"Ask a Scientist"/General Science Discussion

[kristin]

re:

What's the airspeed velocity of an unladen swallow?"

is that african or european?

If anyone really wants to know about this: http://www.style.org/unladenswallow/

[ceastman]

Well, this thread certainly took off! (Maybe it's being carried by swallows. Of course, being laden, they're flying less rapidly than they otherwise might.)

Catharine, I found your answer on the chromosomes very interesting, but it kind of raised another question in my mind. Specifically, I was wondering if there was any *evolutionary* reason why the Y chrom is smaller than the X.

My reason for asking this is that I seem to recall that in birds, the sex determination operates in the opposite way from mammals -- i.e., if there are two "W" chromosomes, you get a male, while if there's a "W" and a "Z", you get a female. And I seem dimly to recall that the "Z" is usually pictured as smaller than the "W".

Is that just a convention? Or do the "anomalous" chromosomes that determine gender (Y --> male in mammals; Z --> female in birds) really tend to be smaller than the "normal" versions (X and W, respectively)? If so, does anyone have any idea why?

So, this isn't exactly my area of expertise. There's a good for-the-intelligent-nonscientist article here at the University of Chicago's Hospitals page. I'll summarize it below.

The sex chromosomes likely started off as another pair of same-length chromosomes just like the other autosomes. (There's a number of genes on the Y chromosome that are very closely related to genes on the X chromosome - that's the basis of the theory.) The scientists postulate a few stages in the evolution of the Y chromosome.

(1) About 300 million years ago, a gene on what later became the Y chromosome mutated to become the SRY gene - the Sex-determining Region Y, which acts as a master switch for creating a male organism.

(2) Over time, the X and the Y-protochromosome gradually lost the ability to recombine. (During the cell division process that takes place when eggs or sperm are formed, chromosomes in a pair basically trade bits back and forth with each other; the process is called recombination.) I don't think it's known exactly how this process took place. But the gradual inability to trade material back and forth meant that each member of the increasingly badly-named 'pair' was essentially free to go its own way.

(3) In the case of the Y chromosome in particular, four different inversions took place. An inversion is basically a long chunk of DNA 'inverting' its direction: the sequence that was at the front of the segment is found at the back end, and vice versa.

(4) These inversions tended to introduce really terrible mutations into the genes that they happened in. (These are in addition to spontaneous point mutations that must have happened in the parts of the Y chromosome that didn't invert.) The bad segments then tended to get deleted over time - the DNA in those areas was physically removed. This resulted in the Y chromosome getting shorter and shorter over time.

So... The naming is simply history and convention. But yes, there's biological reasons as to why the difference exists in the first place.

-Catharine

[ceastman]

For example, why is carbon black? Is it the way that groups of carbon atoms are physically arranged? Does it have something to do with the properties of the electrons around its orbit? What is it about carbon that makes it so that no light, or very little light escapes it?

I know even less about this question; a chemist I mostly ain't. But consider this:

Graphite (a form of pure carbon in which the atoms are arranged in sheets that slide across each other) is black.

Diamond (a form of pure carbon in which each carbon atom is linked by four single bonds to other carbon atoms all the way through the crystal) is clear and colorless.

I'm guessing it has to do with the physical arrangement and bondings between the carbon atoms.

[ahab]

Well, this thread certainly took off! (Maybe it's being carried by swallows. Of course, being laden, they're flying less rapidly than they otherwise might.)

Catharine, I found your answer on the chromosomes very interesting, but it kind of raised another question in my mind. Specifically, I was wondering if there was any *evolutionary* reason why the Y chrom is smaller than the X.

My reason for asking this is that I seem to recall that in birds, the sex determination operates in the opposite way from mammals -- i.e., if there are two "W" chromosomes, you get a male, while if there's a "W" and a "Z", you get a female. And I seem dimly to recall that the "Z" is usually pictured as smaller than the "W".

Is that just a convention? Or do the "anomalous" chromosomes that determine gender (Y --> male in mammals; Z --> female in birds) really tend to be smaller than the "normal" versions (X and W, respectively)? If so, does anyone have any idea why?

So, this isn't exactly my area of expertise. There's a good for-the-intelligent-nonscientist article here at the University of Chicago's Hospitals page. I'll summarize it below.

