Negative mass

From Wikipedia, the free encyclopedia

In theoretical physics, negative mass is matter whose mass is of opposite sign to the mass of normal matter, e.g. −1 kg.^[1][2] Such matter would violate one or more energy conditions and show some strange properties, stemming from the ambiguity as to whether attraction should refer to force or the oppositely oriented acceleration for negative mass. It is used in certain speculative theories, such as on the construction of traversable wormholes and the Alcubierre drive. Originally the closest known real representative of such exotic matter is a region of negative pressure density produced by the Casimir effect. In 2017, researchers at Washington State University realized negative effective inertial mass experimentally by cooling rubidium atoms with lasers, although this is not negative mass in the fundamental sense.^[3]

General relativity describes gravity and the laws of motion for both positive and negative energy particles, hence negative mass, but does not include the other fundamental forces. On the other hand, the Standard Model describes elementary particles and the other fundamental forces, but it does not include gravity. A unified theory that explicitly includes gravity along with the other fundamental forces may be needed for a better understanding of the concept of negative mass.

In general relativity

Negative mass is any region of space in which for some observers the mass density is measured to be negative. This could occur due to a region of space in which the stress component of the Einstein stress–energy tensor is larger in magnitude than the mass density. All of these are violations of one or another variant of the positive energy condition of Einstein's general theory of relativity; however, the positive energy condition is not a required condition for the mathematical consistency of the theory.

Inertial versus gravitational mass

Ever since Newton first formulated his theory of gravity, there have been at least three conceptually distinct quantities called mass:

• inertial mass – the mass m that appears in Newtons second law of motion, F = m a
• “active” gravitational mass – the mass that produces a gravitational field that other masses respond to
• “passive” gravitational mass – the mass that responds to an external gravitational field by accelerating.

Einstein’s equivalence principle postulates that inertial mass must equal passive gravitational mass. The law of conservation of momentum requires that active and passive gravitational mass be identical. All experimental evidence to date has found these are, indeed, always the same. In considering negative mass, it is important to consider which of these concepts of mass are negative. In most analyses of negative mass, it is assumed that the equivalence principle and conservation of momentum continue to apply, and therefore all three forms of mass are still the same.

In his 4th-prize essay for the 1951 Gravity Research Foundation competition, Joaquin Mazdak Luttinger considered the possibility of negative mass and how it would behave under gravitational and other forces.^[4]

In 1957, following Luttinger's idea, Hermann Bondi suggested in a paper in Reviews of Modern Physics that mass might be negative as well as positive.^[5] He pointed out that this does not entail a logical contradiction, as long as all three forms of mass are negative, but that the assumption of negative mass involves some counter-intuitive form of motion. For example, an object with negative inertial mass would be expected to accelerate in the opposite direction to that in which it was pushed (non-gravitationally).

There have been several other analyses of negative mass, such as the studies conducted by R. M. Price,^[6] however none addressed the question of what kind of energy and momentum would be necessary to describe non-singular negative mass. Indeed, the Schwarzschild solution for negative mass parameter has a naked singularity at a fixed spatial position. The question that immediately comes up is, would it not be possible to smooth out the singularity with some kind of negative mass density. The answer is yes, but not with energy and momentum that satisfies the dominant energy condition. This is because if the energy and momentum satisfies the dominant energy condition within a spacetime that is asymptotically flat, which would be the case of smoothing out the singular negative mass Schwarzschild solution, then it must satisfy the positive energy theorem, i.e. its ADM mass must be positive, which is of course not the case.^[7][8] However, it was noticed by Belletête and Paranjape that since the positive energy theorem does not apply to asymptotic de Sitter spacetime, it would actually be possible to smooth out, with energy-momentum that does satisfy the dominant energy condition, the singularity of the corresponding exact solution of negative mass Schwarzschild-de Sitter, which is the singular, exact solution of Einstein's equations with cosmological constant.^[9] In a subsequent article, Mbarek and Paranjape showed that it is in fact possible to obtain the required deformation through the introduction of the energy-momentum of a perfect fluid.^[10]

Runaway motion

Although no particles are known to have negative mass, physicists (primarily Hermann Bondi in 1957,^[5] William B. Bonnor in 1989,^[11] then Robert L. Forward^[12]) have been able to describe some of the anticipated properties such particles may have. Assuming that all three concepts of mass are equivalent the gravitational interactions between masses of arbitrary sign can be explored, based on the Einstein field equations and the Equivalence principle:

• Positive mass attracts both other positive masses and negative masses.
• Negative mass repels both other negative masses and positive masses.

For two positive masses, nothing changes and there is a gravitational pull on each other causing an attraction. Two negative masses would repel because of their negative inertial masses. For different signs however, there is a push that repels the positive mass from the negative mass, and a pull that attracts the negative mass towards the positive one at the same time.

Hence Bondi pointed out that two objects of equal and opposite mass would produce a constant acceleration of the system towards the positive-mass object,^[5] an effect called "runaway motion" by Bonnor who disregarded its physical existence, stating:

“	I regard the runaway (or self-accelerating) motion […] so preposterous that I prefer to rule it out by supposing that inertial mass is all positive or all negative.	”
— William B. Bonnor, in Negative mass in general relativity.[11]

Such a couple of objects would accelerate without limit (except relativistic one); however, the total mass, momentum and energy of the system would remain 0.

