Physics – Cosmic Reflections

The Electromagnetic Spectrum

What do light, radio waves, and x-rays have in common? Everything! They are all electromagnetic radiation which differ only in their frequency, wavelength, and energy. All electromagnetic radiation travels at the speed of light, which in a vacuum is 186,282 miles per second or almost 671 million mph.

There are no distinct boundaries between different regions of the electromagnetic spectrum. The spectrum can, however, be conveniently divided into the following seven general categories (in order of decreasing energy and increasing wavelength): gamma rays, x-rays, ultraviolet, visible, infrared, microwave, and radio.

Depending on how it is observed, all electromagnetic radiation has a dual nature, capable of behaving either as a wave or as a particle. We call these particles photons. The highest energy photons are gamma ray photons, and the lowest energy photons are radio wave photons.

The highest-energy, shortest-wavelength electromagnetic radiation (gamma rays and x-rays) usually behaves like a particle rather than a wave in experiments, whereas the lowest-energy, longest-wavelength electromagnetic radiation (radio waves) usually behaves like a wave in experiments.

Is there a limit to how far the electromagnetic spectrum extends? On the high energy end, we think so. On the low energy end, not really. Let’s explore both extremes.

Gamma rays are produced by the most energetic processes in the universe. The highest energy gamma rays yet detected are 2.5 PeV (2.5 peta-electronvolts or 2.5 × 10¹⁵ eV). The theoretical limit to the energy of a gamma ray is the Planck energy, 12.2 ReV (12.2 ronna-electronvolts or 1.22 × 10²⁸ eV). Any gamma ray having this much energy would have a wavelength of a Planck length (1.6 × 10^-35 meter). At this wavelength, standard quantum physics breaks down and the gamma ray photon would concentrate so much energy into so small a space that it would likely collapse into a microscopic black hole, ending its existence as radiation.

There is another, much lower energy limitation, however. At gamma ray photon energies around 0.1 to 100 PeV (10¹⁴ – 10¹⁷ eV), the universe becomes almost completely opaque over large distances. A gamma-ray photon at this extreme energy will travel only a short distance before colliding with a photon from the pervasive cosmic microwave background. This collision converts the original photon into an ultra-relativistic electron-positron pair. As these new particles hurtle through space, they smash into additional background photons via inverse Compton scattering, boosting them into high-energy gamma rays. This triggers an energy-diluting electromagnetic cascade: photons create particle pairs, and particle pairs produce new photons. The chain reaction repeats until the individual photon energies drop below the threshold required for pair production. The end result: the original high-energy gamma ray photon never reaches your detector.

Now, let’s talk about the opposite end of the spectrum, the radio waves. What do we know about the least energetic of photons? To put things in perspective, you may be interested to know that national and international standards have been established for the orderly use of the radio spectrum down to a frequency of 3 kHz (3000 Hz), although the military is using 40 – 80 Hz radio waves for global submarine communications, and every electrical wire in your house is generating low levels of 60 Hz radio waves due to alternating current flipping back and forth 120 times each second.

Below 40 Hz exists a strange realm indeed. The Earth and its atmosphere act like a giant, natural radio transmitter. Lightning storms continuously bombard the upper atmosphere and trigger extremely low frequency radio waves (known as Schumann resonances) that ring around the globe at roughly 7.5 Hz.

To generate radio waves with even lower frequencies than 7.5 Hz, the scale of the “antenna” must grow to planetary proportions. In nature, this occurs when exceptionally energetic lightning events or the shockwaves from large meteors dump enormous energy into the atmosphere, creating radio waves with frequencies of 3 Hz or lower. The immense mechanical stress of shifting fault lines before an earthquake can turn miles of quartz-rich bedrock into a colossal subterranean transmitter, broadcasting seismic radio pulses down to a fraction of a hertz via the piezoelectric effect. Humans can replicate this feat artificially, but the physical constraints are staggering. For example, a 1 Hz wave has a wavelength of nearly 300,000 kilometers. Efficient transmission requires an antenna of comparable size.

There is no theoretical lower limit to a radio wave’s frequency. Any accelerating electrical charge can produce one, and if the system that produces it is large enough, natural processes can generate radio waves with frequencies as low as a nanohertz (a billionth of a Hz), or even lower, resulting in wavelengths measured in light-years.

