Winter chill got you down? Tired of all that snow and slush? Think of it this way: Things could be much worse. Nighttime temperatures on the moon, for example, plunge to –250 degrees Fahrenheit. When Voyager 2 passed by Neptune’s moon Triton in 1989, it recorded the lowest known temperature in our solar system: –391 degrees F. That might be expected because Triton is 30 times as far from the sun as Earth is. But even on Mercury, the sun’s closest planet, the thermometer drops below –300 degrees F because its dark, airless nights last the equivalent of three of our months.
Technically, heat (or cold) is merely the motion of atoms and molecules. At room temperature they jiggle at about 1,000 miles per hour. Go outside on a hot August day and they move up to 3 percent faster. On a cold February night, they may move some 5 percent slower. In the midst of an extreme Antarctic freeze, atmospheric molecules lose roughly 20 percent of their room-temperature velocity. That seemingly small decrease corresponds to temperatures of around –120 degrees F.
Lower temperatures mean increasingly lethargic movements, until molecular motion essentially stops at –459.67 degrees F. Because there’s nothing slower than stopped, this is the lowest possible temperature—absolute zero. Until the 1960s, astronomers thought that empty space hovered at just that point. Now they know that heat from the Big Bang, greatly diluted by cosmological expansion, yields an all-pervasive 4.91 degrees F warmth, meaning that the temperature of space, on average, is –454.76 degrees F.
You might think this is as cold as nature gets, but it’s not. Astronomers recently discovered a more frigid place: the Boomerang nebula in the constellation Centaurus, located about 5,000 light-years away. This gas cloud is expanding and, like the coolant in a refrigerator’s heat exchanger, growing colder in the process. As a result, the nebula hovers at just 2 degrees F above absolute zero—the coldest known spot in the universe. It is chillier than space itself !
By far, however, the lowest recorded temperatures are not found in distant places but rather in laboratories here on Earth. Research into cold became a hot topic starting in the 1920s, when studies of quantum theory showed that the characteristics of atoms and subatomic particles are inherently uncertain. Unlike planets, whose location and movement are easily determined, little things do not have both a precise position and momentum. Pin one down and the other grows indistinct.
In 1925 Albert Einstein and Indian physicist Satyendra Nath Bose realized that absolute zero seemed to provide an escape clause. If atoms were sufficiently cooled, their motion would grind to a halt. At this point you would know their exact momentum: zero. You would also know their exact location, since they would sit inertly in front of you. It might seem, then, that a high-tech freezer could let you outsmart quantum laws. Bose and Einstein disagreed, arguing that the atoms find a new way to hide their characteristics by blending into a strange collective state of existence.
Curiosity about this issue drove researchers to build elaborate devices that use lasers to drain almost all the motion from atoms. Finally, a decade ago, two research teams chilled matter to 20 billionths of a degree above absolute zero. As Bose and Einstein predicted, the atoms merged into a blurry blob, losing their separate identities and thereby shedding specific information about their position and momentum. The matter was not solid, liquid, gas, or plasma. It was something new, a Bose-Einstein condensate.
Many unusual things happen at extremely low temperatures. Shine a light into a Bose-Einstein condensate, for instance, and light screeches almost to a stop. Some hypercooled materials become superconductors, losing all resistance to the movement of electricity. Helium—the only element without a natural solid state—becomes a frictionless fluid that seemingly defies gravity by flowing upward in a beaker.
To study these bizarre phenomena, physicists keep pushing for ever-lower temperatures. In recent years, MIT physicists have cooled sodium gas to half a billionth of a degree above absolute zero, at which point the atoms in the gas slow to speeds of just one inch per 30 seconds. Compare that to a winter’s day.
What's up in February
February 2: Groundhog Day marks the midpoint between winter and spring. Odds favor morning clouds in Pennsylvania, hence no shadow.
Mid-February: Sirius, the brightest star, and the Orion nebula, the brightest interstellar gas cloud, rise to their highest, most prominent point in the south between 8 and 9 p.m. Also, Jupiter now rises before 10 p.m., shining brilliantly in the eastern sky.
February 26: The waning gibbous moon forms a tight pair with Jupiter, part of a series of repeating monthly close encounters this year.
February 28: The zodiacal light—the faint glow of sunlight reflected off interplanetary dust—is easiest to see about now. Look to the west one to two hours after sunset under clear, dark skies.
All month: Saturn is at its best. You can easily spot this bright, steady, yellowish “star” almost directly overhead, in Gemini, around 9 p.m.