Wikipedia:Reference desk/Archives/Science/2014 December 10

= December 10 =

Relativity Question
Consider two identical space ships A and B. Each ship has similar vertical as well as horizontal light clocks.

Vertical light clock; consists of two mirrors, one at/on the ceiling and the other on the floor of ship, between which a light pulse is bouncing.

Horizontal light clock; consists of two mirrors, one in the prow and the other in stern of ship, between which a light pulse is bouncing.

All clocks are equal in lengths and synchronized on ground.

Both pilots set out on their computerized synchronize journey in the same direction into space. Both ships are moving parallel to each other and separated by conspicuous perpendicular distance.

After some time, let both ship attained a constant of speed of say 0.8c in space. Since both pilots can’t tell that they are moving therefore time dilation and length contraction wouldn’t be observed by each pilot from their respective frame of reference

Although they wouldn't see length contraction LC but horizontal clock would be contracted due to LC therefore I can say that they could still observe LC with the help of difference in bouncing of light pulse between the respective mirrors of both vertical and horizontal clocks. So isn’t this against the Einstein's first postulate, which says that the laws of physics are the same in all inertial frames of reference.162.157.238.164 (talk) 06:06, 10 December 2014 (UTC)Eclectic Eccentric Kamikazi


 * It is not correct to say that their horizontal clocks are, in general, length contracted, because their non-accelerating reference frame is no more or no less preferred than any other, including the one from which they launched. We might commonly think of their launch platform as being stationary and the rockets to be moving at 0.8c, but, in the reference frame of the rockets (which is as correct as any other), they are stationary and their launch platform is receding at 0.8c.  Their horizontal clock is length contracted in the reference frame of the launch platform, but not in their own reference frame, where, instead, one dimension of the launch platform is length contracted.  Measurements of time and distance between events in spacetime are relative to the reference frame in which they are taken, hence relativity.
 * The horizontal and vertical clocks remain in synchronization in all reference frames. For that of the rocket it is because there is no length contraction.  For that of the launch platform it is precisely because there is length contraction, since otherwise it would have taken longer for light to travel between the two horizontal mirrors (moving parallel to their light pulse) than between the vertical ones (moving perpendicular to their light pulse).
 * Note that the problem, as you stated it, made no use of your second rocket. -- ToE 07:13, 10 December 2014 (UTC)


 * You wrote: "After some time, let both ship attained a constant of speed of say 0.8c in space". No, you can not say that! You can only say a speed relative to something else! You can never say relative to space, there is simply no such concept. So when you compare the speed of the two ships to EACH OTHER, the are stationary, so there is no LC. Ariel. (talk) 11:41, 10 December 2014 (UTC)

OK thanks for your answer. Since the bouncing of the light pulse of H clock would be changed for stationary observer due to LC as compared to V clock therefore my another question is would both H and V clocks still remains synchronized for stationary observer for calculation purpose?162.157.238.164 (talk) 19:08, 10 December 2014 (UTC)EEK


 * Two light clocks with the same velocity will tick at the same rate regardless of orientation. Your intuition is that they won't, but if you work it out numerically you'll see that that intuition is wrong. A clock oriented perpendicular to the motion ticks slower (by a factor of $$\sqrt{1-(v/c)^2}$$) because the light has to travel farther. A clock oriented parallel to the motion ticks faster (by a factor of $$1/\sqrt{1-(v/c)^2}$$) because its length is shorter, but slower (by a factor of $$1-(v/c)^2$$) because the light has to catch up to the receding mirror. -- BenRG (talk) 23:46, 10 December 2014 (UTC)


 * this doesn't seem correct. Clocks moving away/toward each other at a relative velocity will never appear to be the same from an oberver in either frame.  It's possible that a 3rd frame observer would say the clocks tick exactly but it would be disagreeing with observers in the other clock frames. --DHeyward (talk) 21:02, 11 December 2014 (UTC)

I couldn’t find such derivation on the net so what website do you suggest as I am still not sure whether the contracted length of H clock is included in the math or not. I just asked because the length between mirrors is shortened in the H clock due to LC as compared to V clocks for stationary observer. So how come they tick at the same rate when the distance b/t mirrors of V clock is greater than the distance b/t mirror of H clock. I can look by myself if provided any link related parallel clocks only162.157.238.164 (talk) 02:38, 11 December 2014 (UTC)EEK


 * Suppose you can run 10 m/s. (That's pretty fast.)  If two cars are parked along the street with 100 m of open space between their bumpers, it will take you 10 s to run from one to the other, or 20 s to make a round trip.  Now imagine the same situation, but with the two cars traveling at 5 m/s, keeping exactly 100 m open between them, maintaining a taut 100 m measuring tape.  If you start at the front bumper of the trailing car, in 10 s you will cover 100 m, but the cars themselves will have covered 50 m and you will only be half way to the rear bumper of the lead car.  You will need another 10 s to reach that bumper and start back.  It will then take you only 6 2/3 seconds to make it back to the trailing car because during that time you will cover 66 2/3 m but the trailing car will advance 33 1/3 m.  So your total round trip time is 26 2/3 s.
 * But that is only half of the story. There is a third car pacing the trailing car, along side it but with 100 m of open space between them.  (It's a very wide street!)  I can also run 10 m/s, and I am running back and forth between those two cars.  Back when they were stationary, I could make the round trip in 20 s.  Now that they are moving, I need to run at an angle so that for every 1 s during which I run 10 m along the hypotenuse, I also track 5 m down the street (pacing the cars), and thus make good 5 √3 m across the street (perpendicular to the cars' motion).  (The angle I run is 30° from the perpendicular.)  Since the perpendicular component of my speed is only 5 √3 m/s, the trip across and back (while pacing the cars), will take 200 / (5 √3) s = 40 / √3 s = (40/3) √3 s = (13 1/3) √3 s = 23.09 s.
 * If you and I were in synch when the cars were still, both taking 20 s round trip, we should fall out of synch when they are moving because your round trip will take about 26.67 s while mine will take only 23.09 s. If we lived in some crazy universe where we somehow stayed in synch, both as observed by the drivers of the cars and the observers standing along the side of the street, something strange must be going on.
 * Length contraction is what we need to explain the continued synchronization from the point of view of the observers on the street. If, as observed from those on the street, the open space between the leading and following car, which is spanned by a taut 100 m tape, somehow shrunk by some factor α (shrinking the tape with it), then the round trip time would be α (26 2/30) s, but this needs to equal (13 1/3) √3 s for us to remain in synch, so solving for α we get α = √3/2 = sqrt(1-52/102) = (1-v2/c2).  Note that length contraction does not apply to the calculation of my round trip time because it is not the observed speed of the cars which is affected; it is just the length of the cars and the 100 m tape stretched between them in line with their direction of motion which is contracted.
 * Rework this with v and c to derive the formula for length contraction. -- ToE 22:32, 11 December 2014 (UTC)

