Lasers in the Movies

The physics of lasers is often incorrectly portrayed, here are a few myths perpetuated by movie special effects, then some common bloopers and a sobering example of science versus science fiction.

  1. Myth # 1 : Slow Laser Beams
  2. Myth # 2 : Visible Laser Beam
  3. Myth # 3 : Recoil
  4. Myth # 4 : Perfect Absorbers
  5. Blooper #1 : Laser Beam Alignment Errors
  6. Reality Check: Science versus Science Fiction

Myth #1 : Slow Laser Beams

In many scenes laser beams emerge from the tip of the laser with velocities less than the speed of light ! This can be confirmed by using a VCR 'freeze frame', the beam seems to emerge as if it were some kind of projectile and only travels a few meters between frames or roughly 10 meters per thirtieth of a second (the interval between NTSC frames). This translates to 300 meters per second of the order of a million times less that the speed of light which is 299,792,458 meters per second.

(This is also slower than a speeding bullet !). The reasons for this distortion of the laws of physics is to give the impression that the 'light' pulses from the laser are some kind of photon 'bullets'.

Myth #2 : Visible Laser Beam

Another myth perpetuated by the silver screen is the typical appearance of a laser beam as a streak of light, especially in space where the density of particles is so low that the beam should be invisible until it hits its target. When a laser beam becomes visible it is scattering off minute particles dispersed along its path. The very fact that we can see the beam indicates that it is loosing energy in directions not aligned with the main beam. This energy loss could severely deplete the energy content of the beam by the time it reached its target. The full power of the laser could not be attained trough the typically very long path length towards the target.

Myth #3 : Recoil

How often do you see victims flung several meters by a laser blast. The total photon momentum from even the highest powered lasers is negligible compared to their energy content. In reality, the target would be almost instantaneously perforated with very little direct impetus from the photons. If the momentum were from the photons, Newton's laws of motion would cause an equal and opposite reaction on the laser rifle which should in theory recoil with the force of a cannon. The only possible momentum transfer is from the plasma plume emerging from the target. However since the target would be perforated these superhot jets of high velocity gas would emerge both from the front and rear of the hole cancelling out the forces. Only in the very initial stages of the laser perforation would there be an asymmetric jet emerging from the front of the target, leading to a net force and possible recoil. However most movie special effects wizards underestimate the size of this plume, which should appear like flames from a rocket nozzle.

Myth #4 : Perfect Absorbers

Another flaw in the portrayal of laser beams is that no substance is a perfect absorber. For example in science fiction movies, the very fact that we see the space ship indicates that it doesn't absorb all the radiation falling on it. For the ship to be visible, a certain fraction of the light from a nearby star, planet or moon must reflect off the ship and make its way to our eyes (or to the virtual 'cameras' eyes). Most space ships are made of metals which typically reflect optical radiation with very high efficiency, especially when they aren't tarnished by oxidation. In the near perfect vacuum of space, oxidation isn't very likely. Depending on processing metals can be made to reflect with high specularity, which means that most of the energy from a laser weapon would simply bounce off with an angle of incidence equal to the angle of reflection. This simple and inexpensive countermeasure has been a strong argument against the use of space based laser weapons in the strategic defense initiative against intercontinental ballistic missiles.

Only a tiny fraction of total laser beam power gets absorbed by a very good reflector. Therefore this small portion of the energy must be deposited rapidly enough so that an extremely thin layer near surface reaches the melting/vaporization temperature before the heat gets conducted towards the interior. Good thermal conductors make this task even more difficult. Diamond has one of the highest thermal conductivities, and with its high index of refraction it could easily be multicoated to increase is reflectivity to nearly 100 percent. Diamond is also is one of the toughest substances around, a laser beam would have to be in 'contact' with the same area much longer or have much higher power to cause significant damage. At very high powers the reflectivity decreases due to non-linear optical effects which begin to dominate such as multi-photon absorption etc...


Bloopers

Laser Beam Alignment Error

Nitpickers have noticed how the beams of hand held laser weapons often aren't aligned with the barrel of the gun.

If you freeze frame some of the famous Star Wars laser battles you can see how special effects wizards render laser beams on film. There are straight beams which are parallel and diverging beams that spread out by the laws of perspective depending on the viewing angle of the camera relative to the beam. However careful examination and visual inspection or ray-tracing reveals that some of the beams aren't aligned with the barrel of the gun but seem to go off at some other angle. If the alignment was off by some fixed amount due to shoddy workmanship, at least the gunner could take the error into account and compensate in his next shot; unfortunately the angular error is random and changes after every shot !

A resonable explanation for this blooper lies with the method used to create laser beams. If a low power visible light laser is attached to a laser rifle the beam is invisible unless the room is very smoky or cloudy. Therefore laser beams are 'simulated' in the post production phase. The actors and stunt people aim their laser rifles and pretend to fire them, the beams are later overlaid by dark room tricks or computer processing.

The principal problem lies with the excellent aim of the laser battle participants. It turns out that unfortunately in many of the shots the stunt people playing the storm troopers were excellent shots and accurately aimed their 'fake' laser weapons onto their 'theoretical' targets (especially at close quarters).

This poses an embarrassing problem : The villains naturally outnumber the heroes in any good 'western' style sci-fi shoot out. Therefore if the simulated laser beams are reconstructed 'too' accurately, most of the heroes would be killed in the first few salvos.

The solution is simply to 'fudge' the beam to avoid 'sensitive' targets. Not too difficult considering the problem of re-creating a 3D beam on a 2D medium without the aid of a computer or accurate landmarks. The beam errors are noticeable when the opponents are very close or there is too much perspective in the beam.

A simpler solution to this problem is to do away with conspicuous laser guns barrels, as in Star Trek the Next Generation, the hand held phaser barrels are much shorter hence less problems with discernible beam alignment. The original Star Trek series used phasers that resembled more the original outline of a handgun, and consequently the angle error caused more headaches.


Reality Check : Science versus Science Fiction

As an amusing exercise let's speculate on what would actually happen if you were a member of a large fleet of friendly ships engaging enemy ships in a laser beam battle in space. The following two versions of this story underscore the differences between science fiction and science fact: A single stray reflection from your laser has bounced off one of those highly polished enemy ships and neatly slices through your partners ship. You wouldn't even know what hit them because This situation would be quite embarrassing for sci-fi authors and directors who would rather 'bend' the laws of physics rather than explain why the space fleet appears to have been neatly dissected by a giant food processor.


REFERENCES

  1. Science Fiction: Constructing Artificial Laser Stars
  2. Krauss, Lawrence M.: 1995, The Physics of Star Trek, BasicBooks.
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