A brief history of time (Edition 1996 ) Stephen HAWKING,

after years of research in mathematics and cosmology, Stephen HAWKING has decided to present the state of knowledge of cosmology and physics. His book became, upon its release in 1988, a best-seller. Surprising for a work on themes, a priori, reserved for scientific elites.

Hawking wrote about it: « I sells more books on physics than Madonna on sex! « He explains this phenomenon by the renewed interest of the readers for questions about our origins, on the functioning of the universe or the place of God. HAWKING is recognized as one of the most brilliant physicists of his generation. It is especially great theorist of black holes.

 To begin with, a little reminder to the standard image atom model:

Theoretical model the atom


The historical universe

Since antiquity, philosophers thought they could explain, definitively, how our universe woks. ARISTOTLE (-340) and Ptolemy saw the universe as « finite » and placed the Earth at the center and the man at the center of the earth. This vision of « geocentric » and finished our universe perfectly agreed with the Christian church who could not imagine God’s creature other than the center of everything.

COPERNICUS (1514), the first (in fact there was a previous, little known in Greece: Aristarchus of Samos (-280)), risked his life puting the sun in the center (Heliocentrism).

NEWTON (1687), the first true physicist, described the force (gravity) that governs the movement of the star but also explains the apples fall.


At that time, the question of the infinite (already put on the table by Giordano Bruno (1548)) was discussed : if the universe is infinite, there are an infinity of stars. Then why the noght night is not illuminated? The question of the infinite also covers the time: is there a beginning ? And if that was the case, what was there before ? The only answers were brought by metaphysic : according to the Bible ( Genesis ): there was God, the first cause  (Aristotle).

Already SAINT-AUGUSTIN (354), during his short life, had noticed the progress of humanity: new tools are invented, sails were improving. His conclusion was obvious: « if there is improvemen, all did not always exist and, therefore, there was a beginning » (QED).

Kant (1781) saw things differently, starting from the principle of causality (already stated by Aristotle), which states that « nothing happens without a reason » :

  1. if the world had a beginning. The principle of causality is then not respected because we have a first event (the world) not caused.
  2. if the world has no beginning. Then there is always a previuos cause, till infinity : for Kant it was absurd.

In short, Kant did not advance the debate.

In 1929 Hubble demonstrated that all the stars and all the galaxies are moving away from the earth and what is more, more quickly they are more distant. The universe is expanding like the sparks in a firework. But to conclude that in the distant past, all this material was concentrated on one point, there was short

Doppler effect

Aside from the author of the blog : How HUBBLE diescovers the flight of distant galaxies? Imagine an ambulance, siren screaming. Even with eyes wide shut, you know, thanks to changes in the sound of a siren, if the ambulance goes away or closer. How you do so ? Sound is a wave that travels through the air, like a wave traveling on the surface of the sea. When the siren is approaching, it « cup » the airwaves. The sound is more acute. Conclusions ; the ambulance is approaching. Hubble used the same method : he uses the feature of light waves that tend to turn red when light objects recede.

The twentieth century was the century of the great advances. To name a few :

  • EINSTEIN who with his theory of relativity, described in 1906 the laws thar govern the « great masses » : the stars and galaxies and
  • Max Planck who was interested, instead, in small items (atomic and subatomic) through his theory of quanta.

We had two antagonistic ways of explaining the universe. The challenge faced then was to find a theory of everything, a theory capable of explaining the laws of galaxies but also that of light and ants running around us …


Space and time

Aristotle thought that objects had a natural tendency to rest. Their « natural »position was fixed (in a hole for a stone) and their speed nul.

GALILEO and NEWTON question this theory (which was still the church therory) : all depends on the observer ! Indeed, from the moon, for example, or from a train, a stone, even in its hole, is in motion (relative to the locomotive). This was the first great vexation of humanity, according to Freud, (there will be many others): earth can not be considered as a particular point, a privileged place in the universe and it is not still, for example for a Martian.

In 1865 , Maxwell showed that light has a speed. But a velocity relative to what? To avoid the trap, we invented the concept of the ether: a kind of fluid, of force that fullfill Universe, such a force that have also described Obiwan KENOBI, a force absolutely still. Any movement could then be described with respect to this reference.

In 1887, Albert Michelson showed that the speed of light is fixed regardless of the speed of the observer: even for Usain BOLT : when he runs behind a ray of light, he always seen that this ray has the same speed 300 000 km per second! Exit the ether!

space-time distortion

In 1905 , Einstein gave him the coup de grace. After dismissing the idea of a fixed place, he dared to advance the idea of the lack of an absolute time! Render unto Caesar what belongs to Caesar, it was Poincaré (the president’s brother) who was the first to adance this theory. In short, a twin traveling in a high-speed rocket would age slower than his brother remained on earth!

