Extreme Gamma Rays

The highest-energy gamma ray photon ever recorded was recently observed by the Large High Altitude Air Shower Observatory (LHAASO) on Haizi Mountain, Sichuan province, China, during its first year of operation.

1.42 ± 0.13 PeV

That is 1.4 petaelectronvolts = 1.4 × 1015 eV! The origin of this fantastically energetic photon hasn’t been localized, but possible candidates are the Cygnus OB2 young massive cluster (YMC), the pulsar PSR 2032+4127, or the supernova remnant candidate SNR G79.8+1.2.

The LHAASO observatory, in China, observes ultra high-energy light using detectors spread across a wide area that will eventually cover more than a square kilometer. Institute of High Energy Physics/Chinese Academy of Sciences

How much energy is 1.4 PeV, actually?

We can calculate the frequency of this photon using


h = Planck’s constant = 4.135667696 × 10-15 eV·Hz-1
ν = the photon’s frequency
E = the photon’s energy

Solving for ν, we get

ν = 3.4 × 1029 Hz

Next, we’ll calculate the photon’s wavelength using

c=\lambda \nu

c = the speed of light = 299792458 m·s-1
λ = the photon’s wavelength

Solving for λ, we get

λ = 8.9 × 10-22 m

To give you an idea of just how tiny 8.9 × 10-22 meters is, the proton charge radius is 0.842 × 10-15 m, so 1.9 million wavelengths of this gamma ray photon would fit inside a single proton! An electron has an upper limit on its radius—if it can be said to have a radius at all—between 10-22 and 10-18 m. So between 1 and 2000 wavelengths of this gamma ray photon would fit inside a single electron.

Using Einstein’s famous equation E = mc2 we can find that each eV has a mass equivalent of 1.78266192 × 10-36 kg. 1.4 PeV then gives us a mass of 2.5 × 10-21 kg. That may not sound like a lot, but it is 1.5 million AMUs (Daltons), or a mass comparable to a giant molecule (a protein, for example) containing ~200,000 atoms.

This and other extremely high energy gamma ray photons are not directly detected from the Earth’s surface. The LHAASO detector array in China at 14,500 ft. elevation detects the air shower produced when a gamma ray (or cosmic ray particle) hits an air molecule in the upper atmosphere, causing a cascade of subatomic particles and lower-energy photons, some of which reach the surface of the Earth. It is the Cherenkov photons produced by the air shower secondary charged particles that LHAASO collects.

Conover, E. (2021, June 19). Record-breaking gamma rays hint at violent environments in space. Science News, 199(11), 5.

Z. Cao et al. Ultrahigh-energy photons up to 1.4 petaelectronvolts from 12 γ-ray Galactic sourcesNature. Published online May 17, 2021. doi: 10.1038/s41586-021-03498-z.

The Laws of Physics and the Existence of Life

George F. R. Ellis writes in Issues in the Philosophy of Cosmology:

The first requirement is the existence of laws of physics that guarantee the kind of regularities that can underlie the existence of life.  These laws as we know them are based on variational and symmetry principles; we do not know if other kinds of laws could produce complexity.  If the laws are in broad terms what we presently take them to be, the following inter alia need to be right, for life of the general kind we know to exist:

  • Quantization that stabilizes matter and allows chemistry to exist through the Pauli exclusion principle.

  • The neutron-proton mass differential must be highly constrained.  If the neutron mass were just a little less than it is, proton decay could have taken place so that by now no atoms would be left at all.

  • Electron-proton charge equality is required to prevent massive electrostatic forces overwhelming the weaker electromagnetic forces that govern chemistry.

  • The strong nuclear force must be strong enough that stable nuclei exist; indeed complex matter exists only if the properties of the nuclear strong force lies in a tightly constrained domain relative to the electromagnetic force.

  • The chemistry on which the human body depends involves intricate folding and bonding patterns that would be destroyed if the fine structure constant (which controls the nature of chemical bonding) were a little bit different.

  • The number D of large spatial dimensions must be just 3 for complexity to exist.

It should not be too surprising that we find ourselves in a universe whose laws of physics are conducive to the existence of semi-intelligent life.  After all, we are here.  What we do not know—and will probably never know: Is this the only universe that exists?  This is an important question, because if there are many universes with different laws of physics, our existence in one of them may be inevitable.  If, on the other hand, this is the only universe, then the fantastic claims of the theists, or at least the deists, become more plausible.

