The article's discussion of golf balls reads completely opposite to standard well-accepted theory. The author writes:
> [The "drag catastrophe"] is when an object is falling so fast that the boundary layer of gas separates off the object and the drag force suddenly drops by a factor of almost 10. The reason why golf balls have dimples is to cause this drag catastrophe to happen at slightly slower speeds, so the ball will travel a lot farther.
This is very confused. Golf balls have dimples to prevent flow separation. The dimples are turbulators meant to induce turbulent flow around the golf ball before the laminar flow would otherwise give way to flow separation. Far from decreasing the drag force, flow separation increases it substantially.
[Also, the term "drag catastrophe" appears to have no relevant hits on Google other than this one article.]
[Edit 1]: The author is well qualified and unlikely to be confused himself; so I don't doubt his conclusion. Reading charitably, turbulent flow might be called a form of separated flow, and this must be what the author means. His coefficient of drag graph supports this interpretation as his "drag catastrophe" would be happening when you would expect a transition flow (from separated laminar to turbulent). Pedagogically he should have more clearly distinguished it from the typical laminar separated flow.
I'd never heard of "drag catastrophe" before either, but I think you're right that he's referring to the onset of turbulent flow that reduces drag on a bluff body.
That's the sudden drop at the right in all of these graphs: www.google.ro/search?q=drag+reynolds+number (same as his own Figure 9, really).
There are a lot of surprising nonlinearities in those graphs. I wish there was a more detailed article/paper somewhere.
I'm confused if a low-enough Reynolds number implies that laminar flow is still happening, how would the dimples on a golf ball even create turbulent flow given they have such a low Reynolds number?
EDIT: Ah, the velocity determines increases so does the Reynolds number. So at around 55 mph, the Reynolds number is high enough to induce turbulent flow.
This article is fascinating and now I understand why golf balls have dimples.
Submissions like this is the reason I frequent HN! A thorough well written and well researched article that point by point dissects an interesting event and comes to a surprising conclusion.
If you come across other articles of this calibre please submit them!
They took a few seemingly unrelated things and uncovered just how intimately intwined they actually are. Although abstract, isn't that essentially what a good hacker does on the daily? I think so.
I came here to post that it was a great article - except for the needless invocation and anthropomorphization of Occam's Razor.
I. It is not clear that a rock falling out is _simpler_ than a meteorite falling out. How exactly is a meteor less simple? Sure, it's rarer and less likely, but it is not simpler.
II. A simple explanation of a how an iPhone works is "magic." But we know that's not the case. Our invocation of Occam is tautological - we only call on it when we think it's right, not when we're making a decision. We note that the article did not begin with: "there must be a simple explanation, because Occam's Razor always works." Instead, it invoked Occam after the fact.
Conclusion: Occam's Razor is a fetishized social construction and carries no magical properties and is of no use in predicted phenomena using a scientific model. It is simply a feel-good mantra like prayer and rosary beads.
"Simpler" is how it was formulated at the time... nowadays we'd probably formulate it in terms of going with the possibility that least amount of "luck" in its explanation. Something that requires four events with 99% probability is still more likely than an explanation that requires one at 0.004%, even if it is not "simpler". (Those numbers are just made up, not my guess at the odds in question... in reality, the gulf is probably even larger.)
I don't think the odds on filming a meteorite are so bad that it will absolutely never happen, low probability events do happen in the real world (people have been directly hit by meteorites, after all [1], and that's much lower probability than a meteorite passing through a camera's field of vision, which has a much larger volume per second), but for a given claimed meteorite filmed, the mundane explanations are more likely.
Restrict yourself to explanations which can be rendered as a series of individually simple steps--something you could reduce to math; something you could program into a computer. Note that "magic" does not qualify, here.
The shortest such series of steps that successfully outputs your observation is the most likely explanation for those observations. In this case, we would either need an exception to well-known rules of fluid dynamics, or an explanation why 2 years of searching in a small area yielded no meteorite. Either one of those is an added complication to the explanation which predicts stowaway gravel: P(A & B) <= P(A).
Unless you know magic exists and works, postulating "magic" fails Occam's Razor, since you're needlessly postulating that magic exists as part of your explanation. The hypothesis that an iPhone works by elecricity and clever engineering- since we have similar objects like TVs that work that way- is favored by Occam's razor.
You're right that it doesn't seem like you can apply Occam's razor to the falling rock. Really it seems like an example of Bayesian reasoning: we know the prior probability of someone coming anywhere near a falling space rock is very, very small. We know the probability of a piece of grit falling out of a chute is orders of magnitude larger. Shakey video evidence by itself barely shifts our confidence that it actually was a meteor.
