For centuries, scientists have sought to understand the nature of light -
© Christophe Daussy / Université Sorbonne Paris Nord
A better understanding of our world has arisen from the study of light, according to a study published by our partner The Conversation.
Research is still ongoing and should one day make it possible to formulate a quantum and relativistic theory of gravity.
The analysis of this evolution was carried out by Christophe Daussy, physicist, teacher-researcher at the Laboratory of Lasers Physics, CNRS, Sorbonne Paris Nord University - USPC.
Technologies based on knowledge and mastery of light have become essential: screens, telecommunications, imaging or laser and its many applications.
In the field of research, optics and photonics have provided in recent decades high sensitivity instruments which have enabled, for example, the detection of a black hole in the center of our galaxy (thanks to adaptive optics), detection of gravitational waves (thanks to giant interferometers) or even the creation of the most stable clocks in the world.
Beyond these remarkable successes, work on light - or with light - has marked our representations of the physical world for centuries.
We will focus here on showing the close links that exist between the discoveries in this field of research and the emergence in the twentieth century of theories that have turned our understanding of light upside down, but also of space, time and matter. .
Light, particle or wave?
The story that interests us begins in the 17th century.
The quantitative study of the deviation of the rays of light then allows Snell and Descartes to establish the laws of geometrical optics.
Modeling this phenomenon leads scientists to take a position on two fundamental questions: what is the nature of light and at what speed does it propagate?
These two problems will be debated for more than two centuries and will lead physicists towards two revolutions: the theory of relativity and the quantum theory.
In the 17th century, for Descartes and Newton, light was corpuscular in nature and it propagated faster in glass than in air.
For his part, Huygens developed a wave theory which led him to consider that light propagates more slowly in glass than in air.
So wave or particle?
The overwhelming success of Newtonian mechanics will totally eclipse the wave model for over a century.
At the beginning of the 19th century, the light interference experiments carried out by Young put Newton's particle theory at fault.
The debate then seems definitively settled when in 1850, Fizeau and Foucault experimentally show that the speed of light is lower in water than in air.
Huygens was therefore right: light is indeed a wave!
Our understanding of the nature of this wave then owes a lot to the work of Maxwell who associates electrical, magnetic and optical phenomena and shows that light is an electromagnetic wave.
Better, its speed in a vacuum must be invariant: it is always
equal to
c
, whatever the speed of the source or of the observer.
And here things get complicated!
Indeed this invariance of the speed of light is incompatible with a central law in Newtonian mechanics, the Galilean law of composition of speeds.
Despite this black spot and another unresolved problem, calculating the amount of light emitted by thermal radiation (the black body problem), many scientists at the end of the 19th century considered our representation of the physical world to be fully accomplished. and physics practically finished.
However, the answers which will be brought to these two problems will be the bases of a true revolution.
Relativity
The solution which will make it possible to reconcile electromagnetism and mechanics is provided in 1905 by Einstein.
Galilean invariance gives way to relativistic invariance to give rise to special relativity.
This new theory will upset our vision of space, time, and matter.
It
is no longer simply the speed of all electromagnetic waves in a vacuum, it becomes an insurmountable limit speed.
It will acquire in 1983 the status of fundamental constant of reference when it is fixed to redefine the unit of length of the new International System.
The existence of such a limit makes it necessary to review the notion of simultaneity of two events: It depends on the movement of the observer.
As a consequence, space and time are no longer absolute and must be rethought in terms of a new concept, space-time.
Lengths and durations are no longer absolute.
Rules contract and clocks slow down when in motion!
The concept of mass must also be redefined in the light of the famous relation
E
=
mc
2 which poses the equivalence between mass and energy.
The birth of the concept of photon
In 1900, the solution to the problem of the black body was provided by Planck by means of a disconcerting hypothesis: light is emitted discontinuously by grains of energy, the quanta.
In the process, Einstein tackles the photoelectric effect.
Based on the work of Planck, he interprets this effect by considering that light is composed of grains of energy
hν
(
h
is Planck's constant and
ν
the frequency).
It is the birth of the concept of photon and with it a new page of the debate on the nature of light.
How to reconcile the electromagnetic waves of Maxwell and the photons of Einstein?
The solution to this question finds its roots in the 17th century.
Fermat formulates the principle of "minimum time" which allows him to find the laws of geometrical optics.
This principle is then extended to mechanics by Mauperthuis to give the principle of “least action”: The trajectory of a material point is always that which minimizes the action, a magnitude obtained by multiplying the quantity of movement and the distance traveled.
We can see in these two principles a form of similarity between the treatment of light and that of matter, a similarity which will be formalized by Hamilton in a new writing of the laws of mechanics.
It will then be necessary to await the work of Planck and the introduction of the quantum of action
h
, of Einstein which quantifies the light, so that in 1924 de Broglie brings the unexpected solution which will definitively reconcile wave and photon: the light behaves sometimes like a wave, sometimes like corpuscles, everything depends on the conditions of the experience.
We speak of wave-particle duality.
Even more confusing, he generalizes this duality to matter: we must therefore be able to transpose to electrons and atoms, the experiments of wave optics.
It is on this basis that a new mechanics will be developed, quantum mechanics of which Schrödinger establishes the central equation, the wave equation for a particle.
Our "Physics" file
Quantum mechanics
This new theoretical framework in turn upsets our representation of matter and interactions.
The notion of trajectory of a particle must be rethought in light of the probability amplitude of the wave function whose squared modulus represents the probability of finding the particle at a point in space.
Classical deterministic predictability then leaves room for quantum probabilistic predictability.
Finally the conditions of the measurement, the observer, must be integrated into the description of the quantum system.
In this context,
h
acquires the status of fundamental constant to become the “action quantum” which translates the discontinuity of interactions and represents the lower limit of any action.
In 2019,
h
even becomes a fixed reference constant to define the unit of mass.
Taking into account the constants
h
and
c
in the same theoretical framework
will then lead, during the second half of the twentieth century, to the unification of relativistic and quantum theories to lead to theories of electromagnetic interaction, of interaction weak and strong interaction.
The Standard Model brings together these three theories and today defines our representation of elementary particle physics and interactions.
At the ultimate level of this unifying approach, the taking into account of the gravitational constant
G
should make it possible to formulate a quantum and relativistic theory of gravitation.
This theory, which still remains to be constructed to this day, will in turn shake up our representation of the world.
Science
The "cosmic voids", these immense regions between galaxies, would conceal dark energy
Science
Why Sherlock Holmes would have been (much) less great in the 1960s
This analysis was written by Christophe Daussy, physicist, teacher-researcher at the Laboratory of Lasers Physics, CNRS, Sorbonne Paris Nord University - USPC. The original article was published on The Conversation website.
Physical
Albert Einstein
Science
Video
History
Research