In the name of physics decency, to protect the minds of children everywhere, so that they may grow up in a world where they know the difference between speed and velocity, we have taken the responsibility to rate movies for their portrayal of excessively bad physics. The system is as follows:

• GP = Good physics in general

  About Our Site Since its start-up in 1997, "Insultingly Stupid Movie Physics" has received hundreds of thousands of visitors including NASA scientists, high school and university physics teachers and at least one published science fiction writer. We have received many helpful suggestions and constantly consider them as we improve our site. "Insultingly Stupid Movie Physics" has been reviewed or listed in  Physics Today Magazine, The News Letter of the American Physical Society, The New York Times, and The Hollywood Reporter, as well as numerous other publications. It has been listed on Fark and Slashdot and been featured on the Osgood Files and NPR as well as many other radio stations. The vast majority of comments we receive are positive. However, for the others we offer the following: Yes, we do have lives and also do enjoy movies. No, we are definitely not "normal" but then we have never met a "normal" person, although we're sure there is one somewhere in the world.

  
  

• PGP = Pretty good physics (just enough flaws to be fun)
  
  
• PGP-13 = Children under 13 might be tricked into thinking the physics were pretty good; parental guidance is suggested
  
  
• RP = Retch
  
  
• XP = Obviously physics from an unknown universe
  
  
• NR = Unrated. When a movie is obviously a parody, fantasy, cartoon or is clearly based on a comic book it can't be rated but may still have some interesting physics worth discussing.

GENERIC BAD MOVIE PHYSICS

Some insultingly stupid movie physics are so commonplace as to make it inefficient for us to rail about individual instances. They have become visual clichés and do for movies what verbal clichés do for literature. Really good movies like Casablanca don't need visual clichés to build excitement. They depend on less glitzy techniques like good plot, character development, and sparkling dialog. To avoid repetition, we have listed the following generic bad physics items:

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:

KE = ½mv²

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

Ever notice how cars in movies always burst into flames the instant they collide with anything? Our favorite is when a car falling from a high place explodes the instant before it hits the ground. It's as though its gas tank gets panicky and detonates at the mere thought of striking Earth. Fortunately, the physics are not so cooperative.

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.

The Mac 10 Problem

Although 9 mm submachine guns such as the Uzi (which fires 600 rounds per minute) are popular in movies, everyone knows that real action heroes prefer .45 cal Mac 10's. These fire bigger bullets at rates of 1000 rounds (in other words bullets) per minute. They have a thirty-round magazine (the long black thing that stores the bullets) and are by any measure a deadly weapon.

Movies are filled with scenes of good guys and bad guys blazing away for minutes at a time. Of course, no one is overly concerned with reloading or lack of ammunition, but then that's been true since the days of singing cowboys such as Roy Rogers who smiled a lot and engaged in friendly gunplay between musical numbers. So why would we bother to mention what is common knowledge? We can't help but be impressed by the weight of the matter.

First, let us point out that the thirty-round magazine in a Mac 10 will be expended in a mere 1.8 seconds of sustained fire! If our shooter blazes away steadily for a total of only 3 minutes, his or her Mac 10 will spit out around 3000 chunks of lead at roughly 15 grams a piece. This amounts to 45 kilograms or a little less than 100 pounds of lead. And that doesn't account for the weight of the 3000 cartridge cases or 100 empty magazines scattered on the ground.

Second, bullets are, after all, propelled by some very hot gasses which exert high pressures that create high stresses in gun parts. A firearm can withstand the high pressures and stresses only if the blasts of high temperature gasses don't happen too many times before the firearm has time to cool off.  Running 3000 of these temperature cycles back-to-back would turn a light weight submachine gun, like a Mac 10, into a red hot piece of scrap metal, that is, if it even lasted for 3000 rounds.

Yes, 9 mm submachine guns with slower firing rates would reduce weight problems, but it seems that real action heroes use Mac 10s, preferably one in each hand. We can't help asking where the sidekicks are with wheelbarrows to carry the ammo, let alone the cooling systems.

Problems with Windows

No, we're not referring to Bill Gates' woes (or lack of them), but to the ways movie windows refuse to obey simple laws of physics. Apparently no one in Hollywood has ever picked up a piece of broken glass and suffered the inevitable bloodied finger.

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 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. This greatly reduces the severity of lacerations but can't completely eliminate them. 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 four to ten times stronger than an equal thickness of ordinary glass and is hardly a soft surface to run into. 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 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. The movie True Lies has a good example of the former situation. When Arnold Schwarzenegger is nearly assassinated by a terrorist in the men's room, the terrorist runs into a store and leaps through the store's display window in order to escape. If the scene is run in slow motion it's possible to see one of the small explosive charges going off in the middle of the left edge of the window just before the terrorist crashes through the glass.

