Wikipedia:Reference desk/Archives/Science/2014 August 4

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August 4

Are Mathematical Dimensions incorrect?

This is probably not the best place to debate or question the validity of the current mathematical dimensions up to 4-dimensional, but I was trying to imagine some spacetime concepts last week, and reading up on the mathematical 4D, it seems that some people like to refer to the fourth dimension as "Time".

I'd like to take a moment to define three dimensional space and somehow propose that 4D is not explicitly "time" in the way it is currently understood. In 3D, much like 2D, there are just points on planes; but with the added Z plane to the established XY planes, right? But the thing about analyzing depth is that it already inherently requires a factor of time to be aware of it. From a static view from any static point in an XYZ 3D Graph, all other points and their lines would just appear flat. The main reason depth can be analyzed at all is because either the points can move at a speed, or the entire 3D graphical presentation can be moved and manipulated to display different angles, and movement is a factor of time.

This has lead to problems with visualizing 4D Shapes using the current time-related Fourth Dimensional view because of this apparent overlooking of "time" having already been implicitly established in the Third Dimension view. I have proposals for a restructred dimensional understanding from 1D to 4D, But again i'm not sure this is the place to best deliver the idea.

So really i'm just wondering if any mathematicians out there doubt the established order and definitions of Mathematical Dimensions? Fbushnik (talk) 02:52, 4 August 2014 (UTC)

Before you advance your proposals in a more appropriate forum (Wikipedia isn't such a forum), it would probably help for you to read Dimension (mathematics and physics). RomanSpa (talk) 04:34, 4 August 2014 (UTC)

It's not a question of doubting it. Euclidean 4-space is as self-consistent a mathematical system as Euclidean 3-space. But we know for a fact that neither of them describes the physical world correctly. To see an abstract mathematical model of relativistic space-time, check out Minkowski space. AlexTiefling (talk) 04:43, 4 August 2014 (UTC)

Treating time as "The Fourth Dimension" is a mathematical convenience. It doesn't mean that it is literally "the fourth dimension of space" in terms of physics...that's not even a particularly reasonable presumption. There are plenty of scientific theories ("String theory" being the most prominent) that actually require there to be more than three "spatial" dimensions. It's then necessary to question why we can only perceive three of them...but there are ways to do that.

This question about extra spatial dimensions comes up quite a bit - and I like to point out that we do not in fact "see" the world in three dimensions in the first place. Our retinas are two dimensional imaging devices, just like cameras or an artist's canvas. We see the world in TWO dimensions and attempt to infer the third from the way that the 2D image changes as we move around - from the fact that we have two, slightly different 2D images to compare - from the fact that we have to adjust the focal length of the lenses in our eyes to bring nearby objects into clear focus...that kind of thing.

An analogy with how a computer draws 3D objects in computer games and such is worth explaining here. The third dimension is basically used only indirectly. It's used to reduce the size of objects as they get further away (perspective) and to hide distant objects behind nearer ones (hidden-surface elimination). However, we can change that...it's easy enough to toss out the perspective thing and draw an "orthographic" display. Lots of "god games" like SimCity do that, and we have no problems with it. You can also draw just the outlines of objects, making them transparent - and still, we can recognize 3D objects. So simply ignoring the 3rd dimension doesn't seriously impact our ability to see. We can use the third dimension in other ways...modifying the color of objects as they get further away allows you to simulate fog...which actually improves depth perception quite a bit if used subtly. Mixing in a bit of blue creates the effect of Raleigh and Mie scattering...same deal.

You can also do what doctors do with MRI machines - they photograph a bunch of slices of an object and change them over time. That's using time as the THIRD dimension...which is another kind of projection.

So-called "3D displays" like in movie theatres basically use the third dimension to produce a spatial shift between two images where the amount of shift varies with range...since our visual system naturally does that in the real world, it works reasonably well.

You could (theoretically) make a display where the blurriness of the image varied according to the third dimension to allow your eye's focussing mechanism to be used to get us that extra dimension.

In mathematical terms, these are all kinds of "projection" - how do you project three dimensions into two? And things like perspective, orthographic, fog and hidden-surface are all ways to drop that extra dimension.

So to view a 4D world, it's just a matter of deciding how you're going to deal with the extra two dimensions. You could, for example, use one of the extra dimensions to do a perspective projection and the other dimension to produce color shifts like fog or spatial shifts or whatever.

The most common way to try to help people see in 4D is to use the third dimension as perspective and the fourth as time...but it's certainly not the only choice.

