Trait leadership is defined as integrated patterns of personal
characteristics that reflect a range of individual differences and
foster consistent leader effectiveness across a variety of group and
organizational situations.
The theory is developed from early leadership research which
focused primarily on finding a group of heritable attributes that
differentiate leaders from nonleaders. Leader effectiveness refers to
the amount of influence a leader has on individual or group performance,
followers’ satisfaction, and overall effectiveness.
Many scholars have argued that leadership is unique to only a select
number of individuals, and that these individuals possess certain
immutable traits that cannot be developed.
Although this perspective has been criticized immensely over the past
century, scholars still continue to study the effects of personality
traits on leader effectiveness. Research has demonstrated that
successful leaders differ from other people and possess certain core
personality traits that significantly contribute to their success.
Understanding the importance of these core personality traits that
predict leader effectiveness can help organizations with their leader
selection, training, and development practices.
History of research
The emergence of the concept of trait leadership can be traced back to Thomas Carlyle's "great man" theory, which stated that "The History of the World [...] was the Biography of Great Men". Subsequent commentators interpreted this view to conclude that the forces of extraordinary leadership shape history. Influenced by Carlyle, Francis Galton in Hereditary Genius
took this idea further. Galton found that leadership was a unique
property of extraordinary individuals and suggested that the traits that
leaders possessed were immutable and could not be developed.
Throughout the early 1900s, the study of leadership focused on traits.
Cowley commented that the approach to the research of leadership has
usually been and should always be through the study of traits.
Many theorists, influenced by Carlyle and Galton, believed that trait
leadership depended on the personal qualities of the leader, however,
they did not assume that leadership only resides within a select number
of people.
This trait perspective of leadership was widely accepted until the late
1940s and early 1950s, when researchers began to deem personality
traits insufficient in predicting leader effectiveness.
In 1948, Stogdill stated that leadership exists between persons
in a social situation, and that persons who are leaders in one situation
may not necessarily be leaders in other situations.
This statement has been cited ubiquitously as sounding the death knell
for trait-leadership theory. Furthermore, scholars commented that any
trait's effect on leadership behavior will always depend on the
situation.Subsequently, leadership stopped being characterized by individual
differences, and instead both behavioral and situational analyses of
leadership took over. These analyses began to dominate the field of
leadership research.
During this period of widespread rejection, several dominant theories
took the place of trait leadership theory, including Fiedler's
contingency model, Blake and Mouton's managerial grid, Hersey and Blanchard's situational leadership model, and transformational and transactional leadership models.
Despite the growing criticisms of trait leadership, the purported
basis for the rejection of trait-leadership models began to encounter
strong challenges in the 1980s.
Zaccaro pointed out that even Stogdill's 1948 review, although cited as
evidence against leader traits, contained conclusions supporting that
individual differences could still be predictors of leader
effectiveness. With an increasing number of empirical studies directly
supporting trait leadership,
traits have reemerged in the lexicon of the scientific research into
leadership. In recent years, the research about leader traits has made
some progress in identifying a list of personality traits that are
highly predictive of leader effectiveness. Additionally, to account for
the arguments for situational leadership, researchers have used the
round-robin design methodology to test whether certain individuals
emerge as leaders across multiple situations. Scholars have also proposed new ways of studying the relationship of certain traits to leader effectiveness. For instance, many suggest the integration of trait and behavioral theories to understand how traits relate to leader effectiveness.
Furthermore, scholars have expanded their focus and have proposed
looking at more malleable traits (ones susceptible to development) in
addition to the traditional dispositional traits as predictors of leader
effectiveness. Context is only now beginning to be examined as a contributor to leaders' success and failure. Productive narcissistic CEOs like Steven Jobs of Apple and Jack Welch of GE have demonstrated a gift for creating innovation, whereas leaders with idealized traits prove more successful in more stable environments requiring less innovation and creativity.
Cultural fit and leadership value can be determined by evaluating
an individual's own behavior, perceptions of their employees and peers,
and the direct objective results of their organization, and then
comparing these findings against the needs of the company.
Leadership traits
The investigations of leader traits are always by no means exhaustive. In recent years, several studies have made comprehensive reviews about leader traits that have been historically studied. There are many ways that traits related to leadership can be
categorized; however, the two most recent categorizations have organized
traits into (1) demographic vs. task competence vs. interpersonal and
(2) distal (trait-like) vs. proximal (state-like):
Demographic, task competence and interpersonal leadership
Based on a recent review of the trait leadership literature, Derue et al stated that most leader traits can be organized into three categories: demographic, task competence, and interpersonal attributes.
For the demographics category, gender has by far received the most
attention in terms of leadership; however, most scholars have found that
male and female leaders are both equally effective. Task competence
relates to how individuals approach the execution and performance of
tasks. Hoffman et al grouped intelligence, conscientiousness, openness to experience, and emotional stability
into this category. Lastly, interpersonal attributes are related to how
a leader approaches social interactions. According to Hoffman et al, Extraversion and Agreeableness should be grouped into this category.
Distal (trait-like) vs. proximal (state-like)
Recent
research has shifted from focusing solely on distal
(dispositional/trait-like) characteristics of leaders to more proximal
(malleable/state-like) individual differences often in the form of knowledge and skills.
The hope is that emergence of proximal traits in trait leadership
theory will help researchers elucidate the old question whether leaders
are born or made. Proximal individual differences suggest that the
characteristics that distinguish effective leaders from non-effective
leaders are not necessarily stable through the life-span, implying that
these traits may be able to be developed. Hoffman et al examined
the effects of distal vs. proximal traits on leader effectiveness. They
found that distal individual differences of achievement motivation, energy, flexibility, dominance, honesty/integrity, self-confidence, creativity, and charisma
were strongly correlated with leader effectiveness. Additionally, they
found that the proximal individual differences of interpersonal skills,
oral communication, written communication, management skills, problem
solving skills, and decision making were also strongly correlated with
leader effectiveness. Their results suggested that on average, distal
and proximal individual differences have a similar relationship with
effective leadership.
