Absolute Zero: Temperature & Definition

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  • 0:00 Definition
  • 0:48 Temperature Limits
  • 2:09 Examples
  • 9:10 Lesson Summary
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Lesson Transcript
Instructor: Elena Cox
There is a limit to how cold matter can get. Learn more about what happens when the temperature gets down to absolute zero and cannot get any lower. Test your knowledge with quiz questions.


As you remove heat from a substance, its temperature drops. The particles of the substance move slower and slower. What happens if you keep removing heat from the substance? You will reach a temperature at which all particle motion almost stops. The temperature is called absolute zero, and it's the lowest possible temperature. It is equal to -273oC (-459oF). You cannot have a temperature lower than absolute zero. You can think of absolute zero as the temperature where molecules are completely frozen, with no motion. Technically, molecules never become absolutely motionless, but the kinetic energy is so small it might as will be zero.

Temperature Limits

You might say that a fire is hot and a freezer is cold. But the temperatures of everyday objects are only a small part of the wide range of temperatures present in the universe, as shown in this figure:

Some different temperatures present in the universe

Temperatures in the universe range from just above absolute zero to more than 10 to the 10th power K. Temperatures do not appear to have an upper limit. The interior of the Sun is at least 1.5x10 to the 7th power C. Supernova cores are even hotter. On the other hand, liquefied gases can very cold. For example, helium liquefies at -269oC. Even colder temperatures can be reached by making use of special properties of solids, helium isotopes, and atoms and lasers.

Temperatures do, however, have a lower limit. Generally, materials contract as they cool. If an ideal atomic gas in a balloon were cooled to -273.15oC, it would contract in such a way that it occupied a volume that is only the size of the atoms, and the atoms would become motionless. At this temperature, all the thermal energy that could be removed has been removed from the gas, and the temperature cannot be reduced any further. Therefore, there can be no temperature less than -273.15oC, which is called absolute zero.


The Fahrenheit and Celsius temperature scales have little or nothing to do with the fundamental nature of the concept of temperature. After all, the freezing point of water at Earth's atmospheric pressure has no obvious relationship to any basic aspect of the Universe. Presumably, an alien scientist on a planet that has no water will devise a thermometer that measures temperature equally well. More to the point, could there be a universal zero of temperature linked to the very essence of matter and energy, a zero that all scientists (human or otherwise) might discover? The answer is yes, and it is called the absolute zero of temperature.

Let's look at the history of temperature measurements to better understand the nature of absolute zero. Like so many other physical quantities, temperature was measured long before it was understood. Galileo appears to have invented (ca.1592) the first device for indicating 'degrees of hotness'. As shown in the image below, he simply placed the end of an inverted narrow-necked flask, warmed in his hands, into a bowl of water (or wine). As it cooled, the liquid was drawn up, partially filling the neck.

The thermoscope invented by Galileo

This was Galileo's thermoscope. It is not called a thermometer, because the scale was arbitrary. The egg-sized globe at the top is the sensor. The gas within it expands or contracts, and the liquid level rises and falls.

Air captured in the bulb at the top either expanded or contracted when subsequently heated or cooled, and the column fell or rose proportionately. Much the same effect can be seen by putting an inflated balloon in the freezer. The gas molecules inside the balloon keep it puffed up by constantly bombarding the inner walls. Cooling the gas, removing kinetic energy from its molecules, lessens their bouncing and causing the balloon to collapse.

The medical applications of the thermometer were recognized almost immediately, and normal body temperature became a focus of interest. In 1631, J. Rey, a French physician, inverted Galileo's device, filling the bulb with water and leaving much of the stem with air in it. In that configuration, which is more like today's thermometer, the liquid's expansion operates the device. Within 70 years of Galileo's invention, sealed pocket-sized thermometers containing either alcohol or mercury were in use.

By exploring the behavior of matter with the aid of the thermometer, it soon became evident that there were a number of physical occurrences that happened at fixed temperatures. In 1665, Boyle, Hooke, and Huygens, European natural scientists, independently recognized that that fact could provide a reliable reference point for any thermometer. Hooke suggested using the freezing point of water, and Huygens offered its boiling point. In 1694, an Italian scientist Carlo Rinaldini used both the freezing and boiling points of water to standardize two widely spaced points on his thermometer.

As we will see, the manner in which any material - mercury, alcohol, water, glass, whatever - expands on heating is characteristic of that material. What this means practically is that thermometers that differ in their physical construction and yet have the same two fixed reference points, will necessarily agree exactly only at those two points.

The range of temperatures we deal with is extensive, and there is now a whole arsenal of different kinds of thermometers, each with its own virtues. The familiar mercury-in-glass instrument is only useful between the points where mercury freezes and glass melts. Moreover, it can be used reliably only when its presence doesn't affect the temperature being determined. If you want to measure the temperature of a flea or a thermonuclear fireball, the old mercury-in-glass standby will not be of much help. Accordingly, there are electrical resistance thermometers, optical thermometers, thermocouples, and constant-volume gas thermometers, to name a few. Of these, the standard for accuracy and reproducibility is still the constant-volume gas instrument as shown in the figure below, though it is large, slow, delicate, and inconvenient.

The constant-volume gas thermometer
constant volume thermometer

Sometime around 1702, Guillaume Amontons devised an improvement on Galileo's thermoscope that has evolved into the modern constant-volume gas thermometer shown here.

Let's have a look at what happens with a typical gas as its temperature is lowered.

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