It can happen in physics only, that we can argue with Newton, and question things that we can't see in our lifetimes, like what could be the hottest objects in the universe. It is only possible in physics, that by reading just a few equations, your mind can travel anywhere in the universe without any passport, a journey beyond all horizons.

WHAT DOES TEMPERATURE MEAN?

In physics, a selective, confined space separated from the surrounding by the means of a boundary on which all the observations take place, is known as the system, now a system can be anything, even a planet can be a system.

Usually, the degree of hotness or coldness of the system is referred to as the temperature, in day-to-day life temperature is measured in the Degree-Fahrenheit scale or the Degree-Celsius scale.

These measuring scales tell us the amount of heat or energy contained in any object, the temperature, and the more heat or energy it will contain, the more the temperature it will have.

On the molecular level, one can say that temperature is nothing but a measure of how fast the particles of that material are moving randomly i.e the Kinetic energy of the particles.

In physics, the coldest temperature any object can hit is zero Kelvin or -273 Degree-Celsius, it is also referred to as an absolute zero but theoretically speaking there is no such upper limit, on how hot an object can get because theoretically, we can add up as much energy as we want in a system.

But how hot it can get, let's see!

WIEN'S LAW, THE RELATIONSHIP BETWEEN WAVELENGTH AND TEMPERATURE

All objects possess a certain amount of energy, so they have a temperature, whether it will be your body touching a deadly fever of 100 degrees Celsius or the core of the sun that will hit 3 billion Kelvin when it will collapse in the future, every object have a temperature

Any Object above zero Kelvin will emit some kind of radiation, we generally emit infrared, which we can't able to see through our eyes, generally, you have to heat an object to a certain temperature, at which it started to emit visible light, the temperature at which this phenomenon happens is called as draper point.

In physics, according to Wien's law radiation having shorter and shorter wavelength i.e higher frequency, will contain more energy, than radiation having a larger wavelength

This indicates that Objects having very high temperatures will emit radiation of very short length.

PLANKS TEMPERATURE AND ABOVE IT

Now we know according to wien's law that, the shorter the wavelength of the radiation emitted by any object, the greater will be its temperature or energy content, or heat. So you will ask this question? What could be the shortest possible wavelength

In 1899, Max Planck was also asking similar types of questions, in the search of that, he ended up proposing a set of units to simplify the laws of physics around units.

By using fundamental constants like the universal gravitational constant 'G' and speed of light 'C' and mathematics of dimensional analysis, he ends up calculating the Planck length, Planck mass, Planck temperature, Planck time, and Planck charge.

These values are purely theoretical but physicists believe they are quite accurate, as the maths behind them makes sense, such a value is Planck's length which is the shortest possible length of anything that can exist in this universe, meaning things shorter than Planck's length will not make sense in reality, and this length comes out to be 1.66× 10^-26 nanometers.

So according to this, the smallest possible wavelength can't get smaller than Planck's length, on doing some more maths we can calculate the temperature of the object which would emit such small wavelengths, which comes out to be 1.44 x 10^32 Kelvin, which is nothing but the value of Planck's temperature.

So theoretically, we can still add up more energy to any object or system, even if it ends up hitting the plank's temperature mark, so what will happen after that, what will happen above the Planck's temperature?

In reality, we don't know anything about it, it will make no sense at all, some physicists have suggested interesting answers but they all are speculations only because we have no factual or observed evidence about it.

One of the most famous solutions to this problem is that adding up more energy after reaching the plank's temperature will end up creating a black hole, out of pure energy, which is also known as kugelblitz.

So, in the end, there is no upper limit to the energy supply, we can add up as much energy as we wish but after reaching Planck's temperature, adding more energy can be fun but in physics, it makes no sense.

And as Dr. Neil DeGrasse Tyson said, "The Universe is under no obligation to make sense to you."