How does an embryo know where to grow limbs etc
Questiion by Ram nareshji deleted as probable copyvio Nil Einne (talk) 14:51, 16 December 2014 (UTC)
 * Not an expert, but embryonic cells "know" where they are relative to each other by chemical signals, same as in adults. These molecules are known as morphogens (specific examples include the sonic hedgehog and β-catenin). The amount of morphogen in a region of cells determines which gene gets turned on and thus what it develops into. And the amount varies by how far they are from the source of the morphogen. Though AFAIK, scientists don't fully understand their mechanisms yet (like how they are dispersed, how the different concentrations are kept rigidly, etc.).


 * As for arms, etc., the earliest cells are unspecialized (stem cells). They "define" which becomes what early on by how they are arranged. Remember that their "up, down, left, and right" is relative to them, not to their surroundings. The first multicellular hollow ball of stem cells, the blastula, separates into distinct germ layers during gastrulation - the ectoderm, mesoderm, endoderm. Each of these layers develop into something different upon further division. What develops after that is basically a worm, composed of a series of repeating "segments" (somites) with a distinct head and tail area. Somites are initially undifferentiated. The job of determining which segment becomes what falls on Hox genes. So that for example, one segment becomes the head, another becomes the arm and pectoral girdle, another becomes a rib+vertebra, etc. Hox genes are arranged in clusters within the DNA relative to which gets expressed first. The first hox gene is for the head for example, while the last is for the tip of the tail (the tipmost segment of the coccyx in humans). Though these clusters themselves are often repeated into a kind of redundant failsafes. This page from the University of Utah explains it pretty well. It helps if you view embryogenesis as more or less echoes of evolution in fast forward. -- O BSIDIAN  †  S OUL  12:01, 10 December 2014 (UTC)


 * The reference to "Ontogeny recapitulates phylogeny" is useful, but bear in mind that even though the principle often seems an informative guide it tends to be looked down on because it isn't exceptionally accurate. I have a soft spot for Ernst Haeckel but some are critical of his confirmation bias.  Evolution can alter an embryonic stage (for example, human embryos don't develop fully open gill slits); it just often has no need to, leaving interesting relics.  In general morphogenesis progresses much like the way the Europeans divided up the New World among themselves, with a series of vague and rough divisions followed up by ever more precise and local distinctions.  Except of course that after the initial egg begins to grow, those divisions can cause actual changes in the "landscape" when certain parts are instructed by those genetic cues to divide.  You may find Drosophila embryogenesis, gap gene, pair rule gene etc. to be interesting in the case of the fly embryo (even though Drosophila is such a specialized system, which did evolve to develop differently during embryogenesis than its ancestors, that little else is much like it even among other insects.  It allows free diffusion of transcription factors in a massive syncytium whereas almost anything else uses morphogens passed between separate cells to create that pattern indirectly) Wnt (talk) 14:21, 10 December 2014 (UTC)


 * Besides chemical signalling between cells, some recent research has shown that mechanical forces between cells also matter. The combination of chemical signals and the physical pressure of neighboring cells helps individual cells "figure out" where they are in the body, which triggers them to develop in a particular way. --Srleffler (talk) 17:51, 10 December 2014 (UTC)


 * Good point.  is one review of some of these cases. Wnt (talk) 18:06, 10 December 2014 (UTC)

Do mammals develop tolerance to anticholinergics?
Question by Ram nareshji deleted as probable copyvio Nil Einne (talk) 14:53, 16 December 2014 (UTC)


 * One group of researchers has found that (in the rat brain model) tolerance to the antimuscarinic agent scopolamine is paralleled by increased binding to muscarinic receptors and increased behavioral response to the centrally active muscarinic agonist pilocarpine. The conclusion they reached was that decreased availability of ACh agonists can significantly increase the availability and functional activity of central muscarinic ACh receptors to reflect "disuse supersensitivity."


 * That ought to give you a direction to do further research on your own (I noticed you've got this question out in Yahoo and one of the stack sites, too). Good luck. loupgarous (talk) 20:49, 12 December 2014 (UTC)