This behavior is completely inconsistent with a common-sense approach and the expected behaviour of 'normal' matter; but is completely mathematically consistent and introduces no violation of conservation of momentum or energy. If the masses are equal in magnitude but opposite in sign, then the momentum of the system remains zero if they both travel together and accelerate together, no matter what their speed:

${\displaystyle p_{\mathrm {sys} }=mv+(-m)v={\big (}m+(-m){\big )}v=0\times v=0.}$

And equivalently for the kinetic energy:

${\displaystyle E_{\mathrm {k,sys} }={\tfrac {1}{2}}mv^{2}+{\tfrac {1}{2}}(-m)v^{2}={\tfrac {1}{2}}{\big (}m+(-m){\big )}v^{2}={\tfrac {1}{2}}(0)v^{2}=0}$

However, this is perhaps not exactly valid if the energy in the gravitational field is taken into account.

Forward extended Bondi's analysis to additional cases, and showed that even if the two masses m⁽⁻⁾ and m⁽⁺⁾ are not the same, the conservation laws remain unbroken. This is true even when relativistic effects are considered, so long as inertial mass, not rest mass, is equal to gravitational mass.

This behaviour can produce bizarre results: for instance, a gas containing a mixture of positive and negative matter particles will have the positive matter portion increase in temperature without bound. However, the negative matter portion gains negative temperature at the same rate, again balancing out. Geoffrey A. Landis pointed out other implications of Forward's analysis,^[13] including noting that although negative mass particles would repel each other gravitationally, the electrostatic force would be attractive for like charges and repulsive for opposite charges.

Forward used the properties of negative-mass matter to create the concept of diametric drive, a design for spacecraft propulsion using negative mass that requires no energy input and no reaction mass to achieve arbitrarily high acceleration.

Forward also coined a term, "nullification", to describe what happens when ordinary matter and negative matter meet: they are expected to be able to cancel out or nullify each other's existence. An interaction between equal quantities of positive mass matter (hence of positive energy E = mc²) and negative mass matter (of negative energy −E = −mc²) would release no energy, but because the only configuration of such particles that has zero momentum (both particles moving with the same velocity in the same direction) does not produce a collision, all such interactions would leave a surplus of momentum, which is classically forbidden. So once this runaway phenomenon has been revealed, the scientific community considered negative mass could not exist in the universe.

Arrow of time and energy inversion

In 1970, Jean-Marie Souriau demonstrated, through the complete Poincaré group of dynamic group theory, that reversing the energy of a particle (hence its mass, if the particle has one) is equal to reversing its arrow of time.^[14][15]

The universe according to general relativity is a Riemannian manifold associated to a metric tensor solution of Einstein’s field equations. In such a framework, the runaway motion prevents the existence of negative matter.^[5][11]

Some bimetric theories of the universe propose that two parallel universes instead of one may exist with an opposite arrow of time, linked together by the Big Bang and interacting only through gravitation.^[16][17][18] The universe is then described as a manifold associated to two Riemannian metrics (one with positive mass matter and the other with negative mass matter). According to group theory, the matter of the conjugated metric would appear to the matter of the other metric as having opposite mass and arrow of time (though its proper time would remain positive). The coupled metrics have their own geodesics and are solutions of two coupled field equations:^[19][20]

${\displaystyle R_{\mu \nu }^{(+)}-{\tfrac {1}{2}}\,R^{(+)}g_{\mu \nu }^{(+)}=\chi \left(T_{\mu \nu }^{(+)}+{\sqrt {\frac {g^{(-)}}{g^{(+)}}}}T_{\mu \nu }^{(-)}\right)}$

${\displaystyle R_{\mu \nu }^{(-)}-{\tfrac {1}{2}}\,R^{(-)}g_{\mu \nu }^{(-)}=-\chi \left({\sqrt {\frac {g^{(+)}}{g^{(-)}}}}T_{\mu \nu }^{(+)}+T_{\mu \nu }^{(-)}\right)}$

The Newtonian approximation then provides the following interaction laws:

• Positive mass attracts positive mass.
• Negative mass attracts negative mass.
• Positive mass and negative mass repel each other.

Those laws are different to the laws described by Bondi and Bonnor, and solve the runaway paradox. The negative matter of the coupled metric, interacting with the matter of the other metric via gravity, could be an alternative candidate for the explanation of dark matter, dark energy, cosmic inflation and accelerating universe.^[19][20]

In Gauss's law of gravity

In electromagnetism one can derive the energy density of a field from Gauss's law, assuming the curl of the field is 0. Performing the same calculation using Gauss's law for gravity produces a negative energy density for a gravitational field.

Gravitational interaction of antimatter

The overwhelming consensus among physicists is that antimatter has positive mass and should be affected by gravity just like normal matter. Direct experiments on neutral antihydrogen have not been sensitive enough to detect any difference between the gravitational interaction of antimatter, compared to normal matter.^[21]
Bubble chamber experiments provide further evidence that antiparticles have the same inertial mass as their normal counterparts. In these experiments, the chamber is subjected to a constant magnetic field that causes charged particles to travel in helical paths, the radius and direction of which correspond to the ratio of electric charge to inertial mass. Particle–antiparticle pairs are seen to travel in helices with opposite directions but identical radii, implying that the ratios differ only in sign; but this does not indicate whether it is the charge or the inertial mass that is inverted. However, particle–antiparticle pairs are observed to electrically attract one another. This behavior implies that both have positive inertial mass and opposite charges; if the reverse were true, then the particle with positive inertial mass would be repelled from its antiparticle partner.

