Refraction of brachistochrones and time-like geodesics
The brachistochrone problem can be solved either by variational calculus or by a skillful application of the Snellius’ law of refraction. This suggests the question whether also other variational problems can be solved by an analogue of the refraction law. In this paper we investigate the physically interesting case of free fall in General Relativity that can be formulated as a variational problem w. r. t. proper time. We state and discuss the corresponding refraction law for a special class of spacetime metrics including the Schwarzschild metric…
Read more at https://arxiv.org/pdf/1809.00355.pdf
We analyze in this work some analogies between thermal emission of nano objects and Hawking’s radiation. We first focus on the famous expression of the black hole radiating temperature derived by Hawking in 1974 and consider the case of thermal emission of a small aperture made into a cavity (Ideal Blackbody). We show that an expression very similar to Hawking’s temperature determines a temperature below which an aperture in a cavity cannot be considered as standard blackbody radiating like T^4. Hawking’s radiation therefore appear as a radiation at a typical wavelength which is of the size of the horizon radius. In a second part, we make the analogy between the emission of particle-anti particle pairs near the black hole horizon and the scattering and coupling of thermally populated evanescent waves by a nano objects. We show here again that a temperature similar to the Hawking temperature determines the regimes where the scattering occur or where it is negligible.
Read more at https://arxiv.org/pdf/1808.08037.pdf
Originally an empirical law, nowadays Malus’ is seen as a key experiment to demonstrate the traversal nature of electromagnetic waves, as well as the intrinsic connection between optics and electromagnetism. More specifically, it is an operational way to characterize a linear polarized electromagnetic wave. A simple and inexpensive setup is proposed in this work, to quantitatively verify the nature of polarized light. A flat computer screen serves as a source of linear polarized light and a smartphone is used as a measuring instrument thanks to its built-in sensors. The intensity of light is measured by means of the luminosity sensor with a tiny filter attached over it. The angle between the plane of polarization of the source and the filter is measured by means of the three-axis accelerometer, that works, in this case, as an incliometer. Taken advantage of the simultaneous use of these two sensors, a complete set of measures can be obtained just in a few seconds. The experimental light intensity as a function of the angle shows an excellent agreement with standard results…
…. Read more at https://arxiv.org/ftp/arxiv/papers/1607/1607.02659.pdf
Joshua M. Grossman
During a recent ride in an elevator, I was startled by an observation. Once the door closed, the features on the back wall of the elevator were evident in a reflection on the door; however, my own reflection appeared absent . How could that be? What physics caused this curious phenomenon? The elevator had wooden molding, including horizontal strips that ran all the way around the back and sides . These horizontal strips were what showed up most clearly in the reflection. The door’s surface was brushed metal with the brush marks all running vertically. Therein lay the solution…
…Read more at scitation.aip.org
by Michael Slezak
Isaac Newton just got cheated. Laser pulses have been made to accelerate themselves around loops of optical fibre, seeming to break the physicist’s law that every action must have an equal and opposite reaction. The work exploits a trick with light that only makes it appear to have mass, so it is a bit of a cheat, but it may one day lead to faster electronics and more reliable communications.
According to Newton’s third law of motion, when one billiard ball strikes another, the two balls should bounce away from each other. But if one of the billiard balls had a negative mass, then when the two balls collide they will accelerate in the same direction. This effect could be useful in a diametric drive, a speculative “engine” in which negative and positive mass interact to accelerate forever. NASA explored using the effect in the 1990s in a bid to make a diametric drive for better spacecraft propulsion. But there was a very big fly in the ointment: quantum mechanics states that matter cannot have a negative mass. Even antimatter, made of particles with the opposite charge and spin to their normal matter counterparts, has positive mass.
“Writing a negative mass in quantum field theory doesn’t make any difference,” says Archil Kobakhidze at the University of Sydney, Australia. The equations involve terms that are always squares of mass, so any negative mass will become positive anyway. “It has no observable meaning.”
Now Ulf Peschel at the University of Erlangen-Nuremberg in Germany and his colleagues have made a diametric drive using “effective mass”. As photons travel at the speed of light they have no rest mass. But if you shine pulses of light into some layered materials, such as crystals, some of the photons can be reflected backwards by one layer and then reflected forwards again by another. That delays part of the pulse, causing it to interfere with the rest of the pulse as it propagates more slowly through the material.
“It’s a bit like what happens with a stroboscope,” says Dragomir Neshev at the Australian National University in Canberra, who was not involved in the study. If you watch a spoked wheel turning under a strobe it can appear to move at a different speed or even backwards.
When a material slows the speed of the pulse proportional to its energy, it is behaving as if it has mass – called effective mass. Depending on the shape of the light waves and the structure of the crystal, light pulses can have a negative effective mass. But to get such a pulse to interact with one with a positive effective mass requires a crystal that is so long it would absorb the light before the two pulses could show a diametric drive effect.
To get around this, Peschel created a series of laser pulses in two loops of fibre-optic cable. The pulses get split between the loops at a contact point, and the light keeps moving around each loop in the same direction. The key is that one loop is slightly longer than the other, so light going around the longer loop is relatively delayed (see diagram, above right). When that pulse comes back around and splits at the contact point, it shares some of its photons with pulses in the other loop. After a few round trips, the pulses develop an interference pattern that gives them effective mass.
The team created pulses with positive and negative effective mass. When the opposing pulses interacted in the loops, they accelerated in the same direction, moving past the detectors a little bit sooner on each round trip.
“By having these loops you can loop it forever – it’s equivalent to having enormously long crystals,” says Neshev, whose group has also tried to create a diametric drive. “It is nice physics and a very clever apparatus.”
Electrons in semiconductors can also have effective mass, so the loops could be used to speed them up and boost processing power in computers, says Peschel. And in some fibres the speed of a light pulse is equivalent to its wavelength, which means the loops could be used to control a fibre’s colour output. Neshev says the method could increase the bandwidth of optical communications or even help create bright displays like laser screens. But he cautions that it will not be easy to adapt the loops for practical purposes.
Journal reference: Nature Physics, DOI: 10.1038/NPHYS2777
Ramis Movassagh and Steven G. Johnson
By Bernoulli’s law, an increase in the relative speed of a fluid around a body is accompanies by a decrease in the pressure.
Therefore, a rotating body in a fluid stream experiences a force perpendicular to the motion of the fluid because of the unequal relative speed of the fluid across its surface. It is well known that light has a constant speed irrespective of the relative motion.
Does a rotating body immersed in a stream of photons experience a Bernoulli-like force?
We show that, indeed, a rotating dielectric cylinder experiences such a lateral force from an electromagnetic wave.
In fact, the sign of the lateral force is the same as that of the fluid-mechanical analogue as long as the electric susceptibility is positive (ε>ε0), but for negative-susceptibility materials (e.g. metals) we show that the lateral force is in the opposite direction.
Because these results are derived from a classical electromagnetic scattering problem, Mie-resonance enhancements that occur in other scattering phenomena also enhance the lateral force.
Read more at http://arxiv.org/pdf/1305.0317v2.pdf