"Magic" itself isn't an explanation, it's an excuse for not seeking an explanation. It can also be a category of explanation (that is you could develop an explanation which involves magic), but any explanation in that category is unlikely to actually be particularly simple in the sense of Occam's razor, in that it will involve invoking new entities that are not already needed to explain other phenomena. Occam's razor is simply a test of parsimonious when considering expansion of an effect in the context of all other effects.
weeeeeellll... I used to think along similar lines, until an entomologist explained to me that Occam's razor has a very valid place in the branch of biology called "systematics":
Systematics is the branch of biology that attempts to establish genealogical relationships among organisms. It is also concerned with their classification. There are three primary camps in systematics; cladists, pheneticists, and evolutionary taxonomists. The cladists hold that genealogy alone should determine classification and pheneticists contend that similarity over propinquity of descent is the determining criterion while evolutionary taxonomists say that both genealogy and similarity count in classification.
It is among the cladists that Occam's Razor is to be found, although their term for it is cladistic parsimony. Cladistic parsimony (or maximum parsimony) is a method of phylogenetic inference in the construction of types of phylogenetic trees (more specifically, cladograms). Cladograms are branching, tree-like structures used to represent lines of descent based on one or more evolutionary changes. Cladistic parsimony is used to support the hypotheses that require the fewest evolutionary changes. For some types of tree, it will consistently produce the wrong results regardless of how much data is collected (this is called long branch attraction). For a full treatment of cladistic parsimony, see Elliott Sober's Reconstructing the Past: Parsimony, Evolution, and Inference (1988). For a discussion of both uses of Occam's Razor in biology, see Sober's article "Let's Razor Ockham's Razor" (1990).
Other methods for inferring evolutionary relationships use parsimony in a more traditional way. Likelihood methods for phylogeny use parsimony as they do for all likelihood tests, with hypotheses requiring few differing parameters (i.e., numbers of different rates of character change or different frequencies of character state transitions) being treated as null hypotheses relative to hypotheses requiring many differing parameters. Thus, complex hypotheses must predict data much better than do simple hypotheses before researchers reject the simple hypotheses. Recent advances employ information theory, a close cousin of likelihood, which uses Occam's Razor in the same way.
(end quote)
Of course, if you read the above and maybe follow a few related links from the WP article, you'll see that it's not quite as straight forward as that, Occam's razor / cladistic parsimony isn't always valid in systematics, but it's an indispensable tool because the search space of valid solutions is so huge, they need a kind of heuristic to guide the way to the most scientifically interesting results (this then becomes not about finding the one correct truth, but about deciding which part of the bigger truth to uncover first).
This isn't just about the Occam's Razor. The author goes through a thorough analysis of the evidence. Surely a forged video is a simpler explanation, but given the fact that the trajectory and the timing of the rock matches a simulated timing and trajectory and timing of a gravel falling out of the parachute, than the most probable explanation is the stowaway piece of gravel.
That's my not-so-subtle way of claiming a late victory on an internet argument since in this thread all the people claiming that it was more likely that it was something else were systematically downvoted.
On a less narcissistic note it's interesting to see the effect of different articles taking different angles on the comment section, I guess even HN is not proof to confirmation bias and hivemind.
"It turns out that when the rock size is set to about 3 centimeters in the simulation, it passes the skydiver at 12 seconds. This rock size just happens to have the same terminal velocity as the solution we found by matching the velocity seen in the video."
Hm. Poking around[1], drogue/pilot chutes seem to be around 30 inches or 75 centimeters in diameter. I wonder how you'd get a 3cm piece of "gravel" mixed up in that?
The pilot chute can be hastily scrunched up and shoved into the bottom of the container ("backpack") without any adverse effects to parachute deployment, so it's plausible that the rock was stowed by accident.
“It can’t be anything else. The shape is typical of meteorites – a fresh fracture surface on one side, while the other side is rounded,” said geologist Hans Amundsen.
Every hoax just needs one credible expert to put their name to it. This story may not have been a deliberate hoax but it ultimately was a hoax. It is good to see something debunked with some good, old-fashioned science!
I'm pretty sure a hoax requires a deliberate deception.
From the definition:
Noun: something intended to deceive or defraud
Otherwise it's just a mistake, misunderstanding, or other form of "just being incorrect". If they said "we know it's not a meteor, let's tell everyone it is anyway" - then it's a hoax.
> [The "drag catastrophe"] is when an object is falling so fast that the boundary layer of gas separates off the object and the drag force suddenly drops by a factor of almost 10. The reason why golf balls have dimples is to cause this drag catastrophe to happen at slightly slower speeds, so the ball will travel a lot farther.
This is very confused. Golf balls have dimples to prevent flow separation. The dimples are turbulators meant to induce turbulent flow around the golf ball before the laminar flow would otherwise give way to flow separation. Far from decreasing the drag force, flow separation increases it substantially.
[Also, the term "drag catastrophe" appears to have no relevant hits on Google other than this one article.]
[Edit 1]: The author is well qualified and unlikely to be confused himself; so I don't doubt his conclusion. Reading charitably, turbulent flow might be called a form of separated flow, and this must be what the author means. His coefficient of drag graph supports this interpretation as his "drag catastrophe" would be happening when you would expect a transition flow (from separated laminar to turbulent). Pedagogically he should have more clearly distinguished it from the typical laminar separated flow.