A major breakthrough in cosmic ray physics
was recently achieved by the IceCube Neutrino Observatory, a major astrophysics
project funded by the U. S. National Science Foundation and constructed between
2004 and 2010 at the
Amundsen-Scott South Pole Station in
The new IceCube discovery sheds light on one of the major unsolved
problems of contemporary astrophysics: "What is the origin of ultra-energetic
In the July-August-1999 issue of Analog, just before the
end of the 20th century, I wrote an AV column entitled What We Don't
column, written over 18 years ago, listed what in my opinion at the time were
the top seven major unsolved problems in physics and astrophysics.
These seven, in no particular order, were: (1) What are dark matter and
dark energy?; (2) What's behind
particle physics' Standard Model?; (3) What's the origin of gamma ray
bursts?; (4) What's the origin of ultra-energetic cosmic rays?; (5) Why do we
detect only 1/3 of the Sun's neutrinos?; (6) Why does the Universe contain
more matter than antimatter?; and (7) What
is the origin of the arrow of time?
As of last week, problem (5) the solar
neutrino problem (see
AV#123) and problem (3) the origin of gamma ray bursts (see
AV#142) had both been satisfactorily solved. A
good but incomplete start was also made at solving problem (4), the origin of
ultra-high energy cosmic rays (see
AV#142). That came from work by
the Pierre Auger Collaboration, which operates the Auger Observatory, an array
of 1,600 large particle detectors spaced 1.5 kilometers apart and occupying
3,000 square kilometers of the Argentine pampas.
Active galactic nuclei (AGN) are super-active galaxies probably powered
by very large central black holes. The
group compared the sky coordinates of their 27 most energetic cosmic ray events
(all with energies greater than the GZK Limit, the energy at which cosmic ray
protons should interact and lose energy from collisions with the photons of the
cosmic microwave background) with the positions of known AGNs that are within
about 240 million light years from Earth. The
Auger Group demonstrated an excellent correlation between known AGN positions
and the incoming directions of these highest energy cosmic rays, suggesting that
these highest energy particles are coming from AGNs.
But this correlation with AGNs raised
questions. There is no known
physical mechanism that could accelerate particles to such high energies.
Further, the only explanation of how the GZK limit is being breached
across such large separation distances is that the 27 detected particles with
these extreme energies must have been super-energetic iron nuclei rather than
protons. However, that remains a
hypothesis that has never been verified. The
new Ice Cube result the sheds more light on the question of whether AGNs are
indeed source of ultra-high-energy cosmic rays.
Neutrinos are among the most elusive stable particles presented to us by
nature. They are spin ½ fermions,
leptons with no electric charge, their three flavors (ne, nm,
and nt) all with the smallest of non-zero rest masses (around 0.001
electron-volts), and they interact with other particles of matter only through
the weak and gravitational forces. If
a ne electron-neutrino with a kinetic energy of one million electron-volts
was passing through solid lead, its range (i. e., average distance traveled
before an interaction) would be about one light-year.
Therefore, the detection of neutrinos presents a very formidable
experimental challenge, usually requiring huge detectors with tons of active
detector material and subject to very low counting rates.
The neutral Z0 and the charged W± are the mediating particles of the weak interaction. Typically, a neutrino is detected either by a Z0-exchange scattering from an electron or nucleus, or by a charge-changing W±-exchange interaction in which the electrically-neutral neutrino is converted to its electrically-charged lepton twin. For example, in an interaction with a nucleus, a nm neutrino might be converted to a m lepton.
The most energetic particles in nature are ultra-high-energy cosmic rays, some of which have been observed with kinetic energies of over 1020 electron-volts. These highest energy objects may be charged particles, particularly protons and iron nuclei, but they may also be neutrinos. The study of neutrino cosmic rays is particularly interesting because, having no electric charge, neutrinos will not be deflected in flight by galactic magnetic fields, as charged particles would be. Therefore, their flight paths point directly back to the site of their origin and can be used to identify their source.
The most ambitious attempt to detect cosmic ray neutrinos is the
previously mentioned IceCube Neutrino
Observatory. The ice near the
Why is ice a good detector
medium for neutrinos?
