Science desk
< October 23	<< Sep \| October \| Nov >>	October 25 >

Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is an archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

October 24

UDFy-38135539 both exists AND may have been seen by space aliens, or something

The new article on UDFy-38135539 states that the galaxy is "the most distant object in the universe known to have existed and which has been observed from Earth." As opposed to objects that have been observed but are NOT known to have existed? I assume the qualifier is meant to exclude hypothetical objects, but it's redundant for something that has been scientifically observed to be further qualified as "known to have existed." Comments? Clarifications? (Also, why "observed from earth"? Isn't "observed," by convention, a human-centric concept? A bit pedantic to worry that saying "observed" instead of "observed from earth" might leave out "observed by aliens." And clearly "from earth" isn't meant as opposed to "from a space telescope.") So why not: "the most distant object in the universe that has been observed."? 63.17.82.132 (talk) 01:03, 24 October 2010 (UTC)[reply]

Sounds like reasonable objections. Now head over to the article and improve it! –Henning Makholm (talk) 01:09, 24 October 2010 (UTC)[reply]

Agree – and maybe also consider raising the issue for discussion on that article's talk page just as you have here – happy editing! :) WikiDao ☯ (talk) 01:58, 24 October 2010 (UTC)[reply]

Scientists--real scientists--prefer precision. Given that we do not know if other species exist out there, it would be silly to say simply 'observed,' because that presupposes we are the only observers. Actually by the same token, 'observed from earth' is a bit of a presupposition. → ROUX ₪ 01:34, 24 October 2010 (UTC)[reply]

OK, "real scientist." I suppose when a particle is said to be the smallest particle yet observed, it's necessry to add "by human beings" and "on earth"? Seriously, this is the sort of thing that gives pendantry a bad name. Anybody else? 63.17.82.132 (talk) 02:01, 24 October 2010 (UTC)[reply]

Well, one might consider it perhaps a bit "pedantic" just to raise the issue you are raising yourself, though (but I am sure you are doing so in good faith, and it is an awkward sentence that ought to be improved, so I am glad you mentioned it and would encourage you, again, to go ahead and make the change yourself and/or raise the issue on the talk page there). Let's not get into a "real scientist" contest here, though; let's just focus on making this improvement to this article. WikiDao ☯ (talk) 02:15, 24 October 2010 (UTC)[reply]

It seems a little unfair, WikiDao, to characterise an attempt to remove pedantry and tautology as itself "a bit pedantic". -- Jack of Oz ... speak! ... 02:36, 24 October 2010 (UTC)[reply]

Um, I have no argument there, really, Jack, was just trying to be, um, "fair" is all. WikiDao ☯ (talk) 02:55, 24 October 2010 (UTC)[reply]

To clarify, by 'real scientist' I meant actual researchers, not the professional sound-bite providers to news organisations that elide details. → ROUX ₪ 02:23, 24 October 2010 (UTC)[reply]

So which version of this sentence do you prefer, Roux? WikiDao ☯ (talk) 02:27, 24 October 2010 (UTC)[reply]

The "known to have existed" thing could be referring to the fact that, just because we've detected the light from this object, doesn't necessarily mean it still exists anymore. All we know for sure is that it was there 13 billion years ago, so it is known to have existed. But a lot of water can flow under the cosmic bridge in that time. Heck, we never even see the Sun as it is right now, but as it was 9 minutes ago. -- Jack of Oz ... speak! ... 02:36, 24 October 2010 (UTC)[reply]

That's funny! There should be a category of "most distant objects that DO exist," and we could quote Descartes. Not a very great distance ....63.17.51.20 (talk) 03:51, 24 October 2010 (UTC)[reply]

Okay, I made this edit; how's that? WikiDao ☯ (talk) 03:12, 24 October 2010 (UTC)[reply]

Stay with me, buddy; we're gonna make it through this

On television and in movies, wounded people are often encouraged to stay concious. Is this necessary to improve chances of survival? Why not just let them be unconscious and continue rescue efforts while they sleep? --90.215.213.167 (talk) 01:51, 24 October 2010 (UTC)[reply]