We can only study these ultra low frequency radio waves which are of a cosmic origin from space because the Earth’s ionosphere reflects away everything below about 5 MHz. But we’ll have to travel far from the Sun if we wish to listen to cosmic radio frequencies below about 20 kHz because the solar wind acts as a reflective shield. Only by traveling beyond the heliopause and into interstellar space will we escape the Sun’s plasma shield, though the ambient interstellar medium itself will still block frequencies below 2 kHz. In order to listen to the cosmic din and chatter at sub-Hertzian frequencies, you’d have to find a completely empty intergalactic void entirely cleared of plasma.

Here are the wavelength ranges of our currently operating general purpose space telescopes (1 Å = 1 angstrom = 0.1 nm = 0.1 nanometer = 10^-10 m).

Fermi Gamma-ray Space Telescope
0.000000041 → 1.55 Å

Chandra X-Ray Observatory
1.20 → 120 Å

Hubble Space Telescope Visible Spectrum
1150 → 17,000 Å 3800 → 7500 Å (blue → red)

James Webb Space Telescope
6000 → 283000 Å

There are currently no active general purpose space telescopes operating in the 120 Å to 1150 Å range (between Chandra and Hubble). This is known as Extreme Ultraviolet (EUV), the transition zone between the ultraviolet and x-ray parts of the electromagnetic spectrum. However, space telescopes that observe the Sun do observe in the EUV region.

Because neutral hydrogen in the interstellar medium is highly efficient at absorbing EUV radiation, galactic and extragalactic objects are difficult or impossible to observe at these wavelengths, especially between 400 Å and 912 Å.

There are currently no general purpose space telescopes observing at wavelengths longer than 283000 Å (James Webb), in the far-infrared and radio part of the spectrum.

To detect far-infrared and submillimeter (shortest-wavelength radio) radiation requires a space telescope to be cooled to nearly absolute zero. This is challenging but not impossible.

The Earth’s atmosphere is adequately transparent to most radio waves, so observing this part of the electromagnetic spectrum from the Earth’s surface is easier and cheaper than building a radio space telescope. However, the far side of the Moon would be an even better location to observe almost the entire radio spectrum (except for the short-wavelength submillimeter radiation which requires extreme cooling as mentioned above), both because the Moon lacks an atmosphere but also because terrestrial radio interference would be completely blocked.

Gravitational Lenses and Caustics

A massive foreground object such as a galaxy or a galaxy cluster (including, of course, dark matter) can brighten and magnify a distant object. This is called gravitational lensing. Light from the distant object radiates out in all directions, but the massive foreground object bends some of these light rays towards the observer that normally would have continued on in a different direction, as shown in the illustration above.

The image we see is brighter because more light rays are directed our way. The image we see is also magnified because the gravitational lens gives the distant object a larger angular size, making it appear to be much closer to us. Again, the illustration above will help you understand why these effects occur.

Of course, unless the foreground object is a star or black hole or some other small spherical object with a reasonably uniform mass distribution, the gravitational lens effect will be complex and distorted, as illustrated below.

Gravitational lenses produce different shaped images depending on the shape of the lensing body. If the lens is spherical then the image appears as an Einstein ring (in other words as a ring of light) (top); if the lens is elongated then the image is an Einstein cross (it appears split into four distinct images) (middle), and if the lens is a galaxy cluster, then arcs and arclets (banana-shaped images) of light are formed (bottom). **Credit:** European Space Agency

There can be certain locations in a gravitational lens where light from a small region in the background becomes enormously magnified by a factor of up to 10,000 times. These regions are called caustics. Though the concept of a caustic is a bit difficult to describe or illustrate, here is a video of optical caustics caused by a laser pointer shining through a plastic disc with a lumpy surface.