Time and space were finally linked. For Einstein, gravity of Newton is not a force. It’s a distortion of space-time: Imagine a ball placed on a trampoline. The canvas is « distorted » by the mass of the ball. Another ballis put on the canvas. It falls ont ther first ball as if ab strength attracted her. Our sun does the same. It distorts, by its mass, space-time. Earth, located in the warped space-time, seems attracted by the sun. It was (it seems to me) the first example of a theory explained a posteriori by the practice: an expedition was launched to observe the night sky during a solar eclipse. Photos were taken and it was found that in the periphery of the eclipsed sun, some stars were not in their place: their light was deflected by the mass of the sun. Kant, in the critique of pure reason, had asked: « synthetic a priori judgments are they possible? « Einstein proved that yes.


The expanding universe

In 1929 , Hubble showed that the universe was (and still is) growing.

In 1922, Friedman had already predicted theoretically that expansion. It is ironic that Einstein (as Hoyle and others) was still firmly opposed to this principle. FRIEDMAN deduced that:

  • the universe was between 10 and 15 billion years;
  • Originally, it was concentrated in a single point of zero dimension, and therefore infinite density (all the matter in the universe in a handkerchief). Such a point is called a mathematical singularity ;
  • all started with a « BIG BANG » (the name given by critics of the theory), a first explosion.

In 1965, two experts (Penzias and Wilson) from BELL measure a fossil radiation. This radiation was uniform in all directions : it was a fossil radiaton ; a radiation that had been emmited by the first explosion; Friedman’s theory was confirmed.

In 1965 again, PENROSE (an English mathematician friend of Hawking) put in eqautions the singularity of FRIEDMAN. This singularity is a point of zero dimension and infinite density. It is so dense that not even light can escape. It is black. It will be called black hole. At this time our hero (HAWKING) married Jane Wilde. He also discovered his illness that leaves him less than two years of life expectancy.

In 1970, the Big Bang theory was accepted. But to be consistent with the obesrvation, it requires more matter in Universe. A secobnd problem arised. It lacks to explian the so-called  singularity : all laws of physics, including relativity, collapsed across the border of a black hole or before the Big Bang. It is as if God (HAWKING makes a lot of reference to God) hid to the man what happens before or beyond


 The uncertainty principle (or rather indeterminacy)

In the nineteenth century, Laplace said that if we knew all the laws of physics, the position and velocity of all the elements, we would be able to determine the future, including human behavior : an observer  noting the speed of a ball can, without much error, knows his future position.

In 1900, Max Planck showed that light (for example) is both:

  • a particle (a photon massless and speed of 300 000 km per second);
  • a wave (like a radio wave).

It shows that energy can not be absorbed by « package » or quanta. A particle (an atom, electron, neutron, …) may not have any energy level. Planck inaugurates the construction of a new mechanical, one suited to particle physics: quantum mechanics.

Aside from the author of the blog: the atom is the elementary particle of matter. It consists of neutrons (zero charge) and protons (positively charged) that form the core. Around this nucleus revolve electrons (negative charge). They are the same quantity as the proton which gives a total load atom zero.

1926 a great date (for me the greatest). Werner Heisenberg (see my article « Has it proved the existence of God? ») showed that a particle can not have both a fixed location and a fixed speed regardless of the measurement method. It boggles the mind ! But we must integrate once and for all, that in quantum mechanics, this type of situation is common. For example, we now know that a particle can be in two places at once! But back to Heisenberg : principle (also valid for fields) for the proposition that for a particle having a specific speed and precise position is meaningless in nature. If we make the analogy with a car we can say:

  • it has a  speed of  100 km per hour;
  • whether it is at New-York;

but not both at once.


The forces of nature

Aside from the author of the blog: to understand what follows, we must introduce two or three concepts. It was long believed that the atom (Greek for « that one can not separate ») was the basic element of matter, the Lego brick that could not be divided. Democritus (-370) had already stated this principle (of the atom, not the lego!). Then, the existence of neutrons and protons has been highlighted by Rutherford. Finally, MURAY showed that protons and neutrons were themselves each consisting of three quark. After that ? Then, the physical became interested in the cohesive strength of these structures, « how this little world holds together? « It can be seen that

  • the electron revolves around the nucleus of the atom;
  • neutrons are bonded to protons;
  • quarks are glued together;
  • atoms are bonded together to form molecules, as in the water (two hydrogen atoms and an oxygen atom).

It is in this point 5 that Hawking provides answers.

