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The Robertson–Walker (RW) metric allows us to apply general relativity to model the behavior of the Universe as a whole (i.e., cosmology). We can properly interpret various cosmological observations, like the cosmological redshift, the Hubble parameter, geometrical distances, and so on, if we identify fundamental observers with individual galaxies. That is to say that the interpretation of observations of modern cosmology relies on the RW metric. The RW model satisfies the cosmological principle in which the 3-space always remains isotropic and homogeneous. One can derive the cosmological redshift relation from this condition. We show that it is still possible for us to obtain consistent results in a specific time-varying speed-of-light model without spoiling the success of the standard model. The validity of this model needs to be determined by observations.

It is known that dimensional constants, such as ℏ, c, G, e, and k, are merely human constructs whose values and units vary depending on the chosen system of measurement. Therefore, the time variations in dimensional constants lack operational significance due to their dependence on these dimensional constants. They are well structured and represent a valid discussion. However, this fact only becomes a meaningful debate within the context of a static or present Universe. As theoretically and observationally well established, the current Universe is undergoing accelerated expansion, wherein dimensional quantities, like the wavelength of light, also experience redshift phenomena elongating over cosmic time. In other words, in an expanding Universe, dimensional quantities of physical parameters vary with cosmic time. From this perspective, there exists the possibility that dimensional constants, such as the speed of light, could vary with the expansion of the Universe. In this review paper, we contemplate under what circumstances the speed of light may change or remain constant over cosmic time and discuss the potential for distinguishing these cases observationally.

Describes how Danish astronomer Ole Romer measured the speed of light using a crude telescope and a mechanical timepiece.

The work presents the concept of a device for the measurement of the one-way speed of light. The minimum parameters of this device have also been determined based on the Special Theory of Ether without transverse contraction. The Special Theory of Ether is a relativistic theory of kinemat-ics with a universal frame of reference in which light propagates and is an alternative explanation for the null result of the Michelson-Morley experiment. An experiment based on the proposed device can be one of the ways to falsify the Special Theory of Relativity and Special Theory of Ether.

There is an ever-growing demand for artificial intelligence. Optical processors, which compute with photons instead of electrons, can fundamentally accelerate the development of artificial intelligence by offering substantially improved computing performance. There has been long-term interest in optically constructing the most widely used artificial-intelligence architecture, that is, artificial neural networks, to achieve brain-inspired information processing at the speed of light. However, owing to restrictions in design flexibility and the accumulation of system errors, existing processor architectures are not reconfigurable and have limited model complexity and experimental performance. Here, we propose the reconfigurable diffractive processing unit, an optoelectronic fused computing architecture based on the diffraction of light, which can support different neural networks and achieve a high model complexity with millions of neurons. Along with the developed adaptive training approach to circumvent system errors, we achieved excellent experimental accuracies for high-speed image and video recognition over benchmark datasets and a computing performance superior to that of cutting-edge electronic computing platforms.Linear diffractive structures are by themselves passive systems but researchers here exploit the non-linearity of a photodetector to realize a reconfigurable diffractive ‘processing’ unit. High-speed image and video recognition is demonstrated.

We present a Relativistic Newtonian Dynamics ( R N D ) for motion of objects in a gravitational field generated by a moving source. As in General Relativity ( G R ), we assume that objects move by a geodesic with respect to some metric, which is defined by the field. This metric is defined on flat lab spacetime and is derived using only symmetry, the fact that the field propagates with the speed of light, and the Newtonian limit. For a field of a single source, the influenced direction of the field at spacetime point x is defined as the direction from x to the to the position of the source at the retarded time. The metric depends only on this direction and the strength of the field at x. We show that for a static source, the R N D metric is of the same form as the Whitehead metric, and the Schwarzschild metric in Eddington–Finkelstein coordinates. Motion predicted under this model passes all classical tests of G R . Moreover, in this model, the total time for a round trip of light is as predicted by G R , but velocities of light and object and time dilation differ from the G R predictions. For example, light rays propagating toward the massive object do not slow down. The new time dilation prediction could be observed by measuring the relativistic redshift for stars near a black hole and for sungrazing comets. Terrestrial experiments to test speed of light predictions and the relativistic redshift are proposed. The R N D model is similar to Whitehead’s gravitation model for a static field, but its proposed extension to the non-static case is different. This extension uses a complex four-potential description of fields propagating with the speed of light.

