【推荐1】The universal speed limit of any kind of wave — whether electromagnetic or gravitational travelling through a vacuum has been known since Albert Einstein developed his theory of special relativity in 1905. But the maximum speed of sound moving through a solid or a liquid has just been calculated for the first time. It is about 36 kilometre per second, more than 8,000 times lower than the speed of light in a vacuum.

To make this calculation, Kostya Trachenko at Queen Mary University of London and his colleagues started with two well-known physical constants: the ratio of proton mass to electron mass, and the fine structure constant, which characteristics the strength of interactions between charged particles.

Trachenko says we have a pretty good idea of these values, because if they were changed even a bit, the universe wouldn’t look at all like it does.” If you change these constants by a few percent, then the proton might not be stable anymore, and you might not even have the processes in stars resulting in the combination of heavy elements, so there would be no carbon, no life,” he says.

Sound is a wave that spreads by making neighbouring particles interact with one another, so its speed depends on the density of a material and how the atoms within it are bound together. Atoms can only move so quickly, and the speed of sound is limited by that movement.

“The common wisdom was that diamond has the highest speed of sound, because it is the hardest material, but we didn’t know whether there was a theoretical fundamental limit to it,” says Trachenko. The theoretical bound is about twice the speed of sound in a diamond.

The speed of sound is also dependent on the mass of the atoms in the material, so there searchers predicted that solid metallic hydrogen — a material that theoretically exists at the centre of giant planets, but for which laboratory evidence has been hotly questioned — should have the highest speed of sound. They calculated that it should be close to the theoretical limit. They also looked at experimental data for 133 materials and found that none of them broke the limit.

【推荐2】Most people aren’t particularly fond of needles. But to a significant number of people, the fear of needles goes beyond anxiety into a more dangerous area, in which the fear prevents them from seeking out needed medical care.

And as the world’s hopes of returning to a post-pandemic normal rest largely on people’s willingness to take a COVID-19 vaccine (疫苗), experts and health care professionals are assuring those people that there are ways to overcome a fear of needles. Experts say it is a problem that can be overcome, whether the fear is keeping you from getting the vaccine or just causing your distress. Here are the steps they suggest taking.

A therapist (治疗师) can help people with the most severe fears, using some of the techniques that help people conquer other fears that can affect their lives. Dr. Chambless suggested working on your comfort levels by first looking at photos of needles, then photos of someone getting a shot, and working up to videos. In addition, a therapist can offer a fuller plan. If you can’t see a therapist, self-help books on overcoming fears could be a quicker choice, she said.

The whole thing will be over in seconds, and a distraction can help you get through it. It could be a YouTube video on your phone, or your favorite song playing. You could practice deep-breathing, or move your toes up and down, or look around and count all of the blue items you can see in the room.

The media can do its part by showing fewer images of people looking uncomfortable while a needle goes into their skin, which can increase feelings of anxiety, Dr. Rajapakse said. A good strategy is all of the positive photos appearing on social media of people holding their vaccination cards, she said. The more selfies (自拍), stickers and grateful posts people see, the more likely they are to associate the vaccine with positive feelings, she said.

【推荐3】Maitland Jones Jr. is one of the nation’s top organic chemistry professors. He taught the subject for decades, first at Princeton and then at New York University, and wrote an influential textbook. He received awards for his teaching, as well as recognition as one of NYU’s coolest professors.

But last spring, as the campus emerged from pandemic restrictions, 82 of his 350 students signed a petition (请愿书) against him. Students said the course-negatively known for ending many students’ dream of medical school-was too hard, blaming Dr. Jones for their poor test scores. “We are very concerned about our scores, and find that they are not an accurate reflection of the time and effort put into this class,” the petition said. “A class with such a high percentage of withdrawals and low grades has failed to make students’ learning and well-being a priority and reflects poorly on the chemistry department.”

Dr. Jones, 84, who pioneered a new method of instruction that relied more on problem solving, defended his standards. “Students were misreading exam questions at an astonishing rate,” he wrote to the university. Grades fell even as he reduced the difficulty of his exams. After several years of Covid learning loss,   the students not only didn’t study, they didn’t seem to know how to study.

The officials decided to end the contract with Jones and offered the students a chance to review their grades and withdraw from the class.

The university’s handling of the petition set off equal and opposite reactions from both the chemistry professors and pro-Jones students. “They are obviously going for some bottom line, and they want happy students who are saying great things about the university so more people apply and the U.S. News rankings keep going higher,” said Paramjit Arora, a chemistry professor.

“I don’t want my job back,” Dr. Jones said, adding that he had planned to retire soon anyway.“I just want to make sure this doesn’t happen to anyone else.”