Wednesday, April 8, 2009

Tropic Thunder and the Inevitability of Physics

In the movie “Tropic Thunder”, the character played by Tom Cruise makes a statement: “Speedman is a dying star. A white dwarf headed for a black hole. That's physics. It's inevitable.” He then proceeds to dance while his goofy yes-man also dances in the background while shoving money in the face of Speedman’s agent to get him to stop worrying about Speedman’s plight. It’s funny as hell. I laughed a considerable amount. However, in the back of my mind, I kept thinking “You're misrepresenting physics!!!” Maybe I’m taking too much from a blockbuster absurdist comedy, but I couldn’t help but think, “Maybe this is how people perceive physics. Maybe they think it’s inevitable, certain, and complete -- movies are a decent representation of mass cultural attitudes” So... is physics inevitable?

While a goal of physics is to predict what will happen in the physical world based upon observations of what the physical world has done so far, it is far from inevitable and complete. Physics, like all sciences, is based upon a unifying idea (or theory) supported by observations (empirical results). The results presented in classical physics have been confirmed time and again, but that does not mean that they are not subject to change.

Now, did Newton start with observation and then move to creating a system to explain those observations? I have no idea. In fact, it doesn’t matter. What does matter is that the theory is backed by observations.

Observations of what? Well, glad I could write in a format to force that question from you! Classical physics, and physics in general, is an exercise in constructing a system of understanding of the entire physical universe. Usually physics takes a “subtractive” approach – it tries to find a root cause for events. We could construct an understanding of the physical world by testing ideas that deal with, say, the chemical make-up of a substance. While that is important to physics, it’s not the end. Instead, it tries to see what all objects do (Though the cross-over between physics and chemistry is great, they’re still very different. More to come in future blog posts!).

So, to give a starting point, from general observations we can say that all objects seem to be subject to movement. They move from point A to point B. There isn’t a single object that doesn’t seem to do this. While it is possible that such an object exists, we have no reason to believe that it does, and if something doesn’t move, from our everyday experience, it’s usually because something is in its way, not because the object itself simply does not move by virtue of a characteristic internal to itself (something usually referred to as an “intrinsic” or “intensive property”).

So, all objects move, that’s terrific. But how? What causes an object to move? Well, this can be answered in several ways, but I think the simplest answer is “ a Force”. You may be familiar with the term from high school physics and Newton’s Second Law expressed mathematically: F = ma. But if not, the concept is fairly simple and associated with the normal definition: Force is something that pushes an object. You force a door open. You shove a box. You pull a rope. Gravity pulls you towards the ground. All of those are forces, and "Force" is just the general concept for anything such as that. You’ll notice that Force (the F) is defined here by two terms, m and a, defined as mass and acceleration respectively.

Well, no shit!, you say, of course objects move because of acceleration! Hold your horses, I ain't done. If we divide the equation by the “m” term, then we get

F = a

Now, to help with the explanation to come, I want to go over the classical physicist’s definition of mass:

Mass is the property internal to an object that resists changes in movement.

It's a different definition than what is normally used, namely because it's not as intuitive, nor does it really reference things we think about on a day to day basis. Normally, mass is explained simply as "Stuff", or "Matter". But there is a reason for this particular definition – because we’re dealing with movement of an object, we define mass in terms that includes only that object, instead of the “stuff” that an object might be made of. You’ll note, if we somehow had a way to quantify Force (and we do), that if we have a larger force then we’ll have a larger acceleration, because “Force“ is on top. Simultaneously, if we had a way to quantify mass (which we also do), then we’ll have a smaller acceleration. Intuitively speaking, this is because the object we’re dealing with is heavier. Think of shoving a basketball. Now think of shoving a bowling ball. Which seems harder to move? Which moves faster if you were to shove them both with the exact same amount of Force? Well, the basketball would accelerate more of course.