It's more dramatic to see the Wounded Hero struggle. However, some injuries--e.g. concussion--carry a danger if the patient loses consciousness. → ROUX ₪ 01:54, 24 October 2010 (UTC)[reply]

WRONG —Preceding unsigned comment added by Kj650 (talk • contribs) 03:29, 24 October 2010 (UTC)[reply]

Could you be more specific, please? → ROUX ₪ 03:31, 24 October 2010 (UTC)[reply]

WP:OR here: I suspect that, because an injured person who is verifiably conscious, is not dead, this is reassuring to the others around. Fighting to stay conscious also demonstrates that the injured person has not "given up". Bielle (talk) 05:27, 24 October 2010 (UTC)[reply]

With a (suspected) concussion loosing consciousness or sleeping is not dangerous. The problem is that going to sleep can mask serious injury. That's why the recommendation is not to let the person go to sleep have a head injury - you want to make sure they are not seriously injured (which you would notice by altered consciousness states, pupils of different sizes, etc.). From this people extrapolate a little too far.

The only other thing I can think of is that a conscious person can help in their recovery, but for example pinpointing injuries (pain), and giving a medical history. But besides that I can not think of any reason not to let the person become unconscious - in fact one of the first treatments for serious head wounds is a medically induced coma. Physically forcing (shaking) a person to stay awake can probably injure them more! (However all that said, I would be reassured if a doctor could confirm/deny what I wrote.) Ariel. (talk) 08:02, 24 October 2010 (UTC)[reply]

There can be a number of reasons to keep the casualty awake. For example a study discussed here into pre-hospital deaths from road accidents "showed that at least 39% and up to 85% of preventable pre-hospital deaths may be due to airway obstruction”. Much easier to monitor and remedy airway problems in a conscious person. Another serious risk with accident victims or people suffering from blood loss is hypothermia. At least two reasons you want to keep someone awake who's at risk of this, firstly again problems are easier to monitor in a conscious patient, and secondly the body temperature naturally decreases during sleep, so thereby heightening risks of hypothermia. Of course once a person has been transferred to professional care and a proper medical facility the equation changes completely. --jjron (talk) 11:55, 24 October 2010 (UTC)[reply]

U.S. Army FM 21-11 First Aid for Soldiers [1] has a special wounds chapter entitled Proper First Aid for Head Injuries. This chapter outlines numerous procedures if the victim loses consciousness - but at no time does it suggest trying to keep the victim awake. Nimur (talk) 16:53, 24 October 2010 (UTC)[reply]

The most significant mortal risk in loss of consciousness is loss of airway. An unconscious person cannot protect their airway, so no gag reflex. That means you can inhale stuff (blood, snot, vomit, bone, brain, whatever) into your lungs (aspiration pneumonia) and this can progress to ARDS. Then you have a really good chance of dying. In hospital we have plenty of technology at our disposal to artificially maintain airways. We just need to be aware of the problem! But in the field, the best means of protecting an airway is to keep the victim conscious. Failing that there's airway manoeveres like jaw thrust. If there 's a seriously life-threatening head injury, then unconsciousness may be as preventable as nightfall, but deterioration of milder injuries may be prevented when the victim stays awake. Mattopaedia ^{Say G'Day!} 05:38, 25 October 2010 (UTC)[reply]

Large rigid structures on the ocean

I've heard that some of the large ocean rigs/cruise ships nowadays are so huge that you can barely notice any rocking, even when the waves are quite large. Is this true, and if so, how much so? I looked for some description of the stability of these structures, and I found mega-float, which sat in Tokyo bay for a while, and it claims on that page that pilots didn't notice anything different from a landing on a land-airport. I'm wondering if the same would apply to someone standing on the platform, and if not, if the structure was a few kilometers in size, is it feasible to think that it could be build so that there was no noticeable "seismic activity", at least for a human? This is assuming a rigid (within reason) structure! Thanks! 173.180.219.65 (talk) 04:45, 24 October 2010 (UTC)[reply]

