It cannot get colder than -273.15 degrees Celsius (0 Kelvin, -459,67 Fahrenheit). The reason physics gives for this is that temperature is a measure of the kinetic energy of particles, i.e. it tells us something about how fast they are moving. When all motion stops, we have reached the minimum of the temperature scale, which by definition is 0. But is there also a highest temperature? One could assume that, because there is not only a minimum speed (0), but also a maximum speed (the speed of light c). But then it is not so simple, because the energy of massed particles, which approach c, goes towards infinity. Thus their temperature would be also infinite.

So we start best in known areas. Water evaporates at normal pressure at 100 degrees Celsius (211 F). At 5930 degrees Celsius (10,700 F) tungsten boils, the metal with the highest boiling point (200 degrees warmer than at the surface of the sun). So at more than 6000 degrees Celsius (10,830 F) and normal pressure, solid or liquid matter no longer exists (it’s a different story at high pressure). If you heat further, atoms lose their electrons and become ions forming plasma, the fourth state of matter. In the core of the sun it is over 15 million Kelvin (27 million F) hot. A supernova reaches around 10 billion degrees Celsius (18 billion F).

But that’s not the end. If you keep adding heat, the heated plasma will eventually reach the Hagedorn temperature, which is 1.7 trillion (10¹²) Kelvin. Something strange happens here: the temperature does not rise any further at first, just as the temperature does not rise any more when water boils. But the cause is different: At this enormous heat, there is so much energy that quark-antiquark combinations can be created out of nothing. Their production on the one hand swallows a part of the supplied energy, but also leads to the fact that the system gets new degrees of freedom, which can also absorb energy. All hadrons (elementary particles made of quarks) dissolve into their components and swim together with their glue, the gluons, and many quarks added from nowhere in an ultra-hot soup, the quark-gluon plasma. In the laboratory (namely in particle accelerators) such temperatures of several trillion Kelvin are reached today regularly.

But also this quark-gluon soup can be heated up further – at the latest, when there is not enough space left to add new quarks, because quarks are not point-like, but occupy a certain space (even if it is very small). Now it is a long way to the next limit. This is given by the string theory, which assumes that all matter consists of vibrating strings. These extend over more than our four known dimensions – over up to 21 dimensions in total. The reason that we do not notice anything about these additional dimensions is that they are “coiled up” to tiny dimensions, the detection of which is not yet possible for us today.

But there are physicists who suspect something else: The extra dimensions could also be very large, so that they are found only at high energies (and temperatures). If one would reach these temperatures, all basic forces would unite to one. This would have to happen then at about 100 quadrillion degrees (10¹⁷) which are almost tangible in the particle accelerator today. This is exactly why it does not look good for this theory. So far it could not be confirmed in the experiment, on the contrary, the researchers could already exclude some versions of the theory.

But this leaves string theory with tiny extra dimensions in play. It would lead to the unification of all basic forces at about 10³⁰ Kelvin. With it it lies, however, still a piece under the Planck temperature of 10³² Kelvin (a 1 with 32 zeros) which must have prevailed shortly before the big bang in the universe.

Is this then the highest temperature? In this universe perhaps, but in the universe next door with its completely different physical constants it can look differently.