THE NOBLE CAUSE

Intuitor Strikes a Blow for Decency in Movie Physics!

Technonerds go to movies strictly for entertainment, and of course, the most entertaining part comes after the movie when they can dissect, criticize, and argue the merits of every detail. However, when supposedly serious scenes totally disregard the laws of physics in blatantly obvious ways it's enough to make us retch. The motion picture industry has failed to police itself against the evils of bad physics. This page and its companion book. is provided as a public service in hopes of improving this deplorable matter. The minds of our children and their ability to master vectors are (shudder) at stake.

While many movies do fall short there are example of good one. See Intuitor's Recommendations for Movies With Good Movie Physics

PERSPECTIVE

The book is also intended to give aspiring filmmakers, science fiction writers, and just plain moviegoers a helpful perspective. We continue to think that art, physics, and mathematics are all part of the same creative human experience. What's more, we continue to think that all can be entertaining and enlightening.

Read past perspectives in the Perspective Archives

GENERIC BAD MOVIE PHYSICS

Flashing Bullets

The terrorist unleashes a lengthy burst of submachine gunfire as the hero runs along a gangway in an industrial plant. Bullets bounce everywhere. This would be a dramatic event for almost anyone, yet moviemakers feel it must be enhanced. The special effects representing impacting bullets give off bright flashes of light. Normal bullets, especially handgun bullets, do not.

Typical handgun bullets are made of copper-clad lead or lead alloys. They simply don't create bright flashes of light when they strike objects, even if the objects are made of steel. In the chemical industry it's commonplace to limit maintenance workers to copper-alloy or lead hammers when they are working in areas where flammable fumes may be present. Hammers made of these materials do not produce sparks when they strike objects, while steel hammers can. If you've never noticed this phenomenon with steel hammers, don't be surprised, the sparks generally are barely visible even under ideal lighting conditions.

The non-sparking tendencies of copper versus steel can be seen if the metals are ground. Grinding a piece of steel will produce a copious quantity of sparks even in bright lighting. Small hot particles of the steel actually burn. If copper tubing is ground it may produce an occasional spark due to a contaminate on the grinding wheel or copper, but will be virtually spark free.

(Note: grinding is potentially hazardous. It generates hot, high velocity metal particles. The grinding wheel can also fly apart at high velocity. Soft metals can clog a grinding wheel. It should only be done with proper safety equipment and precautions.)

We definitely don't recommend grinding lead because it produces toxic particles in addition to the other hazards of grinding. However, when ground under controlled conditions to prevent lead contamination, lead performs similarly to copper.

Bullets do get hot when they strike solid objects. The worst case would be if all of a bullet's kinetic energy were instantly converted to thermal energy when a bullet struck its target and all the thermal energy remained inside the bullet. This is highly unlikely but easy to calculate.

A .45 cal handgun bullet, for instance, has a mass of 0.015 kg and a muzzle velocity of around 288 m/s (at the upper end of velocity for commercially available ammunition). Kinetic energy is calculated from the mass and the magnitude of the velocity of an object using the following equation:

We calculate that such a bullet has a kinetic energy of 619 J. If this kinetic energy is all converted to thermal energy, the temperature rise can be calculated as follows:

Using a specific heat of lead equal to 0.128 J/g/K we get a temperature increase of 324° Celsius. If the bullet starts at room temperature (24° C) it will end up at 348° C (659° F). The melting point of lead is 328° C.

For the moment let's not worry about whether the bullet melts but what it might look like if it did. Molten lead looks like newly polished silver and is sometimes used in movies to represent molten silver, for example, when someone casts a silver bullet to kill a werewolf. Molten silver, on the other hand, glows red (melting temperature = 962° C). At its melting temperature,  lead does not glow with visible light.

The analysis would be similar for most common handgun or sub-machine gun bullets. (Note, submachine guns use handgun bullets.) On the other hand , High powered rifles contain much more kinetic energy and have the potential to get a lot hotter.