Experimentation

Physicist Peter Engels and a team of colleagues at Washington State University claimed to have observed negative mass behavior in rubidium atoms. On 10 April 2017 Engels team created negative "effective" mass by reducing the temperature of rubidium atoms to near absolute zero, generating a Bose-Einstein condensate. By using a laser-trap, the team were able to reverse the spin of some of the rubidium atoms in this state, and observed that once released from the trap, the atoms expanded and displayed properties of negative mass, in particular accelerating towards a pushing force instead of away from it.^[22][23] This kind of negative effective mass is analogous to the well-known apparent negative effective mass of electrons in the upper part of the dispersion bands in solids.^[24] However, neither case is negative mass for the purposes of the stress–energy tensor.

Some recent work with metamaterials suggests that some as-yet-undiscovered composite of superconductors, metamaterials and normal matter could exhibit signs of negative effective mass in much the same way as low temperature alloys melt at below the melting point of their components or some semiconductors have negative differential resistance.^{[25] [26]}

In quantum mechanics

In 1928, Paul Dirac's theory of elementary particles, now part of the Standard Model, already included negative solutions.^[27] The Standard Model is a generalization of quantum electrodynamics (QED) and negative mass is already built into the theory.

Morris, Thorne and Yurtsever^[28] pointed out that the quantum mechanics of the Casimir effect can be used to produce a locally mass-negative region of space–time. In this article, and subsequent work by others, they showed that negative matter could be used to stabilize a wormhole. Cramer et al. argue that such wormholes might have been created in the early universe, stabilized by negative-mass loops of cosmic string.^[29] Stephen Hawking has proved that negative energy is a necessary condition for the creation of a closed timelike curve by manipulation of gravitational fields within a finite region of space;^[30] this proves, for example, that a finite Tipler cylinder cannot be used as a time machine.

Schrödinger equation

For energy eigenstates of the Schrödinger equation, the wavefunction is wavelike wherever the particle's energy is greater than the local potential, and exponential-like (evanescent) wherever it is less. Naively, this would imply kinetic energy is negative in evanescent regions (to cancel the local potential). However, kinetic energy is an operator in quantum mechanics, and its expectation value is always positive, summing with the expectation value of the potential energy to yield the energy eigenvalue.

For wavefunctions of particles with zero rest mass (such as photons), this means that any evanescent portions of the wavefunction would be associated with a local negative mass–energy. However, the Schrödinger equation does not apply to massless particles; instead the Klein-Gordon equation is required.

In special relativity

One can achieve a negative mass independent of negative energy. According to mass–energy equivalence, mass m is in proportion to energy E and the coefficient of proportionality is c². Actually, m is still equivalent to E although the coefficient is another constant ^[31] such as −c².^[32] In this case, it is unnecessary to introduce a negative energy because the mass can be negative although the energy is positive. That is to say,

${\displaystyle {\begin{aligned}E&=-mc^{2}>0\\m&=-{\frac {E}{c^{2}}}<0 aligned="" annotation="" end="">$

Under the circumstances,

${\displaystyle dE=F\,ds={\frac {dp}{dt}}\,ds={\frac {ds}{dt}}\,dp=v\,dp=v\,d(mv)}$

and so,

${\displaystyle {\begin{aligned}-c^{2}\,dm&=v\,d(mv)\\-c^{2}(2m)\,dm&=2mv\,d(mv)\\-c^{2}\,d(m^{2})&=d(m^{2}v^{2})\\-m^{2}c^{2}&=m^{2}v^{2}+C\end{aligned}}}$

When v = 0,

${\displaystyle C=-m_{0}^{2}c^{2}}$

Consequently,

${\displaystyle {\begin{aligned}-m^{2}c^{2}&=m^{2}v^{2}-m_{0}^{2}c^{2}\\m&={\frac {m_{0}}{\sqrt {1+{\frac {v^{2}}{c^{2}}}}}}\end{aligned}}}$

where m₀ < 0 is invariant mass and invariant energy equals E₀ = −m₀c² > 0. The squared mass is still positive and the particle can be stable.

From the above relation,

${\displaystyle p=mv={\frac {m_{0}v}{\sqrt {1+{\frac {v^{2}}{c^{2}}}}}}<0 annotation="">$

The negative momentum is applied to explain negative refraction, the inverse Doppler effect and the reverse Cherenkov effect observed in a negative index metamaterial. The radiation pressure in the metamaterial is also negative^[33] because the force is defined as F = dp/dt. Interestingly, negative pressure exists in dark energy too. Using these above equations, the energy-momentum relation should be

${\displaystyle E^{2}=-p^{2}c^{2}+m_{0}^{2}c^{4}}$

Substituting the Planck–Einstein relation E = ħω and de Broglie's p = ħk, we obtain the following dispersion relation

