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Point particles RIP

John Healy
Classical wave packet
John Healy using Apple’s Grapher

I typically add samples of books to my Kindle library when considering purchases. While examining my Kindle library yesterday, I started reading a sample of Art Hobson’s 2017 book Tales of the Quantum: Understanding Physics’ Most Fundamental Theory and then became interested in his background. A Google search found biographical information, references to his books, YouTube videos, and his 2013 paper “There are no particles, there are only fields.” 1

Well, that paper was useful, because it says what I might say if I had the proper physics “chops.”

To find out what textbooks say, I perused the 36 textbooks in my university’s library having the words “quantum mechanics” in their title and published after 1989. 30 implied a universe made of particles that sometimes act like fields, 6 implied the fundamental constituents behaved sometimes like particles and sometimes like fields, and none viewed the universe as made of fields that sometimes appear to be particles. Yet the leading quantum field theorists argue explicitly for the latter view (Refs. 10-18). Something’s amiss here.

In physics lab at Caltech in the early years of the “red book” The Feynman Lectures on Physics class, we did the double-slit experiment (not the “dim” beam version, as I recall). That experience left me dissatisfied. My takeaway was “okay, light acts as both a particle and a wave — what’s next?” My gut feel over the years was that the photons (or electrons in a similar experiment) interacted with the slits. But the duality was left hanging — nothing beyond paradox.2

In this chapter we shall tackle immediately the basic element of the mysterious behavior in its most strange form. We choose to examine a phenomenon which is impossible, absolutely impossible, to explain in any classical way, and which has in it the heart of quantum mechanics. In reality, it contains the only mystery. We cannot make the mystery go away by “explaining” how it works. We will just tell you how it works. In telling you how it works we will have told you about the basic peculiarities of all quantum mechanics. — The Feynman Lectures on Physics, vol III, p. 1-1 (1965).

So, here the Quantum Field Theory (QFT) context offers some closure. A career physicist (Hobson) explains something that I’d not encountered so clearly in my reading the last few years.

In the 2-slit experiment, for example, the quantized field for each electron or photon comes simultaneously through both slits, spreads over the entire interference pattern, and collapses non-locally, upon interacting with the screen, into a small (but still spread out) region of the detecting screen.

Lately while reviewing the saga of infinities in modern physics and continuing to ponder how to visualize a classical Coulomb field for an electron using a QFT framework (with virtual particle screening, photon exchange in Feynman diagrams, vector potential field, etc.), I was really frustrated with the notion of point particles. 🐟

Perhaps we can say “point particles RIP,” eh (especially oscillating ones).4

For example, now we can explore how energy is “bundled” into discrete quanta (Feynman’s wiggles or excitations of spatially unbounded continuous fields), and how an electron — as a unified bundle of field which hits like a particle — interacts with other field bundles, universal fields, and the vacuum.

Hobson notes:

“How can any physicist look at radio or microwave antennas and believe they were meant to capture particles?” It’s implausible that EM signals transmit from antenna to antenna by emitting and absorbing particles; how do antennas “launch” or “catch” particles? In fact, how do signals transmit?

The superposition principle should have been a dead giveaway: A sum of quantum states is a quantum state. Such superposition is characteristic of all linear wave theories and at odds with the generally non-linear nature of Newtonian particle physics.

A benefit of QFTs is that quanta of a given field must be identical because they are all excitations of the same field, somewhat as two ripples on the same pond are in many ways identical. Because a single field explains the existence and nature of gazillions of quanta, QFTs represent an enormous unification. The universal electron-positron field, for example, explains the existence and nature of all electrons and all positrons.

… Einstein’s goal of explaining all fields entirely in terms of zero-rest-mass fields such as the gravitational field has not yet been achieved, although the QFT of the strong force comes close to this goal of “mass without mass.”

And I started my physics blog for the same reason Hobson notes in the preface to his book, namely, as someone “who would like to better fathom, before they depart this mortal coil, what makes the universe tick.” 3

[1] Submitted 2012; published March 2013. https://arxiv.org/abs/1204.4616 (https://arxiv.org/pdf/1204.4616.pdf).

[2] As Feynman said, “It is what makes physics fascinating.” — The Feynman Lectures on Physics, vol III, p. 18-9 (1965).

In his paper, Hobson notes:

For Richard Feynman, this paradox was unavoidable. Feynman was a particles guy. As Frank Wilczek puts it, “uniquely (so far as I know) among physicists of high stature, Feynman hoped to remove field-particle dualism by getting rid of the fields” (Ref. 16).

