birefringence are two of those quasi-obscure technical words derived from
physical optics. Quantum physics
tells us that a beam of light is a stream of photons, but at the same time it is
a travelling wave made of oscillating electric and magnetic fields vibrating at
right angles to the direction of wave motion and to each other.
The light is said to be linearly polarized if the electric field always vibrates in the same
plane, and we call that its plane of polarization. (Light can also be circularly
polarized, elliptically polarized, or unpolarized, but we will ignore those
complications for the present discussion.)
Birefringence refers to a special property of some
transparent crystals within which the speed of a light beam passing through
depends on the direction of polarization and on the direction of motion with
respect to the axes of the crystal structure.
Solid materials have atoms in their interiors containing lots of positive
and negative electric charges. Because
of all the possible electrical interactions, such materials should, in
principle, stop a light beam dead in its tracks.
However, some insulating solid materials like glass and crystals are transparent
to light because the electrons they contain are bound (not free to move around
and conduct electricity). As light
passes by, these bound electrons are pushed-on by the passing electric field and
vibrate like tiny masses-on-springs in time with the wave frequency.
The coherent motion of these electrons, all vibrating together, captures
and then re-emits the light energy, forming a sort of bucket-brigade
transmission of the light through the material.
In this process the speed of light is slowed to less than c
(its velocity in vacuum), but the light is passed through the material
without significant absorption. In
glass this slowing of light makes possible refractive devices like lenses and
best-known example of birefringence is the polarized double images that are seen
when a printed page is viewed through a transparent crystal of calcite.
In the optics laboratory birefringence is very useful, because it can be
used to split a light beam into two separate beams with complementary
polarizations. It can also be used to rotate the plane of polarization of a
light beam or to change linear polarization to and from circular polarization.
However, birefringence is not confined completely to the domain of
crystals. Under extreme conditions
it can become a property of the vacuum itself.
In 1936, quantum mechanics pioneer Werner Heisenberg and his colleague
Hans Euler reported their investigation of the influence of strong electric and
magnetic fields on light. In this
work, they used the then-emerging theory now known as quantum electrodynamics
(QED). The QED theory depicts the
quantum vacuum as full of virtual electrons and positrons, charged particles of
somewhat indefinite mass that spontaneously appear, briefly interact with their
environment, and then vanish. Heisenberg
and Euler reasoned that the behavior of these virtual particles would have to be
modified by a sufficiently strong magnetic field, changing the properties of the
vacuum. They showed that when a
region of "empty" space contains a magnetic field of greater than the
critical value of 4.41 × 1013
gauss (or 4.41 billion tesla), the familiar linearity of the Maxwell theory of
light moving in vacuum gives way to a quite unfamiliar nonlinear behavior.
Later theorists showed that, in particular, the magnetically perturbed
quantum vacuum becomes birefringent, so that light rays with planes of
polarization parallel and perpendicular to a large external magnetic field will
move with differing speeds. These
speeds can both be significantly less than c, the standard vacuum velocity of light.
By ordinary standards, the magnitude of the magnetic field at which this
vacuum birefringence phenomenon occurs is very large.
Until fairly recently, the vacuum birefringence prediction of high-field
QED was considered to be merely an interesting but completely untestable
curiosity of the theory, because the required magnetic field strength was many
millions of times larger than any conceivable magnetic field that might be
created in a physics laboratory.
However, there is a domain in which the requirement of a 1013
gauss field does not look so formidable. That domain is the astrophysics of
neutron stars. When a star of about
ten times the mass of our Sun undergoes a supernova and collapses to a neutron
star, the parent star's preexisting magnetic field is trapped in the
collapsing medium by circulating currents that become superconducting.
The existing magnetic field is compressed to occupy a much smaller space,
a spherical volume having a diameter of about 20 km or less.
The magnetic field lines are squeezed together within this volume, and
the field strength becomes very much larger.
The result of this compression is that the surface magnetic field of an
average neutron star is estimated to be about 1012 gauss or more.
There is also a particularly x-ray-active class of neutron stars called magnetars
that includes stars with even more intense magnetic fields, ranging of up to
about 1015 gauss. Thus,
the large magnetic fields provided by neutron stars would be expected to provide
fertile testing ground for investigating the QED predictions of vacuum
In particular, we would like to determine if the visible light emitted
by neutron stars shows any indication of linear polarization.
However, that is not as easy as it sounds.
