What is "Absolute Zero"?

What is "absolute zero"?  This is a question that many curious young scientists may ask. To put it simply, absolute zero is the lowest temperature that can possibly be reached.  We could stop there, but the actual thermodynamics of absolute zero are a little more complicated.

Conventional sciences states that everything in the universe contains some heat.  That is, its particles are constantly in motion.  Kinetic Energy is the energy of moving particles, also called energy of motion or heat energy. It is this heat energy that allows particles to move and change, creating the various chemical reactions that occur every second.  Even the coldest temperatures contain some heat, though particles will have little motion.  What happens, though, when all that can possibly be removed is taken away?  This is what we call absolute zero.

At absolute zero, no more heat can be removed from a system.  Particles are moving at the slowest speed they possible can.  A system that is at absolute zero still contains some heat - the energy of its ground state.  If we were to measure this as a temperature, it would be equal to 0 Kelvin, or -273.15 degrees Celsius.  


More specifically, absolute zero is when the entropy (randomness or disorder) of a system is nonexistent (ΔS = 0).  Disorder within a system is caused by the rapid movement of particles.  When almost all of the heat is removed, disorder will decrease, and particles will behave in a very different way.


Now that we know a little bit about absolute zero, we can ask a few more questions, such as:

- How do particles behave at very low temperatures?
- Have scientists ever been able to achieve absolute zero?
- What is the coldest temperature ever recorded in a lab?
- Is it possible to go below absolute zero?

For the answer to these questions and more, please see the sections on "The Bose-Einstein Condensate" and "What is the coldest temperature ever created?".  These are located at the top of the page.