Wikipedia:Reference desk/Archives/Science/2008 April 9

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April 9

*Urgent Question*

Will someone please explain to me why bands occur in nanocrystals containing only a few hundred to a few thousand atoms? The article says the number of atoms must be on the order of $10^{20}$ so why does this occur with so few? Zrs 12 (talk) 00:49, 9 April 2008 (UTC)

Which article? and what type of bands are you talking about? Infrared?--Shniken1 (talk) 01:10, 9 April 2008 (UTC)

Sorry. In the article entitled electronic band structure and I'm talking about bands like contain electrons (kind of like electron orbitals). Zrs 12 (talk) 01:15, 9 April 2008 (UTC)

That article suggests that 'bands' are simply a term used to describe the electronic energy levels of a solid when lots of atomic orbitals (with similar but slightly different energy levels) come together. One atom will still have a conduction 'band' but rather than a band of different energy levels it will be one discrete electronic transistion (an electron jumping from ground to excited state). When lots of the discrete energy levels come together they produce bands of slightly different energies. Hope that makes sense.--Shniken1 (talk) 04:13, 9 April 2008 (UTC)

I know, but why are there bands (continuums of electron orbitals) when there is such a small congregation of atoms? Zrs 12 (talk) 18:53, 9 April 2008 (UTC)

The article appears very clear about this: sharing of electrons between multiple atoms "produces a number of [...] orbitals proportional to the number of atoms. When a large number of atoms (of order 10²⁰ or more) are brought together to form a solid, the number of orbitals becomes exceedingly large, and the difference in energy between them becomes very small, so the levels may be considered to form continuous bands of energy." Thus a "band" is never a true continuum, it is always a collection of individual levels, with the number of levels similar to the number of atoms. If there are "many" atoms, and thus "many" levels, it will be convenient to ignore the discrete nature, and think of the band as a continuum, but there is no general definition of what constitutes "many" in this sense. Only in the context of a particular experiment will there exist a definition of "many", given by how small an energy difference between successive levels can be and still affect the outcome of that experiment. The number 10²⁰ in the article is meant as an indication of the vast number of atoms in a "typical chunk of solid", not as a definition of "many". If the number of atoms is small enough that it makes sense to think of the individual energy levels within it, as may be the case in some nanocrystals, people may still choose to refer to this collection of levels as a "band". --mglg_(talk) 17:40, 10 April 2008 (UTC)

Well, I thought it was something about breaking the lattice symmetry of the crystal or something. Can anyone maybe expand on this (if it is in fact correct)? Zrs 12 (talk) 03:25, 11 April 2008 (UTC)

Current electricity

In current electricity questions, we frequently come across questions in which it is given that say a positive charge is moving from left to right or a positive charge is moved in an electric field, etc.. i want to know that how can a positive charge be moved?? because positive charge is present on protons and the protons are bounded inside the nucleus so how can we move the protons inside a conductor or elsewhere when they have been bounded by the nucleus. also as i have been taught that current is caused by flow of free electrons, do we have free protons of this type too, which can be moved?? —Preceding unsigned comment added by GK ROCKS (talk • contribs) 03:55, 9 April 2008 (UTC)

The direction of an electrical current is defined as the direction a positive charge would flow. This was defined before the electron and proton were discovered and now that we know that it is the negative electon that carries the current it has become quite confusing. It is however possible to move a positive charge, if you put electrodes in a salt solution the cations (+) will move to the negative electrode. There are also carbon nanotube wires that may be able to carry protons as a current, or in particle accelerators.--Shniken1 (talk) 04:20, 9 April 2008 (UTC)

The nucleus as a whole has a positive charge, so if you move a bare nucleus a charge will flow too. Graeme Bartlett (talk) 04:31, 9 April 2008 (UTC)

