eli5: Why is Absolute Zero the coldest anything can be? Is all matter just solid at that temperature? Is there a corresponding upper boundary?

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The first thing to get out of the way is that “coldness” does not exist. There are things that are “more hot” or “less hot”. When something is “zero hot”, that is absolute zero. ~~Heat~~ Temperature is a measure of the microscopic motions/vibrations of particles. For a particle to have zero heat, it would be not moving/vibrating at all with respect to other particles. If you want to get technical, nothing can actually be absolute zero, because the laws of thermodynamics and quantum mechanics don’t allow removing all of the energy from it. The other end of the spectrum is called the Kugelblitz. Essentially, if you pack enough heat into a small enough space, it becomes a black hole.

SchiferlED

To go with real 5 year old level: heat comes from (and, in fact, essentially is) energy and absolute zero is the temperature of something with zero energy. You literally cannot get colder than that because something can’t have negative energy.

Potatopolis

Temperature is the average kinetic energy (motion) of the molecules that make up a substance. At absolute zero, all molecules are absolutely still. You can’t get less motion than stillness.

edrenfro

Theoretically negative temperatures are possible in that a system can decrease in entropy for an increase of energy, since thermodynamically 1/T=dS/dE, this gives a negative temperature. A system with a negative temperature is *hotter* than any positive temperature because if put in contact with a system with any positive temperature energy will flow from the negative temperature system into the positve temperature system. You can actually see this effect in a limited way in some lasers (population inversion) and Nuclear Magnetic Resonance Spectroscopy. In 2019 Scientists in the Max-Planck Institute for Quantum Optics and the Ludwig-Maximilians University Munich showed negative temperatures in a Bose Einstien condensate of potassium atoms (39K).

MansfromDaVinci

I want to mention that there are different temperature scales because so many people are talking about negatives in temperature. Temperatures are just like any other scale. You need to set a starting point in order to measure how far from the starting point you are. You can have scales that are always the same, no matter how big the number gets, or you can have scales that kind of… slide further and further the bigger your number gets, so each number gets further and further apart from each other. We use negative numbers in Fahrenheit and Celcius because these are convenient temperatures for measuring the things around us. Fahrenheit was designed to be numbers that corresponded well to the human body. (yes, look it up) and celcius is set so 0C is the freezing temperature of water and 100C is the cooling temperature of water. You can get negative numbers with these scales because temperatures can become colder than ice freezing and colder than the temperature needed to freeze a ammonia chloride brine mixture (again, look it up). Once you get past those 2 points, you have negative numbers. So what is absolute zero? It’s the temperature where all motion in atoms stops. It’s a very difficult thing to imagine, but this temperature is so cold that not even the vacuum of space reaches it. So, someone figured out (Kelvin) that you could make one of those sliding scales and have the biggest and smallest numbers reach out to the very highest and lowest temperatures that the universe is capable of reaching. Absolute Zero is Zero K. No chance for negative numbers there. <EDIT> I just want to mention I said that the method of measuring temperature may have a uniform scale but that does NOT mean that there is uniform energy needed to change the temperature at different points. <EDIT 2> The absolute maximum temperature in Kelvin is 142 nonillion K. That’s 142,000,000,000,000,000,000,000,000,000,000. This is the theteotical maximum temperature the universe can achieve. Any higher and you break physics.

Yosho2k