Readers reply: is a ‘negative microwave’ – a device that quickly cools food and drink – possible?

·8-min read

Ive been grappling for decades about how you’d get a “negative
microwave” to work, a device that very quickly cools things such as food or drinks without having to pre-fill it with something that’s already cold. I understand many of the reasons why it’s near impossible but is it actually impossible? Maybe quantum physics can mysteriously do it. George Stewart

Send new questions to nq@theguardian.com.

Readers reply

I believe the most promising method to cool food or drink rapidly would be to put the plate/cup/glass in a box from which air could be pumped. The resulting increased evaporation of water would carry away heat. Advantage: No chemical contamination, no large magnetic fields etc, no need for the pressured storage of gas. Drawback: depending on the applied low pressure, evaporation may result in the explosion of parts of the meal one wants to cool, drinks could boil and spill over, which means that for safety reasons, the pressure drop should be limited. This method would be inefficient for dry food.

Alternative method: the plate/cup/glass could be placed in a volume where pressurised air from a storage bottle is allowed to expand. Just like a CO2 fire extinguisher cools, the expanding gas (which should be ambient air for safety reasons) would be very cold. Drawback: cooling would be achieved over a rapid airflow, which may be inconvenient if one wants to keep a meal on a plate or a liquid in a cup. The large thermal gradients involved may be destructive for the meal or the plate (especially if ice formation occurs). A stock of pressurised ambient air would be required.

Maybe the safest method is still simply to wait for the meal to cool down “naturally” while talking with a nice person. Michael Bremer

A blast chiller is the appliance you need. No quantum physics: it is a sort of super refrigerator. Use of blast chillers is prescribed in European restaurants, but it is not uncommon in private households. I am Italian and my husband has recently bought one. When he cooks too much lasagne, he uses the blast chiller to cool it down quickly, stopping bacterial growth. Food will last longer, it will be safer and when you reheat it, it will taste much much better. Valeria Andreoli

Plunging food into liquid helium will do the job for you. It’s what is used to cool the MRI scanners in hospitals. At a guess, I would say each time you used it in a domestic kitchen would cost about £50, so if you think it’s worth it, there’s your answer. Terry Eaton

Microwave ovens heat food by colliding microwaves with water molecules, exciting the electrons and making them warm up food and drink. This energy needs to be removed in some way to de-excite the electrons. In quantum mechanics, this could be achieved by increasing the wavelengths of electron vibration, making them oscillate less vigorously, so theoretically it is possible. But, as we all know, heat naturally goes from warm to cool. Warm things cool to the ambient temperature as governed by second law of thermodynamics. So, I think the question is: what is the point of creating a negative microwave if the arrow of time dictates that things will cool naturally anyway? Elvis

This has been known since the 17th century, thanks to Robert Boyle. Hold the pressure constant and decrease the pressure; temperature drops. Increase the pressure, and the temperature rises. Takes a lot of energy to do it though. Kevin Aston

I’ve been grappling with this idea for many years. Tinkering in the garden shed hasn’t produced any results yet. Once you have solved this, perhaps you can help me with my other project, the dark torch, which projects a field of “no light”. David Sogan

Sure, it’s possible, but not with microwaves. Instead use liquid pressurised gas that is released into a vacuum cooling chamber. This will create a very frigid environment due to the gas increasing its volume. Google “the coldest place in the universe”, and you will find it’s a rapidly expanding gas cloud with lower temperature than the rest of the universe. The effect can among others be felt when you refill a lighter with butane gas. The refiller bottle will get very cold. OJ Nordhagen, Norway

I’m a nuclear scientist with a physics PhD. As you have been grappling with this for a while, I imagine you have done some research yourself so forgive me if I rehash stuff you have learned. Microwaves heat food as the electromagnetic radiation interacts with the water molecules in the food. This wavelength interacts with different vibrational modes in the molecule within the food, the energy from the microwave excites the molecule and causes it to vibrate faster. This heats the food. It especially interacts with hydrogen bonds, such as water, hydrocarbons in oil etc.

