First, the obligatory Real Genius reference:

Of course, the conceit of that movie was that it could make a human being vaporise from orbit.

Well, this laser ain’t gonna do it.

Nor will it, as this PowerPoint presentation implies, allow one to kill someone with plausible deniability.

First, one needs to understand the language of weaseling in defense procurement, and PowerPoint presentations are the slate upon which weaseling is written.

Then, there is the baisc physics: Human beings are basically water bags, and water is hard to heat, the specific heat of water is: 4.186 joule/gram-°C, which means that the laser, which is supposed to be in the 100KW range, would take about 3.14 seconds to heat up 75 kg (165 pounds) of water from body temp to boiling.

Human reaction time is around 200ms, so in order to burn a hole through someone (100cm deep x 1cm 1cm, 100 g water), you would need about 2.09kw, but once the tissue started to ablate, the resultant steam cloud would start degrading the beam.

Also note that you would carbonize the tissue, and carbon melts at 3500°C, and a person being lasered would be a reducing atmosphere, so it would burn.

Additionally, you would need to hit a vital part. When you hit a liver or a femoral artery with a bullet, death follows in minutes (or less) with a bullet, but a laser wound would be cauterized, so basically, you need a heart or a head hit, and even then the damage is localized along the beam path.

The physical characteristics of a laser wound mean that any post mortem would point to a laser, which in turn, given the cost of lasers, would point back to the US, after all who is going to drop all that money on a few tons of chlorine, hydrogen peroxide, and iodine.

So this won’t be a people killer.

Basically, it’s easier to poke holes in or cut tissue, and more difficult to vaporize tissue relative to metal (Steel’s specific heat is about 0.5 joule/gram-°C, 1/8 that of water)

On the other hand, if you used it on something like a SAM missile motor to trigger an explosion, it might work, or to set off an explosion in an ammo dump, it might very well work.

As the technology gets smaller, and lighter, and cheaper, I could see more applications though.

Posts by David Hambling here and here, and Boeing’s press release here.

I would say, as is de regeur with the USAF, they are overselling the technology.

What Mr. Hambling noted as a scenario, from an orbiting aircraft, with all the shaking involved, along with a claimed 20km of air currents bending the light, is not credible:

According to the developers, the accuracy of this weapon is little short of supernatural. They claim that the pinpoint precision can make it lethal or non-lethal at will. For example, they say it can either destroy a vehicle completely, or just damage the tires to immobilize it. The illustration shows a theoretical 26-second engagement in which the beam deftly destroys “32 tires, 11 Antennae, 3 Missile Launchers, 11 EO devices, 4 Mortars, 5 Machine Guns” — while avoiding harming a truckload of refugees and the soldiers guarding them. It reminds me of how the Lone Ranger could always shoot the gun out an opponent’s hand without injuring them; if that could really be done from an aircraft circling overhead, it would certainly be an impressive feat.

It is potentially useful in limited circumstances, but in most circumstances, something like a missile will do just as well for a lot less money.

When a solid state laser can do something north of 5KW, you will start to see wider application of the technology.