Wikipedia:Reference desk/Archives/Science/2013 November 1

= November 1 =

space objects swift motion
hi, why are certain space objects moving so fast in space, while most just float?


 * You're probably thinking that things float in orbit. They don't. They're actually zipping along pretty quickly. For example, the International Space Station (according to the infobox anyway) is going 27,600 km/h or 17,100 mph around the Earth. It's just that there's not a lot that is stationary up there to give you visual cues.
 * It's all relative anyway. The Earth is speeding around the Sun, the Sun around the Milky Way galaxy, etc. Clarityfiend (talk) 07:30, 1 November 2013 (UTC)


 * The orbital speed of a an object orbiting Earth depends on the radius of its orbit and its eccentricity. The closer to Earth an object orbits the faster it must be going to stay in orbit.  Objects in eccentric orbits speed up as they approach Earth and slow down as they move away.  Spinning  Spark  09:59, 1 November 2013 (UTC)

i was talking about space debris. In "gravity" movie, when many things float here and there, debris come in great speed. Is that because they are propelled or some objects travel in that speed in space. Not about orbiting, about some moving randomly. — Preceding unsigned comment added by 122.164.80.142 (talk) 13:39, 1 November 2013 (UTC)


 * Objects in orbit are subject to the earth's gravity at a fairly significant fraction to what you are. That is, the gravitational forces on you if you were, say, on the International Space Station would not be significantly lower than what it is where you are sitting now.  So why do objects float along side of you when you let go?  For the same reason they would if you jumped out of a plane.  Imagine, if you will, that you jump out of a plane.  Now, imagine you're holding a pen when you do so.  While you're faling towards the earth, let go of the pen.  Will the pen go rushing towards the earth as soon as you let go?  Well, not any faster than you already are.  That is, the pen is already being pulled towards the earth along with you before you let it go, so when you let it go, it keeps the same motion as it already had.  When you look at the pen, it appears to float right next to you, but of course, it's rushing towards the ground along side of you, just as you are.  Orbit works the same way, except you have enough tangental velocity relative to the earth to avoid hitting it.  That is, you're essentially falling in a circle and missing the earth on every pass.  If you let go of your pen in orbit, it floats along side of you for the same reason it did when you jumped out of the plane.  Because you were both moving together before you let it go, and nothing changed about the forces acting on it and on you when you let it go.  -- Jayron  32  14:43, 1 November 2013 (UTC)


 * Adding to that, if something is moving fast relative to something else in space (that is, not floating along with it), it is because it is on a significantly different orbit. I haven't seen Gravity, so I don't know the context for the fast moving objects, but things with intersecting orbits can pass each other VERY quickly. 2009 satellite collision is a recent example of a collision. They collided at 26,000 mph. K ati e R  (talk) 14:53, 1 November 2013 (UTC)


 * BTW, the perpetually falling aspect is the same feeling as real falling whence the large rate of nausea for astronauts and the vomit comet training. The tangential velocity keeps you from hitting but doesn't offset the feeling.  --DHeyward (talk) 09:47, 3 November 2013 (UTC)

evolution of cancer cells
hi, are cancer cells evolving? They are like bacterial cells, adapting and managing to sustain all these years. — Preceding unsigned comment added by Anandh chennai (talk • contribs) 06:26, 1 November 2013 (UTC)


 * They do indeed evolve, but the nature of their evolution is a bit different. When lifeforms evolve, they generally don't have a lot of "unused circuitry" to draw on; typically genes that are unused are quickly lost in the course of evolution.  But cancer cells have many more genes and developmental programs to draw upon than what they should be using, because they are differentiated and, typically, want to become dedifferentiated, calling on abilities from other tissues such as the ability to degrade extracellular matrix with matrix metalloproteinases, turn on telomerase, and of course to undergo rapid cell cycling and growth with oncogenes. Wnt (talk) 07:14, 1 November 2013 (UTC)


 * This is the second time you have asked this exact same question - you asked it here on 23 October. It attracted sevearal good answers.  What was wrong with them?  — Preceding unsigned comment added by 120.145.135.143 (talk) 07:17, 1 November 2013 (UTC)

thanks for assisting. Last time, my question was deleted immediately wanting to discuss such things in an outside forum. I was not aware that it was reactivated. — Preceding unsigned comment added by 122.164.80.142 (talk) 13:35, 1 November 2013 (UTC)
 * Your previous question is in the archives at WP:Reference desk/Archives/Science/2013 October 23. Red Act (talk) 14:41, 1 November 2013 (UTC)