In locally interacting quantum many-body systems, the velocity of information propagation is finitely bounded, and a linear light cone can be defined. Outside the light cone, the amount of information rapidly decays with distance. When systems have long-range interactions, it is highly nontrivial whether such a linear light cone exists. Herein, we consider generic long-range interacting systems with decaying interactions, such asR−αwith distanceR. We prove the existence of the linear light cone forα>2D+1(D, the spatial dimension), where we obtain the Lieb-Robinson bound as∥[Oi(t),Oj]∥≲t2D+1(R−v¯t)−αwithv¯=O(1)for two arbitrary operatorsOiandOjseparated by a distanceR. Moreover, we provide an explicit quantum-state transfer protocol that achieves the above bound up to a constant coefficient and violates the linear light cone forα<2D+1. In the regime ofα>2D+1, our result characterizes the best general constraints on the information spreading.

Optical to near-infrared observations of a transient coincident with the detection of the gravitational-wave signature of a binary neutron-star merger and a low-luminosity short-duration γ-ray burst are presented and modelled. When neutron stars collide Merging neutron stars are potential sources of gravitational waves and have long been predicted to produce jets of material as part of a low-luminosity transient known as a 'kilonova'. There is growing evidence that neutron-star mergers also give rise to short, hard gamma-ray bursts. A group of papers in this issue report observations of a transient associated with the gravitational-wave event GW170817—a signature of two neutron stars merging and a gamma-ray flash—that was detected in August 2017. The observed gamma-ray, X-ray, optical and infrared radiation signatures support the predictions of an outflow of matter from double neutron-star mergers and present a clear origin for gamma-ray bursts. Previous predictions differ over whether the jet material would combine to form light or heavy elements. These papers now show that the early part of the outflow was associated with lighter elements whereas the later observations can be explained by heavier elements, the origins of which have been uncertain. However, one paper (by Stephen Smartt and colleagues) argues that only light elements are needed for the entire event. Additionally, Eleonora Troja and colleagues report X-ray observations and radio emissions that suggest that the 'kilonova' jet was observed off-axis, which could explain why gamma-ray-burst detections are seen as dim. The merger of two neutron stars has been predicted to produce an optical–infrared transient (lasting a few days) known as a ‘kilonova’, powered by the radioactive decay of neutron-rich species synthesized in the merger 1 , 2 , 3 , 4 , 5 . Evidence that short γ-ray bursts also arise from neutron-star mergers has been accumulating 6 , 7 , 8 . In models 2 , 9 of such mergers, a small amount of mass (10 −4 –10 −2 solar masses) with a low electron fraction is ejected at high velocities (0.1–0.3 times light speed) or carried out by winds from an accretion disk formed around the newly merged object 10 , 11 . This mass is expected to undergo rapid neutron capture (r-process) nucleosynthesis, leading to the formation of radioactive elements that release energy as they decay, powering an electromagnetic transient 1 , 2 , 3 , 9 , 10 , 11 , 12 , 13 , 14 . A large uncertainty in the composition of the newly synthesized material leads to various expected colours, durations and luminosities for such transients 11 , 12 , 13 , 14 . Observational evidence for kilonovae has so far been inconclusive because it was based on cases 15 , 16 , 17 , 18 , 19 of moderate excess emission detected in the afterglows of γ-ray bursts. Here we report optical to near-infrared observations of a transient coincident with the detection of the gravitational-wave signature of a binary neutron-star merger and with a low-luminosity short-duration γ-ray burst 20 . Our observations, taken roughly every eight hours over a few days following the gravitational-wave trigger, reveal an initial blue excess, with fast optical fading and reddening. Using numerical models 21 , we conclude that our data are broadly consistent with a light curve powered by a few hundredths of a solar mass of low-opacity material corresponding to lanthanide-poor (a fraction of 10 −4.5 by mass) ejecta.