If you knock an electron off an atom, say in a semiconductor, it will leave behind a positive charge, sometimes described metaphorically as an electron "hole". If a nearby electron then "fills" that hole (takes the place of the first electron), then it will leave behind a positive charge. So no, the positive charge itself doesn't move, but the result is the same as if it did. It's like a sliding puzzle, where it is the plastic numbers you slide around, but from a distance it looks as though the hole is moving around. Also, in an acid battery, both positive and negative ions are traveling in opposite directions. The details of whether electrons are traveling in metal or ions are moving through a solution are not relevant to describing the circuit. kwami (talk) 07:26, 9 April 2008 (UTC)

You might find the article on ion very helpful. In most cases around the home, current is the flow of electrons through a metal conductor (wire). The reason these charges move is because they are subject to a force due to an electric field - this the easiest way to cause a charge to move. In many chemical batteries, current also includes ions in solution undergoing an electrochemical reaction. In some more obscure cases, such as Earth's Van Allen belts, current can be a complicated interaction of ions, electrons, and time-varying electromagnetic fields. In this case, yes, we have free protons caused by ionized hydrogen; there are many other types of positive ions. Nimur (talk) 15:06, 9 April 2008 (UTC)

Note that in Electrophoresis (and other situations where you have current passing through a salt solution) in addition to the negative ions moving one way, you'll have positive ions moving the other way as well. In a solution of common salt, for example, it is not free electrons which travel to make the current, but the Cl^- and Na⁺ ions which move back and forth. -- 128.104.112.85 (talk) 17:23, 9 April 2008 (UTC)

For an analogy, consider warm and cold air. If you leave the wooden front door of your house open, but the glass storm door is still closed, the house will cool down more quickly. Some people might call this "letting the cold in", although technically it's "letting the heat out", since heat is the vibration of atoms and cold is the lack of this vibration. However, you can think about it either way, it doesn't really matter on a practical level. Similarly, you can think of protons flowing from one location to another, even though they really don't do that, if you like. One exception would be ionized normal hydrogen, where you do indeed have flowing electrons and protons. StuRat (talk) 15:46, 9 April 2008 (UTC)

StuRat, I have a long-standing complaint against the anecdote of "letting the cold in/letting the heat out." The flaw in the analogy is that it fails to consider convection, or bulk motion of air mass. In the purely thermodynamic sense, it is true that heat always flows from hot to cold reservoirs; but in the unique case of fluid motion, cold air can move due to a pressure gradient, viscous dragging, and a wide range of complicated interactions. So, in fact, opening a door is neither letting heat in or out; it is letting air in or out; and that air carries a certain amount of heat with it convectively. Nimur (talk) 16:04, 9 April 2008 (UTC)

That would be why I included the glass storm door in the example, to prevent any air from entering or leaving the house. Thus, only heat transfer occurs. StuRat (talk) 17:03, 9 April 2008 (UTC)

If you rub a glass rod with a piece of silk the glass will acquire a positive charge. (while the silk acquires a negative charge). If you move the glass rod from left to right, you are moving a positive charge. What is complicated about this? Edison (talk) 18:03, 9 April 2008 (UTC)

The fact that a proton isn't flowing from one atom to the next maybe. But anyway as someone said it current was said to flow positive to negetive by convention. Once we discovered protons and electrons we realised they got it wrong. Electrons flow from negetive to positive in a circuit while the protons just sit there. Or a 'hole' (a proton with no matching electron) moves from one atom to the next flowing positive to negetive--155.144.40.31 (talk) 03:30, 10 April 2008 (UTC)

I have a real problem with the notion of protons jumping from atom to atom. That would constitute transmutation. Add a proton to an atom and it becomes a differnet element. Add or subtract an electron and it stays the same element. The change in binding energy would be large if protons jumped from atom to atom to effect a flow of current. That is not what hole current is. Edison (talk) 04:19, 10 April 2008 (UTC)

Indeed. In hole current, an electron moves into the empty space but leaves behind another empty space where the electron used to be. Kwami's sliding puzzle analogy is a good one. AlmostReadytoFly (talk) 08:18, 10 April 2008 (UTC)