Trait-leadership model
Figure 1: Model of Trait Leadership.
Zaccaro et al created a model to understand leader traits and their influence on leader effectiveness/performance. This model, shown in figure 1, is based on other models of leader traits and leader effectiveness/performance.
and rests on two basic premises about leader traits. The first premise
is that leadership emerges from the combined influence of multiple
traits as opposed to emerging from the independent assessment of traits.
Zaccaro argued that effective leadership is derived from an integrated
set of cognitive abilities, social capabilities, and dispositional
tendencies, with each set of traits adding to the influence of the
other. The second premise is that leader traits differ in their proximal
influence on leadership.
This model is a multistage one in which certain distal attributes (i.e.
dispositional attributes, cognitive abilities, and motives/values)
serve as precursors for the development of proximal personal
characteristics (i.e. social skills, problem solving skills and
expertise knowledge).
Adopting this categorization approach and based on several
comprehensive reviews/meta-analysis of trait leadership in recent years,
we tried to make an inclusive list of leader traits (Table 1). However,
the investigations of leader traits are always by no means exhaustive.
Table 1. Leader Traits based on Zaccaro's (2004) Model
One dimension of Big-Five Personality Model; represents the tendency
to be sociable, assertive, active, and to experience positive affects,
such as energy and zeal. In Judge et almeta-analysis, Extraversion was significantly positive related to leadership (r = .31).
One dimension of Big-Five Personality Model; refers to the tendency
to be trusting, compliant, caring, and gentle. The relationship between
Agreeableness and leadership is still ambiguous. In Judge et al meta-analysis, Agreeableness was not significantly related to leadership (r = .08).
One dimension of Big-Five Personality Model; it comprises two related facets, namely achievement and dependability. In Judge et al meta-analysis, Conscientiousness was significantly positively related to leadership (r = .28).
One dimension of Big-Five Personality Model; the disposition to be
imaginative, nonconforming, unconventional, and autonomous. In Judge et al meta-analysis, Openness was found to be significantly positively related with leadership (r = .24).
One dimension of Big-Five Personality Model; represents the tendency
to exhibit poor emotional adjustment and experience negative affects,
such as anxiety, insecurity, and hostility. In Judge et al meta-analysis, Neuroticism was significantly negatively correlated with leadership (r = -.24).
Defined as the correspondence between work and deed, and as being truthful and non deceitful. Honesty/integrity was found to be positively related to leadership effectiveness of others and surrounding factors (r = .29).
Charismatic leaders are able to influence followers by articulating a
compelling vision for the future, arousing commitment to organizational
objectives and inspiring commitment and a sense of self-efficacy among
followers. It has a significant influence on leadership (r = .57).
Intelligence is regarded as the most important trait in psychology.
It has been identified as one of the most critical traits that must be
possessed by all leaders.
Creativity has been proposed as an important component of effective
leadership. A significant relationship was found between creativity and
leader effectiveness (r = .31).
Achievement motivation (Distal - Motive/Value)
The motivation to achieve has been proved to have significant relationship with leader effectiveness (r = .23).
Characterized by the satisfaction leaders derive from exerting
influence over the attitudes and behaviors of others. Need for power has
a positive relationship with leader effectiveness.
Oral/written communication (Proximal - Social Skills)
Oral and written communication skills are found to be significantly correlated with leader effectiveness.
Including a broad range of skills associated with un understanding of human behavior and the dynamics of groups, interpersonal skills were found to be significantly correlated with leader effectiveness.
Technical knowledge includes methods, processes, and equipment for
conducting the specialized activities of the managers’ organizational
unit. It has been proved to be positively correlated with leader effectiveness.
Given that leaders’ key responsibilities involve coordinating the
work of multiple constituents, the ability to manage is likely crucial
to leader effectiveness. This relationship has also been proved
significant.
Other models of trait leadership
Multiple
models have been proposed to explain the relationship of traits to
leader effectiveness. Recently, integrated trait leadership models were
put forward by summarizing the historical findings and reconciling the
conflict between traits and other factors such as situations in
determining effective leadership.
In addition to Zaccaro's Model of Leader Attributes and Leader
Performance described in the previous section, two other models have
emerged in recent trait leadership literature. The Leader Trait
Emergence Effectiveness (LTEE) Model, created by Judge, Piccolo, &
Kosalka in 2009, combines the behavioral genetics and evolutionary
psychology theories of how personality traits are developed into a model
that explains leader emergence and effectiveness. Additionally, this
model separates objective and subjective leader effectiveness into
different criterion. The authors created this model to be broad and
flexible as to diverge from how the relationship between traits and
leadership had been studied in past research.
Another model that has emerged in the trait leadership literature is
the Integrated Model of Leader Traits, Behaviors, and Effectiveness.
This model combines traits and behaviors in predicting leader
effectiveness and tested the mediation effect of leader behaviors on the
relationship between leader traits and effectiveness. The authors found
that some types of leader behaviors mediated the effect between traits
and leader effectiveness. The results of a Derue et al study supported an integrated trait-behavioral model that can be used in future research.
Criticisms of trait leadership
Although
there has been an increased focus by researchers on trait leadership,
this theory remains one of the most criticized theories of leadership.
Over the years, many reviewers of trait leadership theory have commented
that this approach to leadership is "too simplistic", and "futile".
Additionally, scholars have noted that trait leadership theory usually
only focuses on how leader effectiveness is perceived by followers rather than a leader's actual effectiveness. Because the process through which personality predicts the actual effectiveness of leaders has been relatively unexplored.
these scholars have concluded that personality currently has low
explanatory and predictive power over job performance and cannot help
organizations select leaders who will be effective. Furthermore, Derue et al found that leader behaviors are more predictive of leader effectiveness than are traits.
Another criticism of trait leadership is its silence on the influence of the situational context surrounding leaders. Stogdill found that persons who are leaders in one situation may not be leaders in another situation.