The sex chromosomes likely started off as another pair of same-length chromosomes just like the other autosomes. (There's a number of genes on the Y chromosome that are very closely related to genes on the X chromosome - that's the basis of the theory.) The scientists postulate a few stages in the evolution of the Y chromosome.

(1) About 300 million years ago, a gene on what later became the Y chromosome mutated to become the SRY gene - the Sex-determining Region Y, which acts as a master switch for creating a male organism.

(2) Over time, the X and the Y-protochromosome gradually lost the ability to recombine. (During the cell division process that takes place when eggs or sperm are formed, chromosomes in a pair basically trade bits back and forth with each other; the process is called recombination.) I don't think it's known exactly how this process took place. But the gradual inability to trade material back and forth meant that each member of the increasingly badly-named 'pair' was essentially free to go its own way.

(3) In the case of the Y chromosome in particular, four different inversions took place. An inversion is basically a long chunk of DNA 'inverting' its direction: the sequence that was at the front of the segment is found at the back end, and vice versa.

(4) These inversions tended to introduce really terrible mutations into the genes that they happened in. (These are in addition to spontaneous point mutations that must have happened in the parts of the Y chromosome that didn't invert.) The bad segments then tended to get deleted over time - the DNA in those areas was physically removed. This resulted in the Y chromosome getting shorter and shorter over time.

So... The naming is simply history and convention. But yes, there's biological reasons as to why the difference exists in the first place.

-Catharine

Fantastic, Catharine! A great explanation -- thanks.

[Jc]

My question, though, is why nitrogen and oxygen preferentially scatter blue light.

I would dig out my Waves, Light, and Modern Physics notes, but I think I've thrown them away. From what I remember, it had to do with electrons. The energy in an electron varies depending on the layer it orbits on, ie electrons nearer to the nucleus have less (i think) energy than electrons further from the nucleus. When a photon hits the electron, it jumps on a higher energy level (goes away from the nucleus) and releases a photon when it goes back to its ground state (unexcited state). Electrons on different layers release different wavelengths of photons when returning to their ground state, hence different colors.

For example, why is carbon black? Is it the way that groups of carbon atoms are physically arranged? Does it have something to do with the properties of the electrons around its orbit? What is it about carbon that makes it so that no light, or very little light escapes it?

As for why things are of certain color, my guess would be because it depends on its atomic structure.I
let's say that particle A1 has 1 electron and is yellow, and particle B2 has 2 electrons and is blue, and when they react, produce particle C which is purple. Well, due to the bonds between the atoms, the energy of the 3 electrons have changed, thus releasing a different wavelength when returning to their (new) ground state.

As for carbon being black, i think it's because the molecule is quite stable and it's hard to excite the eletrons, or rather, it's hard for "normal light" to excite the electrons, which would explain why it doesn't release any visible light (though it migh always be releasing infrared light or UV...)

And why Diamonds are transparents, it might be because the cristalline structure allows light to go through quite easily?

[Jc]

I have a genetics question for our resident geneticist!!

How do they test DNA for genetically transmitted diseases? What I mean is, do they splice the chromosomes and run it through gel to check if you have such and such bit, or do they take a bit and actually decode it?

[Planish]

cs technician.)

Analogy - Sound ... sound is also a wave form ... if you or I spoke ( in a normal voice ) directly at a brick wall a person on the other side would NOT hear the sound as the wall would absorb some of the sound and reflect the rest .... And now enters the elephant .... Elephants can generate infra sound a very low frequency sound wave .... this infra sound will pass through solid objects ( our brick wall ) unhampered !!

Not "unhampered". In order to pass through a brick wall (assuming it's infinitely wide and high so as not to allow for any sound passing around it) some of the mass of the brick would actually have to be moved back and forth. As with all mechanical systems, there would be some loss of kinetic energy (of which acoustic energy would be a special type, I suppose), into heat. As far as that goes, even when passing through air there are losses. Not to be confused with the decrease in sound pressure level with distance, according to the inverse square law.

However, if I recall correctly, low-frequency sound involves more kinetic energy in order to be measured as the same level as mid-range or high frequency sound. That's why sub-woofers need a lot of power and cubic volume (meaning the spatial dimensions of the speakers, compared to, say, piezoelectric alarm beepers) to be able to push enough air back and forth. That's the kind of energy you get when you hear a van driving by going "thumpa-thumpa-thumpa...".