Genes & Memory
I watched a documentary recently about a child in Scotland who displayed very detailed knowledge of a 'past life' in a remote corner of Scotland. Lots of research went into this child's 'memories' (many of which turned out to be correct, although some didn't) but nobody brought up the idea of whether the mother or father was related in some way to the family the boy was describing. I was wondering whether there has been any research on whether memories can be passed down through genes. I recently read something about a type of bird which flies south in the winter, has its babies, then dies after rearing them to adolescence, then the adolescent birds fly back up north to the exact same spot their parents came from (despite never having been there before). I wonder if there is a link here. KägeTorä - ( 影 虎 ) ( Chin Wag )  10:03, 10 December 2014 (UTC)
 * There was a guy named Lamarck who proposed just that, early in the 19th century, as an evolutionary theory. See Inheritance of acquired characteristics. ←Baseball Bugs What's up, Doc? carrots→ 10:10, 10 December 2014 (UTC)
 * Thanks, BB. I am aware of the theory of evolution. Certain behaviours become instinctual, for example. A catterpillar becomes a butterfly and can fly almost immediately after leaving the coccoon, without ever having seen another butterfly before in its entire life. Your link is helpful and relevant, though. I was wondering more, however, if instinct is just inherited memory, and how it could relate to children's supposed 'memories' of 'past lives', and if there has been any research done on this. I've read a lot about the subject of past lives - not for any kind of mystical reassurance or anything, but purely because it's a phenomenon that seems to exist.  KägeTorä - ( 影 虎 )  ( Chin Wag )  10:56, 10 December 2014 (UTC)
 * You may find Past life regression interesting. ←Baseball Bugs What's up, Doc? carrots→ 11:20, 10 December 2014 (UTC)
 * You may wish to read Genetic memory (psychology). -- ToE 11:28, 10 December 2014 (UTC)
 * That's perfect, but a little short. I will keep looking around. Thanks.  KägeTorä - ( 影 虎 )  ( Chin Wag )  11:49, 10 December 2014 (UTC)
 * I'm not aware of a case of bird migration or other animal navigation that matches what you describe; please link or specify species if you can! Though I don't know of birds that die on cue like spawning salmon, I know better than to think anything is impossible in biology.  Regarding the paranormal, I'd say that precognition can be used to explain anything. Wnt (talk) 14:57, 10 December 2014 (UTC)
 * A more extraordinary example might be the Monarch butterfly, which finds its way along it migration path to and from Mexico despite having no apparent overt knowledge of where they're going or why. ←Baseball Bugs What's up, Doc? carrots→ 14:59, 10 December 2014 (UTC)
 * Definitely extraordinary in its way, but to match the OP's particular claim you'd have to be able to take some specific Monarchs, let them loose from a location well removed from their usual haunts, and have the progeny of that butterfly return to the same location. So far I haven't seen that suggested as a means for dealing with threats to their habitat; the assumption has been that they will return to the same location no matter what.  But I don't know if that assumption has been tested.... and this is biology, after all, and so you never really know until you do. Wnt (talk) 18:09, 10 December 2014 (UTC)


 * Recently some work appeared in Nature that is highly relevant to notions of inherited or "genetic" memory, see here . Basically, mice can inherit fears and associations for multiple generations, in response to stimuli they have never received. This is thought to be an epigenetic effect. This post (a nutrition blog, but with 10 good scientific refs) talks about epigenetic effects in general, and discusses a few famous examples, notably that starvation of a grandparent can affect obesity/nutrition in a grandchild: SemanticMantis (talk) 15:47, 10 December 2014 (UTC)
 * Right...but the situation in those mice is like this: The momma mouse is starving...her cells, being short on nutrition...turn off some metabolic processes and turn on others by switching bits of DNA on and off.   If she gets pregnant at the time she's starving, her egg cells have those genetic switches set to "Starvation Mode" and so the baby mouse is born with all of it's cells running in starvation mode.  This effect doesn't rely on the brain of the momma mouse passing memories of being hungry onto the baby mouse...it only works because every cell in the momma mouse's body was in starvation mode at the time that conception occurred.   This mechanism really can't explain how a young child can remember the vocal patterns making up the name of some distant ancestor (or whatever) because memories of ancestor names simply aren't stored in every cell of the body...they can't possibly be - there isn't remotely enough storage space.  (See my post below).    Epigenetics can explain how some seemingly lamarkian effects are possible - but only when the change to the ancestor's body reacts as a whole to some event that has to affect every single cell...and the complexity of the 'racial memory' has to be exceedingly low...starvation, perhaps some specific reaction to a disease, that kind of thing.   It's *NOT* the same as the instinctual memory that allows my pet dog do know how to do a "play bow" when he wants to play.   That has to be handed down in the full genome (where there is much more storage capacity) - *and* it's the same for all dogs over thousands of years so there is no issue of how the structure of the momma dog's brain affected the play-bow gene in the puppy's DNA.  Epigenetics simply can't do what is being asked of it by the OP...it just doesn't work like that. SteveBaker (talk) 17:17, 10 December 2014 (UTC)
 * I suggest you read the nature article I reference above (ideally the research article, not just the news blurb) before you dismiss inheritance of learned behavior. I was not invoking anything to do with instinct, nor do the authors of that paper. SemanticMantis (talk) 17:31, 10 December 2014 (UTC)
 * Yeah - I did. It's one study that's never been repeated and everyone finds to be extremely unlikely.  But it's not inconceivable that some aspect of the biochemistry here is an epigenetic effect like the starvation thing.  We don't know whether that one chemical has a specific genetic meaning to the mouse.  But it's one hell of a leap from "Mouse epigenetics passing fear of a nasty smell onto offspring" to "Child remembering past lives"...and the numbers simply don't allow the latter to be the case.  SteveBaker (talk) 18:07, 10 December 2014 (UTC)
 * Epigenetics is a path to inheritance of acquired characteristics. But the intelligence passed on would be expected to be the intelligence of a single cell, not an entire brain.  A single cell might well remember whether a smell is delicious, sickening, or fearful simply by putting the right marks to cover and uncover the sites of specific transcription factors that determine where those particular olfactory neurons send their axons (that's rhetorical speculation - I don't know of such data).  To remember a sequence of memories, in order... it's harder.  I'm not going to say impossible, not in biology, but I would really need some convincing; it would need some truly remarkable mechanism.  (For example, there was a 1960s suggestion of "memory RNA", because the genetic code is just that remarkable; but so far I've never heard a threat of proof for it.  Which direction a planarian circles doesn't cut it, since a single cell could learn that also) Wnt (talk) 18:15, 10 December 2014 (UTC)


 * But there are sharp limits on what genetic memory can do - even in principle. There are 3.2 billion base-pairs in a DNA strand, each of which can take on four possible values.  So in computer terms, there are only 6.4 gigabits (rather less than 1 gigabyte) of storage space on a DNA strand...and 99.6% of that is identical between all humans, and cannot be storing stuff like remembering who you were in a previous life.  So we have about 4 megabytes of "personal" DNA storage that has to contain everything that makes you different from me (eye color, hair color, lactose intolerance...etc) - including whatever of these hypothetical past-lives there are.  That's not enough room to store very much information.


