Wikipedia:Reference desk/Archives/Science/2009 May 4

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May 4

Charging small objects

What is the most efficient way of charging styrofoam balls around a centimeter in diameter? That is, how do I deliver the largest amount of electric charge possible using household equipment? Thanks in advance. --99.237.234.104 (talk) 00:24, 4 May 2009 (UTC)

The simplest thing is to just have them rub against a dry surface made of rubber, latex, or wool (don't remember which works best). Using a hair-dryer to blow the air past the styrofoam balls may also work. The less moisture there is in the air and on the surfaces, the better it should work. Now, if you want more than that, you can always build a small Van de Graaf generator at home using a rubber or polyethylene band, a 3V DC electric motor pulled from any toy, and a metal bell or hemisphere. I built one when I was in high school, 20 years ago (sigh...). That was fun. --Dr Dima (talk) 00:58, 4 May 2009 (UTC)

A small object can be charged very effectively by stroking a length of PVC pipe with a piece of wool, then holding the PVC near the object, and repeating.I found that PVC and wool works better than silk and glass. But a styrofoam ball, being an excellent insulator, will not be easy to charge uniformly. If a charge is applied to one spot on a styrofoam ball and the ball is suspended from an insulating silk or polyester thread, it will rotate so the charged spot is on the opposite side from the approaching charged PVC. With styrofoam, the charge stays at the spot it is applied and does not spread around the surface. A piece of wood or a piece of bread or a piece of cereal is conductive enough that the charge will be able to uniformly cover the surface. A styro ball covered with carbon or graphite might also work better than plain styro to become uniformly charged. Classic experiments like 2 similarly charged objects suspended from strings moving away from each other do not work as well as with lightweight slightly conductive objects. Edison (talk) 01:41, 4 May 2009 (UTC)

By the way, the multiple repeated transfer of charge described by Edison is the basis of the Wimshurst machine which essentially automates this process. Many years ago we built one using unwanted LPs instead of glass discs. What's an LP? For the iPod generation, see LP album SpinningSpark 08:09, 4 May 2009 (UTC)

In the latest edition of "Make" magazine (which I strongly recommend to any tinkerers) there are complete instructions for building a Wimshurst machine. It doesn't look terribly difficult - and they are wonderful things to mess around with. SteveBaker (talk) 12:31, 4 May 2009 (UTC)

One of my personal heroes, Stephen Gray (scientist), (1666-1736), was an impoverished English electrical experimenter in the early 1700's. He characterized conductors and insulators, and used insulated conductors to carry electrical signals to great distances. His "electrical machine" was merely a glass tube and a piece of silk. A 1 meter length of PVC pipe is an excellent electrostatic generator and takes little time or little money to create. It can charge a Leyden jar to many thousands of volts and electrify small objects hung from silk threads. It will pick up pieces of paper and attract metal cans free to roll. Edison (talk) 02:52, 5 May 2009 (UTC)

Interesting ideas! Rubbing the styrofoam spheres against other objects doesn't work well because they're too small, but induction seems to work, if not well. I also loved the Van de Graaf generator and Wimhurst machine ideas. They're simply fascinating, and I'll definitely try making one or the other. --99.237.234.104 (talk) 04:29, 8 May 2009 (UTC)

Lenz's law and equilibrium

There's one thing I don't get ...

As an orgo student, I'm going to use NMR for my example. So I have an aromatic ring, like benzene, or perhaps a carbon nanotube, which has conductive pi electrons around the sp2 carbons. So in response to the strong magnetic field, the electrons circulate diamagnetically, opposing the magnetic field. So, as evident on the NMR readout, the local field drops in intensity and the protons outside of the skeleton are shown to be more downfield as evidence of this. But that means the inductive effect on the electrons should fall too, right? So the electrons move slower, the diamagnetic effect decreases and the local field moves up in intensity again. Clearly I know there must be some sort of equilibrium here.