${\displaystyle \omega ^{2}=-k^{2}c^{2}+\omega _{\mathrm {p} }^{2}\,,\quad \left(E_{0}=\hbar \omega _{\mathrm {p} }=-m_{0}c^{2}>0\right)}$

when the wave consists of a stream of particles whose energy-momentum relation is ${\displaystyle E^{2}=-p^{2}c^{2}+m_{0}^{2}c^{4}}$ (wave–particle duality) and can be excited in a negative index metamaterial. The velocity of such a particle is equal to

${\displaystyle v=c{\sqrt {{\frac {E_{0}^{2}}{E^{2}}}-1}}=c{\sqrt {{\frac {\omega _{\mathrm {p} }^{2}}{\omega ^{2}}}-1}}}$

and range is from zero to infinity

${\displaystyle {\begin{aligned}{\frac {\omega _{\mathrm {p} }^{2}}{\omega ^{2}}}&<2 _="" amp="" c="" frac="" mathrm="" mbox="" omega="" p="" quad="" v="" when="">2\,,\quad {\mbox{when }}v>c\end{aligned}}}$

Moreover, the kinetic energy is also negative

${\displaystyle {\begin{aligned}E_{\mathrm {k} }&=E-E_{0}\\&=-mc^{2}-\left(-m_{0}c^{2}\right)\\&=-{\frac {m_{0}c^{2}}{\sqrt {1+{\frac {v^{2}}{c^{2}}}}}}+m_{0}c^{2}\\&=m_{0}c^{2}\left(1-{\frac {1}{\sqrt {1+{\frac {v^{2}}{c^{2}}}}}}\right)<0 aligned="" annotation="" end="" left="" m_="" mbox="" quad="" right="" where="">$

In fact, negative kinetic energy exists in some models^[34] to describe dark energy (phantom energy) whose pressure is negative. In this way, the negative mass of exotic matter is now associated with negative momentum, negative pressure, negative kinetic energy and faster-than-light phenomena.

John Archibald Wheeler

From Wikipedia, the free encyclopedia

John Archibald Wheeler
John Archibald Wheeler before the Hermann Weyl-Conference 1985 in Kiel, Germany
Born	July 9, 1911 Jacksonville, Florida, United States
Died	April 13, 2008 (aged 96) Hightstown, New Jersey, United States
Residence	United States
Nationality	American
Alma mater	Johns Hopkins University (Ph.D.)
Known for	Breit–Wheeler process Wheeler–DeWitt equation Popularizing the term "black hole" Nuclear fission Geometrodynamics General relativity Unified field theory Wheeler–Feynman absorber theory Wheeler's delayed choice experiment One-electron universe Geon S-matrix Quantum foam Coining the term "neutron moderator" Coining the term "superspace" Coining the term "wormhole" Lorentzian wormhole "It from bit" Participatory anthropic principle
Spouse(s)	Janette Hegner
Awards	Albert Einstein Award (1965) Enrico Fermi Award (1968) Franklin Medal (1969) National Medal of Science (1970) Oersted Medal (1983) J. Robert Oppenheimer Memorial Prize (1984) Albert Einstein Medal (1988) Matteucci Medal (1993) Wolf Prize in Physics (1997) Einstein Prize (APS) (2003)
Scientific career
Fields	Physics
Institutions	University of North Carolina Princeton University University of Texas at Austin
Thesis	Theory of the dispersion and absorption of helium (1933)
Doctoral advisor	Karl Herzfeld
Doctoral students	Jacob Bekenstein Claudio Bunster Demetrios Christodoulou Ignazio Ciufolini Hugh Everett Richard Feynman Kenneth W. Ford Robert Geroch John R. Klauder Bahram Mashhoon Charles Misner Milton Plesset Benjamin Schumacher Kip Thorne Jayme Tiomno Bill Unruh Robert Wald Katharine Way Arthur Wightman

John Archibald Wheeler (July 9, 1911 – April 13, 2008) was an American theoretical physicist. He was largely responsible for reviving interest in general relativity in the United States after World War II. Wheeler also worked with Niels Bohr in explaining the basic principles behind nuclear fission. Together with Gregory Breit, Wheeler developed the concept of the Breit–Wheeler process. He is best known for linking the term "black hole" to objects with gravitational collapse already predicted early in the 20th century, for coining the terms "quantum foam", "neutron moderator", "wormhole" and "it from bit", and for hypothesizing the "one-electron universe".

Wheeler earned his doctorate at Johns Hopkins University under the supervision of Karl Herzfeld, and studied under Breit and Bohr on a National Research Council fellowship. In 1939 he teamed up with Bohr to write a series of papers using the liquid drop model to explain the mechanism of fission. During World War II, he worked with the Manhattan Project's Metallurgical Laboratory in Chicago, where he helped design nuclear reactors, and then at the Hanford Site in Richland, Washington, where he helped DuPont build them. He returned to Princeton after the war ended, but returned to government service to help design and build the hydrogen bomb in the early 1950s.

For most of his career, Wheeler was a professor at Princeton University, which he joined in 1938, remaining until his retirement in 1976. At Princeton he supervised 46 PhDs, more than any other professor in the Princeton physics department.

