Is there such a thing as absolute hot – the hottest you can possibly get? And no, I’m not talking about some it-person de jour being voted ‘hottest’ on the planet by some scatalogically-minded magazine trying to up its sales figures. I’m talking about the laws of physics. Temperature. And temperature beyond… er …. temperature. Let me explain.
First, let’s think of the opposite. Is there absolute cold, from a physics standpoint? You betcha. It’s well known, so much so that the Kelvin temperature scale starts with it – in short, absolute zero is just that in Kelvin. It’s also −273.15° Celsius (Centigrade) and −459.67° on the scale I was brought up on, Fahrenheit. Absolute zero is the coldest it’s possible to get – it’s where all molecular movement stops and is, of course, impossible to reach. Experiments have got close – and have produced what’s known as a Bose-Einstein Concentrate. This is a peculiar form of matter that our friend Dr Albert Einstein predicted had to exist at close on absolute zero, after reading a paper by Satyendra Bose. Einstein was, of course, absolutely right.
It stands to reason that absolute zero must exist, because things move more slowly as they get cold. And yes, if things get cold enough – even particles stop. But is there an upper limit to temperature? And if there is, how can we calculate it?
It turns out that there is indeed such a limit, and it doesn’t have much to do with velocity. You might think that this was a limit – particles move faster as they become more energetic, and obviously a particle can’t exceed light-speed. But it turns out that this doesn’t actually define absolute hot. Instead, the limit is all to do with the way the particle re-radiates energy put into it. It works like this. If you put energy into a particle, you get energy out, typically electromagnetic – heat, for instance. Add more energy, and that turns into heat and light. Light is simply electromagnetic energy with a shorter wavelength than heat – technically, exactly the same thing apart from the wavelength.
This is the issue; the more energy you put into a particle, the shorter the wavelength at which it re-radiates it. If you add yet more energy to the particle after it’s radiating visible light, it’ll radiate in ultraviolet, then in soft X-rays, and so on. Every time, the wavelength of the emitted energy is shorter.
There is, of course, an end point. That happens when the particle reaches a temperature of about 1.416833(85) x 1032 degrees Kelvin – calculations vary a bit, so other figures around 1032 Kelvin are also cited. At that point, the wavelength of the emitted energy hits 1.6 x 10-35 metres. It’s infintesimally small. This is what’s known as the Planck Length, and it’s the doorway to quantum mechanics.
And that’s it. According to this theory (which is one of several competing ones) nothing can get hotter. Well, it can – but at that temperature, the laws of physics as we know it cease to apply. We don’t know what happens.
In case you’re wondering, there is actually a temperature scale known as the Planck scale. It has two numbers in it. The first is 0. This defines absolute zero – and the other is 1. That’s the Planck temperature. Everything between is expressed as a fraction of it.
How cool is that? Er – I did just say ‘cool’ didn’t I…
Copyright © Matthew Wright 2019