So the science regions of the blogosphere recently exploded with a new paper claiming to have made a material with a negative absolute temperature. Those last three words together may sound weird to the uninitiated (and residents of the Plains and Northeast may wonder why physicists need all that newfangled equipment to go below zero degrees), but they need to be combined in that phrase in physics. If you’ve ever been prone to intense existential thought over your weather forecast, you might realize the way we think of 0 degrees in everyday life seems a bit arbitrary compared to what 0 means for other units. Most of the time when we say something is 0, we mean there’s none of that quality. A length or mass of 0 means there is nothing there. A speed of 0 means something isn’t moving. Zero current means there’s no electricity. A bank account with $0 means you have no money. And in units that allow negative amounts, these counteract positive values. A negative velocity means you’re going in the direction opposite your reference. A negative financial statement means you’ve lost money or owe money (and so take away from positive values of money you had).
So what do these mean for temperature? First, it’s important that while we deal with temperature everyday, it’s actually kind of an ephemeral unit. It’s not the same thing as energy, but it is related to that. And in scientific contexts, temperature is not the same as heat. Heat is defined as the transfer of energy between bodies by some thermal process, like radiation (basically how old light bulbs work), conduction (touching), or convection (heat transfer by a fluid moving, like the way you might see soup churn on a stove). So as a kind of approximate definition, we can think of temperature as a measure of how much energy something could give away as heat* (Edit: A friend of mine has come up with a good analogy: Temperature is like pressure, but for heat. Mass moves from high to low pressure areas, and heat flows from high to low temperature objects). And so now, we realize there’s nothing particularly special about 0 degrees in our everyday scales, in both Fahrenheit and Celsius. I mean, sure, there is a marker that these definitions came from. Fahrenheit’s 0 point is the temperature which a mixture of water ice and ammonium chloride (which Scandinavians might recognize as the flavoring of salty licorice) always stabilizes to. And as nearly all middle schoolers know, Celsius is 0 at the temperature at which water freezes.
While these are cold temperatures to our human flesh, you know things at 0 degrees in either scale can still manage to give up heat (just not to you and your warm 98.6 degree F body). This is where absolute temperature comes into play. For the field of thermodynamics, where temperatures and heat transfer are important in studying energy, scientists quickly realized that their old zero points were meaningless when trying to talk intelligently about energy. So they made new scales with “absolute zero”, a point where there is no more energy (at least from the motion of atoms). This point is about -273 °C and -460 °F. This is also partially why anyone attempting to cook a recipe twice as fast by “doubling” the temperature on their oven/stove/whatever is doomed to failure, because going from say 250 °F to 500 °F on your oven hasn’t doubled the temperature. Instead, you would need to go from 710 absolute degrees Rankine (the absolute temperature scale based on the Fahrenheit intervals; ask a chemical engineering friend if they’ve worked in Rankine) to 1420 °R, which would be like 960 °F and would lead to a really amusing story with your oven manufacturer and/or the fire department.
You’ll notice a first on the blog here, and that’s the presence of a “part 1”. I haven’t actually gotten to the new science of the article yet, but I wanted to set up the background here. Scientific units actually have fascinating histories and I got incredibly caught up on researching temperature for this part, so I ended up with way more information than I originally planned on.
*This is a really loose definition of temperature. Don’t ever use this on homework, ever. Consider it more of a guideline.