What was not really explored in physics class is where exactly I set this equilibrium, by what mechanism and how I determine it. Or maybe it's been too long. The equilibrium must be fairly constant, otherwise the affected protons would be moving all over the place (upfield, downfield) in the readout. John Riemann Soong (talk) 05:16, 4 May 2009 (UTC)

I am sure there is some "bouncing around" but the timescale is much smaller than what you are able to work with. When your molecule first hits the magnetic field, it may "shake" a bit, but equilibrium is probably established within microseconds. By the time you fire up the NMR to take a reading, everything has settled out. Look at it this way: NMR works, right? So any supposition that it shouldn't work out the way that it actually does can be taken as a false assumption on face value... --Jayron32.talk.contribs 05:28, 4 May 2009 (UTC)

I know. I actually also know from real life that Lenz's law applied elsewhere (NMR is just my most recent need for it) and I don't get wildly fluctuating local magnetic fields, so I'm just trying to work out what exactly 'stabilises' both the current and the local field. I think I may have forgotten perhaps some key differences between concepts. In fact, what causes a changing magnetic flux to flow through the benzene ring? Does the ring current in fact, not flow that much? The problem would be having the downfield effects on local protons outside the ring remain constant. (I mean, the peaks are even sharper than for alcohols!) (hence why proton NMR readouts have aromatic downfield effects of 7 ppm and not like say, 20000 ppm?)

Also, non-chemist physicists shouldn't be intimidated by the discussion. This would apply to anything undergoing Lenz's law. John Riemann Soong (talk) 05:46, 4 May 2009 (UTC)

Applying Lenz's Law to electrons flowing in a wire involves billions and billions of electrons. Applying it to a benzene ring involves, um, 12 electrons. See the difference? How do you generate significant current with 12 electrons? The only way would be to move them at astronomically high speed; greatly exceeding the limits allowed by the basic laws of physics, like the speed of light. The downfield shift caused by this flow of electrons is miniscule because the maximum current generated is miniscule. --Jayron32.talk.contribs 17:00, 4 May 2009 (UTC)

Well naturally. But benzene electrons are moving within the molecule, so their velocity is much higher than electrons within a wire to begin with. But anyway, that's not my question -- my question is how the pi electrons arrive at an equilibrium in the first place. (In fact, for all the diamagnetic localised electrons too.) If a constant magnetic field is flowing through the benzene ring, what generates the /changing/ magnetic flux? John Riemann Soong (talk) 17:06, 4 May 2009 (UTC)

I am not understanding your question then. There is an interaction between the NMRs magnetic field and that generated by the delocalized pi electrons in the benzene ring. That interaction casues a change in the electron density profile of the C-H bond, withdrawing some of the electron density away from the H, and causing a small amount of deshielding, creating a small downfield shift for aromatic-bonded protons vis-a-vis those bonded to sp3 carbons... I don't see the source of the confusion here. However, this effect is evident even in non-aromatic sp2 carbons, which also have downfield shifting on the H's bonded to the sp2 carbons. Perhaps if you could explain in more detail where the paradox is? --Jayron32.talk.contribs 17:34, 4 May 2009 (UTC)

Actually I had the impression that the "downfield" effect was simulated. The electron density doesn't change so much more as the local magnetic field increases, as though there was less local electron density there. In actuality I suppose, all electrons happen to be moving in a sort of current, just that the aromatic current is greater. My question isn't NMR-specific. It has to deal with any application of Lenz's law, but it was inspired by thinking about NMR. That is, since the local magnetic field drops around the circulating electrons, that decreases the magnetic field felt by the circulating electrons. That means the electrons feel less of a magnetic field, so they circulate more slowly. That restores the local magnetic field closer to the global value. But that increases the magnetic field felt by the electrons, so they circulate more quickly....

Surely it's not simple harmonic motion at work here, and there must be a fixed, static equilibrium, as dictated by knowledge. So how is a fixed, constant equilibrium theoretically possible?

Also, reviewing my notes again, it appears the sample column spins within the field, which is possibly a source of magnetic flux. Yet, dissolved constituents are likely to be bouncing about Brownianly.... so anyway, I do have a feeling it might have to do with understanding the changing magnetic flux flowing through say, an aromatic ring, but ahhh, I can't seem to work it out at the moment. John Riemann Soong (talk) 08:07, 5 May 2009 (UTC)

Any physicists ... guys? I'm sure this is quite simple, I'm just missing something. John Riemann Soong (talk) 04:17, 6 May 2009 (UTC)

Not sure if my mental picture is the same as yours, but I think your "local magnetic field" effect doesn't happen. Consider an electron moving in a static magnetic field; it has a cyclotron resonance frequency. Change B and the frequency changes with it instantaneously -- it's not affected by the field it sets up. Also consider a conductive loop in a magnetic field. - mako 07:01, 7 May 2009 (UTC)

Do container ships carry containers in their holds?

