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Joseph Weber's Contribution to Gravitational Waves and Neutrinos Firenze University Press www.fupress.com/substantia Historical Article New Astronomical Observations: Joseph Weber’s Contribution to Gravitational Waves Citation: S. Gottardo (2017) New Astronomical Observations: Joseph and Neutrinos Detection Weber’s Contribution to Gravitational Waves and Neutrinos Detection. Sub- stantia 1(1): 61-67. doi: 10.13128/Sub- stantia-13 Stefano Gottardo Copyright: © 2017 S. Gottardo.This European Laboratory for Nonlinear Spectroscopy (LENS), 50019 Sesto Fiorentino (Flor- is an open access, peer-reviewed arti- ence), Italy cle published by Firenze University E-mail: [email protected] Press (http://www.fupress.com/substan- tia) and distribuited under distributed under the terms of the Creative Com- Abstract. Joseph Weber, form Maryland University, was a pioneer in the experimen- mons Attribution License, which per- tal research of gravitational waves and neutrinos. Today these two techniques are very mits unrestricted use, distribution, and promising for astronomical observation, since will allow to observe astrophysical phe- reproduction in any medium, provided nomena under a new light. We review here almost 30 years of Weber’s career spent the original author and source are on gravity waves and neutrinos; Weber’s experimental results were strongly criticized credited. by the international community, but his research, despite critics, boosted the brand Data Availability Statement: All rel- new (in mid-sixties of last century) research field of gravity waves to become one of evant data are within the paper and its the most important in XXI century. On neutrino side, he found an unorthodox way Supporting Information files. to reduce the size of detectors typically huge and he claimed to observe neutrinos flux with a small pure crystal of sapphire. Competing Interests: The author declares no competing interests. Keywords. Gravitational wave, Neutrino, Joseph Weber, Bar detector, Torsion balance. INTRODUCTION The astronomic observations will grow rich, in the next few years, two new methods of investigation. Today the sky is observed and measured almost exclusively by electromagnetic radiation. Until 1950 the available radiation was only the visible or near-infrared. Then in the second half of the last cen- tury, thanks to enormous technological advances, we added X-ray radiation, microwaves, radio-waves that almost complete the electromagnetic spectrum. These frequencies allowed to discover objects like pulsars, quasars, neutron stars and cosmic background radiation. Many things are still hidden to elec- tromagnetic radiation. An example is the photons (the quanta of electromag- netic field) that come from the sun. The earth is illuminated by a “old” radi- ation, about 100,000 years old. The photons are created in the center of the sun, but employ about 100,000 years to arrive on Earth surface. The reason is the very high temperature inside our star. Matter is not what we know at such temperature. The core of the sun has a density 150 times larger than water and a temperature of 1,5x107 °C. The core is formed by a plasma of ions and electrons, which traps the light. Photons cannot escape from the core, except after a long time, this is because the plasma of ions and electrons is opaque to Substantia. An International Journal of the History of Chemistry 1(1): 61-67, 2017 DOI: 10.13128/Substantia-13 62 Stefano Gottardo electromagnetic radiation (the scattering cross section of from black hole, neither light. But an exception are gravi- photons with plasma is much higher with respects to the tational waves that can be observed during the merging ordinary matter, where nuclei and electrons form atoms). of black holes binary systems. One type of particles, however, manages to escape quick- Gravitational waves were predicted in 1916 by Ein- ly from the solar core and to get on the Earth after only 8 stein, by finding that they are the carrier of the energy minutes, with a velocity very close to the speed of light. of the gravitational field, as electromagnetic waves trans- These particles are neutrinos. Hypothesized by Austrian port the energy of electromagnetic field. In 1915 Einstein physicist Wolfgang Pauli and structured theoretically developed, after seven years, the theory of general relativ- by Enrico Fermi in the 1920s, neutrinos are elusive par- ity2 that fixed a lot of paradoxes present in the old New- ticles that do not interact electromagnetically, but only tonian gravitational theory (see Ref. 3 for an introduction through the weak interaction. They are produced during to general relativity). The gravity force in general relativ- the nuclear fusion process that occurs in the sun and tell ity is due to the curvature of the spacetime that is gener- us the types of nuclear reaction that is occurring inside ated by masses (as in Newtonian theory) but also by any our star. The typical decay that involves neutrino is the so form of energy and momentum. Einstein field equation called β-decay, where a free neutron n decays into a pro- in tensorial notation has a simple form of: - 1 ton p, an electron e and an electronic antineutrino νe . 