Stephen Hawking’s 4 questions 6 years after his death – 03/18/2024 – Science

March 14th marked six years since the death of Stephen Hawking.

On his personal website (now replaced), Hawking defined himself as a cosmologist, space traveler and hero. It was, in fact, all three.

He claimed to be working on the basic laws that govern the Universe and described his three main discoveries.

Let’s summarize them here. And we will address, based on them, four transcendental questions to which Hawking gave answers: where are time and space born? Where do they die? Is the Universe finite? Does humanity have a future in this?

Einstein as a basis

Hawking’s work takes as its starting point the extraordinary theory we have about the cosmos: Albert Einstein’s general relativity.

A revolutionary conception of time and space (space-time), based on the principle of equivalence of forces. It is the result, in turn, of a eureka moment: a free fall imagined by Einstein from the top of the roof of his house.

From his pervasive logic, based on incredibly obvious principles, emerged a single possible law for the cosmos, given by his relativistic field equations.

Which, for our Universe, according to the cosmological principle, admits only one solution: that of Friedmann-Lemaître-Robertson-Walker.

Exceptional result: a single equation and possible solution for the Universe!

Hawking started from there and was able to go further, with surprising singularity theorems (Hawking-Penrose) and results on the thermodynamics of space-time (Bekenstein-Hawking).

With them, he was able to answer eternal questions.

Where are time and space born?

Using Einstein’s theory, Hawking concluded that our Universe had an origin in the past, a beginning. This was his first discovery.

The Hawking-Penrose singularity theorems state that spacetime began in a singularity: the Big Bang.

This is demonstrated by going back in time, using the theory’s equations and the conditions that occurred in the primitive universe (recreated, in part, in large laboratories, such as Cern).

There comes a moment when time and space disappear: it is no longer possible to go back. That’s where it comes from! It’s like looking for the sources of the Nile, as Livingstone did, the exact place where it originates.

And where do they end up?

Spacetime comes to an end at each of the black holes that are constantly forming in the cosmos.

These are the other singularities that arise from the Hawking-Penrose theorems. Sinkholes where time and space disappear forever.

Such results have led to the need to quantify severity in order to be able to validate them on very small scales.

Only in this way could they be considered definitive, and not mere classical approximations of reality. In other words, it was necessary to combine Einstein’s general relativity and quantum physics: the two great theories of the 20th century.

A first step in this direction was taken by Hawking himself, with his second great discovery: Hawking radiation.

“Every Schwarzschild black hole, of mass M, emits electromagnetic radiation as if it were a perfect black body at the temperature: T = ħc³/8πGMk.”

Magic formula, which combines the most important fundamental constants in nature: Planck’s (quantum physics); the speed of light, c (relativistic physics); that of universal gravitation, G (Newtonian physics), that of Boltzmann, k (thermodynamics) and the number π (mathematics).

All the main physical theories brought together in a simple formula. A beautiful universal symphony!

With this, Hawking opened new paths that were still unexplored. One, towards the quantization of gravity. Another is the nascent quantum information theory and its surprising cosmic paradoxes.

Is our Universe finite?

This question has intrigued many generations. For centuries it was believed that the Universe was eternal, infinite in space and time. Then he changed his mind several times.

Until Hawking and Hartle showed that, if time was imaginary at the beginning, then the Universe would have no edges and limits.

It would be finite but unlimited, and there would be no initial singularity. How it originated would then be determined by the laws of physics.

This was his third discovery, the Hartle-Hawking conjecture about a self-sustaining Universe, in a way.

Does humanity have a future in the Universe?

In the last years of his life, Hawking analyzed the very serious problems we face.

He concluded that they could become lethal for humanity if we don’t solve them in the next hundred years.

Hawking was optimistic: he predicted that, by then, we would have established self-sustaining colonies outside of Earth.

And advances in intelligence (human and artificial) will have found the solution to the multiple problems that are now hidden. Opening a new horizon, of centuries of hope.

His last piece of advice to young scientists (who have always been his favorite audience) was to keep that wonderful feeling alive as they contemplate our vast and complex Universe: “There is nothing comparable to experiencing a eureka moment, to discovering for the first time something that no one knew.”

Until the universe disappears, or we humans disappear, the extraordinary discoveries that Stephen Hawking bequeathed to us — like his beautiful formula, now engraved on his tombstone — will generate new knowledge.

In this singular space-time in which today, six years after his death, we remember him.

_{This article was published on The Conversation. You can read the original version here.}