Four forces were identified:

  • gravity , which governs the operation of large bodies (planets, stars, galaxies). This is the lowest (in termes of strength). But this is the only acting at great distances. As it concerns many particles (all atoms of the sun and all of earth, for example), it will eventually have enormous power ;
  • the electromagnetic force, which acts between particles carrying an electrical charge; this is what makes the electron does not escape from his atom, attracted by the charges of the proton;
  • the weak nuclear force, which, as its name suggests, is the nucleus force: it is responsible in particular of radioactivity;
  • the strong nuclear force that holds quarks together.

At each of these four forces associated particles:

  1. gravity: the graviton;
  2. the electromagnetic force : the photon;
  3. the weak nuclear force : the Higgs;
  4. the strong nuclear force: the gluon.

The idea of physicists since Einstein was to develop a theory that would unite the four forces: in other words, to enclose the universe in a series of equations.

Some progress have been made : 2 and 3, for example, were gathered in a theory called GUT (Grand Unified Theory). But never four. GUT has however identified a surprising result: the discovery of antimatter (Dirac 1932). An anti-neutron is exactly the opposite of a neutron. If you put them together, they destroy one another and give vaccul. It works for all types of particles. Another result is the exclusion principle of Pauli (1925): two particles can not have the same position or the same speed. One of the great faults of this great unified theory is that it is not unified at all: it forgot gravity always the ugly duck of physics.


Black holes

This is the field of research of HAWKING (still alive five years after his life expectancy was fixed at two years). He explains why stars produce light (like our sun). They consist essentially of hydrogen (1 proton, 1 neutron and one electron). Hydrogens, a little tight, come under the influence of the enormous gravity, collision and thus cause an increase in temperature. They fuse into helium (two protons) to produce heat. The pressure increases inside the sun but gravity manages to avoid any blows. When hydrogen was completely burned, the temperature drops. The sun shrinks. As the Pauli exclusion principle prevents particles have the same position, they depart. The sunbegin an expansion phase. As far as particles are limited (in terms of energy), gravity still manages to keep everything. It even win. The star contracts. Depending on the quantity of material present, three fates are possible.

It can become:

  • <1.5 solar mass: white dwarfs (volume of the soil);
  • > 1.5 solar mass: neutron stars (15 km in diameter
A black Hole

, but very dense (one billion ton per cm3)) > 10 solar masses black holes (an infinitely dense point)

On the left, an optical image from the Digitized Sky Survey shows Cygnus X-1, outlined in a red box. Cygnus X-1 is located near large active regions of star formation in the Milky Way, as seen in this image that spans some 700 light years across. An artist’s illustration on the right depicts what astronomers think is happening within the Cygnus X-1 system. Cygnus X-1 is a so-called stellar-mass black hole, a class of black holes that comes from the collapse of a massive star. New studies with data from Chandra and several other telescopes have determined the black hole’s spin, mass, and distance with unprecedented accuracy.

HAWKING focused primarily black holes. The difficulty lies in the fact that they are black and therefore can not be seen. They are essentially defined theoretically. Their density is such that light can not escape.


Black holes are not black

HAWKING was interested in the features of the Event Horizon (EH), the boundary from which light remains trapped. Based on the uncertainty principle of Heisenberg, he showed that the vacuum of space is not completely empty. Indeed, if it was empty, the field that characterizes the void  would remain so. He would then have a position and a velocity determined and exactly equal zero. Which is not possible. The field thus fluctuates around zero. How is this possible? The vacuum is not empty ? HAWKING showed that particles and antiparticles constantly arise from nothingness, meet and disappear. And this is also true at the EH. When the antiparticle appears, it is immediately swallowed by the black hole and thus can not be negated by the particle which, in turn, is ejected into space as radiation. HAWKING has shown that the black hole was not black but it emitted radiation and even  heat! As it absorbs antiparticles flows, it grows and can even explode.


Rise and fall of the universe

HAWKING describes the formation of the universe from the Big Bang to black holes. The starting point is an infinitely dense point with zero dimension where all the laws of physics are destroyed. The temperature began to drop and appeared electrons and photons (the famous emitting radiation captured by Penzias and Wilson) and neutrinos and their congeners (antiparticles); The temperature dropped further: the pair formation rate of particle / antiparticle became greater than their annihilation rate.

Afficher l'image d'origine

Then were formed:

  • the first cores (the energy of the neutrons and protons was no longer sufficient to cons-carrer weak nuclear force)
  • and the first atoms (the electrons no longer had enough energy to cons-carrer electromagnetic force).