And, of course, the electrons do not actually "flow" in any normal sense when a current flows through a wire, so we would be "wrong" whichever direction we agreed on by convention. 78.32.74.48 (talk) 15:35, 12 April 2008 (UTC)

Solid and Solid Shape

what is the differencebetween a solid and a solid shape —Preceding unsigned comment added by 72.27.45.24 (talk) 12:27, 9 April 2008 (UTC)

Typically, solid refers to a phase of matter while a solid shape refers to a more abstract mathematical concept. Do you have more context where these terms were used? Nimur (talk) 15:08, 9 April 2008 (UTC)

Nitrogen requirement of microorganisms

I know that some microorganisms can get their nitrogen requirements met with inorganic substances such as nitrate salts or urea. Others need organic nitrogen sources such as peptones or amino acids. I am looking for some rules of thumb along these lines, e.g. "most fungi can be cultured with nitrates" or "gram-positive bacteria usually require amino acids". Any little bit of information you can offer could help; maybe we can come up with a rule of thumb if none exists. ike9898 (talk) 14:57, 9 April 2008 (UTC)

There's some information in our article on nitrogen fixation; that doesn't fully answer your question, though. Nitrogen fixation refers to conversion of near-inert atmospheric nitrogen into bioavailable forms. I gather that you're looking for the broader set of organisms that can use any bioavailable inorganic nitrogen to generate all the amino acids de novo. Unfortunately, our article on amino acid synthesis doesn't list the organisms with these capabilities, though the navboxes at the bottom of that page cover all the important biochemical pathways involved. TenOfAllTrades(talk) 15:45, 9 April 2008 (UTC)

Orbit

If an object on a satellite is propelled radially toward the mass being orbited what will be its path? Will it orbit at a more eccentric orbit, or will it return to the same orbit as the satellite from which it was thrown? Cslloyd (talk) 15:17, 9 April 2008 (UTC)

To make sure I'm understanding the suggestion, we're talking about a guy on the space station throwing a baseball at the Earth (more or less), right? In that case, and neglecting out air resistance and such, the baseball will orbit with a slightly different eccentricity (though not necessarily more, depending on the orbit of the space station) while still crossing the orbit of the space station. Fully raising or lowering an orbit is generally accomplished by complementary thrusts at opposing points in the orbit, not with a single thrust. — Lomn 15:43, 9 April 2008 (UTC)

If the baseball is thrown with sufficient speed towards earth, it will enter an eccentric orbit with a collision to the planet's surface. This is almost intuitive; throwing a ball down does make it go down; the trick is that it is thrown from a moving "platform" and already has a (very significant) angular velocity / angular momentum. You can compute the trajectory if you know its initial orbit and the velocity it is thrown at. It is also worth noting that large transfer of momentum to the baseball will also result in equal and opposite reaction on the astronaut, to conserve momentum for the complete system. Nimur (talk) 15:52, 9 April 2008 (UTC)

(Addendum) As Lomn pointed out above, it is also possible that the baseball's new orbit does NOT collide with the planet (neglecting air resistance and only considering "hard impacts"). In that case, the new orbit will be more eccentric, as mentioned by Lomn, but will intersect the original point where it was thrown from. Nimur (talk) 16:00, 9 April 2008 (UTC)

Incidentally, this experiment has been conducted from the ISS. — Lomn 17:35, 9 April 2008 (UTC)

If an object were projected from a satellite in low earth orbit in a retrograde direction with about 1% of the satellite's orbital velocity, it would reenter the earth's atmosphere and burn up or land depending on its aerodynamics. A velocity of "throwing" of 170 mph or thereabouts should suffice. This would be at a right angle from the direction posited by the questioner. Edison (talk) 17:56, 9 April 2008 (UTC)

...but the golf ball that was hit from the ISS in Nov 2006 is still in orbit nearly 30 18 months later - see this tracker. Apparently NASA predicted it would stay in orbit for only 3 days ... Gandalf61 (talk) 19:58, 9 April 2008 (UTC)

