inherent productivity in a plot that is more diverse. This provides for
a composition effect on productivity, rather than diversity being a
direct cause. However, the sampling effect may in fact be a compilation
of different effects. The sampling effect can be separated into the
greater likelihood of selecting a species that is 1) adapted well to
particular site conditions, or 2) of a greater inherent productivity.
Additionally, one can add to the sampling effect a greater likelihood of
including 3) a pair of species that highly complement each other, or 4)
a certain species with a large facilitative effect on other members of
the community.
Review of data
Field
experiments to test the degree to which diversity affects community
productivity have had variable results, but many long-term studies in grassland ecosystems have found that diversity does indeed enhance the productivity of ecosystems. Additionally, evidence of this relationship has also been found in
grassland microcosms. The differing results between studies may
partially be attributable to their reliance on samples with equal
species diversities rather than species diversities that mirror those
observed in the environment. A 2006 experiment utilizing a realistic variation in species composition for its grassland samples found a positive correlation between increased diversity and increased production.
However, these studies have come to different conclusions as to whether the cause was due more to diversity or to species composition.
Specifically, a diversity in the functional roles of the species may be
a more important quality for predicting productivity than the diversity
in species number. Recent mathematical models have highlighted the importance of
ecological context in unraveling this problem. Some models have
indicated the importance of disturbance rates and spatial heterogeneity of the environment, others have indicated that the time since disturbance and the habitat's carrying capacity can cause differing relationships. Each ecological context should yield not only a different relationship,
but a different contribution to the relationship due to diversity and
to composition. The current consensus holds at least that certain
combinations of species provide increased community productivity.
Future research
In
order to correctly identify the consequences of diversity on
productivity and other ecosystem processes, many things must happen.
First, it is imperative that scientists stop looking for a single
relationship. It is obvious now from the models, the data, and the
theory that there is no one overarching effect of diversity on
productivity.
Scientists must try to quantify the differences between composition
effect and diversity effects, as many experiments never quantify the
final realized species diversity (instead only counting numbers of
species of seeds planted) and confound a sampling effect for
facilitators (a compositional factor) with diversity effects.
Relative amounts of overyielding
(or how much more a species grows when grown with other species than it
does in monoculture) should be used rather than absolute amounts as
relative overyielding can give clues as to the mechanism by which
diversity is influencing productivity, however if experimental protocols
are incomplete, one may be able to indicate the existence of a
complementary or facilitative effect in the experiment, but not be able
to recognize its cause. Experimenters should know what the goal of their
experiment is, that is, whether it is meant to inform natural or
managed ecosystems, as the sampling effect may only be a real effect of
diversity in natural ecosystems (managed ecosystems are composed to
maximize complementarity and facilitation regardless of species number).
By knowing this, they should be able to choose spatial and temporal
scales that are appropriate for their experiment. Lastly, to resolve the
diversity-function debate, it is advisable that experiments be done
with large amounts of spatial and resource heterogeneity and
environmental fluctuation over time, as these types of experiments
should be able to demonstrate the diversity-function relationship more
easily.
Effects on community stability
Averaging Effect
If all species have differential responses to changes in the ecosystem
over time, then the averaging of these responses will cause a more
temporally stable ecosystem if more species are in the ecosystem. This effect is a statistical effect due to summing randomvariables.
Negative Covariance Effect If some species do better when
other species are not doing well, then when there are more species in
the ecosystem, their overall variance will be lower than if there were fewer species in the system. This lower variance indicates higher stability. This effect is a consequence of competition as highly competitive species will negatively covary.
Insurance Effect If an ecosystem contains more species then it will have a greater likelihood of having redundant
stabilizing species, and it will have a greater number of species that
respond to perturbations in different ways. This will enhance an
ecosystem's ability to buffer perturbations.
Resistance to Invasion Diverse communities may use resources
more completely than simple communities because of a diversity effect
for complementarity. Thus invaders
may have reduced success in diverse ecosystems, or there may be a
reduced likelihood that an invading species will introduce a new
property or process to a diverse ecosystem.
