Table of contents

Introduction
About gamma photons
Solar gamma photons

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INTRODUCTION

Rays, radiation, particles, waves, fields … what’s in a name, eh? There’s a history [1], an evolution of discovery and understanding. So that sometimes a “particle” by another name is something more interesting [2].

While quantum field theory (QFT) has penetrated the way many physicists think (and sometimes even talk) about fundamental so-called particles, I hope that the language of QFT will enlighten the common vernacular as well (for example, in popsci articles).

Let’s start with the electromagnetic spectrum. And using the term photon at least – gamma photons (aka “rays” in 1900).

This vintage NASA video visualizes the “chaos” of EM waves around us, all the time.

• YouTube > NASA > ScienceAtNASA > “Tour of the EMS 01 – Introduction” (May 6, 2010)

The complete range of photons in the electromagnetic field has “no known limit for long and short wavelengths.” And the “gamma” energy band has “no precisely defined boundary.” With extreme photon energy – “the most energetic form of electromagnetic radiation” [that is, excitations of the electromagnetic (EM) field].

• NASA > “Tour of the Electromagnetic Spectrum – Gamma Rays” (2016) [includes YouTube video]

Unlike optical light and x-rays, gamma rays cannot be captured and reflected by mirrors. Gamma-ray wavelengths are so short that they can pass through the space within the atoms of a detector. Gamma-ray detectors typically contain densely packed crystal blocks. As gamma rays pass through, they collide with electrons in the crystal. This process is called Compton scattering, … These collisions create charged particles that can be detected by the sensor.

A recent Quanta Magazine article about solar gamma photons (below) reminded me that I’d not really written a post about gamma photons per se. Other posts discuss gamma photons in various contexts [3].

Wiki notes that: “In 1914, gamma rays [photons] were observed to be reflected from crystal surfaces, proving that they were electromagnetic radiation” (waves) – rather than “particles” (as for alpha and beta “rays” / radiation).

A gamma decay was then understood to usually emit a gamma photon.

So, the extreme energy side of the electromagnetic spectrum is hard to imagine. So beyond our everyday experience with the photon energies of visible light (1.6 eV to 3.4 eV). Wiki notes that a TeV (one trillion electronvolts) photon (far from the highest energy) – about the kinetic energy of a flying mosquito – is able to penetrate our bodies and damage cells, and requires shielding by dense material such as lead, steel, or concrete.

Section notes

[1] An etymology also related to the term cosmic ray:

The term ray (as in optical ray) seems to have arisen [historically] from an initial belief, due to their penetrating power, that cosmic rays were mostly electromagnetic radiation. Nevertheless, following wider recognition [of] various high-energy particles with intrinsic mass, the term rays was still consistent with then known particles such as cathode rays, canal rays, alpha rays and beta rays. Meanwhile “cosmic” ray photons, which are quanta of electromagnetic radiation (and so have no intrinsic mass) are known by their common names, such as gamma rays or X-rays, depending on their photon energy.

[2] Something more interesting indeed, as in these questions.

• West Texas A&M University > Science Questions with Surprising Answers by physics professor Dr. Christopher S. Baird > “How do you make a one-photon-thick beam of light?” (July 23, 2014)

There is no such thing as a one-photon-thick beam of light. … The number of photons in a light beam is more an indication of the beam’s brightness than of the beam’s width. … we could construct a light source that only emits one photon of light every second. But we discover that as soon as the photon travels out into free space, the single photon spreads out into a wave that has a non-zero width and acts just like a coherent beam containing trillions of photons. Therefore, even if a beam has a brightness of only one photon per second, it still travels and spreads out through space like any other light beam.

• Is it possible to directly measure the frequency of a single photon?

Here’s one answer:

It is not actually possible to directly measure the frequency of a single photon of light. This is because a single photon is going to behave more like a particle than a wave, and the concept of frequency (cycles or alternations per second) only applies to waves.

• How can a (single) photon have a “frequency”?

Cf. Is it possible to measure the frequency of a single photon?

As opposed to “a coherent signal which is the aggregate of many photons.”

Some answers use these methods:

A. Computations (indirect method)

E=hf

where E = photon energy (measured), h = Planck constant, f = frequency

B. Statistics

However, if you only have one photon to work with you can’t do it, because only the probability is determined. You would need to repeat the experiment with a large sample of identical photons to get the answer statistically.

So the correct way to do it is to measure the photon’s momentum, for example by colliding it with a target in zero gravity. The frequency of the photon is known to be proportional to the momentum, and the wavelength is inversely proportional.

