In chemistry, molecules are normally found in a stable state of balance called equilibrium. Scientists often poke or prod these molecules to see how they react. They might blast the sample with a quick pulse of a laser or a strong magnetic field.
This sudden burst of energy forces the molecules out of their comfortable resting state. They might start spinning faster, vibrating harder, or pointing in the wrong direction. Eventually, the external energy source is turned off.
The molecules immediately begin to dump their extra energy and settle back down. The specific amount of time this recovery takes is called the relaxation time. Students sometimes think all molecules in a sample relax at the exact same speed.
In reality, different types of motion relax at completely different rates. A molecule might stop spinning very quickly, but take much longer to stop vibrating. By carefully measuring these different recovery speeds, chemists can learn amazing details. They can figure out the exact shape of a molecule or how thick a liquid is.