If we make similar calculations for a high-powered rifle bullet such as the .223 Remington ₁ (similar to the 5.56 NATO round used in M-16 rifles) but also account for lead's heat of fusion, we find that at point blank range there is enough energy in the bullet to easily melt the bullet's lead core and most likely also its copper jacket. The temperature of the combined metals in this case would exceed 1000° C and glow a bright orange. However, while a blob of molten lead could produce some nasty surface injuries, it would have reduced  penetration power.

Do bullets really get as hot as our calculations suggest? No. Most of the kinetic energy leaves a bullet between the time it hits and comes to rest. Part leaves in a shock wave transmitted into the object the bullet strikes. Part goes into deforming and/or breaking up both the bullet and the object. In addition, bullets make good thermal contact with the object they hit, causing heat to be transferred out of the bullet.

We should add that all of our calculations are made for point blank ranges. In other words, at zero distance from the end of the gun barrel. Bullets tend to slow down significantly at greater distances due to air resistance and in the process lose much of their kinetic energy before they reach their targets.

We've shot numerous rifle and pistol bullets and have yet to see a bright flash of light from an ordinary bullet. Yes, a bullet can obtain a static electric charge as it flies through the air. On impact, it can also cause pieces of rock to strike each other and produce sparks. A small, high-powered bullet like a 5.56 NATO round could potentially get hot enough to cause some level of flashing when fired against an unyielding steel barrier. Steel-jacketed or steel core bullets are available in many types of military ammunition and can also cause some sparks. However, sparks caused by ordinary bullets are not as dramatic as the large flashes of light depicted in movies and generally can't be seen in daylight conditions.

Bullets containing incendiary material such as white phosphorous are the exception. We've seen these bullets fired, and they do indeed produce bright flashes of light when they hit as opposed to ordinary bullets which do not. Incendiary rounds were originally designed to start fires when riddling the fuel tanks of enemy vehicles but also make it easy to see where bullets are striking. Even heavy machine gun bullets are not reliable sources of ignition unless they contain incendiary materials. While incendiary ammunition can be found, it's not commonly used outside of the military. (Note: incendiary bullets are different from tracer bullets which produce a streak of light as they travel to the target. These can also light fires but are designed mostly to help gunners see where their bullets are going before they hit.)

At best, flashing bullets aren't needed. At worst, they detract. Serious movies generally don't use them. Subtlety can be far more dramatic. For example, merely hearing bullets thunk against steel beams as the troops waded ashore in the movie Saving Private Ryan was positively chilling.

Flaming Cars

Gasoline has a very narrow flammable range of about 1.4 to 7.6% gasoline vapor in air ₂. In other words, the vapor-air mixture must be exactly as specified or the gas will not burn, let alone explode! Note that we say vapor. Liquid gasoline must change into vapor before it can burn (although this is no huge problem since it easily vaporizes).

For a car to explode during impact the tank must catastrophically rupture and spew a fine mist of gasoline over a large area so it can vaporize and mix with air in exactly the right proportions. The mixture must then find a source of ignition. Automobile gas tanks are built to withstand a considerable impact force and are usually located in a protected area between the beams of a car's frame. Common ignition sources in the car's engine are generally at the other end of the vehicle.

As portrayed in movies, gasoline tanks are fragile and gasoline so volatile that the vaporizing and mixing process occurs in milliseconds. It always results in an explosive mixture which always finds a source of ignition. Thank heavens it's not so easy or people would regularly be blowing themselves up while refueling at the pump.

Even when a wrecked car catches on fire it rarely explodes. A gas tank can explode if it contains an explosive mixture and there's an opening for the flames to enter. More likely, fire would have to impinge on the outside of the gas tank, vaporizing the gasoline in the tank and eventually causing it to overpressure and explode. However, if the vapors escaped fast enough the tank would not rupture. Most fires start in the engine compartment and will not spread backwards to the gas tank area unless the tank is leaking fuel on the ground. Again a whole series of events has to be just right for an explosion to occur.