In ice refraction effects slow light to 76.3% of its speed in vacuum, so
any high-energy charged particle that is moving through ice at near vacuum
lightspeed will be travelling at much greater than the local speed of light in
the medium and will produce the electromagnetic equivalent of a sonic-boom shock
wave called Cerenkov radiation. The
blue Cerenkov photons form a cone-shaped shock wave centered on the path of the
particle and propagating outward, and these photons are efficiently detected by
IceCube's photomultipliers, allowing the parent particle's trajectory to be
accurately reconstructed. Therefore,
if any incoming neutrino has an interaction that produces a fast charged
particle, IceCube will detect it and measure its energy and direction.
In particular, if an incoming mu-neutrino (nm)
has a charge-exchange interaction in the ice that converts it into a m-lepton,
that particle will have a very long straight flight path through the ice, making
Cerenkov light all the way. This is
exactly what happened on September 22, 2017, when an event given the name IceCube-170922A
occurred. A cosmic ray
mu-neutrino from the northern sky having a kinetic energy of about 2.9 x 1011
electron-volts passed all the way through the Earth before it was converted to a
within the IceCube detector volume. The
long range of the produced m-lepton
allowed the determination of the direction of origin within a small circle in
the northern sky spanning half a degree of arc and located at right ascension
77.33° and declination +5.72°.
With these coordinates, IceCube's automatic notification system went
into action, notifying ground-based and space-based observatories around the
world of their estimate of the coordinates of the event.
Within a few seconds the Fermi Gamma-ray Space Telescope turned to
examine this location and detected an outburst of x-rays and gamma rays coming
from the so called "Texas Blazar" TXS 0506+056, an object that fell within
the circle determined by IceCube. Optical
astronomers have also observed increased activity from the "Texas"
blazar TXS 0506+056 near the time of the IceCube detection.
What is a blazar?
Blazars are the brightest objects in the sky.
They are active galactic nuclei powered by large central black holes.
The gravitational energy liberated by the black hole is observed to
create jets of relativistic particles that are beamed out perpendicular to the
plane of the AGN, probably because of magnetic and rotational angular momentum
effects. But there is another
requirement for an AGN to qualify as a blazar: the
AGN must have one of its relativistic beams pointing directly at the Earth.
The probability of such an accidental alignment is small, so there are
only a few dozen known blazars. The
name is derived from the first blazar ever recognized, an AGN object with the
name BL Lacertae. That
identifier suggested the generic name "blazar".
After the discovery that event IceCube-170922A
can be tracked back to the
So we now know that at least one blazar had beamed ultra-high-energy
neutrinos in our direction more than once. The
implication of this work is that AGNs in our universe are probably the source of
all ultra-high-energy cosmic rays.
However, this conclusion is not without its own problems.
We do not presently understand how such extremely high energy particles
could be created from the gravitational energy released by a black hole.
In other words, the basic acceleration mechanism remains a mystery and a
challenge for astrophysical theory. Therefore,
we cannot yet say that problem (4), the source of ultra-high-energy cosmic rays,
has been completely solved, but only that significant progress has been made.
Watch this column for further developments.
John G. Cramer's 2016 nonfiction book (Amazon gives it 5 stars) describing his transactional interpretation of quantum mechanics, The Quantum Handshake - Entanglement, Nonlocality, and Transactions, (Springer, January-2016) is available online as a hardcover or eBook at: http://www.springer.com/gp/book/9783319246406 or https://www.amazon.com/dp/3319246402.
SF Novels by John Cramer: Printed editions of John's hard SF novels Twistor and Einstein's Bridge are available from Amazon at https://www.amazon.com/Twistor-John-Cramer/dp/048680450X and https://www.amazon.com/EINSTEINS-BRIDGE-H-John-Cramer/dp/0380975106. His new novel, Fermi's Question may be coming soon.
Alternate View Columns Online: Electronic reprints of 212 or more "The Alternate View" columns by John G. Cramer published in Analog between 1984 and the present are currently available online at: http://www.npl.washington.edu/av .
UHE Neutrino Detection:
"Neutrino emission from the direction of the blazar TXS 0506-056 prior to the IceCube-170922A alert", IceCube Collaboration, Science 361, 147-151 (2018).