 * The brain, on the other hand has 1,000 trillion synapses - which could in principle store around 100 terabytes of data.


 * Worse still, a memory of a past life would have to be turned into brain cell connections as a single sperm or egg cell develops into a living human being. This would be exceedingly hard to imagine.  To remember a name from a past life, for example, would require creating brain structure to store that name long before the baby has learned language.   This seems impossibly unlikely.


 * Even harder is how the distant ancestor who saw that information managed to transfer the brain structures corresponding to those memory patterns into a DNA strand somewhere in a sperm or egg cell.  For the female line, this is an incredible stretch because women are born with all of the egg cells they'll ever have already formed and stuffed with DNA.  To change that DNA at the base-pair level is inconceivable.  The epigenetics that cause starving mice to have offspring that get obese happens because sections of DNA ("genes") are turned on and off by a single base-pair that controls some aspect of metabolism...there are even fewer of those switches than there are genes - and we only have 30,000 genes.  So now we're down to about 3,000 bytes of information - most of which can't be inherited without killing the offspring!


 * So if (and it's a huge IF) we had literal memories stored in DNA, they would have to be extremely minimal in nature...I doubt very much that anything as complex as remembering names and faces, places and events could conceivably be stored that way.  The numbers simple don't add up.


 * My bet is that these stories of children remembering past lives are the usual kind of nonsense that people come up with that result in stories of being abducted by aliens - the same kinds of story that lead people to believe that their children have indigo auras that confer magical powers on them.  If you tell a child that they remembered something amazing and praise them for it - you can be sure they'll dream up more stuff to please you.


 * SteveBaker (talk) 16:56, 10 December 2014 (UTC)
 * My point was that inheritance of learned behavior (i.e. a memory) is now well documented in the mouse model. I don't think counting DNA base pairs is a very profitable line of attack on the issue of how much can be inherited, but that's far beside the point. (briefly, animals are not computers, and you're ignoring the vast complexity and informational storage afforded by gene expression and Gene_regulatory_networks, multi-function genes, DNA, function associated with placement of loci within secondary, tertiary and quaternary structures, etc. etc. If you insist on a computer analogy, then a more appropriate concept might be e.g. a 42kb Zip bomb that unpacks to 4.5 petabytes) I agree that it is highly unlikely and indeed implausible that a human could somehow inherit memories of specific places, names or faces, but the mouse work linked above is very intriguing to me nonetheless. This is all rather new stuff, so we'll just have to keep our eyes on that research over the next few years and see what they turn up. SemanticMantis (talk) 17:29, 10 December 2014 (UTC)


 * Absolutely no it's not! "Well documented" would means something that would pass WP:MEDRS and this one isolated mouse study doesn't remotely do that!   This is incredibly tenuous evidence - and (as we should all say 10 times before breakfast) - extraordinary claims demand extraordinary evidence...and we don't have that.


 * Whatever is going on, you have to explain how the memory is stored in the single-cell embryo since that's the only link between mother/father and child. The only storage mechanisms that there are are inside that teeny-tiny cell.  You can count the number of bits in that data stream and it's flat out not enough.   So unless there is magic going on - or some incredibly unknown storage system at the cellular level - that isn't remotely enough to explain memories of past lives.  Not by many orders of magnitude.  So, yeah, we have to count base pairs and the other stuff.


 * There are other possibilities - like gut flora passing from mother to offspring and all of their DNA - but that doesn't explain the case with in-vitro fertilization. You can't compress petabytes of arbitrary data down to 42Kb...that's bullshit.  A Zip bomb can indeed unpack to a vast amount of data - but it's a vast amount of zeroes or some very simple repeating pattern.  Data compression arguments are entirely fallacious when storing arbitrary data.


 * Then, it's plausible to imagine an epigenetic effect producing hormonal changes in reaction to chemical stimuli - which could certainly result in the hormonal effects of fear being produced in response to the detection of some chemical at the cellular level. But the creation of memories in the brain is a massively less likely thing!  You'd need to have trillions of neurons connecting up just right to produce all of the memories that are claimed for these children.   Worse still, those connections would have to be made as the brain is forming - so the child would have the memory of someones name before they'd learned language.   It's just crazy to imagine that's even remotely possible.  SteveBaker (talk) 18:22, 10 December 2014 (UTC)
 * By "well documented" I meant that it has been documented well, i.e. it has been accepted for publication in one of the world's most competitive and prestigious journals. That is a high bar to pass, I know in part because I've reviewed articles for a Nature journal and got to see the process up close.
 * I did not say that these inherited behaviors are 100% unassailable fact, or that the process was completely understood - it is not either. It is a bit of a misrepresentation to say it's just a single work though, even though it is recent it already has 97 citations, and some of those are follow up and related work. I also never said that arbitrary data could be compressed with compression ratios similar to a zip bomb - but recall that there are many repeated structures in an animal body. SemanticMantis (talk) 16:17, 11 December 2014 (UTC)

Speed of Light
If two people in a spacecraft were travelling at (or faster than) the speed of light, would they be able to see each other, the spacecraft, their surroundings, or even themselves? KägeTorä - ( 影 虎 ) ( Chin Wag )  11:20, 10 December 2014 (UTC)