Although it's actually quite rare, exploding cars are a common excuse for not wearing seat belts. Onlookers at crash sites are often so concerned about explosions that they unnecessarily jeopardize a person with a spinal injury by pulling them out of a wrecked car. The common Hollywood depiction fuels these harmful misconceptions.

Problems with Windows

Saying that shards of broken glass are razor sharp is an understatement. A shattered window contains thousands of incredibly sharp edges and dagger-like points. It takes almost no force for one of these points or edges to cause a laceration. However, people in movies routinely jump through plate glass windows without receiving a single scratch.

Broken glass has at least two mechanisms for slashing a person diving through a window: its weight and its inertia. First, large heavy shards of glass can fall like guillotines, slicing off body parts. Second, when a person jumps or, even worse, drives a motorcycle through a window, the shards of glass tend to stay in place due to their inertia. The only way to move them is to apply a force. If the person's body provides this force by pushing on the edge of a piece of glass, it can slice right through clothing, skin, and flesh. In the real world, jumping or driving through a plate glass window would be suicidal.

There are individuals who have accidentally fallen through windows without sustaining serious injuries. There are also people who have survived the Ebola virus. However, in both cases the odds are not particularly good.

Safety glass helps considerably because it's designed to completely shatter into small pieces with low amounts of weight and inertia, not to mention rounded rather than sharp edges. Laminated safety glass adds a thin layer of plastic sandwiched between glass layers. This helps keep pieces of broken glass from becoming projectiles.

Safety glass  is not a soft surface. All car windows are made of one or the other form of safety glass. Nevertheless, when craniums impact car windows it often results in head injuries, including lacerations and broken bones or teeth.

A person who jumps through a safety glass window would be far more likely to avoid serious injury than if he jumped through a plate glass window, but would still likely sustain at least minor cuts. All it takes is one minor cut on the head or face to make a person look like a bloody mess.

If it's so hard to jump through windows and not look like you just took a catsup bath, then how do they do it in movies? Simple, they either use real safety glass but break it ahead of time with small explosive charges, an instant before the stunt man reaches it, or they don't use glass at all.

At times, moviemakers have used panes made of sugar in glass-breaking scenes. That's right, candy windows! These look like glass and break like glass but have no sharp edges. More recently the candy has been replaced by a commercially available product called SMASH! plastic which simulates glass without all the safety problems. The manufacturer of this product recommends that panes of the material be no more than 3.2 mm (1/8 in.) thick to avoid impact injury. The same company makes a clear form of silicone rubber which looks like broken glass and can be walked on with no risk of injury.

The Attractive Force of Glass

If he were not on the sidewalk by a display window, then invariably he'd be blown into a rack of whisky bottles, a giant mirror, or some other large glass object. This happens so often that if we didn't know better we'd think Hollywood had discovered a new principle of physics: the attractive force of glass for shooting victims.

Hollywood apologists would explain that the hero was blown backwards by the force of the shotgun blast, and glass objects are in the way 98% of the time due entirely to random chance. Unfortunately conservation of momentum says it can't happen. A bullet or even shotgun blast simply does not have enough moment to propell a victim violently backwards.

There is one other possible mechanism for being blown through a window: involuntary muscle contraction. The victim could be so stunned by being shot that he involuntarily jumps backwards. Since we haven't run this experiment, and have no desire to do so, we can't totally rule it out, but it does seem unlikely.

For more complete details see our site's companion book .

Scaling Problems

Let's start with the density problem. Ordinary matter is mostly empty space, and so it is conceivable that an object could be shrunk or expanded by somehow adjusting the amount of empty space inside it. Unfortunately, this would leave the weight exactly the same.

Expanded objects or persons would have such low densities that they would be blown away in the wind like big balloons. Tiny people would suddenly exert huge pressures under their little feet since the area of their feet would be miniscule but their weight the same.

