Hollywood under the microscope
Would real space battles look and sound just like they do in ‘Star Wars’? And could Bruce Willis’s bomb save us from an asteroid? Adam Weiner puts screen science to the test
Wednesday 30 January 2008
In a galaxy far, far away, Luke Skywalker and Princess Leia are locked in a battle for the freedom of the Republic. Within the numerous battles that ensue, ships of every size, shape and political affiliation launch their attack across the vastness of space. The Millennium Falcon and the Death Star – as well as all kinds of spacecraft in between – are featured heavily as deafening lasers whizz and screech across the dark, starry backdrop.
Now, let's not be silly about this: you have to assume that a long, long time ago, in a galaxy far, far away, they knew how to build these space ships. But here's the thing – sound can't travel in a vacuum, so the dramatic inter-galactic combat wouldn't be quite as breathtaking as it seems in the film. There is no sound in space, no matter what George Lucas might have you believe. In a nutshell, the speed of sound is 340 metres per second through air. It travels much faster in water or solids, but in a vacuum there is no matter for sound waves to travel through.
And it's not just George who's guilty of perpetuating the myth – Star Trek, Galaxy Quest, and Starship Troopers all fall prey to the same notion. The only notable exception is 2001: A Space Odyssey, which uses the stillness of space to its advantage – the strains of "Daisy, Daisy, give me your answer, do" wouldn't sound quite so menacing against a backdrop of anything but deathly silence.
The1989 Batman, with Michael Keaton, is a great example to showcase film physics. When Batman and Kim Basinger are dangling over a ledge, they lose their grip and fall before Batman’s retractable rope-hook catches on a gargoyle, saving them from crashing to death on the ground below. Would it really save them, though? Remember that the principal feature that distinguishes Batman from other superheroes is that he has no superpowers. So here’s where the physics doesn’t add up. It doesn’t matter whether a fall is interrupted beforeimpact with the ground; if the deceleration is sharp enough, severe injury is just as likely as in hitting the ground. To alleviate the effects of the forces and resulting negative acceleration (or deceleration), their magnitude must be reduced by increasing the time over which the forces occur. The rope does no good unless it is very elastic, like a bungee cord. Batman’s rope isn’t; he and Kim Basinger are brought to an abrupt halt.
Rapid accelerations and decelerations in humans would cause large bones to break, and internal injuries are possible. This is a result of Newton’s First Law: internal organs aren’t fixed to the frame of the body, so they will continue to move “at a constant speed in a straight line until acted on by a net external force”. That is, until they splat on your ribcage
The scenario: scientists have discovered an asteroid on a direct collision course with Earth. If the asteroid hits, it will destroy all life on the planet. Billy Bob Thornton and his Nasa staff have to divert the asteroid before it hits. The solution involves flying a space shuttle out to the asteroid, carrying a nuclear bomb to be placed in a hole that Bruce Willis is going to drill into the rock. The explosion will blow the asteroid in half, and each half will be deflected to either side of Earth. We are told the asteroid is “the size of Texas”, which means it has a diameter of about 700 miles.
Bruce Willis and his crew drill a hole that is only 800 feet deep to get the bomb inside the asteroid. If you draw a figure to scale, you can see that for all practical purposes they are exploding the bomb on the surface. Not only that but the asteroid must also split into just two roughly equal-size halves in order to change from its Earth-bound course. So does the bomb have enough energy to ensure that a) the asteroid will split into just two fragments instead of shattering into space rubble, and b) the fragments would be blown apart with enough force to miss the Earth? No problem. Bruce Willis and his crew have a 100-megaton nuclear bomb, after all. But hang on, according to my calculations that would explode with only one-hundred-millionth of the energy needed. Assuming all the energy of a nuclear bomb can be converted into the kinetic energy of the asteroid, and assuming the bombs are able to split the asteroid into fragments, that would be blown far enough from their current course to miss the Earth, another 70 or 80 million bombs of that size would be needed.
The Day After Tomorrow
The Day after Tomorrow is an end-of-the-world disaster movie, and this time it’s all about bad weather. The premise is based loosely on a controversial theory that global warming could end up triggering a global deep-freeze. Any discussion of weather clearly calls for a little foray into the principles of thermodynamics. The controversial theory in The Day after Tomorrowis that global warming causes melting of glacial ice, and that the influx of fresh water reduces the salinity of the oceans in those areas. This could affect something called thermohaline circulation, which affects ocean convection. The amount of warm water flowing into the North Atlantic is reduced, cooling parts of the northern hemisphere.
The film slams this theory in overdrive as it leads to an “instant ice age”. Probably the most dramatic phenomenon in The Day after Tomorrow is the famous masses of air descending, at -100C, to the surface from the upper layer of the Earth’s atmosphere, causing everything to freeze instantly. But these temperatures in the upper part of the troposphere, the lowest layer of the atmosphere, only range from around -45C to -75C. And even if the air magically got that chilly at the top of the troposphere, its descent would warm it up. It seems that there might be a moment of physics clarity when Ian Holm, as meteorologist Terry Rapson, asks, “Shouldn’t the air warm up as it descends?” Yes, it should! On its way down, the cold air has to be compressed, and that means it heats up.
In this 1996 version of the “aliens invade Earth” plot – and in spite of the fact that the aliens’ technology is incomprehensibly advanced – the humanssomehow hack into the alien computer system and blow up the gigantic mother ship. Along the way, we’re told the mother ship has a mass equal to one-quarter the mass of the Moon and is in geosynchronous orbit above the Earth, more than 10 times closer than the Moon.
The ship’s gravity would cause huge tides, totally destroying coastal areas, and probably flex the Earth’s crust enough to cause catastrophic earthquakes. Still, the aliens send out smaller craft (each about 15 milesacross) to hover before unleashing flaming death-rays. Here, we cannot ignore Newton’s Third Law: to allow a ship to hover, there must be an upward force equal to the weight acting on it. So the ship must be exerting a downward force. If this involves air, the city beneath will be crushed by air pressure. So why waste energy on those death rays?
Adam Weiner is the author of Don’t Try This At Home: The Physics of Hollywood Movies, published in the US by Kaplan. Excerpted by permission from Kaplan Publishing, a division of Kaplan, Inc. © 2007 Kaplan, Inc
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