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Why do we need antiparticles in a relativistic quantum theory? I will review the usual arguments based on spacetime symmetries and CPT invariance and show that antiparticles are not particles that travel backwards in time as Feynman claimed. In a recent twitter thread [1], Martin Bauer states that antiparticles are needed in a relativistic quantum theory because if we swap space and time in a quantum scattering process, the particles would travel backward in time and this is a puzzle, forcing us to reinterpret those exotic particles that travel backward in time as antiparticles that travel forward in time. His argument is not valid. First, because applied to classical scattering events it would lead to the conclusion that antiparticles are also necessary in the classical theory, which is not the case. Secondly, because relativity does not establish that space and time are equivalent and can be freely interchanged. It is a common misunderstanding of relativity that space

We saw in a previous post that the Casimir effect cannot be used to prove the existence of the quantum vacuum. In this post, I will explain why the quantum vacuum is not a true state of emptiness and why it is an artifact of quantum field theory. Reference [1] states: "In the old days of classical mechanics the idea of a vacuum was simple. The vacuum was what remained if you emptied a container of all its particles and lowered the temperature down to absolute zero. The arrival of quantum mechanics, however, completely changed our notion of a vacuum. All fields – in particular electromagnetic fields – have fluctuations." A similar misunderstanding is found in [2] and many other places. Quantum mechanics did not change our notion of a vacuum. The concept of emptyness in quantum mechanics is the same concept that is used in classical mechanics. The modern concept of quantum vacuum was introduced by quantum field theory, but quantum field theory is not the application of quant

The Casimir effect is an excellent example of the sad state of postmodern theoretical physics. I will show in this post why this effect cannot be used to prove the existence of the speculative quantum vacuum. The Casimir effect is the phenomenon of attraction between two nearby uncharged parallel conducting plates. The corresponding force is called the Casimir force. According to folklore, this force is "caused by quantum vacuum fluctuations of the electromagnetic field" and is often invoked as proof of the existence of the quantum vacuum [1,2]. This is wrong for at least three reasons. First, the term "quantum vacuum" is a misnomer because it is anything but empty. Second, the claim that the "arrival of quantum mechanics" completely changed our notion of the concept of vacuum is physically and historically incorrect, because there is no vacuum in quantum mechanics and the concept of quantum vacuum was introduced with quantum field theory [3]. Third, an

The Heisenberg uncertainty principle is one of the most famous elements of quantum theory, not only mentioned in academic papers and described in quantum mechanics textbooks, but also present in popular treatises. Well, the fact is that " Heisenberg uncertainty principle " is a misnomer because it is neither a principle nor about uncertainties. Brian Randolph Greene, a leading theoretical physicist, mathematician, and string theorist, often mentions the Heisenberg uncertainty principle. He does so in his popular books and he did again in a recent tweet . The " Heisenberg uncertainty principle " is usually abbreviated as HUP and the first thing to clarify about it is that it is not a principle, but a theorem derived from the postulates of quantum mechanics. A derivation of this theorem can be found in book [1]. The result for two arbitrary quantum operators \( \hat{A} \) and \( \hat{B} \) associated to the observables \( A \) and \( B \) is \[ \sigma(A) \cdot \si

Experience shows that some people likes to rewrite history and make heroes, and when Steven Weinberg died in 2021, some obituaries could not resist the temptation. In 2005, Weinberg wrote an article titled "Einstein's Mistakes". I am not Weinberg and he was not Einstein, but I will write about Weinberg's mistakes in this post. Weinberg justified his review [1] of Einstein's mistakes in the following terms: "Perhaps most important, by showing that we are aware of mistakes made by even the greatest scientists, we set a good example to those who follow other supposed paths to truth. We recognize that our most important scientific forerunners were not prophets whose writings must be studied as infallible guides—they were simply great men and women who prepared the ground for the better understandings we have now achieved". I will not review all of Weinberg's mistakes, but only those which appear in the obituary written by Nima Arkani-Hamed: "How St

There is a myth, perpetuated in physics textbooks and popular treatises, that states that general relativity is the theory of a gravitational field. The myth was started by Einstein, who often used the term gravitational field during the development of the theory. The pioneers make a lot of mistakes because the territory is unknown, but there is no reason to continue perpetuating myths a century after Albert Einstein, Marcel Grossmann, and David Hilbert developed the theory of general relativity. Let me quote a recent tweet from cosmologist Will Kinney: in general relativity, the gravitational field doesn’t really exist . Understanding that general relativity is not a field theory is not only desirable for reasons of rigor and consistency, but has profound implications for research. For example, the sad status of quantum gravity research is partially because some physicists such as Feynman and Weinberg have tried to apply quantum field theoretic methods to a theory in which there is

The second law is one of the most popular laws of nature, because it is often discussed in popular science treatises and educative videos, but what is the more correct and general formulation of this law? According to P. W. Bridgman (1946 Nobel Prize in Physics) " There have been nearly as many formulations of the second law as there have been discussions of it ". We can find many historical verbal statements of the law, from when the science of thermodynamics was developing in the 19th century. Some examples: " A transformation whose only final result is to transfer heat from a body at a given temperature to a body at a higher temperature is impossible. " " It is impossible, by means of inanimate material agency, to derive mechanical effect from any portion of matter by cooling it below the temperature of the coldest of the surrounding objects. " " It is impossible to construct an engine which will work in a complete cycle, and produce no effect

I recently watched a talk by John Baez titled " Symmetric Monoidal Categories " [1]. Baez presents new mathematical material that he considers provides a common foundation for the description of different scientific and engineering topics, material that he considers to be a kind of " Rosetta stone ". Baez is correct that scientists and engineers like to describe processes or composite systems using diagrams: flow charts, Petri nets, electrical circuit diagrams, signal-flow graphs, chemical reaction networks, Feynman diagrams, etc. He claims that many of these diagrams fit into a common framework, the mathematics of symmetric monoidal categories, and that when we accept this achievement, we begin to see connections between seemingly different topics. Baez also claims that this new viewpoint introduces a paradigm shift in science. Let us go over all those interesting claims. My comments will focus on the scientific side of his talk. To get started, Baez says that s

This book does not offer an idealized view of physics, but a realistic view with its problems, inconsistencies, and limitations. Popular science books written by physicists, physics textbooks, and the professional physics literature contain misleading or easily misinterpreted claims, and such misconceptions and myths are preventing a fundamental understanding of Nature. This book identifies over two hundred common misconceptions in classical electrodynamics, thermodynamics, statistical mechanics, general relativity, quantum mechanics, and quantum field theory. The claims that the Coulomb gauge is not a physical gauge, that mass increases with velocity, that entropy is a quantity characterizing disorder, that general relativity is equivalent to the theory of a massless self-interacting spin-2 field, that molecular chaos is the source of irreversibility, that electrons and protons are sometimes particles and sometimes waves, and that relativistic quantum electrodynamics is a Lagrangian