Wikipedia:Reference desk/Archives/Science/2008 December 5

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December 5

Einstein's Theory Of Relativity

Can you explain to me what The Theory Of Relativity states?

"Everything is relative".

Seriously, did you read the theory of relativity article? ~Amatulić (talk) 00:32, 5 December 2008 (UTC)

I thought spacetime intervals were absolute.--GreenSpigot (talk) 02:05, 5 December 2008 (UTC)

Actually Einstein himself thought "relativity" was a bad name for it, because what really matters about his theory are the few invariants that muck around with a relative universe. --98.217.8.46 (talk) 02:08, 5 December 2008 (UTC)

You may want to see Introduction to special relativity or simple:Special relativity. — DanielLC 17:15, 5 December 2008 (UTC)

Designing difficultiy

Why is it much harder to design/propose something simple then something complicated? —Preceding unsigned comment added by 79.75.83.53 (talk) 01:53, 5 December 2008 (UTC)

Simplicity requires being able to see through to the very essence of what one is getting at and to strip away everything inessential to that point. To do that requires knowing said essence and what is essential—a great amount of complete understanding that, especially on the first go of things, is usually quite hard to attain. When I write papers, the first draft are always overstuffed and complicated. Only after lots of revisions do I see what the core essence of it is and try to strip away everything else. Even then, it's hard to do it—there's a temptation to include all sorts of extra bits, just because they have grown on one. --98.217.8.46 (talk) 02:12, 5 December 2008 (UTC)

"I didn't have time to write a short letter, so I wrote a long one." (attr. Mark Twain, orig. by Blaise Pascal). DMacks (talk) 05:13, 5 December 2008 (UTC)

I don't think that's always true...but certainly it sometimes is. A car is more complicated than a skateboard - I would certainly never claim that a car was easier to design than a skateboard. But to design a car that's significantly simpler than current cars (and performs the same functions) would be extremely hard. It goes both ways.

I design software systems (computer games) - and very often, I come up with an initial design for some part of the system - but I may regard it as too complex and then have to put in more effort to simplify it. But if I saw that my design was actually too simple (and therefore couldn't do some of the things it needed to do) - it could easily take significant effort to make it sufficiently complex. Adding simplicity is very satisfying though...particularly if simplifying something makes it easier to make...the best computer programmers are lazy people!

SteveBaker (talk) 03:18, 5 December 2008 (UTC)

Keep design as simple as you can, but not simpler :) - manya (talk) 04:11, 5 December 2008 (UTC)

It think it should be "complex is easier than simple ... for equivalent functionality". If you started to attach an engine, breaks, air conditioning, leather seats, safety systems, etc. to the skateboard, things start to change in the simple/complicated analysis. I think one of the reasons that complex is easier than simple relates to whole epicycle issue- if you need to add functionality, it's easiest to just tack it on top of the already existing structure. Do that enough times, and you end up with a rats' nest. It then takes a lot of time, effort, and ingenuity to pare back the complexity and cleanly integrate the functionality at the base level, rather than having it be a tacked-on afterthought. -- 128.104.112.113 (talk) 17:31, 5 December 2008 (UTC)

Speed though time

At what speed do we all progress through time? —Preceding unsigned comment added by 79.75.83.53 (talk) 01:55, 5 December 2008 (UTC)

"Speed" is a concept which requires reference to time (e.g. miles per hour), so it's not a physically sensible question. The idea that time itself could have a speed is an illusion brought on by things like film technology which apparently let us manipulate the speed of time, but this is just an visual illusion related to how vision works. Time does not work the way it appears in a cinema. --98.217.8.46 (talk) 02:03, 5 December 2008 (UTC)

Yes but we all travel forward through time dont we? So, at what speed do we go. Its a reasonable question —Preceding unsigned comment added by 79.75.83.53 (talk) 02:10, 5 December 2008 (UTC)