References
How gravitational lensing acts as a magnifying glass — diagram

Different types of gravitational lenses

Caustic Projection Optical Element

Rodríguez, J. M. D. (2026). The First Stars. Scientific American, 334(2), 38. https://doi.org/10.1038/scientificamerican022026-2z1ygyIpj7gCMVn01NrfpU

Oxygen Speaks with an Accent

There are three stable (non-radioactive) isotopes of the element oxygen:

¹⁶O has 8 protons and 8 neutrons
¹⁷O has 8 protons and 9 neutrons
¹⁸O has 8 protons and 10 neutrons

All the oxygen in our solar system was forged in stars that existed before the birth of our Sun. The fusion processes that create oxygen from lighter elements require both high temperature and pressure. These conditions exist deep within a star. Different isotopes are created. A nucleus of an atom containing 8 protons identifies it as an oxygen atom, but it is the number of neutrons in the nucleus that determines which isotope it is. Not all isotopes are created in equal abundance.

When the solar system was forming, the oxygen in the “solar nebula” no doubt originally came from various progenitors. A supernova here or there, a planetary nebula somewhere else, and so on. As the solar nebula collapsed to form the Sun and planets, the relative abundance of oxygen to the other elements may or may not have been different in different parts of the solar nebula. Similarly, the relative abundances of the three stable isotopes of oxygen may also have been different in different parts of the solar nebula.

When we measure the relative amounts of the three oxygen isotopes in a terrestrial rock, ocean water, moon rocks, or the solar wind, it may tell us where the oxygen in those materials came from. It may also tell us something about the “life experiences” of the oxygen since the solar system formed. For example, water molecules containing ¹⁶O are more likely to evaporate than those water molecules containing the heavier isotopes ¹⁷O or ¹⁸O. Thus, ground water in the middle of a continent has a higher abundance of ¹⁶O than does water in the ocean.

When we look at the solar system today, we find significant differences in the relative abundances of the oxygen isotopes depending on where the material came from. On Earth, 99.75% of the oxygen atoms are of the ¹⁶O variety, 0.04% are ¹⁷O, and 0.21% are ¹⁸O, on average. We see very similar oxygen abundance ratios in moon rocks, indicating perhaps a common origin, but the oxygen abundance ratios in meteorites and solar wind particles are significantly different from this. For example, if you plot the ¹⁷O/¹⁶O ratio vs. the ¹⁸O/¹⁶O ratio for a bunch of terrestrial rocks, you get pretty much a straight line. Moon rocks fall along the same line. The calcium-aluminum-rich inclusions (CAI) and iron-magnesium-silicon chondrules in meteorites also form a straight line on this plot, but it has a distinctly different slope.

The solar wind samples collected by the Genesis spacecraft yielded abundances that fall along the same line as the CAIs and chondrules. Mars rocks fall on a line that parallels the Earth-Moon line, but is shifted upwards, indicating that for a given abundance of ¹⁸O, the Mars rocks will have a relatively higher abundance of ¹⁷O.

Superheavy Elements

There are currently 118 known chemical elements. The most recent, 118 Oganesson (chemical symbol Og), was first synthesized in 2002 . Its only known isotope, $\mathbf{\frac{294}{118}\textrm{\textbf{Og}}}$ (118 protons + 176 neutrons = 294 nucleons), has a half-life of just 0.0007 seconds, and to date only five oganesson atoms have been produced.

It is possible, given our current knowledge of nuclear physics, that there is at least one island of nuclear stability where stable or quasi-stable isotopes of superheavy elements exist. One such island might exist around Z = 164, that is an element having 164 protons and something like 246 neutrons.

Are any superheavy elements stable enough to be found in nature? Is there any astrophysical process that could produce them? If superheavy elements exist, we would expect such matter to have a mass density in excess of the densest-known stable element, osmium (element 76), 22.59 g/cm³. Superheavy elements around Z = 164 are expected to have a mass density between 36.0 and 68.4 g/cm³.

Researchers at the University of Arizona in Tucson explain that superheavy elements might exist in nature, either in the exotic form of extremely dense alpha matter — nuclear matter composed of alpha particles in a Bose-Einstein condensate-like configuration — or as standard matter. Though a long shot, they suggest looking at asteroids (and other objects) possibly having anomalously high densities, which they call Compact Ultradense Objects (CUDOs).

In order to calculate the density of an asteroid, you need to measure its volume and its mass. The volume can be calculated if you know the size and shape of the asteroid, and the mass can best be calculated if the asteroid has a satellite (either natural or artificial), or from a spacecraft flyby. A less certain mass can be calculated by measuring how an asteroid gravitationally perturbs a neighboring asteroid as they both orbit around the Sun. We must keep in mind that any asteroids that presently appear to have an unusually high density may later be found to have a more normal density upon better estimates of the size and shape of the asteroid, and especially its mass.