Containers in the hold?

I work in a factory near a major container port, and large container ships pass by several times a day. These carry a good number of containers on deck ). I was wondering - do these ships carry all their containers on deck, or are some carried in the holds as well? — QuantumEleven 13:16, 4 May 2009 (UTC)

Good question - whatever the answer, it should be added to Container ship. Tempshill (talk) 13:52, 4 May 2009 (UTC)

At least some ships have room for containers in the hold. See the image at right. This page describes their boat: "The units are arranged in six tiers of 13 rows on deck as well as six tiers of 11 rows in the hold". I don't know how typical that is, or whether having them in the hold is universal. --Sean 14:34, 4 May 2009 (UTC)

I would say that they simply have to judging by how top-heavy many of them seem to be, or else they will have to carry a huge amount of ballast. They look like even mildly rough weather would capsize them. SpinningSpark 14:41, 4 May 2009 (UTC)

Yes they store them in the hold too. One of the more interesting things about them is the amount of work and cooperation that goes on to ensure the least amount of messing around, they try to ensure that the containers that are to be offloaded are at the top rather than at the bottom of the pile, that no shuffeling has to be done. It takes quite a bit of working out on a computer so the turnaround time can be minimized. Dmcq (talk) 21:31, 4 May 2009 (UTC)

Yes, large containerships such as Emma Maersk store containers in the hold. You can pay extra to have your containers in the hold instead of on deck. The actual deck consists of heavy sections of girders and sheet steel. Each section has lift points so the container cranes can be used to lift the deck sections. The deck is removed and then the containers are put into the hold and the deck replaced. Then the on-deck containers are loaded. The deck is load-bearing, so the six-high stacks below the deck do not bear the weight of the six-high stacks above the deck. The ship has a number of independent cross-sectional "slices" of containers. A crane works on one "slice" at a time, handling both the above-deck and below-deck containres (and the deack sections) for that slice. The crane is then repositioned handle the next slice. -Arch dude (talk) 02:56, 5 May 2009 (UTC)

Thank you very much everyone, that's very helpful! — QuantumEleven 07:47, 5 May 2009 (UTC)

dimensions

Hi, I know this is no place for homework solving, but I just can't solve this question. I have to find the dimensions of a, b, c and d. x = distance and t = time

Power of a body, P = [[at – ^x]] + c$x$ + d b^t —Preceding unsigned comment added by 122.50.142.12 (talk) 13:56, 4 May 2009 (UTC)

I'm afraid I don't understand your notation, what are all those superscripts and subscripts meant to mean? Regardless, the first step in answering this question is to find out what the dimensions of power are, do you know that? --Tango (talk) 14:26, 4 May 2009 (UTC)

I don't understand your notation either, but since you are going to solve the problem, not me, I can still give you some clues. In all dimensional analysis problems you start by writing down the dimensions of the left side of the equation in basic units (that is metres, seconds and kilograms). You should know what the units of power are, and you should know how to express that in basic units. Each term in the polynomial on the right must be in the same units individually. So long as there is only one arbitrary constant in each term then you should be able to determine its dimensions by equating the dimensions of the term with the dimensions of the right hand side. SpinningSpark 14:38, 4 May 2009 (UTC)

foods that are unsafe COMBINED in stomache?

are there any foods that are fine and safe, except when COMBINED, even in the stomache? You would think out of all the things people could eat, at least 2 would react chemically with each other and form something toxic, explosive, etc...any such cases? (literally please! no bean jokes...) 79.122.30.40 (talk) 19:03, 4 May 2009 (UTC)

There's a classic urban legend about Pop Rocks and Coke, but it's not true. -- Coneslayer (talk) 19:09, 4 May 2009 (UTC)

