Cold

Cold is the presence of low temperature, especially within the atmosphere. While people often describe cold as a subjective feeling, it is actually a measurable physical state. Temperature is directly related to thermal energy. This energy is the kinetic energy created by the random motion of particles within matter. When an object is hot, its particles move quickly. When an object is cold, it has less thermal energy, and its particles move more slowly.

There is a theoretical limit to how much an object can be cooled. This limit is known as absolute zero, which is defined as 0.00K on the Kelvin scale. At this point, the motion of particles in a sample of matter would cease entirely in a classical sense. However, quantum mechanics shows that matter still possesses zero-point energy even at absolute zero due to the uncertainty principle. Scientists use several methods to reach low temperatures. Cooling can occur by removing heat from a system or exposing it to a colder environment. One method is conduction, where heat transfers from a warm object to a cold coolant like ice or liquid nitrogen. Another method is air cooling, which works if the air is colder than the object. This process can be improved by increasing the surface area or the flow rate of the air. Scientists also use advanced techniques like laser cooling and magnetic evaporative cooling to reach extremely low temperatures.

Humans have used the properties of cold for thousands of years. In ancient times, Romans used ice to chill wine. Emperor Nero is said to have invented the ice bucket so he could chill wine without diluting it. Around 1700 BC, King Zimri-Lim of the Mari Kingdom used an "icehouse" called a bit shurpin. By the 7th century BC, the Chinese were using icehouses to preserve fruits and vegetables. During the Tang dynasty, workmen were employed in an "Ice-Service" to freeze items ranging from wine to dead bodies.

Scientific understanding of cold changed significantly during the 17th century. Cornelis Drebbel performed a famous demonstration at Westminster Abbey in 1620. He created a chill in the hall that was so intense the king and his entourage shivered and ran out. While many viewed this as magic, thinkers like Francis Bacon and Robert Boyle sought the real science behind it. Boyle conducted extensive experiments on how cold transmits between materials. He proved that water was not the only source of cold, as metals like gold and silver could also become severely cold. This research helped lay the groundwork for 19th-century physics.

In the 19th century, technology made cold more useful for everyday life. In 1810, Thomas Moore created an ice box for butter using a wooden tub with a metal lining and rabbit skin insulation. In 1825, Nathaniel J. Wyeth invented a horse-drawn device to harvest uniform blocks of ice. By the mid-1800s, ice export in the United States was a massive industry, second only to cotton. The 20th century brought even more change. Home refrigerators were invented in 1913, and the first self-contained unit arrived in 1923. By the 1940s, separate home freezers allowed frozen foods to become a common part of life.

Cold has significant effects on the human body. One of the first responses is shivering. The body may also produce goose bumps to help reduce heat loss.

However, extreme cold can be dangerous. It can disrupt blood circulation and cause extracellular water to freeze, which destroys tissue. This can lead to frostnip, frostbite, or even the death of body parts like fingers and toes. Very extreme temperatures can cause sepsis or hypothermia. It is a common myth that cold weather itself causes the common cold, but science shows no evidence for this, even though such diseases are more common in winter.

Space contains some of the coldest environments known. The Boomerang Nebula is the coldest natural location in the universe, with an estimated temperature of 1 K. In our solar system, the lunar crater Hermite is one of the coldest known places, at 26 kelvins. The dwarf planet Haumea has a temperature of −241°C. Even the space between stars has a temperature of roughly 2.725 kelvins due to cosmic microwave background radiation.

Scientists have even achieved a state of matter called a Bose–Einstein condensate by cooling rubidium atoms to below 170 nK. This represents one of the coldest temperatures ever reached in a laboratory setting.