Physics conundrums

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AuthorTopic: Physics conundrums
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quote:
Originally written by Victim Rich Environment:

Latour picks some good examples, among them the structure of DNA. There's really no need to worry about the structure of DNA anymore. It's so proven that trying to re-evaluate makes everyone think you're a lunatic. Working with that as a solid building block gives your own research a very solid foundation and makes your work harder to undermine.
And this example actually does a very good job of proving SoT's point, because we're just now learning that DNA doesn't always form a classical double helix structure and sometimes this is of biological significance. :P

One of the most frustrating things about science is constantly learning that things everybody knows aren't actually true, and that very often this is because people made simplifying assumptions that they knew probably wouldn't hold in all cases even back when they made them.

[ Wednesday, October 11, 2006 23:43: Message edited by: Thuryl ]

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SoT:
quote:
Molecules do not feel pressure, any more than shopkeepers feel GNP. Pressure is a concept that only makes sense for large numbers of particles treated collectively. Individual molecules do not follow the streamlines of hydrodynamics, and do not simply 'flow past' the wing surfaces.

(gives thick head a shake) I need to break the problem down past particle motion into molecular motion. Much more difficult, because as you pointed out Bernoulli applied his knowledge to fluid work and particles in streamlines. My explanation looked at the particles, which is why I couldn't separate Bernoulli from the answer.

We need to explain how large numbers of molecules can be treated collectively?

Well, I won't/can't answer that but I will state that in practice airplane wings are tilted at an angle, so in a windstream more of the bottom surface is exposed to contact with the air mass than the top surface, causing more molecular collisions to occur. But this only produces part of the lift.

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SoT:
quote:
we're not a bunch of engineers who turn out working products without necessarily understanding our own tools.
Do I need to know how to tune a piano in order to make good music?

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quote:
Originally written by Student of Trinity:

We're supposed to be able to explain everything from first principles; we're not a bunch of engineers who turn out working products without necessarily understanding our own tools.
But those two statements are entirely unrelated! Unless you work in a field that involves a lot of Newtonian mechanics, relating straight-line motion and angular motion is totally unrelated to the actual products (laws, equations) that you end up turning out.

Put another way: you can adequately understand quantum chromodynamics without an advanced knowledge of how a trebuchet works.

quote:
And the impulse that makes a physicist go, "Hey! Why can't I explain that precessing top?" is precisely the same emotion that makes them try to explain things that no-one can yet explain.
That just means that physicists are supposed to want to be able to explain everything, not that they are supposed to be able actually to do it. Having the impulse to know how everything works isn't the same as knowing how everything works.

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And my favorite example of one of Latour's "black boxes" that caused a fuss was Einstein's physics disagreeing with Newton's. The point is that a conclusion drawn from the double helix of DNA or one that can be explained by the double helix structure is not immediately subjected to queries of O RLY? The double helix is not a question.

Now I may be getting my history wrong, but when someone first proposed ssDNA, assuming it was after Watson, Crick, Wilkins and Franklin, the first reaction was probably not "sure." It was probably "go back and justify how you can possbily imagine a single, non-helical strand of DNA."

—Alorael, who considers the first chapter of Science in Action by Latour mandatory reading for anyone writing with citations. It's a how-to guide for disagreeing with people and making people agree with you whether they want to or not. On further thought, maybe it's actually mandatory reading for politicians. No, that's really the third chapter on how to get people to pay for you to break their knees so you can sell them wheelchairs.
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2bit: by 'particles' I guess you mean 'fluid elements' or something like that; in usual terminology, 'molecule' and 'particle' are interchangeable in the context of gas dynamics. No you don't need to be a piano tuner to make music. But theoretical physics is the lowest level science. Our instrument is maybe more like a guitar or a violin than a piano. And to play a violin well, you do need to know how to tune it.

I don't think you really can understand QCD without also understanding how a trebuchet works. In fact the QCD theta-vacuum involves stuff that's mathematically just like a trebuchet; but that's sort of a fluke, and not really a good counterargument to Kelandon's argument in general. The more serious point, I think, is that the theory of a trebuchet isn't that hard to absorb; if you come at all near it, you kind of can't avoid picking it up in passing. And come near it you will, because the first starting point towards QCD will be the harmonic oscillator, and soon after that, the physical pendulum as an anharmonic oscillator. QCD is an uncountable number of trebuchets, all hooked up together in tricky way, and quantized.

Physics really isn't very modular. Specialized and advanced stuff diverges, but everybody shares the same base, and the base stays active. You still use the stuff you learn in freshman year, forever afterwards. There's a good bit of F = m a even in string theory.

Now it's true that the people who can't explain why a spinning top precesses mostly manage to do other good work nonetheless. But it bugs people when they realize this is something they thought they understood, and they realize they don't. I think most people in physics do feel that they should be able to answer my five questions, even if they can't.

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quote:
Originally written by Student of Trinity:

1) How do airplanes really fly? The answer must be in terms of molecules bouncing off the wings, and not in terms of hydrodynamics! Lots of people know some pat answer about Bernoulli's principle. That's not what I want.