Resistance to Disease A decreased number of competing plant
species may allow the abundances of other species to increase,
facilitating the spread of diseases of those species.
Review of temporal stability data
Models have predicted that empirical relationships between temporal
variation of community productivity and species diversity are indeed
real, and that they almost have to be. Some temporal stability data can
be almost completely explained by the averaging effect by constructing
null models to test the data against. Competition, which causes negative covariances, only serves to strengthen these relationships.
Review of resistance and resilience stability data
This
area is more contentious than the area of temporal stability, mostly
because some have tried generalizing the findings of the temporal
stability models and theory to stability in general. While the
relationship between temporal variations in productivity and diversity
has a mathematical cause, which will allow the relationship to be seen
much more often than not, it is not the case with resistance/resilience
stability. Some experimenters have seen a correlation between diversity and reduced invasibility, though many have also seen the opposite. The correlation between diversity and disease is also tenuous, though theory and data do seem to support it.
Future research
In
order to more fully understand the effects of diversity on the temporal
stability of ecosystems it is necessary to recognize that they are
bound to occur. By constructing null models to test the data against (as
in Doak et al. 1998)
it becomes possible to find situations and ecological contexts where
ecosystems become more or less stable than they should be. Finding these
contexts would allow for mechanistic studies into why these ecosystems
are more stable, which may allow for applications in conservation management.
More importantly more complete experiments into whether diverse
ecosystems actually resist invasion and disease better than their less
diverse equivalents as invasion and disease are two important factors
that lead to species extinctions in the present day. In order to address
these problems specifically, future work should focus on practical
methods to increase the successful establishment of the poor performing
but desirable species.
Theory and preliminary effects from examining food webs
One
major problem with both the diversity-productivity and
diversity-stability debates discussed up to this point is that both
focus on interactions at just a single trophic level. That is, they are concerned with only one level of the food web,
namely plants. Other research, unconcerned with the effects of
diversity, has demonstrated strong top-down forcing of ecosystems (see keystone species).
There is very little actual data available regarding the effects of
different food webs, but theory helps us in this area. First, if a food
web in an ecosystem has a lot of weak interactions between different species, then it should have more stable populations and the community as a whole should be more stable. If upper levels of the web are more diverse, then there will be less biomass in the lower levels and if lower levels are more diverse they will better be able to resist consumption
and be more stable in the face of consumption. Also, top-down forcing
should be reduced in less diverse ecosystems because of the bias for
species in higher trophic levels to go extinct first. Lastly, it has recently been shown that consumers can dramatically change the biodiversity-productivity-stability relationships that are implied by plants alone. Thus, it will be important in the future to incorporate food web theory
into the future study of the effects of biodiversity. In addition this
complexity will need to be addressed when designing biodiversity
management plans.
Quantum mechanics is the study of matter and matter's interactions with energy on the scale of atomic and subatomic particles. By contrast, classical physics
explains matter and energy only on a scale familiar to human
experience, including the behavior of astronomical bodies such as the
Moon. Classical physics is still used in much of modern science and
technology. However, towards the end of the 19th century, scientists
discovered phenomena in both the large (macro) and the small (micro) worlds that classical physics could not explain. The desire to resolve inconsistencies between observed phenomena and
classical theory led to a revolution in physics, a shift in the original
scientific paradigm: the development of quantum mechanics.
Many aspects of quantum mechanics yield unexpected results, defying expectations and deemed counterintuitive. These aspects can seem paradoxical as they map behaviors quite differently from those seen at larger scales. In the words of quantum physicist Richard Feynman, quantum mechanics deals with "nature as She is—absurd". Features of quantum mechanics often defy simple explanations in everyday language. One example of this is the uncertainty principle: precise measurements of position cannot be combined with precise measurements of velocity. Another example is entanglement: a measurement made on one particle (such as an electron that is measured to have spin
'up') will correlate with a measurement on a second particle (an
electron will be found to have spin 'down') if the two particles have a
shared history. This will apply even if it is impossible for the result
of the first measurement to have been transmitted to the second particle
before the second measurement takes place.
