Hello there, I'm Dr. [你的名字] , a physicist specializing in low-temperature physics and quantum mechanics. I've dedicated my career to understanding the fascinating behavior of matter at extremely low temperatures.

Let's dive into the fascinating world of coldest known substances!

When we talk about "coldest", we're referring to the temperature of a substance, which is a measure of the average kinetic energy of its constituent particles. The lower the temperature, the less kinetic energy the particles have, and the slower they move. At extremely low temperatures, the behavior of matter can become quite peculiar and counterintuitive.

So, what is the coldest known substance? This is a surprisingly complex question with no simple answer. It depends on how we define "coldest". Are we interested in the lowest temperature ever achieved in a laboratory setting? Or are we looking for the substance with the lowest possible ground state energy, which represents the lowest energy state that the substance can occupy?

Let's first consider the lowest temperature ever achieved:

* Absolute Zero: The theoretical lower limit of temperature is absolute zero, which is defined as 0 Kelvin (or -273.15 °C). This is the point where all thermal motion ceases, and particles theoretically have zero kinetic energy. However, reaching absolute zero is physically impossible due to the limitations of quantum mechanics.

* Record-Low Temperatures: Physicists have gotten remarkably close to absolute zero. The coldest temperature ever recorded in a laboratory setting was achieved in a research group led by Wolfgang Ketterle at the **Massachusetts Institute of Technology (MIT)** in 2003. They cooled a gas of sodium atoms to a mere 0.45 nanokelvin (4.5 x 10^-10 Kelvin). That's **450 billionths of a degree above absolute zero**!

Now, let's look at the lowest possible ground state energy:

* Bose-Einstein Condensate (BEC): A Bose-Einstein condensate (BEC) is a state of matter in which separate atoms cool down and start to act like a single entity, forming a "super atom". BECs are characterized by their extremely low temperature, often just a few nanokelvins above absolute zero. However, the key factor here is the quantum state of the system, rather than just its temperature. BECs are examples of systems that can be cooled to extremely low energies.

* Fermionic Condensate: While BECs are created from bosons, a class of particles that can occupy the same quantum state, fermions are particles that cannot occupy the same quantum state. Surprisingly, even fermions can form condensates at extremely low temperatures, called fermionic condensates. These condensates are often used in quantum computing research.

So, to answer the question definitively, the "coldest" known substance depends on your definition. If you're looking for the lowest temperature ever achieved, the record goes to the MIT team for their sodium atom condensate. However, if you're interested in the lowest possible ground state energy, Bose-Einstein condensates and fermionic condensates are excellent contenders.

The study of these ultra-cold substances is crucial for our understanding of quantum mechanics and its potential applications in fields like quantum computing and nanotechnology.

I hope this information has been enlightening! Remember, the world of cold is full of mysteries waiting to be unlocked.

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Liquid hydrogen is the coldest substance known to man, minus 400 degrees. ALSO: A Bose-CEinstein condensate (BEC) is a state of matter of bosons confined in an external potential and cooled to temperatures very near to absolute zero (0 K, ?273.15 --C, or ?459 --F ).read more >>