 * You can only measure speed when comparing two items. So when you compare the speed of the travelers and their ship there is no speed difference between them, so they are not traveling at the speed of light and there is no problem. You might then decide to compare the ship with a star and find that it could be traveling at near the speed of light compared to that star - but you can also compare that ship with a different ship and find that the speed compared to that other ship is half of the speed of light.
 * Or in other words speed is relative: That means you can only compare two items to find a speed, there is no overall speed you can ever assign to an item, it's always in comparison with something else.
 * You can even have such oddities as two ships going in opposite directions from a star each going at almost the speed of light compared to the star - but when the two ships compare speeds to each other they are not going at double the speed of light as you might expect! But only at near the speed of light! Strange stuff. Speed does not add together in logical ways, it's just how the universe works. If you wish to learn how to add speed, and why it works when it appears to make no sense you can, it's not especially difficult but will need some math. As just a brief explanation: This is why time slows down - so that when you add speed together you get results - instead of double the speed of light, you slow down time so you end up with a speed lower than the speed of light. (There is more to it, but I don't want to overdo the explanation.) Ariel. (talk) 11:30, 10 December 2014 (UTC)
 * Yes, it's basically the impossibility of crossing the road by forever going in half increments - you never get there. My question was more about the people and ship may be all travelling at the same speed, but how would they be able to see anything else, as the light eminating from them would be surely distorted.  KägeTorä - ( 影 虎 )  ( Chin Wag )  11:42, 10 December 2014 (UTC)
 * No, it is not that at all, it's not even similar to that. Read my text again. The light from them is not distorted, it is perfectly ordinary because when they compare speeds with each other, they are still, so there is nothing distorted at all. Ariel. (talk) 11:46, 10 December 2014 (UTC)
 * Thanks. I think I understand. This wasn't a joke question, by the way. It was an actual query. I do feel a little silly, now, though.  KägeTorä - ( 影 虎 )  ( Chin Wag )  11:53, 10 December 2014 (UTC)
 * There is a valid question regarding the visual appearance of objects moving at high relativistic speeds (with respect to the observer, of course). I don't know if we have an article which addresses this, but Special relativity gives a lot of external links, including spacetimetravel.org. -- ToE 12:49, 10 December 2014 (UTC)
 * Something I've wondered about: If a photon could "see", what would it see? ←Baseball Bugs What's up, Doc? carrots→ 13:10, 10 December 2014 (UTC)
 * My understanding is it would be a brief show indeed. Lorentz contraction increases with the speed of an object, so to the photon the entire [finite observable; you're on your own about the rest] universe is flattened to a disk.  It is emitted at a point in the disk and absorbed at the same point in the disk, and so its entire trip takes no time at all. Wnt (talk) 14:05, 10 December 2014 (UTC)
 * Anything travelling at the speed of light experiences the entire life of the universe in zero time. So the photon wouldn't have time to process anything before the universe ended...no matter how long the universe might last.  A photon would also have no sense of place in the universe because the universe itself will have been compressed into zero size in whichever direction it's heading...but what if it's bounced off of a mirror at 45 degrees to it's direction of motion...it's really hard to wrap your head around.   (A yet harder question is how black holes feel about that...given that the universe passes them by in zero time too...yet they are capable of evaporating gradually over time). SteveBaker (talk) 16:28, 10 December 2014 (UTC)
 * Hmmmm, I'm thinking anything moving at the speed of light would experience the whole lifetime of the universe in zero time, but also experiences any shorter interval in zero time. Clearly photons that are emitted and absorbed fly for only a finite elapsed duration.  A mirror imposes what seems like an infinite acceleration, and I don't claim to guess the general relativity of that.  (I'm not sure, maybe even if you count near field effects it's still infinite???)  It would be interesting to see that broken down here.  But I don't get the part about black holes ... what part of a black hole is moving at the speed of light?  Unless maybe the event horizon, but besides being a purely mathematical object it too can have finite duration like a photon. Wnt (talk) 16:59, 10 December 2014 (UTC)
 * Hint: Gravity also distorts time. SteveBaker (talk) 16:48, 11 December 2014 (UTC)


 * This question connects well with the rule that nothing can travel faster than light. In the "observer"'s frame (whoever it is who says that the spacecraft is traveling superluminally), light emitted/reflected from one person will be left behind and never reach the other (unless the other is aft; then consider the reverse direction instead).  In the spacecraft's frame, of course vision works normally.  This is in contradiction to the literal principle of relativity, which is that the same rules of physics should apply according to observers in either frame.  --Tardis (talk) 14:20, 10 December 2014 (UTC)

To directly answer the OP's question:


 * "If two people in a spacecraft were traveling at (or faster than) the speed of light..."

Well, right there we have a problem. Nothing whatever can travel faster than light. Even hypothetically. The mathematics of time and space break down at speeds that are faster than light...the math requires you to take the square root of a negative number if that were to happen - and that's simply not possible (try it on your calculator and see what it says!). Actually, nothing that has any mass can even travel *at* the speed of light because it would take an infinite amount of energy to push it up to that speed. Hence, the situation you're asking about simply cannot come about.

So this is a question about an impossible situation...and there is no reasonable answer to that. In order to produce an answer, you have to make drastic assumptions about what would happen if the rules of physics were very different than they appear to be - and as soon as you do that, we can no longer use the laws of physics that we know and understand to form a reasonable answer. So as soon as we break the rules to allow the question, we have no way to form an answer. I could say "Well, *IF* you could go faster than light, then the entire visible universe would spontanously turn into a slightly overripe banana."...and that's every bit as good an answer as anything else that can be said on the matter.