You may wonder why I call the human race semi-intelligent.  Rest assured, I am not being sarcastic or sardonic.  I say “semi-intelligent” to call attention to humanity’s remarkable technological and scientific achievements while also noting our incredible ineptness at eradicating war, violence, greed, and poverty from the world.  What is wrong with us?

G.F.R. Ellis, Issues in the Philosophy of Cosmology, Philosophy of Physics (Handbook of the Philosophy of Science), Ed. J. Butterfield and J. Earman (Elsevier, 2006), 1183-1285.

Probing the Proton

We have known since 1968 that protons are not elementary particles and are comprised of three persistent quarks: two up quarks with charge +⅔e, and one down quark with charge -⅓e, where e is the charge of the electron (which is an elementary particle).

But in a brilliant illustration of E = mc2, we now know that very little of the proton’s 938 MeV rest mass comes from the rest mass of its component up and down quarks.

The mass composition of a proton is:

½% up quark rest mass (¼% × 2)

½% down quark rest mass

8% sea quarks (virtual quark-antiquark pairs created by the strong nuclear force)

23% quark-gluon interaction energy (the trace anomaly)

32% kinetic energy of the up and down quarks

36% momentum energy of the massless gluons that hold the up and down quarks together within the proton

We thus see that only 1% of the rest mass of a proton is provided by the rest mass of its three component valence quarks (2 up + 1 down), and the other 99% is interaction energy!

Even though protons are in the nucleus of every atom of matter in the universe, we still do not fully understand them.  For example, there is the proton radius puzzle, the proton spin puzzle, and the important question of proton decay.

E. Conover. How the proton’s mass adds up. Science News. Vol. 194, December 22, 2018, p. 8.
E. Conover. There’s still a lot we don’t know about the proton. Science News. Vol. 191, April 29, 2017, p. 22.
Y.B. Yang et al. Proton mass decomposition from the QCD energy momentum tensor. Physical Review Letters. Vol. 121, November 23, 2018, p. 212001. doi: 10.1103/PhysRevLett.121.212001.

Constants of Nature

We continue our series of excerpts (and discussion) from the outstanding survey paper by George F. R. Ellis, Issues in the Philosophy of Cosmology.

The constants of nature are indeed invariant, with one possible exception: the fine structure constant, where there is claimed to be evidence of a very small change over astronomical timescales.  That issue is still under investigation.  Testing such invariance is fundamentally important, precisely because cosmology usually assumes as a ground rule that physics is the same everywhere in the universe.  If this were not true, local physics would not guide us adequately as to the behaviour of matter elsewhere or at other times, and cosmology would become an arbitrary guessing game.

The fine structure constant (α) is a unitless number, approximately equal to 1/137, that characterizes the strength of the electromagnetic force between electrons.  Its value is the same no matter what system of measurement one chooses.  If the value of α were just a little smaller, molecular bonds would be less stable.  If the value of α were just a little larger, carbon—which is essential to life—could no longer be produced inside of stars.

Do constants of nature, specifically dimensionless physical constants such as α, the fine structure constant, and μ, the proton-to-electron mass ratio1, vary with time?  This is an active topic of investigation.  If constants of nature change at all, they change so slowly that it presents a formidable challenge to measure that change.  But if they do indeed change, it would have profound implications for our understanding of the universe.  A lot can happen in 13.8 billion years that might not be at all obvious in the infinitesimal interval of a human life or even human civilization.

“Despite the incessant change and dynamic of the visible world, there are aspects of the fabric of the universe which are mysterious in their unshakeable constancy.  It is these mysterious unchanging things that make our universe what it is and distinguish it from other worlds we might imagine.” – J.D. Barrow, The Constants of Nature. (Vintage, 2003).

I’d like to conclude this discussion of constancy and change with a poem I wrote about the possibility of sentient life having a very different sense of time than we humans do.

Life On a Cold, Slow World

Life on a cold, slow world
On Europa, perhaps, or even Mars
On distant moons and planets of other stars.

A minute of time for some anti-freeze being
Might span a year for us human folk
(A greeting could take a week, if spoke.)

How fast our busy lives would seem to pass
Through watchful eyes we cannot see
Curious about our amative celerity.

The heartbeat of the universe runs slow and deep
We know only violent change, the sudden leap
But that which is most alive appears to sleep.

David Oesper

1μ = mp / me ≅ 1836

Barrow, J.D., Webb, J.K., 2005, Scientific American, 292, 6, 56-63

Ellis, G. F. R. 2006, Issues in the Philosophy of Cosmology, Philosophy of Physics (Handbook of the Philosophy of Science), Ed. J. Butterfield and J. Earman (Elsevier, 2006), 1183-1285.