Yes, the wider the structure the more waves and swells it spans and the less the influence of each wave/swell. Even the heaving (up and down) motion would become insignificant if the dimensions of the structure were large enough. Unfortunately I don't have any measurements. Dolphin (t) 05:09, 24 October 2010 (UTC)[reply]

What "up/down" motion? I thought that was just the motion of the boat on top of waves with large wavelengths. 173.180.219.65 (talk) 05:56, 24 October 2010 (UTC)[reply]

The law of thermodynamics

Defend the statement that 100%of the electrical energy that goes into lighting a lamp is converted to thermal energy.Are the first and the second laws of thermodynamics violated? —Preceding unsigned comment added by 99.28.147.106 (talk) 05:48, 24 October 2010 (UTC)[reply]

I don't wish to defend the statement (or to do your homework for you), but I think a very tiny amount goes to produce sound. I suppose that this eventually dissipates as thermal energy, as does the light output. Dbfirs 07:43, 24 October 2010 (UTC)[reply]

I'm not so sure that it goes into sound...the filaments in light bulbs are in a vacuum, as far as I can recall, and thus there would be no medium within which sound could travel (inside the light bulb, at least). Ks0stm ^(T•C•G) 02:27, 25 October 2010 (UTC)[reply]

Could at least be vibration of the filament itself even if vibrations don't propagate. Our incandescent light bulb article explains why they are usually filled with some gas rather than being high-vacuum. DMacks (talk) 02:58, 25 October 2010 (UTC)[reply]

What about those photons which make it out the window, through the atmosphere, and into space? How long will it take for their energy to be converted to thermal energy? But isn't the light emitted by the filament itself black body thermal radiation? So why should that original escaped photon be considered any different from those photons which are emitted by the roof shingles which were, rather indirectly, heated by light from the bulb? -- 124.157.218.132 (talk) 02:08, 25 October 2010 (UTC)[reply]

Underwater wrecks and pressure

How come that fragile wooden shipwrecks stay intact deep underwater, despite the pressure? —Preceding unsigned comment added by Jib-boom (talk • contribs) 07:33, 24 October 2010 (UTC)[reply]

There is no air left in a wreck, so it won't be crushed by the pressure as, say, a submarine would be if it exceeded its crush depth. Any gaps in the wood will be filled by water, and the individual fibres of the wood will not be affected by the external pressure. A wooden wreck is very fragile if it is recovered and allowed to dry out (see Mary Rose#Conservation), but if left deep underwater, where the cold and dark will slow down decay, then I imagine it is quite tough. Gandalf61 (talk) 07:53, 24 October 2010 (UTC)[reply]

It depends greatly on the oxygen level of the water; the Black Sea is well known for well preserved shipwrecks. Wnt (talk) 09:26, 24 October 2010 (UTC)[reply]

is this real?

I only watched the first video here: http://forum.grasscity.com/science-nature/686839-we-being-watched.html out of that big crowd of people, news cameras, etc, for hours and hours, did anyone film that object with a telescopic lens? How about just renting a helicopter and flying up closer and taking a closeup? Is that video real? What is it? Why do we just get such a small, far-away version when obviously a few hundred dollars of equipment will give you a clear closeup? (Or you can use a $20 pair of binoculars and put your iphone camera to them...) 84.153.221.42 (talk) 10:42, 24 October 2010 (UTC)[reply]

I think the answer is obvious, if anyone did actually take a closer look they would have seen exactly what they were. My money is on something completely innocuous such as some balloons. But then there wouldn't be any news story and you wouldn't here about it. Theresa Knott | Hasten to trek 10:54, 24 October 2010 (UTC)[reply]

Of course are it real, the question is what it was. It was per definition an UFO. I saw a interview with some on school in the area that had released a number of party balloons by mistake, it was probably what they observed. --Gr8xoz (talk) 14:34, 24 October 2010 (UTC)[reply]

"There's a UFO over New York, and I ain't too surprised," said John Lennon. ("Nobody told me") WikiDao ☯ (talk) 20:26, 24 October 2010 (UTC)[reply]

follicles

Does the use of tweezers damage hair follicles over time? —Preceding unsigned comment added by 178.72.214.137 (talk) 13:02, 24 October 2010 (UTC)[reply]