I encountered this characterization of Feynman as a particles guy at least once before. But currently I cannot find such a citation in my notes. So, I cannot say whether others besides Wilczek held that opinion. Wilczek’s original 1999 article “The persistence of ether” is archived here.

And Hobson further goes on to quote from Feynman’s introduction to one of his lectures (The Character of Physical Law, The MIT Press, Cambridge, MA, 1965) where he says:

I am going to tell you what nature behaves like. … Do not keep saying to yourself, … “But how can it be like that?” because you will get “down the drain,” into a blind alley from which nobody has yet escaped. Nobody knows how it can be like that.

One may consider the tone of Feynman’s discussion of wave-particle duality by reading from The Feynman Lectures on Physics online — from Volume 3, Chapters 1 and 2 (the same lectures in Chapters 37 and 38 of Volume 1): Quantum Behavior and The Relation of Wave and Particle Viewpoints.

[3] Hobson, Art. Tales of the Quantum: Understanding Physics’ Most Fundamental Theory (Kindle Locations 96-98). Oxford University Press 2017. Kindle Edition.

[4] There were rebuttals to Hobson’s paper, for example:

Massimiliano Sassoli de Bianchi, “Quantum fields are not fields.”

Because of the experimentally verified presence of entanglement, the so- called fields of several quantum entities are certainly not fields that can be defined in a three-dimensional space, but only in a higher dimensional configuration space. … quantum non-spatiality … what quantum mechanics teaches us is that not all of physical reality is contained within space, and that we need to drop the preconception that so-called microscopic “particles” and quantum “fields” would necessarily be spatial entities.

Am. J. Phys., Vol. 77, No. 10, October 2009. Letters To the Editor. “The Real Scandal of Quantum Mechanics.” Richard Conn Henry, The Johns Hopkins University.

We know for a fact that the universe is not “made of” anything. Get it through your heads, physicists! It is sometimes said that the only thing that is real are the observations, but even that is not true: observations are not real either. They, and everything else, are purely mental.

6 thoughts on “Point particles RIP”

  1. I’m getting near the end of Hobson’s book Tales of the Quantum, and today’s Space.com article “There and Back Again: Scientists Beam Photons to Space to Test Quantum Theory” exemplifies his point about how wave-particle dualism is treated in textbooks. Add another media reference to that list of sources which have yet to view “the universe as made of fields that sometimes appear to be particles [as ripples in a field which hit like particles].”

    Researchers have taken a famous quantum-physics experiment to new heights by sending light, in the form of photons, to space and back, demonstrating the dual-particle-wave nature of light over much greater distances than scientists can achieve on Earth.

    The light split into two beams, like a wave and, at the same time, stayed together as a single photon, until the end, when the liquid crystal device forced it to behave as one or the other right before hitting the detector. The predictions of quantum theory were vindicated, Vallone said — and the surreal nature of quantum mechanics was reaffirmed.

    The article provides some historical background for this latest earth-space form of the famous double-slit experiment (expansively discussed by Hobson). And notes that the duality depends on how scientists measure photons (which introduces the measurement problem). Then discusses how the behavior is “decided” (or chosen) in the experiment: “Does light commit to one behavior at the beginning of an experiment, when it’s produced; at the end, when it’s detected; or some time in between?” (There is no mention of wave function collapse in the article. And nothing about quantum unity as well.1)

    Superposition is mentioned once: “Vallone’s group … were able to keep the light in its bizarre double state, called a superposition, for 10 milliseconds.” But that statement only confuses the matter, since superposition is a wave phenomenon — some readers’ takeaway might be that the superposition is “wave and particle” rather than a spatially extended quantum.

    The new findings suggest that the behavior of objects in the universe is fundamentally undetermined until something forces them to behave a certain way. Particles propagate like waves, waves coalesce into particles and nothing can be predicted with certainty, only a probability.

    Vallone approaches the concept in a similar manner. “When we think of a photon as a particle, as a little ball, we are [making a] mistake. When we think of a photon like a water wave, we are [also making] a mistake,” he said. “The photon, in some cases, seems to behave like a wave or seems to behave like a particle. But actually, it’s neither.”

    [1] “Despite being extended spatially, a quantum is a single thing, not made of parts. You cannot alter a quantum at just one place. Whatever happens to it happens to the entire quantum.” — Hobson, Art. Tales of the Quantum: Understanding Physics’ Most Fundamental Theory (Kindle Locations 926-928). Oxford University Press. Kindle Edition.

