How can the strange, bizarre world of the smallest, of which everything is made up, be so fundamentally different from the perception of reality we experience every day?
Carnot and the birth of thermodynamics
Nicolas Léonard Sadi Carnot was an officer in the French army at the time of Napoleon, but also a physicist and mathematician. To help France win its wars, he did research on heat exchange and ways of making steam engines more efficient. Previously, James Watt had been able to increase their efficiency from 1% to 19%. Carnot’s sole publication in 1824, Sur la puissance motrice du feu (‘On the motive power of fire’), received little attention during his lifetime, but he nevertheless laid the foundation for the science of thermodynamics. His work was later used by Rudolf Clausius and Lord Kelvin to formalise the second law of thermodynamics and to introduce the concept of entropy and define it as a measure of the disorder in a system. According to the second law of thermodynamics, the entropy of an isolated physical system can never decrease. This law is regarded as the physical law with the greatest impact outside the realm of physics itself.
Boltzmann, Shannon, and the information view of entropy
Ludwig Boltzmann refined the concept in 1877 and characterised entropy as the total number of distinct microscopic states in which particles composing a parcel of matter can exist without altering the external appearance of the parcel. Based on this, Claude Shannon introduced in 1948 the concept of entropy as a measure of information content: the Shannon entropy. The number of states calculated from the Boltzmann entropy reflects the amount of Shannon information required to bring about any specified arrangement of particles. Rolf Landauer showed that for information storage and transfer, physical systems are always needed—information has to obey physical laws. He demonstrated that the destruction of information costs energy because discarding information has fundamental physical consequences. Later on, Lucas Céleri demonstrated that the ‘Landauer Principle’ also stands in a system obeying the mysterious laws of quantum physics.
Black holes and the holographic principle
Gerard ’t Hooft and Leonard Susskind showed that even in black holes, information cannot be lost. In 1986, Rafael Sorkin proved that the entropy of a black hole is precisely one-quarter of the area of the event horizon of the black hole as measured in Planck surfaces. Jacob Bekenstein introduced the generalised second law of thermodynamics as the statement that the sum of the entropies of black holes and the ordinary entropy of the universe never decreases. Gerard ’t Hooft and Leonard Susskind postulated the holographic principle, according to which the information content of a region of space and time is given by a quarter of the area of its horizon. It ensues from this that all the information describing the visible universe is encoded on a quarter of the de Sitter horizon.
Entropy as information: A new foundation
Erik Verlinde and emergent gravity
The second law of thermodynamics could be redefined as: entropy, a measure of the amount of information in a system, must always increase. Erik Verlinde puts information at the centre of his new theory of gravity, an interesting and innovative concept—but the theory still has many issues to solve. He describes the universe in terms of information, or more exactly, in terms of quantum mechanical entanglement entropy. Gravity arises from microscopic quantum information and quantum entanglement, with its associated entanglement entropy playing a central role in this. One qubit consists of two entangled particles and fits in the smallest possible volume: a Planck volume. For every bit of information you throw into a black hole, the surface of its horizon expands by one Planck length, suggesting that information is a fundamental building block of the universe.
Information density and the emergence of gravity
Changes in the density of information play the same role in the emergence of gravity as molecules do in the rising of temperature; entropy equals the total amount of information, and energy is the speed at which information gets processed. In this, temperature equals the energy per amount of information. Spacetime and matter are all the same; they exist one by one out of the same building blocks. Information is located not only on the surface bounding a specific space but within the space itself as well, giving rise to the fact that entangled quantum information is not only responsible for the emergence of spacetime but also leads directly to the arising of ordinary matter, dark matter, and dark energy. The larger the region of space, the more important the volume relative to the surface. For very large regions of space, where relativity theory requires dark matter and dark energy to ensure that calculations agree with observations, Verlinde’s model describes the universe in terms of the distribution of information without the need for dark matter or dark energy. Erik Verlinde: “And from a theoretical perspective, insights from black hole physics and string theory indicate that our macroscopic notions of spacetime and gravity are emergent from an underlying microscopic description in which they do not have a priori meaning”.
A history of radical shifts in understanding
Our understanding of reality has had to be radically and drastically revised a number of times. The Earth of the ancient Greeks was the centre of the universe, and most certainly flat, or else you would fall off. Logically, it was dangerous to approach the edge. Until it became apparent that the Earth was demonstrably round, and, remarkably, we did not appear to fall off after all. Then it transpired that the Earth also rotated and moved around the Sun, which in those days was contrary to all sense and reason, for movement, after all, was something you could feel.
Later, it became apparent that space as we thought we knew it, and time, which is so familiar and which we thought we could measure so precisely with our accurate clocks, were relative as well. However, things became completely baffling when the world that had always been hidden from us, the world of the infinitesimal, gradually began to reveal itself. The microscopic world proved to be fundamentally different from the world as it was normally perceived and described by physical laws. This world obeyed very different laws, and nothing was certain anymore. However, all the calculations of quantum mechanics are so amazingly accurate that there must be a reality behind it, but one that is hidden from our direct perception. There proved to be a bizarre but clear correlation between measurement or observation and the behaviour of the smallest.
Quantum measurement and the nature of reality
Niels Bohr and the Copenhagen School expressed it in the words: a microscopic property has no value unless it is a measured value, and the outcome depends on the measurement procedure. Perception, observation, and measurement lead to quantum decoherence with the leaking of information and to quantum collapse with the release of all the information contained in the system. This nonrealism, which Albert Einstein found so difficult to accept, has recently been unequivocally demonstrated—like nonlocality, which he called ‘spooky action at a distance’.
Descartes had already feared it in the 17th century: “My senses deceive me”.
Wheeler and the primacy of information
John Wheeler was the first to start using information to describe reality. He argued that the universe is characterised by ‘it from bit’. In other words, a physical object always consists of information, bits. For scientists such as John Wheeler, Gerard ’t Hooft, Leonard Susskind, Juan Maldacena, and Erik Verlinde, bits of information or qubits are a more fundamental building block of reality than quarks and electrons, although a qubit is not a physical object but contains information about the physical object. They argue that this deeper layer of information in all probability exists and that this insight alone has significant consequences for our understanding of reality.
Sci 2021, 3,35. https://doi.org/10.3390/sci3040035
