The Coldest Cold Versus The Hottest Hot

laser trap


A basic and surprisingly subtle concept in thermal physics is that of temperature. Two questions that tend to arise in any discussion of temperature is what is the coldest possible temperature and what is the hottest possible temperature. The coldest temperature is known and is absolute zero, 0 K ≅ -273.15 oC. This temperature corresponds to a macroscopic system being in its lowest quantum energy state (ground state), which also corresponds to zero entropy. (Contrary to what many high school science texts say, the coldest temperature does not correspond to the absence of motion, you will learn in Physics 363 that it is possible to have motion, even bulk motion, at absolute zero.)

Using experimental "traps" based on magnetic fields and laser beams like that shown in the above image, scientists have created clouds of atoms suspended in space with temperatures less than 0.1 nanokelvin (10-10 K), extremely close to absolute zero. These temperatures are far far colder than outer space, which is a relatively toasty 3 millikelvin (3×10-3 K) because of the cosmic microwave background radiation (another topic we will discuss during the semester), a remnant of the Big Bang that occurred 14 billion years ago. Scientists using these innovative traps have made remarkable discoveries of new states of matter at ultra-low temperatures, including Bose-Einstein condensates (which we will discuss in the course) and strongly interacting systems of fermions.

As to the question of the hottest possible temperature: you will learn in Physics 363 that temperatures for magnetic spin systems can be negative which implies that such a spin system is hotter than any system with a positive temperature (heat will always flow from a negative temperature system to a positive temperature system). If you feel magnetic spins are "cheating" (you can't disintegrate some object by bringing it in touch with a negative temperature system), then physicists believe that the largest possible positive temperature is likely the Planck temperature TP, which is obtained by combining four of the fundamental constants of nature: TP = ( h c5 / (G k2) )1/2 ≅ 1032 K where h is Planck's constant fundamental to quantum mechanics, c is the speed of light, G is the universal gravitational constant associated with Newton's universal law of gravity, and k is Boltzmann's constant, of central importance to thermal physics. At such immense temperatures, which presumably occurred at the earliest moment of the Big Bang, massive particles move ultrarelativistically and collisions between such particles lead to the creation of matter, antimatter, and even tiny black holes that distort local space-time in a complicated mess that currently lies beyond what the known laws of physics can handle. Advances in string theory and in other efforts to unify gravity with quantum mechanics may possibly resolve the question of whether there is a largest temperature and what is its value.

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