The most recent available table of asteroid bulk densities can be found on the SiMDA (Size, Mass, and Density of Asteroids) web site. In that table, a bulk density accuracy rank of A (most accurate) to E (least accurate), and X (unrealistic) for each object is given. Among the A-rank densities, we find that 16 Psyche is listed as having the highest bulk density of 3.90 ± 0.29 g/cm³. NASA’s Psyche robotic spacecraft was launched on October 13, 2023 and is expected to begin orbiting 16 Psyche in August 2029.

Among the B-rank densities, two asteroids have nominal bulk densities higher than 16 Psyche’s: 135 Hertha at 4.45 ± 0.63 g/cm³ and 192 Nausikaa at 4.10 ± 0.70 g/cm³.

Among the C-rank densities, 21 asteroids have nominal bulk densities higher than 16 Psyche’s:

Rank "C" Asteroid Densities (> 16 Psyche)

206 Hersilia	6.08 ± 2.55
181 Eucharis	5.46 ± 2.43
410 Chloris	4.96 ± 2.41
679 Pax	4.95 ± 1.45
110 Lydia	4.88 ± 1.75
97 Klotho	4.80 ± 1.01
124 Alkeste	4.74 ± 2.22
275 Sapientia	4.69 ± 1.12
92 Undina	4.64 ± 1.75
34 Circe	4.63 ± 1.21
56 Melete	4.57 ± 1.07
102 Miriam	4.46 ± 1.88
680 Genoveva	4.37 ± 2.06
129 Antigone	4.35 ± 2.14
69 Hesperia	4.33 ± 1.11
709 Fringilla	4.12 ± 1.98
89 Julia	4.01 ± 1.61
675 Ludmilla	3.99 ± 1.94
201 Penelope	3.99 ± 1.97
455 Bruchsalia	3.93 ± 1.29
354 Eleonora	3.93 ± 1.84

Among the D-rank densities, 16 asteroids have nominal bulk densities higher than 16 Psyche’s:

Rank "D" Asteroid Densities (> 16 Psyche)

250 Bettina	7.84 ± 5.42
138 Tolosa	7.69 ± 4.39
360 Carlova	6.62 ± 4.51
388 Charybdis	5.80 ± 3.66
43 Ariadne	5.54 ± 2.84
536 Merapi	5.39 ± 4.77
172 Baucis	5.34 ± 3.31
420 Bertholda	4.94 ± 4.44
103 Hera	4.78 ± 2.87
491 Carina	4.58 ± 3.11
683 Lanzia	4.49 ± 2.69
849 Ara	4.29 ± 2.18
506 Marion	4.16 ± 2.29
363 Padua	4.10 ± 2.25
705 Erminia	4.02 ± 2.39
786 Bredichina	3.91 ± 2.28

Among the E-rank densities, 7 asteroids have nominal bulk densities higher than 16 Psyche’s:

Rank "E" Asteroid Densities (> 16 Psyche)

2004 PB108	6.74 ± 7.23
1013 Tombecka	6.39 ± 53.43
306 Unitas	6.23 ± 6.77
132 Aethra	5.09 ± 7.72
445 Edna	4.60 ± 4.91
147 Protogeneia	4.18 ± 5.03
769 Tatjana	4.09 ± 4.38

Among the X-rank densities, 14 asteroids have nominal bulk densities higher than 16 Psyche’s:

Rank "X" Asteroid Densities (> 16 Psyche)

1686 De Sitter	430.61 ± 213.19
33 Polyhymnia	75.32 ± 9.72
1428 Mombasa	43.03 ± 14.78
152 Atala	42.29 ± 10.80
949 Hel	12.31 ± 5.14
582 Olympia	9.98 ± 27.31
61 Danae	9.74 ± 9.45
665 Sabine	9.05 ± 5.19
217 Eudora	8.94 ± 0.64
204 Kallisto	8.89 ± 26.79
234 Barbara	8.89 ± 29.30
202 Chryseis	8.66 ± 1.63
126 Velleda	8.64 ± 106.21
67 Asia	8.59 ± 1.23

