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Lasers and other heat-based weapons in sci-fi are often misrepresented in how they deal damage. A common reaction to heat based weapons like lightsabers, laser rifles and plasmaweapons is to say they cauterize the wound by themselves. However having undergone a medical procedure with a heat-based cuttingtool the tool did not cauterize the wound, it bled immensely until the end of the procedure where they lowered the temperature and used heat over an extended period of time to actually cauterize the wound with the same instrument. Now such a small tool does not mean that a sci-fi heat weapon would not cauterize, but it does bring doubt that just because its heat-based that it will cauterize. And any information I found on the subject was either an echo-chamber or unclear about how it would really do against human flesh. I looked at plasma-cutting wounds for example but they are almost always pictures of secondary burns (in both senses) or people burning things like processed meat.

The weapon one of my characters is about to get hit with will vaporize a sizeable amount of meat somewhere on their torso through one pulsed blast, and I want to see what kind of wound this would realistically leave. My current idea looks like this:

  • the laser vaporizes the skin and some meat below, creating essentially an explosion.
  • the meat that isnt vaporized is burned and charred. This portion will expand due to the heat and tighten the area as it has few places to go. This would cauterize the wound were it not for the water. The water inside is boiled and creates rips and tears in the burned area as it expands and escapes. Further movement will likely break the more brittle charred area and cause more rips and tears.
  • beyond the 3rd degree burn section there is a second and first degree burn as the heat tapers off.
  • the shockwave from the explosion rips and tears the burned section as it is being burned. The shockwave goes deeper and causes bloodvessles to burst while bruising any tissue it comes across. The blood that isnt left to pool internally will find its way through the rips and tears made in the burned sections, causing the wound to bleed.

Now this is my guess and I have nothing really to support it. So my question is: can anyone give a credible account of what would really happen here?

Keep in mind that most heat-based wounds in our modern day do not vaporize parts of the flesh and are from longer exposures.

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  • $\begingroup$ Have you done any research on your own? I am pretty sure there are pages detailing the mechanism of laser induced wounds in fairly deep details. $\endgroup$ – L.Dutch - Reinstate Monica Nov 9 '20 at 13:41
  • $\begingroup$ I thought the "pulse" had more to do with things like the mechanics of creating the laser can't withstand continuous use (they need a cool-down period) or the application needs to be "slowed down" because continuous use would cause too much damage (medical applications). I can't imagine anything but SciFi thinking a pulse laser was better than a continuous later unless it had the same mechanical limitations (the mechanics, like the optics, overheat due to impurities). $\endgroup$ – JBH Nov 9 '20 at 15:31
  • $\begingroup$ @L.Dutch I have looked around occasionally and not found any real explanation whst kind of wounds laser or other heat based weapons would create. At best I have found, including on this site, people restating that it cauterizes wounds but without backing it up. I think there was one youtuber (Kyle Hill or something? have to check) who also said that it wouldnt actually cauterize. But I cant remember if he had any actual proof for it. As mentioned I looked at current day heat-based injuries but found nothing concrete. As aside: who's Monica and what does she need to be reinstated for? $\endgroup$ – Demigan Nov 9 '20 at 15:31
  • $\begingroup$ But, to back up @L.Dutch-ReinstateMonica's comment, consider reading this chapter from Princeton Univ. and this Worldbuilding question. (Who's Monica?... has it been that long? Monica Ciello was one of the most respected people in the SE universe, a moderator on a number of sites and remarkably level-headed. Basically, she asked how to implement SE's new we-tolerate-everyone program [including use of preferred pronouns] and got instantly fired for it. Led to a lawsuit. Bad juju.) $\endgroup$ – JBH Nov 9 '20 at 15:34
  • $\begingroup$ @JBH in Atomic Rockets they mention some of thr characteristics of a working laser weapon, such as using hundreds to thousands of smaller pulses to maximise damage. However I would prefer to keep the weapon closer to sci-fi workings with a single powerful energy pulse dealing the damage rather than a laser pulse that basically digs into your armor and body. That would also make it easier to transfer the knowledge between lasers and plasma weapons for example. $\endgroup$ – Demigan Nov 9 '20 at 15:35
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Laser light has the peculiar feature of being made of photons all having the same frequency and phase. This makes for some pretty intense interaction between radiation and matter.

For a more precise answer one would need to know the substrate being irradiated, the wavelength of the laser, its duty cycle, repetition rate and the power settings. Then for very high pulse energy one has also to take into account non linear effects which start to become significant.

To stay on a generic level, once photons are absorbed by the superficial layer of the target they create a local excess of energy. This local excess can have some interesting consequences, some of which you have already outlined in your question:

  • it can turn the superficial layers to plasma, leading to the so called laser ablation (more likely with UV and above lasers, where each photon can easily strip electrons from the outer shells)
  • it can increase the local temperature (more likely with IR lasers), vaporizing low boiling materials like water. The sudden expansion of water can then create shock-waves into the material, blasting away surface layers and/or "hammering" the deeper ones.

Laser eye surgery uses the above principle: creating shock-waves into the liquid medium of the eye the retina is "hammered" back in place, or shock-waves in the cornea can strip it of some layers to change its profile.

How extended the effects are depends on the combination of the pulse energy and the duration of the exposure. For surgical usage of course the effect is tailored to the desired outcome while minimizing the collateral damage.

For accidental/harmful exposure the situation changes. For example in my university years I have accidentally placed my forearm in the optical path of a Nd-YAG laser, firing nanosecond pulses of some nano-joule each, catching a few seconds worth of pulses: it felt like being stung by a wasp, plus the smell of burnt hair, while the same laser hitting a copper target sounded like someone using a hammer on it. Then I have seen the footage of a chicken breast being exposed to an infrared laser firing few kilowatts of power: a lot of smoke and a passing hole burnt in seconds.

The above is far from being an exhaustive answer, but it should give you a generic idea of what is playing on stage when we talk about laser matter interaction.

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  • $\begingroup$ Would it simplify the answer if I changed the question to a heated plasma? As that is essentially the end result of the laser in use in my story at the impact site. The amount of damage would naturally depend on the energy content, but I would assume that at a certain point the higher energy content would simply mean the same kind of wound but scaled up. Just like a 5.56 rifle bullet would have similar damage profiles to a 7.62 bullet, only scaled up. That is what I aim to achieve: a general idea of what kind of wound a meat flash-vaporization weapon would cause. $\endgroup$ – Demigan Nov 9 '20 at 17:46
  • $\begingroup$ @Demigan, for a plasma to be in contact with an object for a significant time it need to be in a vacuum, which of course makes other types of damages $\endgroup$ – L.Dutch - Reinstate Monica Nov 9 '20 at 17:54
  • $\begingroup$ Hmmm. This makes me want to do my "hotdog vs laser cutter" test that I've been thinking about for a couple years. It would be simple, just throw a thawed hotdog into a laser cutter and run a cut and an etch, but I still haven't taken the time. I know an 80w laser will cleanly cut through 1/8" leather with only minor discoloration, but that's way thicker than human skin and also treated, so I expect a clean cut that isn't cauterized. $\endgroup$ – computercarguy Nov 9 '20 at 23:15

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