Many neutron stars are pulsars, producing a tight beam of radio waves,
light, and x-rays that is briefly visible to astronomers on Earth when the beam
sweeps by as the neutron star rotates on its spin axis. The beam-sweep appears
as a pulse of radiation, with a very precise interval between successive pulses
that ranges from milliseconds to seconds,. The
pulsar's visible light is not of interest for vacuum birefringence tests
because it is emitted along the direction of the magnetic field, where QED
predicts that no birefringence polarization effects should be observed.
Instead, we are interested in the non-pulsed ambient light from neutron
stars, preferably emitted in a direction perpendicular to the magnetic field.
The problem with observing such light is that neutron stars are very dim
except when pulsing. While they are
often surrounded by highly visible supernova remnants, neutron stars on their
own emit very little non-pulsed visible light.
This means that measuring the polarization of the non-pulsed visible
light coming from a neutron star is very difficult.
Fortunately, there are a few neutron stars within our immediate galactic
neighborhood that happen to be close enough to the Earth to be visible with the
most powerful telescope complexes presently available.
of those powerful telescope complexes is the Very Large Telescope (VLT) of the
Paranal Observatory operated by the European Southern Observatory (ESO) in
Recently, a team of astronomers led by Roberto Mignani of the INAF Milan and Poland's Zielona Gora University used the VLT to observe the weak (magnitude 25.5) visible light from the neutron star RX J1856.5-3754, which is located about 400 light years from Earth. Assuming that the neutron star has a dipole magnetic field with magnetic north and south poles, the surface light from the star's equator passing through this field will tend to be linearly polarized along the star's magnetic field lines by vacuum birefringence. Indeed, Mignani's group report observations made at a wavelength of 555.0 nanometers that showed a linear polarization of 16.4±5.3%. As a control, they applied the same measurements to light from 42 nearby (normal) stars and found that all were consistent with zero linear polarization. Their work constitutes the faintest star ever measured for its optical polarization,
This result, when compared with various QED theoretical models, can be considered only as indirect observational evidence suggesting the presence of vacuum birefringence. By itself, it is not definitive proof of the existence of the phenomenon, because there are a few unlikely scenarios that could account for the observed polarization.. The work will need to be followed up with more observations of RX J1856.5-3754 involving longer observation durations and measurements at more wavelengths of visible light.
In addition, the work suggests a test of vacuum birefringence with more energetic photons. Magnetars, which are exotic high-field neutron stars that are strong sources of x-rays, should be studied. Magnetars are expected to have magnetic fields that are one to two orders of magnitude larger than the field of RX J1856.5-3754. Magnetic fields of such strength would be expected to produce dramatic vacuum birefringence polarization effects on light at both visible and x-ray wavelengths.
space missions presently in the planning stages are designed to provide
polarization-sensitive x-ray images of astronomical objects.
For example, the Imaging X-ray Polarimetry Explorer, to be launched in
2021, has three identical telescopes designed to measure the polarization in the
photon energy range of 2 to 8 keV x-rays. With
these, more definitive demonstrations of the presence of the vacuum
birefringence effect can be expected, along with accurate determinations of
magnetic field strength in this extreme region and improved modeling of neutron
stars. Watch this column for further
"Consequences of Dirac Theory of the Positron", W. Heisenberg and H. Euler, Z. Phys. 98, 714-732, (1936); (English translation arXiv: 0605.038 [physics.hist-ph]).
birefringence in strong magnetic fields: (I) Photon polarization tensor with all
the Landau levels", Koichi Hattori and Kazunori Itakura, Annals of Physics
330, 23-54, (March 2013), arXiv:1209.2663
of vacuum birefringence from the polarisation of the optical emission from an
Isolated Neutron Star", R. P.
Mignani, V. Testa, D. Gonzalez Caniulef, R. Taverna, R. Turolla, S. Zane, K. Wu,
and G. Lo Curto, arXiv:1710.08709 [astro-ph.HE].
John Cramer's book on Quantum Mechanics: a non-fiction work describing his Transactional Interpretation of quantum mechanics, The Quantum Handshake - Entanglement, Nonlocality, and Transactions, (Springer, January-2016) is available for purchase online as a printed or eBook at: http://www.springer.com/gp/book/9783319246406 or https://www.amazon.com/dp/3319246402.
SF Novels by John Cramer: my two hard SF novels, Twistor and Einstein's Bridge, are available as eBooks by Book View Cafe and are available at : http://bookviewcafe.com/bookstore/?s=Cramer .
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