Temperature is one of those fundamental concepts that despite our everyday experience with it, we can easily be baffled by. And this is not just true for non-experts. Temperature has been a crucial scientific concept for centuries and understanding its limits impacts us far beyond pure science.

It all boils down (pardon the pun) to thermodynamics, the study of energy, temperature, heat, and work and how they relate to each other. The four laws (from zeroth to third) are so fundamental that they pop up in completely different disciplines. And people have dedicated their lives trying to disprove them, with no success.

The zeroth law affirms that temperature is an important empirical parameter and that thermal equilibrium is a transitive relation. So if object A and object B are in thermal equilibrium with object C, they are also in thermal equilibrium with each other. That is pretty much saying that thermometers are indeed an accurate way to measure things, and if one says yesterday was X degrees and then says that today is also X degrees, that means that both days had the same temperature.

One of our favorite analogies for the other three laws is to picture the universe as a gambling table. The first law is the conservation of energy and it is equivalent to knowing that you cannot win at this table because you can’t create something out of nothing. The second law tells us that you can’t even draw. No system is 100 percent efficient and entropy always increases in an isolated system. Sorry, perpetual motion machine fans, it can’t be done.

The third one says that you can’t leave the table. You can’t choose not to play this game. You are subjected to the laws of thermodynamics wherever you go and those laws suggest that there is an ultimate lowest possible temperature: absolute zero.

What is absolute zero?

The temperature of an object or substance is due to the motion of its molecules. The hotter it is, the more the molecules shake. As energy is removed from a system through thermodynamical processes (like in a fridge for example), the molecules slow down.

And that’s where absolute zero comes in. There is going to be a point when molecules are still, motionless. There is no way to slow them down further. No further lower temperature can be reached.

The value of absolute zero is −273.15°C (−459.67°F) or simply 0 Kelvin in the International System of Units scale. The record for the coldest temperature ever achieved was broken just over a year ago with the cooling of rubidium gas to 38 picokelvins (3.8 * 10^-11 K), truly just a fraction above absolute zero.

What’s the hottest temperature in the Universe?

Humans like symmetry, so if there is a lower limit, is there also an upper limit? Well, things are not so clear-cut when it comes to how hot something can be. The hottest temperature ever created in the lab was 5 trillion Kelvin. It was created in the Large Hadron Collider and it was the temperature of the Universe a few instants after the Big Bang.

But can we go hotter than that? It might be possible for sure. When it comes to the physics of the hottest, we are yet to find something as strict as absolute zero. Absolute hotness has several possibilities, it could be 10,000 times hotter than what we have achieved in particle colliders for example. But it is not strict.

The only limit that can be found in physics depends on the so-called Planck scale. This set of units of measurement depends exclusively on physical constants and tends to signal where physics as we know it breaks apart. Planck temperature is equivalent to 1.4 x 10³² K. That’s 100 billion billion times what you can get in a particle accelerator. Scientists don’t believe it’s possible to get any hotter than that, but the true limit may be much lower.