What is the zeroth law of thermodynamics?

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The zeroth law of thermodynamics states that if two bodies are each in thermal equilibrium with some third body, then they are also in equilibrium with each other. Thermal equilibrium means that when two bodies are brought into contact with each other and separated by a barrier that is permeable to heat, there will be no transfer of heat from one to the other.

In other words, the zeroth law means the three bodies are all the same temperature, according to NASA. James Clerk Maxwell put this perhaps more simply when he said, "All heat is of the same kind." (Longmans, Green, and Co. 1875). What is most important is that the zeroth law establishes that temperature is a fundamental and measurable property of matter.

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When the zeroth law was originally conceived in the 18th century, there were already two laws of thermodynamics. .However, this new law, which presented a formal definition of temperature, actually superseded the existing laws and should rightfully be at the head of the list, according to OpenStax, an educational organization run by Rice University. This created a dilemma: The original laws were already well known by their assigned numbers, and renumbering them would create a conflict with the existing literature and cause considerable confusion. One scientist, Ralph H. Fowler, came up with a solution to the dilemma: He called the new law the "zeroth law." (Cambridge University Press, 1939). (Interestingly, science-fiction writer Isaac Asimov appropriated the idea for a zeroth law in his 1985 novel "Robots and Empire" when he found that he needed to add a new law to the Three Laws of Robotics that superseded the First Law.)

The most notable early efforts to standardize temperature measurement were those of German instrument maker Daniel Gabriel Fahrenheit. In the early 18th century, Fahrenheit invented the familiar glass-tube-type thermometers using both alcohol and mercury. He also invented the Fahrenheit scale, which sets the freezing and boiling points of water at 32 degrees Fahrenheit and 212 degrees Fahrenheit, respectively, and is still used to this day, particularly in the United States. Most of the rest of the world uses the Celsius scale, which assigns values of 0 degrees for the freezing point of water and 100 degrees for its boiling point at mean sea level.

All measuring scales used in science and engineering start at a value of zero. The concept of zero length, mass or time is relatively easy to grasp; however, zero temperature, or absolute zero, where there is absolutely no heat energy whatsoever, is a bit more difficult to grasp. This is because such a temperature has never been observed in nature or in the laboratory, and it is generally believed that it never will be; however, scientists have gotten very close.

The unit for thermodynamic temperature is the kelvin (K) and was originally defined according to the triple point of water. The triple point is defined as "

the condition of temperature and pressure under which the gaseous, liquid, and solid phases of a substance can exist in equilibrium," according to Merriam-Webster. While this standard was convenient for ambient temperatures, it made measurements at very high and very low temperatures uncertain. So in 2019, The International Bureau of Weights and Measures changed the standard to be based on the Boltzmann constant (k), which relates temperature to the average energy of the particles in a substance. That definition acknowledges that temperature is a measure of the average energy of molecular motion, according to the U.K.'s National Physical Laboratory.

For instance, some types of thermometers use a liquid, typically alcohol or mercury, which expands or contracts with increasing or decreasing temperature. This small expansion is amplified by having a relatively large reservoir of liquid in a glass bulb connected to a long and very narrow glass tube. In this way, a small change in the volume of the liquid in the bulb can cause a large change in the level of the liquid in the tube; the temperature can then be determined by reading the height of the liquid against a calibrated scale, according to Harper College.

Another type of thermometer relies on the thermal expansion of metal. Again, the problem is how to amplify a very small change in size so it can be read on a scale. One method involves a coil with many loops, so that even a slight change in length gets multiplied by the number of loops and so is readable. Another method exploits the fact that different metals expand at different rates when heated. Strips of two different metals with different coefficients of expansion — the degree to which the material expands for a certain amount of temperature change — can be laminated together so that the assembly will curl when heated. This "curling" can move a needle that can be read against a scale, according to The Pennsylvania State University.

Another method for measuring temperature involves color changes in temperature-sensitive organic materials. These are typically useful only for measuring limited temperature ranges, such as indicating fever or monitoring room temperature. Another device, called a thermistor, works based on changes in the electrical resistivity of a semiconducting material due to its temperature, according to Penn State. These devices can detect extremely small temperature changes and are used in bolometers — which measure radiation based on a change in temperature — and to monitor laboratory experiments. However, no measurement would be possible without relying on the principle described in the zeroth law.

Jim Lucas is a contributing writer for Live Science. He covers physics, astronomy and engineering. Jim graduated from Missouri State University, where he earned a bachelor of science degree in physics with minors in astronomy and technical writing. After graduation he worked at Los Alamos National Laboratory as a network systems administrator, a technical writer-editor and a nuclear security specialist. In addition to writing, he edits scientific journal articles in a variety of topical areas.

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Ashley Hamer PritchardLive Science Contributor

What is the zeroth law of thermodynamics?

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