Thursday, May 29, 2008

Physics errors in Indiana Jones 4

This article is driven by the discussion stimulated by an article by Matt in "Built on Facts". There were minor spoilers in his discussion, and a few more in mine.

That discussion was centered on Newton's First Law (which I will repeat here with some links), but a reply from Matt included a side remark about nuclear physics. I posted one comment on his blog but saved a more specific bit of physics for here.

Newton's First Law:

The biggest joke in the entire movie was when they were riding the jet powered sled and stayed right where they were sitting after it came to an abrupt stop as a result of hitting a water tank braking zone. This is clearly modeled after the rocket sled used by Col John Stapp in the late 1940s to explore the limits of safe deceleration of a human wearing a seat belt. According to wiki, his record was about 45 times that of gravity.

(For reference, since the braking distance was no more than 14 m, he would have to go from 111 m/s to zero to have a constant acceleration of 45 g over that distance. A speed of 111 m/s, more than a football field in one second, is about 250 mph. This corresponds to a 250 mph race car stopping in 0.25 seconds while covering a distance of 14 m, about 45 feet.)

The only way you are going to stay in one place on that sled when it goes from 250 mph to zero in a quarter of a second is if you are belted in place. Otherwise, all that happens is that the sled stops and you keep on going at 250 mph. This event in the movie is totally bogus.

(We will leave out the continuity error of this happening at Area 51 in Nevada, allegedly within walking distance of an active part of the Nevada Test Site, whereas the rocket sled was at Edwards Air Force Base in California.)

On the flip side, the lead lined refrigerator he hides in apparently has almost no inertia. Despite being the densest object around, the blast wave tosses it much more easily than it does the lighter materials around, such as the house and its contents. The physics here is that of drag forces (which depend on the area and shape of an object) overcoming the inertia of the mass of an object. The basic rule is that the force of the wind will have a bigger effect on a whiffle ball than on a baseball, because the baseball has more mass while the wind forces will be similar.

Nuclear Weapon Physics:

I mentioned in passing that I was disappointed in the rendering of the mushroom cloud from the nuclear explosion. They actually got most of this physics quite correct, which is not surprising given all of the film and photos of these various tests, as well as books like Glasstone's "The Effects of Nuclear Weapons", published by the US Department of Defense. (You can find much of the relevant information in the Nuclear Weapon Archive or in books such as Richard Rhodes' "The Making of the Atomic Bomb".)

Of course, it would not be the 21st century if you could not see the film from the 1955 civil defense test on YouTube. I want to call your attention to the "after" pictures of the frame house at 3:23 in the film, since I will come back to it, and how well a first floor reinforced bathroom shelter (3:43) stood up to this blast, given the tornado damage we have seen recently. You might also notice that this was done in 1955, two years before it happens in the movie!

The important effect they get right in the Indiana Jones movie can be seen around 2:20 in the movie. The light, heat, and gamma radiation all travel at the speed of light. They get there first, and burn the paint off of the building and the skin off of people. The blast wave travels at the speed of sound (actually a bit faster due to the overpressure, as I recall) and arrives much later. Notice that it does two things: it knocks down the building, but it also blows out the fire. (The house you see at 3:23 did not burn down. It is not the case that a firestorm is a necessary result of a nuclear blast.)

The part they could have done a better job on concerns the colors seen in and around the fireball and mushroom cloud. See, for example, the witness statement in a paragraph at the bottom of page 673 in "The Making of the Atomic Bomb": "Then, as a climax, which was exceedingly impressive in spite of the fact that the blinding brightness had subsided, the top of the slender column seemed to mushroom out into a thick parasol of a rather bright but spectral blue..." from one report, and "When the red glow faded out ... a most remarkable effect made its appearance. The whole surface of the ball was covered by a purple luminescence, like that produced by the electrical excitation of the air, and caused undoubtedly by the radioactivity of the material in the ball" from another.

There is a picture of the Ranger/Fox test from 1951 in Nevada that gives a hint of what colors can be present around the fireball, but most color pictures do not capture this effect. I suspect it might be seen more easily by eye than by a photograph.

The spectra of ionized nitrogen gas is a beautiful violet color. (This is why pictures of lightning bolts have a violet hue around them.) What is happening is that the intense radiation from the results of the explosion is ionizing the air and making it glow. You can also get Cherenkov light (typically a light blue) when particles travel faster than the speed of light in a medium, such as water or air. It would have been very cool if they had included this as the fireball cooled, although I have no idea if it is visible in a daylight explosion.

Radiation from a Nuclear Weapon:

The alert spectator will notice the sticker that says the refrigerator Indy hides in is Lead lined. This would protect him from x-ray and gamma-ray radiation, just as it would make it unlikely that the refrigerator would travel very far from the blast, but it would not protect him from neutrons. Heavy elements shield you from photons like gamma rays, but you need light elements to protect you from neutrons. That is why concrete and dirt are used to shield a shelter.

The biggest unexpected effect of a nuclear explosion turned out to be radiation from neutrons released in the explosion. These could have affected him even if he was inside that lead-lined refrigerator, and the damage would not wash off.

Semi-funny story. Jet aircraft were used to collect samples of radioactive material from within the mushroom cloud of the first thermonuclear test, Ivy Mike, in November 1952. I had the privilege of talking with a number of "Atomic Veterans" at one of their reunions, some of whom witnessed this test and one of the problems of neutron radiation. (Thermonuclear weapons, the H bomb, produce even more neutrons.) They knew how to decontaminate aircraft from practice runs during fission weapon tests. However, as much as they tried, they could not wash off the contamination from the planes used for Ivy Mike. My guess is that the aircraft materials had been "neutron activated", that is, the elements transmuted to a radioactive isotope as a result of neutron capture. One guy said the planes were still sitting at the end of a runway on Kwajalein a decade later, while another said he thought some had been pushed off of an aircraft carrier into the ocean.


Matt said...

Quite thorough! While the film was definitely no Armageddon, I think for an action blockbuster it did all right.

You're dead on about the fridge being of limited effectiveness as a neutron shield, but Indy certainly didn't help matters by climbing out before the mushroom cloud even stopped glowing. While I guess it's better than suffocation, both the prompt radiation still being emitted by the fission products and the early fallout would probably at minimum prevented him from being healthy enough to chase Communists around the world!

Doctor Pion said...

The neutrons come out in two groups.

There are the prompt neutrons, responsible for driving the chain reaction through 80 or so generations in a few milliseconds. These are the main threat to people and they have gone through you by the time the blast gets there, traveling at maybe c/5 to c/10 or so.

The rest are significantly delayed (they are what make it relatively easy to control a nuclear reactor) and are considered part of the fallout. My recollection is that they are a minor part of the fallout risk, but it has been ages since I looked at that sort of civil defense information.

ricardo said...

Besides that... u would not have enough space to number the historical and geograpgical mistakes on this blockbuster fiasco

Kai said...

I was annoyed at the lapse in Physics in the scene where Indiana uses gunpowder and the buckshot to find a magnetized artifact. As far as I know there is no iron or magnetic material in gunpowder, and buckshot is lead, iisn't it". Additionally there was no hint of magnetic attraction to a steel jjeep! Once you lose believability there, the deceleration Gs on the sled just adds to the annoyance!