I have two: A classic one is the coprine and ethanol mix. Coprine is a compound found is a few of the Coprinus mushroom species, several of which are of culinary interest like Coprinopsis atramentaria. Consumed on their own, gastrointestinal discomfort is relatively rare but when taken with alcohol, "symptoms include facial reddening, nausea, vomiting, malaise, agitation and palpitations which arise 20 minutes to 2 hours after consumption." In effect the mushroom slows down the metabolism of alcohol leading to a buildup of acetaldehyde - a compound responsible for a lot of hangover symptoms. "Although very unpleasant, the syndrome has not been associated with any fatalities. The symptoms can occur if even a small amount of alcohol is consumed up to 3 days after eating the mushrooms and continue for over a week."

My second one is less exciting. Sodium benzoate, a preservative, and Vitamin C can react together to produce benzene, a carcinogen. Heat increases the rate of benzene formation (as does acidic conditions for the decarboxylations I think - I might be wrong on this) so I suppose benzene would be formed faster "in vivo" than in vitro. According to our article Benzene in soft drinks, the amount of benzene formed in say a can of a Vitamin-C-containing soft drink with sodium benzoate preservative "is unlikely to pose a significant health hazard to a particular individual. However, spread out over millions of people consuming soft drinks each day, there might be a small number of cancers caused by this exposure." The Vitamin C and the benzene can come from two different sources to satisfy the combination requirement. Sifaka ^talk 19:37, 4 May 2009 (UTC)

There are warnings against alchohol and some medications, but that is not technically food. 65.121.141.34 (talk) 19:12, 4 May 2009 (UTC)

Any food high in tyramine may cause high blood pressure and headaches when the metabolism of tyramine is affected by Monoamine oxidase inhibitors. A number of consumable plants (for example: pepper, turmeric, tobacco, hypericum) contain substances that inhibit monoamine oxidase to some extent. Not sure if the quantities are sufficient, though. --Dr Dima (talk) 20:13, 4 May 2009 (UTC)

The reaction between tyramine and Monoamine oxidase inhibitor is called Cheese syndrome, where there is a list of many food that can cause Hypertensive crises if ingested with Monoamine oxidase inhibitor Maen. K. A. (talk) 21:59, 4 May 2009 (UTC)

I've heard that the cheese from a cheese fondue can harden dangerously in the stomach if you consume chilled white wine with it. I don't think it's exactly dangerous - but it can be exceedingly uncomfortable. (It's possible that this is some variety of old-wife's tale - but I've heard it independently from quite a few old wife's - so it may still be true). SteveBaker (talk) 22:13, 4 May 2009 (UTC)

Yeah, I've heard that too. Cheese fondue can create a whole new level of indigestion (worth it, though!), but I've never found wine to make it any worse, despite the conventional wisdom. --Tango (talk) 17:07, 5 May 2009 (UTC)

There is something in South America which is harmless, until you eat some other local plant too, then it forms a very potent hallucinogen (which could be considered quite dangerous). But dang if I can find the reference right now: after a half hour of searching, I have to give up! Franamax (talk) 09:34, 5 May 2009 (UTC)

A variety of plants can do this, although they are normally mixed together outside of the stomach (the mixture is called ayahuasca). The main objective is to take a DMT-containing plant mixed with an MAOI-containing plant, as DMT is rapidly metabolized by MAO in the gut (see ayahuasca article for list of plants). --Mark PEA (talk) 12:56, 5 May 2009 (UTC)

And there is also substances that combine elsewhere in the body, for example oxalic acid and calcium making a calcium oxalate kidney stone, frpom eating tofu and spinach. Graeme Bartlett (talk) 21:22, 6 May 2009 (UTC)

See Nitrosamine for one more. (e.g. cured bacon and cheese). 71.236.24.129 (talk) 06:18, 10 May 2009 (UTC)

shelf life of ammo

Does ammunition have a shelf life? How long is it? This is in reference to .306, and shotgun ammunition.65.121.141.34 (talk) 19:08, 4 May 2009 (UTC)