Early life and education

Wheeler was born in Jacksonville, Florida on July 9, 1911 to librarians Joseph Lewis Wheeler and Mabel Archibald (Archie) Wheeler.^[1] He was the oldest of four children, having two younger brothers, Joseph and Robert, and a younger sister, Mary. Joseph earned a Ph.D. from Brown University and a Master of Library Science from Columbia University. Robert earned a Ph.D. in geology from Harvard University and worked as a geologist for oil companies and at colleges. Mary studied library science at the University of Denver and became a librarian.^[2] They grew up in Youngstown, Ohio, but spent a year in 1921 to 1922 on a farm in Benson, Vermont, where Wheeler attended a one-room school. After they returned to Youngstown he attended Rayen High School.^[3]

After graduating from the Baltimore City College high school in 1926,^[4] Wheeler entered Johns Hopkins University with a scholarship from the state of Maryland.^[5] He published his first scientific paper in 1930, as part of a summer job at the National Bureau of Standards.^[6] 

He earned his doctorate in 1933. His dissertation research work, carried out under the supervision of Karl Herzfeld, was on the "Theory of the Dispersion and Absorption of Helium".^[7] He received a National Research Council fellowship, which he used to study under Gregory Breit at New York University in 1933 and 1934,^[8] and then in Copenhagen under Niels Bohr in 1934 and 1935.^[9] In a 1934 paper, Breit and Wheeler introduced the Breit–Wheeler process, a mechanism by which photons can be potentially transformed into matter in the form of electron-positron pairs.^[5][10]


Early career

The University of North Carolina at Chapel Hill made Wheeler an associate professor in 1937, but he wanted to be able work more closely with the experts in particle physics.^[11] He turned down an offer in 1938 of an associate professorship at Johns Hopkins University in favor of an assistant professorship at Princeton University. Although it was a lesser position, he felt that Princeton, which was building up its physics department, was a better career choice.^[12] He remained a member of the faculty there until 1976.^[13]

In a 1937 paper "On the Mathematical Description of Light Nuclei by the Method of Resonating Group Structure", Wheeler introduced the S-matrix – short for scattering matrix – "a unitary matrix of coefficients connecting the asymptotic behavior of an arbitrary particular solution [of the integral equations] with that of solutions of a standard form."^[14][15] Werner Heisenberg subsequently developed the idea of the S-matrix in the 1940s. Due to the problematic divergences present in quantum field theory at that time, Heisenberg was motivated to isolate the essential features of the theory that would not be affected by future changes as the theory developed. In doing so he was led to introduce a unitary "characteristic" S-matrix, which became an important tool in particle physics.^[14]

Wheeler did not develop the S-matrix, but joined Edward Teller in examining Bohr's liquid drop model of the atomic nucleus.^[16] They presented their results at a meeting of the American Physical Society in New York in 1938. Wheeler's Chapel Hill graduate student Katharine Way also presented a paper, which she followed up in a subsequent article, detailing how the liquid drop model was unstable under certain conditions. Due to a limitation of the liquid drop model, they all missed the opportunity to predict nuclear fission.^[17][18] The news of Lise Meitner and Otto Frisch's discovery of fission was brought to America by Bohr in 1939. Bohr told Leon Rosenfeld, who informed Wheeler.^[12]
Bohr and Wheeler set to work applying the liquid drop model to explain the mechanism of nuclear fission.^[19] As the experimental physicists studied fission, they uncovered puzzling results. George Placzek asked Bohr why uranium seemed to fission with both very fast and very slow neutrons. Walking to a meeting with Wheeler, Bohr had an insight that the fission at low energies was due to the uranium-235 isotope, while at high energies it was mainly due to the far more abundant uranium-238 isotope.^[20] They co-wrote two more papers on fission.^[21][22] Their first paper appeared in Physical Review on September 1, 1939, the day Germany invaded Poland, starting World War II in Europe.^[23]

Considering the notion that positrons were electrons that were traveling backwards in time, he came up in 1940 with his one-electron universe postulate: that there was in fact only one electron, bouncing back and forth in time. His graduate student, Richard Feynman, found this hard to believe, but the idea that positrons were electrons traveling backwards in time intrigued him and Feynman incorporated the notion of the reversibility of time into his Feynman diagrams.^[24]


Nuclear weapons

Manhattan Project

Soon after the Japanese bombing of Pearl Harbor brought the United States into World War II, Wheeler accepted a request from Arthur Compton to join the Manhattan Project's Metallurgical Laboratory at the University of Chicago. He moved there in January 1942,^[23] joining Eugene Wigner's group, which was studying nuclear reactor design.^[25] He co-wrote a paper with Robert F. Christy on "Chain Reaction of Pure Fissionable Materials in Solution", which was important in the plutonium purification process.^[26] It would not be declassified until December 1955.^[27] He gave the neutron moderator its name, replacing the term "slower downer" used by Enrico Fermi.^[28][29]



Loading tubes of the Hanford B Reactor

After the United States Army Corps of Engineers took over the Manhattan Project, it gave responsibility for the detailed design and construction of the reactors to DuPont.^[30] Wheeler became part of the DuPont design staff.^[31] He worked closely with its engineers, commuting between Chicago and Wilmington, Delaware, where DuPont had its headquarters. He moved his family to Wilmington in March 1943.^[32] DuPont's task was not just to build nuclear reactors, but an entire plutonium production complex at the Hanford Site in Washington.^[33] As work progressed, Wheeler relocated his family again in July 1944, this time to Richland, Washington, where he worked in the scientific buildings known as the 300 area.^[26][32]

