In search of the "ultimate equation" at the origins of the universe

Starbirth storm, in the Large Magellanic Cloud, captured from the Giant Hubble Telescope.

-

• The infinitely large and the infinitely small are two “universes” governed by their own physical laws.

• For several decades, scientists have been trying to "find the ultimate equation", making it possible to marry Einstein's relativity and quantum physics, explains Jean-Pierre Luminet.

• The astrophysicist evokes the various hypotheses that attempt to answer this enigma in 

  L'Ecume de l'Espace-temps

  (Editions Odile Jacob), in bookstores this Thursday.

Since the dawn of time, humans have looked at the stars and wondered.

How did galaxies come into being?

What is the shape of the universe?

The big names in science have tried to provide answers to these dizzying questions.

But physics is faced with an enigma 

how to reconcile the infinitely large and the infinitely small?

“I dared to take the gamble of making the dizzying mystery of the world that surrounds us and makes us less unspeakable,” writes astrophysicist Jean-Pierre Luminet.

On the occasion of the release this Thursday of his latest book,

(Editions Odile Jacob),

interviewed this specialist in black holes and cosmology.

You recall it at the beginning of the book: two physical theories allow us to understand our universe.

On the scale of "the infinitely large", stars, galaxies, it is Albert Einstein's general relativity that applies.

How could we sum it up?

It is a theory of gravity, which governs the universe on very large scales.

Before Einstein, gravity was a universal force of attraction, discovered by Newton, responsible for the fall of bodies, the famous apple, and the movement of the planets.

Einstein makes gravity a manifestation of the curvature of space-time.

Space is no longer a rigid framework, with forces inside that link objects, as in Newton's work, but an elastic fabric, which is deformed by the mass of objects, in particular the planets.

And the movement of objects is at the same time dictated by the curvatures of this space-time.

We can imagine it like the curvatures that balls make on a stretched sheet, for example.

This theory made it possible to predict unimaginable things: gravitational waves, the Big Bang, black holes.

At the other end of the scale, we have quantum theory, which describes the behavior of atoms and particles.

You say that its principles “offend common sense”.

Why ?

The world of elementary particles actually obeys quantum laws, which are counter-intuitive.

At the particle scale, if you want for example to locate an electron in space, you will inevitably have some uncertainty about its speed, which does not exist at our macroscopic scale.

This principle of uncertainty greatly annoyed physicists, in particular Einstein.

Moreover, quantum mechanics is probabilistic: an object is not unique but in a superposition of states at the same time.

This is Schrödinger's famous cat: as long as it is not measured, it is both in a dead and alive state.

These quantum laws are very different from those that apply in our observable world, but this theory is verified experimentally with phenomenal precision.

Why do you want to reconcile these two theories?

For a very long time, these two communities of physicists, relativists and quantum, worked on their own, because there are in truth very few situations where one needs to marry the two theories.

But we realized that this union was necessary to deal with the singularity of the Big Bang [thanks to Einstein's equations, we can go back to time 0 of the universe, but the laws then describe an energy density and a infinite temperature, physical nonsense] or understand what's going on at the center of black holes.

In these examples, we must simultaneously deal with gravity, mass, energy, but in a very small, very dense space, so the universe is also quantum.

The marriage of the two theories therefore becomes necessary.

In your book, you explain that several theories are in progress to attempt this union, in particular string theory or loop quantum gravity ...

In the 1960s, we thought we could achieve this union with string theory, which starts from quantum theory and reaches gravity.

She explains that at the most fundamental level, there are one-dimensional objects called strings, and that it is the vibration of these strings that defines, in a way, the type of particle.

It's a nice theory but one of the problems is that it doesn't work in the usual framework of space and time, and you have to add additional dimensions, up to 8 or 9 ...

Quantum loop theory is in the spirit of Einstein's general relativity, which geometries space-time.

It proposes to quantify space and time themselves.

Space-time would thus not be continuous but composed of atoms of space and time distinct, discontinuous, linked together like a network of neurons.

But this theory also has problems.

All of these leads are intellectually exciting, but none has come to fruition today.

If these two theories work on their own scale, why absolutely want to unify them?

The theorist's fantasy is to find the ultimate equation.

But this fantasy also leans on the past, as the history of science shows that the successful unification of seemingly disparate theories is a natural form of knowledge progression and engenders more comprehensive insight and more questions. fundamental to the functioning of our universe.

Such an equation would assume that the universe itself is written in mathematical language.

One might also think that mathematics is only a means found by man to best explain physical reality.

What do you think ?

Mathematics is an irreplaceable tool for understanding reality.

I think mathematics is a form of universal language, that it is embedded in the very fabric of the universe.

But on the other hand, we must get rid of the worn idea, although attractive, according to which the beauty of a theory proves its veracity, that the beauty of a mathematical formula would be the key to unification.

Science

The Nobel in chemistry awarded to the French Emmanuelle Charpentier and the American Jennifer Doudna

Science

Space: Six galaxies trapped in a supermassive black hole dating from the beginning of the Universe

• Science

• Space

• Universe

• Black hole

• Big Bang

• Albert Einstein