I don't know nearly enough about physics to answer this properly, but I like to think of planes as self-throwing machines. :)

[ Thursday, October 12, 2006 13:16: Message edited by: Ash Lael ]

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quote:
Originally written by Student of Trinity:

Specialized and advanced stuff diverges, but everybody shares the same base, and the base stays active. You still use the stuff you learn in freshman year, forever afterwards.
Absolutely. But what you're asking doesn't get taught in freshman year. It's not now part of that base. Are you saying that it should be?

quote:
But it bugs people when they realize this is something they thought they understood, and they realize they don't.
And this, I think, is the reason for the disquiet that I'm reading in your posts. I don't think you're really startled by the fact that people ought to know these things and don't; I think you're startled by the fact that people think that they know these things and only realize that they don't when asked to explain them.

It's of course unsettling for people to realize that they don't know what they thought that they did, but I think that's all that you've found here.

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quote:
Originally written by Victim Rich Environment:

Now I may be getting my history wrong, but when someone first proposed ssDNA, assuming it was after Watson, Crick, Wilkins and Franklin, the first reaction was probably not "sure." It was probably "go back and justify how you can possbily imagine a single, non-helical strand of DNA."
I wasn't actually thinking of single-stranded DNA; I was thinking of the really freaky structures like DNA/RNA triple helices that seem to form in certain parts of the genome under some circumstances. But I take your point.

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quote:
quote:
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we're not a bunch of engineers who turn out working products without necessarily understanding our own tools.
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Do I need to know how to tune a piano in order to make good music?

I thought you were poking fun at engineers. I acted mildly miffed. I wasn't offended. A failed attempt to inject humor into this topic.

I expressed my understanding of the first two problems w/o resorting to texts etc. Perhaps that is why my explanations are in part convoluted. I do not appreciate how molecules act in a streamline as opposed to larger sized elements. I do not appreciate at what point you cannot apply concepts of pressure and streamlines to elements. I.e. Radius of curvature of the wings works because it establishes different streamlines above and below the wing, setting up a low pressure zone which results in experiencing lift. But apparently (from my reading of your post) molecules within the streamline are not affected by either the streamline or differences in pressure.

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quote:
But it bugs people when they realize this is something they thought they understood, and they realize they don't.
Ignoring for a moment Kel's consideration that certain elements may no longer for the base of learning and accepting that at one time a piece of knowledge was "learned" it is not disquieting for me to meet someone who can no longer express understanding of it.

I accept that a piece of knowledge can at one point be proven to be true to the satisfaction of an individual. However, if that individual has no cause to interact with that knowledge regularly it is conceivable that the ability to interact with that piece of knowledge is forgotten. When forced to confront that knowledge again, the indivdual knows from previous experience that it is true and doesn't need to reprove it to use it - unless there is a necessity to show the connections (justify result back to first principles).

I think that the examples given can be interesting tests of what we remember (if we have been schooled in those areas) but failure to express the process shouldn't necessarily be considered as evidence of a failure to understand.

There are many things I work with that I merely look up in tables or refer back to texts/source material rather than commit to memory as a method of allocating scarce resources (time, memory) in order to accomplish the tasks set out before me.

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This is the point of my conundrums, apart from whatever interest they have as individual puzzles. There are different levels of description, for instance angular momentum and torque for rigid bodies versus momentum and force for particles, or pressure and streamlines for a fluid versus free motion and collisions for particles.

Having different levels of description is a profound thing in general, of course. Introducing a new level that actually works is one of the basic intellectual achievements. Like most other fields, physics is quite happy with having different levels of description. Where physics is perhaps special is that we insist that all levels be understood in terms of the bottom one. In practice we cannot always achieve that goal, but that is the goal. Higher levels of description are not autonomous in physics. To the extent that they are, one has ceased to do physics.

This does not mean that one must do every calculation in the most fundamental terms possible. It means that one should understand qualitatively whatever is going on in fundamental terms, and only rely on the higher level theory as an approximation technique for getting precise numerical values. So when one encounters a problem that one cannot answer even qualitatively in fundamental terms, this is a failure.

EDIT: This was written before 2bit's last post. As an addendum to react to that as well, I could mention that Einstein once cheerfully admitted that he did not know the speed of sound in air. He observed, though, that it was a simple fact which anyone could simply look up. What Einstein certainly must have known, however, was the basic idea of what sound is and how it propagates, in terms of molecular motion.

It is important that somebody is able to compute the precise speed of sound, and it would probably be good for everybody to do it once. It is not important to remember what it is forever after. But it is important to remember the conceptual relationship between molecules and sound. This is something basic and profound, an example of an enormous class of things in physics. Forgetting such a thing would be severe damage.

[ Friday, October 13, 2006 06:25: Message edited by: Student of Trinity ]

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SoT: You didn't really answer my previous post, so I'm going to re-state and rephrase it. You've already said that specialized and advanced branches of physics diverge from basic branches of physics. It seems obvious, then, that the questions you've posed come from specialized branches of physics, which means that they've diverged from everyone's basic knowledge, which is fine.

You've said that these things are basic and fundamental, but you've never really said why you think that they are. Clearly they are not, unless you can provide some reason to believe they are. In that case, why should physicists know them?