It's those new graphite drivers, they've ruined space golf ;) Franamax (talk) 21:02, 9 April 2008 (UTC)

Nov 2006 was only 18 months ago, not 30. --ChokinBako (talk) 23:11, 9 April 2008 (UTC)

Gandalf is from the future and momentarily forgot this is the past aicmfp 130.88.140.121 (talk) 15:56, 10 April 2008 (UTC)

Hertz

Our article Hertz has a table conveniently telling us all the forms of "hertz" combined with all the SI prefixes. The familiar ones like kilohertz, megahertz, and gigahertz are there, but so are units smaller than the hertz, from the decihertz (0.1 Hz, i.e. once every 10 seconds) all the way down to the yoktohertz (10^–24 Hz, i.e. once approximately every 3 x 10¹⁷ years (about 23 million times longer than the age of the universe). My question is, are these units of frequency smaller than the hertz ever actually used in real-life applications? I don't just mean the really implausible ones like the yoktohertz, I mean even things like the millihertz (once every 16'20"), microhertz (once approximately every 11.5 days), and nanohertz (once approximately every 31.7 years). Is there anything in real life whose frequency is actually measured in those units? —Angr 16:53, 9 April 2008 (UTC)

A quick google search (ignoring the definition pages) yields the following scientific topics (primarily journal article titles):

• Millihertz Quasi-Periodic Oscillations from Marginally Stable Nuclear Burning on an Accreting Neutron Star
• Isolation transformer passes millihertz signals.
• Millihertz Oscillation Frequency Drift Predicts the Occurrence of Type I X-ray Bursts
• The Last Few Microhertz: Eliminating Remaining Discrepancies Between Observed and Calculated Solar Oscillation Frequencies
• Golf: Solar Signal for Frequencies Below 5 microHertz
• T violation and microhertz resolution in a ring laser
• Pulsar Timing Array -- A Nanohertz Gravitational Wave Telescope
• Probing the Nanohertz Gravitational Wave Background

It looks like the units are primarily used in astronomy, although the sample set may be biased. -- 128.104.112.85 (talk) 17:30, 9 April 2008 (UTC)

(ec) mHz, nHz. In astrophysics, orbital periods (if expressed as a frequency) will be in these ranges. -- Coneslayer (talk) 17:32, 9 April 2008 (UTC)

That makes sense, thanks! And to judge from one of the titles listed above, pulsars' radiation emissions can also be measured in Hz. That article says pulsars' periods range from 1.5 ms to 8.5 s, but presumably it could also be stated in terms of frequency ranging from 120 mHz to 670 Hz. Next question: in my original post I wrote that 1 nHz corresponds to a frequency of approximately 31.7 years; am I right in thinking that 12:00 midnight on New Year's Day also occurs at a frequency of approximately 31.7 nHz? Or have I screwed up my math somewhere (all too possible)? —Angr 18:06, 9 April 2008 (UTC)

I think it depends on the definition of year you use. It looks like most are around 3.2×10⁷ s though, and inverting this gives 3.1×10⁻⁸ Hz or 31 nHz. --Prestidigitator (talk) 18:25, 9 April 2008 (UTC)

The traditional approximation for astronomers is π × 10⁷
s. -- Coneslayer (talk) 19:02, 9 April 2008 (UTC)

What I was mostly interested in is knowing whether I got it right that when 1 nHz corresponds to a frequency of x years, then years occur at a frequency of x hertz. I wasn't expecting that apparent coincidence. —Angr 21:06, 9 April 2008 (UTC)