It seems as though this is a question like "What does it sound like to two people who are inside a plane flying faster than the speed of sound?" (which is a question we can certainly answer - and as someone who flew that fast on Concorde, I can tell you that it's a bit of an anticlimax because nothing whatever happens!) But this question isn't like that because light and sound work VERY differently. SteveBaker (talk) 16:28, 10 December 2014 (UTC)
 * Exactly. Any question about faster-than-light travel is based on hypothetical science.  Science fiction involving faster-than-light travel involves assumptions about future discoveries in physics, but the question doesn't appear to have to do with science fiction.  As SteveBaker points out, objects with mass cannot travel at the speed of light, because they would have to pick up infinite kinetic energy.  Photons, which do travel at the speed of light, have no rest mass, so that their energy is a zero-times-infinity (depending on the frequency of the wave).  There are theories that there may be particles called tachyons which travel faster than light only, because they are theorized to have imaginary rest mass, but there are criticisms of these theories.  The question isn't answerable within the scope of relativistic physics, which is the best physics that we have, because it is about things that don't and can't happen.  Robert McClenon (talk) 16:49, 10 December 2014 (UTC)
 * Coming up with an imaginary number doesn't always mean the end of the story; for example, a hyperbola is an ellipse with an imaginary minor axis, but both are perfectly good orbits. I'm sure there must be various speculations in Arxiv, which has a wide range of hits for "faster than light" at least, that try to delve into this question one way or another.  But, yes, we're scraping for speculation; the best we can hope for is sourced speculation. Wnt (talk) 17:16, 10 December 2014 (UTC)
 * The imaginary rest mass isn't the problem with tachyons. The problem with tachyons is that, if they interact with tardyons (regular particles with mass), then they permit faster-than-light communication.  Robert McClenon (talk) 17:19, 10 December 2014 (UTC)
 * Well, to me causality seems more like religion than science; though of course it is contentious precognition would be a counter-example. Wnt (talk) 18:00, 10 December 2014 (UTC)
 * I don't understand. The discussion about tachyons doesn't get into causality.  Robert McClenon (talk) 18:35, 10 December 2014 (UTC)
 * Tachyons are like unicorns...mythical beasts. We don't know how unicorns affect causality either.
 * The issues of causality are certainly real - they are a rock-solid consequence of basic relativity. Doubting that they exist isn't really a tenable position unless you're going to deny an enormous amount of established, experimentally-verified science.  When the rate of passage of time depends on relative speeds, the idea that event A happens before event B is kinda meaningless.
 * However, trying to argue in favor of precognition is getting into crazy-territory. You need things to be moving incredibly fast relative to each other to get any measurable issues of causality - so if someone here on Earth is claiming precognition more than a microsecond in advance of an actual event, then I call "bullshit".  You can't use the fact of relativity to explain away fringe theories and blatant pseudoscience like that. SteveBaker (talk) 20:54, 10 December 2014 (UTC)
 * There are lot's of things that seem tied to this. Namely, time always flows forward, net entropy always increases,  and universe handedness (i.e. disparate amounts of matter/antimatter, gravity always attracts, no magnetic monopoles.)   --DHeyward (talk) 03:32, 11 December 2014 (UTC)

The OP's question can be made to be more physically possible by making the craft travel in a medium that is not a vacuum, so that light doesn't go as fast as c in it. (Actually, then what do they see? Is it just like SteveBaker's example of Concorde: it doesn't look very interesting?) Double sharp (talk) 02:19, 11 December 2014 (UTC)
 * Cherenkov radiation is charged particles moving through a dielectric faster than light would move through the medium. The speed limit is only in a vacuum. It would be interesting if permittivity of free space could be manipulated.  No idea what that would do.  --DHeyward (talk) 03:41, 11 December 2014 (UTC)

Vaska's complex and its amazing triple-bonded oxygen
At work we were doing the typical Wikipedia surfing, and ran across Vaska's complex, which is interesting to me because, among other reasons, it contains a triple-bonded oxygen atom.

At the electron level, how is such a thing possible while keeping the rest of the molecule electrically neutral?

– ClockworkSoul


 * Triple bonds are somewhat rare, but not so rare as to be amazing. For example, carbon monoxide is also a C≡O triple bond.  The most common triple bonds are C≡C (e.g. alkynes), C≡O, C≡N (e.g. cyanates) and N≡N (e.g. dinitrogen).  Dragons flight (talk) 20:39, 10 December 2014 (UTC)
 * Take a look at metal carbonyl for a more electronic perspective. --OuroborosCobra (talk) 21:15, 10 December 2014 (UTC)
 * Dinitrogen is one of the strongest bonds known. That is the real reason why atmospheric nitrogen is unreactive.  Breaking the very strong triple bond is endothermic.  Robert McClenon (talk) 21:21, 10 December 2014 (UTC)
 * The molecule is electrically neutral because the triple bond is polar, with the oxygen being positive (one electron shifted to the carbon) and the carbon being negative. Robert McClenon (talk) 21:21, 10 December 2014 (UTC)


 * Normally, structural formulae of complexes are drawn using crystal field theory as a basis, meaning that ligands are considered to be bound entirely electrostatically. Consequently, the bonding within the ligand remains unaffected by ligation, and M-L bonds are always single. Vaska's complex may actually be a ketene that has been given the CFT treatment. Plasmic Physics (talk) 21:36, 10 December 2014 (UTC)
 * Pi backbonding is probably a better description of the situation than really "ketene". The C=O IR frequency of ketenes seems typically 2100 cm–1 or greater, similar to carbon monoxide, whereas the carbonyl ligand of M(CO) noted in that article is typically notably less. DMacks (talk) 22:54, 10 December 2014 (UTC)


 * Tomato, toematoe. The IR frequency you gave is only really valid when the carbon is doubly bound to another carbon. In any case, it was just a thought. Plasmic Physics (talk) 23:51, 10 December 2014 (UTC)

One very cool thing about this compound is that it binds reversibly to O2 and H2, and was apparently one of the first compounds discovered to have the oxygen-binding property of haemoglobin! However, given Ir's low cosmic abundance, several orders of magnitude below V, Fe, and Cu (which are actually used by terrestrial organisms) and Mn and Co (which would also work, but AFAIK are unattested in terrestrial organisms), it seems rather unlikely that this compound is used in any alien species' blood. Double sharp (talk) 03:18, 11 December 2014 (UTC)

