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.
- Myth # 1 : Slow Laser Beams
- Myth # 2 : Visible Laser Beam
- Myth # 3 : Recoil
- Myth # 4 : Perfect Absorbers
- Blooper #1 : Laser Beam Alignment Errors
- Reality Check: Science versus Science Fiction
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'.
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.
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.
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
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.
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:
- Science Fiction:
You fire a laser weapon at the enemy and you 'see' and
'hear' the beam emerging from your laser gunports (which violently recoil)
and watch the 'beam' travel very fast towards the enemy ship which promptly bursts into
flames and explodes very loudly (a sound that you hear instantaneously).
- Science Fact:
You quietly fire an invisible beam from laser weapon without recoil and
instantaneously your wingman's ships is split in half !
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
- (a) Your laser beam travels at the speed of light.
- (b) The beam would be invisible until it struck its target.
- (c) Highly reflective surfaces efficiently reflect laser light.
- (d) Sound doesn't travel trough a vacuum (even in an atmosphere
the sound would take a certain amount of time to reach you,
therefore you would see the damage before you could hear it).
- (e) Most space ships don't 'burst' into flames since there's no
air in space to sustain such explosive combustion and the
amount of air already in the ship quickly disperses into
space by explosive decompression.
Most futuristic space ships don't carry flammable fuels like
present day rocket technology. If the antimatter fuel were
accidentally released it would appear more like a blinding
flash.
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
- Science Fiction: Constructing Artificial Laser Stars
-
Krauss, Lawrence M.: 1995, The Physics of Star Trek, BasicBooks.
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