Obviously, most—if not all—of the asteroids listed above will eventually be found to have bulk densities less than that of 16 Psyche as more accurate masses and volumes are determined. Presently, only the following asteroids have minimum bulk densities greater than that of 16 Psyche, assuming the mean error listed is correct:

Asteroid Densities > 16 Psyche (within error)

1686 De Sitter	430.61 ± 213.19
33 Polyhymnia	75.32 ± 9.72
1428 Mombasa	43.03 ± 14.78
152 Atala	42.29 ± 10.80
949 Hel	12.31 ± 5.14
217 Eudora	8.94 ± 0.64
202 Chryseis	8.66 ± 1.63
67 Asia	8.59 ± 1.23

LaForge, Price, and Rafelski choose 33 Polyhymnia as the current best candidate to search for superheavy elements. Even a small amount of superheavy elements (especially in the alpha matter state) could significantly raise the bulk density of the asteroid as a whole. Kretlow lists the mass of 33 Polyhymnia as (6.20 ± 0.74) × 10¹⁸ kg and its volume-equivalent diameter as 54.0 ± 0.9 km, giving a bulk density around 75 g/cm³.

This finding is not without controversy, however. See the following discussion:

https://groups.io/g/mpml/topic/33_polyhymnia/101917502

References
Kretlow, M. Size, Mass and Density of Asteroids (SiMDA) – A Web Based Archive and Data Service” (2020). https://astro.kretlow.de/?SiMDA

LaForge, E., Price, W. & Rafelski, J. Superheavy elements and ultradense matter. Eur. Phys. J. Plus 138, 812 (2023). https://arxiv.org/abs/2306.11989
https://doi.org/10.1140/epjp/s13360-023-04454-8

Light Blue Blob in a Daytime Sky

Joan Oesper photographed this anomalous light blue patch on
April 13, 2023 at 1:04 p.m. CDT (1804 UT) from Alpine, TX

See the light blue blob in the photograph above? Even though it is partly cloudy, the light blue blob is decidedly different in color from the nearby patches of blue sky. Is this some unusual atmospheric phenomenon, or was there a daytime on orbit rocket burn (such as an apogee kick motor)? If the latter, I have not been able to find any evidence online of a rocket firing around 1804 UT on 13 Apr 2023.

Joan Oesper took this photo from the campus of Sul Ross State University in Alpine, TX at 1:04 p.m. CDT (1804 UT) on Thursday, April 13, 2023. The exact coordinates where the photograph was taken are 30° 21′ 54″ N, 103° 39′ 00″ W. She was facing an azimuth of approximately 161° (SSE) and the altitude of the blue blob was approximately 15° above the horizon.

Joan writes, “The people I saw it with said they’d been watching it and that it had moved eastward during the 5-10 minutes they were watching. It seemed to be behind the clouds.”

Has anyone seen something like this in the past? Was there an on-orbit daytime rocket firing at this time?

Infinity

George F. R. Ellis weighs in on the concept of infinity in his excellent paper, Issues in the Philosophy of Cosmology, available on astro-ph at https://arxiv.org/abs/astro-ph/0602280. He writes:

9.3.2 Existence of Infinities

The nature of existence is significantly different if there is a finite amount of matter or objects in the universe, as opposed to there being an infinite quantity in existence. Some proposals claim there may be an infinite number of universes in a multiverse and many cosmological models have spatial sections that are infinite, implying an infinite number of particles, stars, and galaxies. However, infinity is quite different from a very large number! Following David Hilbert, one can suggest these unverifiable proposals cannot be true: the word “infinity” denotes a quantity or number that can never be attained, and so will never occur in physical reality.³⁸ He states:

Our principal result is that the infinite is nowhere to be found in reality. It neither exists in nature nor provides a legitimate basis for rational thought . . . The role that remains for the infinite to play is solely that of an idea . . . which transcends all experience and which completes the concrete as a totality . . .

This suggests “infinity” cannot be arrived at, or realized, in a concrete physical setting; on the contrary, the concept itself implies its inability to be realized!

Thesis I2: The often claimed physical existence of infinities is questionable. The claimed existence of physically realized infinities in cosmology or multiverses raises problematic issues. One can suggest they are unphysical; in any case such claims are certainly unverifiable.