Note that the oxygen could be written with a formal positive charge to preserve the octet (which is real in the case of carbon monoxide), with a formal negative charge on the iridium. However, as the metal carbonyl article explains, you can write several resonance structures, even one with negative charge on the oxygen and a single bond. The article on iridium lists the oxidation state of iridium in this compound as +1; that's due to the chlorine's electronegativity, whereas the carbon monoxide is treated like the phosphine ligands, I assume. Wnt (talk) 11:02, 11 December 2014 (UTC)

Did my stain evaporate ?
I spilled some butter on my shirt, took it off and left it slung over the edge of the hamper until laundry day (to remind me to treat the stain). Come laundry day, I was quite surprised to find the dark stain was almost gone. The hamper is positioned so that the furnace vent blows right on it, so it's frequently hotter than room temp. So, did most of the butter just evaporate in a few days ? The only other explanation I can come up with is that the butter evenly distributed itself on the shirt, so as to have less apparent edges. I'm guessing a bit of both occurred. StuRat (talk) 20:17, 10 December 2014 (UTC)


 * Water in the butter may evaporate after a few days but the fat won't. Capillary action and diffusion would probably account for blurring of the edges. You may have issues with Rancidification. I'm not sure if that can change the appearance of fats but it can certainly change the smell. SemanticMantis (talk) 20:30, 10 December 2014 (UTC)

"Artificial" smallpox
Is it technically possible with current biotechnology to create the smallpox virus (variola) artificially? For example, I could imagine genetic modification of a close relative (such as vaccinia), but maybe it's not a close enough relative to make this possible. Are there other ways it could be done? Would oligonucleotide synthesis/artificial gene synthesis work for creating the whole viral genome? 108.2.194.62 (talk) 23:57, 10 December 2014 (UTC)


 * Possibly, yes, if we know the genome for smallpox. I'm not sure if this has been found yet.  Considering that that information would be perhaps more dangerous than the virus itself, it seems unwise to me to even determine it.  It's presumably easier to confine the virus to the lab than to confine the virus genome to a flash drive.  Keeping in mind how some hacker gets access to supposedly secure info every day, having the genome available in digital form seems to incur more risk than reward, to me. StuRat (talk) 00:12, 11 December 2014 (UTC)


 * Somehow I missed this when reading the smallpox article the first time: "Concern has been expressed by some that artificial gene synthesis could be used to recreate the virus from existing digital genomes, for use in biological warfare".  That partially answers my question, and I'm off to read the cited reference.  108.2.194.62 (talk) 00:20, 11 December 2014 (UTC)


 * Apparently that train has already left the station ... Variola major virus (strain Bangladesh-1975) complete genome 108.2.194.62 (talk) 00:35, 11 December 2014 (UTC)


 * Wow, that was seriously irresponsible. I can appreciate the fact that nobody could synthesis a virus back in 1995, but you'd think they would know that at some point in the near future this would be possible, and eventually inexpensive and easy.  They handed a weapon of mass destruction to all the terrorists out there. StuRat (talk) 01:59, 11 December 2014 (UTC)
 * You can see here for the views of one participant as to why the smallpox genome was published. (N.B. Some of the claims in that source related to other issues may be contentious. I don't think this affects the person's explaination however.) There may be more detail in the earlier book mentioned, but I'm lazy to find it. Nil Einne (talk) 02:41, 11 December 2014 (UTC)
 * Stu, it's obvious that the human genome encodes the human brain (because, obviously, and also other primates don't have the human brain due to a different genome.) It's also obvious that it would be useful to have an adult brain under one's control.  Is it therefore irresponsible to publish the human genome, since presumably at some future point (decades from now? centuries)? someone will go aahead and entrap a brain for whatever they want?  The point is, if something doesn't exist today it is hard to worry about preventing a matrix-like dystopia. . .  212.96.61.236 (talk) 03:59, 11 December 2014 (UTC)
 * I had a look and found the earlier book. It has some more on the politics of the sequencing. The Russian and US efforts were semi-coordinated with the WHO.  One of the plans (also mentioned in the earlier source) is that this may lead to the destruction of smallpox samples. There was a Soviet scientist on the American team (as well from China & Taiwan), who eventually left due to problems with alcohol.
 * Craig Venter (the author of both books, who's also seen as one of the key people behind the Mycoplasma laboratorium efforts) was eventually opposed to destruction of smallpox . That part of the book also basically says the same thing as the earlier sources, any thought of classfying the US sequence data was abandoned once the Russian effort said they would publish theirs. It also includes mention of some discussion in papers around reproducing the genome at the time (although the what Economist are said to have said doesn't seem to actually contradict Craig Venter's we now know correct view). Nowadays, as mentioned in our Smallpox virus retention controversy, the availability of the genome is one reason given by the US against destruction of the smallpox samples.
 * Not available in the Google previews but this quote may be of interest:
 * "Although we had rehearsed several times what he should and should not say, he seemed to forget all that when he was asked by one reporter about the possibility of making deadly pathogens. After Ham blurted out that "we could make the smallpox genome," I interrupted to point out that while that was indeed possible, it was known that smallpox DNA is not infective on its own, attempting to pour at least a little cold water on Ham's speculation. Ham interjected, "But you and I discussed ways to get around that," and then turned toward me and said with a sheepish grin, "I probably shouldn't have said that, huh?" Fortunately, our exchange did not go further than a paragraph in The New York Times, and the coverage was mostly favorable."


