How Low Can Temperature Go? Lord Kelvin and the Science of Absolute Zero

Temperature might be the second most measured bodily amount in our trendy world — after time. 

Once I get up within the morning, the very first thing I often examine is the time (to see if I ought to return to sleep), however the second factor I examine is the temperature exterior (in order that I understand how to decorate). 

Temperature is such a typical measurement that we typically overlook how vital it’s. From dairy farming to rocketry, from local weather science to climate prediction, so many issues require an correct data of temperature.

The metric (SI) unit for temperature is named the kelvin, after Lord Kelvin, whose two hundredth birthday we have a good time at present. 

Lord Kelvin and the Early Science of Temperature 

Lord Kelvin, or William Thomson, labored in what was then the rising subject of thermodynamics — remodeling warmth into dynamical movement. He did this each as a scholar on the College of Cambridge and as a younger professor on the College of Glasgow. Collectively along with his shut collaborator, James Joule, he researched all types of issues in thermodynamics, together with temperature scales.

On the time, the scientifically accepted scale for temperature was the Celsius scale, with zero temperature being the freezing level of water and 100 levels being the boiling level of water. However after finding out how gases modified quantity and stress in response to altering temperature, Thomson, Joule and different scientists realized that there was an absolute coldest temperature that could possibly be reached.

To know how they reached this conclusion, take into account a gasoline in a balloon. If you happen to cooled the balloon, the gasoline inside would exert much less stress in opposition to the balloon itself and in opposition to the ambiance exterior it, inflicting the balloon’s quantity to shrink. 

Don’t consider me? Inflate a balloon and stick it in your freezer. If you pull it out, you’ll be able to really feel the balloon broaden. Now extrapolate: How chilly would it’s important to make the balloon to make its quantity go to zero (ignoring the truth that the gasoline inside will finally condense right into a liquid)? That should be the coldest potential temperature as a result of the balloon can not have a unfavorable quantity.

In 1848, Lord Kelvin used comparable reasoning to precisely calculate absolutely the coldest temperature as unfavorable 273.15 Celsius (or unfavorable 459.67 levels Fahrenheit). It could be roughly one other decade earlier than scientists like Lord Kelvin and Ludwig Boltzmann understood that at absolute zero, the molecules in the gas stop moving. 

Since 2019, all three of those scientists have been immortalized within the SI. The kelvin is our SI unit of temperature, outlined via the Boltzmann constant, which relates temperature to power, the SI unit of which is the joule.

In the present day, atomic physicists like myself use a method partly pioneered at NIST referred to as laser cooling, which makes use of lasers to chill clouds of between 100,000 and 1 billion atoms to temperatures of about 100 microkelvin. This temperature is 1/10,000th of a level Celsius above absolute zero. 

And we measure these ultracold temperatures in a means that may not be stunning to Lord Kelvin (though making such chilly gases may be!). 

We measure the common pace of the atoms within the gasoline. Researchers at NIST use such laser-cooled atoms for all types of functions, from atomic clocks to hoover requirements.

Vacuum Customary

Laser cooling atoms to close absolute zero solely works inside a chamber the place nearly all of the air has been eliminated by a pump to isolate the atoms from the encompassing surroundings. Such vacuum chambers are frequent and are utilized in industries resembling semiconductor manufacturing. 

Many of the parts in your cellphone have been out and in of no less than one vacuum chamber. The core parts, just like the central processing unit, have in all probability been via a chamber that has produced among the finest vacuums on Earth. For each trillion gasoline molecules that began within the chamber, all have been eliminated however one. Such beautiful vacuums are required as a result of leftover gasoline molecules can each contaminate the chip and scatter the ultraviolet gentle that’s used to imprint the designed circuit. This could trigger the chip to be ruined.

Amazingly, the present finest strategy to measure such pure vacuums is through the use of what’s successfully a vacuum tube. However now, the laser-cooled atoms in my lab could also be the most effective sensor of ultralow vacuum pressures on Earth. 

After the sensor atoms are cooled to close absolute zero, we maintain the sensor atoms in a “lure” that’s made totally of magnetic fields. This lure may be very weak, solely capable of maintain onto the ultracold sensor atoms. The vacuum sensor works as a result of if a chilly sensor atom is struck by a leftover gasoline molecule, it’ll nearly at all times be ejected from the weak lure. The speed at which this course of happens is dependent upon the variety of gasoline molecules the pump has left behind. Thus, figuring out the variety of leftover gasoline molecules simply entails counting the variety of sensor atoms that stay after a while. 