This applies in principle to both small and large scales in any single universe:

The existence of a physically existing spacetime continuum represented by a real (number) manifold at the micro-level contrasts with quantum gravity claims of a discrete spacetime structure at the Planck scale, which one might suppose was a generic aspect of fully non-linear quantum gravity theories. In terms of physical reality, this promises to get rid of the uncountable infinities the real line continuum engenders in all physical variables and fields⁴⁰. There is no experiment that can prove there is a physical continuum in time or space; all we can do is test space-time structure on smaller and smaller scales, but we cannot approach the Planck scale.

Infinitely large space-sections at the macro-level raise problems as indicated by Hilbert, and leads to the infinite duplication of life and all events. We may assume space extends forever in Euclidean geometry and in many cosmological models, but we can never prove that any realised 3-space in the real universe continues in this way—it is an untestable concept, and the real spatial geometry of the universe is almost certainly not Euclidean. Thus Euclidean space is an abstraction that is probably not physically real. The infinities supposed in chaotic inflationary models derive from the presumption of pre-existing infinite Euclidean space sections, and there is no reason why those should necessarily exist. In the physical universe spatial infinities can be avoided by compact spatial sections, resulting either from positive spatial curvature, or from a choice of compact topologies in universes that have zero or negative spatial curvature. Machian considerations to do with the boundary conditions for physics suggest this is highly preferable; and if one invokes string theory as a fundamental basis for physics, the “dimensional democracy” suggests the three large spatial dimensions should also be compact, since the small (“compactified”) dimensions are all taken to be so. The best current data from CBR and other observations indeed suggest k = +1, implying closed space sections for the best-fit FL model.

The existence of an eternal universe implies that an infinite time actually exists, which has its own problems: if an event happens at any time t₀, one needs an explanation as to why it did not occur before that time (as there was an infinite previous time available for it to occur); and Poincaré eternal return will be possible if the universe is truly cyclic. In any case it is not possible to prove that the universe as a whole, or even the part of the universe in which we live, is past infinite; observations cannot do so, and the physics required to guarantee this would happen (if initial conditions were right) is untestable. Even attempting to prove it is future infinite is problematic (we cannot for example guarantee the properties of the vacuum into the infinite future—it might decay into a state corresponding to a negative effective cosmological constant).

It applies to the possible nature of a multiverse. Specifying the geometry of a generic universe requires an infinite amount of information because the quantities necessary to do so are fields on spacetime, in general requiring specification at each point (or equivalently, an infinite number of Fourier coefficients): they will almost always not be algorithmically compressible. All possible values of all these components in all possible combinations will have to occur in a multiverse in which “all that can happen, does happen”. There are also an infinite number of topological possibilities. This greatly aggravates all the problems regarding infinity and the ensemble. Only in highly symmetric cases, like the FL solutions, does this data reduce to a finite number of parameters, each of which would have to occur in all possible values (which themselves are usually taken to span an infinite set, namely the entire real line). Many universes in the ensemble may themselves have infinite spatial extent and contain an infinite amount of matter, with all the problems that entails. To conceive of physical creation of an infinite set of universes (most requiring an infinite amount of information for their prescription, and many of which will themselves be spatially infinite) is at least an order of magnitude more difficult than specifying an existent infinitude of finitely specifiable objects.

One should note here particularly that problems arise in the multiverse context from the continuum of values assigned by classical theories to physical quantities. Suppose for example that we identify corresponding times in the models in an ensemble and then assume that all values of the density parameter and the cosmological constant occur at each spatial point at that time. Because these values lie in the real number continuum, this is a doubly uncountably infinite set of models. Assuming genuine physical existence of such an uncountable infinitude of universes is the antithesis of Occam’s razor. But on the other hand, if the set of realised models is either finite or countably infinite, then almost all possible models are not realised. And in any case this assumption is absurdly unprovable. We can’t observationally demonstrate a single other universe exists, let alone an infinitude. The concept of infinity is used with gay abandon in some multiverse discussions, without any concern either for the philosophical problems associated with this statement, or for its completely unverifiable character. It is an extravagant claim that should be treated with extreme caution.

³⁸An intriguing further issue is the dual question: Does the quantity zero occur in physical reality? This is related to the idea of physical existence of nothingness, as contrasted with a vacuum. A vacuum is not nothing!