The absolute altitude record for a manned spacecraft

 * Apollo 13
 * Because Apollo 13 followed the free-return trajectory, its altitude over the lunar far side was approximately 100 km greater than the orbital altitude on the remaining Apollo lunar missions. Due to this fact, Apollo 13 holds the absolute altitude record for a manned spacecraft, reaching a distance of 400,171 kilometers from Earth on 7:21 pm EST, April 14, 1970.


 * Moon
 * Apogee 405,503 km
 * Perigee 363,295 km

Apollo 8 entered moon orbit. Apollo 13 only uses moon to capture them and sent them home.

Is it a good idea to say that merely 100 km farther away from the dark side of the moon than previous missions could earn them this world record? -- Toytoy (talk) 11:18, 1 November 2013 (UTC)


 * Good spot, that's somewhat misleading (it beats Apollo 8 by about 2,800km, not 100km). It was not that they were further from the moon which gained them the record, it was that they were futher from the Earth (and, as it happened, the moon was further from Perigee than during other missions) The fact is cited to Guiness World Records 2010, so I can't be sure if the inference is theirs or not. I don't seem to be able to find it on the GWR website.  Either way, it should be changed to remove the misleading "due to this fact". MChesterMC (talk) 13:44, 1 November 2013 (UTC)

Reflection
At quantum level, what causes the difference in reflection of visible light from polished surfaces of, say, silver and copper? Paul venter (talk) 12:13, 1 November 2013 (UTC)


 * Copper explains the color of copper on a quantum level, at least rudimentarily. That's the only difference I can think of.  -- Jayron  32  14:30, 1 November 2013 (UTC)


 * Specifically, from Copper : "Together with caesium and gold (both yellow), and osmium (bluish), copper is one of only four elemental metals with a natural color other than gray or silver.[8] Pure copper is orange-red and acquires a reddish tarnish when exposed to air. The characteristic color of copper results from the electronic transitions between the filled 3d and half-empty 4s atomic shells – the energy difference between these shells is such that it corresponds to orange light. The same mechanism accounts for the yellow color of gold and caesium." loupgarous (talk) 00:12, 2 November 2013 (UTC)


 * Red tarnish ? I always think of copper turning green. StuRat (talk) 15:56, 2 November 2013 (UTC)


 * It has both kinds. Copper (I) compounds are reddish, while copper (II) compounds are often blue or green.  The red tarnish so mentioned is usually something like copper (I) oxide, while the green stuff is patina, which are mostly copper (II) compounds like copper(II) carbonate.  -- Jayron  32  01:32, 3 November 2013 (UTC)

Greatest and Least Surface gravity on inhabitable planet?
Assume a planet with a core and mantle somewhat similar to Earth or one of the other inner planets. What is the Greatest and Least possible *surface* gravity for a planet that Human beings and Earth plants would be otherwise be able to inhabit? I'm wondering because if the planet is much larger than earth then it would seem to be likely to keep it's Hydrogen in a way that would make the planet otherwise unusable and if it was much lower, then the Oxygen would be able to escape relatively quickly in the planet's life. (See Mars, which would be worse if it were closer, I think.)Naraht (talk) 15:17, 1 November 2013 (UTC)


 * You must take air temperature into consideration.


 * If the planet is hot (but not too hot to boil living things), then gas molecules would be more likely escape from it.


 * If the planet is cold (but not too cold), then gas molecules would be less likely escape from it.