.... all things are comprised more of space ...empty space than of particulate matter ... my guess is that the higher frequency needs more of this space to move through than does the lower frequency ... and then there is the factor of volume for if you throw enough sound or light at a solid object some of it starts to leak through ( put you hand over a flash light and you will see this effect - the low frequency redness ))

The redness from shining a flashlight through your hand is due to the white light passing through the blood and being filtered from the non-red wavelengths being absorbed, just like through red glass. If a Vulcan tried it, it would come out green.

Still, it is possible to make analogies between sound and light. Interference patterns of light produce fringing effects, with light and dark areas. Similarly, with pure tone audio sources (single frequency, sinusoidal) the reflections off of walls and objects can produce interference patterns giving you in "dead spots" in a room where an alarm is beeping. Especially so if it's up around several thousand kilohertz. Move or turn your head just a few inches, and it becomes markedly louder or softer. A real pain when you're trying to locate the source, like an unattended wristwatch alarm somewhere in the room.

With many ultrasound applications, the wavelength of the sound (inversely proportional to the frequency) is typically smaller than the object of interest. 20 kHz (in air) has a wavelength of 17 millimeters, 40 kHz is 8.6 mm, 100 kHz is 3.4 mm. That's about the size of a bug that a bat would be looking for, and bats can make squeaks from 14 to 100 kHz.
Because the human body is mostly water, and the velocity of sound is much greater in water than in air, medical ultrasound scanning frequencies need to be much higher than bat squeaks, on the order of 2 to 15 megaHertz, in order that the wavelengths are closer to (or less than) the sizes of the "objects of interest".

Getting back to light frequencies, and passing through stuff, it gets pretty complicated. Broilers (infrared) heat up the surface of food, and then the heat is conducted inward. Microwave ovens generate an electromagnetic frequency of (typically) 2.4 GHz, which is a bit lower in frequency (and longer in wavelength) than infrared, such as you would get from a broiler. the longer wavelength allows it to penetrate a bit deeper before it's absorbed and does the cooking thing.

So ... now I'm confused too, because visible light (also electromagnetic radiation) does not penetrate very far. On the other hand, X-rays are much higher frequency than visible light, mostly pass through meat-like materials, and are used for imaging peoples innards. Communications microwaves (such as C-band satellite, 6 GHz going up, 4 GHz coming down) are more affected by "rain fade" than, say commercial FM radio (88 - 108 MHz) because the wavelength of FM radio is so much greater than the dimensions of a raindrop.

I confess I'm out of my league here. There must be several things going on here that I'm missing. Let's see, wavelengths, density, refraction, and -whoops- transparency! I forgot about transparency, and I can't recall why several feet of glass will pass light (most of it) that would be stopped by a few nanometers of aluminum. I should have remembered that when graphite vs. diamond was mentioned. Scattering too. I forgot about scattering.

On the other hand, my sons and I have worked out several of the features and behaviours of a "four-dimensional cheese slicer". This started out over dinner as a thought experiment, and got quite elaborate as we figured out what phenomena would be observed when you used it in different ways, extrapolating from uses of a conventional cheese slicer. No idea of how to make one, we just know what it could do, and what the cheese would look like on video. We came to some very alarming conclusions, including a way to use it as a kind of explosive device.

[LeonMire]

Carl Sagan gave a really nice illustration of the concept of a fourth dimension. If you have a glass cube, with clearly marked edges, it will cast a two-dimensional shadow of its three-dimensional shape. The shadow of the glass cube is the shape made when you try to draw a cube on paper, like in math textbooks. Now we can't see a four-dimensional glass "cube," but we can see its shadow. Its shadow is the glass cube itself. Just as three dimensional objects cast two dimensional shadows, so four dimensional objects cast three dimensional shadows. Real wiggity.

However, that doesn't mean I have any idea what you mean by a four-dimensional cheese grater, nor do I understand how it could be used as an explosive.

[Jc]

A fourth dimension would be one that is "perpendicular" to all 3 of our dimensions: in the same way the y-axis is perpendicular to the x-axis, and the z-axis perpendicular to both the x and y axis, the axis for the 4th spacial dimension would have to be perpendicular to all x,y,and z.

