Discussing physical extremes that are difficult to understand from an 8th scale perspective can be challenging. Regardless, I’ll do my best to briefly explore the concept of the hottest temperature. Fortunately, cosmologists have already set the boundaries on what is possible using advanced physics. This means the smallest and largest units are based on very sound mathematical logic, which is good because these things are difficult to imagine, to say the least. Consider the fact that the energies needed to smash particles, to within a Planck length of each other, were available to the local universe at a time equal to the Planck length divided by the speed of light. This 1st scale unit of time is called the Planck time, which can be expressed in scientific notation as:
This is important to understand because, at the start of any new universe, in a literal instant (the Planck time), a natal spacetime continuum attains a state known as “absolute hot”. As the name would suggest, no matter what, nothing can ever get any hotter than that for any reason whatsoever. This is because, at the maximum temperature, the wavelength of electromagnetic radiation is the shortest distance possible, which is the Planck length, or the length of a string in string theory. This results from a temperature of 1.41 x 10³² K (Kelvin), which is equivalent to about 255,000 billion billion billion degrees Fahrenheit. This is why and how the notion of the highest temperature gets to the heart of current inquiries and proposed hypotheses in theoretical physics.
Among scientists, the Planck temperature is thought to be the upper limit of heat, but there’s really no way of knowing for certain, which is a known unknown that philosophers are quick to point out. Nonetheless, the best models of the origin of the universe based on Big Bang theory assume that the universe passed through the Planck temperature during the Planck time as a result of enormous entropy expansion. So, in the Planck temperature scale, the magnitude of the Planck temperature is equal to 1, while that of the coldest possible temperature is equal to 0, which is absolute zero. Thus, absolute hot is the opposite of absolute cold.
In line with this particular cosmological and ontological model of the world, not that long ago, cutting edge experimental scientists created the hottest temperature ever achieved in the lab at CERN, the renowned European Organization for Nuclear Research. To do this, they smashed gold particles together, for a split second, and as a result, the temperature reached nearly ten trillion degrees Fahrenheit. This is utterly astounding because that’s hotter than a supernova explosion and close to the temperature generated during the creation of the local universe. That’s mind-blowingly hot, and its not nearly the end of the search for the hottest thing in existence.
As time goes on, and new facilities are built, the achievable temperature could rise beyond what’s called the Hagedorn temperature, which is about 2×10¹² K. The strange thing is that instead of temperature rising at that point, at the Hagedorn temperature, more and heavier particles are produced by pair production, thus preventing effective further heating in the standard sense, given that only hadrons are produced. This is a key element to consider when trying to define the hottest possible temperature. Of course, further heating is possible with pressure if the matter undergoes a phase change into a quark-gluon plasma, which is exactly what happens during a Big Bang.
With that being said, the way it seems, the thing to note is that the Hagedorn temperature is more akin to a boiling point than an insurmountable upper limit. This is a very important distinction to make in future experimental endeavors. As it currently stands, for hadrons, the Hagedorn temperature has been reached and even exceeded in LHC and RHIC experiments. However, the thing to realize is that a separate temperature can be defined using string theory, where strings similarly provide the extra degrees of freedom. The problem is that the ultimate goal is so high that no foreseeable experiment could ever conceivably reach the ultimate Planck temperature. Only time will tell though…