Complementing this situational theory of leadership, Murphy wrote that
leadership does not reside in the person, and it usually requires
examining the whole situation.
In addition to situational leadership theory, there has been growing
support for other leadership theories such as transformational,
transactional, charismatic, and authentic leadership theories. These
theories have gained popularity because they are more normative than the
trait and behavioral leadership theories.
Previously, studies failed to uncover a trait or group of traits
that are consistently associated with leadership emergence or help
differentiate leaders from followers, but more recent research supports a link between narcissism and the emergence of leadership.
Additionally, trait leadership's focus on a small set of personality
traits and neglect of more malleable traits such as social skills and
problem solving skills has received considerable criticism. Lastly,
trait leadership often fails to consider the integration of multiple
traits when studying the effects of traits on leader effectiveness.
Implications for practice
Given the recent increase in evidence and support of trait leadership theory,
scholars have suggested a variety of strategies for human resource
departments within organizations. Companies should use personality
traits as selection tools for identifying emerging leaders.
These companies, however, should be aware of the individual traits that
predict success in leader effectiveness as well as the traits that
could be detrimental to leader effectiveness. For example, while Derue et al
found that individuals who are high in Conscientiousness, Extraversion,
and Agreeableness are predicted to be more likely to be perceived as
successful in leadership positions, Judge et al
wrote that individuals who are high in narcissism are more likely to be
a liability in certain jobs. Narcissism is just one example of a
personality trait that should be explored further by HR practitioners to
ensure they are not placing individuals with certain traits in the
wrong positions.
Complementing the suggestion that personality traits should be
used as selection tools, it was found that the Big Five Personality
traits were more strongly related to leadership than intelligence. This
finding suggests that selecting leaders based on their personality is
more important than selecting them based on intelligence. If
organizations select leaders based on intelligence, it is recommended
that these individuals be placed in leadership positions when the stress
level is low and the individual has the ability to be directive.
Another way in which HR practitioners can use the research on
trait leadership is for leadership development programs. Although
inherent personality traits (distal/trait-like) are relatively immune to
leadership development, Zaccaro suggested that proximal traits
(state-like) will be more malleable and susceptible to leadership
development programs. Companies should use different types of
development interventions to stretch the existing capabilities of their
leaders.
There is also evidence to suggest that Americans have an
Extrovert Ideal, which dictates that people, most times unconsciously,
favor the traits of extroverted individuals and suppress the qualities
unique to introverts.
Susan Cain's research points to a transition sometime around the turn
of the century during which we stopped evaluating our leaders based on
character and began judging them instead based on personality. While
both extroverted and introverted leaders have been shown to be
effective, we have a general proclivity towards extroverted traits,
which when evaluating trait leadership, could skew our perception of
what's that important.
Bohr developed the Bohr model of the atom, in which he proposed that energy levels of electrons are discrete and that the electrons revolve in stable orbits around the atomic nucleus
but can jump from one energy level (or orbit) to another. Although the
Bohr model has been supplanted by other models, its underlying
principles remain valid. He conceived the principle of complementarity: that items could be separately analysed in terms of contradictory properties, like behaving as a wave or a stream of particles. The notion of complementarity dominated Bohr's thinking in both science and philosophy.
Niels Henrik David Bohr was born in Copenhagen, Denmark, on 7 October 1885, the second of three children of Christian Bohr,a professor of physiology at the University of Copenhagen, and his wife Ellen née Adler, who came from a wealthy Jewish banking family. He had an elder sister, Jenny, and a younger brother Harald. Jenny became a teacher, while Harald became a mathematician and footballer who played for the Danish national team at the 1908 Summer Olympics in London. Niels was a passionate footballer as well, and the two brothers played several matches for the Copenhagen-based Akademisk Boldklub (Academic Football Club), with Niels as goalkeeper.
Bohr was educated at Gammelholm Latin School, starting when he was seven. In 1903, Bohr enrolled as an undergraduate at Copenhagen University. His major was physics, which he studied under Professor Christian Christiansen, the university's only professor of physics at that time. He also studied astronomy and mathematics under Professor Thorvald Thiele, and philosophy under Professor Harald Høffding, a friend of his father.
Bohr as a young man
In 1905 a gold medal competition was sponsored by the Royal Danish Academy of Sciences and Letters to investigate a method for measuring the surface tension of liquids that had been proposed by Lord Rayleigh
in 1879. This involved measuring the frequency of oscillation of the
radius of a water jet. Bohr conducted a series of experiments using his
father's laboratory in the university; the university itself had no
physics laboratory. To complete his experiments, he had to make his own glassware, creating test tubes with the required elliptical
cross-sections. He went beyond the original task, incorporating
improvements into both Rayleigh's theory and his method, by taking into
account the viscosity
of the water, and by working with finite amplitudes instead of just
infinitesimal ones. His essay, which he submitted at the last minute,
won the prize. He later submitted an improved version of the paper to
the Royal Society in London for publication in the Philosophical Transactions of the Royal Society.
Harald became the first of the two Bohr brothers to earn a master's degree,
which he earned for mathematics in April 1909. Niels took another nine
months to earn his on the electron theory of metals, a topic assigned by
his supervisor, Christiansen. Bohr subsequently elaborated his master's
thesis into his much-larger Doctor of Philosophy thesis. He surveyed the literature on the subject, settling on a model postulated by Paul Drude and elaborated by Hendrik Lorentz,
in which the electrons in a metal are considered to behave like a gas.
Bohr extended Lorentz's model, but was still unable to account for
phenomena like the Hall effect,
and concluded that electron theory could not fully explain the magnetic
properties of metals. The thesis was accepted in April 1911, and Bohr conducted his formal defence on 13 May. Harald had received his doctorate the previous year.
Bohr's thesis was groundbreaking, but attracted little interest outside
Scandinavia because it was written in Danish, a Copenhagen University
requirement at the time. In 1921, the Dutch physicist Hendrika Johanna van Leeuwen would independently derive a theorem in Bohr's thesis that is today known as the Bohr–Van Leeuwen theorem.