Now it's hard to imagine what a 4th dimension would look like, since we only live in 3, but we can "project" a 4D object (for instance, a cube) into 3D, just as we can draw a 3D cube on a sheet of paper. I saw an animation of a tessaract (or hypercube/4Dcube) once, but I can't find the website...

[Planish]

EDIT : "four-dimensional cheese slicer" - this I'm curious about ... ( not sure how it fits into the light/sound wave thing ) ...

No connection whatsoever. It was a bit of misdirection on my part, so that I could hold forth on something that I thought might be amusing and that I knew a bit about.

For the purposes of the discussion, yes, we used time as the fourth dimension.

It requires also that you have a block cheese that normally comes into existence at some time, and then ceases to exist ... poof, like that. We can then introduce the term "volume-life", where "life" is the duration of that given volume of cheese. The unit "litre-hours" was convenient. It's similar to "foot-pounds" for torque, or "man-hours" for labour.

A conventional 3DCS ("three-dimensional cheese slicer") take a single block of cheese of some volume, and severs part of it such that some of it may be translated in space (say, to a sandwich) and the remainder of it may stay where it was, or be translated to a different place, such as the fridge. You now have two volumes of cheese separated by 3D space, but the total volume of cheese is the same.

With a 4DCS, a portion of the block of cheese's volume-life (say, 10 cm by 10 cm by 10 cm for 10 hours - that's 10 litre-hours) may be severed such that you then have a litre of cheese that pops into existence for 9 hours and 30 minutes, disappears *poof* for a while, pops back into existence for 30 minutes more, and then *poof* disappears forever. There is still only a total of 10 litre-hours of cheese.

If you cut the block of cheese a bit crooked, what you get is a block that exists for a while, then begins to shrink to nothing. Later, it re-appears as a tiny dot and expands to full volume, then stays at that volume for a while, and finally disappears *poof*. The rate of contraction and later expansion is directly related to the "angle" of the 4D cut.

If you cut diagonally, it begins shrinking to nothing immediately, does not exist for a while, and then expands out of nothingness until it's full volume again, at which point it disappears *poof*.

We're not sure what happens if you cut it in half the 4DCS equivalent of "lengthwise", whatever that is. We speculate that each half goes to a different parallel universe for 5 hours, simultaneously.

If you slice the entire block into enough (say, several thousand) pieces, it will appear to "strobe" in and out of existance. If the intervals between the resultant volume-lives are 1/60th of a second, and it's in fluorescent lighting, it may take on a colour shift, because fluorescent lighting often flickers sort of orange to blue 60 times a second (or 50 per second in the UK and other places, I gather). If they happen to exist only while the lights are in the blue phase, the cheese slices will appear bluish as well, but 50% transparent. If the interval is ever so slightly greater or less than the frequency of the mains voltage, then the slices will shift in colour slowly.

How is this a useful machine, you ask? Well, suppose you are living in an area where there is an energy shortage, and they cut the power for 6 hours a day as part of a rotating blackout. You don't want the cheese to spoil while the refrigerator is not running, so you cut it into 18 hour pieces, and separate them by 6 hours so that they only exist while the fridge is operating. It's like not having a container big enough to hold something, so you cut the object into pieces to fit several containers that you do have.

As for the explosive part, I don't recall exactly how it was to be done. I'm waiting for a reply email from one or both of my sons who were in on the original discussion. I think it involved the notion that when cutting the volume-life into pieces, you could also have it automatically set the time displacement for the slices, essentially sending it off to materialize in the future.

If so, then cut the 10 litre-hour block into ... uh, *calculate* *calculate* *calculate*... 1,000,000 pieces, at a slight angle. Get them all to begin rematerializing at the same time. For almost 36 milliseconds, there will exist a cube of cheese 10 meters on a side, rapidly expanding from nothingness, and then just as rapidly shrinking out of existence. That's gotta do some damage. If you had cut the slices straight, the appearance and disappearence would instaneous, with the full volume (of 1 million litres) existing for the full 36 milliseconds. Want it bigger? Cut them only 36 microseconds long, for a cube that's 100 meters on a side.

It's possible that 4D cheese can spontaneously sever itself into multiple shorter-duration volume-lives, but we would be hard-pressed to tell because if they did not also spontaneously translate the pieces in time, we might not be able to tell the difference from the uncut 10 litre-hour block. It's like cutting a slice of cheese with a 3DCS, and not moving the pieces elsewhere. The block looks the same.

