THE BLOG
11/23/2012 06:40 pm ET Updated Jan 23, 2013

Particles Are for Real

In an article in the December 2012 Scientific American, physicist David Tong makes the following statement:

Physicists routinely teach that the building blocks of nature are discrete particles such as the electron or quark. That is a lie. The building blocks of our theories are not particles but fields: continuous, fluidlike objects spread throughout space.

This is highly misleading. No one has ever observed a quantum field. Quantum fields are purely mathematical constructs within quantum field theory.

Every quantum field has associated with it a particle that is called the "quantum of the field." The photon is the quantum of the electromagnetic field. The electron is the quantum of the Dirac field. The Higgs boson is the quantum of the Higgs field. In other words, like love and marriage, you can't have one without the other. The building blocks of our theories are fields and particles.

But note that what Tong calls "a lie" is the notion that the building blocks of nature are discrete particles while it is fields that are the building blocks of our theories. That is, he is equating nature to theory.

Tong is revealing his acceptance of a common conception that exists among today's theoretical physicists (Paul Dirac and Richard Feynman, among other twentieth-century greats, were not part of that school). That is, the symbols that appear in their mathematical equations represent "true reality" while our observations, which always look like localized particles, are just the way in which that reality manifests itself.

The association of mathematical symbols with true reality is precisely the philosophy that was proposed by Plato 2,400 years ago in his theory of forms. Platonism is more theology than science, and Plato was no scientist. He claimed that a divine craftsman called the Demiurge constructed the cosmos according to a divine plan. There are two realms: a realm of forms or ideas that is perfect and a material realm in which these forms or ideas are imperfectly replicated (Lindberg, p.36).

As someone who 40 years as an experimental elementary particle physicist, I carry a different perspective. I started in the 1960s analyzing bubble chamber pictures where one sat at a scanning table and traced the beautiful tracks of bubbles photographed during the time the superheated liquid in the chamber was exposed to a pulsed beam from an accelerator.

It never occurred to my collaborators and me to describe what we observed as some kind of field in an abstract multidimensional space. (Quantum fields do not live in familiar space-time, despite Tong's assertion). We interpreted the trails of bubbles as the effect of charged particles ionizing the atoms of the liquid and producing localized boiling along their paths. The paths were curved because of an external magnet that produced a magnetic field inside the chamber, and by measuring the curvature of an individual path the momentum of the particle could be determined. From the rate at which the particle slowed down as it lost energy to ionization we could also determine its speed and, from these two measurements, determine the particle's energy.

These measurements enabled us detect the presence of many new objects, which we also called "particles," that had never before been observed. Some were so unstable that they did not live long enough to leave a measurable track in the chamber. They were detected by the missing mass calculated from the measurements of the particle tracks for a complete event.

For example, suppose the chamber is filled with hydrogen and exposed to a beam of very high-energy pions. Then you will observe the track of a pion coming in and colliding with a proton in the hydrogen. That is called an "event." You then measure the momenta and energies of the outgoing tracks and use energy and momentum conservation to determine any missing mass.

While the detectors found at today's giant colliders, such as the Large Hadron Collider, are far more sophisticated than the bubble chamber, they operate on the same general principle. They measure momentum and energy by magnetic curvature and energy deposit.

And, the workers at the LHC do not talk about colliding the quantum fields of two protons together to measure the wavelengths of some abstract wave oscillating in some imaginary aether. They speak of banging particles together and measuring the particles they see coming out.

The point is, while our mathematical theories are expressed in terms of abstract fields, what we always measure is best described as particles.

For example, consider the famous double slit experiment. Light described as an oscillating electromagnetic field or "wave" passes through two narrow slits producing an intensity pattern on a screen that is characterized by a series of alternating, equally spaced bright and dark "interference" fringes. The distance between the fringes can be used to calculate the wavelength of the oscillating field.

However, if in place of the screen you have an array of photodetectors sensitive at the one-photon level, you will detect individual photons -- the particles that constitute light according to Einstein's 1905 theory of the photoelectric effect. That is, you do not detect a field or a wave. You detect a particle. The fringe pattern does not appear until you have accumulated the statistical distribution of a large number of photons. The so-called "wave nature" of light is thus not a characteristic of a single photon; it is a characteristic of the behavior of an ensemble of many photons.

Now, other philosophical schools exist besides Platonism. Most experimentalists, if dragged kicking and screaming into having to think about a philosophical question, would probably say that the objects they observe, which they call particles, are "real." But then, they have no better means to prove that assertion than their theoretical colleagues have to insist that only fields are real.

Questions on the nature of reality are metaphysics. They entail no observations that can adjudicate the questions scientifically. If they did, they would be physics and not metaphysics. I know of only two other ways by which we might determine the nature of reality outside the realm of observation: revelation and reason. I doubt many theoretical physicists believe in revelation, and those few that do make no attempts to apply it to their work and thereby invite ridicule from their colleagues.

That leaves reason. Plato, Aristotle, St. Augustine, St. Thomas Aquinas, Immanuel Kant, and many other great thinkers throughout history have claimed that reason absent observation is capable of discovering truths about the world. The trouble is, despite thousands of years of trying, they haven't yet come up with a single example of such a truth that has been objectively verified.

No one doubts the moon is real, and it's just a particle when viewed from far enough away. How is it any different from a photon registered in a photodetector? So, even if I can't prove it, it seems reasonable that photons, electrons, quarks, neutrinos, and Higgs bosons are for real.

Reference

David C. Lindberg. The Beginnings of Western Science: The European Scientific Tradition in Philosophical, Religious, and Institutional Context, Prehistory to A.D. 1450. 2nd ed. (Chicago: University of Chicago Press, 2007).

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