Local fluctuations of matter density has caused local contractions under the action of gravity. Of galaxies formed. The local concentrations of hydrogen atoms formed stars that began to shine when in their heart, fusion into helium began. Elements increasingly heavy were created by fusion of lighter elements. When the fuel was exhausted, the stars contracted into black holes (if any), to finish in supernova (an explosion) who sent in the Universe heavy elements. A second generation of stars was created from these elements and planets. The solar system has been formed from a supernova and the formation of stars of second generations.

This story involves « initial conditions » . There is thus a series of constants whose « good setting » is essential for life to appear. It may well include such

  • the speed of light (c);
  • the gravitational constant (g);
  • Planck’s constant;
  • the mass of quarks;
  • Avogadro’s number;
  • the Faraday constant, …

Aside from the blog author

If the gravitational constant (g) had been a little more:

  • low , only few massive stars would have formed; there would have been no supernova and no heavy elements, so no life
  • powerful ; more gas would have condensed in the stars that would have been so much more massive. Their life was short because of an accelerated fusion of hydrogen into helium atoms; too short, in any case, to let the life time to appear.

It therefore seems that g was « well settled » for life to appear neither too strong nor too weak.

If the electromagnetic force was more:

  • low, the electrons would have been less firmly bound to the nuclei of atoms. The atoms were unstable, preventing the emergence of life;
  • powerful, the electrons would have been firmly secured to the nuclei of atoms. But the formation of molecules (such as water, for example) requires exchange of electrons between atoms. This exchange would have been impossible. No water, no life.

If the strong nuclear force was more:

  • low , protons and neutrons would not have merged in heavy elements, such as carbon particular, one of the building blocks essential to life;
  • powerful , hydrogen would have completely merged in heavy elements and water (requiring hydrogen) would not have existed.

God has set the constants of Universe

Dozens similar examples can be drawn. The question of « setting of these constants » thus inevitably arises. By who ?

Or could be other universes? Are there been an infinity of universes with lots of different set of constants, most have disappeared due to lack of stability? This last assumption leads HAWKING to describe the anthropic principle: « to ask such questions, we must be in a universe with such constants ».

It remains to find the Grail: the theory of everything. HAWKING suggests : Universe is finite but without boundary. How is this possible? He drew an analogy with the surface of the earth. An man who follows equator would never happen at the end (no border), while he walls on a finished surface.


The arrow of time

HAWKING holds three definitions of time:

1. The thermodynamic time

It is based on the second law of thermodynamics. The principle is that in a closed system (eg room), disorder always increases (dirty socks that accumulate), unless you inject work (the cleaning lady). It is because of this principle that we see vases will break into pieces and that we have rarely seen the reverse. The thermodynamic time presupposes an increase in disorder.

2. Psychological time

It explains why you remember your day yesterday and not tomorrow’s.

3. The cosmological time

It is illustrated by the expansion of the universe.

The human being is a being ordered. His brain is probably the most ordered structure of the universe. How is this possible in a system that wants the mess, still more disorder? We must in fact consider the whole universe as a system. For example, it may be that your home is generally untidy but locally in your wardrobe for example, ties are carefully arranged. That is why man can still live in a world where disorder believes. When we eat, we eat organized systems (a banana) and we turn them into heat (energy disorganized) but also in tissues (organized) or information (organized). Overall the system « man » leads to a bigger mess because the heat takes precedence over the rest.


 Wormholes and time travel

HAWKING describes the possibilities for time travel. This can be achieved by:

  • going faster than light (a priori impossible according to Einstein);
  • bend the space-time. It would then have negative density region of space (emptier than empty). It seems to be possible and was experienced (CASIMIR effect 1948). Between two metal plates, the vacuum fluctuates (as Heisenberg said). The plates are close together. Pairs of particles and antiparticles are created in space. But those having too large wavelengths are discharged outside. The pressure outside becomes higher than the inside. This is the vacuum energy!


Unification of physics

Max BORN, in 1928, announced the end of physics in six months. Today it seems that one is away from the goal. HAWKING sees three scenarios:

  • we will get there (but we can not prove it by experience);
  • there are endless theories that lead to a description of increasingly fine of the universe;
  • we will not succeed because there is no theory.

The theory of everything will in any event if it exists, allow the unification of the four fundamental forces.

After dropping the super gravity (too complex), it advocates string theory (which involves several dozen dimensions).



HAWKING is this more philosophical conclusion that physicist. He finally asked whether the real question is not « why is there a universe? « He returns to LAPLACE (who told Napoleon that the hypothesis « God » had no place in his model) emphasizing the fallacy. The French in particular had forgotten the initial conditions (that chose them and fixed the laws governing their evolution?)