In 1910, Bohr met Margrethe Nørlund, the sister of the mathematician Niels Erik Nørlund. Bohr resigned his membership in the Church of Denmark on 16 April 1912, and he and Margrethe were married in a civil ceremony at the town hall in Slagelse on 1 August. Years later, his brother Harald similarly left the church before getting married. Bohr and Margrethe had six sons. The oldest, Christian, died in a boating accident in 1934,
and another, Harald, was severely mentally disabled. He was placed in
an institution away from his family's home at the age of four and died
from childhood meningitis six years later. Aage Bohr
became a successful physicist, and in 1975 was awarded the Nobel Prize
in physics, like his father. A son of Aage, Vilhelm A. Bohr, is a
scientist affiliated with the University of Copenhagen and the National Institute on Aging in the U.S. Hans [da] became a physician; Erik [da], a chemical engineer; and Ernest, a lawyer. Like his uncle Harald, Ernest Bohr became an Olympic athlete, playing field hockey for Denmark at the 1948 Summer Olympics in London.
Bohr returned to Denmark in July 1912 for his wedding, and
travelled around England and Scotland on his honeymoon. On his return,
he became a privatdocent at the University of Copenhagen, giving lectures on thermodynamics. Martin Knudsen put Bohr's name forward for a docent, which was approved in July 1913, and Bohr then began teaching medical students. His three papers, which later became famous as "the trilogy", were published in Philosophical Magazine in July, September and November of that year. He adapted Rutherford's nuclear structure to Max Planck's quantum theory and so created his Bohr model of the atom.
Planetary models of atoms were not new, but Bohr's treatment was.
Taking the 1912 paper by Darwin on the role of electrons in the
interaction of alpha particles with a nucleus as his starting point, he advanced the theory of electrons travelling in orbits
of quantized "stationary states" around the atom's nucleus in order to
stabilize the atom, but it wasn't until his 1921 paper that he showed
that the chemical properties of each element were largely determined by
the number of electrons in the outer orbits of its atoms. He introduced the idea that an electron could drop from a higher-energy orbit to a lower one, in the process emitting a quantum of discrete energy. This became a basis for what is now known as the old quantum theory.
where λ is the wavelength of the absorbed or emitted light and RH is the Rydberg constant.
Balmer's formula was corroborated by the discovery of additional
spectral lines, but for thirty years, no one could explain why it
worked. In the first paper of his trilogy, Bohr was able to derive it
from his model:
where me is the electron's mass, e is its charge, h is Planck's constant and Z is the atom's atomic number (1 for hydrogen).
The model's first hurdle was the Pickering series, lines which did not fit Balmer's formula. When challenged on this by Alfred Fowler, Bohr replied that they were caused by ionisedhelium, helium atoms with only one electron. The Bohr model was found to work for such ions. Many older physicists, like Thomson, Rayleigh and Hendrik Lorentz, did not like the trilogy, but the younger generation, including Rutherford, David Hilbert, Albert Einstein, Enrico Fermi, Max Born and Arnold Sommerfeld saw it as a breakthrough.
The trilogy's acceptance was entirely due to its ability to explain
phenomena which stymied other models, and to predict results that were
subsequently verified by experiments.
Today, the Bohr model of the atom has been superseded, but is still the
best known model of the atom, as it often appears in high school
physics and chemistry texts.
Bohr did not enjoy teaching medical students. He later admitted
that he was not a good lecturer, because he needed a balance between
clarity and truth, between "Klarheit und Wahrheit". He decided to return to Manchester, where Rutherford had offered him a job as a reader
in place of Darwin, whose tenure had expired. Bohr accepted. He took a
leave of absence from the University of Copenhagen, which he started by
taking a holiday in Tyrol with his brother Harald and aunt Hanna Adler. There, he visited the University of Göttingen and the Ludwig Maximilian University of Munich,
where he met Sommerfeld and conducted seminars on the trilogy. The
First World War broke out while they were in Tyrol, greatly complicating
the trip back to Denmark and Bohr's subsequent voyage with Margrethe to
England, where he arrived in October 1914. They stayed until July 1916,
by which time he had been appointed to the Chair of Theoretical Physics
at the University of Copenhagen, a position created especially for him.
His docentship was abolished at the same time, so he still had to teach
physics to medical students. New professors were formally introduced to
King Christian X, who expressed his delight at meeting such a famous football player.
Institute of Physics
In April 1917, Bohr began a campaign to establish an Institute of
Theoretical Physics. He gained the support of the Danish government and
the Carlsberg Foundation, and sizeable contributions were also made by
industry and private donors, many of them Jewish. Legislation
establishing the institute was passed in November 1918. Now known as the
Niels Bohr Institute, it opened on 3 March 1921, with Bohr as its director. His family moved into an apartment on the first floor. Bohr's institute served as a focal point for researchers into quantum mechanics
and related subjects in the 1920s and 1930s, when most of the world's
best-known theoretical physicists spent some time in his company. Early
arrivals included Hans Kramers from the Netherlands, Oskar Klein from Sweden, George de Hevesy from Hungary, Wojciech Rubinowicz from Poland, and Svein Rosseland from Norway. Bohr became widely appreciated as their congenial host and eminent colleague. Klein and Rosseland produced the institute's first publication even before it opened.
The Bohr model worked well for hydrogen and ionized single-electron helium which impressed Einstein
but could not explain more complex elements. By 1919, Bohr was moving
away from the idea that electrons orbited the nucleus and developed heuristics to describe them. The rare-earth elements
posed a particular classification problem for chemists because they
were so chemically similar. An important development came in 1924 with Wolfgang Pauli's discovery of the Pauli exclusion principle,
which put Bohr's models on a firm theoretical footing. Bohr was then
able to declare that the as-yet-undiscovered element 72 was not a
rare-earth element but an element with chemical properties similar to
those of zirconium. (Elements had been predicted and discovered since 1871 by chemical properties), and Bohr was immediately challenged by the French chemist Georges Urbain, who claimed to have discovered a rare-earth element 72, which he called "celtium." At the Institute in Copenhagen, Dirk Coster
and George de Hevesy took up the challenge of proving Bohr right and
Urbain wrong. Starting with a clear idea of the chemical properties of
the unknown element greatly simplified the search process. They went
through samples from Copenhagen's Museum of Mineralogy looking for a
zirconium-like element and soon found it. The element, which they named hafnium (hafnia being the Latin name for Copenhagen), turned out to be more common than gold.
