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Dear Evil Engineer: Could I build a freeze ray appropriate to my icy new persona?

Image credit: Jon Garrison - Unsplash

This month’s correspondent wants to dispatch his enemies in a suitably chilling manner.

Hail Evil Engineer,

I have recently undergone a chilling experience, which has left me a different man. I hope you may be able to help me adjust to my new life.

Soon after I had broken the ice with the most exceptional woman I had ever met, she fell sick with an incurable condition, and volunteered for cryogenic preservation in the hope that she may be revived when medical science has advanced far enough to give her a second chance at life. An interpersonal dispute at her freezing ceremony led to an accident in which I was soaked with mysterious cooling chemicals, which unexpectedly transformed me into a blue-skinned villain who must wear a sub-zero suit to stay alive.

I was given the cold shoulder by my peers, leaving me to scheme in my frigid lair with snowone else around.

It has taken some time, but I have now embraced my cool new personal brand, such as by choosing to speak almost entirely in ice-related puns. I feel that my choice of weaponry, however, needs updating to reflect the new me; what I really want is a ray gun that can freeze my enemies in an instant. Could you advise me on how to do this?


A frosty villain

Dear villain,

I am impressed by your ability to keep a cool head through such an unusual turn of events. Of course, I would be happy to provide some technical guidance.

Some weapons already exist which can ‘freeze’ people; for instance, a stun gun like a Taser can be fired from a distance and deliver an electric current to temporarily disable the target (‘neuromuscular incapacitation’). However, given that you seem to have immersed yourself in this new persona so thoroughly, I suspect that you are being quite literal when you write of your desire to ‘freeze’ your enemies.

The difficulty in building a freeze ray is that a ray (beam of radiation) almost always adds thermal energy to the target, while you want to draw thermal energy out of the target. A family of techniques called laser cooling – which rely on atoms absorbing and re-emitting light to shed energy – is an exception to the rule. Laser cooling is useful for keeping objects as still as possible, including extremely sensitive measuring equipment and objects which exhibit quantum behaviours at very low temperatures.

This is no longer restricted to the nano- and micro-scale; in 2007, MIT researchers led by Professor Nergis Mavalvala demonstrated laser cooling on the macroscale for the first time when they cooled a coin-sized object to less than a degree above absolute zero. Even if laser cooling could be scaled up to freeze a human body, however, I suspect that it would not be the best route to head down, given that it would still require tightly-controlled conditions – such as the target being isolated in an optical cavity, making it unsuitable as the basis for a ranged weapon.

I suspect that the best approach is to go basic and messy by covering your target in a sub-zero substance, which rapidly lowers their temperature. Let’s say a typical do-gooder weighs 80kg with typical body temperature of 37°C. In order to ‘freeze’ your enemy in the literal sense of the word, you need to cool them to the freezing point of water: 0°C. The average specific heat capacity of a human is 3,500J/(kg.K), so you need to remove approximately 107J of energy from your target.

First let’s consider using liquid nitrogen to freeze your target; as it reaches the target, it rapidly boils off, drawing latent heat from your target and their surroundings. The specific latent heat of evaporation for nitrogen is about 2×105J/kg, so a whole human body could be frozen with a minimum of 1×107/2×105 = 50kg of liquid nitrogen under perfect conditions. However, as the liquid nitrogen will begin to boil off even before it reaches the target, and more will be blown away by gaseous nitrogen boiling off the target, you are likely to be looking at around 5 per cent efficiency at best, requiring a tonne of liquid nitrogen or more. Pumping out a tonne of liquid nitrogen fast enough to incapacitate your enemy before they can run away would require something approximately as powerful as a water cannon of the sort used to fight forest fires.

However, if you are not insistent on freezing your enemies evenly throughout their entire body, a smaller device, which uses rapidly expanding gas to pump liquid nitrogen through an insulated metal barrel, could do the trick. If you can fire the liquid out of the weapon at high enough pressure, this would allow you to partially freeze your target as the liquid boils off them, rather than just giving them a pleasant cryotherapy experience and mild frostbite. Using a liquid nitrogen weapon indoors would also displace oxygen, causing possible inert gas asphyxiation.

It would be easier still to use pellets of dry ice instead of liquid nitrogen; the specific heat of sublimation for dry ice is 5.7×105J/kg, so the mass required to reduce a human body to freezing is considerably less than that for liquid nitrogen. You can acquire a basic dry-ice blaster (typically used for industrial cleaning) from specialist suppliers for around £10,000. With a few small adaptations, a dry-ice blaster – while not technically a ‘freeze ray’ – would make an effective and practical freeze weapon with low overheads.

Good luck!


The Evil Engineer

PS: You mentioned that this all began when you were soaked in mysterious cooling chemicals; perhaps this substance would be worth considering as the basis for your freeze weapon?

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