For instance, a normal-sized person exerts a pressure of about 2 pounds per square inch with their feet when they are standing on both feet. If their weight stayed the same and they were shrunk by a factor of 100, a six foot tall person would now be about 0.72 inches tall. Their foot pressure, however, would rise by a factor of 10,000 or in other words become 20,000 psi.

Such a person would instantly sink if they stepped on mud. The pressure under their feet would exceed the compressive strength of concrete (typically 3000 to 4000 psi) and would likely mar the surface of sidewalks. How could such pressures be possible? To explain it we must first look at the mathematical model for pressure:

P = F/A

Where P is pressure, F is the magnitude of the force (in this case weight), and A is area (in this case the area of the bottoms of a person's feet). Note that when weight remains the same and area decreases pressure increases. Since pressure and area are inversely proportional, decreasing area by a factor of 10,000 increases pressure by a factor of 10,000.

Reducing a person's size by a scaling factor of 100 decreases the area of their feet by a factor of 10,000, since area scales up and down with the square of the scaling factor. If this seems strange, then consider the fact that the area of a rectangle is the width times the length. If both dimensions are decreased by a factor of 100 then the new area is decreased by a factor of 100 times 100 or 10,000.

The density problem could perhaps be solved by removing molecules when reducing size and adding them in when increasing size. This would be an inordinately complex process, because it would be extremely difficult to make sure that molecules were removed in exactly the correct proportions.

However, if we assume that this problem could somehow be overcome, serious problems would still remain. A creature's legs (human or otherwise) are similar to the columns which hold up Greek temples. Their strength is directly proportional to their cross-sectional area. This, in turn, is proportional to the square of the radius of the column, according to the equation:

A = pr²

V = (4/3)pr³

With constant density, the weight therefore increases with the cube of the scaling factor. Hence, weight increases by a factor of 1000 cubed, or 1 billion. This means weight increases 1000 times faster than leg strength. In other words, the ant would probably collapse under its own weight.

The ant's mass, and hence inertia, also increase about 1000 times faster than muscle strength. So, if the ant could still stand, it would barely be able to move.

Scaling downward, or shrinking, avoids some of the weight problems. However, it has problems of its own, especially for warm-blooded creatures.

Heat loss is related to the ratio of bodily surface area to mass. In other words, a creature with a high ratio of surface area to mass will cool off much faster than one with a low ratio. Such a creature would need a higher metabolism rate and to eat more food to maintain body temperature.

Small creatures have high surface-area-to-mass ratios, which explains why shrews must eat several times their bodyweight in food everyday. They must do so to maintain body temperature.

Surface-area-to-mass ratios scale up or down inversely proportional to the scaling factor. In other words, shrinking a human by a factor of 100 would increase the surface-area-to-mass ratio by a factor of 100. Such a person would have to eat continually or risk death from hypothermia even in 70-degree Fahrenheit weather.

None of the above discussion even mentions the fact that the design of lungs, hearts, brains, blood cells, etc. is very specific to relative size and does not scale up and down well if at all. Physiology changes dramatically with size due to basic laws of physics. Although it might be entertaining, the prospects of big bugs and tiny humans will have to wait until the laws of physics are substantially altered.

 For more complete details scaling problems see our site's companion book .

Outerspace Explosions

Sound is a pressure wave which requires matter of some sort to propagate it. It moves along at a rather sedate velocity of 340 m/s (1120 ft/s) in atmospheric-pressure air. Light, on the other hand, is an electromagnetic wave and needs no matter for transmission. It moves in a vacuum at 300,000,000 m/s (186,000 mi/s).

Yes, an explosion probably would create an expanding cloud of gases which would eventually impact a spaceship in its path. However, in the vacuum of space this expanding cloud of gas would have a very low density. When it hit a ship some distance from the explosion it would probably sound like a gust of wind blowing against the spacecraft.

Unfortunately, even the Star Trek writers eventually succumbed to market pressures and began adding sound effects to explosions. To make matters worse, the sounds were portrayed as traveling at the speed of light, since they always arrived simultaneously with the image of the explosions.