Even before the Hanford Site started up the B Reactor, the first of its three reactors, on September 15, 1944, Wheeler had been concerned that some nuclear fission products might turn out to be nuclear poisons, the accumulation of which would impede the ongoing nuclear chain reaction by absorbing many of the thermal neutrons that were needed to continue a chain reaction.^[34] In an April 1942 report, he predicted that this would reduce the reactivity by less than one percent so long as no fission product had a neutron capture cross section of more than 100,000 barns.^[35] After the reactor unexpectedly shut down, and then just as unexpectedly restarted about fifteen hours later, he suspected iodine-135, with a half life of 6.6 hours, and its daughter product, xenon-135, which has a half life of 9.2 hours. Xenon-135 turned out to have a neutron capture cross-section of well over 2 million barns. The problem was corrected by adding additional fuel rods to burn out the poison.^[36]

Wheeler had a personal reason for working on the Manhattan Project. His brother Joe, fighting in Italy, sent him a postcard with a simple message: "Hurry up".^[37] It was already too late: Joe was killed in October 1944. "Here we were," Wheeler later wrote, "so close to creating a nuclear weapon to end the war. I couldn't stop thinking then, and haven't stopped thinking since, that the war could have been over in October 1944."^[36] Joe left a widow and baby daughter, Mary Jo, who later married physicist James Hartle.^[38]


Hydrogen bomb

In August 1945 Wheeler and his family returned to Princeton, where he resumed his academic career.^[39] Working with Feynman, he explored the possibility of physics with particles, but not fields, and carried out theoretical studies of the muon with Jayme Tiomno,^[40] resulting in a series of papers on the topic,^[41][42] including a 1949 paper in which Tiomno and Wheeler introduced the "Tiomno Triangle", which related different forms of radioactive decay.^[43] He also suggested the use of muons as a nuclear probe. This paper, written and privately circulated in 1949 but not published until 1953,^[44] resulted in a series of measurements of the Chang radiation emitted by muons. Muons are a component of cosmic rays, and Wheeler became the founder and first director of Princeton's Cosmic Rays Laboratory, which received a substantial grant of $375,000 from the Office of Naval Research in 1948.^[45] He received a Guggenheim Fellowship in 1946,^[46] which allowed him to spend the 1949–50 academic year in Paris.^[47]



The "Sausage" device of Ivy Mike nuclear test on Enewetak Atoll. The Sausage was the first true hydrogen bomb ever tested.

The 1949 detonation of Joe-1 by the Soviet Union prompted an all-out effort by the United States, led by Teller, to develop the more powerful hydrogen bomb in response. Henry D. Smyth, Wheeler's department head at Princeton, asked him to join the effort. Most physicists were, like Wheeler, trying to re-establish careers interrupted by the war and were reluctant to face more disruption. Others had moral objections.^[48] Those who agreed to participate included Emil Konopinski, Marshall Rosenbluth, Lothar Nordheim and Charles Critchfield, but there was also now a body of experienced weapons physicists at the Los Alamos Laboratory, led by Norris Bradbury.^[49][50] Wheeler agreed to go to Los Alamos after a conversation with Bohr.^[48] Two of his graduate students from Princeton, Ken Ford and John Toll, joined him there.^[51]

At Los Alamos, Wheeler and his family moved into the house on "Bathtub Row" that had been occupied by Robert Oppenheimer and his family during the war.^[52] In 1950 there was no practical design for a hydrogen bomb. Calculations by Stan Ulam and others showed that Teller's "Classical Super" would not work. Teller and Wheeler created a new design known as "Alarm Clock", but it was not a true thermonuclear weapon. Not until January 1951 did Ulam come up with a workable design.^[53]

In 1951 Wheeler obtained permission from Bradbury to set up a branch office of the Los Alamos laboratory at Princeton, known as Project Matterhorn, which had two parts. Matterhorn S (for stellarator, another name coined by Wheeler), under Lyman Spitzer, investigated nuclear fusion as a power source. Matterhorn B (for bomb), under Wheeler, engaged in nuclear weapons research. Senior scientists remained uninterested and aloof from the project, so he staffed it with young graduate and post-doctoral students.^[54] In January 1953 he was involved in a security breach when he lost a highly classified paper on lithium-6 and the hydrogen bomb design during an overnight train trip. This resulted in Wheeler being given an official reprimand.^[55] Matterhorn B's efforts were crowned by the success of the Ivy Mike nuclear test at Enewetak Atoll in the Pacific, on November 1, 1953,^[56][54] which Wheeler witnessed. The yield of the Ivy Mike "Sausage" device was reckoned at 10.4 megatons of TNT (44 PJ), about 30 percent higher than Matterhorn B had estimated.^[57] Matterhorn B was discontinued, but Matterhorn S endures as the Princeton Plasma Physics Laboratory.^[54]


Later career in academia

After concluding his Matterhorn Project work, Wheeler resumed his academic career. In a 1955 paper, he theoretically investigated the geon, an electromagnetic or gravitational wave that is held together in a confined region by the attraction of its own field. He coined the name as a contraction of "gravitational electromagnetic entity."^[58] He found that the smallest geon was a toroid the size of the Sun, but millions of times heavier.^[59]