Quantum mechanics helps people understand chemistry, because it explains how atoms interact with each other and form molecules. Many remarkable phenomena can be explained using quantum mechanics, like superfluidity. For example, if liquid helium cooled to a temperature near absolute zero
is placed in a container, it spontaneously flows up and over the rim of
its container; this is an effect which cannot be explained by classical
physics.
James C. Maxwell's unification of the equations
governing electricity, magnetism, and light in the late 19th century
led to experiments on the interaction of light and matter. Some of these
experiments had aspects which could not be explained until quantum
mechanics emerged in the early part of the 20th century.
The seeds of the quantum revolution appear in the discovery by J.J. Thomson in 1897 that cathode rays were not continuous but "corpuscles" (electrons). Electrons had been named just six years earlier as part of the emerging theory of atoms. In 1900, Max Planck, unconvinced by the atomic theory, discovered that he needed discrete entities like atoms or electrons to explain black-body radiation.
Black-body radiation intensity vs color and temperature. The rainbow bar represents visible light; 5000 K
objects are "white hot" by mixing differing colors of visible light. To
the right is the invisible infrared. Classical theory (black curve for
5000 K) fails to predict the colors; the other curves are correctly
predicted by quantum theories.
Very hot – red hot or white hot – objects look similar when heated to
the same temperature. This look results from a common curve of light
intensity at different frequencies (colors), which is called black-body
radiation. White hot objects have intensity across many colors in the
visible range. The lowest frequencies above visible colors are infrared light,
which also give off heat. Continuous wave theories of light and matter
cannot explain the black-body radiation curve. Planck spread the heat
energy among individual "oscillators" of an undefined character but with
discrete energy capacity; this model explained black-body radiation.
At the time, electrons, atoms, and discrete oscillators were all exotic ideas to explain exotic phenomena. But in 1905 Albert Einstein
proposed that light was also corpuscular, consisting of "energy
quanta", in contradiction to the established science of light as a
continuous wave, stretching back a hundred years to Thomas Young's work on diffraction.
Einstein's revolutionary proposal started by reanalyzing Planck's
black-body theory, arriving at the same conclusions by using the new
"energy quanta". Einstein then showed how energy quanta connected to
Thomson's electron. In 1902, Philipp Lenard
directed light from an arc lamp onto freshly cleaned metal plates
housed in an evacuated glass tube. He measured the electric current
coming off the metal plate, at higher and lower intensities of light and
for different metals. Lenard showed that amount of current – the number
of electrons – depended on the intensity of the light, but that the
velocity of these electrons did not depend on intensity. This is the photoelectric effect.
The continuous wave theories of the time predicted that more light
intensity would accelerate the same amount of current to higher
velocity, contrary to this experiment. Einstein's energy quanta
explained the volume increase: one electron is ejected for each quantum:
more quanta mean more electrons.
Einstein then predicted that the electron velocity would increase
in direct proportion to the light frequency above a fixed value that
depended upon the metal. Here the idea is that energy in energy-quanta
depends upon the light frequency; the energy transferred to the electron
comes in proportion to the light frequency. The type of metal gives a barrier, the fixed value, that the electrons must climb over to exit their atoms, to be emitted from the metal surface and be measured.
Ten years elapsed before Millikan's definitive experiment verified Einstein's prediction. During that time many scientists rejected the revolutionary idea of quanta. But Planck's and Einstein's concept was in the air and soon began to affect other physics and quantum theories.