  2. On August 25, 2017, I commented on the post “GR: Chicken or egg redux” that I sometimes think that generations of scientists raised in space might help advance physics, having lived in a world dominated by inertia (rather than friction or gravity).

    Similarly, in his January 26, 2016, talk “Quantum is Different: Part 2 – One Entangled Evening,” physicist John Preskill made a prediction about future generations of physicists who played with quantum toys as kids (much like those today who have used smartphones since pre-K) — a visceral understanding of quantum physics.

    In the future quantum physicists who have been playing quantum games since they were three years old will not be able to understand why 20th century scientists thought quantum mechanics was weird. Our digital technology has launched a gaming culture which continues to accelerate. with quantum technologies we’ll be able to play new kinds of games that we haven’t imagined before, and kids will play those games, and they’ll develop a visceral understanding of quantum physics that we lack and that will lead them to new discoveries. [See the YouTube video https://youtu.be/lN8zT_Yk5sg — published on Feb 14, 2016, the IQIM Caltech channel.]

  3. Regarding point particles [1], particularly the electron, seasoned science communicator Ethan Siegel [2] took a stab at explaining what an electron is in this article: “Ask Ethan: What Is An Electron? Sometimes, the simplest questions of all are the most difficult to meaningfully answer” (April 13, 2019).

    A Patreon supporter asked:

    Please will you describe the electron… explaining what it is, and why it moves the way it does when it interacts with a positron. If you’d also like to explain why it moves the way that it does in an electric field, a magnetic field, and a gravitational field, that would be nice. An explanation of charge would be nice too, and an explanation of why the electron has mass.

    Well, Ethan summarized the codex for the Standard Model well enough — the Standard Model’s characterization of an electron, its properties. I found these bits interesting:

    The particles that make up matter, known as the fermions, all have antimatter counterparts: the anti-fermions. The bosons, which are responsible for the forces and interactions between the particles, are neither matter nor antimatter, but can interact with either one, as well as themselves.

    It’s important, before we enumerate what all the properties of the electron are, to note that this is merely the best understanding we have today of what the Universe is made of at a fundamental level. We do not know if there is a more fundamental description …

    If an electron and a positron (which has some of the same quantum numbers and some quantum numbers which are opposites) interact, there are finite probabilities that they will interact through either the electromagnetic or the weak force.

    Most interactions will be dominated by the possibility that electrons and positrons will attract one another, owing to their opposite electric charges. They can form an unstable atom-like entity known as positronium, where they become bound together similar to how protons and electrons bind together, except the electron and positron are of equal mass.

    However, because the electron is matter and the positron is antimatter, they can also annihilate. Depending on a number of factors, such as their relative spins, there are finite probabilities for how they will decay: into 2, 3, 4, 5, or greater numbers of [gamma ray] photons. (But 2 or 3 are most common.)

    When you subject an electron to an electric or magnetic field, [virtual?] photons interact with it to change its momentum; in simple terms, that means they cause an acceleration.

    Why electrons have these particular properties is beyond the scope of the Standard Model, though.

    So, my puzzlement about the electron remains, that it’s “assumed to be a point particle with a point charge and no spatial extent.”

    Wiki: The admission of the hypothesis of a finite radius of the electron is incompatible to the premises of the theory of relativity. On the other hand, a point-like electron (zero radius) generates serious mathematical difficulties due to the self-energy of the electron tending to infinity. … the electron is the least massive particle with non-zero electric charge, so its decay would violate charge conservation.

    [1] As well as puzzlement in my post on Virtual attraction. And then there are neutrinos …

    Wiki: In a simplified picture, every photon spends some time as a combination of a virtual electron plus its antiparticle, the virtual positron, which rapidly annihilate each other shortly thereafter.

    The apparent paradox in classical physics of a point particle electron having intrinsic angular momentum and magnetic moment can be explained by the formation of virtual photons in the electric field generated by the electron. These photons cause the electron to shift about in a jittery fashion (known as zitterbewegung), which results in a net circular motion with precession. This motion produces both the spin and the magnetic moment of the electron.

    Wiki: The mass of the neutrino is much smaller than that of the other known elementary particles.

    [2] “I am a Ph.D. astrophysicist, author, and science communicator, who professes physics and astronomy at various colleges.” He posts articles under the rubric “Ask Ethan” on Forbes.com, Medium.com, etc.

  4. So, referencing my comment above (April 15, 2019), research on the electron electric dipole moment [EDM] is interesting. No experiment has found a non-zero electron EDM.

