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Comment by robomartin | Hacker News Reader

The first thought that popped into my punny little brain was: Why didn't they use a centrifuge to accelerate results?

The one variable that could be controlled is the gravitational force on the substance. If we were on the moon the experiment, as designed, would take far longer to produce a drop. By using a centrifuge they could have easily simulated significantly greater gravitational forces and arrived at a result much sooner.

EDIT:

The Queensland experiment data says that it takes about 13 years for a drop to form and fall [1]. You'd need less than 5,000 g's to make it happen within a day or about 160 g's to get results in 30 days.

[1] http://www.nature.com/news/world-s-slowest-moving-drop-caugh...

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Chemist here. We use centrifuging to separate components of a suspension by weight.

Pitch is a suspension. The heavy stuff would drop to the outside, the viscosity would be affected, no experiment.

Indeed, it occurs to me that one reason to keep watching the experiment is to see whether heavy pitch migrated to the bottom. If so, the rate should speed up in a few hundred years, assuming the hermetic seal is good and nothing is volatilizing off the top.

This is unlikely, however. Many suspensions have a critical pressure below which they simply will not settle. Homogenized milk, for a familiar example.

> Chemist here. We use centrifuging to separate components of a suspension by weight. Pitch is a suspension.

The last bit is puzzling: Are all suspensions fluids? And all fluids are viscous (even non-newtonian)?

If so, why do we need an experiment to prove that pitch --a suspension-- is a fluid and is viscous?

On the point of using a centrifuge. Is there something particularly special about gravity on earth that does not affect pitch? In other words, if we exposed pitch to 0.1x, 1x, 10x, 100x and 1000x g, at what point would the fundamental characteristics of the material invalidate the experiment. Are fluids and their viscosity only defined on earth (ignoring thermal and other non-gravitational effects)? Perhaps pitch is a viscous fluid on earth but a solid on the moon, where gravity is 1/6 of ours?

Is this experiment only valid at 9.8 m/s^2 then? What's the tolerance based on the substance under test?

No, all suspensions are not fluids. Amorphous rocks, for an example, often contain microcrystals in suspension. They will not flow under any circumstances. Glass can have all sorts of powders suspended in it, and is not a fluid.

Pitch can be proven to be a suspension via distillation. To prove it's a fluid, it must be observed to flow, to form a droplet, and to detach that droplet in the manner of a fluid.

Mathematician here. Yes, this is about to be a nitpick, and I apologize in advance.

However:

    > No, all suspensions are not fluids.

I think what you mean is "No, not all suspensions are fluids."

Interpreting what you said literally would imply that anything that is a suspension is definitely not a fluid. And I know one can interpret what you said according to common sense to deduce that you didn't mean that, and that since some suspensions definitely are fluids, then you must have meant that there are suspensions that are not fluids.

But this is about science, and where possible, I believe we should be precise.

I believe it matters.

Linguist here.[0] Yes, this is a nitpick, but no apologies, because this is Hacker News. :)

Unfortunately, languages don't always work the way that logicians would like them to do. Yes, we can read that sentence as the English-language summary of a logical formula (something like ∀x [Sx -> ¬Fx]), but English (and many other languages) treat the scope of the "not" as ambiguous. It's not a matter of "interpreting what you said literally", because there are multiple correct literal interpretations of what he said---he is not using a metaphor or anything. There are many cases where "not" can scope out to an enclosing quantifier (one of the better-known is "All that glitters is not gold", famously by Tolkien), and this is not just because all these speakers are in error.

[0] Also computer scientist, but I'll wear my linguist hat for now.

The mathematician had a better point.

For me, It violates the principle of least surprise to encounter a sentence like "all birds do not fly". I have to do a double take and reinterpret the sentence to realise its intended meaning.

IMO, if "all these speakers" collectively make the language ambiguous, then clearly someone is in error, because unambiguity[0] is essential to the primary purpose of language: communication. This has nothing to do with whether one is a logician or mathematician.

A sentence like "all X are not Y" naturally associates like "all X are (not Y)". Usually modifiers attach to the closest thing they can, as a matter of well-established convention. So if someone decides they want that to mean "not (all X are Y)" and expects me to understand it, I'm going to go ahead and say they're the one in error. Sorry, Tolkien.

[0] Is that a word? Chromium spell-check doesn't like it. I have lightened up on neologisms, as long as it's clear what they mean.

Despite the primary purpose of languages being communication, they occasionally fail at it. Does this surprise you? Have you never heard someone say something and completely misinterpreted them?

Ambiguity does exist in natural languages, and it is obviously no big problem.[1]

Unfortunately, technically inclined people like to apply their domain specific "knowledge" (compiler cosntruction?) to everything and come to incredibly bad conclusions. Ex falso quodlibet.

It gets especially bad when technical guys talk about law and juristic methodology. Demonstrated daily here on HN.

[1] BTW, linguistics is descriptive, not prescriptive. You immediately disqualify yourself from any linguistic discussion when you claim that native speakers are in error.

> IMO

Note: science is not about your opinion.

> unambiguity is essential

Is it? We function in the presence of ambiguity all the time. Sometimes, we let the ambiguity ride. Sometimes, we take a sentence that is (by itself) ambiguous and let the context disambiguate it. Sometimes, we use the ambiguity for some rhetorical purpose and then resolve it later. There are many sources of ambiguity in natural language---it's one of the things that makes NLP so damn hard---and you can't wave that away by saying that everybody is doing it wrong. (Not that generations of prescriptivist grammarians haven't tried.)