We would also like to point out that observing an exploding spacecraft in outerspace would be quite dangerous compared to observing one on Earth. The shrapnel and debris from exploding spacecraft would attain very high initial velocities just like they do on Earth. However, with no gravity to pull them to the ground and no air drag to slow them down, the debris would travel outward in straight lines virtually forever until they hit something.

Distance from the explosion would reduce the number of projectiles striking a spaceship. However, impacting pieces would have the same kinetic energy they had right next to the blast. A spacecraft would have to use the time afforded by distance from the explosion to raise its shields or risk annihilation. Being in a desperate battle surrounded by exploding ships and having no shields would be certain death.

For more complete details on space battles and how they might unfold see our site's companion book .

Visible Laserbeams

Multi-beamed laser security systems are a frequent Hollywood plot device. Again and again movies feature tension-filled scenes in which characters snake their way through mazes of laserbeams artistically arranged in random patterns by professional security fools to entertain us by making would-be thieves do contortions. A simple arrangement of closely-spaced parallel beams would be contortion-proof but certainly not as much fun.

Unfortunately the tension-filled fun requires visible beams. And anyone who's used a typical red laser pointer knows that visible red laserbeams are as commonplace as the quintessential dimly-lit smoke-filled room. Shine a pointer under normal conditions and you get a puny dot of light, not a visible beam extending dramatically across the room. It's only when the laserbeam hits a diffuse surface that its light is scattered in all directions, some towards your eyes, allowing you to see the dot.

The only way to "see" a red laser pointer's beam is to shine it through a cloud of smoke, chalk dust, mist, etc. in a dimly-lit space. The small particles in the cloud act as tiny diffuse surfaces which scatter part of the beam toward your eyes. Dust particles usually create a sparkling effect as they float through the beam. Sunbeams and moonbeams are created in the same way. Technically, what you actually see are the particles in the cloud, not the beam itself.

With the correct wavelength of light, laserbeams can make air in their path glow. If a photon of the correct wavelength hits an electron in the air it can "bounce" it to a higher energy level. Eventually the electron returns to its normal level by emitting a photon. The light emitted by the electrons in the air is not laserlight because it's not all going in the same direction, but it is all the same color as the laserbeam shining through it. However, it's hard to see in a lighted room unless the laser has a very high power level.

We might applaud Hollywood for often making security-system laserbeams invisible, but alas, it's a plot gimmick used only when needed for dramatic tension. Movie characters typically respond in some clever but unrealistic fashion.

Sometimes they spray aerosols. In theory this could make beams visible, but in actual practice it's hard to find a spray that both works and persists in the air. The spray itself could trip a sensor with high sensitivity and would only work in dimly-lit spaces.

More recently, Hollywood actors have started using special glasses. Again, light must shine into the eyes to be seen. Unlike us humans, photons don't fall for gimmicks, and the glasses don't cause those in laserbeams to veer off course towards the actor's eyes. Glasses can only alter light already shining into your eyes.

Yes, night vision equipment could amplify laserlight scattered by dust. Infrared (IR) goggles could make it possible to see otherwise invisible IR lasers. However, both still require particles in the air and could often be defeated, simply by providing bright ambient lighting.

Perhaps the biggest problem with multi-beamed laser security systems is that in the real world they're rarely used. Systems with active light sources typically use inexpensive infrared LEDs. They give off invisible infrared light much like an ordinary light bulb gives off visible light. Intruders trip these systems by creating a shadow on a detector. By comparison, a laserbeam is expensive and requires precise alignment.

Passive infrared devices are even cheaper because they require no special IR light sources. Human beings are like walking infrared light bulbs. A single inexpensive passive sensor can be used to detect the presence of human motion for an entire room. While multi-beamed laser security systems are not impossible, there's usually no reason to use one.

