Wikipedia:Reference desk/Archives/Science/2013 June 4

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June 4

Gravitational time dilation

Can anyone tell me a bit more aboutGravitational time dilation. Particularly how time speeds up on an aircraft. Clover345 (talk) 00:00, 4 June 2013 (UTC)

Time doesn't speed up on a moving aircraft, it slows down -- only by a minuscule amount, but enough to be measurable by a super-accurate atomic clock. 24.23.196.85 (talk) 00:45, 4 June 2013 (UTC)

Gravitational time dilation has the opposite effect. An object farther from the surface of the earth will have less gravitational time dilation, and thus at the same velocity, a plane flying at a higher altitude will have a slightly faster clock than one flying at a lower altitude at the same velocity. --Jayron32 04:37, 4 June 2013 (UTC)

Some decades ago, some source (probably the Guinness book) discussed the fact that the Apollo astronauts' clocks during their journeys to the moon had gone slightly faster than earth-bound clocks, by some number of microseconds. The book remarked that despite that fact, "No formal overtime claim was filed." ←Baseball Bugs ^{What's up, Doc?} carrots→ 13:13, 4 June 2013 (UTC)

Also interesting is that at full Moon the clocks on Earth slow down by a measurable amount. This can be detected by measuring the arrival times of pulses of pulsars. When astronomers started to use pulsars as accurate clocks, they forgot to correct for the fact that the distance of pur cocks from the Sun have monthly periodic oscillation, due to the Earth's center of mass orbiting the center of mass of the Earth-Moon System. At Full Moon we are a bit closer to the Sun than at New Moon, thefore due to gravitational time dilation caused by the Sun's gravitational field, our clocks run a bit slower during full Moon, therefore the pulses of pulsars will speed up relative to our clocks. All other effects where corrected for except this effect, so the data showed a mysterious monthly period in the delays of pulses all the observed pulsars. Count Iblis (talk) 13:28, 4 June 2013 (UTC)

I assume "all other effects" includes things like the variable sun-earth and earth-moon distances caused by the elliptical nature of the orbits in question, which will also cause variations in gravitational time dilation. --Jayron32 13:30, 4 June 2013 (UTC)

Yes, all the effects, the most important one is variable Doppler effect due the Earth's motion around the Sun (the Earth-Pulsar distance will have a periodic oscillation on top of the steady change), the next important effect is due the electromagnetic waves not traveling in a straight line but it is deflected in the gravitaional field on the Sun and the planets, then you have the time dilation effect due the the varying speed of the Earth relative to the pulsar, but this included in the relativistic Doppler effect. The gravitational time delay effects due to the elliptic orbit of the Earth, also that clocks located at different places on Earth will have relative delays w.r.t. each other. Everything has to be corrected for and if you miss one effect, you can see that. Count Iblis (talk) 13:42, 4 June 2013 (UTC)

I have no doubt the effect Count Iblis describes does actually exist. I am skeptical that any clock - experimental or otherwise, hand-held clock or physics-laboratory full of ultra-precise time-keeping equipment - has ever measured this specific time-dilation effect on perceived pulsar rates, because its magnitude must be very small. If you know of some published work, I would be happy to read about it!

For what it is worth, the LIGO - two very large and specialized facilities - are the most precise astrophysical time-keeping observatories that I am aware of. They are looking for general-relativistic time dilations due to propagation of gravitational disturbance - "gravitational waves" - and to date there have been no remarkable discoveries or observations of such dilation. I would suspect that if Count Iblis' description of gravtiational-potential time dilation is a measurable phenomenon, then LIGO should either measure it or compensate to remove it, because the magnitude of the signal they seek is even smaller. Nimur (talk) 16:00, 4 June 2013 (UTC)

The gravitational time dilation factor is (approximately) 1 − M/r, where M is the Sun's mass in this case. The variation in that due to oscillation of Earth's center relative to the Earth-Moon barycenter is ±Mr/R² where r is the distance to the barycenter and R is the average distance to the Sun. That's about ±3×10⁻¹³. The period of PSR B1913+16 is given to 13 decimal places in the article, with no explicit error margin. So they might have noticed this effect, just barely.