Foucault pendulum swings

I'm curious to estimate the number of swings that Foucault pendulum can do in one day, but not sure if this would be an appropriate formula?

n={\frac {T_{E}}{T_{p}}}\approx {\frac {T_{E}}{2\pi }}{\sqrt {\frac {g}{\ell }}}

where:

$T_{E}$ is the one day period or about 86,400 seconds,
$T_{p}$ is the period of Foucault pendulum in seconds, assuming simple pendulum relation,
$\ell$ is the length of Foucault pendulum in meters, and
$g$ is the Earth's gravity in m/s².

can you give me a hand? (Posted by EMail4mobile, who forgot to sign.)

Looks correct to me. Red Act (talk) 15:17, 24 October 2010 (UTC)[reply]

Don't forget that your equation will give you the number of full cycles of the pendulum: the full cycle includes both the "forward" swing and the "backwards" swing. Physchim62 (talk) 15:23, 24 October 2010 (UTC)[reply]

measuring id, ego, etc.

I was just reading a recent response in the Ref desk re: the use of evidence and reasoning rather than "feelings" or "gut feelings" to "prove" something. This led me to wondering about some things that have been "accepted" as truth. For example, many psychologists (as well as the public in general) accept the "truth" or "validity" of such thikngs as id, ego, superego and other such psychological concepts. However, I have spoken to many psychologists (I work with many of them)and they are not able to provide support for any evidence for these terms/concepts yet they still generally accept them. So, I am wondering: is there actual way of measuring these terms/concepts or have they just been referenced so much in common vernacular that they are simply accepted as fact? What about the concepts put forward by other theorists (such as Piaget, Jung, et al)?

Asssuming that these are just frameworks used to try to better understand human behavior, why are they so well accepted? 99.250.117.26 (talk) 16:21, 24 October 2010 (UTC)[reply]

We also accept 'love' 'anger' and 'that pleasurable feeling of comfort you get when you flip over your pillow to reveal the cool underside' as truth, and they are just as immeasurable. The basis for accepting those names--ego, superego, id--is that they serve to describe concepts well enough that we may as well treat them as real. Other paradigms in psychotherapy use different terminology. Freud just happened to publish first, so got to be the dominant nomenclaturist in the public eye. → ROUX ₪ 16:41, 24 October 2010 (UTC)[reply]

I agree with Roux here: in a sense, the necessary-and-sufficent "evidence" for these things is simply that they are there and that whatever they are (in themselves) can be described or defined to some extent. It is a good question, though, and also true that the naming and defining of "intangibles" can cause them to take on a reality that is different from what they are "in themselves", which can be a source of systemic error in some cases. The formal "defining" of mental illnesses is another example. WikiDao ☯ (talk) 16:54, 24 October 2010 (UTC)[reply]

That's just tautological reasoning, though. It's not scientific. Saying, "we know it's there, that's the evidence" is fairly silly with regards to things like the id or the ego. Do we know they are there because we "feel" them, or because we've been told they are there? People think auras and psychic activity and superstition and ghosts "feel" real too, but it doesn't make them so. "Anger" and "love" both correlate with distinct emotional states that can be quantified and measured (adrenaline, facial temperature, activity in the Limbic system). They are much stronger empirical grounds than are Freud's concepts, which have essentially no empirical backing, and there is no reason other than "as part of a therapeutic regimen it has a good track record" to believe in them, and even there you're going to have a hard time convincing a skeptical outsider that they do in fact have a good track record. The validity of psychoanalytical concepts (and their inability to be falsified) has been a major point of contention in the mind sciences for a long time and I don't think you can just waive them away by saying we "know" they are true. We know no such thing. --Mr.98 (talk) 17:44, 25 October 2010 (UTC)[reply]

But, that's not what I am saying.

I am not a Freudian myself, btw, 98, and certainly have no bone to pick with all the very reasonable criticism of Freudianism. But I think a lot of criticism of Freud's basic concepts and theories would benefit from greater familiarity with what is being criticized.