Yes it does. There are multiple concerns. The main concern is the propellant. It will slowly break down. Another concern is the metal, which may corrode. When storing for long term (many years), you need a desiccant to avoid both problems, allowing the shelf life to be nearly indefinite. Note that some propellants do have a rather short shelf life (some as low as 10 years). However, I don't know of any shotgun shells that use that sort of propellant. -- kainaw™ 20:03, 4 May 2009 (UTC)

Is a desiccant needed if one lives in a dry environment, say Arizona? 65.121.141.34 (talk) 20:05, 4 May 2009 (UTC)

Have you seen rust? If so, there is moisture in the air at some point to cause corrosion. Of course, you need to decide how long you want to store the ammo. If you are looking at 10 years, don't worry. If you are looking at 1,000 years, then you need to concern yourself with keeping it as cool and dry as possible. -- kainaw™ 20:09, 4 May 2009 (UTC)

1,000 years, are you serious? If you did manage to keep ammunition for that long, technology would have moved on so far that there would be nothing still around that you could fire it from. For instance, would the ammunition from this gun fit any modern firearm if we had managed to store any of it? SpinningSpark 21:16, 4 May 2009 (UTC)

I was referring the shelf life of ammunition. A thousand years is possible for standard shotgun shells as long as they are kept in a cool and very dry place. The questioner would be better off asking what needs to be done to make the ammo last for a specific number of years, such as: Can I store my ammo for 10 years? -- kainaw™ 02:43, 5 May 2009 (UTC)

There's also the matter of what the distribution of round failure is over time. That is, if I put a box of machine gun rounds into storage, what percentage of them can I expect to fail if I try to fire them in 10 years? It seems to me like that would be the most pertinent question to at least military ordinance officials, who would want to know when it's worth throwing out wartime overstocks. Someguy1221 (talk) 09:49, 5 May 2009 (UTC)

Time travel (sorry)

I'm sure you probably get loads of speculative questions about time travel so apologies. But if someone travels back in time, what happens to the people he leaves in (what is then) the future? Could he potentially return to that time/place at all? Or would his actions reset the timeline in some way? And, if you could visit the future, would the future you see be different from what would actually occur, by virtue of your not taking any actions between getting in the time machine and visiting the future (so then when you go back your actions are preventing what you saw from happening)? How should time travel work if it worked at all? 86.8.176.85 (talk) 21:27, 4 May 2009 (UTC)

It is a basic principle of logic that if A is false, then A implies B is true regardless of what B says. In other words, if you ask what happens if the impossible is true?, the answer is, anything you say.

Our article on time travel is pretty much all-encompassing, and does have a few possible answers to your queries if you look. For instance, one comment is that most feel travel to the past wouldn't be possible unless you would instead go to a parallel universe. As for the future, though he wasn't a time traveler, IIRC, but insted in a deep sleep orbiting the earth, I'd say Buck Rogers would be a good example. it would be as if you disappeared and suddenly appeared years later.Somebody or his brother (talk) 21:58, 4 May 2009 (UTC)

Indeed - time travel is impossible - so it's meaningless to ask what would happen. If you read enough sci-fi, you'll come across a huge range of answers. You have parallel worlds versions of time travel where you go back in time - but to a parallel universe that already has the time-traveled 'you' in it - so what happens to the people you left behind is that they've always lived with the 'time-line' you're now in so nothing changes. However, in other versions, the future changes as you change things in the past (But oddly, your memories of the future don't change?!?) - then we have the "Back to the Future" version of that where the photos of people from your own time start to gradually fade away as you change the past - but again, your memory doesn't. In yet others, anything that introduces a paradox ends the universe - so "Don't Do That". In yet others, you are physically unable to change the future when you are in your past. Other stories (Doctor Who, for example) are carefully vague about that - and in fact lack any kind of consistency from episode to episode. Then of course there is the utter failure of almost every version of time travel to consider that in the time you jumped back to - the earth - the entire solar system in fact - has moved - so far from arriving back in your own living room - you'd be gasping on vacuum...hence all time machines must also be teleportation devices (actually, Doctor Who gets that bit right - his TARDIS can indeed do teleportation - when it feels like it).

Take your pick...but realize that they are all fiction.