    The Harvard-Yale ACME experiments are referenced in this Space.com article “What a Tiny Electron Reveals About the Structure of the Universe” (by Alexey Petrov on January 06, 2019). The article references this Nature article “Improved limit on the electric dipole moment of the electron” (published by the ACME Collaboration on 17 October 2018), and asks the question: “What is the shape of an electron?” If an electron has a “shape,” then …

    If the shape of an object reflects the distribution of its electric charge, it would also imply that the object’s shape would have to be different from spherical. Thus, naively, the EDM would quantify the “dumbbellness” of a macroscopic object.

    Physicists of the ACME collaboration did not observe the electric dipole moment of an electron — which suggests that its value is too small for their experimental apparatus to detect. This fact has important implications for our understanding of what we could expect from the Large Hadron Collider experiments in the future.

    The article also includes a link to this YouTube video [visualization] “The ACME Search for the Electron EDM” on the ElectronEDM channel (published on Dec 13, 2013).

    … the electron’s electric dipole moment … is the distance between the electron’s center of mass and its center of charge … [What does “center of mass” or “center of charge” mean, if anything, for an excitation in QFT?]

    Wiki: In a simplified picture, every photon spends some time as a combination of a virtual electron plus its antiparticle, the virtual positron, which rapidly annihilate each other shortly thereafter.

    The apparent paradox in classical physics of a point particle electron having intrinsic angular momentum and magnetic moment can be explained by the formation of virtual photons in the electric field generated by the electron. These photons cause the electron to shift about in a jittery fashion (known as zitterbewegung), which results in a net circular motion with precession. This motion produces both the spin and the magnetic moment of the electron.

  5. So, I’ve noticed that physicists, especially particle physicists like Don Lincoln at Fermilab, both understand quantum field theory (QFT) and have no problem calling the objects of their research particles: “the particles we see are just localized vibrations in the field.” And I do like the term “localized vibrations” better than Feynman’s wiggles.

    But whether called localized vibrations or particles, my puzzlement about the electron remains, that it’s “assumed to be a point particle with a point charge and no spatial extent” — despite issues with the theory of relativity and mathematical infinities. So, this Phys.org article “The geometry of an electron determined for the first time” by University of Basel (May 23, 2019) caught my attention.

    [Graphic] An electron is trapped in a quantum dot, which is formed in a two-dimensional gas in a semiconductor wafer. However, the electron moves within the space and, with different probabilities corresponding to a wave function, remains in certain locations within its confinement (red ellipses). Using the gold gates applied electric fields, the geometry of this wave function can be changed. (Image: University of Basel, Departement of Physics)

    A quantum dot is a potential trap which allows confining free electrons in an area which is about 1000 times larger than a natural atom. Because the trapped electrons behave similarly to electrons bound to an atom, quantum dots are also known as “artificial atoms.”

    Based on their theoretical model, it is possible to determine the electron’s probability density and thus its wave function with a precision on the sub-nanometer scale.

    “To put it simply, we can use this method to show what an electron looks like for the first time,” explains Loss.

  6. This Phys.org article “Watching electrons using extreme ultraviolet light” (August 20, 2019) by Denis Paiste, Massachusetts Institute of Technology, discusses research on the interaction of photons and electrons in the surface of materials — using extreme ultraviolet photons to measure the dynamics of electrons (point particles) on a femtosecond timescale.

    A new technique developed by a team at MIT can map the complete electronic band structure of materials at high resolution. This capability is usually exclusive to large synchrotron facilities, but now it is available as a tabletop laser-based setup at MIT. This technique, which uses extreme ultraviolet (XUV) laser pulses to measure the dynamics of electrons via angle-resolved photoemission spectroscopy (ARPES), is called time-resolved XUV ARPES.

    The MIT team evaluated their instrument resolution using four exemplary materials representing a wide spectrum of quantum materials: a topological Weyl semimetal, a high-critical-temperature superconductor, a layered semiconductor, and a charge density wave system.

    The technique is described in a paper appearing in the journal Nature Communications …

    A central goal of modern condensed-matter physics is to discover novel phases of matter and exert control over their intrinsic quantum properties.

    For example, anyone can drop a pebble on the surface of water and watch how the ripples decay to observe the surface tension and acoustics of water. The difference in the MIT setup is that the researchers use infrared light pulses to “pump” the electrons to the excited state and the XUV light pulses to “probe” the photoemitted electrons after a time delay.

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