The observation that we're ALREADY missing one of the three dimensions because of our two-dimensional retinas is a useful one. There is nothing magical about having extra dimensions - it's just a matter of how you discard them in order to produce an effect that our 2D retinas can cope with.

SteveBaker (talk) 15:12, 4 August 2014 (UTC)

It is very much of a shorthand, and a somewhat misleading one, to label time as Dimension 4 along with the three known spatial dimensions. Another treatment is to use a Minkowski treatment that views time as Dimension -1 as being time-like rather than space-like. (In a black hole, of course, things are different.) Robert McClenon (talk) 17:10, 4 August 2014 (UTC)

• Doesn't gravitational lensing count as a directly visible spatially fourth-dimensional phenomenon?

• Is there any possibility of "exposure" being a dimension in itself?

For example, hubble deep field images. We have the telescopic power to look immense distances to recognize extremely distant galaxies; but only with the factor of exposure. IIRC the eXtreme Deep Field took upwards of 20 hours of exposure. The way i see it; real 3D points are static even with regard to changing an outside viewing angle of their 3D graph, and only with movement do they become "4D", which would fully envelope our universal existence and the way we perceive motion in our evermoving universe. But although those distant galactic sources of light are always sending their light to us at a relatively constant rate, it takes an extra dimension of recorded time in "exposure" to actually view them. Imagine light photons on the Edge surface of a wheel, and this wheel will roll along the the line-of-sight from the focal point (xtreme deep field), to our point of observation (hubble). We normally cant see the distant galactic objects (imagined by the edge surface of the wheel not intersecting our point of observation), but if we set exposure, it allows that wheel to "roll" towards us and eventually the edge will intersect our point of observation, to which after having recorded for 20 hours we now get a visible image. Something we cannot experience "right now", but can experience over a length of time. Fbushnik (talk) 19:41, 4 August 2014 (UTC)

Effects of faking muteness

I recently saw a Chinese TV series where one particular character had faked being mute at the request of her mother. This charade lasted for several years until she saw an urgent situation involving a couple of other characters and suddenly spoke again (and the words came out flawlessly), much to the surprise of the other two characters that were with her. I wonder how realistic this scenario would be in real life: if someone actually faked being mute for years and the suddenly decided to start speaking again, wouldn't that person initially have trouble speaking coherently and thus require therapy/training in order to regain his/her formerly coherent speech? 69.120.134.125 (talk) 03:26, 4 August 2014 (UTC)

I wouldn't think so, since they would still be thinking the words all that time, perhaps even forming them with the mouth but not sounding them. This would keep them in practice. StuRat (talk) 05:23, 4 August 2014 (UTC)

There are cases of people being genuinely mute or voluntarily choosing not to speak for decades and suddenly being able to speak again...it does seem to be something you don't forget. SteveBaker (talk) 14:50, 4 August 2014 (UTC)

One famous example is Maya Angelou, who was mute for five years as a child. Ghmyrtle (talk) 16:25, 4 August 2014 (UTC)

Yes I was also wondering about those who took a long term vow of silence although they're generally much older. However I didn't find anything about what it's like when they speak again. Nil Einne (talk) 18:39, 4 August 2014 (UTC)

Selective mutism. --TammyMoet (talk) 18:09, 4 August 2014 (UTC)

I don't know if that's particularly relevant. (Selective mutism could cause problems like [1]) but if the person is still speaking then it probably won't have the same effect on their ability to speak as someone who never speaks, presuming there is an effect.)

Although it could be. One thing that isn't clear from the OP's post is whether the person was truly mute. It seems easily possible they spoke to their mother when alone, perhaps when she gave permission. Or alternatively secretly to a friend or even to herself. There are also other factors that may come in to play, e.g. what age was she when she stopped speaking and what was her level of speech? Of course, may be the story didn't go in to such details.

Nil Einne (talk) 18:39, 4 August 2014 (UTC)

Can a person get Ebola if they have sex

Can a human being contract Ebola if they have sex with someone with the Ebola virus? If so, should the person be wearing a condom to protect themselves? 202.177.218.59 (talk) 03:51, 4 August 2014 (UTC)

Actually, I think this is one case where a gendered pronoun might work better than singular they. :) Though I might still take some flack from trannies for being politically incorrect. :) Wnt (talk) 20:12, 4 August 2014 (UTC)

(I'll assume that this is a hypothetical question, rather than a request for medical advice) If you know your partner has Ebola, you shouldn't be getting that close anyway. It is usually transmitted through "direct contact (through broken skin or mucous membranes) with the blood, secretions, organs or other bodily fluids of infected people" [2] While a condom might provide some protection, it certainly wouldn't be enough to make it safe. Abstinence in such circumstances would seem sensible... AndyTheGrump (talk) 04:06, 4 August 2014 (UTC)

