Physics Goes for a Vacation

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As avid movie-goers and people of science, we would have all been emphatically pushed out from the immersive experience that movies offer with sequences that stretch the laws of nature beyond belief and compromise the entire experience itself. By ‘laws’, I mean ‘physical laws’. A practitioner of science would surely find such sequences silly and careless from the side of the creators. Hence, what follows is a brief introduction to the several ways in which the laws of physics are violated in movie sequences, with a brief discussion on how the sequence would play out realistically. I am not considering movies where the plot establishes some logical or fictional point as to how the protagonist can yield such power but those where your everyday commoner wields it without any explanation as such. This only aims to cultivate an audience who cannot be tricked and who would in turn demand more from the makers themselves. Above all, its ultimate purpose would be entertainment alone. 

  1. Walking away from an explosion: Several movies portray this particularly interesting sequence where a protagonist walks away from an explosion in the background. Shockwaves from an explosion are pressure waves that travel at the speed of sound and usually have an amplitude of around 10 atmospheric pressure. In a realistic scenario, the blast would result in a vacuum followed by strong winds that pushes against objects, deforming and displacing them. In certain studies involving US soldiers in Afghanistan, the effects of the shockwave on the human body were studied, ranging from bursting of eardrums to extreme pressure on bones, breaking them, and deep neurological stress from the intense pressure experienced by the brain and its displacement with respect to the skull. Another source of danger is the projectiles i.e., the objects that may be launched by the explosion which can penetrate the human body. These projectiles acquire high momentum from the force associated with the drop in pressure due to the explosion and follow simple rules of momentum conservation. Hence, if one is anywhere near an explosion, it’s best to lie on the ground and take cover if possible.
  1. The Overpowered Jumper: The protagonist can be seen jumping over extraordinarily high walls and somehow changing their direction mid-flight by just sheer will. Changing direction mid-air requires one to generate enough thrust to change one’s linear momentum. However, generating this thrust is simply impossible without some solid body on which the person can exert a force and hence, in turn, propel themselves in a different direction. We can’t generate a force large enough mid-air so that the reaction force from the nearby air molecules can change our momentum. 

  1. Pressure control: In a famous sequence from the movie Mass (dubbed in Hindi as ‘Meri Jung One Man Army’), the main character is seen creating a mini-tornado, merely by moving his leg around in circles, thus creating a low-pressure region. To put things in perspective, a hurricane is expected to have an energy output of the order of 1017 joules per day. In a single second, this would be about 1013 joules. Humans can comfortably sustain an energy output of 200-300 joules per second and even an output as high as 2000 watts. Hence, generating a pressure difference that is enough to sustain a tornado is impossible.
  1. Controlled descent when falling: This isn’t exactly overlooking the laws out of a creative reason but a more practical one. When people rebound after landing a punch or jumping off a building, their bodies should fall at the acceleration due to gravity. However, what our eyes are treated to is a controlled descent, which is usually hidden by making the entire fall play out in slow-motion. This is usually done with strings as falling at the acceleration due to gravity would cause injuries to the actor. The primary reason for the same is the change in momentum they undergo and how fast this change happens. If one acquires a large velocity over time and their motion is arrested rapidly, the momentum change happens over a shorter time (i.e., a greater impulse is exerted on the body), resulting in some broken bones at the very least. However, with a controlled descent, as in the case of a parachute, the momentum is brought down or maintained in such a way that bringing the body to rest wouldn’t damage it. 
  1. Movies with elastic goons: A common trope observed in many action sequences is the goon who bounces. The protagonist kicks the goon and the goon lands with a thud on the ground. However, what follows is an immediate rebounding as they bounce off the floor and are raised to the ground only to be put back in place by the protagonist. The human body, in general, will bounce when the heights involved are sufficiently large or the surface of contact is itself elastic. But when the surface involved is dirt, rebounding is perhaps not the way to go for the scales involved and what one should expect to see is an effect similar to throwing mud against a rock.
  1. Falling into a black hole: Obviously, this article wouldn’t be complete without a reference to the (in)famous scene from ‘Interstellar’ where the protagonist Cooper’s ranger vessel falls into a black hole with him inside it. Although the depth one can reach in a black hole and survive depends on the size of the black hole, the eventual result would be that the differential in gravity i.e., the difference in the gravitational force as experienced by different parts of the body that are at varying distances from the singularity would be so high that the human body would be stretched or ‘spaghettified’ until it becomes impossible for any biological process to continue to operate. In short, one would die. Now, if the black hole were smaller, the differential is much larger even outside the point of no return (the event horizon) and one would die before reaching the event horizon. However, for a supermassive black hole, this differential isn’t as high outside the event horizon and hence a person can fall beyond the event horizon and survive. Nonetheless, the differential would soon become high enough for ‘spaghettification’ to occur. Moreover, the accretion disk (the ring of matter orbiting the black hole at relativistic speeds) would have had an inhomogeneous brightness with light coming towards Cooper looking brighter than the light moving away from him due to Doppler beaming. This means that the light looks brighter when it’s moving towards us and dimmer when it’s moving away from us. This is why the light from a lighthouse looks faint at an angle but extremely bright when directed towards us. 
Black Hole from Interstellar

How it would have looked realistically

All of this is not to say that there aren’t sequences where the laws have been respected. However, they are few and far between. Alas, the movies reflect the vision of the maker and any comments made here only reflect my interpretation of the same. There are several more inconsistencies that one can observe if one pays close attention. What we have covered so far is but the tip of the iceberg. I hope this only makes one appreciate the greater effort in making a movie coherent with the laws of physics and that I have not ruined your experience of classic sequences from movies.