EDIT: Am I wrong in thinking that the most important thing is that new information be possible to be explained in terms of fundamental principles, not that it actually be explained that way, which is completely different? For it to be possible to explain new information in terms of old information, it's plenty just to know that someone has written a proof somewhere connecting angular momentum and Newton's Laws, although one may never have seen it.

[ Friday, October 13, 2006 07:09: Message edited by: Kelandon ]

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quote:
It seems obvious, then, that the questions you've posed come from specialized branches of physics...
The questions posed are not overly specialized. Theoretically someone with an undergraduate degree in physics or a similar field should, in principle, be able to answer them. They do not require any esoteric knowledge beyond basic, freshman and maybe sophomore level physics.

In practice, answering them in terms of the basic fundamental laws is difficult. The problem is that things get complicated very fast, so we create more "useful" concepts such as torques, fluids, etc. to make an explanation tractable. Solving the equation of motion for every molecule/atom in a system is too difficult most of the time.

To rephrase, F = dp/dt is always true, but it is not always useful or tractable when solving a problem. That's why, for example, we create things such as fluids, temperatures, pressures, densities, and enthalpies to do things like thermal hydraulics problems.

Another example is a simple inelastic collision. To solve this we can write down the conservation of momentum and the conservation of total energy (this includes the irreversibilities of the inelasticity of the collisions). The conservation of momentum follows directly from F = dp/dt as does the conservation of mechanical energy*.

What most people know is how to get initial and final states given some information. The details of the collision itself are really a black box in many of these problems. Another "simple" question would be to explain this collision without using conservation of momentum and energy. This requires a much more detailed insight into the actual collision itself to determine the final states.

* Conservation of total energy is a bit more ad hoc in some ways, but one could, again, reduce it all to basic molecular interactions if one had enough insight and computing time.

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Do molecular collisions have to be explained in terms of the repulsion of their respective electron orbitals?

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Oh my, cant we all just leave the science stuff on the midterm and exams. Physics is the last thing I want to think about on my off time.

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The physics of ice hockey maybe?

Now that they "improved" hockey and made it "family friendly," there is rarely a need for physiology in the shport.

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quote:
Originally written by Kelandon:

Well, I'm at least pretty sure that Salmon is losing.


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quote:
Originally written by VCH:

Oh my, cant we all just leave the science stuff on the midterm and exams. Physics is the last thing I want to think about on my off time.
You know, there are some people who take physics for more than just to fill some course requirements. It's too bad a lot of people feel that way; physics really is quite powerful and allows us to understand a lot.

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No one's forcing you to read the thread, either. It's quite clearly marked that it has to do with physics, so you had to know what the thread was about. If it doesn't look interesting, just ignore it.

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If you don't like physics, just think: every instant of your life, the electromagnetic fields and waves that are penetrating your brain, and every particle in the air you breathe, and in your entire body, are all practising physics, whether you like it or not!

Otherwise, I'm thinking about some of the above posts.

[ Saturday, October 14, 2006 07:14: Message edited by: Student of Trinity ]

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From the restroom graffiti in the physics building:

Drugs are for those people who can't handle reality.
Physics is for those people who can't handle drugs.

Still going back to first principles to use as a starting point for a problem is hard even at graduate level. There is a lot where you are taught to do things because someone else proved that this step has already been rigorously solve and not to waste time rederiving the result. I did have one professor give an example where one of these derivations did have a minor error that had been propagated through out several textbooks in electromagnetic scattering.
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I am absolutely serious when I say that introductory physics may have saved my life. I was standing on a roof painting when wasps atacked me, and I was able to quickly calculate roughly how fast I'd be going if I jumped off the roof and whether I would be in more pain than if I let the wasps get me. I jumped and suffered nothing more than wounded dignity.

Also, physics can explain why you don't spontaneously disassemble into your component molecules. That's pretty neat.

—Alorael, whose calculations while pursued by wasps used a great deal of approximation, including the perhaps worrisome ad hoc creation of an upper bound on speed at which humans want to hit the ground in free-fall.
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quote:
Originally written by Student of Trinity:

If you don't like physics, just think: every instant of your life, the electromagnetic fields and waves that are penetrating your brain, and every particle in the air you breathe, and in your entire body, are all practising physics, whether you like it or not!
Yes. Similarly, the invisible four-stomached steak flow is constantly affecting what you think and do, not least by its influence on the udder particles of which we are composed, and which are constantly bowed in the direction of the Nine-Headed Cave Cow, whether you like it or not!

...I'm not disagreeing, but I think you get that point :)

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quote:
Originally written by Ex Omnia Paratus:

—Alorael, whose calculations while pursued by wasps used a great deal of approximation, including the perhaps worrisome ad hoc creation of an upper bound on speed at which humans want to hit the ground in free-fall.
In practice, the maximum safe falling distance depends very heavily on your stopping distance, which depends on the surface you're landing on and the position you're landing in. If you don't know how to land properly, even falling half a metre onto a hard surface is enough to break something.

[ Saturday, October 14, 2006 17:33: Message edited by: Thuryl ]

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