Reality Is Not What It Seems: The Journey to Quantum Gravity

by Carlo Rovelli

An elementary structure of the world is emerging, generated by a swarm of quantum events, where time and space do not exist. Quantum fields draw space, time, matter and light, exchanging

information between one event and another. Reality is a network of granular events; the dynamic which connects them is probabilistic; between one event and another, space, time, matter and energy melt in a cloud of probability.

The idea of Democritus’s system is extremely simple: the entire universe is made up of a boundless space in which innumerable atoms run.

Enlightenment. The ethical ideal of Democritus is that of a serenity of mind reached through moderation and balance, by trusting in reason and not allowing oneself to be overwhelmed by passions.

loss of the works of Democritus in their entiretyfn2 is the greatest intellectual tragedy to ensue from the collapse of the old classical civilization.

Einstein resuscitated the ‘living proof’ presented by Lucretius, and probably first conceived of by Democritus, and made it solid by translating it into mathematical terms, thus managing to calculate the size of the atoms.

words of Democritus: ‘To a wise man, the whole earth is open, because the true country of a virtuous soul is the entire universe.’25

Plato carved on the door of his school the phrase, ‘Let no one enter here who is ignorant of geometry.’

Mathematics allows the world to be described and the future to be predicted:

In India, where Greek learning had arrived, thanks to rich commercial and cultural exchanges, these books were studied and understood. From India this knowledge returned to the West, thanks to learned Persian and Arab scientists who were able to understand and preserve

Kepler in the cold north calculates movements in the sky,

Pythagorean-Platonic vision that nature is understandable through mathematics, Galileo decides to study how objects move on Earth when they are set free

Three centuries have passed, but it’s still thanks to theories based upon Newton’s equations that today we build bridges, trains and skyscrapers, engines and hydraulic systems; that we know how to fly planes, make weather forecasts, predict the existence of a planet before seeing it and send spaceships to Mars

Maxwell’s equations are used daily to describe all electric and magnetic phenomena, to design antennae, radios, electric engines and computers. And

We see the world around us in colour. What is colour? Put simply, it is the frequency (the speed of oscillation) of the electromagnetic wave that light is. If the wave vibrates more rapidly, the light is bluer. If it vibrates a little more slowly, the light is redder.

Colour, as we perceive it, is the psychophysical reaction of the nerve signal generated by the receptors of our eyes, which distinguish electromagnetic waves of different

All modern communications technology – radio, television, telephones, computers, satellites, wi-fi, the internet, etc. – is an application of Maxwell’s prediction; the Maxwell equations are the basis for all calculations made by telecommunications engineers.

The theories of Newton and of Maxwell appear to contradict each other in a subtle way. Maxwell’s equations determine a velocity: the velocity of light. But Newton’s mechanics is not compatible with the existence of a fundamental velocity, because what

enters Newton’s equations is acceleration, not velocity.

contradiction between Maxwell’s equations and Newton’s mechanics.

the greater the distance of the event from you, the longer the duration of the extended present.

This means we can say that, on Mars, there are events that in this precise moment have already happened, events that are yet to happen, but also a quarter of an hour during which things occur that are neither in our past nor in our future.

Without the notion of fields introduced by Faraday, without the spectacular power of mathematics, without the geometry of Gauss and Riemann, this ‘certain physics’ would have remained incomprehensible. Empowered by new conceptual tools and by mathematics, Einstein writes the equations which describe

in a world such as that of science, where often, too frequently, disputes over precedence become poisonous – the two gave a truly wonderful example of wisdom, clearing the field of all negative tension.

To comprehend how light may be simultaneously an electromagnetic wave and a swarm of photons will require the entire construction of quantum mechanics.

Since colour is the frequency of light, light is emitted by substances at certain fixed frequencies.

The set of the frequencies that characterizes a given substance is known as the ‘spectrum’ of this substance.

Werner Heisenberg is twenty-five years old when he writes the equations of quantum mechanics, the same age as Einstein was when he wrote his three major articles.

The quantum leaps from one orbit to another constitute their way of being real: an electron is a combination of leaps from one interaction to another. When nothing disturbs it, an electron does not exist in any place.

For him, the world is not made of things, it’s constituted of an abstract mathematical structure which shows us how things appear and how they behave

Dirac’s quantum mechanics is the mathematical theory used today by any engineer, chemist or molecular biologist. In it, every object is defined by an abstract spacefn20 and has no property in itself, apart from those that are unchanging, such as mass. Its position and velocity, its angular momentum and its electrical potential, and so on, acquire reality only when it collides – ‘interacts’– with another object.

While Newton’s physics allows for the prediction of the future with exactitude, if we have sufficient information about the initial data and if we can make the calculations, quantum mechanics allows us to calculate only the probability of an event.

The apparent determinism of the macroscopic world is due only to the fact that the microscopic randomness cancels out on average, leaving only fluctuations too minute for us to perceive in everyday life.

The general form of quantum theory compatible with special relativity is thus called quantum field theory, and it forms the basis of today’s particle physics. Particles are quanta of a field, just as photons are quanta of light. All fields display a granular structure in their interactions.

I think that quantum mechanics has revealed three aspects of the nature of things: granularity, indeterminacy and the relational structure of the world.

Quantum mechanics reveals to us that, the more we look at the detail of the world, the less constant it is. The world is not made up of tiny pebbles. It is a world of vibrations, a continuous fluctuation, a microscopic swarming of fleeting micro-events.

The theory does not describe things as they are: it describes how things occur and how they interact with each other. It doesn’t describe where there is a particle but how the particle shows itself to others. The world of existent things is reduced to a realm of possible interactions. Reality is reduced to interaction. Reality is reduced to relation.

Speed is not a property of an object on its own: it is the property of the motion of an object with respect to another object.

The world of quantum mechanics is not a world of objects: it is a world of events.

Granularity (figure 4.8). The information in the state of a system is finite, and limited by Plank’s constant. Indeterminacy. The future is not determined unequivocally by the past. Even the more rigid regularities we see are, ultimately, statistical. Relationality. The events of nature are always interactions. All events of a system occur in relation to another system.

This argument can be made more precise with a little mathematics. The result is general: quantum mechanics and general relativity, taken together, imply that there is a limit to the divisibility of space. Below a certain scale, nothing more is accessible. More precisely, nothing exists there.

Physical space is the fabric resulting from the ceaseless swarming of this web of relations. The lines themselves are nowhere; they are not in a place but rather create places through their interactions. Space is created by the interaction of individual quanta of gravity.

Why? Because time does not pass in the same way everywhere in the world. In some places, it flows more quickly; in others, more slowly. The closer you get to the Earth, where gravity

And to think that, initially, the phrase ‘theory of the Big Bang’ was coined by opponents of the theory, to mock an idea that seemed outlandish … Instead, in the end, we were all persuaded:

Verifiable quantitative predictions are instruments to validate hypotheses.

The objective of scientific research is not just to arrive at predictions: it is to understand how the world functions; to construct and develop an image of

the world, a conceptual structure to enable us to think about it. Before being techni...

The verifiable predictions are the sharpened tool which allows us to find out when we hav...

What Copernicus, Newton, Einstein and many others did was to build upon pre-existing theories which synthesized empirical knowledge across vast fields of nature, and to find a way of combining and rethinking them to improve the general picture.

As every researcher working in every laboratory throughout the world knows, doing science means coming up hard against the limits of your ignorance on a daily basis – the innumerable things which you don’t know, and can’t do.