 * This was at a press conference held in 2003 or 2004. That's also mentioned here but not the bit about ways to get around it. So yes, the fact that smallpox could be created isn't something that's a particular new thought. While this is a concern mentioned in numerous sources (e.g. the ones earlier or ), I would say the far bigger modern concern is that as technologies advance, someone could create a designer pathogen that's even worse than smallpox.
 * In terms of the publication if we ignore the politics, this ultimately comes down to a security through obscurity debate IMO. I.E. whether having the genome published and therefore easy to be studied with work such as this and better understanding of its mechanisms of action is better, or whether it's better to keep it a secret and hope that the "bad guys" aren't able to get their hands on it. If they had agreed to it, this would require for example, both the Russian and US teams who did the sequence to sufficiently control their sequence data, including any they give it to to study.
 * One of the obvious risks of such an approach is that it's easily possible such a leak wouldn't be known about. We're talking about something which is about the same size as this page even if stored in a fairly inefficient method of using 8 bit characters to store the individual base pairs. (There is also the possibility someone could sequence it anew. But getting a smallpox sample would probably be more difficult than getting the sequence data. Even considering all those samples stored in forgotten places, it would need to make it's way to someone interested in and able to do that whether for good or malicious purposes, rather than someone who'd just try to use it maliciously straight away edit: including reproducing it as needed or someone who'd make sure it's destroyed.)
 * Nil Einne (talk) 07:08, 11 December 2014 (UTC)


 * Well considering you can synthesise the entire 580kbp genome of Mycoplasma genitalium with some minor additions in the form of Mycoplasma laboratorium, I don't see that the 186kbp of the smallpox genome is really going to be a problem if you know what you're doing and have the appropriate resources. Heck it's even a dsDNA so less difference with genitalium. Of course you still need to find somehow to put in an an appropriate virion, or otherwise infect someone with it to produce these, but I don't think these will really be a major barrier if you know what you're doing and have sufficient resoures. Edit: See, in particular Epstein-Barr virus with 170k base pairs was produced in 2006. Nil Einne (talk) 02:23, 11 December 2014 (UTC)


 * Yes, and not only is this a question of kilo-base-pairs, its a question of getting a simple long loop of smallpox DNA into an existing cell, as opposed to getting 23 pairs of functional chromosomes into a nucleus of a nice, fat, working, totipotential eukaryote cell close to a uterine wall. Even then you have to get the cell to mature and its descendants to replicate for two decades while you educate it to the point it can kill a few dozen unarmed-people before itself being killed.  Stu's answer is many orders of magnitudes more reality-relevant than IP 212's. μηδείς (talk) 04:56, 11 December 2014 (UTC)


 * Smallpox is still an immensely large virus. Do note that the Mycoplasma sequence was no simple tabletop synthesis, but a major project.  Nor has the price of synthetic oligonucleotides dropped anywhere near as much as the price of sequencing over time, nor do they come ready-made as a single complete genome sequence.  You can't just pipe your smallpox text to the printer and ready to go.  Someone with such resources might have reasonable hope of finding remains with intact DNA (even though they are ... knock on wood ... no longer infectious) and sequencing the genome from scratch.  Indeed, I'd sort of expect a "responsible bioterrorist" to do so just in case those early DNA sequences had a few typos, inadvertent or otherwise.  (And in 1995, how many published sequences didn't?)  So I think that the published sequence has little real use to terrorists, who would have to have substantial resources to try such a project; but if a nation really budgets many millions toward biowar then they would be expected to come up with something.  Probably it would be cheaper to tamper with monkeypox, passaging it through a ten or twenty 'generations' of political prisoners... smallpox might actually be counted as the more moderate option because they could be more confident that their vaccines would protect them.  Compared to so many goals readily achievable with those resources... let's hope bioterrorists are so uncreative as to spend their money on this. Wnt (talk) 10:49, 11 December 2014 (UTC)


 * While synthesizing a virus may be expensive and require PhD's today, I'd expect the price and expertise required to steadily decrease to where any terrorist organization can easily do it. StuRat (talk) 23:02, 12 December 2014 (UTC)


 * Well, hopefully by that time their medi-nanotech will laugh at smallpox, no doctor visit required. Or at the very least we'll have immediate access to antibody injections for passive immunity that are custom-made on the spot to deal with whatever virus is troubling us. Wnt (talk) 00:42, 13 December 2014 (UTC)


 * I'm not confident that a cure could be produced, distributed, and prescribed quickly enough to prevent massive casualties, especially if the target was in a Third World area. Look at all the trouble the West has in just dealing with Ebola. StuRat (talk) 04:55, 13 December 2014 (UTC)


 * Well, smallpox has been stopped in the Third World before. Stockpiles of vaccines exist against terrorist possibilities now.  So even under present circumstances there's reason to be hopeful.  But premising a future where it is cheap and easy to synthesize a giant virus like that, I'd hope it would also be cheap and easy to synthesize any given antibody on the spot, and to copy the sequence from any survivor of any infectious disease, such that it would be feasible to fight most new plagues almost as quickly as smallpox can be fought now.
 * To indulge in a bit of the speculation and forum chat that some here protest against, but I should add that my mental image of the future tech discussed here, where such syntheses become easy for any random person, might involve a workhorse strain of bacteria which possesses the ability to sense several colors of strongly monochromatic light and to reliably implement logical commands based on them. So you might send them a message (clear) (synthesize) (three nucleotides) (checksum) (checksum nucleotide) (... more nucleotides ...) (integration site tag) (...integration site nucleotides with checksum...) (integration site end) (reverse transcribe) (integrate) (synthesize) (... first few nucleotides from sequence ...) (sequence from primer) (bacteria would emit lower light frequencies in response, receiving a pulsed confirmation signal; those failing to match the sequence signal die)  After some replication, use a chemical cue to activate strong transcription and translation from the antibody complex you've integrated your sequence into; the bacteria would be predesigned to be able to add the proper sugars to these to create an antibody that can pass for human inside the body.  However, current efforts in this direction are quite rudimentary, proof of principle only - see, for example. Wnt (talk) 13:35, 13 December 2014 (UTC)