More on this book
Community
Kindle Notes & Highlights
Read between
April 9 - April 30, 2023
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.
There are no longer particles which move in space with the passage of time, but quantum fields whose elementary events happen in spacetime. The world is strange, but simple
The first is the existence of a fundamental granularity in nature. The granularity of matter and light is at the heart of quantum theory.
the first meaning of quantum mechanics is the existence of a limit to the information that can exist within a system: a limit to the number of distinguishable states in which a system can be.
The world is a sequence of granular quantum events. These are discrete, granular and individual;
Quantum mechanics introduces an elementary indeterminacy to the heart of the world. The future is genuinely unpredictable. This is the second fundamental lesson learned with quantum mechanics.
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 same appearance of probability at an elementary level, is the second key discovery about the world that quantum mechanics expresses.
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.
Quantum mechanics extends this relativity in a radical way: all variable aspects of an object exist only in relation to other objects. It is only in interactions that nature draws the world.
In the world described by quantum mechanics there is no reality except in the relations between physical systems.
The world of quantum mechanics is not a world of objects: it is a world of events.
A stone is a vibration of quanta that maintains its structure for a while, just as a marine wave maintains its identity for a while before melting again into the sea.
What is a wave, which moves on water without carrying with it any drop of water? A wave is not an object, in the sense that it is not made of matter that travels with it. The atoms of our body, as well, flow in and away from us. We, like waves and like all objects, are a flux of events; we are processes, for a brief time monotonous
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.
Quantum mechanics teaches us not to think about the world in terms of ‘things’ which are in this or that state but in terms of ‘processes’ instead. A process is the passage from one interaction to another.
I think that the obscurity of the theory is not the fault of quantum mechanics but, rather, is due to the limited capacity of our imagination.
And yet between the two theories there is something that grates. They cannot both be true, at least not in their present forms, because they appear to contradict each other. The gravitational field is described without taking quantum mechanics into account, without accounting for the fact that fields are quantum fields – and quantum mechanics is formulated without taking into account the fact that spacetime curves and is described by Einstein’s equations.
Einstein understood that space and time are manifestations of a physical field: the gravitational field. Bohr, Heisenberg and Dirac understood that physical fields have a quantum character: granular, probabilistic, manifesting through interactions. It follows that space and time must also be quantum entities possessing these strange properties. What, then, is quantum space? What is quantum time? This is the problem we call quantum gravity.
Newton discovered universal gravity precisely by combining Galileo’s physics of how things move on Earth with Kepler’s physics of the heavens. Maxwell and Faraday found the equations of electromagnetism by bringing together what was known about electricity and what was known about magnetism. Einstein found special relativity in order to resolve the apparent conflict between Newton’s mechanics and Maxwell’s electromagnetism – and then general relativity in order to resolve the resulting conflict between Newton’s mechanics and his own special relativity.
we are speaking of gravity and, therefore, as Einstein understood, we are not speaking of fields immersed in space but of the very structure of space itself.
When we say that the volume of a room is, for example, 100 cubic metres, we are in effect counting the grains of space – the quanta of the gravitational field – which it contains.
General relativity taught us that space is something dynamic, like the electromagnetic field: an immense, mobile mollusc in which we are immersed, which stretches and bends. Quantum mechanics teaches us that every field of this sort is made of quanta, that is to say, it has a fine, granular structure. It follows that physical space, being a field, is made of quanta as well. The same granular structure characterizing the other quantum fields also characterizes the quantum gravitational field, and therefore space. We expect space to be granular. We expect quanta of gravity, just as there are
...more
Yet we develop our conceptual schema for understanding the world by exploring new ideas but also by building on the powerful intuitions of giant figures from the past.
The crucial difference between photons (the quanta of the electromagnetic field) and the nodes of the graph (the quanta of gravity) is that photons exist in space, whereas the quanta of gravity constitute space themselves.
Just as an electron is in no place but diffused in a cloud of probability in all places, space is not actually formed by a single specific spin network but rather by a cloud of probabilities over the whole range of all possible spin networks.
Space is created by the interaction of individual quanta of gravity.
Just as the idea of the space continuum containing things disappears, so, too, does the idea of a flowing continuum ‘time’ during the course of which phenomena happen.
time no longer exists in the fundamental theory: the quanta of gravity do not evolve in time. Time just counts their interactions.
The moment we take quantum mechanics into account, we recognize that time, too, must have those aspects of probabilistic indeterminacy, granularity and relationality which are common to all of reality.
We have known for more than a century that we must think of time instead as a localized phenomenon: every object in the universe has its own time running, at a pace determined by the local gravitational field.
At the extremely small scale of the quanta of space, the dance of nature does not develop to the rhythm kept by the baton of a single orchestral conductor: every process dances independently with its neighbours, following its own rhythm. The passing of time is intrinsic to the world, it is born of the world itself, out of the relations between quantum events which are the world and which themselves generate their own time.
we can’t ever measure the true time t but, if we assume that it exists, we can set up an efficient framework to describe nature.
The idea of a time t which flows by itself, and in relation to which all things evolve, is no longer a useful one.
In the example of the pulse and the candle chandelier, we will not have the pulse and the candelabrum evolving in time, but only equations which tell us how the two variables evolve with respect to each other. That is to say, equations which tell us directly how many pulse-beats there are in an oscillation, without mentioning t. ‘Physics without time’ is physics in which we speak only of the pulse and the chandelier, without mentioning time.
We must learn to think of the world not as something which changes in time but in some other way. Things change only in relation to one another. At a fundamental level, there is no time. Our sense of the common passage of time is only an approximation which is valid for our macroscopic scale.
Just as a calm and clear Alpine lake is made up of a rapid dance of a myriad of minuscule water molecules, the illusion of being surrounded by continuous space and time is the product of a long-sighted vision of a dense swarming of elementary processes.
The world, particles, light, energy, space and time – all of this is nothing but the manifestation of a single type of entity: covariant quantum fields.
Covariant quantum fields have become today the best description that we have of the ἄπειρον, the apeiron, the primal substance of which everything is formed hypothesized by the man that could perhaps be called the first scientist and the first philosopher, Anaximander.fn45
The separation between the curved and continuous space of Einstein’s general relativity and the discrete quanta of quantum mechanics which dwell in a flat and uniform space has dissolved.
Continuous space and time are an approximate large-scale vision of the dynamic of quanta of gravity.
The conceptual price paid is the relinquishing of the idea of space, and of time, as general structures within which to frame the world.
Space and time are approximations which emerge at a large scale.
The main reward of this kind of physics is that, as we shall see in the next chapter, infinity disappears.
The singularities which render Einstein’s equations absurd when the gravitational field becomes too strong also disappear: they are only the result of neglecting the quantization of the field.
A quantum repulsion pushes away the electron when it gets too close to the centre. Thus, thanks to quantum mechanics, matter is stable. Without it, electrons would fall into nuclei, there would be no atoms and we would not exist.
Our universe could thus be the result of the collapse of a previous contracting universe passing across a quantum phase, where space and time are dissolved into probabilities.
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 technical, science is visionary.
The verifiable predictions are the sharpened tool which allows us to find out when we have misunderstood something.