LIGO is an interferometer and won't notice this effect since it won't cause a fringe shift. If they ever detect gravitational waves they might, in principle, notice the frequency variation due to this effect, but only if the true frequency varies by less than one part in 10¹² over the course of a month and they can measure it to a similar precision. So probably not. Pulsars don't emit detectable gravitational radiation. -- BenRG 21:17, 4 June 2013 (UTC)

I'd be careful about assuming that LIGO will or won't notice something simply because it is an interferometer. Based on a talk I attended, LIGO notices continental drift, the solid Earth tides created by the moon, and for a while they noticed tumbleweeds hitting the outside of their facility (until more distant catch fences were erected). When you are designed to measure ridiculously tiny effects, the list of not-quite-so-tiny systematics can grow very large indeed. You are right though to the extent that LIGO wouldn't have mistaken any of those effects for gravitational waves since they each have very different physical and temporal signatures. Dragons flight (talk) 09:42, 6 June 2013 (UTC)

Second reflection in mirror

I noticed that my bathroom mirror gives a second, "ghostly" reflection slightly offset from the main one, looking like a faint, barely noticeable image of the object, obvious only for shiny reflective surfaces. How does it form? - Sikon (talk) 03:53, 4 June 2013 (UTC)

A typical mirror is a reflective surface covered with glass. (The purpose of the glass is to keep the reflective surface from being damaged, tarnished, etc., and to allow for easy cleaning.) However, the surface of glass is slightly reflective itself, especially when viewed from a shallow angle, as opposed to straight on. You've probably seen your reflection in a glass window before, so know this to be true. I suspect that this is the ghost reflection you see, with the offset being due to the thickness of the glass. StuRat (talk) 04:22, 4 June 2013 (UTC)

(edit conflict) From the front surface of the glass. Glass mirrors will reflect from the back "silvered" surface mostly, but there will always be a faint reflection from the front of the glass as well. A back-silvered mirror has the advantage of protecting the reflective surface from oxidation, but the disadvantage of the problems you note, of the double image. See This article describing the difference between "front" and "back" silvered mirrors, also called "first surface mirrors, and second surface mirrors. --Jayron32 04:28, 4 June 2013 (UTC)

The partial reflection of glass etc is an extremely facinating subject see the book (QED: The Strange Theory of Light and Matter) for a good introductory discusion! OneMadScientist (talk) 10:20, 4 June 2013 (UTC)

But is the reflected light polarized? Nimur (talk) 15:41, 4 June 2013 (UTC)

Ah... Feynman, what a champion. OneMadScientist (talk) 09:15, 5 June 2013 (UTC)

There are (in principle) even more reflections than that - a reflection from the back of the mirror can be partially reflected again by the front of the glass, then again off the back surface and finally out to your eyes. You don't notice them because the fraction of the energy of the light that is reflected at each turn becomes less and less - but if you have a really bright light source (like a laser pointer) and a dark room, and play around with the angle of the laser and the mirror, you can see more than two reflections. (Be careful not to stare directly into the laser when you do this though!) SteveBaker (talk) 15:56, 4 June 2013 (UTC)

You will certainly notice multipath reflections off undesired, non-ideal optical surfaces if you ever try to build a high-quality camera! Many types of lens flare are caused by multipath reflections off the wrong surface of an optical element - a lens, a mirror, the film or sensor... the edges of the camera case... Nimur (talk) 16:25, 4 June 2013 (UTC)

Scientific instruments, cameras and optical instruments often use a First surface mirror to avoid the extra reflection. The downsides to using them is that they cost more and the reflective coating can get scratched. Edison (talk) 20:31, 5 June 2013 (UTC)

Malignant lump

so a friend of mine had a lump tested and it was a "malignant lump" could it be anything other than cancer?--Irrational number (talk) 10:55, 5 June 2013 (UTC)

Medical advice! Plasmic Physics (talk) 11:25, 5 June 2013 (UTC)