From the standpoint of visibility, laser gunfights are usually depicted realistically. We must also admit there's something ominous about seeing a little red dot on a person and knowing a bullet could soon follow. However, laser sights are used in some ridiculous situations; for example, on sniper rifles.

When a sniper looks through the telescopic sight on his rifle, he knows where the bullet is going to go relative to the crosshairs. Adding a laserbeam would do nothing except tip off the victim that he's about to be shot and give him time to duck before the bullet arrived. It would also help reveal the sniper's location.

Figure 1: A Conveniently Visible Hollywood Laser

Figure 2: A Real Laser

Figure 3: A Real Laser Shining Through a Cloud

Hitting a moving target using a laser sight would be extremely difficult. The sniper would have to lead the subject and so the red dot would be projected in front of the target where it could easily be lost in the background.

High-powered laser blasters or deathrays would be easier to see than the low-powered versions used in security systems and gunfights. The light reflected by particles in the air would be brighter since the laserbeam itself would be brighter. And as mentioned earlier, even a low-powered laser of the right wavelength would cause the air to glow. A high-powered laser would make it glow even brighter because it emits far more photons to collide with electrons in the air .

Outerspace lasers are another matter. There's no air and few particles to make them visible. To make matters worse, some movies show laserbeams shooting through outerspace like glowing spears. All light, including laserlight, travels at 3×10⁸ m/s or 186,000 mi/s (in a vacuum), so fast that the human eye couldn't possibly detect the motion of a laserbeam even if it were in the form of a glowing spear. The afterimage of the moving light source would make it appear as a continuous beam from the source to the target.

Yes, a blaster or deathray could be something other than a laser. It could be a high-energy particle beam. The beam might be visible but would travel at such high velocities, it would look like a continuous beam from the source to the target.

Moviemakers generally throw in enough mumbo jumbo to obscure the mechanisms behind their fictional weapons, leaving some room for imagination. We also have to admit that a cool-sounding, glowing spear-like blast does have dramatic appeal. However, such blasts are speculative if not outright silly from a scientific standpoint.

The Sound of Movies

A dastardly terrorist wanders into view. The secret agent raises his weapon and coolly squeezes the trigger. On the street below onlookers hear an innocuous "fut" sound. The secret agent steps back from the window undetected, his assignment completed.

Unfortunately for the secret agent, he's not so likely to go undetected. A 7.62 NATO round is supersonic and would cause a miniature sonic boom even if the muzzle blast from the rifle was muffled ₄. Yes, the miniature sonic boom is not as easy to pinpoint as a muzzle blast but does produce a very noticeable noise which can draw attention to a shooter.

Even silencing the muzzle blast to a mere "fut" is next to impossible. Muzzle blast noise can exceed 150 decibels ₅ (measured at the shooter's location) and is one of the loudest sounds humans are likely to hear. Silencers, suppressors, or cans as they are sometimes called ₆ have to be precision made using very exacting technology to have any hope of quieting such a loud noise.

Considering that the threshold of pain is only 130 dB, we're actually glad Hollywood sound tracks don't accurately reproduce the noise of muzzle blasts. If they did, the only sound action movie fans would hear as they staggered out of the theater would be the ringing in their ears. In Blackhawk Down, the soldier who had an automatic weapon fired near his ears really would have been left temporarily, if not permanently, deafened.

SWAT teams sometimes use silencers, not for stealth, but to insure that they will be able to hear if one of the SWAT team members fires a shot inside the confined space of a room. Discharging an unsilenced firearm in a room can cause temporary deafness. Silencers are also sometimes used in raids on clandestine methamphetamine labs. Discharging a normal firearm produces a muzzle flash which can set off volatile fumes. Silencers act as flash suppressors.

Sound is a form of energy transfer and we could define loudness in terms of the energy per unit of time or power output, but it wouldn't give the complete picture. Sound waves travel outward like balloons expanding around their source. Some amount of a sound's energy  is distributed on the surface of each wave. Since a wave's sound energy is fixed and its area increases with the square of the distance from its source, the amount of energy per unit of area in the wave declines rapidly as the wave moves away from the source. About the same wave area contacts a person's ear regardless of how far she or he is away from the source. The area, however,  contains significantly less energy when the source is far away rather than up close, which is why a distant sound is not as loud.