I am also not a clinical psychologist or a psychiatrist, much less a psychoanalyst, but the OP implies that they are still teaching the concepts of id, ego, and superego to students in those fields. If so, there is obviously some reason to do so – ie., presumably those concepts are still useful for some purposes. And it is utility and not "scientific validation" that is what's important in such general conceptual frameworks as this. It is important to note the distinction, though, and avoid making misleading assumptions about the framework under the misunderstanding that it has been "scientifically validated" in ways that it has not been. WikiDao ☯ (talk) 20:23, 25 October 2010 (UTC)[reply]

The experimental method is the gold standard of finding out if something is true or not. If a conceptual model has been verified by experiment, then I'm willing to accept it as probably true. A lot of people such as Freudian psychologists do not require experimental verification and just accept things on faith or the assertion of others. Its not too difficult to create a model which is intenally consistent but still untrue: eg. Phlogiston theory or Aether theories. People without scientific training will accept all sorts of twaddle as being true if it seems superficially coherent and it is supported by others. People may have invested years of training and experience in Freudian psychology and get a good income and prestige from it, so they are not going to give it up. I understand that experiments to confirm the truth of Freudian psychology have not verified it. 92.15.31.47 (talk) 17:15, 24 October 2010 (UTC)[reply]

There is no experimental verification that 'love' actually exists. It is the name given to a complex set of social norms coupled with some verifiable brain activity plus feelings, which likewise cannot be independently verified by experiment in the way you propose. Do you say that love does not exist? → ROUX ₪ 17:28, 24 October 2010 (UTC)[reply]

That's a glib saying: are you sure there have been no experiments regarding love? I think I can vaguely recall at least one, to do with scanning the brain. I expect if you search the scientific literature then there have been many experiments regarding love. You might as well claim that pain or pleasure do not exist. 92.15.31.47 (talk) 17:32, 24 October 2010 (UTC)[reply]

(edit conflict)

Yes. But there are many "conceptual models" that have not been "verified by experiment" (at least, not in the sense of "experimentation" as used in the physical sciences) but are nevertheless not only "probably true" but quite useful as well!

One really must try to be as "scientific" as possible in the face of material that by its very nature evades "scientific investigation" – which is what Freud (who wanted to be a scientist but became a medical doctor for the $$$), for one, always tried to do. WikiDao ☯ (talk) 17:35, 24 October 2010 (UTC)[reply]

See also Psychoanalyst#Criticism, in particular Psychoanalyst#Scientific_criticism and as an example of the consequences of psuedoscience beliefs Emma Eckstein. 92.15.31.47 (talk) 18:00, 24 October 2010 (UTC)[reply]

lol - I always get a kick out of this misconception. 92.15, experimentation is (literally) the antithesis of a truth claim. Non-scientifists make claims about truth or falsehood; scientists create theoretical structures and create evidence-based arguments that those structures should be accepted by others. No scientist would ever claim that the (massive amount of) experimental verification of the Theory of Gravity makes the Theory of Gravity true. All such experimental verification does is make the ToG believable.

If you think this is ever about truth, you misunderstand science drastically. The entire rasion d'etre of science is to replace unfounded truth-claims with founded beliefs, where the foundation of the new beliefs lies in empirical evidence.

Back on the original topic: Freud's inner structures (id, ego, superego, cathexes, neuroses, etc...) are mostly important for the insight that there are inner structures. Prior to Freud, people had a very superficial understanding of the workings of the mind (assuming that every outward action was either the result of a conscious choice or the manifestation of a physical defect). Freud created the understanding that there was an internal mental life that was neither strictly conscious nor strictly biological. There are certainly problems with Freud's theory, which even Freud recognized (he was constantly revising his conception over his lifetime), but the basic idea of id, ego, and superego (corresponding to to the tension between purely egoistic interests and purely social constraints, moderated by conscious rational processes) is appealing and intuitive, and a helpful tool for understanding both functional and dysfunctional behavior. No doubt that particular terminology will disappear over time, but the concept behind it is too useful to disappear. --Ludwigs2 18:33, 24 October 2010 (UTC)[reply]

"Experimentation is (literally) the antithesis of a truth claim". Already been aware of that for a very long time thanks. Was not the place to mention Karl Popper, falsifiability and all that. 92.28.246.6 (talk) 19:26, 24 October 2010 (UTC)[reply]

If you are already aware of this, then please don't say things like "The experimental method is the gold standard of finding out if something is true or not"[2], which would tend to imply the opposite. clarity in communication is important.