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

Our hero stands innocently on the sidewalk as a sinister car approaches with a shotgun protruding from the window. Suddenly he sees it, but—blam— it's too late. He's blown violently off his feet and flies several feet backward through the nearest display window. Fortunately, he's wearing his bulletproof vest and survives.

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 the current laws of physics don't agree.

A load of buckshot hitting a vest can be considered an inelastic collision. This means that the kinetic energy of the victim with the buckshot stuck on his vest is less than the original kinetic energy of the buckshot before the collision. The "lost" kinetic energy is not really lost, it has just changed forms. Some of it becomes a shock wave in the victim that creates bruises and possibly cracked ribs. Some is converted to heat.

Even though kinetic energy is "lost" during the collision, momentum is not. The momentum of the victim is the same as the original momentum of the buckshot.  So, the collision can be analyzed using conservation of momentum. This will let us estimate the backwards velocity of the shooting victim and judge whether he would indeed be thrown violently backwards.

To make the analysis we have to decide on some simplifying assumptions. As a rule of thumb, physicists and engineers (who should be considered applied physicists) generally start with the simplest reasonable calculation or model when analyzing whether an event will occur. They will also attempt to make assumptions which favor the event's occurrence. The reasoning is that if a simple model with favorable assumptions shows there could be no effect then there's no point in making a more rigorous model.

We'll make a simplifying assumption that there is no friction to impede the backward motion of the victim. This would favor the event's occurrence.

To calculate the momentum of an object we use the following equation:

p = mv

Where p is momentum, m is mass, and v is velocity.

Before the buckshot collides with the victim, the victim's momentum is zero, since he is not moving. This means that we only have to consider the momentum of the buckshot. For simplicity we will treat the buckshot as though it is a single object rather than calculating individual momentums for each pellet and adding them together. Both of these methods give the same result.

After the collision the victim and buckshot stick together and so, again, we only have to calculate the momentum of their combined mass. We'll use a subscript of 1 to indicate conditions before the collision and a subscript of 2 to indicate conditions after the collision. Hence:

p₂ = p₁

By substitution:

m₂v₂ = m₁v₁

Solving for the velocity of the victim after the collision gives:

v₂ = (m₁/m₂)v₁

Note that the velocity of the victim is proportional to the buckshot's mass to victim's mass ratio. This ratio is going to be tiny. Using the following values: the mass of the man with buckshot stuck to his vest equals 80 kg and the mass of the buckshot alone equals 0.0318 kg with a velocity of 486 m/s, we obtain:

This is about 0.4 miles per hour. Keep in mind that humans can walk about 4 miles per hour. Since our model was set up with favorable assumptions, we have to conclude that shooting victims aren't going to be blown backwards through display windows by the force of a shotgun blast.

There's yet another way to view the problem. Conservation of momentum works for shooters as well as victims. In other words, recoil from firing a weapon will give a shooter backward momentum equal to the forward momentum of the bullet and hot gasses from burning gun powder exiting the gun barrel. (Note: buckshot will also include a light weight, fibrous wad placed between the powder and buckshot.) When the bullet strikes the victim he'll end up with only the momentum the bullet had immediately before striking. The magnitude of the victim's  backwards momentum will be less than  the magnitude of the shooter's backward momentum because the victim will not be hit by the firearm's hot gasses. Also, thanks to air resistance, the bullet will be moving more slowly and have less momentum than when it first exited the gun barrel. If the recoil from discharging a firearm is insufficient to throw the shooter backwards through the nearest window then the bullet also will not throw the victim backwards through the nearest window.

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.

Falls

Excuse us if we sound like worried mommies, but we can't help pointing out that falls from high places are a bit more serious than the sniffles. It's a simple equation, the gravitational potential energy stored in an object becomes kinetic energy during a fall. This is the same type of kinetic energy which makes bullets deadly.

Our .45 cal bullet, for instance, has a mass of 0.015 kg and a muzzle velocity of around 288 m/s. Kinetic energy is calculated from the mass and the magnitude of the velocity of an object using the following equation:

KE = ½mv²

We calculate that such a bullet has a kinetic energy of 619 J. For comparison, let's assume that we have a wiry hero with a mass of 63.2 kg (139 lbs) and is in bed (sleeping of course). The bed is an old-style double-post model like one Lincoln slept in, and hence is a little higher than normal, say 1 m high. Our hero, due to some devious plot by the villain, falls out of bed. His (gravitational) potential energy in bed can be calculated from the equation:

PE = mgh

Where m is mass, g is the magnitude of acceleration due to gravity (9.8 m/s²), and h is height. Thus, his (gravitational) potential energy in bed is 619 J. Since this is converted into kinetic energy during the fall, our hero hits the ground with the kinetic energy of a .45 cal bullet!