Experiments with light and matter in the late 1800s uncovered a
reproducible but puzzling regularity. When light was shown through
purified gases, certain frequencies (colors) did not pass. These dark
absorption 'lines' followed a distinctive pattern: the gaps between the
lines decreased steadily. By 1889, the Rydberg formula predicted the lines for hydrogen gas using only a constant number and the integers to index the lines.The origin of this regularity was unknown. Solving this mystery would
eventually become the first major step toward quantum mechanics.
Throughout the 19th century evidence grew for the atomic
nature of matter. With Thomson's discovery of the electron in 1897,
scientist began the search for a model of the interior of the atom.
Thomson proposed negative electrons swimming in a pool of positive charge. Between 1908 and 1911, Rutherford showed that the positive part was only 1/3000th of the diameter of the atom.
Models of "planetary" electrons orbiting a nuclear "Sun" were
proposed, but cannot explain why the electron does not simply fall into
the positive charge. In 1913 Niels Bohr and Ernest Rutherford
connected the new atom models to the mystery of the Rydberg formula:
the orbital radius of the electrons were constrained and the resulting
energy differences matched the energy differences in the absorption
lines. This meant that absorption and emission of light from atoms was
energy quantized: only specific energies that matched the difference in
orbital energy would be emitted or absorbed.
Trading one mystery – the regular pattern of the Rydberg formula –
for another mystery – constraints on electron orbits – might not seem
like a big advance, but the new atom model summarized many other
experimental findings. The quantization of the photoelectric effect and
now the quantization of the electron orbits set the stage for the final
revolution.
Throughout the first and the modern era of quantum mechanics the
concept that classical mechanics must be valid macroscopically
constrained possible quantum models. This concept was formalized by Bohr
in 1923 as the correspondence principle. It requires quantum theory to converge to classical limits. A related concept is Ehrenfest's theorem, which shows that the average values obtained from quantum mechanics (e.g. position and momentum) obey classical laws.
Stern–Gerlach experiment:
Silver atoms travelling through an inhomogeneous magnetic field, and
being deflected up or down depending on their spin; (1) furnace, (2)
beam of silver atoms, (3) inhomogeneous magnetic field, (4) classically
expected result, (5) observed result
In 1922 Otto Stern and Walther Gerlachdemonstrated that the magnetic properties of silver atoms defy classical explanation, the work contributing to Stern’s 1943 Nobel Prize in Physics.
They fired a beam of silver atoms through a magnetic field. According
to classical physics, the atoms should have emerged in a spray, with a
continuous range of directions. Instead, the beam separated into two,
and only two, diverging streams of atoms. Unlike the other quantum effects known at the time, this striking result involves the state of a single atom. In 1927, T.E. Phipps and J.B. Taylor obtained a similar, but less pronounced effect using hydrogen atoms in their ground state, thereby eliminating any doubts that may have been caused by the use of silver atoms.
In 1924, Wolfgang Pauli called it "two-valuedness not describable
classically" and associated it with electrons in the outermost shell. The experiments lead to formulation of its theory described to arise from spin of the electron in 1925, by Samuel Goudsmit and George Uhlenbeck, under the advice of Paul Ehrenfest.
In 1924 Louis de Broglie proposed that electrons in an atom are constrained not in "orbits" but as
standing waves. In detail his solution did not work, but his hypothesis –
that the electron "corpuscle" moves in the atom as a wave – spurred Erwin Schrödinger to develop a wave equation for electrons; when applied to hydrogen the Rydberg formula was accurately reproduced.
Example original electron diffraction photograph from the laboratory of G. P. Thomson, recorded 1925–1927
Max Born's 1924 paper "Zur Quantenmechanik" was the first use of the words "quantum mechanics" in print. His later work included developing quantum collision models; in a footnote to a 1926 paper he proposed the Born rule connecting theoretical models to experiment.
In 1927 at Bell Labs, Clinton Davisson and Lester Germerfired slow-moving electrons at a crystallinenickel target which showed a diffraction pattern indicating wave nature of electron whose theory was fully explained by Hans Bethe. A similar experiment by George Paget Thomson and Alexander Reid, firing electrons at thin celluloid foils and later metal films, observing rings, independently discovered matter wave nature of electrons.