 * The air temperature is determined by TOO MANY factors other than surface gravity:


 * It may be affected by: the planet's rotational axis, the sun's strength, the distance between the planet and the sun, whether it has a moon or moons, the color of the ground, the existence of sea and the composition of the sea (salinity and color), mountain ranges, the air's color (see Jupiter), greenhouse gases ...... blah blah ...... -- Toytoy (talk) 15:31, 1 November 2013 (UTC)
 * I agree that temperature would need to be taken into account. However the specifications for survival of both Humans *and Earth Plants* would keep the temperature within a *relatively* narrow range. (Assume on one end for temperature a strip with weather no worse than Calgary along the Equator and on the other with Poles no warmer than Jakarta). The Star would also have to generate enough energy in wavelengths that Earth plants could use to survive. (Let's say F, G, or K class, though that may be too broad).Naraht (talk) 15:43, 1 November 2013 (UTC)


 * Surface gravity per se is probably not the dominant factor. For example, Saturn actually has a lower surface gravity than Earth (about 90%, because of its very low density).  However, the rate of decrease in gravity with altitude is far slower than on Earth, so Saturn is much better at holding on to its atmosphere. Looie496 (talk) 16:04, 1 November 2013 (UTC)


 * Saturn is the first thing that came to mind, but if we assume your ecosystem needs ground to stand on (otherwise, Venus is nearly habitable, except for the nasty sulfur making its clouds acid instead of water). So for the largest planet, we're basically supposing it has density of 1 g/cc all the way down to the core, so you can stand on the surface.  (The planet could be made out of any number of things, but for purposes of discussion I'll say "turtles".)  That means the planet has mass M = 1 g/cm3 r^3, where r is your distance from the center.  Mass of Earth is 6 x 1027 g and its radius is 6.36 x 108 cm.  For Earth mass of our light planet, you have radius 1.8 x 109 cm (cube root of the volume), which is 2.9 times further, which means you'd actually have only 1/8 the gravity!  Yet we know full well the gravity well is as deep, so gas molecules at the top of the atmosphere would need the same escape velocity, and so the planet would lose atmosphere at the same rate as Earth... wait, erm, no, because the top of the atmosphere is so much bigger, it would lose it 8x faster.  But then again, it would probably have 24x the gas to start with inside of it.  But how much reaches the surface?  Hrm, the ugly head of reality noses unwanted into the tent.  But gas release ought to be at least 8x more, so surface gravity could be 1/8 Earth's with the same atmosphere. oops, wait, it's just as deep at the center of the planet, but ...  The scale height of the atmosphere will be a little teeny bit different, but not much - Earth's atmosphere is so thin that the difference in gravity between top and bottom is miniscule, and for the bigger planet it is even less. Wnt (talk) 16:37, 1 November 2013 (UTC)
 * But you have a relatively narrow line to walk between losing Hydrogen fast enough and not losing Oxygen. If the functional atmosphere is that much be that much bigger won't it lose Oxygen faster as well?Naraht (talk) 16:46, 1 November 2013 (UTC)
 * Erm, I might have gone "out of my depth" on this one. Every time I think about it there's some factor I forgot about that throws the whole result off by a factor of 3. Wnt (talk) 17:03, 1 November 2013 (UTC)


 * The ability of the planet to retain hydrogen and oxygen really isn't much to do with gravity. Both hydrogen and oxygen are highly reactive elements and won't remain in the atmosphere as gasses for very long.  On earth and other planets that we can study easily, the oxygen reacts with elements such as carbon, iron and hydrogen to make CO2, rust and water - and hydrogen reacts to make methane, water and such like.  There may once have been free oxygen on Mars - but it's long been combined with the iron and carbon in the crust - which is why Mars is such a pretty shade of red and has a CO2 atmosphere.  There would be no oxygen in the air here on Earth if it were not for plants doing photosynthesis to continually regenerate it - and there isn't much hydrogen in our atmosphere because it reacts with the oxygen too easily.


 * Obviously if a planet's gravity is too low, then it won't be able to retain any useful amount of atmosphere at all - and the threshold for that is somewhere between Mars (which has an atmosphere) and our moon (which has a very tenuous one).


 * But as others have pointed out - it's not all about mass and gravity. Look at Venus, its mass and surface gravity are a little less than ours yet it has an atmospheric pressure about 100 times greater than Earth.  Divers can survive for long periods at up to 7 times atmospheric pressure providing they breathe the right mix of gasses and decompress slowly enough when they return...but above that is dangerous.  So Venus is an example of a near-1g world that has way too much atmosphere.