One second per second. Algebraist 02:11, 5 December 2008 (UTC)

We don't travel through time. Time is not physical space. We use the metaphor of physical space to help us make sense of it with our clumsy human brains, but that is not what time is. (There is a relationship between time and physical space—spacetime—but that is not what time is.) --98.217.8.46 (talk) 02:14, 5 December 2008 (UTC)

Time is the 4th dimension. We travel through it. At what speed? —Preceding unsigned comment added by 79.75.83.53 (talk) 02:23, 5 December 2008 (UTC)

I told you already. What is the problem? Algebraist 02:34, 5 December 2008 (UTC)

You can think of time as the 4th dimension - but if you start imagining it's "just like another spatial dimension" then you're missing what's going on. Forget that analogy - it sucks and it's confusing and doesn't fit reality well. There are three spatial dimensions and one time dimension...don't roll them together - it just doesn't work.

But you only "move" when you are changing where you are in both space and time simultaneously. When you are travelling at 30 miles per hour - that means that as you travel along the time dimension - you also travel along the space dimension. If you plotted your 'time' and your 'distance' as points on a graph, the "speed" would be the slope of that graph. So you don't have speed in one axis at a time - speed is something that requires change in both the time dimension and one or more of the distance dimensions. If you don't move in the spatial dimension - the line is flat and your speed is zero - if you could somehow move in one of the distance dimensions without moving in time - the line would be at right angles to the time axis and your speed would be infinite.

So beyond the interesting philosophical issue of whether we should consider time to be the 4th dimension - it's truly a meaningless question. 03:06, 5 December 2008 (UTC)

According to Brian Cox, when we stand still in all three spacial dimensions we are moving through the fourth dimension (time) at the speed of light, and if we move very fast in any of the space dimensions we "use up" or transfer some of that "speed" in the forth dimension to the space dimension,so move more slowly through time.Trevor Loughlin (talk) 03:05, 5 December 2008 (UTC)

That's a bit garbled, but more or less true. See special relativity. However, from our own perspective we are always at rest, hence moving forward in time at one second per second (which translates, in this slight garbling, as the speed of light). From the point of view of observers moving at high speeds relative to us, we are going forward in time at much less than one second per second. Algebraist 03:08, 5 December 2008 (UTC)

Thanks a lot for this! Now I'm always at rest. How do I get to the bathroom? Wanderer57 (talk) 06:21, 5 December 2008 (UTC)

Cause it to move towards you. Duh. Algebraist 06:27, 5 December 2008 (UTC)

Oh good idea! Except with time moving at the stupendous rate of one second per second, I'm not sure I can hold out that long. Wanderer57 (talk) 07:21, 5 December 2008 (UTC)

I disagree that "from our own perspective we are always at rest". You can adopt whatever perspective you like. I personally don't think of myself as always at rest. I'm at rest as I write this, but when I go walking outside I'll be moving. I adopt the Earth (including all the stuff attached to it, like buildings and trees) as my reference body most of the time, because it's a more useful reference body than my body for most purposes.

Some people have gotten the idea that special relativity requires you to use rest frames. They get into horrible difficulties with what should be very simple problems involving accelerating objects because they believe that they have to work with uncountably many reference frames, one for each point on the object's worldline. Sometimes they even claim that special relativity can't handle acceleration at all, which is like saying that Cartesian coordinates can't handle circles. So I try to discourage the you-are-always-at-rest perspective because I think it interferes with learning special relativity. -- BenRG (talk) 15:47, 5 December 2008 (UTC)

There is no way to answer this question (except with jokey tautologies like : 'Sixty minutes per hour'). The word speed is defined as distance per unit time. What you're asking for is "distance (through time) per unit time" which doesn't make sense, because that just simplifies down to "time per time", which is silly.

"Speed" is what you get when you compare a movement against time. Comparing something with itself doesn't give you a usable answer.