I'm not asking for advice, I'm not asking what should I do. I'm just asking what could it be. The doctors haven't said anything more yet. I'm just asking whether it could be anything else. In other words I'm asking what are the types of malignant lumps I don't know how that's asking for advice.--Irrational number (talk) 11:21, 5 June 2013 (UTC)

<deleted medical advice> SteveBaker (talk) 15:14, 5 June 2013 (UTC)

No, this is still medical advice. We're not allowed to diagnose, offer treatment advice or give a prognosis. You're asking for a diagnosis ("what could it be?") and we're explicitly not allowed to do that. SteveBaker (talk) 15:14, 5 June 2013 (UTC)

Apart from everything else, there is so little information there, heard thirdhand, that we couldn't do any better than the search bar at answering you. Wnt (talk) 16:39, 5 June 2013 (UTC)

• The OP's asking whether the term "malignant lump" can refer to anything besides cancer. Calling that medical advice is just plain hilarious. I don't have the answer, but you can look under malignancy and malignant tumor which seem to imply the term is pretty much restricted to cancers. μηδείς (talk) 16:40, 5 June 2013 (UTC)

Let's call a spade a spade. A malignant tumor is another name for cancer. This is not a medical advice or diagnose. We are not saying anybody has cancer (no diagnose) and we are not telling anybody that does have cancer what to do with their lives (no advice). This is a matter of semantics. What is the meaning of the term malignant tumor. Answering that question is within bonds. The medical-advice-police needs to get a grip. Dauto (talk) 17:05, 5 June 2013 (UTC)

The problem is not the strict application of the rules--they don't apply here. The problem is the inability to make the logical distinction between us offering a diagnosis and us commenting whether a term has any other meaning. μηδείς (talk) 18:03, 5 June 2013 (UTC)

Fat chance of that happening. Right now they're pushing at Wikipedia:Reference_desk/Refdesk_reform_RFC for a more strict policy against anything "remotely related". Heck, I just tried to add links to two indexes of clinical trials for Tourette Syndrome only to get policies brandished at me. [1] What we need to understand is that, the right to treat, to talk about, to know about disease is the sole property of the cartel, and so we are all utterly unethical and evil for daring to think about such things. Wnt (talk) 17:54, 5 June 2013 (UTC)

Can we offer veterinary advice? HiLo48 (talk) 05:33, 6 June 2013 (UTC)

Are you thinking of "You can't ride a dead horse"? 93.132.177.75 (talk) 23:25, 6 June 2013 (UTC)

Ears, temperature, and balance

Why is it that when there is a pronounced temperature difference between your middle ears, that you experience nausea and imbalance? Plasmic Physics (talk) 11:15, 5 June 2013 (UTC)

The fluid in your inner ear ("endolymph") flows around the Semicircular canal and stimulates small hairs there that convey your sense of motion and balance. Temperature variations alter the volume, viscosity and electrical conductivity of that fluid which can result in balance issues - and nausea from motion sickness. It's easy to imagine how getting different balance information from one ear versus the other might worsen this effect. Sadly I don't have reference for the fluid viscosity/conductivity thing - it's what my doctor told me the last time I had this problem. SteveBaker (talk) 15:11, 5 June 2013 (UTC)

Thanks. Plasmic Physics (talk) 23:50, 5 June 2013 (UTC)

Our article on the caloric reflex test explains the basic mechanism. Looie496 (talk) 05:28, 6 June 2013 (UTC)

Very interesting. Auditory irrigation with warm water is part of my personal hygiene routine, but in a recent instance, circumstances required me to use cold water on one side, immediately following the use of hot water on the other side. Plasmic Physics (talk) 09:21, 6 June 2013 (UTC)

Dot markings on Torx screws

I've just bought a new Seagate hard disk drive. The top plate of this is secured by six Torx T8 screw fasteners (the normal kind, not the tamper-resistant ones). I've noticed that four of the screws are marked with four dots, and two with only a single dot. I'm confident that these are simply markings - at less that 1mm in diameter, they're too small and too shallow to have a mechanical purpose. Presumably they denote different characteristics of the two screws. I've been unable to find any information about these markings online. One can speculate that they denote different screw lengths (that's my guess), screw diameters or pitches, or the required torque (but I'd expect that to be marked on the receptacle instead). Here is a photo of the two. Is this a standard marking, where is it documented, and what to the dots denote? I don't intend to dismantle the drive to find out. -- Finlay McWalterჷTalk 12:02, 5 June 2013 (UTC)