⁴⁰To avoid infinities entirely would require that nothing whatever is a continuum in physical reality (since any continuum interval contains an infinite number of points). Doing without that, conceptually, would mean a complete rewrite of many things. Considering how to do so in a way compatible with observation is in my view a worthwhile project.

So, given this discussion of infinities, the answer to the doubly hypothetical question, “Can God make a rock so big he can’t pick it up?” is likely a “Yes”! – D.O.

Emergence

Physics is the fundamental science in that it describes the workings of the universe at all scales. No other science is so comprehensive.

Will our knowledge of physics finally lead us to a “Theory of Everything”? Perhaps, but the Theory of Everything alone will not be able to describe, predict, or explain its full expression upon/within the universe—no more so than our musical notation system can explain how a Brahms symphony was composed, nor its effect upon the listener.

Reductionism states that the whole is the sum of its parts, but emergence states that the whole is more than the sum of its parts.

There are many examples of emergent properties in the natural world, what one might call radical novelty. Some examples: crystal structure (e.g. a salt crystal or a snowflake), ripples in a sand dune, clouds, life itself. Social organization (e.g. a school of fish or a city), consciousness.

John Archibald Wheeler (1911-2008) created a diagram that nicely illustrates an emergent property of the universe that is important to us.

The universe viewed as a self-excited circuit. Starting simply (thin U at right), the universe grows in complexity with time (thick U at left), eventually giving rise to observer-participancy, which in turn imparts “tangible reality” to even the earliest days of the universe.

Richard Wolfson writes,

At some level of complexity, emergent properties become so interesting that, although we understand that they come from particles that are held together by the laws of physics, we can’t understand or appreciate them through physics alone.

I like to think of emergence as an expression of creativity. Our universe is inherently creative, just as we humans express ourselves creatively through music, art, literature, architecture, and in so many other ways.

Creativity is the most natural process in the universe. It’s in our DNA.

But DNA alone can’t explain it.

References

Richard Wolfson, The Great Courses, Course No. 1280, “Physics and Our Universe: How It All Works”, Lecture 1: “The Fundamental Science”, 2011.

“And the end of all our exploring will be to arrive where we started and know the place for the first time.” – T. S. Eliot

Antistars

Do stars made of antimatter exist in the universe? Possibly.

One of the great mysteries of cosmology and astrophysics is that even though equal quantities of matter and antimatter appear to have been produced during the “Big Bang”, today there is only a negligible quantity of antimatter in the observable universe. We do not appear to live in a matter-antimatter symmetric universe.

If antimatter stars, “antistars”, do exist, how could we distinguish them from stars made of normal matter? The light emitted from an antistar would look identical to the light emitted by a normal-matter star.

But if normal matter were infalling upon an antistar, the contact between matter and antimatter would generate an annihilation spectrum of gamma ray photons that peaks around energy 70 MeV (half the mass of a neutral pion) up to a sharp cutoff around 938 MeV (mass of the proton).

A recent analysis of data collected by the Fermi Gamma-ray Space Telescope found fourteen possible antistars. These fourteen point sources produce a gamma-ray signature indicative of matter-antimatter annihilation. These point sources do not exhibit the characteristics of other known gamma-ray sources. For example, they are not, ostensibly, pulsars, active galactic nuclei, or black holes.

The positional error ellipses for these fourteen point sources range from 11×10 arcminutes up to 128×68 arcminutes (95% confidence). Here are optical images of these sources from the Palomar Digital Sky Survey, in order of right ascension (epoch 2000 coordinates).

4FGL J0548.6+1200
5 48 38.8 +12 00 10
29.6’×23.6′ error ellipse
field of view 48.5′, Orion
bright star near crosshairs is HD 38797

4FGL J0948.0-3859
9 48 03.6 -38 59 57
53.7’×45.9′ error ellipse
field of view 48.5′, Antlia
bright star near crosshairs is TYC 7693-3238-1 ;
nebulous streak through the field is unidentified, 11˚ from the galactic plane

4FGL J1112.0+1021
11 12 03.1 +10 21 31
128.3’×67.9′ error ellipse
field of view 1.63˚, Leo
brightest star in field is HD 97502