I rather doubt that anyone suffering from Ebola would be in any condition to engage in sexual activity. Roger (Dodger67) (talk) 07:15, 4 August 2014 (UTC)

Apparently, there's a week or so at the start where you're not a "sufferer" yet. Looking normal and feeling normal leads to normal sex. InedibleHulk (talk) 08:12, 4 August 2014 (UTC)

I've just been reading an article based on the diary of a doctor working with the World Health Organisation in Africa at the heart of the ebola outbreak. For reference it's here but you need a subscription to read it all. He says that ebola has a 21 day incubation period when people are infected but symptom free and that the virus has been isolated from almost every body fluid including sweat, semen, blood, urine, oral secretions and tears. Doctors and nurses caring for the patients wear full body protection, like space suits, and yet a number of them have contracted the disease and died. A condom would provide little protection in this situation as you would still be exposed to bodily fluids other than semen. Richerman (talk) 18:04, 4 August 2014 (UTC)

While I don't disagree that a condom is unlikely to do much, how infectious are people during the period they are symptom free, particularly the early part? While I'm not sure if anyone knows for sure, it seems easily possible many of the infections of medical workers are coming from caring for patients with symptoms rather than patients who are currently symptom free. Particularly in the early stage of infection (amongst other things, it seems unlikely many of these are patients, actually I think that's a big part of the problem). Nil Einne (talk)

Many of the media reports have been reassuring us that people aren't infectious until they show symptoms. Then again, these are the same media reports that reassured us that Ebola could only spread through backward African customs of washing the dead and so Americans in Liberia would be at no risk of catching it. So I tend to think somebody is trying to quell panic, and might be making broader generalizations than what is known of the biology of the virus can really support. On the other extreme there's Richard Preston in The Hot Zone claiming that Ebola Reston passaged in a few generations to become a sort of "Ebola flu" being passed through ventilation shafts from room to room. The virus is new to humans, still getting accustomed to its new surroundings, so I wouldn't hastily rule out such a thing, but it still is a pretty extraordinary claim. Bottom line: when the virus is ready, it will let us know... Wnt (talk) 20:09, 4 August 2014 (UTC)

Sounds unlikely as symptomless carriers like bats transfer the virus just fine. Viruses that kill too quickly before patient death don't last long. The most lethal viruses are transferrable for a long period when the host is alive. --DHeyward (talk) 12:09, 5 August 2014 (UTC)

Men who have recovered from the disease can still transmit the virus through their semen for up to 7 weeks after recovery from illness. Count Iblis (talk) 21:22, 4 August 2014 (UTC)

Exercise and immune system

In what ways does exercise help the immune system? And does this include all exercise? (Cardio, muscle strength training, walking etc) — Preceding unsigned comment added by 82.132.216.37 (talk) 13:11, 4 August 2014 (UTC)

Here is a quick overview from the National Institutes of Health, a U.S. Government agency. It's got references too, if you want to follow up for more details. --Jayron32 23:43, 4 August 2014 (UTC)

Objects hitting the Moon?

I don't know if this has already been addressed, but what about the claims of objects hitting the Moon in this video? Bubba73 ^{You talkin' to me?} 19:59, 4 August 2014 (UTC)

I'm not going to watch that whole video, but things hit the moon quite frequently. It has a minimal atmosphere so nothing "burns up" as it does in the earth; any time we have a meteor shower on earth, all those streaks you see are little rocks that, on the moon, strike the ground. See Moon#Impact_craters. Here is a page at NASA.com discussing a 2006 impact on the moon which was watched from Earth; it discusses lunar impacts in some detail as well. --Jayron32 20:35, 4 August 2014 (UTC)

I didn't watch the whole thing, because 19 minutes is way too long to spend watching a UFO conspiracy theorist's video. But at one point of the skipping around that I did, it looked like the guy was being amazed by what to me just looked like some compression artifacts. Red Act (talk) 20:51, 4 August 2014 (UTC)

I didn't watch the whole thing, but about 8 minutes in he shows what he is talking about. In two of them I could see a dark streak going across. I didn't see the crater he was talking about. Could the dark streak be a compression artifact? Bubba73 ^{You talkin' to me?} 21:47, 4 August 2014 (UTC)

Yes. It looks to me like a meteor or some other object passed in between the telescope and the moon, without touching the moon. Some of the pixels in the image along the line where the object passes aren't the same color after the object passes over as they were before the object passes over, and the guy interprets that as indicating an object skipping across the moon, leaving craters in its wake. My presumption is that those changed pixels are compression artifacts. When a video is compressed, most of the frames of the video aren't compressed completely independently of other frames, because that would be too inefficient. Instead, only infrequent I-frames are encoded completely independently of previous frames. All other frames are either P-frames, which are encoded in terms of data from past frames, or B-frames, which are encoded in terms of data in both past and future frames. The result of that inter-frame data dependence can result in a tiny object flying across the image leaving some "ghost" pixels behind it where the object's image used to be in recent frames. Red Act (talk) 22:57, 4 August 2014 (UTC).

