|MadSci Network: Physics|
First, a couple thermal physics basic facts for anyone who reads this question and response to set the stage: 1) Absolute Zero (0 Kelvin) is unattainable by any finite number of steps in a cooling process. This is known as the Third Law of Thermodynamics. 1a) Associated grammar fact: Since Kelvin is the name of the unit in the absolute temperature scale, it's just "Kelvin" and not "degrees Kelvin." Celsius and Fahrenheit are the names of temperature scales, and the units of variation in these scales are degrees, hence "degrees Fahrenheit" and "degrees Celsius." Almost no one, even experienced physicists, gets this right. 2) There's really no upper limit on temperature. 10 million Kelvin is hot, but there are stars that are hotter and explosions (supernova) that make 10 million Kelvin look cool in comparison. The basic process of measuring such extreme temperatures is the same, to observe light either reflected from or emitted by such matter. To observe extremely cold matter (nanoKelvin), we observe how the matter disperses after it's cooled. The speed of the atoms is a direct measurement of the temperature. We can't touch the cooled matter with a thermometer, because the thermometer itself would heat it...so we have to cool the matter in a vacuum and just watch it. As we go up in temperature, we can apply physical probes to matter. We know the relationship between electrical conductivity and temperature for a variety of materials. For very cold materials (liquid helium, etc...), we use small probes at the end of very long, thin wires (so as not to heat the material we want to measure the temperature of). This requirement is situation-dependent and mostly just a matter of proper engineering of experiments. Once matter is hot enough that it emits a spectrum of light that we can easily observe, we can simply look at the light that it gives off. This can be from microwaves (cosmic background radiation tells us the basic temperature of the universe...about 4 Kelvin) to x-rays and gamma-rays (for high-energy exploding matter in supernova or for accretion discs surrounding black holes). The basic blackbody spectrum of radiation emitted tells us very accurately the temperature of the matter we're observing. There's enough overlap in these methods to measure matter at practically any achievable temperature. But at very low and very high temperatures the "thermometers" we use aren't directly in contact with the hot material, they're observations. Any precautions? Yes, matter at low and high temperatures can create hazards...but if you're simply observing matter from a distance you're pretty safe. The measurement of temperature is one of the newer and more transforming of the fundamental measurements in physics (length and time came much sooner). "A Matter of Degrees" by Gino Segre is an excellent popular book on the subject. Technical information on temperature measurement is a vast field of information, and depends largely on what you're trying to measure the temperature of. A Google search of "measuring the temperature of X," where "X" is what you want to measure the temperature of, is a good start.
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