Planck and Einstein started the revolution with quanta that broke
down the continuous models of matter and light. Twenty years later
"corpuscles" like electrons came to be modeled as continuous waves. This
result came to be called wave-particle duality, one iconic idea along
with the uncertainty principle that sets quantum mechanics apart from
older models of physics.
In 1923 Compton
demonstrated that the Planck-Einstein energy quanta from light also had
momentum; three years later the "energy quanta" got a new name "photon". Despite its role in almost all stages of the quantum revolution, no explicit model for light quanta existed until 1927 when Paul Dirac began work on a quantum theory of radiation that became quantum electrodynamics. Over the following decades this work evolved into quantum field theory, the basis for modern quantum optics and particle physics.
The concept of wave–particle duality says that neither the classical
concept of "particle" nor of "wave" can fully describe the behavior of
quantum-scale objects, either photons or matter. Wave–particle duality
is an example of the principle of complementarity in quantum physics. An elegant example of wave-particle duality is the double-slit experiment.
The
diffraction pattern produced when light is shone through one slit (top)
and the interference pattern produced by two slits (bottom). Both
patterns show oscillations due to the wave nature of light. The double
slit pattern is more dramatic.
In the double-slit experiment, as originally performed by Thomas Young in 1803, and then Augustin Fresnel a decade later, a beam of light is directed through two narrow, closely spaced slits, producing an interference pattern
of light and dark bands on a screen. The same behavior can be
demonstrated in water waves: the double-slit experiment was seen as a
demonstration of the wave nature of light.
Variations of the double-slit experiment have been performed using electrons, atoms, and even large molecules,and the same type of interference pattern is seen. Thus it has been demonstrated that all matter possesses wave characteristics.
If the source intensity is turned down, the same interference
pattern will slowly build up, one "count" or particle (e.g. photon or
electron) at a time. The quantum system acts as a wave when passing
through the double slits, but as a particle when it is detected. This is
a typical feature of quantum complementarity: a quantum system acts as a
wave in an experiment to measure its wave-like properties, and like a
particle in an experiment to measure its particle-like properties. The
point on the detector screen where any individual particle shows up is
the result of a random process. However, the distribution pattern of
many individual particles mimics the diffraction pattern produced by
waves.
Suppose it is desired to measure the position and speed of an
object—for example, a car going through a radar speed trap. It can be
assumed that the car has a definite position and speed at a particular
moment in time. How accurately these values can be measured depends on
the quality of the measuring equipment. If the precision of the
measuring equipment is improved, it provides a result closer to the true
value. It might be assumed that the speed of the car and its position
could be operationally defined and measured simultaneously, as precisely
as might be desired.
In 1927, Heisenberg proved that this last assumption is not correct. Quantum mechanics shows that certain pairs of physical properties, for
example, position and speed, cannot be simultaneously measured, nor
defined in operational terms, to arbitrary precision: the more precisely
one property is measured, or defined in operational terms, the less
precisely can the other be thus treated. This statement is known as the uncertainty principle.
The uncertainty principle is not only a statement about the accuracy of
our measuring equipment but, more deeply, is about the conceptual
nature of the measured quantities—the assumption that the car had
simultaneously defined position and speed does not work in quantum
mechanics. On a scale of cars and people, these uncertainties are
negligible, but when dealing with atoms and electrons they become
critical.
Heisenberg gave, as an illustration, the measurement of the position and momentum
of an electron using a photon of light. In measuring the electron's
position, the higher the frequency of the photon, the more accurate is
the measurement of the position of the impact of the photon with the
electron, but the greater is the disturbance of the electron. This is
because from the impact with the photon, the electron absorbs a random
amount of energy, rendering the measurement obtained of its momentum
increasingly uncertain, for one is necessarily measuring its post-impact
disturbed momentum from the collision products and not its original
momentum (momentum which should be simultaneously measured with
position). With a photon of lower frequency, the disturbance (and hence
uncertainty) in the momentum is less, but so is the accuracy of the
measurement of the position of the impact.
