Ok let's pretend that you can get over the opening sequence wherein Delta Force buddy's HUD sees a shadow and has too much RAM tied up in displaying the audio waveform at the bottom of the display to run the facial-recognition sequence on the ghost that kills him, especially because later the damn thing can pick up motion signatures. Also pretend that facial recog also isn't the first thing the military does to confirm the existence of ghosts. BUT that's ten minutes in. Let's go back to what our hero Homenugget does in his introductory shot.
Homenugget handbombs a cubic foot of ice onto an aluminum shelf.
Homenugget uses a standard vernier dial to line up his lenses AND THEN DOESN'T RUN A TEST BEAM THROUGH IT TO CHECK THE CALIBRATION BUT WHATEVER.
Homenugget fires up what we can only assume is a finite energy source cuz IT'S PLUGGED INTO THE FECKIN WALL.
... and the chunk of ice is INSTANTLY LIQUIFIED.
I nearly shat my pants. Like, I actually had to stand up and check. Almost as badly as what the fictional ERB for that lab would've if some suit had been like "well test it on animals" and Homenugget's boss agreed without a second thought. Maybe DARPA doesn't have one. Fuck it, I don't know your rules. Anyways, not the point.
The thing is that the temperature field in any material can be expressed as a solution (or approximate solution) to a set of governing partial differential equations that depend on boundary conditions (temperatures and heat losses at all points along the surface of the ice), the density of the material (ice), and a property called thermal conductivity which dictates the rate at which heat can actually pass through a material. How this temperature field changes when exposed to a heat source is highly dependent on time as well unless you're operating at what we call "steady state" or balance between the heat input to the system and the system's heat losses. Generally we think about steady-state as static and unchanging. Given the sudden, dynamic, and oh-so-very dramatic way that the ice turns into water we can safely say that this is a highly temporal problem.
Except... not the way it's shown in the movie. You see, the melting of ice involves a very special solution to the above-mentioned PDEs called the Free Boundary Problem because melting involves materials in two phases (solid and liquid), and requires some of the energy in that system to go into latent heat to actually cause the phase change. Because thermal conductivity is a thing, you can't actually liquify something all at once with a finite amount of heat - you would have to conduct sufficient heat to overcome latent heat while simultaneously providing the same amount of heat to the next little bit of material, and so on and so on throughout the body all at the same time.
The "sufficient heat" here is literally infinity. Given how we already said Homenugget plugged this thing into the wall, the guys running the local nuclear power plant might have a thing or two to say about that.
There is always the option that Homenugget's government-funded weapon of mass destruction creates such a great heat source that the time it takes for latent heat absorption and diffusion through the material is so little that we couldn't perceive it. This is why the aluminum frame is suddenly of interest - if you're suddenly and violently creating a point source that is basically a tiny sun in your lab, you're probably not gonna do it on a brushed metal bookshelf you bought at the local Ikea. You sure as fuck aren't gonna do it while you flounce around in a v-neck T-shirt behind exactly zero permanent heat shield. Having a tan is great. Instantaneous destruction of your skin and the underlying tissue by means of a semi-infinite amount of radiant heat is not. That's to say nothing about the concussive shock likely to occur from the rapid expansion of the air around the ice due to the presence of this same heat source.
Long story short, Homenugget's plasma gun is bullshit. I love a good action flick as much as most (Christopher Nolan basically inspired my thesis project) and I thoroughly enjoyed the rest of the movie, but this one really ground my gears. Don't blame me, blame the laws of thermodynamics.