15 December 2017
Star Trek has been using phasers to delete objects for over fifty years. Here's an early example.
So what happened to this homeless person who accidentally annihilates himself after stealing McCoy's phaser? One explanation is that he was converted into energy. Unfortunately E=mc2 disagrees. Assuming the person weighs 60kg, then his relativistic mass-energy equivalent is 5x1018 Joules, which is 1,289 megatons of TNT -- just shy of the combined output of all the world's 15,000 nuclear warheads. Clearly that's not happening, or else the collateral damage would be somewhat greater than shown on screen.
A more plausible explanation is that these people are being vaporized. Vaporized means (obviously) being turned into vapor. Humans are "ugly bags of mostly water" (according to the aliens in TNG's Home Soil), so let's just look at what it takes to vaporize water. 60kg of water takes 1.53x108 Joules to boil. That's much more reasonable, it's the energy output of 4.5 liters of gasoline.
One problem with this is that the matter still has to go somewhere. The mass of air in an average room is around 100kg. Suddenly adding another 60kg of vapor to the room will increase the pressure by 60 kPa, which will cause eardrum rupture.
The appearance of all this steam will also heat up the room's air, resulting in a temperature increase to 52°C (125°F) ((23°C * 100kg + 100°C * 60kg) / 160kg). That's assuming that the phaser is 100% efficient in vaporizing the individual in question. The room would turn into an instant sauna. Hope you enjoy breathing Klingon.
Writers for Star Trek (and science fiction in general) really need to understand that objects can't just vanish without consequences. At the very least, have a big pile of ash left behind after the disintegration. And leave some charred boots behind too. That would go a long way towards making this trope more believable.