I am not a physicist, so the following may be my misunderstanding. Symmetry seems theoretically impossible, except at one instant. If there was a perfectly symmetric piece of matter (after rotating or reflecting it around some axis, the set of locations of its atoms would be the same as before, just a possibly different atom in each location), then in the next instant of time, its atoms would move to unpredictable locations by the Heisenberg uncertainty principle (the location and momentum of a particle cannot be simultaneously determined). This is because the locations of the atoms would be known by symmetry in the first instant, thus their momenta unknown.
Symmetry may not provide complete information about the locations of the atoms, but constrains their possible locations. Such an upper bound on the uncertainty about locations puts a lower bound on the uncertainty about momenta. Momentum uncertainty creates location uncertainty in the next instant.
Symmetry is probably an approximation: rotating or reflecting a piece of matter, its atoms are in locations close to the previous locations of its atoms. Again, an upper bound on the location uncertainty about the atoms should put a lower bound on the momentum uncertainty. If the atoms move in uncertain directions, then the approximate location symmetry would be lost at some point in time, both in the future and the past.
I have not tried this, so it is just speculation. There are many claims online that a recumbent bike cannot be bunny hopped. However, lifting the front wheel should be possible while sitting on the bike, because lifting the front caster of an office chair is possible without touching the floor. Lean forward, then slam your torso back against the backrest – careful that you don’t tip over backward. Your legs may be lifted or the feet may rest on top of the “spider” at the bottom of the chair.
On a recumbent, a further boost comes from suddenly pedalling hard in low gear, which accelerates the rear wheel forward and under, rotating the front wheel up around the pivot of the rear wheel.
Lifting the rear wheel of a recumbent should be possible while seated, because popping your butt off the floor when sitting with straight legs is possible without using your leg muscles. Put your fists on the floor slightly behind and to the side of your hips. Bend your elbows, then suddenly straighten them, pushing explosively against the floor. Your butt and your fists lift a few inches. Keep your legs locked straight. Very strong people can do this with legs lifted (in boat pose: body in V-shape with only the butt and fists touching the floor).
Because lifting each wheel is possible and the movements do not directly oppose each other, a recumbent should be bunnyhoppable. Lift first the front and then the rear wheel.
Casiraghi et al. (2021) in Science Advances (DOI: 10.1126/sciadv.abe0465) show that human sleep duration and onset depends on the phase of the moon. Their interpretation is that light availability during the night caused humans to adapt their sleep over evolutionary time. Casiraghi et al. fit a sine curve to both sleep duration and onset as functions of the day in the monthly lunar cycle, but their Figure 1 A, B for the full sample and the blue and orange curves for the rural groups in Figure 1 C, D show a statistically significant deviation from a sine function. Instead of same-sized symmetric peaks and troughs, sleep duration has two peaks with a small trough between, then a large sharp trough which falls more steeply than rises, then two peaks again. Sleep onset has a vertically reflected version of this pattern. These features are statistically significant, based on the confidence bands Casiraghi and coauthors have drawn in Figure 1.
The significant departure of sleep patterns from a sine wave calls into question the interpretation that light availability over evolutionary time caused these patterns. What fits the interpretation of Casiraghi et al. is that sleep duration is shortest right before full moon, but what does not fit is that the duration is longest right after full and new moons, but shorter during a waning crescent moon between these.
It would better summarise the data to use the first four terms of a Fourier series instead of just the first term. There seems little danger of overfitting, given N=69 and t>60.
A questionable choice of the authors is to plot the sleep duration and onset of only the 35 best-fitting participants in Figure 2. A more honest choice yielding the same number of plots would pick every other participant in the ranking from the best fit to the worst.
In the section Materials and Methods, Casiraghi et al. fitted both a 15-day and a 30-day cycle to test for the effect of the Moon’s gravitational pull on sleep. The 15-day component was weaker in urban communities than rural, but any effect of gravity should be the same in both. By contrast, the effect of moonlight should be weaker in urban communities, but the urban community data (Figure 1 C, D green curve) fits a simple sine curve better than rural. It seems strange that sleep in urban communities would correlate more strongly with the amount of moonlight, like Figure 1 shows.
The reason why incomplete cleaning may increase the visual perception of dirt is by increasing the contrast between the patches of thicker grime and the normal colour by removing a uniform covering of thinner dirt. If something is uniformly grimy, then the colour of the covering dirt may be perceived as the thing’s normal hue. Cleaning may remove approximately the same thickness of dirt from all points on the surface. If some patches initially have a thicker layer, then these remain the colour of the dirt after the cleaning, but other areas may be fully cleaned and revert to the original look of the surface. The human visual system mostly perceives contrast, not the absolute wavelength of the reflected light, as various optical illusions demonstrate. Higher contrast between the thicker patches of grime and the rest of the surface then enhances the perception of dirtiness.
The paper by Qiao et al. (2020) in Science Advances shows that unicellular algae injected near a hypoxic tumour photosynthesise oxygen in the body in response to infrared light with wavelength 660nm that penetrates >4mm into tissues. The oxygen saturation of the tumour rises from 6.2% to 30% in 2 hours after the algae receive a 5-minute laser exposure. The oxygen sensitises the tumour to radiation therapy. No side effects were found from the algae in this or previous research. The performance of the algae stayed the same when these were coated with red blood cell membranes to delay their clearance from the body.
