The State of Physics in the 900 '

      At the beginning of the twentieth century, physics was in full swing. After the invention of the spectroscope made by Fraunhofer (1814) and the discovery of absorption lines, dark lines, visible both in the spectra of sunlight and stellar light and in the light reflected from the surface of the Moon and some other brighter planets, the studies by the Kirchhoffs and Bunsen quickly led to the discovery and study of emission lines in the light spectra of chemicals (Fig. 1).

  Fig. 1

 

The correspondence of some of the visible emission lines with the absorption ones, as in the case of the fundamental H-alpha line of hydrogen (red line in Fig. 5), has led to a qualitative leap in the study of the cosmos, opening the way to analyze the composition of stellar atmospheres: it was now clear that nature, so jealous in keeping the secrets of matter, could hide them for a while longer.

The discovery and study of natural radioactivity (1896) by H. Becquerel and Pierre and Marie Curie and the following year that of J Thomson's electron, soon led to the conception of new models of matter formed no longer by elementary atoms, therefore indivisible, but by atoms with an internal structure made of positive and negative particles, a model that of the atom that evolved rapidly after a few years, becoming the current Bohr atomic model -Shrœdinger (Fig. 2).

Fig. 2 

Following the studies of J.K. Maxwell (1864) on electromagnetism and those on the black body spectrum by Kirchhoff (1862) reinterpreted in a quantum key by Max Planck, in 1900 the contrast between a wave description and a corpuscular description of light began to emerge, a dichotomy destined to grow more and more until the world view changes.

Indeed, two phenomena continued to resist the wave interpretation of light. In addition to failing to explain the black body spectrum in any way, electromagnetism could not even explain the strange phenomenon of some metals that emit negative electrical charges when illuminated. The photoelectric effect highlighted in 1887 by Hertz and studied by Lenard, led to insuperable inconsistencies with the electromagnetic theory.

Electromagnetic waves or how many of light?

The Maxwellian vision of the world, in addition to providing in a unified electromagnetic key an accurate explanation of all the electrical and magnetic phenomena hitherto seen as separate, described visible and invisible light with a pair of reciprocally induced oscillating electric and magnetic waves (Fig. 3) synchronous at a certain frequency with a propagation speed in vacuum of about 300,000 kilometers per second.

 

Fig. 3

This evidence in itself upsets the eighteenth-century vision of wave propagation, in fact this required the presence of a material medium, not of the vacuum which, being empty by definition, could in no way be considered a medium in which a wave could propagate. In any case, neglecting this dilemma only briefly, it must be said that in Maxwell's theory the transport of energy and momentum can occur continuously, that is, any value of energy and momentum can be emitted by matter, transported into space by the point A to point B and absorbed by other matter.

Fig- 4 Black body spectrum. The intensity corresponds to the number of quanta emitted at a given frequency value. The wavelength is inversely proportional to the frequency and corresponds to the distance between two maxima of the wave.

The explanation of the black body spectrum proposed by Planck (Fig. 4), on the other hand, required to accept for no apparent reason that the exchanges of energy and momentum between electromagnetic radiation and matter in equilibrium at a certain kelvin temperature, took place in a discrete way, therefore not for any value of energy as Maxwell's theory would have it, but for multiples of the values ​​of energy and momentum proportional to the frequency of the wave. In the electromagnetic theory, on the other hand, the energy transported by the wave and transferred to matter is necessarily proportional to the square of the electric and magnetic field of the wave itself, therefore it depends on the square of the wave intensity and not on the frequency as foreseen by the Planck's hypothesis. In any case, despite these theoretical inconsistencies, electromagnetism allowed Guglielmo Marconi to develop radio, while quantum theory, in addition to the explanation of the black body spectrum, allowed Einstein to explain the photoelectric effect, to Bohr and Sommerfeld to formulate an atomic model capable of predicting the energy and wavelength of all the lines observed in the emission and absorption spectra (Fig. 5), all fundamental steps to arrive at the hypothesis of wave-particle dualism proposed by Luis de Broglie which led to interpretation of Copenhagen in 1955, interpretation of Quantum Mechanics still accredited today but not the only one.

Fig. 5 Emission spectra of hydrogen and iron. The lines are due to the atomic de-excitation produced by the passage of an electron from a more energetic atomic orbit to a less energetic one. The different colors are associated with different frequencies. The energy and frequency grows from red to purple.

Of considerable interest in this regard is the quantum interpretation of de Broglie-Bohm which, due to its properties of non-localities verified in entanglement, opens up interesting developments in reference to the EPR paradox and to the Bell theorem violated in many experiments also recently, in contradiction with the local relativistic realism typical of Einstein's relativistic mechanics which would like that two systems, even very distant from each other, could not communicate instantly.

Empty or full? Space-time only 

Let's go back to the problem of the propagation of electromagnetic waves in a vacuum. Attempts to fill empty space with a material medium called Ether, through which electromagnetic waves could actually propagate just like a mechanical wave in a medium, failed, the last, most important experiment conducted by Michelson and Morley did not go to successful but failed not in vain. In fact, there were those who interpreted the proof of the non-existence of the Aether as a valid result and not as a failure. Albert Einstein was one of them. The experiment, in fact, had the great merit of demonstrating unequivocally that the Ether is not a medium that fills empty space but is the space-time itself in which light propagates. In the case of relative motion between source and observer, space-time undergoes a deformation based on their relative speed, allowing light to always have the same speed for all observers. However, space-time not only has this property, in fact it also deforms in the presence of energy, curving in such a way that a trajectory along a geodesic becomes curved because it follows the curvature of space-time. To give a simple example, if we throw a stone in a certain direction with a given speed, in the absence of gravity and other forces, it will continue its rectilinear motion indefinitely at a constant speed. If we throw the same stone in the same direction as before and with the same speed starting from the earth's surface, where the mass energy of the Earth curves the local space-time, it will move along a parabolic trajectory until it touches the ground again in another place, not because it is attracted, but because it follows the geodesic of space-time curved by the effect of the mass energy of the Earth. In this sense, the force of gravity is not a real force as in Newton's universal gravitation, but only the manifestation of a local curvature of the web of space-time that not even light escapes.

 

Fig. 6. Two-dimensional model of the curvature of space-time in the presence of mass energy.

When a ray of light passes near a massive body such as a star or a galaxy, instead of continuing in a straight line it travels a curved trajectory not because it is gravitationally attracted to the mass but because it follows the geodesic curves of space-time, the numerous gravitational lens images that distort or multiply images of distant galaxies (fig. 7-8).

Fig. 7. Distorted image of a galaxy due to a gravitational lensing effect. (Credit: NASA / ESA / STScl)

 

 

 

Fig. 8. The Einstein cross of the quasar G2237 +0305, visible in the four peripheral luminous points, produced by the gravitational lensing effect of the galaxy ZW 2237 +030 (center) (Credit: NASA / ESA / STScl)

Now some problems remain. To unify all forces, gravity must also be a quantized field like the electroweak forces and how they can be traced back to the electromagnetic field. So, can gravity be a quantized field? Is the graviton really the mediator of gravitational force? How is it possible that general relativity is in agreement with quantum gravity? Einstein didn't believe it at all.