The examples you give all differ from the unit of hertz by only a SI prefix. I have most commonly seen scientific notation used to express smaller frequencies in plain hertz like (e.g. 2.3×10⁻³ Hz), but the metric prefixes are systematic and (many of them) common enough that it seems either (e.g. 2.3 mHz) should be clear to the reader, which is what is really important. You may also find that for applications where very low frequencies are common that the measurements used are instead that of periodicity or wavelength rather than frequency. I'm sure you could easily do Google searches for variations of herz ("millihertz", "milli hertz", "mHz", "centihertz", "centi hertz", "cHz", etc.) to see if you come up with anything. What is the purpose of the question? Is it idle curiosity, or are you trying to determine whether it would be proper to use a value in a particular context, or what? --Prestidigitator (talk) 18:04, 9 April 2008 (UTC)

It's mostly idle curiosity, perhaps with an eye to adding a discussion of these units at Hertz#Applications, which at the moment only discusses things measured in hertz and the units larger than the hertz (kHz, MHz, GHz, etc.). —Angr 18:12, 9 April 2008 (UTC)

Heavy water question

Because of the laws of thermodynamics, just having heavy water lying around in a non-sealed container will cause it to react with the light water vapour in the air, gradually causing a situation where I have plain old light water lying in the container and an extremely thin concentration of heavy water vapour in the air around me. Of course given little enough heavy water in the first place, this is perfectly safe, but it will lead to loss of valuable heavy water. So my question is, how quickly will this happen, and what can be done to slow it down? JIP | Talk 17:43, 9 April 2008 (UTC)

This is a very difficult question to answer quantitatively. This page goes into a bit of a discussion about half of the problem—how fast does liquid water move to the gas phase in an evaporative process? (At constant humidity, you have a dynamic equilibrium between evaporation and condensation, such that water is entering and leaving the liquid phase at constant, equal rates.) The rate of exchange of heavy water into the gas phase is going to depend greatly on the surface area of the container, the depth of the container, the amount of mixing and circulation of the liquid in the container and the air above it, the temperature, any agitation of the liquid-air interface, etc..

If you use a broad, shallow pan of warm heavy water and put it into a room with a fan blowing over the surface (or even better, with a bubbler circulating room air through the liquid in the pan) the equilibration between light and heavy water will occur many orders of magnitude more quickly than if you have the liquid in cool bottle with a narrow mouth sitting in still air. TenOfAllTrades(talk) 18:47, 9 April 2008 (UTC)

Just to complicate things still further, once you have a mixture of H₂O and D₂O in the liquid phase, you'll get some HDO (half-heavy water? :-) due to hydrogen ion exchange. That gives you three molecular species to keep track of. JohnAspinall (talk) 20:39, 9 April 2008 (UTC)

And why not add some acid to it so that you get HD2O+, H2DO+, D3O+(?),H3O+,H2O,D2O,HDO.....--Shniken1 (talk) 00:34, 10 April 2008 (UTC)

Is there a purpose to why you have your heavy water just sitting around?--155.144.40.31 (talk) 03:26, 10 April 2008 (UTC)

I don't actually have any heavy water yet. I'm trying to purchase some, in order to conduct experiments on heavy ice in normal water. As I have not done anything like this before, I want to make sure of the necessary precautions in advance. JIP | Talk 20:59, 10 April 2008 (UTC)

In most cases the controlling quantity is average air velocity. In a matter of seconds after being exposed to air, a microlayer of air saturated in heavy water vapor will form above the heavy water liquid. In most environments the rate at which this is transported away from the liquid surface is governed by the average air speed (only in very, very still air is diffusion more important than local turbulence). Because of things like leaky doors and air conditioning, empty indoor environments typically have ambient air speeds of 0.1 - 10 mm/s. To order of magnitude, you might expect to lose heavy water at a rate that is of order the area of your exposed surface times the ambient air velocity times the saturation vapor density. Which suggests a number like 10-70 mg lost per cm^2 of surface area exposed per hour in an empty room. If you are doing silly things like admiring your heavy water and breathing over it, one could easily increase this by one or more orders of magnitude. Obviously the best solution is to keep your heavy water in a sealed container whenever possible. Dragons flight (talk) 06:52, 10 April 2008 (UTC)

What histone modifications exist at <<DNA sequence>>?