4FGL J1232.1+5953
12 32 06.1 +59 53 03
15.4’×13.0′ error ellipse
field of view 24.11′, Ursa Major
brightest star in field is TYC 3847-229-1 ;
the galaxy is LEDA 2595040

4FGL J1348.5-8700
13 48 30.7 -87 00 47
10.6’×9.7′ error ellipse
field of view 11.99′, Octans

4FGL J1710.8+1135
17 10 50.5 +11 35 57
30.7’×26.7′ error ellipse
field of view 48.49′, Ophiuchus
brightest star near crosshairs is HD 155411

4FGL J1721.4+2529
17 21 24.7 +25 29 25
36.4’×25.2′ error ellipse
field of view 48.49′, Hercules
brightest star in field is HR 6455

4FGL J1756.3+0236
17 56 21.2 +02 36 52
19.0’×14.1′ error ellipse
field of view 24.11′, Ophiuchus

4FGL J1759.0-0107
17 59 03.7 -01 07 11
25.7’×22.8′ error ellipse
field of view 24.11′, Serpens
brightest star in field is HD 163914

4FGL J1806.2-1347
18 06 14.7 -13 47 36
19.2’×11.5′ error ellipse
field of view 24.11′, Serpens

4FGL J2029.1-3050
20 29 09.6 -30 50 06
31.0’×21.4′ error ellipse
field of view 48.49′, Microscopium
brightest star in field is HD 194640

4FGL J2047.5+4356
20 47 32.0 +43 56 33
58.9’×34.0′ error ellipse
field of view 1.63˚, Cygnus
brightest star in field is 56 Cyg ;
behind it is the Pelican Nebula (IC 5070)

4FGL J2237.6-5126
22 37 39.4 -51 26 05
20.7’×16.8′ error ellipse
field of view 24.11′, Grus
brightest star near crosshairs is TYC 8452-1160-1 ;
the edge-on galaxy is LEDA 92766

4FGL J2330.5-2445
23 30 35.6 -24 45 15
28.5’×20.5′ error ellipse
field of view 48.49′, Aquarius
brightest star near crosshairs is HD 221258

Since there appears to be no known way to distinguish a star made of antimatter from one made of matter—except for the gamma-ray signature of matter infalling onto the antimatter star, a higher-resolution gamma-ray telescope or interferometer (10 – 1000 MeV) needs to be developed to localize these candidate sources to within a few arcseconds. Higher spectral resolution will help as well, allowing a more detailed characterization of the gamma-ray spectrum.

References

S. Dupourqué, L. Tibaldo and P. von Ballmoos. Constraints on the antistar fraction in the solar system neighborhood from the 10-year Fermi Large Area Telescope gamma-ray source catalog. Physical Review D. Published online April 20, 2021. doi: 10.1103/PhysRevD.103.083016.
https://arxiv.org/abs/2103.10073

M. Temming (2021, June 5). Antistars could lurk in Milky Way. Science News, 199(10), 8-9.
https://www.sciencenews.org/article/antimatter-stars-antistars-milky-way-galaxy-space-astronomy

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

$\textup{E}=h\nu$

where
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

$c=\lambda \nu$

where
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.

References
Conover, E. (2021, June 19). Record-breaking gamma rays hint at violent environments in space. Science News, 199(11), 5.
https://www.sciencenews.org/article/light-energy-record-gamma-ray

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

James Clerk Maxwell

Today we celebrate the 190th anniversary of the birth of Scottish mathematician and physicist James Clerk Maxwell (13 Jun 1831 – 5 Nov 1879). Between 1864 and 1873, Maxwell developed four important mathematical equations that describe the behavior of electric and magnetic fields and their interrelated nature. He showed that any oscillating electric charge produces an electromagnetic field, and that this electromagnetic field propagates outward from the oscillating charge at the speed of light. He then correctly deduced that light itself is an electromagnetic phenomenon, and proposed that since electric charges can oscillate at any frequency, there should be a whole spectrum of electromagnetic waves of which visible light is only a small part. We now know that the electromagnetic spectrum does include many other types of “light”, namely gamma rays, x-rays, ultraviolet, infrared, microwave, and radio waves. They are all exactly the same phenomenon, differing only in their properties of frequency, wavelength, and energy.