--Ludwigs2 19:54, 24 October 2010 (UTC)[reply]

Pedantic nit-picking. 92.28.246.6 (talk) 22:46, 24 October 2010 (UTC)[reply]

regarding use of solumedrol

how useful can solumedrol be in patients suffering from viral fever ( malaria /dengue fever )especially when it is associated with thrombocytopenia ? Does it play a role in such condition ? —Preceding unsigned comment added by Pravinbathe (talk • contribs) 19:09, 24 October 2010 (UTC)[reply]

Our solumedrol article doesn't seem to address this specific symptom/disease situation. If anyone finds anything in some other ref, please do update the article. DMacks (talk) 03:01, 25 October 2010 (UTC)[reply]

glacier grit

The composition of the black grit on top of ice in Southern Patagonian Icefield is ( ) and arrives there from ( ). —Preceding unsigned comment added by Jlawrencen (talk • contribs) 22:33, 24 October 2010 (UTC)[reply]

Wikipedia:Do your own homework--Jac16888 ^Talk 22:38, 24 October 2010 (UTC)[reply]

In Antarctica, occasionally a rock will be found on top of the ice in a place where a rock has no business being. Some Antarctic ice-sheets are kilometers thick - so how would a rock float its way up through 1000 m of ice? In such cases, it is typically assumed to be a meteorite. This assumption can be supported by geochemical analysis of the specimen. Other ways that a rock can "surface" to the top of an ice sheet include being carried there by animals and humans, being blown by wind across the surface (and uphill), or even "convecting" through the ice. Glaciers behave as "fluids" over very long time-scales, and the ice moves and flows very dynamically, carrying debris with it. If the ice is the highest geographical point for many miles around, the number of ways rock can get on top of it are very limited. But bear in mind, Antarctica is not the same as Patagonia - but we do have an article on the Southern Patagonian Ice Field. You might find the article Grey Glacier instructive, especially photos of the gritty surface below a mountainous landscape. We also have an article called colluvium; you might find this term helpful. Nimur (talk) 23:16, 24 October 2010 (UTC)[reply]

There are several volcanoes in the Southern Patagonian Ice Field, some of which have been active within the last century. Lautaro, for example, last erupted in 1979. Gandalf61 (talk) 12:02, 25 October 2010 (UTC)[reply]

Why do humans need potable water?

How did we survive so long without modern methods of obtaining it? 67.243.7.240 (talk) 23:47, 24 October 2010 (UTC)[reply]

Define "potable". The easy answer is that Humans used pre-modern methods to obtain potable water. See spring, well, and aqueduct. Moreover, humans can get used to all but the worst biological contaminations. Most running water in low-population density areas of the world is pretty safe to drink. Indeed, I spend some time on the East coast of Australia, and they put up signs at the one river where it was not safe to drink the water. --Stephan Schulz (talk) 23:54, 24 October 2010 (UTC)[reply]

I'm not sure about how the numbers work out for this, but it's possible that pre-modern societies in some regions can obtain a large percentage of their water from the foods they eat, such as fruits. I know that in pre-Industrial times it was common in Europe, at least, for people to drink beer or wine instead of water because it kept fresh for much longer. --Laryaghat (talk) 00:00, 25 October 2010 (UTC)[reply]

And then there's the simple fact that in many places around the world (China, India, etc.) we've made water far, far more poisoned in the last 100 years than it ever would have been normally. The Masked Booby (talk) 02:20, 25 October 2010 (UTC)[reply]