It's tough to know what's a compression artifact - or some consequence of amateur "enhancement" tricks that actually make exciting images from nothing at all. Before anyone should give an opinion here, we need to see the most original image available. The narrator says "the only change made is adding that reference marker"...yeah - but to add that marker, they had to decompress the video images and then recompress them again. These compress/decompress cycles are "lossy" - so ANYTHING you do to an image with lossy encoding and decoding can obliterate valuable evidence and introduce spurious artifacts - and that's more than enough to create all kinds of junk in the picture.

For a rock to be visible at that scale, it would either have to be VERY close to the camera (ie, not hitting the moon) - or absolutely freaking ungodly huge...in which case we'd have heard about it because anything the size of a large mountain entering inside the moon's orbit would be a major event.

Back in 2009, I showed how this kind of thing happens. I start with a screenshot of the Wikipedia science reference desk (what could be more innocuous?!). I look at a single period ("." character) from that page, I convert between file formats, zoom in, do "enhancement" and wind up with a clear picture of an alien city, hiding inside that period! Here are the stages...

Here is another example where I grab a picture of the moon and "enhance" it to reveal clear signs of an alien rocket attack!

Bottom line, "enhancement" tools are dangerous weapons in the hands of the idiots. SteveBaker (talk) 23:13, 4 August 2014 (UTC)

And you didn't point out the "force fields" around Earth and Moon. With that amount of geometic regularity, they HAVE to be of alien origin! OTOH, the lines look "genuine" but I'd blame scratches or smears on the window rather than missiles.

Introducing said JPEG/MPEG artifacts before enhancement is the high-tech equivalent of rounding intermediate results. That's one of the reasons why popular video formats are not used in scientific imaging: they tend to have a very low data rate but discard useful information. In essence, they try to discard "information the human eye isn't likely to see anyway", while image enhancement can be defined as "enhancing information the human eye isn't likely to see anyway"! It's not hard to see what happens if one compresses and then enhances what's left of the origial data. - ¡Ouch! (^{hurt me} / _{more pain}) 07:20, 5 August 2014 (UTC)

Actually, rather than a meteor, my bet is that the object going across the image is a satellite. Comparing the field of view of the image to a full picture of the moon, it looks like the object is travelling at an angular velocity relative to the Earth such that it subtends the same angle that the moon's diameter does in about 8 seconds. The angular diameter of the moon as seen from the Earth is about 30 arcminutes, so an object in Earth orbit would have that angular velocity if its orbital period is about 96 minutes. Given that 96 minutes is in the range of orbital periods that satellites in low Earth orbit have (88-127 minutes), and given that the large majority of satellites out there are in low Earth orbit, it seems like it'd be too much of a coincidentally correct speed for the object to not be a satellite. If someone wanted to, it should be possible to take the time of the observation and the latitude and longitude of the telescope, and figure out exactly which satellite would be appearing to cross the moon at that time from that location.

Although I have a theoretical interest in astronomy, I'm not a star-gazer at all, so I don't know if typical satellites are as easily visible as in that video (although I'm guessing not). If not, then I'm thinking the observed object was the ISS, which is an unusually visible object among the objects in low Earth orbit. The orbital period of the ISS is 92.88 minutes, which is very close to the 96 minute orbital period that I came up with as an approximation. Red Act (talk) 02:04, 5 August 2014 (UTC)

I am a star gazer and while the ISS is especially bright, there are other many satellites which are also very bright, Some can be even brighter when the angle is right. Vespine (talk) 03:39, 5 August 2014 (UTC)

I am a former star gazer, and in my experience they wouldn't even have to be visible to the naked eye. It's not uncommon while looking at a telescopic view of something to see, in a 20 minute period, 2 or 3 satellites that (by comparison to the stars in the field) must be well under magnitude 6. {The poster formerly known as 87.81.230.195} 212.95.237.92 (talk) 13:23, 5 August 2014 (UTC)