C. Interactions

That takes place when the photon interacts with an electron in a photon detector. A photon has a certain kinetic energy which we can know about when that energy interacts with the electron; the difference between the two levels of energy emerges as a wave front which tells us the frequency of the photon at the moment of detection (interaction). It is commonly thought that the photon has a frequency and therefore a wavelength during transit from the EM field that launched it and the EM field that absorbs its kinetic energy, but that is not a right conception of the nature and structure [?] of a photon.

[This method, as incompletely visualized in a video (Aug 23, 2014), uses the notion of rapid beats blending together (as in a musical tone), perceived as a smooth tone (or single pulse). Topologically, a traveling photon (energy / wave packet) is akin to a moving toroid with a curved surface converging and diverging from a center point – tracing a 3D sinusoidal “tunnel.” Appearing classically (as simplified) like the topology of a nondescript point particle. The quantum field topology is essentially opaque until active in field interactions. At which point we’re “in the weeds” of relational quantum theory.]

Here’s another answer:

The classical electromagnetic wave is built out of [many] photons … So for a wave pulse [packet?] advancing smoothly in some direction, the oscillations inside the pulse correspond to the photon frequency and wavelength. Meanwhile the general movement of the pulse would be associated with the smooth movement of the particle. Quite different. So the photon itself doesn’t wiggle with the photon frequency, just the underlying abstract quantum field used to describe where we can find it.

[3] Contexts such as: Gamma-ray bursts, blazar neutrinos, multi-messenger astronomy / astrophysics, gamma-ray astronomy, the Fermi Gamma Ray Space Telescope, the Milky Way’s “Fermi bubbles,” black holes, space medicine.

Section related posts

• A particle by any other name?

As I noted in that post, an extended spacetime tangle (excitation in a field) might be point-like mathematically (much as a spherical “particle” is topologically the same as a point), but that’s a simplification of reality.

• A boson by any other name would …

• QFT – fields and wave packets

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ABOUT GAMMA PHOTONS

My post on gamma ray bursts [Cosmic retrospective — gamma ray bursts] includes a December 28, 2022, comment which references this Space.com article (below) by Keith Cooper.

As noted there, setting aside Cooper’s popsci legacy characterization of photons (“packets of energy”) as particles which can behave as waves, his article provides a useful recap of gamma rays.

The article covers where these high-energy photons fit in the electromagnetic spectrum (compared to visible light); their energy levels, production, health hazards (protection measures), history of detection; cosmic bursts and current space observatories. As well as some useful references.

• Space.com > “Gamma rays: Everything you need to know about these powerful packets of energy” by Keith Cooper (December 28, 2022) – Gamma rays can only be detected by sensors made of dense metals and takes over six feet (1.8 meters) of concrete to block.

Section related posts

• Cosmic retrospective — gamma ray bursts

• Levels of understanding – what are X-rays?

• Up in the sky — faster than a speeding LHC proton

• Humans fit for space? — NASA’s Twins Study

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SOLAR GAMMA PHOTONS

While “broad spectrum” sunscreen lotions offer protection for our skin from some UV photons (rays), our sun emits “electromagnetic radiation across the electromagnetic spectrum at all wavelengths, from radio waves to gamma rays.” [1]

Solar flares are particularly intense eruptions of electromagnetic radiation.

The persistence of highly energetic (TeV) gamma photons from our Sun may depend on a “sweet spot” in the interaction of cosmic rays with the Sun’s magnetic field. A tale of energy scale [2].

• Quanta Magazine > “Strange Solar Gamma Rays Discovered at Even Higher Energies” by Katie McCormick, Contributing Writer (February 27, 2023) – Our sun’s high-energy radiation is much more than expected by current models, likely indicating that “the propagation of cosmic rays from their sources to us is not [well] understood.”

… researchers with the High-Altitude Water Cherenkov (HAWC) Gamma-Ray Observatory and collaborators report on the abundance of gamma radiation persisting in a range that is two to 10 times more energetic than that of any previously measured solar gamma rays.

[Astrophysicist Annika Peter, Ohio State University] Peter and her colleagues are working on the problem, developing elaborate simulations of the sun’s magnetic fields and the intricate dynamics of the cosmic particles winding around them.

Section notes

[1] Wiki notes: “UV radiation is present in sunlight, and constitutes about 10% of the total electromagnetic radiation output from the Sun.”

[2] The mind-boggling scale, for example, of cosmic gamma-ray bursts: “The so-called long-duration gamma-ray bursts produce a total energy output of about 10^44 joules (as much energy as the Sun will produce in its entire life-time) but in a period of only 20 to 40 seconds.”

Section related posts

• Living with stellar storms – the moods of our Sun