Hot or Not?

Three curious stories about temperature

by Polykum Redaktion

Body temperature decrease

What is the normal temperature of the human body? Most people would probably answer with 37 °C. And a hundred years ago, that answer would have been correct. But not today! Average body temperature seems to be declining steadily since the beginning of records by about 0.03 °C per decade. This means that the current average body temperature is more than half a degree cooler compared to the 19th century and lies closer to 36.6 °C than 37 °C.

This phenomenon still puzzles scientists and many hypotheses are currently considered as possible explanations. It is quite clear though, that the effect cannot simply be traced to inaccurate measurements in the 19th or early 20th century. The small, but statistically significant difference in body temperature can still be detected today when measuring body temperatures of old and young people on the same day, with the same thermometer.

What are the possible explanations for this phenomenon and how cold-blooded is humanity going to become? If you’re interested in finding out more about the curious anomaly, check out the research of Julie Parsonnet at Stanford University in California.

Highest possible temperature

We’ve all heard of the lowest possible temperature, the absolute zero. But is there an equivalent at the other end of the scale? How hot is the hottest possible hot?

The standard model of physics offers an answer here: One Planck time, or 10^(-34) seconds after the Big Bang, when all the energy in the universe had condensed at a spot the size of one Planck length, or 10^(-35) metres, the universe reached the hottest theoretical temperature named – you guessed it – the Planck temperature. It equals 1 0^32 degrees Kelvin, or in other words about 100 million million million million million degrees.

Okay, sounds like a lot, but why is that the absolute limit? As objects get warmer, they give off energy in the form of radiation. The warmer an object is, the smaller the wavelength of that radiation – or in other words, the “bluer” the light it emits. Since the standard model posits that the Planck length is the smallest possible length and nothing in the universe can be closer together than one Planck length, this gives us a smallest possible wavelength for this radiation. Once an object is so hot that the radiation it emits has a wavelength of one Planck length, it simply cannot become any hotter – at least not without breaking the standard model of physics.

Negative temperatures

Did I just claim that we all know about absolute zero being the lowest possible temperature? Well, that depends too. There are systems in which we can actually speak of negative temperatures on the Kelvin scale! With the standard view of temperature as a measure of how much particles are moving, this sounds quite insane. If everything comes to a complete standstill at 0 Kelvin, how could it possibly be any colder?

To understand this, we need to understand that temperature is actually defined by how entropy changes in response to changes in energy. In normal, positive-temperature systems, the entropy increases when we add energy to the system. However, in systems where only certain configurations of the particles are allowed, it is possible to achieve a setting where the entropy decreases as the energy increases – leading to states that have a negative temperature on the Kelvin scale.

Is your mind not sufficiently blown yet? Then consider this: When a system with negative temperature comes in contact with a positive-temperature system, heat will flow from the negative to the positive-temperature system – meaning the negative-temperature system is actually hotter than any system with positive temperature!

Anna Weber, 30, sometimes misses the days when she was still studying Physics.

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