If I had to pick one - I think the parallel universe one is the 'cleanest' because there is zero possibility for paradox or anything of that sort. You arrive in what seems to you to be your past - but which is really just an exact copy of your past. You change things - and things change in the future of this new universe accordingly - but these changes are not in line with your memories/photographs which are taken from your original universe. No messy paradoxes.

SteveBaker (talk) 22:07, 4 May 2009 (UTC)

I think the Doctor manages to explain it rather well: it's just wibbly-wobbly timey-wimey stuff. The Timey wimey Ball can be seen in action across the sci fi universe. Gwinva (talk) 22:28, 4 May 2009 (UTC)

The Earth moving isn't a problem - all motion is relative. Just define your coordinates in terms of where your time machine started and all is well. Actually, pretty much all serious ideas for time machines I've seen only allow you to go back as far as when the time machine was created, so you just need to view the time machine as a 4D object that you are travelling within and the problem disappears. My preferred view of time travel is that described by the Novikov self-consistency principle - basically, you can't go back in time and kill your grandfather because if you did do that, that's how it would have happened "the first time" (because there is only one time), and clearly it didn't (well, that's one view that satisfies the principle, there may be others that would work too). Parallel universes work too, but are unnecessary (Occam's razor, and all that). Of course, my view of time travel would mean that everything in predetermined (if you can work out how to define "pre"), but I don't have a problem with that - there is no observation that would show the difference between a predetermined universe and a non-predetermined universe, so just assume the way that works best. --Tango (talk) 22:53, 4 May 2009 (UTC)

Wouldn't entropy, the Butterfly effect and probably Heisenberg throw a serious lug wrench into "travelling within" the "4D object"? You can maybe move backwards in space-time, but you can't rewind the universe. 76.111.32.140 (talk) 23:32, 4 May 2009 (UTC)

I don't see what chaos theory or the uncertainty principle have to do with it, but entropy is an interesting consideration. I don't see it being a problem, though - as long as both entering and exiting the time machine result in an increase of entropy (or, at least, not a decrease) you're fine. --Tango (talk) 23:53, 4 May 2009 (UTC)

Then you have the 12 Monkeys version, which is probably the most consistant with a possible time travel scenario. Though you can travel in time, there is a master "meta-time" which is unalterable; that is events that you travel back in time to affect, it turns out that you actually did them, and they are unalterable. So you can travel in time, but that is already taken into account in the "metatime" so ultimately, you cannot change anything. --Jayron32.talk.contribs 00:31, 5 May 2009 (UTC)

I recently watched a video in which a man went back a few years in time and met himself as a younger man. That was a novel idea in science fiction, though I'm surprised no one thought of it before. – GlowWorm. —Preceding unsigned comment added by 98.17.39.26 (talk) 05:15, 5 May 2009 (UTC)

Hardly novel. Just off the top of my head, in Heinlein's 1958 story "—All You Zombies—", the traveler goes back and does more than just meet himself. Clarityfiend (talk) 05:27, 5 May 2009 (UTC)

A time travel machine will never be made because of the impossible paradoxes that would be caused by interactions with the traveller. However there is no logical objection to constructing a time viewing machine that could show anything in the past and a limited view of the future. Cuddlyable3 (talk) 10:42, 5 May 2009 (UTC)

The problem with time "viewing" is that a lot of the past becomes fuzzy for conceivable means of viewing, such as determining the history of particles based on their contemporary positions and other parameters. That would be, unless you happen to already be far away from what you're trying to look into the past of, in which case you just build a huge telescope. But showing anything in the past with perfect accuracy is impossible. Such a thing would constitute a way of recording an infinite amount of information. I feel like there should be a better article to point to about that, but all that comes up is Bekenstein bound. Someguy1221 (talk) 10:51, 5 May 2009 (UTC)

As has come up on this forum more than once - you could launch a series of large mirrors off into space (one per day, perhaps) and view the past "as it's happening" on any given day in the past using a powerful telescope. Of course this actually follows one of the common "rules" for time machines - which is that you can't see back further than the day the first mirror was launched. When the first mirror is one light-year away - you can see two years into the past. In this case, the "infinite storage" is not actually infinite (it only has to extend back to the day the time machine was invented) - and that storage medium is space itself, storing the photons emitted by the scene we're watching. SteveBaker (talk) 13:38, 5 May 2009 (UTC)