Power per unit of area (called sound intensity) would be a better measure of loudness than just power alone. Sound intensity accounts for the fact that the ear receives less power when the source is far away rather than close. Unfortunately, human perception of loudness is not linear with respect to sound intensity. In other words, doubling the sound  intensity does not double the perception of loudness. The perception of loudness is, roughly speaking, logarithmic and is represented somewhat better by the decibel scale as follows:

Even the decibel falls short of being a true indicator of perceived loudness. The loudness of a noise also depends on its frequency or pitch. Sound measuring equipment, at least partially, accounts for this fact by using various frequency weighting filters. The dBA scale is the most common of these applications. However, if we assume that a muzzle blast's frequency content is in the general vicinity of optimum hearing and that the blast's frequency content doesn't change with loudness, then the unweighted decibel scale is a reasonable indicator of relative loudness for purposes of discussion.

The logarithmic nature of  hearing makes muzzle blasts even harder to silence. Let's see what happens to the relative loudness level if we reduce the sound intensity of a muzzle blast by a factor of two. This means we're removing half of the energy from the sound waves. Using the above equation we get the following:

We love the "highly effective" makeshift silencers which movie characters cobble together on the spur of the moment. These have been created with everything from pillows to potatoes. Our favorite is a scene from On Deadly Ground where Steven Seagal "effectively" silences a semi-automatic handgun by taping an empty 2 liter soft drink bottle to the end of the gun barrel and gets the usual "fut" sound. At best, jury-rigged silencers can reduce noise levels only slightly. At worst, they can become obstructed causing the silencer or gun barrel causing it to overpressure and burst ₈. We might add that unregistered silencers are also illegal, even if they are relatively ineffective homemade creations.

Since Hollywood isn't overly concerned about loudness, then certainly it's not going to obsess over small details  like decimal points. The speed of sound is roughly 300 m/s while the speed of light is 300,000,000 m/s (both numbers are accurate to one significant figure). Yet, moviemakers consistently think the two speeds have decimal points in the same location. If an artillery shell explodes on a distant hill the sound invariably arrives simultaneously with the image. Lightning typically coincides with thunder. When a car careens off the edge of a cliff and smashes into the boulders below, we instantly hear the explosion.

Perhaps we should just write this one off to dramatic license, but the truth is virtually everyone knows about the time delay between images and sound. In a movie, if a minor mismatch between an event and its sound causes a distraction, then dramatic license justifies eliminating it. Otherwise, why portray anything falsely, especially when most people know and accept the truth.

We have previously mentioned that noises cannot be transmitted through the near vacuum of outer space because sound has to be transmitted through matter. Why then does Hollywood persist in playing engine noises every time a spacecraft passes by. Seeing a giant craft float silently past would be far more dramatic because it would be unexpected in our earthbound lives.

Arguably, the most dramatic scene in the 1968 movie 2001: A  Space Odyssey  occurs when the computer HAL locks Dave out of the spaceship and Dave is forced to enter the ship in a dangerously unorthodox manner.   Even though Dave sets off explosive bolts, the scene is totally silent because there is no air in outer space. Yet, the scene coveys a sense of utter desperation.

2001 is included in most lists of the top 100 movies of all times (#22 on the AFI list of the top 100 films), has an enduring quality, and cult following because it got the physics of space travel essentially right. It's not a particularly strong movie in terms of plot, action, or pacing. Its best dialog comes from its most notable character, a computer portrayed as a disembodied voice and unexpressive camera lens. Its ending is almost incomprehensible. Still 2001 demonstrates that silence is strongly emotional.