----------------
Tesseract animation: http://en.wikipedia.org/wiki/Tesseract

I would also recommend reading FLATLAND - A Romance of Many Dimensions http://xahlee.org/flatland/index.html
By golly, it's at PG too, with ASCII Art illustrations: http://www.gutenberg.org/dirs/etext95/flat10a.txt
I've seen a short animated version of it too.

Space-time: http://en.wikipedia.org/wiki/Space-time
If you can find it, there are recordings of Feynman's lecture on Space-Time around.

[Jc]

man... that cheese slicer thing is confusing!!

If you cut diagonally, it begins shrinking to nothing immediately, does not exist for a while, and then expands out of nothingness until it's full volume again, at which point it disappears *poof*.

From what I've understood of the cheese slicer, it would depend on how far away in time you "carried" the cut half, but basically you'd have half the block disappearing *poof*, the remaining half existing for 10 hours, then the other half reappearing *poof* later, and again existing for 10 hours.

Took me some time to understand it, but I now have a pretty clear mental image of the process. Here's a graph I came up with (the red line would be the cut):

[ahab]

My question, though, is why nitrogen and oxygen preferentially scatter blue light. I've tried to find this out on my own, honestly, but it seems like nobody explains it without assuming you know the fine details of quantum physics. It's part of my more general quest to find out why anything is any color. For example, why is carbon black? Is it the way that groups of carbon atoms are physically arranged? Does it have something to do with the properties of the electrons around its orbit? What is it about carbon that makes it so that no light, or very little light escapes it? Alternatively, what is it about blue things that makes it so that only, or mostly, blue light escapes to reach your eye? I've been so confused about color for so long. Anyone have any idea?

OK -- I will have a go at this. Caveat emptor, though; this is not my area.

Why the atmospheric gases scatter blue light preferentially -- The phenomenon is called Rayleigh scattering. Lord Rayleigh found, empirically, that for very small particles, the amount of light scattered off of them is inversely proportional to the fourth power of the wavelength. Because the wavelength of blue light is shorter than that of, say, red light, and because it varies as the inverse of the fourth power, the intensity of the scattering for blue wavelengths will be much larger than for red wavelengths (I think it works out mathematically to a tenfold difference in intensity).

That is, of course, only a half answer, because it begs the question of *why *the shorter wavelengths should scatter more. This has to do, I believe, with the fact that the length scale of the particles (gas molecules) is much smaller than the wavelengths that make up light, and so the shortest wavelengths are most likely to interact with those molecules. But I am probably on shaky ground here.

As to why carbon is black -- I was going to bring up the graphite-diamond issue, but Catharine beat me to it! But for crystalline solids, crystal structure is indeed a key to many/most of their physical properties, and I suspect that is the case here as well. Diamond, which is a high-pressure phase of pure carbon, is held together by strong covalent bonds in the form of tetrahedra in all directions, and the bonding of the electrons is very tight (which, of course accounts for the extreme hardness and strength of diamonds). As a result, the electrons are not available to interact with passing photons of light, and diamonds are clear and, in their purest form, colorless; colors in some diamonds are the result of unbound impurities in the crystal lattice.

Graphite, by contrast, which is the stable form of native carbon at room temperature and pressure, has a sheetlike crystal morhpology; the atoms are covalently bonded in two dimensions in sheets, but those sheets are held together by much weaker van der Waals forces in the third dimension. This makes the electrons in graphite much more available to interact with incoming light; some of that light can be absorbed, so graphite tends to be dark, but some also can be reflected or re-emitted as a result, so that crystalline graphite has a grayish or metallic luster.

Interestingly, coal, another form of carbon (as well as types of carbon that result from incomplete combustion), is not a "macrocrystalline" solid. It is, instead, a collection or aggregation of much smaller nanocrystals of native carbon (graphite). I believe that, because these tiny bits of graphite don't, in the aggregate, have a "preferred orientation" the way macrocrystalline graphite does -- that is, because they are randomly oriented -- coal is in general more isotropic that native graphite, and thus tends to be darker and to lack native graphite's metallic sheen.

(I'm not even going to *touch* the properties of that other, more exotic, form of carbon, the fullerenes . . .)