At the heart of the uncertainty principle is a fact that for any
mathematical analysis in the position and velocity domains, achieving a
sharper (more precise) curve in the position domain can only be done at
the expense of a more gradual (less precise) curve in the speed domain,
and vice versa. More sharpness in the position domain requires
contributions from more frequencies in the speed domain to create the
narrower curve, and vice versa. It is a fundamental tradeoff inherent in
any such related or complementary measurements, but is only really noticeable at the smallest (Planck) scale, near the size of elementary particles.
The uncertainty principle shows mathematically that the product of the uncertainty in the position and momentum
of a particle (momentum is velocity multiplied by mass) could never be
less than a certain value, and that this value is related to the Planck constant.
Wave function collapse means that a measurement has forced or
converted a quantum (probabilistic or potential) state into a definite
measured value. This phenomenon is only seen in quantum mechanics rather
than classical mechanics.
For example, before a photon actually "shows up" on a detection
screen it can be described only with a set of probabilities for where it
might show up. When it does appear, for instance in the CCD
of an electronic camera, the time and space where it interacted with
the device are known within very tight limits. However, the photon has
disappeared in the process of being captured (measured), and its quantum
wave function
has disappeared with it. In its place, some macroscopic physical change
in the detection screen has appeared, e.g., an exposed spot in a sheet
of photographic film, or a change in electric potential in some cell of a
CCD.
Because of the uncertainty principle, statements about both the position and momentum of particles can assign only a probability
that the position or momentum has some numerical value. Therefore, it
is necessary to formulate clearly the difference between the state of
something indeterminate, such as an electron in a probability cloud, and
the state of something having a definite value. When an object can
definitely be "pinned-down" in some respect, it is said to possess an eigenstate.
In the Stern–Gerlach experiment discussed above,
the quantum model predicts two possible values of spin for the atom
compared to the magnetic axis. These two eigenstates are named
arbitrarily 'up' and 'down'. The quantum model predicts these states
will be measured with equal probability, but no intermediate values will
be seen. This is what the Stern–Gerlach experiment shows.
The eigenstates of spin about the vertical axis are not
simultaneously eigenstates of spin about the horizontal axis, so this
atom has an equal probability of being found to have either value of
spin about the horizontal axis. As described in the section above,
measuring the spin about the horizontal axis can allow an atom that was
spun up to spin down: measuring its spin about the horizontal axis
collapses its wave function into one of the eigenstates of this
measurement, which means it is no longer in an eigenstate of spin about
the vertical axis, so can take either value.
In 1924, Wolfgang Pauli proposed a new quantum degree of freedom (or quantum number),
with two possible values, to resolve inconsistencies between observed
molecular spectra and the predictions of quantum mechanics. In
particular, the spectrum of atomic hydrogen had a doublet, or pair of lines differing by a small amount, where only one line was expected. Pauli formulated his exclusion principle,
stating, "There cannot exist an atom in such a quantum state that two
electrons within [it] have the same set of quantum numbers."
In 1928, Paul Dirac extended the Pauli equation, which described spinning electrons, to account for special relativity.
The result was a theory that dealt properly with events, such as the
speed at which an electron orbits the nucleus, occurring at a
substantial fraction of the speed of light. By using the simplest electromagnetic interaction,
Dirac was able to predict the value of the magnetic moment associated
with the electron's spin and found the experimentally observed value,
which was too large to be that of a spinning charged sphere governed by classical physics. He was able to solve for the spectral lines of the hydrogen atom and to reproduce from physical first principles Sommerfeld's successful formula for the fine structure of the hydrogen spectrum.
Dirac's equations sometimes yielded a negative value for energy,
for which he proposed a novel solution: he posited the existence of an antielectron and a dynamical vacuum. This led to the many-particle quantum field theory.