This “cold-atom vacuum standard (CAVS)” is a brand new means of measuring vacuum stress, which NIST has performed an important position in creating. We anticipate it getting used to measure ultrapure vacuums in semiconductor manufacturing, quantum computer systems and different huge science experiments, resembling an experiment detecting collisions of extraordinarily distant black holes, generally known as the Laser Interferometer Gravitational Wave Observatory (LIGO).

Having an ordinary just like the CAVS that at all times provides the right vacuum stress studying will assist these functions construct higher vacuum chambers, diagnose issues and enhance each reliability and productiveness.

The CAVS is the one experiment that I’m conscious of that should measure two very totally different temperatures on the similar time: the sensor atom temperature of round 100 microkelvin (very chilly!) and the temperature of the leftover gasoline within the vacuum chamber, close to room temperature at 300 kelvin. 

I believe Lord Kelvin could be amazed to be taught that two very totally different temperatures might exist on the similar time, and each have to be measured for a single experiment to work.

Thermometers

One other attention-grabbing analysis pursuit right here at NIST is making an attempt to make use of atoms or molecules to construct a thermometer that truly measures temperature. 

You might be questioning what I imply. 

In any case, you in all probability have a number of thermometers in and round your property, they usually all provide you with some quantity in both Fahrenheit or Celsius. However the reality is all of them measure some other physical quantity — just like the resistance of a platinum wire or the voltage generated between two dissimilar metals — that is dependent upon temperature. 

For these gadgets to learn out a temperature in Fahrenheit or Celsius, they should be calibrated. NIST does such calibrations, and it’s extra doubtless than not that the calibration for the thermometer in your house’s thermostat may be traced via a sophisticated set of steps all the best way again to NIST.

However we could possibly make this entire calibration course of less complicated by making thermometers that straight measure temperature, utilizing strategies that Lord Kelvin would admire. 

For instance, my colleague Daniel Barker and I are engaged on utilizing lasers to measure the distribution of velocities of a gasoline of rubidium atoms at room temperature and above. This method, referred to as Doppler thermometry, will get on the very coronary heart of how Lord Kelvin understood temperature. 

Along with my colleague Eric Norrgard, I’m additionally engaged on two tasks making an attempt to create a new type of infrared thermometer utilizing atoms and molecules. If these efforts are profitable, calibrating our thermometers might get a lot simpler, and it might additional different scientific developments as effectively. 

Conserving It (Very) Cool within the Lab 

I got here to NIST as a postdoctoral researcher in 2012 after ending my graduate work at Yale College. 

As a postdoc, I labored with among the coldest stuff within the universe: Bose-Einstein condensates (BECs). Just like the CAVS, BECs are additionally manufactured from laser-cooled atoms, however they’ve been cooled even additional to lower than 100 billionths (!) of a level above absolute zero. 

After my postdoc, I made a decision to remain at NIST and attempt to use my expertise with ultracold atoms and lasers to understand sensible and helpful requirements, just like the CAVS. 

I achieve an amazing sense of pleasure after I see what look like glowing balls of ultracold atoms — that are actually enjoyable to play with — used to unravel real-world measurement issues. I think that Lord Kelvin could have felt the identical sense of pleasure to see his measurements and theories concerning thermodynamics (which have been in all probability additionally enjoyable to work on) be utilized to make extra environment friendly steam engines.

Joyful Birthday, Lord Kelvin 

Lord Kelvin didn’t simply calculate absolute zero. After his early work in establishing absolute temperature scales, he was instrumental in laying the first telegraph cables throughout the Atlantic Ocean. Lord Kelvin additionally invented a machine that predicted tides and a compass that helped the Royal Navy navigate the seas. Whereas my analysis just isn’t fairly that diversified, one of many methods I combine up my work is by working at each room temperature and temperatures close to absolute zero.

One of many key issues I’ve realized is that measuring temperature, as Lord Kelvin understood it, is sort of at all times more durable than you may assume. Whereas the concepts are easy, making them work in observe is the actual problem. 

And this truth makes it much more spectacular that Lord Kelvin precisely predicted the temperature of absolute zero … in 1848. 

On his two hundredth birthday, I’ll take a second to understand that.