The 1970 movie Tora Tora Tora was nominated for four Academy Awards including the award for sound.  It won the award for Best Visual Effects. The movie was a marvel of special effects for its time and was vastly superior in historical authenticity to the more recent movie Pearl Harbor. Yet to modern viewers it has an annoying audio distraction. The bullets make a fake sounding ricochet noise when they hit. In 1970 this was standard practice but now sounds ridiculous. Movie makers would do well to take note of this fact. Movie history itself shows that the public eventually does reject nonsense.

Cigarettes as Lighters

Before proceeding further we want to emphasize that obviously, under the right circumstances a cigarette can ignite gasoline with horrific results.

Lighting puddles of gasoline with cigarettes in movies is a common device. The character takes a few puffs and tosses the glowing cigarette in the puddle. Immediately, the gasoline ignites. However, numerous readers have written us and said it isn't so. Some have cited experiences where they saw it attempted. Others have said that cigarettes don't get hot enough.

We searched the web and found several sites that say the glowing tip of a lit cigarette is well above the autoignition temperature of gasoline. Normally this information would have convinced us, but as mentioned before, some of the people writing in seemed to have personal experience. Finally, we decided to conduct an experiment.

We poured a very small amount of gasoline in an aluminum pie pan or slightly deeper cake pan and placed it in the middle of a concrete slab. The pie and cake pans were chosen because they allowed the gasoline to spread out into a very shallow puddle the way it would if spilled on the ground. It also pretty much guaranteed that the vapors at some point above the pan would mix enough with air to form an ignitable mixture.

The explosive or flammable range for gasoline is about 1.4 to 7.6% gasoline vapor in air ₂. Outside of these limits, gasoline cannot be ignited. A large amount of gasoline in an enclosed can usually will not form an ignitable mixture since the vapor concentration will be too high.

We lit a cigarette and tossed it into the pan. The cigarette paper wicked up gasoline and quenched the glowing tip without igniting anything (see Figure 4). We tossed in more lit cigarettes. We tried lighting gasoline soaked paper towels. We used long tongs for reaching far away objects to hold glowing cigarettes over the pan at various heights. More than once we placed several glowing cigarettes in the pan (see Figure 5). Our record was 40 glowing cigarettes at one time. In most cases, we allowed the glowing cigarettes to smolder until they went out.

Various experiments were conducted at different times of the day with different air temperatures and humidity. A total of 223 cigarettes of 11 different types were eventually used all without ever igniting the gasoline. Yet, at the end of each experimental session the gasoline was successfully lit using a single match attached to a long pole (see figures 6 and 7). The gasoline would typically ignite just before the match touched it. This indicated that there was an ignitable mixture just above the surface of the gasoline. Numerous lit cigarettes were in this region for significant periods of time.

	Figure 4: Cigarettes snuffed by gasoline	Figure 5 Smoldering cigarettes on a gasoline soaked paper towel
	Figure 6: Gasoline soaked paper towel at the moment of ignition with a match	Figure 7: Vigorously burning gasoline soaked paper towel after lighting with a match

We knew that puffing a cigarette would increase the tip's temperature substantially and would help mix vapor and air together. We became convinced that puffing a cigarette over the gasoline would cause it to ignite. To test it, we built a simple smoking apparatus which could draw air through the cigarette or push it backwards out the tip.

We tested the apparatus repeatedly in both modes without getting ignition. During a test a cigarette was consumed rapidly and glowed brightly. Often sparks shot or fell off the cigarette. They were smoked at various levels above the gasoline to insure that at least part of the time they were in a region with an ignitable mixture. Surprisingly, even when a cigarette was puffed it didn't ignite the gasoline.

As mentioned earlier, we stand firm that under the right circumstances cigarettes can ignite gasoline, however, tossing a lit cigarette into a puddle of gasoline, as is done in many movies, is not a reliable way to do it.

If you want additional information about the reason why cigarettes are not a reliable ignition source please check out our book.

Experiments described above were conducted in a safety conscious manner under the supervision of a qualified professional with years of experience in handling dangerous materials. Do not attempt them on your own.