Why do temperatures get as high as 1 billion degrees, but only as low as -273 degrees?

Temperature is one of those fundamental concepts that, despite our daily experience, we can easily be amazed at. And this doesn’t just apply to non-experts. Temperature has been a crucial scientific concept for centuries, and understanding its limits has an impact on us far beyond pure science.

It all boils down (excuse the pun) on thermodynamics, the study of energy, temperature, heat, and work and how they relate to each other. The Four Laws (from zero to third) are so fundamental that they crop up in completely different disciplines. And people have devoted their lives to disproving them, without success.

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

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

The third says not to leave the table. You cannot choose not to play this game. Wherever you go, you are subject to the laws of thermodynamics, and those laws suggest that there is the ultimate lowest possible temperature: absolute zero.

What is absolute zero?

The temperature of an object or substance is due to the movement of its molecules. The hotter it is, the more the molecules shake. Since energy is removed from a system by thermodynamic processes (like in a refrigerator, for example), the molecules slow down.

And that’s where absolute zero comes in. There comes a time 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 on the International System of Units scale. The record for the coldest temperature ever reached was broken just over a year ago with the cooling of rubidium gas to 38 picokelvin (3.8 * 10^-11 K), really only a fraction above absolute zero.

What is the hottest temperature in the universe?

People like symmetry, so if there’s a lower bound, is there an upper bound? Well, things are not so clear 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 moments after the Big Bang.

But can we go hotter than that? It may certainly be possible. When it comes to the physics of the hottest, we’ve yet to find something as strict as absolute zero. Absolute hotness has several possibilities, it can be 10,000 times hotter than what we have achieved in particle accelerators for example. But it’s not strict.

The only limit found in physics depends on the so-called Planck scale. This set of units of measurement relies solely on physical constants and tends to show where physics as we know it falls apart. Planck temperature is equal 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 hotter than that, but the real limit may be much lower.