Geometrodynamics

During the 1950s Wheeler formulated geometrodynamics, a program of physical and ontological reduction of every physical phenomenon, such as gravitation and electromagnetism, to the geometrical properties of a curved space-time. His research on the subject was published in 1957 and 1961.^[60][61] Wheeler envisaged the fabric of the universe as a chaotic sub-atomic realm of quantum fluctuations, which he called "quantum foam".^[58][62]


General relativity

General relativity had been considered a less respectable field of physics, being detached from experiment. Wheeler was a key figure in the revival of the subject, leading the school at Princeton University, while Dennis William Sciama and Yakov Borisovich Zel'dovich developed the subject at Cambridge University and the University of Moscow, respectively. Wheeler and his students made substantial contributions to the field during the Golden Age of General Relativity.^[63]

While working on mathematical extensions to Einstein's Theory of General Relativity in 1957, Wheeler introduced the concept and word wormhole to describe hypothetical "tunnels" in space-time. Bohr asked if they were stable and further research by Wheeler determined that they are not.^[64][65] His work in general relativity included the theory of gravitational collapse. He used the term black hole in 1967 during a talk he gave at the NASA Goddard Institute of Space Studies (GISS).^[66]

Wheeler was also a pioneer in the field of quantum gravity due to his development, with Bryce DeWitt, of the Wheeler–DeWitt equation in 1967.^[67] Stephen Hawking later described Wheeler and DeWitt's work as the equation governing the "wave function of the Universe".^[68]


Quantum information

Wheeler left Princeton University in 1976 at the age of 65. He was appointed as the director of the Center for Theoretical Physics at the University of Texas at Austin in 1976 and remained in the position until 1986, when he retired^[13] and became a professor emeritus.^[69] Misner, Thorne and Wojciech Zurek, all former students of Wheeler, wrote that:


Looking back on Wheeler's 10 years at Texas, many quantum information scientists now regard him, along with IBM's Rolf Landauer, as a grandfather of their field. That, however, was not because Wheeler produced seminal research papers on quantum information. He did not—with one major exception, his delayed-choice experiment. Rather, his role was to inspire by asking deep questions from a radical conservative viewpoint and, through his questions, to stimulate others’ research and discovery.^[70]

Wheeler's delayed choice experiment is actually several thought experiments in quantum physics that he proposed, with the most prominent among them appearing in 1978 and 1984. These experiments are attempts to decide whether light somehow "senses" the experimental apparatus in the double-slit experiment it will travel through and adjusts its behavior to fit by assuming the appropriate determinate state for it, or whether light remains in an indeterminate state, neither wave nor particle, and responds to the "questions" asked of it by responding in either a wave-consistent manner or a particle-consistent manner depending on the experimental arrangements that ask these "questions".^[71]


Teaching

Wheeler's graduate students included Katharine Way, Richard Feynman, David Hill, Bei-Lok Hu, Kip Thorne, Jacob Bekenstein, John R. Klauder, William Unruh, Robert M. Wald, Arthur Wightman, Charles Misner and Hugh Everett.^[7][72] Wheeler gave a high priority to teaching, and continued to teach freshman and sophomore physics, saying that the young minds were the most important. With Kent Harrison, Kip Thorne and Masami Wakano, Wheeler wrote Gravitation Theory and Gravitational Collapse (1965). This led to the voluminous general relativity textbook Gravitation (1973), co-written with Misner and Thorne. Its timely appearance during the golden age of general relativity and its comprehensiveness made it an influential relativity textbook for a generation.^[73] Wheeler teamed up with Edwin F. Taylor to write Spacetime Physics (1966) and Scouting Black Holes (1996). At Princeton he supervised 46 PhDs, more than any other professor in the Princeton physics department.^[74]

Alluding to Wheeler's "mass without mass", the festschrift honoring his 60th birthday was titled Magic Without Magic: John Archibald Wheeler: A Collection of Essays in Honor of his Sixtieth Birthday (1972). His writing style could also attract parodies, including one by "John Archibald Wyler" that was affectionately published by a relativity journal.^[75][76]


Participatory Anthropic Principle

Wheeler speculated that reality is created by observers in the universe. "How does something arise from nothing?", he asked about the existence of space and time.^[77] He also coined the term "Participatory Anthropic Principle" (PAP), a version of a Strong Anthropic Principle.
In 1990, Wheeler suggested that information is fundamental to the physics of the universe. According to this "it from bit" doctrine, all things physical are information-theoretic in origin.


