Absolute zero is the coldest possible temperature, where particles nearly stop moving altogether.

The longer version

Temperature measures how vigorously the particles in a substance are moving. Cooling removes that motion, and at the lowest limit the motion reaches its minimum rather than vanishing entirely, because quantum rules forbid complete stillness. Nothing can be cooled below that floor, since there is no slower state to reach.

Reaching it is impossible in practice, because any surrounding object radiates energy inward. Scientists have come extremely close using magnetic and optical methods, and matter at such temperatures shows strange behaviour, such as flowing without friction. The absolute temperature scale begins at that point and never reports a negative value.

Where this entry sits

Everyday physics and chemistry hide small wonders in plain sight, from kitchen reactions to the light in the sky. These short facts explain matter, energy, electricity, magnets, sound, and the simple units we use to measure the world. The full list sits on the Science and Physics page, where every entry is listed in order.