Interestingly, it was the invention of alcohol that really let humans congregate in high population density areas [3]. When the water source has cows pooping in it upstream, drinking beer can be a lifesaver. Buddy431 (talk) 02:46, 25 October 2010 (UTC)[reply]

That's not true, though. Not even wine has enough alcohol for antiseptic purposes. Imagine Reason (talk) 12:35, 26 October 2010 (UTC)[reply]

No, but the processing of (for example) beer involves boiling the water: a major danger was the watering down of beer, which added contaminated water to the boiled-water beer. Similarly, drinking tea is considered to have had health benefits for the English working and middle classes, as the water was (again) boiled before drinking. 86.163.212.182 (talk) 18:39, 26 October 2010 (UTC)[reply]

Possibly our ancestors, as well as "primitive tribesmen," were better able to just drink water from whatever creek or puddle they encountered, than the delicate hothouse flower people of today, who would suffer from the contamination in such water sources. Edison (talk) 04:21, 25 October 2010 (UTC)[reply]

Yes, the human gut adapts to minor bacterial contaminants in drinking-water (and the acidity of the stomach kills off most of the bacteria). Many years ago, a farmer was prevented from selling milk because the local water supply was not of a sufficiently high standard for the cows, despite the fact that he and his mother (who lived to 92) had been drinking it all their lives. I regularly drink from streams when walking on the local hills, but I am selective in my choice! Personally, I wouldn't drink from rivers under normal circumstances, but some people drink water from the Ganges! Dbfirs 08:01, 25 October 2010 (UTC)[reply]

Light speed and giant sticks

This is a self-evidently stupid question, because I know what the answer is, but I'd like to know exactly *why* the answer is the one it is from someone who understands this stuff. Here it is:

Say we have two people, Alice and Bob, facing each other but standing a whole light year apart. If Alice were to try to radio signal Bob, it would take a year for that signal to reach Bob. However, let's say that Alice has a giant stick or rod, that is exactly a light year long and is somehow magically incapable of snapping or bending. Assume also that Alice has the unnatural strength that would be required to lift this rod. Now let's say Alice thrusts this rod forward. How long before Bob is poked by the other end of the rod?

My common sense tells me the answer is "immediately", but I know that the answer has to be "a year or more" - I'm just not sure why this is the answer. Can anyone explain this to me? Is the entire premise of the idea so infeasible that the question is meaningless?

Thanks in advance for anyone who has the patience to answer this! --Laryaghat (talk) 23:58, 24 October 2010 (UTC)[reply]

Alice isn't a light year long so she would only be able to lift one end of the rod. The rod would flex slightly and a signal would travel down that rod towards the other end telling it to "lift". Yhe speed that this signal travels at would be ( I think) the speed of sound in the material of which the rod is made. Much slower than lightspeed. Theresa Knott | Hasten to trek 00:02, 25 October 2010 (UTC)[reply]

This is a standard question, and I'm sure we've discussed this before. If you allow magic, all bets are off. On the other hand, if you have a real rod made of matter, then that is held together by electromagnetic forces. That is, if you push one end, the rod does not move as a whole, but the push creates a longitudinal wave that runs down the rod at at most the speed of light (as the electromagnetic force is mediated by photons which travel at the speed of light). --Stephan Schulz (talk) 00:06, 25 October 2010 (UTC)[reply]

Ah, of course! Electromagnetic energy is what's keeping it together in the first place, and that propagates at light speed. Thanks, I think I understand it now. --Laryaghat (talk) 00:11, 25 October 2010 (UTC)[reply]

As Theresa notes, the response of solid objects to being pushed or pulled is generally limited to the speed of sound in the material. For typical solids this is about 3 km / second, which is much less than the 300,000 km / second that light travels at. Dragons flight (talk) 00:17, 25 October 2010 (UTC)[reply]

If "perfectly rigid" means instantaneous communication down the length of an object, then there is no perfectly rigid object. If "perfectly rigid" means transmission down the length of an object, then how does a small segment in the middle know which end has been pushed with greater force? Also see Ehrenfest paradox. Wnt (talk) 15:35, 25 October 2010 (UTC)[reply]