Editing Zero-point energy (section)

=== Vacuum birefringence ===
{{Main|Lorentz-violating electrodynamics|Euler–Heisenberg Lagrangian}}
[[File:Eso1641a.ogg|thumb|Light coming from the surface of a strongly magnetic [[neutron star]] (left) becomes linearly polarised as it travels through the vacuum.]]
In the presence of strong electrostatic fields it is predicted that virtual particles become separated from the vacuum state and form real matter.{{Citation needed|date=May 2019}} The fact that electromagnetic radiation can be transformed into matter and vice versa leads to fundamentally new features in quantum electrodynamics. One of the most important consequences is that, even in the vacuum, the Maxwell equations have to be exchanged by more complicated formulas. In general, it will be not possible to separate processes in the vacuum from the processes involving matter since electromagnetic fields can create matter if the field fluctuations are strong enough. This leads to highly complex nonlinear interaction – gravity will have an effect on the light at the same time the light has an effect on gravity. These effects were first predicted by Werner Heisenberg and [[Hans Heinrich Euler]] in 1936{{sfnp|Heisenberg|Euler|1936}} and independently the same year by Victor Weisskopf who stated: "The physical properties of the vacuum originate in the "zero-point energy" of matter, which also depends on absent particles through the external field strengths and therefore contributes an additional term to the purely Maxwellian field energy".{{sfnp|Weisskopf|1936|p=3}}{{sfnp|Greiner|Müller|Rafelski|2012|p=278}} Thus strong magnetic fields vary the energy contained in the vacuum. The scale above which the electromagnetic field is expected to become nonlinear is known as the [[Schwinger limit]]. At this point the vacuum has all the properties of a [[Birefringence|birefringent medium]], thus in principle a rotation of the polarization frame (the [[Faraday effect]]) can be observed in empty space.{{sfnp|Greiner|Müller|Rafelski|2012|p=291}}<ref>See {{harvp|Dunne|2012}} for a historical review of the subject.</ref>

[[File:Wide field view of the sky around the very faint neutron star RX J1856.5-3754.jpg|thumb|left|Wide field view of the neutron star [[RX J1856.5-3754]]]]
Both Einstein's theory of special and general relativity state that light should pass freely through a vacuum without being altered, a principle known as [[Lorentz invariance]]. Yet, in theory, large nonlinear self-interaction of light due to quantum fluctuations should lead to this principle being measurably violated if the interactions are strong enough. Nearly all theories of [[quantum gravity]] predict that Lorentz invariance is not an exact symmetry of nature. It is predicted the speed at which light travels through the vacuum depends on its direction, polarization and the local strength of the magnetic field.{{sfnp|Heyl|Shaviv|2000|p=1}} There have been a number of inconclusive results which claim to show evidence of a [[Modern searches for Lorentz violation|Lorentz violation]] by finding a rotation of the polarization plane of light coming from distant galaxies.<ref>See {{harvp|Carroll|Field|1997}} and {{harvs|txt|last1=Kostelecký|last2=Mewes|year1=2009|year2=2013}} for an overview of this area.</ref> The first concrete evidence for vacuum birefringence was published in 2017 when a team of [[astronomers]] looked at the light coming from the star [[RX J1856.5-3754]],<ref>See {{harvp|Mignani et al.|2017}} for experiment and {{harvp|Cho|2016}}, {{harvp|Crane|2016}} and {{harvp|Bennett|2016}} for comment.</ref> the closest discovered [[neutron star]] to [[Earth]].{{sfnp|Rees|2012|p=528}}

Roberto Mignani at the [[National Institute for Astrophysics]] in [[Milan]] who led the team of [[astronomers]] has commented that "When Einstein came up with the theory of general relativity 100 years ago, he had no idea that it would be used for navigational systems. The consequences of this discovery probably will also have to be realised on a longer timescale."{{sfnp|Crane|2016}} The team found that visible light from the star had undergone linear polarisation{{clarify|date=May 2018}} of around 16%. If the birefringence had been caused by light passing through [[interstellar gas]] or plasma, the effect should have been no more than 1%. Definitive proof would require repeating the observation at other wavelengths and on other neutron stars. At [[X-ray]] wavelengths the polarization from the quantum fluctuations should be near 100%.{{sfnp|Cho|2016}} Although no [[telescope]] currently exists that can make such measurements, there are several proposed X-ray telescopes that may soon be able to verify the result conclusively such as China's [[Hard X-ray Modulation Telescope]] (HXMT) and NASA's Imaging X-ray Polarimetry Explorer (IXPE).