Space may not be its namesake. That is, instead of being unoccupied and ‘spacious’ it is in fact extremely full – full of energy!
Although to some this may sound like a new idea, this idea has been around in many forms since time immemorial, namely the ether. In more recent times this ether has been referred to as the quantum vacuum, zero-point field or as Nassim Haramein likes to call it – the Planck field.
However, although the assumptions of old and the hypothesis of modern scientists have made such claims, the reality has never been directly observed. Now for the first time a team of European astronomers have measured what they believe to be the signs of this energetic quantum vacuum. Utilizing observations from the Very Large Telescope in Chile, the team were able to measure the polarization of light from an isolated neutron star (INS), the degree of which is highly suggestive of a polarized vacuum structure generating a birefringent effect on light.
What is vacuum birefringence?
Electromagnetic waves are not limited to specific polarizations and generally oscillate at all possible geometrical orientations. The oscillations can be polarized i.e. confined to a single direction – linear polarization – and/or rotation – circular polarization.
When light passes through a boundary between different media it will be refracted according to Snell’s Law,
where its refractive index, n depends on the propagation angle.
In some optically anisotropic mediums – such as crystalline structures – the refractive index is also dependent on the polarization as well as the propagation angle. Since refraction is a function of velocity, the light will decouple into two beams of different velocity with the slow ray lagging in phase relative to the faster ray. Mediums that exhibit this ‘double refraction’ phenomenon are said to be birefringent.
What does this have to do with the vacuum?
In the 1930s, following the birth of quantum theory and the general acceptance of wave-particle duality – that is the transformation of matter into radiation and vice versa – important inferences were made from the quantum theory describing electrodynamics – quantum electrodynamics (QED). These inferences rested on the fact that nonlinear mathematics beyond Maxwell’s equations were needed to describe the interaction of matter and radiation at the quantum level – which Dirac attributed to an indirect consequence of the virtual possibility of pair production, the Dirac sea of potential.
Following from the work of Dirac, Heisenberg showed that this nonlinear mathematics was also required for seemingly ‘empty’ space where if the specific energies needed to create matter were not present this underlying sea of potential would exhibit a sort of vacuum polarization. That is, the electron positron pairs created a ‘virtual’ dipole.
In a paper examining the collective coherent structures of plasma and their interactions, Haramein & Rauscher similarly concluded that “Coherent plasma states could not exist as localized waves due to nonlinear effects unless these nonlinearities and polarization properties existed within the vacuum structure itself.”
The term ‘vacuum polarization’ may sound like a contradiction in terms, as polarizability is a property normally associated with matter. Different types of media and/or anisotropies within the medium can affect the degree of polarizability – but one thing for certain is for polarization to occur there has to be a medium to polarize.
Experimental evidence of vacuum polarization has since been found, first with the measurements of the Lamb shift and later with the Casimir effect (more about this can be found here). It is thus reasonable to conclude that the original idea of an ether as supported by Einstein and the virtual sea of potential as suggested by Dirac do exist.
This sea of potential energy, or quantum vacuum, behaves nonlinearly – thus, due to this anisotropic nature, when it is polarized it will exhibit birefringence effects known as vacuum birefringence. Now for the first time, a team of astronomers have observed these effects – not only confirming the effects of vacuum birefringence but also further supporting the not so ‘empty’ nature of space.
How did they do this?
To start with they needed to find a strong magnetic source – strong enough to significantly polarize the vacuum and generate birefringent effects. So, what better than a rotating ball of charge, commonly known as a star. Although not completely understood, the smallest and densest of stars – the neutron stars are also the fastest, rotating at speeds of up to 70,000 km/s some 350 times faster than the most massive of stars and 35,000 times faster than our sun – thus generating magnetic fields billions of times stronger (~ 1013Gauss).
In the presence of a strong magnetic field – like those of neutron stars – the opacity of ionized matter to the transfer of photons becomes polarization dependent – that is, the degree to which light is allowed to travel through the medium is effected by the polarization. As a consequence, thermal radiation emanating from every point of the neutron star surface will be highly polarized, with the direction of polarization correlated with the direction of the magnetic field.
The thermal emission received by the detectors at an observatory is the net emission emanating from the entire surface of the neutron star. However, as the magnetic field of the neutron star will have different orientations across the surface – although highly polarized – it is assumed that the polarizations will mostly cancel out, significantly reducing the observed net polarization.
However, Heyl and his colleagues suggest otherwise. In a series of papers (1997, 2000, 2002, 2003) they show how the non-linearity of the vacuum is able to decouple the polarization modes, such that their trajectories evolve separately as they travel outwards through the rapidly weakening magnetosphere. The radius at which the modes recouple is frequency dependent where the higher the frequency the further from the neutron star surface that the coupling takes place.
The further a photon travels from the surface of a neutron star the more aligned the magnetic field lines become, such that the polarization direction of the emission originating in different regions will also tend to align together. Thus for higher frequency modes, that travel the furthest before recoupling, such as those of blue, UV and higher – the net observed polarization of the thermal radiation would be dramatically increased.
Heyl and his fellow scientists subsequently predicted that these large polarizations could be observed, and would thus provide the first direct evidence of vacuum birefringence in the strong field regime.
More than a decade later a team of astronomers, led by Mignani, set about doing just that and began by choosing the right star. In isolated neutron stars which aren’t effected by the magnetosphere the highly polarized thermal radiation can be detected at optical wavelengths. With this in mind the team comprising of scientists from UCL Mullard Space Science Laboratory, University of Zielona Gora, University of Padova and Osservatorio Astronomico di Roma, decided on, RX J1856.5-3754 the prototype for the famous group of 7 radio quiet isolated neutron stars known as the Magnificent Seven (M7).
Although one of the closest neutron stars at a mere 400 light-years from Earth, and the brightest of its kind, its faintness required a specific instrument available at the VLT – the Focal Reducer and low dispersion Spectrograph(FORS2) equipped with polarization optics to measure the linear polarization. The degree of polarization is essentially determined by measuring the intensity difference at 2 predefined polarization states of a light modulator, where if the light is un-polarized the intensity will be constant regardless of the modulator state.
After careful analysis, accounting for all known external factors, the team found the degree of linear polarization to be 16% – that is, not all the light received is vibrating in one direction just 16%. Although low, this is much higher than expected and can only be accounted for if the effects of vacuum birefringence are included in the analysis. Much higher degrees of polarization, up to 100% are expected in the X-ray wavelength which is where the team have their sights set on next.
In any case, these are definitely exciting times as it looks like we are finally on our way to observing and understanding the anisotropic and crystal-like structure of the all-pervasive ether!
By: Amira Val Baker