In 1922, Bohr was awarded the Nobel Prize in Physics "for his services in the investigation of the structure of atoms and of the radiation emanating from them."
The award thus recognized both the trilogy and his early leading work
in the emerging field of quantum mechanics. For his Nobel lecture, Bohr
gave his audience a comprehensive survey of what was then known about
the structure of the atom, including the correspondence principle, which he had formulated. This states that the behavior of systems described by quantum theory reproduces classical physics in the limit of large quantum numbers.
The discovery of Compton scattering by Arthur Holly Compton in 1923 convinced most physicists that light was composed of photons and that energy and momentum were conserved in collisions between electrons and photons. In 1924, Bohr, Kramers, and John C. Slater, an American physicist working at the Institute in Copenhagen, proposed the Bohr–Kramers–Slater theory
(BKS). It was more of a program than a full physical theory, as the
ideas it developed were not worked out quantitatively. The BKS theory
became the final attempt at understanding the interaction of matter and
electromagnetic radiation on the basis of the old quantum theory, in
which quantum phenomena were treated by imposing quantum restrictions on
a classical wave description of the electromagnetic field.
Modelling atomic behaviour under incident electromagnetic
radiation using "virtual oscillators" at the absorption and emission
frequencies, rather than the (different) apparent frequencies of the
Bohr orbits, led Max Born, Werner Heisenberg and Kramers to explore different mathematical models. They led to the development of matrix mechanics, the first form of modern quantum mechanics.
The BKS theory also generated discussion of, and renewed attention to,
difficulties in the foundations of the old quantum theory.
The most provocative element of BKS – that momentum and energy would
not necessarily be conserved in each interaction, but only statistically
– was soon shown to be in conflict with experiments conducted by Walther Bothe and Hans Geiger.
In light of these results, Bohr informed Darwin that "there is nothing
else to do than to give our revolutionary efforts as honourable a
funeral as possible".
Quantum mechanics
The introduction of spin by George Uhlenbeck and Samuel Goudsmit in November 1925 was a milestone. The next month, Bohr travelled to Leiden to attend celebrations of the 50th anniversary of Hendrick Lorentz receiving his doctorate. When his train stopped in Hamburg, he was met by Wolfgang Pauli and Otto Stern,
who asked for his opinion of the spin theory. Bohr pointed out that he
had concerns about the interaction between electrons and magnetic
fields. When he arrived in Leiden, Paul Ehrenfest and Albert Einstein informed Bohr that Einstein had resolved this problem using relativity. Bohr then had Uhlenbeck and Goudsmit incorporate this into their paper. Thus, when he met Werner Heisenberg and Pascual Jordan in Göttingen on the way back, he had become, in his own words, "a prophet of the electron magnet gospel".
1927 Solvay Conference in Brussels, October 1927. Bohr is on the right in the middle row, next to Max Born.
Heisenberg first came to Copenhagen in 1924, then returned to
Göttingen in June 1925, shortly thereafter developing the mathematical
foundations of quantum mechanics. When he showed his results to Max Born
in Göttingen, Born realised that they could best be expressed using matrices. This work attracted the attention of the British physicist Paul Dirac, who came to Copenhagen for six months in September 1926. Austrian physicist Erwin Schrödinger
also visited in 1926. His attempt at explaining quantum physics in
classical terms using wave mechanics impressed Bohr, who believed it
contributed "so much to mathematical clarity and simplicity that it
represents a gigantic advance over all previous forms of quantum
mechanics".
When Kramers left the institute in 1926 to take up a chair as professor of theoretical physics at the Utrecht University, Bohr arranged for Heisenberg to return and take Kramers's place as a lektor at the University of Copenhagen. Heisenberg worked in Copenhagen as a university lecturer and assistant to Bohr from 1926 to 1927.
Bohr became convinced that light behaved like both waves and particles and, in 1927, experiments confirmed the de Broglie hypothesis that matter (like electrons) also behaved like waves. He conceived the philosophical principle of complementarity:
that items could have apparently mutually exclusive properties, such as
being a wave or a stream of particles, depending on the experimental
framework. He felt that it was not fully understood by professional philosophers.
In February 1927, Heisenberg developed the first version of the uncertainty principle, presenting it using a thought experiment where an electron was observed through a gamma-ray microscope.
Bohr was dissatisfied with Heisenberg's argument, since it required
only that a measurement disturb properties that already existed, rather
than the more radical idea that the electron's properties could not be
discussed at all apart from the context they were measured in. In a
paper presented at the Volta Conference at Como
in September 1927, Bohr emphasized that Heisenberg's uncertainty
relations could be derived from classical considerations about the
resolving power of optical instruments. Understanding the true meaning of complementarity would, Bohr believed, require "closer investigation".
Einstein preferred the determinism of classical physics over the
probabilistic new quantum physics to which he himself had contributed.
Philosophical issues that arose from the novel aspects of quantum
mechanics became widely celebrated subjects of discussion. Einstein and
Bohr had good-natured arguments over such issues throughout their lives.
In 1914 Carl Jacobsen, the heir to Carlsberg breweries,
bequeathed his mansion (the Carlsberg Honorary Residence, currently
known as Carlsberg Academy) to be used for life by the Dane who had made
the most prominent contribution to science, literature or the arts, as
an honorary residence (Danish: Æresbolig). Harald
Høffding had been the first occupant, and upon his death in July 1931,
the Royal Danish Academy of Sciences and Letters gave Bohr occupancy. He
and his family moved there in 1932. He was elected president of the Academy on 17 March 1939.