In quantum physics, a group of particles can interact or be created together in such a way that the quantum state
of each particle of the group cannot be described independently of the
state of the others, including when the particles are separated by a
large distance. This is known as quantum entanglement.
An early landmark in the study of entanglement was the Einstein–Podolsky–Rosen (EPR) paradox, a thought experiment proposed by Albert Einstein, Boris Podolsky and Nathan Rosen which argues that the description of physical reality provided by quantum mechanics is incomplete. In a 1935 paper titled "Can Quantum-Mechanical Description of Physical
Reality be Considered Complete?", they argued for the existence of
"elements of reality" that were not part of quantum theory, and
speculated that it should be possible to construct a theory containing
these hidden variables.
The thought experiment involves a pair of particles prepared in
what would later become known as an entangled state. Einstein, Podolsky,
and Rosen pointed out that, in this state, if the position of the first
particle were measured, the result of measuring the position of the
second particle could be predicted. If instead the momentum of the first
particle were measured, then the result of measuring the momentum of
the second particle could be predicted. They argued that no action taken
on the first particle could instantaneously affect the other, since
this would involve information being transmitted faster than light,
which is forbidden by the theory of relativity.
They invoked a principle, later known as the "EPR criterion of
reality", positing that: "If, without in any way disturbing a system, we
can predict with certainty (i.e., with probability
equal to unity) the value of a physical quantity, then there exists an
element of reality corresponding to that quantity." From this, they
inferred that the second particle must have a definite value of both
position and of momentum prior to either quantity being measured. But
quantum mechanics considers these two observables incompatible
and thus does not associate simultaneous values for both to any system.
Einstein, Podolsky, and Rosen therefore concluded that quantum theory
does not provide a complete description of reality. In the same year, Erwin Schrödinger used the word "entanglement" and declared: "I would not call that one but rather the characteristic trait of quantum mechanics."
The Irish physicist John Stewart Bell
carried the analysis of quantum entanglement much further. He deduced
that if measurements are performed independently on the two separated
particles of an entangled pair, then the assumption that the outcomes
depend upon hidden variables within each half implies a mathematical
constraint on how the outcomes on the two measurements are correlated.
This constraint would later be named the Bell inequality.
Bell then showed that quantum physics predicts correlations that
violate this inequality. Consequently, the only way that hidden
variables could explain the predictions of quantum physics is if they
are "nonlocal", which is to say that somehow the two particles are able
to interact instantaneously no matter how widely they ever become
separated.Performing experiments like those that Bell suggested, physicists have
found that nature obeys quantum mechanics and violates Bell
inequalities. In other words, the results of these experiments are
incompatible with any local hidden variable theory.
The idea of quantum field theory began in the late 1920s with British physicist Paul Dirac, when he attempted to quantize the energy of the electromagnetic field;
just as in quantum mechanics the energy of an electron in the hydrogen
atom was quantized. Quantization is a procedure for constructing a
quantum theory starting from a classical theory.
Merriam-Webster defines a field in physics as "a region or space in which a given effect (such as magnetism) exists". Other effects that manifest themselves as fields are gravitation and static electricity. In 2008, physicist Richard Hammond wrote:
Sometimes we distinguish between quantum mechanics (QM)
and quantum field theory (QFT). QM refers to a system in which the
number of particles is fixed, and the fields (such as the
electromechanical field) are continuous classical entities. QFT ...
goes a step further and allows for the creation and annihilation of
particles ...
He added, however, that quantum mechanics is often used to refer to "the entire notion of quantum view".
In 1931, Dirac proposed the existence of particles that later became known as antimatter. Dirac shared the Nobel Prize in Physics for 1933 with Schrödinger "for the discovery of new productive forms of atomic theory".
Quantum electrodynamics (QED) is the name of the quantum theory of the electromagnetic force. Understanding QED begins with understanding electromagnetism. Electromagnetism can be called "electrodynamics" because it is a dynamic interaction between electrical and magnetic forces. Electromagnetism begins with the electric charge.