So, I guess you could say that the answer to your question is "1". (no units, just 1). But that doesn't really mean anything. Better to say that there is no answer. APL (talk) 15:12, 5 December 2008 (UTC)

No, better to say that it is an ill-formed question, a question that does not permit valid answering. It's like asking how many pancakes it takes to make a banana. The question is nonsensical. --98.217.8.46 (talk) 15:33, 5 December 2008 (UTC)

There are two different concepts of time in physics, "time as a fourth dimension" and "time as a measure of change". As far as we can tell, though, there's only one kind of time in the real world. You can think of it as a fourth dimension or as a measure of change, but not as both at once, because that would be like having two independent dimensions of time or two independent time parameters. When you ask "how fast do we move through time", you're making that error. You have time as a dimension (which you call "time"), and you also have time as change (implicit in the verb "move" or "progress"), and in order to "move through time" they would have to act independently. You can think of the world as stuff moving through space, or as stuff in spacetime that just is, but you can't think of it as stuff moving through spacetime.

Interestingly enough, reconciling the two notions of time is one of the biggest problems with combining general relativity and quantum mechanics, the so-called "problem of time". Special relativity can function acceptably with either kind of time, but general relativity has problems with time as change, and quantum mechanics has problems with time as a dimension. -- BenRG (talk) 15:47, 5 December 2008 (UTC)

I think some of the recent questions on this ref desk (i.e. the apparent path of mercury question and this one) are arising from people watching the recent horizon program in which Prof. Brian Cox states that we are travelling through time at the speed of light and the passing of time is thus the result of us moving through the time dimension at this speed. See: http://www.bbc.co.uk/programmes/b00fyl5z (This programme is possibly only viewable in the UK) Jdrewitt (talk) 15:50, 5 December 2008 (UTC)

That sounds likely. What he's talking about is the Four-velocity. It is the tangent vector to the World line in Minkowski space-time and represents rate of change in space and time coordinates along the path. This four-velocity is always equal to the speed of light. From that follows that if a particle moves at near the speed of light, its rate of change in time is almost zero, and that if the particle is at rest, it moves at speed c in the time direction. So in that sense, we're progressing at the speed of light through time. This is complicated further when you'r working in a metric as the world line article points out. EverGreg (talk) 16:12, 5 December 2008 (UTC)

Carbon monoxide poisoning

Our article states that undiluted cigarette smoke contains roughly 30,000 ppm of carbon dioxide monoxide (typo). Under the Toxicity header, our article also has a nice little section dedicated to the side effects of CO poisoning and says that "12,800 ppm (1.28%) [causes] Unconsciousness after 2-3 breaths[citation needed]. Death in less than three minutes."

Okay...I'm a bit confused. If undiluted (yeah, I know cigarettes have filters usually, but do they really take that much CO out? Or what if you smoke an unfiltered cigarette?) cigarette smoke contains 30,000 ppm of carbon monoxide, and LESS THAN HALF of that concentration will kill you in less than three minutes, why aren't there a lot more dead cigarette smokers? And even with the breathing in [moderately] unpolluted air in between cigarette hits, why doesn't this acute carbon monoxide poisoning cause severe headaches or nausea, like it supposedly does at 1600-3200 ppm? I'm just a bit confused is all. --71.98.25.115 (talk) 02:36, 5 December 2008 (UTC)

I suspect "undiluted cigarette smoke" means without air you suck through the cigarette, which would be most of the gas you inhale when you takea puff. It could do with some clarification in that article though. --fvw* 02:41, 5 December 2008 (UTC)

Yeah - I think that's the point. If your lungs were COMPLETELY full of air with 12,800 ppm of CO - it would be bad. But you are breathing a mixture of smoke with 30,000ppm of CO and fresh(ish) air that didn't come through the cigarette. The key word is "undiluted" - if you had several lungfuls of undiluted cigarette smoke - you'd be in deep trouble...but in reality, it's diluted...so it's not so terrible. Of course it's still terrible - there are hardly any chemicals that come out of a cigarette that aren't doing you harm. It's just a matter of degree. SteveBaker (talk) 02:47, 5 December 2008 (UTC)

