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They set a limit on the anisotropy of the speed of light resulting from the Earth's motions of Δc/c ≈ 10 −15, where Δc is the difference between the speed of light in the x- and y-directions. [33] As of 2015, optical and microwave resonator experiments have improved this limit to Δc/c ≈ 10 −18.
For example, for visible light, the refractive index of glass is typically around 1.5, meaning that light in glass travels at c / 1.5 ≈ 200 000 km/s (124 000 mi/s); the refractive index of air for visible light is about 1.0003, so the speed of light in air is about 90 km/s (56 mi/s) slower than c.
In theories that do not respect Lorentz invariance, the speed of light is not (necessarily) a barrier, and particles can travel faster than the speed of light without infinite energy or causal paradoxes. [27] A class of field theories of that type is the so-called Standard Model extensions. However, the experimental evidence for Lorentz ...
The Alcubierre drive ([alkuˈβjere]) is a speculative warp drive idea according to which a spacecraft could achieve apparent faster-than-light travel by contracting space in front of it and expanding space behind it, under the assumption that a configurable energy-density field lower than that of vacuum (that is, negative mass) could be created.
In astronomy, superluminal motion is the apparently faster-than-light motion seen in some radio galaxies, BL Lac objects, quasars, blazars and recently also in some galactic sources called microquasars. Bursts of energy moving out along the relativistic jets emitted from these objects can have a proper motion that appears greater than the speed ...
Jets may also be observed from spinning neutron stars. An example is pulsar IGR J11014-6103, which has the largest jet so far observed in the Milky Way, and whose velocity is estimated at 80% the speed of light (0.8c). X-ray observations have been obtained, but there is no detected radio signature nor accretion disk.
The measurement of the speed of gravity with the gravitational wave event GW170817 ruled out many modified gravity theories as alternative explanations to dark energy. [ 35 ] [ 36 ] [ 37 ] Another type of model, the backreaction conjecture, [ 38 ] [ 39 ] was proposed by cosmologist Syksy Räsänen: [ 40 ] the rate of expansion is not homogenous ...
Attempts to incorporate a variable speed of light into physics were made by Robert Dicke in 1957, and by several researchers starting from the late 1980s. VSL should not be confused with faster than light theories, its dependence on a medium 's refractive index or its measurement in a remote observer's frame of reference in a gravitational ...