Electric charges are the sources of and create, electric fields.
An electric field is a field that exerts a force on any particles that
carry electric charges, at any point in space. This includes the
electron, proton, and even quarks,
among others. As a force is exerted, electric charges move, a current
flows, and a magnetic field is produced. The changing magnetic field, in
turn, causes electric current (often moving electrons). The physical description of interacting charged particles, electrical currents, electrical fields, and magnetic fields is called electromagnetism.
In 1928 Paul Dirac produced a relativistic quantum theory of
electromagnetism. This was the progenitor to modern quantum
electrodynamics, in that it had essential ingredients of the modern
theory. However, the problem of unsolvable infinities developed in this relativistic quantum theory. Years later, renormalization
largely solved this problem. Initially viewed as a provisional, suspect
procedure by some of its originators, renormalization eventually was
embraced as an important and self-consistent tool in QED and other
fields of physics. Also, in the late 1940s Feynman diagrams
provided a way to make predictions with QED by finding a probability
amplitude for each possible way that an interaction could occur. The
diagrams showed in particular that the electromagnetic force is the
exchange of photons between interacting particles.
The Lamb shift
is an example of a quantum electrodynamics prediction that has been
experimentally verified. It is an effect whereby the quantum nature of
the electromagnetic field makes the energy levels in an atom or ion
deviate slightly from what they would otherwise be. As a result,
spectral lines may shift or split.
Similarly, within a freely propagating electromagnetic wave, the current can also be just an abstract displacement current, instead of involving charge carriers. In QED, its full description makes essential use of short-lived virtual particles. There, QED again validates an earlier, rather mysterious concept.
The physical measurements, equations, and predictions pertinent to
quantum mechanics are all consistent and hold a very high level of
confirmation. However, the question of what these abstract models say
about the underlying nature of the real world has received competing
answers. These interpretations are widely varying and sometimes somewhat
abstract. For instance, the Copenhagen interpretation states that before a measurement, statements about a particle's properties are completely meaningless, while the many-worlds interpretation describes the existence of a multiverse made up of every possible universe.
Light behaves in some aspects like particles and in other aspects
like waves. Matter—the "stuff" of the universe consisting of particles
such as electrons and atoms—exhibits wavelike behavior too. Some light sources, such as neon lights,
give off only certain specific frequencies of light, a small set of
distinct pure colors determined by neon's atomic structure. Quantum
mechanics shows that light, along with all other forms of electromagnetic radiation, comes in discrete units, called photons, and predicts its spectral energies (corresponding to pure colors), and the intensities of its light beams. A single photon is a quantum,
or smallest observable particle, of the electromagnetic field. A
partial photon is never experimentally observed. More broadly, quantum
mechanics shows that many properties of objects, such as position,
speed, and angular momentum,
that appeared continuous in the zoomed-out view of classical mechanics,
turn out to be (in the very tiny, zoomed-in scale of quantum mechanics)
quantized. Such properties of elementary particles
are required to take on one of a set of small, discrete allowable
values, and since the gap between these values is also small, the
discontinuities are only apparent at very tiny (atomic) scales.
The relationship between the frequency of electromagnetic radiation and the energy of each photon is why ultraviolet light can cause sunburn, but visible or infrared light cannot. A photon of ultraviolet light delivers a high amount of energy—enough
to contribute to cellular damage such as occurs in a sunburn. A photon
of infrared light delivers less energy—only enough to warm one's skin.
So, an infrared lamp can warm a large surface, perhaps large enough to
keep people comfortable in a cold room, but it cannot give anyone a
sunburn.
In even a simple light switch, quantum tunneling is absolutely vital, as otherwise the electrons in the electric current could not penetrate the potential barrier made up of a layer of oxide. Flash memory chips found in USB drives also use quantum tunneling, to erase their memory cells.