Careful - carbon monoxide and carbon dioxide are different - it's the monoxide which is highly toxic, dioxide is harmless until you get up to pretty high concentrations. You meant "monoxide" in your first sentence. --Tango (talk) 12:11, 5 December 2008 (UTC)

Okay I see now. Just curious then, what would the typical concentration of CO be in the average hit of a cigarette? It must be pretty low, because people don't typically complain of headaches and nausea after smoking in my personal experience. --71.98.24.41 (talk) 18:52, 5 December 2008 (UTC)

Is this a legitimate scientific study?

http://www.medicalnewstoday.com/articles/64474.php —Preceding unsigned comment added by 216.239.234.196 (talk) 13:29, 5 December 2008 (UTC)

It appears to be a report about an essay about the effectiveness of an herbal product written by an organization devoted to promoting the use of herbs. An essay is not a scientific study. - Nunh-huh 13:37, 5 December 2008 (UTC)

That summary contained a link to this PDF file on the "study": [herbalgram.org/files/pdfs/5594COLD-fX.pdf], but the link doesn't work. This makes me rather suspicious. StuRat (talk) 14:04, 5 December 2008 (UTC)

I did a search and did find a seemingly valid study: [1], which does conclude that it's safe and effective (although they only claim a 13% reduction in the chances of getting a cold, which isn't all that great). StuRat (talk) 14:15, 5 December 2008 (UTC)

Is "HerbalGram" a peer-reviewed scientific journal? Looks like an in-house publication of a pro-herbal group... --98.217.8.46 (talk) 15:30, 5 December 2008 (UTC)

I don't know about them, but the study they sight seems to be legit, unless they've just made the whole thing up, of course. StuRat (talk) 17:48, 5 December 2008 (UTC)

"HerbalGram" is not peer-reviewed. However the original study is published in the Canadian Medical Association Journal, here. Axl ¤ [Talk] 10:35, 6 December 2008 (UTC)

Short answer: yes, it does appear legitimate. Axl ¤ [Talk] 11:31, 6 December 2008 (UTC)

But also note that a single study is not usually sufficient for a robust result. In this case, I also note that some of the authors work for the company that make the herbal remedy in question. That does not mean that it is false, but it's easy for bias to slip into it - if only by doing 20 studies and publishing the one that will show a 95% significance by chance. --Stephan Schulz (talk) 23:24, 6 December 2008 (UTC)

It bothers me that (according to that article) the manufacturers, the researchers, the peer-reviewers and the authority claiming the advance are ALL 'acknowledged' herbal/natural diet supplement 'experts'. Becoming an 'expert' in this field requires almost no scientific training or formal qualifications - so how do we know that any of these people know what they are talking about? I'd want to see some studies done by mainstream groups...the history of herbal medicines is littered with bogus claims, fraudulent studies, you name it. This one might be OK - but I'd definitely be sceptical. With herbal medicines, the barrier for testing for side-effects and counter-indications is very low indeed - that's because they mostly don't actually do anything. When you find a herbal medicine that actually DOES do something - then is the time to be worried because you have something with some powerful effect on the body - but almost no adequate testing for side-effects, drug interactions - nobody calculating the safe dose - nobody checking that it's not dangerous to children, pregnant women - no regulation of how much is administered, and so forth. The ones that "work" are the ones you should avoid the most!! SteveBaker (talk) 05:10, 8 December 2008 (UTC)

Energy

Do you have a better idea of what energy is, other than the 'ability to do work' or the fact it is scalar and is some form of attribute? My guess is no, but hey, I might be surprised. —Cyclonenim (talk · contribs · email) 14:29, 5 December 2008 (UTC)

It's the conserved charge corresponding (under Noether's theorem) to the symmetry of the laws of physics under time-translation. Algebraist 14:42, 5 December 2008 (UTC)

LED Grow Lights

Are there any LEDs that are created specifically for use as grow lights?