 * As to the survivability of humans, it's clear that we do very badly in zero-g - astronauts who spend more than a few months in space suffer all sorts of dangerous and sometimes irreversible changes. There have been no experiments testing the lower limit however.  It's difficult to produce g-forces other than 0g or 1g - and the length of time astronauts spent on the moon wasn't enough to really understand the effects of 1/6th g.  Presumably there is a lower level of gravity that's tolerable - but we simply don't know what that is.  For higher than 1g, we know that an average person can only tolerate 4 to 6 g for short periods without blacking out - and even that is only tolerable for a few minutes.  But again, it's hard to maintain a continuous g-force of more than 1g - so we really don't know what would happen if someone were to live at 2g for a year.   We really only know that humans need more than zero-g and less than 4g - but whether the practical long-term limit is 0.5g to 2.0g - or 0.9g to 1.1g - or 0.1g to 2.0g...we really have no idea.


 * SteveBaker (talk) 15:49, 2 November 2013 (UTC)


 * SteveBaker, You might want to revisit your claims about hydrogen and oxygen. Oxygen occurs in the atmosphere in significant quantities in gaseous form in direct contradiction of your statement, and as I understand it, even with the high levels of oxygen, which rapidly oxidises hydrogen, ongoing dissociation of water due to solar radiation still results in a steady loss of hydrogen to space as a result of the low atomic weight.  — Quondum 16:55, 2 November 2013 (UTC)

How Can the Most Distant Galaxy be 30 Billion ly away when the universe is only 13 Billion ly old?
I can't wrap my head around it. Recently, a most distant galaxy was discovered and its distance given as 30 billion light years from earth. Given that the universe in "only" 13.8 billion light years old, and doesn't expand faster than light speed, how is that possible? (The article also mentions "another" distance as 13.1 billion light years. So what are these two distances?) Even if space has expanded, wouldn't it have expanded at the same rate, i. e. having a maximum radius of 13.8 billion ly? You couldn't fit 30 billion light years in that... Help, please?! -- megA (talk) 22:06, 1 November 2013 (UTC)


 * Space can expand faster than light. The lead of your linked article Z8 GND 5296 has a link on expansion of the universe which says: "While special relativity constrains objects in the universe from moving faster than the speed of light with respect to each other, it places no theoretical constraint on changes to the scale of space itself." PrimeHunter (talk) 22:28, 1 November 2013 (UTC)
 * Are you saying that space expands faster than light and somehow drags galaxies along with it? ←Baseball Bugs What's up, Doc? carrots→ 23:04, 1 November 2013 (UTC)
 * Yup, that can happen. The schoolbook analogy is a snail crawling on a rubber band. The snail can only crawl at some maximum speed but that speed is independent of and places no limit on how fast you can stretch the rubber band. 88.112.41.6 (talk) 23:15, 1 November 2013 (UTC)
 * Especially that damned inflation. Only I wouldn't characterize it as "dragged". The expansion is AFAIK in all directions, not just one. When a balloon is inflated, you wouldn't say a spot on it was dragged. Clarityfiend (talk) 23:35, 1 November 2013 (UTC)
 * Maybe "pushed" instead of dragged. But either way, the spot on the balloon is just along for the ride. ←Baseball Bugs What's up, Doc? carrots→ 23:56, 1 November 2013 (UTC)
 * To say "space expands faster than light" is a confusing (and inappropriate) way to describe it; a better way of saying it is that the total distance between two particular points can increase at more than the speed of light due to the expansion of space, if the points are far enough apart. A nice way pf thinking of it is that the path that the photon from the galaxy traveled in the past got stretched in the time since it went past, but this stretched path length is what we mean when we determine the (current) distance to the galaxy, which is inherently longer than the distance traveled by the photon.  — Quondum 00:30, 2 November 2013 (UTC)
 * There's something innately confusing about that description, because it describes a spacelike interval between two galaxies that are a long way away from one another. The article even says it is beyond the cosmic horizon - does that mean that light from it "now" (whatever "now" involves it 30 billion ly away...) will never reach us?  If so, I'm really curious to hear just what frame of reference this distance is measured in... Wnt (talk) 01:00, 2 November 2013 (UTC)
 * The universe is changing over time. For example, the CMBR temperature is currently about 2.7 K. "Now" in a distant galaxy is whenever the CMBR temperature measured there is 2.7 K. The distance "now" to that galaxy is the minimum, over all chains of galaxies connecting it to ours, of the sum of the distances between consecutive galaxies in that chain, measured in the local frame where the CMBR is isotropic. There are other ways to define it, of course. The important thing is that the Big Bang breaks the symmetry of reference frames.
 * I'm not sure what it means to say that it's beyond the cosmic horizon, but probably it does mean that light emitted "now" will never reach us. The cutoff distance for that is $$\int_{now}^\infty \frac{c\,dt}{a(t)}$$, where a(t) is the function given here. I can't remember what that evaluates to but it's more than 18 billion light years and likely less than 28. -- BenRG (talk) 05:45, 2 November 2013 (UTC)