> Usually modifiers attach to the closest thing they can, as a matter of well-established convention.

First of all, attachment ambiguity is a difficult NLP problem specifically because modifiers don't always attach to "the closest thing they can". Second, to the extent that the statement is true (that low, i.e. "closer", attachment has a higher prior probability than high attachment), it is not a matter of "convention" but a fact about the grammar of a language. Third, "not" is special in all sorts of ways (as is negation in general, in many languages, not just English), and as I mentioned before, this specific kind of ambiguity is well-known, and interesting because it seems so strange, and once again you can't just wave the logic stick and declare that it ought not to be so.

The question was "are all suspensions fluids"?

My answer was "No, all suspensions are not fluids". The structure depended from the phrasing of the question.

The question asked if all X are Y. You want to assert that there are some X that are not Y. You can retain the original structure, and be correct according to the strict logic, by saying:

    Not all X are Y.

    Some X are not Y.

If someone came along with no knowledge of the field and read your statement: "all suspensions are not fluids." then it would be reasonable for them to deduce that for every suspension they looked at, it would not be a fluid. This is false.

To answer your second question, there is some critical number, which we do not know empirically, after which pressure will separate pitch.

We could apply some higher acceleration for a shorter time, but that acceleration would have to be highly consistent, and there is no way to shorten the experiment enough to keep that equipment from being unproductive to even contemplate.

Sometimes patience really is the best answer.

OK, had a break so I looked around a little to see what I could learn [1], [2], [3] and [4] are my launching points.

So I read that asphalt is actually a colloid and that pitch is not the same as asphalt. The Wikipedia page on pitch [3] says it is a "viscoelastic polymer".

So I have to ask. Can a polymer be a suspension or a colloid on its own or is pitch a polymer colloid in another "host" substance?

[1] http://en.wikipedia.org/wiki/Suspension_(chemistry)

[2] http://en.wikipedia.org/wiki/Asphalt

[3] http://en.wikipedia.org/wiki/Pitch_(resin)

[4] http://en.wikipedia.org/wiki/Colloids

Pitch is also a colloidal suspension, or colloid. The various polymers that make it up are not of equal weight, and there are little bits of graphite in there too. The chemistry of pitch is by no means simple, destructive distillation can go in many directions.

Learned a lot through this thread. Thanks.

Just letting a suspension sit on the laboratory bench does the same thing: gravity separates the components by density (not weight). That's why you stir paint before you use it: it has partially separated, and has done so without the without the use of a centrifuge.

It is in fact density, although in my defense I meant molecular weight. But was still wrong ;-)

However, there are many suspensions which will never settle under standard gravity, but which will rapidly settle in a centrifuge. We call these colloids.

Centrifuges don't provide a uniform field. The centrifugal pseudopotential is V(r) ~ Ω^2 / r^2; in effect this means that the pitch near the outside of the centrifuge experiences more "gravity". If you want to study pitch in a uniform gravitational field, you'd either need an absurdly large centrifuge -- the size of the Pentagon -- or a solar sail to Jupiter.

edit: also, the Earth's gravity might become a problem.

How long would you have to run the centrifuge for? If it were 6-7 times faster, you would have to run a machine nonstop for 10 years. Seems like a tremendous waste of energy, heat from a motor which could affect the results, and a great likelihood of experimental equipment failure.

No, you could go a million times faster. A million!

Here's an interesting resource:

http://www.endmemo.com/bio/grpm.php

Now, there might be reasons why a million g's might be a bad idea. Maybe 100K g's is a better choice. Someone with more expertise in centrifuge application could chip in and clarify this.

I'm not sure it would be valid. At a million g's, I'm a liquid. But most people would broadly speaking classify me as a solid (albeit one containing a lot of liquid).

> At a million g's, I'm a liquid

Not really. You'll disintegrate in strange ways based on the relative density of your constituent parts.

<edit>: Another thought is that if you laid on a bed for 13 years (the time it took for a drop of pitch to form) without moving --ignoring tissue decay and other factors-- you'd probably "flow" just as you might in a high gravity environment. Gravity, like rust, never sleeps. </edit>

Yet, unless a domain expert says otherwise, I think your point is absolutely valid. However, pitch might just be the kind of substance that would be fine at a million g's. I don't know.

That's the reason for my request for comment from someone with centrifuge experience. At a million g's a lot of seemingly strange things can happen. However, perhaps 100, 1000 or 10,000 g's is reasonable. I used the million g example as an illustration of the attainable extreme in response to a comment that spoke about 6 or 7 g's. Clearly we can do better, far better.

In consultation with a domain expert (Chemist?) I would design the experiment to give results in somewhere between a day and a month. Faster if possible. The criteria would be to choose an acceleration that does not fundamentally change the nature of the substance under test. All we want to do is promote faster flow.

I was admittedly being glib; obviously bone has different characteristics than fat. But characteristics can definitely unhappily change and I'd want an expert involved too.

Well, it looks like we have at least one Chemist pitching in (no pun intended). Time to learn a thing or two. Chemistry is one of those things I wish I had devote more time to in school. Lot's of physics and math but never dove deep into chemistry beyond what was absolutely required.

This is Ireland 1949, they probably did not had money for centrifuge.

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