We usually think of archaeology as involving, brave exploration and lots of tiring digging. But today, long-hidden cities are being revealed from the air, where modern archaeologists use laser beams（激光束） to spot evidence of ancient life buried beneath thick plants.

Lidar, short for “light detection and ranging”（and a cousin of radio-based radar）, involves directing a rapid series of laser pulses at the ground from an airplane. Software records the time and wavelength of the pulses reflected from the surface and combines the data with GPS to produce a three-dimensional map of the scene below. These high-tech explorations have revealed long-buried Mayan cities.

In recent years, lidar exposed an ancient city in western Mexico called Angamuco. Built by the Purepecha, who were enemies of the Aztecs, Angamuco was a major civilization in the early 16th century, before Europeans arrived. “To think that this huge city existed in the heartland of Mexico for all this time and nobody knew it was there is kind of amazing”, says Chris Fisher, an archaeologist at Colorado State University who led the team. “You are talking about 40,000 building foundations, which is about the same number of building foundations that are on the island of Manhattan,” says Fisher.

Archaeologists discovered signs of the buried city in 2017 and initially attempted to explore it using a traditional approach. But the team soon realized that with the rough road, it would take at least a decade to outline the entire city. In 2020, they began using lidar to map nearly 14 square miles, revealing an astonishing number of characteristics, from temples to road systems and garden areas. This gave them the “map” they needed to know where to explore further. So far, Fisher and his team have confirmed more than 7,000 architectural remains over a 1.5-square-mile area. All told, researchers now believe that more than 100,000 people lived in angamuco from about 1000 Ce to 1350 CE. That makes it the biggest city in western Mexico at the time.

“Everywhere you point the lidar instrument, you find new things,” says Fisher. “Right now, every textbook has to be rewritten, and two years from now, they're going to have to be rewritten again.”

“How the brain perceives time depends on its expectations. The brain can predict the probability that something is going to occur, given that it hasn’t happened yet,” said Dr. Michael Shadlen, a scientist at Columbia University.

Every thought has various “horizons”, Shadlen told Live Science. In a book, for example, horizons lie at the end of every syllable (音节), the end of every word, the end of the next sentence and so on. Time moves according to how we anticipate these horizons, he said.

When you’re really absorbed in something, the brain anticipates the “big picture”, which makes time seem to fly, Shadlen said. But when you’re bored, you anticipate the closer horizons such as the end of a sentence instead of the end of the story; these horizons aren’t knitted together as a whole, and time slows down.

There isn’t a single spot in the brain that’s responsible for how we perceive time in this way. Rather, any area that gives rise to thought and consciousness is likely involved in this task, Shadlen said. “There are almost certainly a lot of timing mechanisms (机制) in the brain,” added Joe Paton, a scientist at the Champalimaud Foundation in Portugal. One mechanism involves the speed at which brain cells activate one another and form a network when you’re performing an activity. The faster those paths of neurons (神经元) form, the faster we perceive time, Paton and his team have found in some animals, such as rats and squirrels.

Another mechanism involves chemicals in the brain. Again, in rats, Paton and his colleagues found that a set of neurons that releases the neurotransmitter dopamine—an important chemical involved in feeling rewarded—impacts how the brain perceives time. When you’re having fun, these cells are more active. They release a lot of dopamine and your brain judges that less time has passed than actually has. When you’re not having fun, these cells don’t release as much dopamine, and time seems to slow down.

It’s been 50 years since the beginning of bar codes. Scanning an item at checkout is something we take for granted in this age of convenience. 【小题1】

On March 31, 1971, a historic meeting took place in New York City. The meeting agreed to create a system to uniquely identify every single product, calling it the Global Trade Item Number (GTIN) the core of bar codes, according to El Mudo, a Spanish newspaper. 【小题2】 Today, bar codes are scanned over 6 billion times every day, used by 2 million companies worldwide and remain one of the most trusted symbols in the world, PR Newswire, a US news agency, reported.

【小题3】 Where the product comes from? where it has been? its price expiration (过期时间)… you name it. Bar codes don’t only identify the category of goods but also manage the number of goods. For example, if there are 10 cartons of milk and a consumer takes one, it will be registered when it is scanned at the cash counter, so the store owner knows there are nine left. 【小题4】 The unique identification of products at a global level was the key that opened the door to retail (零售) as we know it today.

In the past five decades, bar codes have provided many conveniences for commerce. “The next generation of bar codes, such as QR codes can hold vastly more information. Their use, for example, can tell consumers if a product contains allergens (过敏原) or if it is organic. 【小题5】” according to GS1, the organization that develops and maintains the global bar codes standard.