By 1929 the phenomenon of beta decay prompted Bohr to again suggest that the law of conservation of energy be abandoned, but Enrico Fermi's hypothetical neutrino and the subsequent 1932 discovery of the neutron provided another explanation. This prompted Bohr to create a new theory of the compound nucleus
in 1936, which explained how neutrons could be captured by the nucleus.
In this model, the nucleus could be deformed like a drop of liquid. He
worked on this with a new collaborator, the Danish physicist Fritz
Kalckar, who died suddenly in 1938.
The discovery of nuclear fission by Otto Hahn in December 1938 (and its theoretical explanation by Lise Meitner)
generated intense interest among physicists. Bohr brought the news to
the United States where he opened the Fifth Washington Conference on
Theoretical Physics with Fermi on 26 January 1939. When Bohr told George Placzek that this resolved all the mysteries of transuranic elements,
Placzek told him that one remained: the neutron capture energies of
uranium did not match those of its decay. Bohr thought about it for a
few minutes and then announced to Placzek, Léon Rosenfeld and John Wheeler that "I have understood everything." Based on his liquid drop model of the nucleus, Bohr concluded that it was the uranium-235 isotope and not the more abundant uranium-238 that was primarily responsible for fission with thermal neutrons. In April 1940, John R. Dunning demonstrated that Bohr was correct.
In the meantime, Bohr and Wheeler developed a theoretical treatment
which they published in a September 1939 paper on "The Mechanism of
Nuclear Fission".
Philosophy
Heisenberg said of Bohr that he was "primarily a philosopher, not a physicist". Bohr read the 19th-century Danish Christian existentialist philosopher Søren Kierkegaard. Richard Rhodes argued in The Making of the Atomic Bomb that Bohr was influenced by Kierkegaard through Høffding. In 1909, Bohr sent his brother Kierkegaard's Stages on Life's Way
as a birthday gift. In the enclosed letter, Bohr wrote, "It is the only
thing I have to send home; but I do not believe that it would be very
easy to find anything better ... I even think it is one of the most
delightful things I have ever read." Bohr enjoyed Kierkegaard's language
and literary style, but mentioned that he had some disagreement with Kierkegaard's philosophy.
Some of Bohr's biographers suggested that this disagreement stemmed
from Kierkegaard's advocacy of Christianity, while Bohr was an atheist.
There has been some dispute over the extent to which Kierkegaard influenced Bohr's philosophy and science. David Favrholdt
argued that Kierkegaard had minimal influence over Bohr's work, taking
Bohr's statement about disagreeing with Kierkegaard at face value, while Jan Faye argued that one can disagree with the content of a theory while accepting its general premises and structure.
Quantum physics
Bohr (left) and Albert Einstein (right), pictured on 11 December 1925, had a long-running debate about the metaphysical implication of quantum physics.
There has been much subsequent debate and discussion about Bohr's views and philosophy of quantum mechanics. Regarding his ontological interpretation of the quantum world, Bohr has been seen as an anti-realist, an instrumentalist, a phenomenological realist or some other kind of realist. Furthermore, though some have seen Bohr as being a subjectivist or a positivist, most philosophers agree that this is a misunderstanding of Bohr as he never argued for verificationism or for the idea that the subject had a direct impact on the outcome of a measurement.
Bohr has often been quoted saying that there is "no quantum
world" but only an "abstract quantum physical description". This was not
publicly said by Bohr, but rather a private statement attributed to
Bohr by Aage Petersen in a reminiscence after his death. N. David Mermin recalled Victor Weisskopf
declaring that Bohr wouldn't have said anything of the sort and
exclaiming, "Shame on Aage Petersen for putting those ridiculous words
in Bohr's mouth!"
Numerous scholars have argued that the philosophy of Immanuel Kant
had a strong influence on Bohr. Like Kant, Bohr thought distinguishing
between the subject's experience and the object was an important
condition for attaining knowledge. This can only be done through the use
of causal and spatial-temporal concepts to describe the subject's
experience.
Thus, according to Jan Faye, Bohr thought that it is because of
"classical" concepts like "space", "position", "time," "causation", and
"momentum" that one can talk about objects and their objective
existence. Bohr held that basic concepts like "time" are built in to our
ordinary language and that the concepts of classical physics are merely
a refinement of them. Therefore, for Bohr, classical concepts need to be used to describe experiments that deal with the quantum world. Bohr writes:
[T]he account of all evidence must be expressed in
classical terms. The argument is simply that by the word 'experiment' we
refer to a situation where we can tell to others what we have done and
what we have learned and that, therefore, the account of the
experimental arrangement and of the results of the observations must be
expressed in unambiguous language with suitable application of the
terminology of classical physics (APHK, p. 39).
According to Faye, there are various explanations for why Bohr
believed that classical concepts were necessary for describing quantum
phenomena. Faye groups explanations into five frameworks: empiricism
(i.e. logical positivism); Kantianism (or Neo-Kantian models of epistemology); Pragmatism
(which focus on how human beings experientially interact with atomic
systems according to their needs and interests); Darwinianism (i.e. we
are adapted to use classical type concepts, which Léon Rosenfeld
said that we evolved to use); and Experimentalism (which focuses
strictly on the function and outcome of experiments which thus must be
described classically).
These explanations are not mutually exclusive, and at times Bohr seems
to emphasize some of these aspects while at other times he focuses on
other elements.
According to Faye "Bohr thought of the atom as real. Atoms are
neither heuristic nor logical constructions." However, according to
Faye, he did not believe "that the quantum mechanical formalism was true
in the sense that it gave us a literal ('pictorial') rather than a
symbolic representation of the quantum world." Therefore, Bohr's theory of complementarity
"is first and foremost a semantic and epistemological reading of
quantum mechanics that carries certain ontological implications." As Faye explains, Bohr's indefinability thesis is that
[T]he truth conditions of sentences ascribing a certain
kinematic or dynamic value to an atomic object are dependent on the
apparatus involved, in such a way that these truth conditions have to
include reference to the experimental setup as well as the actual
outcome of the experiment.