A dumbed down Kindergarten level explanation. Let's take the balloon model as explained above by the IP and others. Then the rate at which two points are receding will be proportional to the distance (if every meter is expanding and becoming larger at some rate then if you have twice as many meters between two points, so the distance must grow at twice the rate). So, you then have Hubble's law that says that the speed v between distant galaxies is v = H d, where d is the distance and H the Hubble constant. If we ignore the new results about the acceleration of the expansion rate due to dark energy and simply consider H to be constant, then that is good enough to roughly understand the two numbers 13.1 billion light years and 30 billion light years.

Then consider what happens when we observe distant galaxies. You don't see them as they are now, rather, you see them as they were when the light left the galaxy. If you measure the distance what you get is not the distance they are from us today, rather you get the distance from us to the point where they were when the light left the galaxy. The Hubble law v = H d based on these distance and speed measurements relates the distance in this sense to the velocity. The farthest you can see in theory is that distance where the velocity would be the speed of light. So H = c/(13.8 billion light years) = 1/(13.8 billion years).

Then we can ask how far away would a galaxy be from us today if d = 13.1 billion lightyears. If the distance at time t is x(t), then the speed is the derivative dx/dt and this must equal H x by the Hubble law, so you have dx/dt = H x. This is a differential equation with the solution x(t) = x(0) exp(H t) = x(0) exp[t/(13.8 billion years)]. Then for the galaxy we can take t = 0 the moment when it was at the point where we observe it now. That point is 13.1 billion light years away, so x(0) = 13.1 billion light years. The distance today is then found by taking t = 13.1 billion years, this gives x(13.1 billion years) = 33.8 billion light years. So, this dumbed down model is quite accurate. Count Iblis (talk) 01:21, 2 November 2013 (UTC)


 * Well, it's an interesting way to calculate things. For example, consider a soda can that has fallen into a black hole and is about to hit the singularity.  You track back to a time when it had an observed distance to a spaceship it was tossed out of, track the relative rates of separation, get a distance...  But it's not a distance the can can travel to get anywhere.  Depending on what frame you looked at them in when they were close together, the distance between them could have been foreshortened to any degree from nothing to practically 100%, yet you could still be in the black hole next to the tin can. Wnt (talk) 04:51, 2 November 2013 (UTC)


 * Embedded LambdaCDM geometry.png made this picture years ago. It's an illustration of the shape of the expanding universe (earlier times at the bottom), based on cosmological parameters measured by WMAP. The yellow line on the right is a distant object, the brown line on the left is us, the diagonal red line is light from the distant object to us, and the orange line at the top is the separation "now" between us and the distant object. Each grid rectangle is 1 billion years by 1 billion present-day light years. You can check by counting grid rectangles that the object is 28 billion light years away "now" and the light was emitted 12 billion years ago. The light always travels at a 45° angle to the grid lines. -- BenRG (talk) 05:09, 2 November 2013 (UTC)
 * Thank you very much, BenRG, that picture really makes it clear how these two figures are to be understood. So, to sum it up, the light emitted from "that galaxy" has traveled for 13 million years, but the space it has already traveled through has expanded in the meanwhile, namely to 30 million light years. Ahh, sweet Epiphany! -- megA (talk) 11:11, 2 November 2013 (UTC)
 * I think you mean billion in all cases. Nil Einne (talk) 08:14, 4 November 2013 (UTC)

Could there be an error in the phrasing of the original question? A light year is a measure of distance, not time. So to say that "the universe [is] 'only' 13.8 billion light years old" is incorrect. Perhaps the OP meant something different and everyone else (but me) understood anyway. El duderino (abides) 06:15, 5 November 2013 (UTC)