Would it work to simply use an array of red and blue emitting LEDs that were not created specifically for growing plants?

To put this another way, is this idea promising enough to be worth trying, or is it way off base? Thanks, Wanderer57 (talk) 16:35, 5 December 2008 (UTC)

Google results. --LarryMac | Talk 16:45, 5 December 2008 (UTC)

I've seen UV LED flashlights on ebay. Cool for seeing unspeakable stains on hotel walls! Not sure about growing plants. --70.167.58.6 (talk) 17:59, 5 December 2008 (UTC)

Yes, I have one of those UV LED flashlights. I've taken it on airliners, to examine the seat cushions, blankets, and pillows. I decided later, that what I don't know won't hurt me.

It seems to me that you could create your own grow light by mixing red, yellow, green, blue, white, and UV LEDs in a proportion that approximates a solar spectrum. ~Amatulić (talk) 20:51, 5 December 2008 (UTC)

Addendum: The light-emitting diode article cites this article, which indicates that simple red LEDs make good grow lights for plants. ~Amatulić (talk) 21:04, 5 December 2008 (UTC)

I'll bet those LED grow lights will be popular with those who grow marijuana crops indoors, as heat from regular grow lights is one of the ways cops find them, using infrared cameras. StuRat (talk) 00:43, 6 December 2008 (UTC)

I imagine not being harassed by cops is a plus for anyone who gardens indoors, even honest folk. APL (talk)

Yes, Lots. Typically they use some assortment of LEDs that emit on a specific frequency of red, and some LEDs that emit on a specific frequency of blue. Searching Google for "LED Grow Lights" will give you lots of hits.

If you're more of a do-it-yourself-er. Here are a couple of entries from the Make blog : [2] [3] I've been thinking about making a couple small ones on timers for indoor potted plants that don't get enough light.APL (talk) 00:53, 6 December 2008 (UTC)

Is it possible to smell your own breath?

I see people doing "breath checks" by exhaling into a cupped hand help up to their face. Barring any serious infection or chronic odor, is it possible to really tell if your breath is funky? --70.167.58.6 (talk) 17:52, 5 December 2008 (UTC)

This is a standard way of checking your breath. See "Smelling and tasting"(2002, or [4]. Another is to [5] [6] lick the back of your hand]. There is also the "Fresh Kiss HC-201," an electronic device which gives a reading ranging from "undetectable" to "very bad halitosis." Edison (talk) 19:44, 5 December 2008 (UTC)

Apparent contradiction in information about tumours

Hello; I heard on the radio that tumours grow exponentially, but that they grow more rapidly at the beginning than later. That appears like a contradiction to me. Can someone please clarify this for me? Thanks! Leptictidium (mt) 19:54, 5 December 2008 (UTC)

I'll hazard an educated guess, based on what I've been told by doctors, in relation to uterine fibroid tumors.

A tumor grows exponentially because each cell continues to reproduce. If you have a doubling of cells every constant time interval, you have exponential growth. However, the tumor cells need oxygenated blood supply to survive. At some point the growth becomes too large for the blood to sustain it, and cells start dying off, slowing the growth.

So yes, the growth is exponential, but a limiting factor takes over after a certain point. That's my guess at explaining the apparent contradition you heard. ~Amatulić (talk) 20:47, 5 December 2008 (UTC)

That's well put, and correct to my knowledge. - Draeco (talk) 00:19, 6 December 2008 (UTC)

Wikipedia really does have an article (or at least, section) on everything: Logistic curve#In medicine: modeling of growth of tumors. --Tango (talk) 01:03, 6 December 2008 (UTC)

Thanks, your answer has been very useful! Leptictidium (mt) 08:25, 6 December 2008 (UTC)