Does anyone know a method by which one could ascertain the histone modifications that exist in a population of cells at a particular DNA sequence? I know the reverse is possible (finding out what sequences enjoy the comfort of particular histone modifications). ----Seans Potato Business 17:45, 9 April 2008 (UTC)

Anergy

An article I read from 1993 describes a process of anergy where the immune system's response to a specific antigen can be inhibited (at least temporarily) while the rest of the immune system is left unaffected. This exists naturally to prevent our bodies from destroying themselves, and seems to show promise in tissue grafting and the treatment of autoimmune diseases but our article on anergy is just a stub. I'm assuming that 15 years would allow enough research to substantiate some of its potentials. Is this something that the scientific community hasn't done much research on or have Wikipedians failed to document major studies on it since 1993? — Ƶ§œš¹ _{[aɪm ˈfɻɛ̃ⁿdˡi]} 19:27, 9 April 2008 (UTC)

Or perhaps it's just been unsuccessful? I don't know, just guessing. Imagine Reason (talk) 00:17, 16 April 2008 (UTC)

Disadvantage to bilingualism in evolutionary point of view

It's been shown that babies can distinguish all the phonemes available, and that as they get older they find it more difficult to distinguish phonemes that are not important in their native language. If exposed to two languages, they retain the ability to distinguish the phonemes important to both languages. I understand in a general sense that pruning brain connections is an important thing, but in this case there's no evidence that bilingual (or multilingual) children learn their first language any worse than monolingual kids, or have any other disadvantages. So why do we lose this ability? What are we gaining from that? moink (talk) 21:04, 9 April 2008 (UTC)

I don't think it would be wise to assume that we gain anything from losing the ability to distinguish foreign phonemes. It could simply be a side-effect of the way our brains learn language. (A spandrel (biology).) --Allen (talk) 21:28, 9 April 2008 (UTC)

In fact, according the article I just linked to, Noam Chomsky believes language may itself be a spandrel, which would render the entire question moot. --Allen (talk) 21:30, 9 April 2008 (UTC)

Not only that but higher life expectancey is a relatively mordern thing that may have never had a part to play evolutionarily because we were never able to age to beyond the point. You don't have to go back to far for life expectancy is the low 20's.--155.144.40.31 (talk) 03:23, 10 April 2008 (UTC)

My understanding is that the ability to learn language (not just phoneme distinction, but all aspects of grammar) is directly related to brain plasticity so that, once your brain becomes less plastic, your ability to acquire language diminishes. Diminished plasticity, I'm sure, has a number of evolutionary benefits. — Ƶ§œš¹ _{[aɪm ˈfɻɛ̃ⁿdˡi]} 04:14, 10 April 2008 (UTC)

My favorite (if possibly flawed) explanation for the (relative) loss of ability to learn language before puberty I read in Morgan's "Descent of Woman" -- otherwise mother's would learn babytalk instead of vice-versa. Saintrain (talk) 18:57, 14 April 2008 (UTC)

Phasor diagrams

In a series circuit composed of an a.c. supply, a resistor and a capacitor, how do you form the phasor diagram?Bastard Soap (talk) 21:09, 9 April 2008 (UTC)

The angle of the phasor is inverse tan of (Vl - Vc) / (Vr), (or inverse tan of (Xl - Xc)/ R). Obviously Vr (or R) is the magnitude in real direction and Vl-Vc (or Xl-Xc) is the magnitude in the perpendicular. So you just need to know the capacitor impedance and the resistance--155.144.40.31 (talk) 03:18, 10 April 2008 (UTC)

heat

affects of heat on rubber —Preceding unsigned comment added by 70.226.196.132 (talk) 22:01, 9 April 2008 (UTC)

Melting —Preceding unsigned comment added by 128.163.172.108 (talk) 22:13, 9 April 2008 (UTC)

And hardening. -- kainaw™ 22:54, 9 April 2008 (UTC)

or burning --S.dedalus (talk) 22:55, 9 April 2008 (UTC)

