What Is the Hottest Possible Temperature If Absolute Zero Is the Lowest Temperature?

Yes, there is a hottest possible temperature. But let’s start off at merely warm and gradually turn up the heat.

Heat is the energy that a substance contains within itself, due to the fact that its atoms and molecules are moving. But temperature is a man-made concept, invented so that we can converse among ourselves about how much of that energy a substance has and actually assign numbers to it.

When we say we are “raising the temperature” of an object, we are adding heat energy to its atoms and molecules and making them move faster. The ultimate limit to cooling and slowing them down has to be when they’re not moving at all; that’s absolute zero. Our current question, then, comes down to whether there is any limit to how fast those atoms and molecules can move.

But long before we reach any such speed limit, several things will happen. First, if the substance is a solid it will melt into a liquid. Then at a higher temperature the liquid will boil and become a vapor or gas, a condition in which the atoms or molecules are flitting around freely in all directions. As the temperature gets higher and higher, they flit faster and faster. For example, the nitrogen molecules in the air in your 350-degree-Fahrenheit (177degree-Celsius) oven are flitting about at an average speed of 1,400 miles per hour (2,300 kilometers per hour).

If the substance is made of molecules (clusters of atoms glued together), the molecules will eventually be knocked to pieces, broken apart into smaller fragments or even into their individual atoms by the shattering forces of their violent collisions. In other words, every molecular compound will decompose at a high enough temperature.

Will the individual atoms themselves ever be broken apart? Yes, indeed. At a high enough temperature the atoms’ electrons will be torn off, resulting in a seething, fluid inferno of free electrons and charged atomic fragments, called a plasma. This is the stuff of the interiors of stars, at temperatures in the tens of millions of degrees.

Still higher temperatures? Why not? There would seem to be nothing to prevent us from heating a plasma’s electrons and atomic fragments to faster and faster speeds, except for one thing. There happens to be a speed limit in the universe: the speed of light in a vacuum, which is 671 million miles per hour (1.08 billion kilometers per hour).

Albert Einstein told us that the electrons in a plasma, or any object, for that matter, may approach the speed of light, but can never achieve it. He also told us that as a particle goes faster and faster, it gets heavier and heavier. For example, when cruising along at 99 percent of the speed of light, an electron has 7 times its normal mass; at 99.999 percent of the speed of light, it is 223 times heavier than when it’s not moving.

There must be an ultimate temperature limit, then, lest the particles in a plasma reach the speed of light and become infinitely heavy. Theoretical considerations peg this temperature at around 140,000,000,000,000,000,000,000,000,000,000 degrees, Fahrenheit or Celsius, take your pick.

The next time someone says to you on a blazing summer day, “Whew! How hot can it get?” tell him. But don’t worry. Global warming still has a long way to go.