Faye notes that Bohr's interpretation makes no reference to a
"collapse of the wave function during measurements" (and indeed, he
never mentioned this idea). Instead, Bohr "accepted the Born statistical
interpretation because he believed that the ψ-function has only a symbolic meaning and does not represent anything real." Since for Bohr, the ψ-function is not a literal pictorial representation of reality, there can be no real collapse of the wavefunction.
A much debated point in recent literature is what Bohr believed
about atoms and their reality and whether they are something else than
what they seem to be. Some like Henry Folse argue that Bohr saw a
distinction between observed phenomena and a transcendental reality.
Jan Faye disagrees with this position and holds that for Bohr, the
quantum formalism and complementarity was the only thing we could say
about the quantum world and that "there is no further evidence in Bohr's
writings indicating that Bohr would attribute intrinsic and
measurement-independent state properties to atomic objects [...] in
addition to the classical ones being manifested in measurement."
Second World War
Assistance to refugee scholars
The rise of Nazism
in Germany prompted many scholars to flee their countries, either
because they were Jewish or because they were political opponents of the
Nazi regime. In 1933, the Rockefeller Foundation
created a fund to help support refugee academics, and Bohr discussed
this programme with the President of the Rockefeller Foundation, Max Mason,
in May 1933 during a visit to the United States. Bohr offered the
refugees temporary jobs at the institute, provided them with financial
support, arranged for them to be awarded fellowships from the
Rockefeller Foundation, and ultimately found them places at institutions
around the world. Those that he helped included Guido Beck, Felix Bloch, James Franck, George de Hevesy, Otto Frisch, Hilde Levi, Lise Meitner, George Placzek, Eugene Rabinowitch, Stefan Rozental, Erich Ernst Schneider, Edward Teller, Arthur von Hippel and Victor Weisskopf.
In April 1940, early in the Second World War, Nazi Germanyinvaded and occupied Denmark. To prevent the Germans from discovering Max von Laue's and James Franck's gold Nobel medals, Bohr had de Hevesy dissolve them in aqua regia.
In this form, they were stored on a shelf at the Institute until after
the war, when the gold was precipitated and the medals re-struck by the
Nobel Foundation. Bohr's own medal had been donated to an auction to the
Finnish Relief Fund, and was auctioned off in March 1940, along with the medal of August Krogh. The buyer later donated the two medals to the Danish Historical Museum in Frederiksborg Castle, where they are still kept, although Bohr's medal temporarily went to space with Andreas Mogensen on ISSExpedition 70 in 2023-2024.
Bohr kept the Institute running, but all the foreign scholars departed.
Meeting with Heisenberg
Werner Heisenberg (left) with Bohr at the Copenhagen Conference in 1934
Bohr was aware of the possibility of using uranium-235 to construct an atomic bomb,
referring to it in lectures in Britain and Denmark shortly before and
after the war started, but he did not believe that it was technically
feasible to extract a sufficient quantity of uranium-235. In September 1941, Heisenberg, who had become head of the German nuclear energy project,
visited Bohr in Copenhagen. During this meeting the two men took a
private moment outside, the content of which has caused much
speculation, as both gave differing accounts.
According to Heisenberg, he began to address nuclear energy, morality
and the war, to which Bohr seems to have reacted by terminating the
conversation abruptly while not giving Heisenberg hints about his own
opinions. Ivan Supek, one of Heisenberg's students and friends, claimed that the main subject of the meeting was Carl Friedrich von Weizsäcker, who had proposed trying to persuade Bohr to mediate peace between Britain and Germany.
In 1957, Heisenberg wrote to Robert Jungk, who was then working on the book Brighter than a Thousand Suns: A Personal History of the Atomic Scientists.
Heisenberg explained that he had visited Copenhagen to communicate to
Bohr the views of several German scientists, that production of a
nuclear weapon was possible with great efforts, and this raised enormous
responsibilities on the world's scientists on both sides.
When Bohr saw Jungk's depiction in the Danish translation of the book,
he drafted (but never sent) a letter to Heisenberg, stating that he
deeply disagreed with Heisenberg's account of the meeting, that he recalled Heisenberg's visit as being to encourage cooperation with the inevitably victorious Nazis and that he was shocked that Germany was pursuing nuclear weapons under Heisenberg's leadership.
Michael Frayn's 1998 play Copenhagen explores what might have happened at the 1941 meeting between Heisenberg and Bohr. A television film version of the play by the BBC was first screened on 26 September 2002, with Stephen Rea
as Bohr. With the subsequent release of Bohr's letters, the play has
been criticised by historians as being a "grotesque oversimplification
and perversion of the actual moral balance" due to adopting a
pro-Heisenberg perspective.
The same meeting had previously been dramatised by the BBC's Horizon science documentary series in 1992, with Anthony Bate as Bohr, and Philip Anthony as Heisenberg. The meeting is also dramatized in the Norwegian/Danish/British miniseries The Heavy Water War.
Manhattan Project
In September 1943, word reached Bohr and his brother Harald that the Nazis considered their family to be Jewish,
since their mother was Jewish, and that they were therefore in danger
of being arrested. The Danish resistance helped Bohr and his wife escape
by sea to Sweden on 29 September. The next day, Bohr persuaded King Gustaf V of Sweden
to make public Sweden's willingness to provide asylum to Jewish
refugees. On 2 October 1943, Swedish radio broadcast that Sweden was
ready to offer asylum, and the mass rescue of the Danish Jews
by their countrymen followed swiftly thereafter. Some historians claim
that Bohr's actions led directly to the mass rescue, while others say
that, though Bohr did all that he could for his countrymen, his actions
were not a decisive influence on the wider events. Eventually, over 7,000 Danish Jews escaped to Sweden.