Another application of algae that can produce oxygen in the organism is doping in sports. The algae can be tattooed under skin that is exposed to light containing enough of the wavelengths which the algae use and which can penetrate under the skin. For example, long-distance runners outdoors in warm weather have most of their skin exposed to sunlight, thus have a large surface area suitable for algal oxygen production. The additional oxygen from photosynthesis improves athletic performance. The only question is whether the oxygen generation is quantitatively fast enough to make a difference. In elite sports, every little advantage counts, so athletes are probably willing to use algae tattoos.
The algae are not dangerous even in deep blood vessels and tissues. Eventually the organism clears the algae, but the clearance of foreign particles is slower in the skin than in deep tissues, as evidenced by the persistence of ordinary tattoos. So the algae will last for a daylong competition.
Patients with breathing problems, for example with coronavirus-induced lung inflammation, may also benefit from algae tattooed on a large area of skin which is then illuminated with 660nm light. Such oxygen supplementation reduces the need for mechanical ventilation. Again, the question is the amount of oxygen from a whole-body algal tattoo.
Creating a standing sound wave in the chamber in which CVD occurs may generate interesting patterns in the deposited film like on Chladni plates. A strong enough compression and strain induced by the wave may even change the crystal structure of the deposit. Maybe even form freestanding filaments through the volume of the chamber, not just a flat deposit on a surface.
Diffraction gratings with narrow bars and bar spacing are useful for separating short-wavelength electromagnetic radiation (x-rays, gamma rays) into a spectrum, but the narrow bars and gaps are difficult to manufacture. The bars are also fragile and thus need a backing material, which may absorb some of the radiation, leaving less of it to be studied. Instead of manufacturing the grating out of a solid material composed of neutral atoms, an alternative may be to use many parallel electron beams. Electromagnetic waves do scatter off electrons, thus the grating of parallel electron beams should have a similar effect to a solid grating of molecules. My physics knowledge is limited, so this idea may not work for many reasons.
Electron beams can be made with a diameter a few nanometres across, and can be bent with magnets. Thus the grating could be made from a single beam if powerful enough magnets bend it back on itself. Or many parallel beams generated from multiple sources.
The negatively charged electrons repel each other, so the beams tend to bend away from each other. To compensate for this, the beam sources could target the beams to a common focus and let the repulsion forces bend the beams outward. There would exist a point at which the converging and then diverging beams are parallel. The region near that point could be used as the grating. The converging beams should start out sufficiently close to parallel that they would not collide before bending outward again.
Proton or ion beams are also a possibility, but protons and ions have larger diameter than electrons, which tends to create a coarser grating. Also, electron beam technology is more widespread and mature (cathode ray tubes were used in old televisions), thus easier to use off the shelf.
Many buildings in Australia, especially new developments, are black, dark grey or brown, or at least the roof is. Many cars are black (other dark colours are less prevalent). The dark colouring increases both cooling and heating costs, because it absorbs and emits solar and infrared radiation faster. In addition, the dark buildings are depressing and ugly. Dark-coloured cars are more difficult to notice, especially in low-visibility conditions, thus have more accidents. White or yellow vehicles are the safest (Lardelli-Claret et al. 2002, Solomon and King 1995).
For cars, the choice of black colour is probably caused by the owner’s desire to seem wealthy by making the car look expensive – limousines in films and popular culture are often black. For buildings, the association in people’s minds between colour and price is weak. If anything, light-coloured houses, reminiscent of Mediterranean villas and the White House, may slightly raise the owner’s status. The reason for dark-coloured roofs may be the cost – tar paper is a cheap material, easy to install. Windows may appear dark due to the one-way glass used. However, for walls, the cheapest material is usually bare concrete, as shown by its choice for purely functional structures (warehouses, barriers, piers, military buildings). For private dwellings, wood or brick may be the cheapest. Neither concrete, wood nor brick is particularly dark in colour, so the choice to build black or brown houses is puzzling. Maybe it is an architectural fad – fashions often trump practicality.
Polycarbonate glasses (sunglasses, safety goggles) can be bent to better fit one’s head by heating the thermoplastic polymer with a lighter, gas stove or other heat source. Example photos are below. Polycarbonate is not flammable and tolerates repeated heating and cooling. The only problem is that the paint discolours when heated and the plastic buckles and wrinkles when bent, so the lenses should not be heated, lest they become unusable. However, for the handles, a better fit outweighs cosmetic appearance in many applications.
Inspired by blind taste testing, manufacturers’ claims about clothes could be tested by subjects blinded to what they are wearing. The test would work as follows. People put clothes on by feel with their eyes closed or in a pitch dark room and wear other clothes on top of the item to be tested. Thus the subjects cannot see what they are wearing. They then rate the comfort, warmth, weight, softness and other physical aspects of the garment. This would help consumers select the most practical clothing and keep advertising somewhat more honest than heretofore. For example, many socks are advertised as warm, but based on my experience, many of them do not live up to the hype. I would be willing to pay a small amount for data about past wearers’ experience. Online reviews are notoriously emotional and biased.
Some aspects of clothes can also be measured objectively – warmth is one of these, measured by heat flow through the garment per unit of area. Such data is unfortunately rarely reported. The physical measurements to conduct on clothes require some thought, to make these correspond to the wearing experience. For example, if clothes are thicker in some parts, then their insulation should be measured in multiple places. Some parts of the garment may usually be worn with more layers under or over it than others, which may affect the required warmth of different areas of the clothing item differently. Sweat may change the insulation properties dramatically, e.g. for cotton. Windproofness matters for whether windchill can be felt. All this needs taking into account when converting physical measurements to how the clothes feel.