eli5: Why is it temperatures can go up to the trillions but only down to -273?
In: Physics
I think the best ELI5 level reason comes from thinking about gases. Briefly, what we call absolute zero is the temperature where a perfect gas (an imagined ideal situation where the gas will never become a liquid,and the atoms/molecules take up zero volume on their own) occupies zero volume and exerts zero pressure.
Suppose you have some helium gas in a sealed container. You’ll make sure gas is held at constant pressure and let the volume change as you change the temperature. When you decrease the temperature you’ll see the volume gets smaller. If you decrease the temperature by 10 degrees, the volume will decrease by some amount, say 10 mL. If you decrease it by 20 degrees, the volume decreases by twice that amount, so 20 mL smaller. 30 degrees, three times the decrease. You can do this over a very large range of temperatures. Suppose we plot volume on a vertical axis and temperature on a horizontal one. We’ll see a straight line forms – increasing temperature leads to increasing volume, decreasing temperature leads to decreasing volume.
Your volume can get as large as it wants. We don’t know if there’s a limit to how big space is, so we can imagine taking the temperature and volume as high as we want. But the smallest volume the gas can occupy is zero. Negative volumes have no meaning. This means if we draw our line back to where it hits zero volume, we’ll have the lowest temperature that makes any sense at all.
This would be interesting, but what makes it remarkable is if we switch helium for argon, or neon, or hydrogen, or any gas (as long as we stay far away from where it turns into a liquid) we will find it’s the same lowest temperature for every one. If we change our experiment and hold volume constant while measuring pressure we get a similar argument and we find the same lowest temperature. This temperature seems to be a floor for gases. If we use gases as our thermometers, we’re going to see a lower bound. There are more involved arguments regarding statistical mechanics that show a lower bound on temperature as well, but they are beyond the scope of an ELI5.
I want to address one other thing. Lots of top level comments are talking about temperature as a measure of the motion or vibration of the molecules. This is wrong.
To illustrate my point, consider a pot of water. You heat it up to boiling. Well below the boiling point you see lots of tiny bubbles in the bottom. This is dissolved gas coming out of solution. After a time, the water starts to bubble again and you get into normal boiling. It doesn’t have to be a rolling boil. Just a nice, steady change from liquid to gas with bubbles forming at the bottom of the pot and rising to the top and the temperature holding steady. The gas in these bubbles is (very nearly) pure water vapor. The temperature of the liquid is quite close to 100 C, depending on the air pressure, but so is the temperature of the gas in a bubble. The molecules in both situations are water molecules, but those in the gas phase are moving a heck of a lot faster than those in the liquid phase. If they weren’t, they couldn’t have separated from one another enough to form the bubble. We have a situation where the gas is at 100 C, the liquid is at 100 C, but the kinetic energy of the molecules in the gas phase is tremendously larger than the kinetic energy of the molecules in the liquid phase. So, temperature cannot simply measure how much the molecules are moving. It is far more subtle than the textbook discussions.
The idea that it measures “average kinetic energy” comes from other details about the ideal gas model I used to lay out absolute zero. In an ideal gas the only place you can put energy is in the kinetic energy of the molecules. If we have other buckets to dump the energy into, then the ideal that temperature measures average kinetic energy fails.
Source – I’m a physicist.
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