As an aside I recommend the film Primer on this subject.Popcorn II (talk) 12:13, 5 May 2009 (UTC)

Yes - that's great time-travel movie - possibly the only one to truly explore the subject. Be warned though - you'll need to watch it at least three times...and take notes...with diagrams. SteveBaker (talk) 13:31, 5 May 2009 (UTC)

You could just use someone else's notes. Back to the question: depending on which version of time travel you're using, either a) you can go back and forward as much as you want, and you'll never change anything, b) after you go back in time, you branch off a new universe. The old one still exists, but there's no way to get back, c) you spontaneously come into existence because if you hadn't it would have resulted in you going back in time, or d) you destroy everything in the entire universe and rebuild it how it used to be. — DanielLC 16:18, 5 May 2009 (UTC)

I don't understand (c) or (d)... --Tango (talk) 17:05, 5 May 2009 (UTC)

It seems like there's only paradoxes if you presuppose free will. If both the past and future are predetermined I don't understand why going back in time would cause a paradox. APL (talk) 18:42, 5 May 2009 (UTC)

I agree - that's the version I like. Although I wouldn't equate predeterminism with an absence of free will - just because everything is determined doesn't mean it wasn't your will that determined it. --Tango (talk) 19:43, 5 May 2009 (UTC)

I also agree - but the tough part is that if you clearly remember an older version of yourself popping into existance with your time machine when you were just a kid - and saying "Hello me!" - then when you go back in time - you might be determined to cause a paradox and to not say "Hello me!". It's really hard to imagine yourself being somehow completely unable to say something different "this time around" - or (perhaps) that your lack of free will requires that if you are ever going to become a time traveller then it must never occur to you to attempt to cause a paradox. In fact, for the older you, in the past, might have full knowledge of everything that's about to happen - yet be utterly powerless to change it in any way. Worse still, you're seeing different laws of physics on the second time around. Fundamentally random things like quantum uncertainty and such suddenly aren't random anymore. If a younger you stood side-by-side by the older you, watching the Schrondinger's cat experiment - the older you remembers how it turns out. That's decidedly problematic. All uncertainty in the universe (in principle) goes away when a time traveller jumps back in time...that's hard to swallow. Even ikkier is the issue of conservation laws. If future-you pops into existence and gives you (say) a snow-globe...telling you to keep it safe and bring it with you into the past when you invent your time machine...where did the snow-globe come from? That might be a case where you say "Well, you're simply unable to do that" for some cosmic reason - but it would be really tough for the universe to make 100% certain that nothing you bring back in time with you ever makes it around the loop (so to speak) - what about the atoms of the air in your lungs...there is a good chance that a few of those never existed outside of that 'loop' of time. SteveBaker (talk) 21:04, 5 May 2009 (UTC)

All those issues (except the uncertainty one, which I'm not sure is really an issue) can be resolved quite simply: Every situation you have described starts with somebody appearing in the past and doing something. You have no control (in the present) over whether or not they will appear and what they will do if they do, so we can just say that the situations you describe will never happen. --Tango (talk) 22:52, 5 May 2009 (UTC)

The problem with this is that there doesn't seem to be any local reason for the forbidden sequences of events to be impossible. It can get you a self-consistent storyline, but not one that respects causality the way we normally understand it.

Of course the best way to do it is to have, not one set of rules, but several mutually inconsistent ones, and choose among them according to the needs of the dramatic flow. If you make sure enough stuff is always going on that the audience doesn't have time to think about it, they'll probably let you get away with it. That's what makes Back to the Future a great movie, and I say that without a hint of irony. --Trovatore (talk) 21:17, 6 May 2009 (UTC)

Is Larry Niven's essay "Theory and Practice of Time Travel" currently or recently in print? —Tamfang (talk) 05:42, 10 May 2009 (UTC)