To cool, we would have to extract heat from the food. This is a little trickier and produces heat as a byproduct. One method is the laser. Negative temperatures have been produced in the lab, but don’t result in a cold object as you would want. A more feasible method would involve using cold liquids and pumping heat away, just like a powerful fridge. Perhaps a quick blast of nitrogen gas at extreme cold temperatures would do it. At my work we use cryogenic temperatures, if it was possible to cool an object indirectly using a negative microwave device we would want to know! Rose Brown

A number of methods could be used to ensure that the surface of the food/object radiates heat and receives minimal incoming heat (eg, cool the walls of a metal container it’s in to near absolute zero.) These will rapidly cool the object’s surface (“flash freezing”). However, there is not – and cannot be – any method to cool the interior of the object other than by conduction of its heat to the outside. A microwave sends electromagnetic energy to the interior, but there is no such thing as negative energy (except in weird contexts). MartinMellish

No, and the simple reason is because of the second law of thermodynamics. It’s the only physical process that isn’t fundamentally symmetrical (in this universe, anyway) and therefore is often regarded as defining the arrow of time. It basically states that entropy (disorder) will always increase in an isolated system. Heating increases entropy, you are working with the natural tendency of the universe, so it’s easy – throw any energy (eg, microwaves) at anything and it just becomes more entropic without any further work. Cooling, on the other hand, means putting more order (decreasing entropy) into a system, working in the opposite direction the universe wants to go, as defined by thermodynamics, so that’s a lot of additional work (and therefore energy) needed to reduce entropy (ie cool it) as increase it (heat it) by the same degree. HaveYouFedTheFish

If you put energy into something, no matter how you do it, it’s not going to become colder. As far as I understand, a fridge works by having energy in the air of the box working on some material in the fridge, thereby cooling the air inside. Overall the surrounding air becomes warmer than if you hadn’t turned on the fridge; if you left the door open (a bad idea) it wouldn’t cool the room. So even in that case, although you put energy into the device and it is cooling something, overall a fridge is heating things up.

If you could reverse time, however, you could just use a regular microwave device. You put your hot food into the microwave, wait till the microwaves have left the food to be absorbed by the device surrounding it and take out your cold food. somehowrational

I’m sure that about 30 years ago they had a contraption in Thresher’s off-licence that you could ask them to put your bottle inside to cool it rapidly. If I remember correctly, it rattled a lot in the process. I have sometimes wondered what happened to those machines and why there is no modern equivalent. HotBurrito

The idea of using fluorescence to cool a material under optical excitation was initially proposed in the 1920s, then theoretically validated by Landau in 1946. As George Stewart expected, it relies on quantum mechanical principles: if a laser is used to drive electrons into excited states with non-equilibrium occupancies, then the electrons can absorb phonons (wave-like structural vibrations that carry heat in materials) before relaxing to a low energy state by emitting a higher-energy photon (fluorescing). This results in a net heat loss from the material and is known as “anti-Stokes fluorescence”: for a better description (and an example of cooling a macroscopic object from room temperature to -182C), see here.

However, if this cooling process is to outweigh the usual heating effects caused by electromagnetic wave absorption, then the material that is being irradiated must possess an extremely high quantum efficiency. This means that the electrons must follow precisely the correct sequence of excitation, absorption and emission, rather than absorbing the energy from the laser by non-radiative dissipation. To date, I believe this has only been conclusively demonstrated using ultra-high purity glasses featuring rare earth ions. (In the past decade, scientists have claimed to achieve similar laser cooling in certain semiconductors and perovskites, but the experiments have not been reproduced and remain controversial.) So I am afraid that anyone attempting to chill their gazpacho with a laser will probably be disappointed. furry_marmot

The answer here is, I think, counterintuitive. Rather than recruiting the very finest and most dedicated brains in academia and industry to research a solution to this puzzle, you need to assemble a bunch of feckless former science students who have all been thrown out of university. That way, the team is sure to achieve zero degrees and will therefore be an unqualified success. ThereisnoOwl

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