Fortunately, our hero lives because the energy of the fall is dissipated over a much larger area than the area of a bullet. However, if you ask around, you will probably have little trouble finding a friend or acquaintance who has suffered a broken bone from a fall of similar height.

The general principle is that each additional meter of height is like adding the kinetic energy of another .45 cal bullet. Hence, a mere six-meter (19.8-foot) fall, which would be routine for an action hero, compares to being simultaneously shot by six .45 cal bullets, from a kinetic energy standpoint.

Suppose our hero is a 109 kg (240 lb) body builder instead of the wiry person mentioned. Now a six meter fall is like getting shot simultaneously by eighteen .45 cal bullets at point blank range. Indeed, it's true that the bigger they are the harder they fall.

Yes, bullets are incredibly lethal because they can easily penetrate into vital organs. A fall on a sidewalk would lack the penetration. However, it's pretty hard to completely avoid injury from being shot point blank six times with a .45 let alone 18 times, even when wearing a bulletproof vest.

Scaling Problems

It's an old movie gimmick; a misguided scientist, radioactive fallout, pollution, or some other folly of mankind abnormally shrinks or expands someone or some creature. While we must admit to being entertained by such gimmicks, the physics are another matter.

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

Hence, the strength of legs scales up (or down) with the square of the scaling factor. For instance, suppose we scale up an ant by a factor of 1000. This increases the ant's length from 1/8 inch to about 10.5 feet It increases the strength of the ant's legs by a factor of 1000 squared, or 1 million. This sounds very reassuring until we look at the ant's increase in mass and weight.

Each segment of the ant's body is roughly similar to a sphere whose weight is proportional to its volume given by the equation:

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.

Outerspace Explosions

Star Trek originally got it right. In early episodes, when something exploded in outerspace, it made no sound. That's because there is no air in outerspace to transmit sound.

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.

Star Wars apologists say that the ship's computers detect the explosion and simulate the noise to inform the crew. To us it would be far more useful to have the computer report that a TIE fighter has exploded on the port bow rather than sounding a loud boom.

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.

Visible Laserbeams

From security systems to space adventures, conveniently-visible laserbeams are a common part of our movie experience. Too bad they often don't reflect reality.

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.

Figure 1: A Conveniently Visible Hollywood Laser

Figure 2: A Real Laser

Figure 3: A Real Laser Shining Through a Cloud

Unfortunately the tension-filled fun requires visible beams. And anyone who's used a typical red laser pointer knows that visible 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.

When low-power lasers are used for something like crime-scene investigation they are always clearly visible. In fact if the plot calls for it, security beams will not only be visible, but arranged in an impenetrable grid pattern. In the movie Murder at 1600, Wesley Snipes encounters a visible grid of this type in a tunnel under the White House. Just when the situation looks hopeless, Snipes cleverly sets it off and hides in the tunnel. The Secret Service agents are, of course, distracted by Snipes' associate who leads them on a chase in the opposite direction out of the tunnel.

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.

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

The secret agent fixes his steely gaze on the crowd across the street in a park seven stories below. He methodically assembles his weapon. First he locks together the stock and barrel, then snaps his telescopic sight into position. Lastly, he screws on an oversized silencer ₃ . He carefully selects a shiny 7.62 mm NATO round (chosen, no doubt, for its long range accuracy ) and chambers it using the weapon's bolt action.

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.

Cutting sound intensity in half only reduces the relative loudness by merely 3 dB. This would be barely noticeable. A good set of ear plugs typically reduces noise by about 30 dB and so, would reduce a muzzle blast from 150 to 120 dB, still a very loud noise. We estimate that the innocuous "fut" sound made by a movie silencer is roughly 50 dB ₇, a whopping noise reduction of 100 dB from the dB level of a muzzle blast! In other words, a silencer has to reduce sound intensity of a muzzle blast by a factor of 10¹⁰ to give such a low relative loudness. This can be done with a very well designed and precision made silencer using subsonic ammunition. However, even commercially available silencers are more likely to give a reduction of 30 to 40 dB similar to ear plugs, than the incredible 100 dB reduction frequently portrayed in movies, especially when used on high-powered rifles.

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.

	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 searched the web and found several sites that say cigarettes do get hot enough. In other words 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.

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.

In physics or for that matter any science, when experimental evidence seems to disagree with theory, the theory needs to be revised. Unfortunately we don't yet have a good revised theory to explain this unexpected behavior. We welcome expert opinions or data from experiments with different results.

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.

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

[RP] The Day After Tomorrow (2004) - This movie looked like a contender for distinction of Worst Physics Movie Ever. However, it was mostly be guilty of gross exaggeration as opposed to total nonsense like The Core.

[PGP-13] Pearl Harbor (2001) - Egregious history as well as bad physics