Wheeler: It from bit. Otherwise put, every it — every particle, every field of force, even the space-time continuum itself — derives its function, its meaning, its very existence entirely — even if in some contexts indirectly — from the apparatus-elicited answers to yes-or-no questions, binary choices, bits. It from bit symbolizes the idea that every item of the physical world has at bottom — a very deep bottom, in most instances — an immaterial source and explanation; that which we call reality arises in the last analysis from the posing of yes-no questions and the registering of equipment-evoked responses; in short, that all things physical are information-theoretic in origin and that this is a participatory universe.^[78]

In developing the Participatory Anthropic Principle (PAP), an interpretation of quantum mechanics, Wheeler used a variant on Twenty Questions, called Negative Twenty Questions, to show how the questions we choose to ask about the universe may dictate the answers we get. In this variant, the respondent does not choose or decide upon any particular or definite object beforehand, but only on a pattern of "Yes" or "No" answers. This variant requires the respondent to provide a consistent set of answers to successive questions, so that each answer can be viewed as logically compatible with all the previous answers. In this way, successive questions narrow the options until the questioner settles upon a definite object. Wheeler's theory was that, in an analogous manner, consciousness may play some role in bringing the universe into existence.^[79]

From a transcript of a radio interview on "The Anthropic Universe":


Wheeler: We are participators in bringing into being not only the near and here but the far away and long ago. We are in this sense, participators in bringing about something of the universe in the distant past and if we have one explanation for what's happening in the distant past why should we need more?
Martin Redfern: Many don't agree with John Wheeler, but if he's right then we and presumably other conscious observers throughout the universe, are the creators — or at least the minds that make the universe manifest.^[80]

Opposition to parapsychology

In 1979, Wheeler spoke to the American Association for the Advancement of Science (AAAS), asking it to expel parapsychology, which had been admitted ten years earlier at the request of Margaret Mead. He called it a pseudoscience,^[81] saying he did not oppose earnest research into the questions, but he thought the "air of legitimacy" of being an AAAS-Affiliate should be reserved until convincing tests of at least a few so-called psi effects could be demonstrated.^[82] 

In the question and answer period following his presentation "Not consciousness, but the distinction between the probe and the probed, as central to the elemental quantum act of observation", Wheeler incorrectly stated that J. B. Rhine had committed fraud as a student, for which he apologized in a subsequent letter to the journal Science.^[83] His request was turned down and the Parapsychological Association remained a member of the AAAS.^[82]


Personal life

For 72 years, Wheeler was married to Janette Hegner, a teacher and social worker. They became engaged on their third date, but agreed to defer marriage until after he returned from Europe. They were married on June 10, 1935, five days after his return.^[84] Jobs were hard to come by during the Great Depression, but Arthur Ruark offered Wheeler a position as an assistant professor at the University of North Carolina at Chapel Hill, at an annual salary of $2,300, which was less than the $2,400 Janette was offered to teach at the Rye Country Day School.^[85][12] They had three children: Letitia, James English and Alison Wheeler.^[13]

Wheeler and Hegner were founding members of the Unitarian Church of Princeton, and she initiated the Friends of the Princeton Public Library.^[86] In their later years, Hegner accompanied him on sabbaticals in France, Los Alamos, New Mexico, the Netherlands, and Japan.^[86] Hegner died in October 2007 at the age of 99.^[87][88]


Death and legacy

Wheeler was influential in mentoring a generation of physicists of the Golden Age of General Relativity, who made notable contributions to quantum mechanics and gravitation.

Wheeler won numerous prizes and awards, including the Enrico Fermi Award in 1968, the Franklin Medal in 1969, the Einstein Prize in 1969, the National Medal of Science in 1971, the Niels Bohr International Gold Medal in 1982, the Oersted Medal in 1983, the J. Robert Oppenheimer Memorial Prize in 1984 and the Wolf Foundation Prize in 1997.^[69] He was a member of the American Philosophical Society, the Royal Academy, the Accademia Nazionale dei Lincei, and the Century Association. He received honorary degrees from 18 different institutions. In 2001, Princeton used a $3 million gift to establish the John Archibald Wheeler/Battelle Professorship in Physics.^[13] After his death, the University of Texas named the John A. Wheeler Lecture Hall in his honor.^[69] 

On April 13, 2008, Wheeler died of pneumonia at the age of 96 in Hightstown, New Jersey.^[89]


Bibliography

• Wheeler, John Archibald (1962). Geometrodynamics. New York: Academic Press. OCLC 1317194.
• Misner, Charles W.; Kip S. Thorne; John Archibald Wheeler (September 1973). Gravitation. San Francisco: W. H. Freeman. ISBN 0-7167-0344-0.
• Wheeler, John Archibald (1979). Some Men and Moments in the History of Nuclear Physics: The Interplay of Colleagues and Motivations. Minneapolis: University of Minnesota Press. OCLC 6025422.
• Wheeler, John Archibald (1990). A Journey Into Gravity and Spacetime. -Scientific American Library. New York: W.H. Freeman. ISBN 0-7167-6034-7.
• Taylor, Edwin F.; Wheeler, John Archibald (1992). Spacetime Physics: Introduction to Special Relativity'. New York: W. H. Freeman. ISBN 0-7167-2327-1.
• Wheeler, John Archibald (1994). At Home in the Universe. New York: American Institute of Physics. ISBN 1-56396-500-3.
• Ciufolini, Ignazio; Wheeler, John Archibald (1995). Gravitation and Inertia. Princeton, New Jersey: Princeton University Press. ISBN 0-691-03323-4.
• Wheeler, John Archibald (1998). Geons, Black Holes, and Quantum Foam: A Life in Physics. New York: W.W. Norton & Co. ISBN 0-393-04642-7.
• Taylor, Edwin F.; Wheeler, John Archibald (2000). Exploring Black Holes: Introduction to General Relativity. Addison Wesley. ISBN 0-201-38423-X.