8πG Gµν = 4 Tµν (1) − c n→ p+e +νe where G is the gravitational constant, c the speed of light Neutrinos can also reveal hidden features in superno- in vacuum. A part from the constants, the equation tells vae explosions. The huge flow of neutrinos will invest the that Gμv called the Einstein tensor that contains the spa- Earth when a supernova explodes. The neutrino detectors cetime curvature, is equal to stress-energy tensor Tμv. In on the Earth will alert telescopes and radio telescopes other words, the stress-energy tensor modifies the space- on supernova position. In fact, the neutrinos emitted by time form flat (Tμv = 0) to curved (Tμv ≠ 0). The indexes the explosion will travel undisturbed towards us, while μ,v = 0,1,2,3 (the spacetime has 4 dimensions, 3 space the electromagnetic radiation will need a bit of time, as type, 1 time type and the index 0 is usually the time com- it will encounter in its path the hot plasma of ions and ponent), so the eq. 1 are in practice 16 equations. It is electrons. There is a worldwide alert on neutrinos called interesting to see how gravitational waves emerge from SNEWS (supernova Early Warning System), active since Einstein field equation (1), to estimate the order of mag- 2005 with the participation of seven detectors around the nitude of such space time ripples that can be detected on world including the two Italian LVD and BOREXINO. the Earth. The Einstein tensor depends in a complicated Neutrino detection can give very precious information way by the spacetime metric gμv If the space is flat, called on neutron stars’ structure and on the merger process the Euclidian space, the spacetime metric is identified by between two neutron stars in a binary system. the tensor ημv that is: The very new second method of astronomical inves- ⎛ −1 0 0 0 ⎞ tigation is the detection of gravitational waves. Accord- ⎜ ⎟ 0 1 0 0 ing to the present theories, such types of waves are emit- η =⎜ ⎟ (2) ted by nearly all astrophysical objects and the most vio- µν ⎜ 0 0 1 0 ⎟ ⎜ ⎟ lent ones give off gravity radiations in copious amounts. ⎝ 0 0 0 1 ⎠ Supernova explosion observed via gravitational waves can reveal how the star collapse is going on, what happens We can suppose for the moment that extreme astro- to star core and how the final explosion takes place. The physical events, like binary black holes merging, happen internal part of supernova will be accessible only to grav- very distant form our observation point, and that we itational waves or neutrinos. Another violent astrophysi- are in a place where the stress-energy tensor Tμv = 0 (no cal event is the black holes merging. Two black holes, gravity at all). The flat space will become nearly flat when orbiting one on each other (binary system), release gravi- spacetime ripples, caused by some event, will hit our tational waves when they become more and more close detector. In this weak field hypothesis, the metric tensor and at the end an enormous amount of gravity radiation gμv becomes gμv = ημv + hμv, where hμv is the correction to will be emitted when they will merge into a more massive flat space and we can consider |hμv|<<1. With the latter black hole. Black holes are the only massive astrophysical two hypothesis Einstein field equation eq. (1) becomes: object that cannot be observed directly with electromag- 2 ∂ hµν 16πG netic waves detectors or neutrinos. Nothing can escape 2 = − 4 Tµν = 0 (3) ∂ xµ c New Astronomical Observations: Joseph Weber’s Contribution to Gravitational Waves and Neutrinos Detection 63 Eq. (3) are now 4 equations since only the index v cist, so he decided to design a detector for gravitational survived the index contraction. Eq (3) can be expressed waves. After years of study to understand the best way in a more familiar way: to measure gravitational waves, he decided to use a bar detector, a resonant mass detector that responds to inci- 7 2 dent gravitational waves by vibrating. The detector was ⎛ ∂ 2 ⎞ (4) a simple aluminum cylinder, 2 m long and with diam- ⎜− 2 +∇ ⎟hµν = 0 ⎝ ∂ t ⎠ eter of 96 cm. Gravitational waves, ripples of spacetime, would compress and then tend the bar. Weber chose the that is the standard wave equation. The solution of Eq. size of the bar to reveal the gravitational wave frequen- (4) is: cy of about 1600 Hz. He based the choice on very rough estimate. In the early 1960s a clear picture of which astro- Aµν exp(iωt +ik1x +ik2 y +ik3z) (5) nomical events could emit gravitational waves around 1 kHz was not clear.
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