I dont belive time travel with you being able to see your future self is possible maybe you could see a shadow of yourself - However if we could theoreticly bend space time to reach point C from point A bypassing B then we can be in a place we would not be able to travel to due to the large distances envolved - So Time travel may not be possible but we could be able to do more extrodanry things in the future to come - or we are all dead due to some massive planetary distructive force that hits us - hence no time traverlers visiting us from the future as we didnt make the technology in time - Unless a UFO is from the future just to add some wooooooo to the whole thing Chromagnum (talk) 13:30, 10 May 2009 (UTC)

Processional orbit

I've just watched BBC Sky at Night (on iplayer for those in the UK) and heard that the Earth's processional orbit is 26,000 years. What does this mean? I can't find anything on Earth or orbit. Thanks Smartse (talk) 23:43, 4 May 2009 (UTC)

See Precession (astronomy). --Dr Dima (talk) 23:56, 4 May 2009 (UTC)

I'd also suggest Milankovitch cycles for info on what it causes. Dragons flight (talk) 00:43, 5 May 2009 (UTC)

Spring stiffness

Moving Q here from Misc desk.

I'm working on the controlling of a simple vibratory system ... But to control this system, I need to find some way to control the spring factor in that system, by increasing and decreasing the stiffness of that spring ( manipulating the spring coefficient "K" ), is that possible ?? maybe by using a pneumatic spring or something ?? It is very important to control the system by controlling the spring not by any other vibration elements...

Thanks for your time,

Hope you can help... —Preceding unsigned comment added by 41.237.8.201 (talk) 18:35, 4 May 2009 (UTC)

You can change the k value of a spring by changing its temperature, but I don't know how significantly. 65.121.141.34 (talk) 19:15, 4 May 2009 (UTC)

I think with a mechanical spring you'd be out of luck changing the spring factor. What you could do is change the "x" by having the base of the spring mounted on a moving plate. That way you could control the force exerted by the spring (since F = -kx for a "normal" spring). I don't know exactly what you want to do so I'm not totally sure if that would work for you... You're also right about using a pneumatic spring, the spring factor can be changed on those by modifying the pressure in the spring's cylinder. TastyCakes (talk) 19:20, 4 May 2009 (UTC)

Are you really altering the spring constant by changing the amount of air in the cylinder? I don't think so. I think you're only adjusting its effective length. SteveBaker (talk) 19:34, 4 May 2009 (UTC)

Hmm yes I suppose you're right, it will simply be at the same pressure (giving the same force) with less displacement, but keeping the same ratio of distance to force (pressure) increase. So I guess I don't know... ;) TastyCakes (talk) 20:10, 4 May 2009 (UTC)

Wait I've been thinking about this some more. By my reasoning it's like this:

$PV=NRT$

$P={F}/{A}=\left({\frac {\left(NRT\right)}{V}}\right)$
where

$V=\pi r^{2}x$

So

$F=\left({\frac {\left(ANRT\right)}{\pi r^{2}}}\right)*{\frac {1}{x}}$

So $k=\left({\frac {\left(ANRT\right)}{\pi r^{2}}}\right)$

And since N changes when you change the pressure, k should also change. Or am I completely off? TastyCakes (talk) 20:32, 4 May 2009 (UTC)

difficult to change spring k. However, could you switch another spring into the circuit: either in parallel, series, or as a replacement to original?> —Preceding unsigned comment added by 79.75.105.45 (talk) 00:52, 5 May 2009 (UTC)

I think you can design your own pneumatic spring, not a standard air cylinder that depends directy on the gas laws. Instead, use a coil of elastic tubing. increasing the pressure within the tube should increase the stiffness. -Arch dude (talk) 02:36, 5 May 2009 (UTC)

I think one way to increase the "k" coefficient is by holding a portion of the spring close to its fixed end stationary. That way, every centimeter that the spring is moved from equilibrium would constitute a greater portion of its effective length and cause a greater restoration force. Whether this is practical or even possible for your case, I wouldn't know. --Bowlhover (talk) 04:41, 5 May 2009 (UTC)

Mount two compressive springs in opposition and use their midpoint for your system. Move the bases of the springs closer together to increase the stiffness without displacing the equilibrium position of the midpoint. Cuddlyable3 (talk) 10:07, 5 May 2009 (UTC)

Could you not use a spring and use something along the lines of Air pressure instead?Chromagnum (talk) 13:33, 10 May 2009 (UTC)