This brings us to another one of those stickler points that physics text-books get hung up on: “Acceleration”. In day-to-day speech, it’s actually a lot closer to the physics definition than its given credit, I think. But then, we usually don’t say “I wish that fucker in front of me would accelerate!”, it’s normally more like “Damnit, speed up!”, and that’s where the hang up occurs. Speed, in the jargon of the physicist, can be equated to velocity in some direction, and velocity is the measure of how much time it takes for an object to move from one place to another, or the measure of how far an object would go in a given amount of time if it were to continue going at the same speed in that direction (which is exactly what your speedometer measures). So, again back to the jargon, acceleration is a change in speed. In specific, it’s how much time it takes for you to change your speed from one velocity to another, which itself is just a measure of how fast it takes to get from point A to point B. Think of your speedometer, again. When you start to push the "Go" pedal, usually it only takes a few seconds to reach the "30 mph" mark, but it takes longer to go from "70 mph" to "80 mph" -- your acceleration is less than what it was, but your velocity is greater. This all relates back to the movement of objects -- in physics, we call this displacement -- And now we’re back to where we began: movement! Objects move. How? Force. A force changes an object’s acceleration, a measure of how fast an object changes its speed in some direction, which in turn is a measure of how fast an object goes from point A to point B. And now we have a simplistic sort of explanation that we can attempt to apply to all objects, because all objects move, and all objects have mass.

Now, back to Tropic Thunder and the inevitability of physics: This is a basic explanation of the concepts held in Newton’s Second Law, but it’s not comprehensive. While I’ve made an argument for an explanation to explain all objects, we have to go out and test the ideas. To do a test, we can’t just say “Yeah, looks pretty good”, we have to have measurements. This is an attempt at being objective. People are biased, especially when they develop ideas of their own, so while pure objectivity is impossible, a strong attempt can still be made. Usually, as physicists, we rely upon math to create rules. This allows us to find numbers that, if our original thought is true, should correspond to measurements that we’ll make during the experiment. It also removes a lot of interpretation from data. Now, numbers can be manipulated in a number of ways (pun intended), so as a physicist, you usually try to stick to the raw measurements as much as possible to give support to a mathematical expression of an observation. If those observations don’t match up to what you thought you'd get -- for example, if you push both a bowling ball and a basketaball with "1 Force", and you measured their masses beforehand, you would be able to find what acceleration you should get, but then find that their accelerations are totally different from what you predicted -- then the experiment wins out. The theory (or law, or hypothesis, or whatever) is bunk. The end. Game over. Back to the drawing board. Try something new.

And that’s why the line in Tropic Thunder kind of bugs me – physics is one of the oldest sciences (at least as we currently understand the term "science"), and classical Newtonian Mechanics have a metric-fuck ton (This really should be an SI unit) of experimental support, but that still doesn’t mean “It’s Inevitable”. It means, up to this point, under the assumptions made by Newtonian physics (namely the three laws), our experiments have matched up to the theory. Simultaneously, this doesn’t mean you can just throw out all the experiments that have been collected and start from scratch – that would similarly be neglecting the experimental side of physics (as well as the idea that we usually build upon previous findings, rather than start over) – but it does mean that physics is confirmed by experiment, and is therefore subject to change if further experimentation reveals a flaw in the theory.

I think this point is missed mostly because in class it’s a lot easier to grade tests where there’s a numerical answer at the end. In addition, it’s easier to write tests that require you to apply theory. It’s also easier to teach to a test written in this manner rather than teaching about the thought that goes behind science. Plus, theory is important to understand and implement, so it's not bad to have these skills. But science is not that simple. And I find it rather sad that “Physics” is inevitable, but “Biology”, a science applying the same epistemic principles as physics, is held in contention for its fundamental theories. But, that is the subject of a blog for another day! This one’s already long winded enough.

PS: I say this is a simplistic explanation, because, well... it's simplified – some questions you may have asked during this explanation might include “Where does Force come from?”, “What happens when objects hit each other?”, and “How does that equation explain objects moving in more than one direction?” – and those are great questions to ask. Keep up the inquiry! Personally, I recommend taking a class because nothing can replace a teacher, but you can usually find a cheap physics text book, or check one out from your local library, or find other information on physics on the internet -- or, if you're feeling really daring, you could set up an experiment.


  1. Did you ever end up taking a look at the book 'How to Think About Weird Things'? The idea of building upon previous experiments reminds me of something they said in there. Something like: All other things being equal, a hypothesis that is conservative (i.e. fits with the current model) is a better hypothesis.

    Also, one little quibble. Isn't acceleration change in velocity, not change in speed? I could be wrong, you've taken physics a lot more recently than I have.

  2. This comment has been removed by the author.

  3. Yes, you are completely correct: Acceleration is a change in velocity. I'm still trying to figure out at what point I should use technical jargon and at what point I should explain concepts. It would probably be better, for correctness' sake, to use "velocity".

    And NO I hadn't read it yet! But I was facebook stalking you trying to find the name of the book earlier. Now I have it, and I will find it after I finish reading what I'm reading now.