When the news of Bohr's escape reached Britain, Lord Cherwell sent a telegram to Bohr asking him to come to Britain. Bohr arrived in Scotland on 6 October in a de Havilland Mosquito operated by the British Overseas Airways Corporation (BOAC). The Mosquitos were unarmed high-speed bomber aircraft that had been
converted to carry small, valuable cargoes or important passengers. By
flying at high speed and high altitude, they could cross German-occupied
Norway, and yet avoid German fighters. Bohr, equipped with parachute,
flying suit and oxygen mask, spent the three-hour flight lying on a
mattress in the aircraft's bomb bay.
During the flight, Bohr did not wear his flying helmet as it was too
small, and consequently did not hear the pilot's intercom instruction to
turn on his oxygen supply when the aircraft climbed to high altitude to
overfly Norway. He passed out from oxygen starvation and only revived
when the aircraft descended to lower altitude over the North Sea. Bohr's son Aage followed his father to Britain on another flight a week later, and became his personal assistant.
Bohr was warmly received by James Chadwick and Sir John Anderson, but for security reasons Bohr was kept out of sight. He was given an apartment at St James's Palace and an office with the British Tube Alloys nuclear weapons development team. Bohr was astonished at the amount of progress that had been made. Chadwick arranged for Bohr to visit the United States as a Tube Alloys consultant, with Aage as his assistant. On 8 December 1943, Bohr arrived in Washington, D.C., where he met with the director of the Manhattan Project, Brigadier General Leslie R. Groves Jr. He visited Einstein and Pauli at the Institute for Advanced Study in Princeton, New Jersey, and went to Los Alamos in New Mexico, where the nuclear weapons were being designed. For security reasons, he went under the name of "Nicholas Baker" in the United States, while Aage became "James Baker." In May 1944 the Danish resistance newspaper De frie Danske
reported that they had learned that 'the famous son of Denmark
Professor Niels Bohr' in October the previous year had fled his country
via Sweden to London and from there travelled to Moscow from where he could be assumed to support the war effort.
Bohr did not remain at Los Alamos, but paid a series of extended visits over the course of the next two years. Robert Oppenheimer credited Bohr with acting "as a scientific father figure to the younger men", most notably Richard Feynman. Bohr is quoted as saying, "They didn't need my help in making the atom bomb." Oppenheimer gave Bohr credit for an important contribution to the work on modulated neutron initiators.
"This device remained a stubborn puzzle," Oppenheimer noted, "but in
early February 1945 Niels Bohr clarified what had to be done."
Bohr recognised early that nuclear weapons would change international relations. In April 1944, he received a letter from Peter Kapitza, written some months before when Bohr was in Sweden, inviting him to come to the Soviet Union.
The letter convinced Bohr that the Soviets were aware of the
Anglo-American project, and would strive to catch up. He sent Kapitza a
non-committal response, which he showed to the authorities in Britain
before posting. Bohr met Churchill on 16 May 1944, but found that "we did not speak the same language".
Churchill disagreed with the idea of openness towards the Russians to
the point that he wrote in a letter: "It seems to me Bohr ought to be
confined or at any rate made to see that he is very near the edge of
mortal crimes."
Oppenheimer suggested that Bohr visit President Franklin D. Roosevelt
to convince him that the Manhattan Project should be shared with the
Soviets in the hope of speeding up its results. Bohr's friend, Supreme
Court Justice Felix Frankfurter,
informed President Roosevelt about Bohr's opinions, and a meeting
between them took place on 26 August 1944. Roosevelt suggested that Bohr
return to the United Kingdom to try to win British approval.
When Churchill and Roosevelt met at Hyde Park on 19 September 1944,
they rejected the idea of informing the world about the project, and the
aide-mémoire
of their conversation contained a rider that "enquiries should be made
regarding the activities of Professor Bohr and steps taken to ensure
that he is responsible for no leakage of information, particularly to
the Russians".
Bohr's coat of arms, 1947. Argent, a taijitu (yin-yang symbol) Gules and Sable. Motto: Contraria sunt complementa ("opposites are complementary").
Following the ending of the war, Bohr returned to Copenhagen on 25
August 1945, and was re-elected President of the Royal Danish Academy of
Arts and Sciences on 21 September. At a memorial meeting of the Academy on 17 October 1947 for King Christian X, who had died in April, the new king, Frederik IX, announced that he was conferring the Order of the Elephant
on Bohr. This award was normally awarded only to royalty and heads of
state, but the king said that it honoured not just Bohr personally, but
Danish science. Bohr designed his own coat of arms which featured a taijitu (symbol of yin and yang) and a motto in Latin: contraria sunt complementa, "opposites are complementary".
The Second World War demonstrated that science, and physics in
particular, now required considerable financial and material resources.
To avoid a brain drain to the United States, twelve European countries banded together to create CERN, a research organisation along the lines of the national laboratories in the United States, designed to undertake Big Science
projects beyond the resources of any one of them alone. Questions soon
arose regarding the best location for the facilities. Bohr and Kramers
felt that the Institute in Copenhagen would be the ideal site. Pierre Auger,
who organised the preliminary discussions, disagreed; he felt that both
Bohr and his Institute were past their prime, and that Bohr's presence
would overshadow others. After a long debate, Bohr pledged his support
to CERN in February 1952, and Geneva
was chosen as the site in October. The CERN Theory Group was based in
Copenhagen until their new accommodation in Geneva was ready in 1957. Victor Weisskopf, who later became the Director General of CERN, summed up Bohr's role, saying that "there were other personalities who started and conceived the idea of CERN.
The enthusiasm and ideas of the other people would not have been
enough, however, if a man of his stature had not supported it."
Bohr died of heart failure at his home in Carlsberg on 18 November 1962. He was cremated, and his ashes were buried in the family plot in the Assistens Cemetery in the Nørrebro
section of Copenhagen, along with those of his parents, his brother
Harald, and his son Christian. Years later, his wife's ashes were also
interred there.
On 7 October 1965, on what would have been his 80th birthday, the
Institute for Theoretical Physics at the University of Copenhagen was
officially renamed to what it had been called unofficially for many
years: the Niels Bohr Institute.