Heat transfer. This is fun! --Mdwyer (talk) 23:04, 9 April 2008 (UTC)

Tire wear. --Julia Rossi (talk) 00:36, 10 April 2008 (UTC)

Shrinkage. --Allen (talk) 02:09, 10 April 2008 (UTC)

Also the release of nasty fumes. --Allen (talk) 02:15, 10 April 2008 (UTC)

Vulcanization? — Kieff | Talk 02:49, 10 April 2008 (UTC)

Sterilization Graeme Bartlett (talk) 02:50, 10 April 2008 (UTC)

Perishing. Gwinva (talk) 03:00, 10 April 2008 (UTC)

Connect 4 Diagonal FTW!--Shniken1 (talk) 03:10, 10 April 2008 (UTC)

Degradation and Unplanned pregnancy. DMacks (talk) 04:46, 10 April 2008 (UTC)

Walter Matthau [1] ---Sluzzelin talk 11:44, 10 April 2008 (UTC)

The Persistence of Memory?--Shniken1 (talk) 13:23, 10 April 2008 (UTC)

Tire fire - Gandalf61 (talk) 15:05, 10 April 2008 (UTC)

Bouncing pucks. Franamax (talk) 15:41, 10 April 2008 (UTC)

C-c-c-combo breaker!! —Keenan Pepper 16:07, 10 April 2008 (UTC)

or a new stanza —Preceding unsigned comment added by 79.122.13.205 (talk) 16:46, 10 April 2008 (UTC)

...While we're off topic. Nimur (talk) 16:53, 10 April 2008 (UTC)

Here's a dual meaning: Entropy increases (applies to this thread too:) Franamax (talk) 18:54, 10 April 2008 (UTC)

Smells bad. Edison (talk) 19:06, 10 April 2008 (UTC)

So, chewie. Julia Rossi (talk) 00:05, 11 April 2008 (UTC)

alternative music Gwinva (talk) 03:58, 11 April 2008 (UTC)

Expansion Of The Universe

Scientists say that galaxies are moving away from each other, so the Universe is expanding. If the Universe was expanding, and we are integral parts of it, wouldn't that mean that we were expanding too? If so, wouldn't this mean that there would be no noticeable difference in distance over time? If this is the case, then galaxies must be moving away from each other for a different reason.--ChokinBako (talk) 22:57, 9 April 2008 (UTC)

We would be expanding, but there are forces holding us together - on the small scale, mostly electromagnetic force, and on the larger scale more often gravitational force. Imagine you and a friend are standing, along with lots of other people, on the surface of a giant balloon, and you're holding your friends hand. When the balloon gets inflated, everyone moves away from each other, but you and your friend stay together because you're still connected. Confusing Manifestation(Say hi!) 23:04, 9 April 2008 (UTC)

See Metric expansion of space. Imagine Reason (talk) 23:09, 9 April 2008 (UTC)

But the article does not explain why the "raisins" (in the last illustration) do not expand along with the balloon. Are the "raisins" the galaxies, held together by gravitation? It would still be possible for the metric of space-time to be expanding, or even to be contracting, without any measurement being able to detect this if our unit of measurement is also contracting at the same rate.

Is there any evidence to refute my (perverse) theory that the metric of the universe is actually contracting, but that the space between galaxies is contracting less quickly than our unit of measurement? Would this not also explain the red shift because the light emitted long ago was from a universe with a longer unit of measurement? 78.32.74.48 (talk) 16:12, 12 April 2008 (UTC)

As Confusing explained, there are forces holding nearby objects together. For example, our solar system will stay intact while we fly away from other galaxies. Whether we'll be torn apart eventually, I think, is still up in the air. Note that this would mean intelligent species, if they survive till that time, will have no way of determining that there's anything else in the Universe but their solar system.

As for your perverse theory, wouldn't the CMB lose its intensity much slower and even strengthen? Imagine Reason (talk) 15:13, 14 April 2008 (UTC)