In a larger sense, I suppose the color of an object always boils down to how the atoms or molecules of that particular substance interact with various frequencies of light, something tied in with their length scale, the details of how the electrons are tied to the constitutent atoms, and the surface properties of the object in question. Those, in turn, will govern how efficiently the substance absorbs or scatters individual wavelengths, as well as more exotic phenomena such as fluorescence. It is indeed, as you suggest, a fascinating and complex topic -- and one on which, as the foregoing suggests, I, too, have only a dim understanding.

Cheers,

Ahab

[Aldark]

re:

What's the airspeed velocity of an unladen swallow?"

is that african or european?

It is funny you should ask. Apparently there have been several attempts to answer that....

However, I found this site interesting.
http://www.style.org/unladenswallow/ Look at the pictures of the swallows and then follow this link: http://www.zazzle.com/product/235478732810365750

Now that's science![/url]

[LeonMire]

Leon ... here this will make things even more confusing ....

No, believe it or not, you clarified things rather than confused them. I had never thought of this part in connection with the physics of color:

all things are comprised more of space ...empty space than of particulate matter ... my guess is that the higher frequency needs more of this space to move through than does the lower frequency

But consider this:

Graphite (a form of pure carbon in which the atoms are arranged in sheets that slide across each other) is black.

Diamond (a form of pure carbon in which each carbon atom is linked by four single bonds to other carbon atoms all the way through the crystal) is clear and colorless.

I'm guessing it has to do with the physical arrangement and bondings between the carbon atoms.

I had considered it, and I decided that things were complex enough! But you're right that that consideration actually sheds light on the subject (haha).

From what I remember, it had to do with electrons. The energy in an electron varies depending on the layer it orbits on, ie electrons nearer to the nucleus have less (i think) energy than electrons further from the nucleus. When a photon hits the electron, it jumps on a higher energy level (goes away from the nucleus) and releases a photon when it goes back to its ground state (unexcited state). Electrons on different layers release different wavelengths of photons when returning to their ground state, hence different colors.

I guess it has to do both with atomic structure and the electrons in their orbits? Is what you said true for all color? Is it always a case of absorption and re-emission? I thought that an apple is red, for example, because it reflects red light? Is this just a convenient way of saying that it absorbs and immediately re-emits red light?

That is, of course, only a half answer, because it begs the question of *why *the shorter wavelengths should scatter more. This has to do, I believe, with the fact that the length scale of the particles (gas molecules) is much smaller than the wavelengths that make up light, and so the shortest wavelengths are most likely to interact with those molecules. But I am probably on shaky ground here.

I've often wondered if this might have something to do with it, but I reasoned about it slightly differently. I first drew a line with a really long wavelengths (representing red) and then drew one with really short wavelengths (representing blue). The "stretched out" wavelengths of red light wouldn't interact with as many air particles, whereas the highly compressed blue light would interact with many more particles. But I don't know if this is at all accurate, or if it makes any sense with wave-particle duality.

Diamond, which is a high-pressure phase of pure carbon, is held together by strong covalent bonds in the form of tetrahedra in all directions, and the bonding of the electrons is very tight (which, of course accounts for the extreme hardness and strength of diamonds). As a result, the electrons are not available to interact with passing photons of light, and diamonds are clear and, in their purest form, colorless; colors in some diamonds are the result of unbound impurities in the crystal lattice.

This is so close to being a satisfactory explanation for me! The thing I don't understand are the words "As a result." How exactly do the strong covalent bonds affect the ability of the electrons to interact with passing photons? Is it mostly because electron sharing leaves fewer total electrons for light to interact with? Or does light, in order to interact with electrons, have to somehow overcome the chemical bond of the material, which it simply can't do with diamonds? Or is there some other connection between the two?

Okay, now that I almost understand how the physical structure of a material could make it transparent to light, and how it could absorb all light, I'd like to move on to the (probably much) more complex issue of how the physical structure and/or the electrons in their orbit could cause a material to reflect a particular wavelength of light. What might cause something to reflect specifically green light , for instance? I'm sure there are lots of different causes for different materials, but I just think one good example would help.

This thread is really cool! I've tried Yahoo! Answers for this sort of thing, but they're all mean and unhelpful. I should've figured LibriVoxers would be much more friendly!