Add 'different coating' (galvanization, phosphate coating, zinc) to your speculation. OsmanRF34 (talk) 14:33, 5 June 2013 (UTC)

I've never seen this before either - but yet another possibility is that it's some kind of quality control thing. If they have (let's say) four different machines in some factory, all turning out Torx fasteners, then they get a batch of bad bolts coming back from customers, a pattern of one, two, three or for dots would allow them to identify which machine needs attention.

I did find some comments on (of all places!) a Jeep Wrangler forum that their (very large) Torx bolts have lines marked on them. One person commented: "3 lines is a grade 5 bolt, 5 is a grade 8. no line are cheap junk" - so it's possible that this is some kind of standard quality indicator, and that smaller bolts use dots instead of lines for reasons of space. SteveBaker (talk) 15:00, 5 June 2013 (UTC)

[| 3 lines, grade 5, or 5 lines, grade 8 source] OsmanRF34 (talk) 15:22, 5 June 2013 (UTC)

I should say that the apparent difference in colour between the two is entirely an artefact of my photography (I don't really have the equipment for a decent macro photo); in actuality the finish between the two types appears indistinguishable. -- Finlay McWalterჷTalk 15:24, 5 June 2013 (UTC)

I found a fastener identification guide from a reliable source here [2]. That pdf won't answer your question, because it doesn't cover torx. The point is, they are describing different marking systems for each manufacturer! There are 'tons' of different markings, with very little standardization. The only dots I see are from the Decker manufacturing corp., but I don't think they make torx fasteners... So, I think you'll have to track down who made the fasteners to get a definitive answer. SemanticMantis (talk) 16:53, 5 June 2013 (UTC)

Interesting to note that the first photograph on the Torx page has a torx screw with three dots equidistant around the head of the screw, and it is on a hard disk drive too. 220 of ^Borg 10:44, 6 June 2013 (UTC)

Sahara

Prior to the desertification of the Sahara what was the largest desert? Don't go too far back in time and disregard frozen lands like Antarctica or Greenland. --KAVEBEAR (talk) 07:07, 4 June 2013 (UTC)

You're bumping into a definitional problem here. Our article Desert uses the definition "Deserts generally receive less than 250 millimetres (10 in) of rain (precipitation) each year." That's a kind of standard in international climate science. Naturally it includes cold places. If you have something different you'd like identified, maybe hot places with sandhills, that's a different question. HiLo48 (talk) 07:22, 4 June 2013 (UTC)

That seems to be the question he's asking, then. -- Jack of Oz ^[Talk] 08:41, 4 June 2013 (UTC)

Shouldn't it be "less than 250 millimeters precipitation (rain or snow)?" Or is it meant to exclude snowing? OsmanRF34 (talk) 12:34, 5 June 2013 (UTC)

Is she asking us to don't 1) Disregard frozen lands like Antarctica or Greenland or 2) don't disregard frozen lands like Antarctica or Greenland. Bit ambiguous if you read it. ☯ Bonkers The Clown \(^_^)/ Nonsensical Babble ☯ 08:50, 4 June 2013 (UTC)

Ah, I see what you mean. Yes, it is ambiguous. Maybe this table from our Desert article would be a good starting point...

The ten largest deserts

Rank	Desert	Area (km²)	Area (mi²)
1	Antarctic Desert (Antarctica)	14,200,000	5,500,000
2	Arctic Desert (Arctic)	13,900,000	5,400,000
3	Sahara Desert (Africa)	9,100,000	3,500,000
4	Arabian Desert (Middle East)	2,600,000	1,000,000
5	Gobi Desert (Asia)	1,300,000	500,000
6	Patagonian Desert (South America)	670,000	260,000
7	Great Victoria Desert (Australia)	647,000	250,000
8	Kalahari Desert (Africa)	570,000	220,000
9	Great Basin Desert (North America)	490,000	190,000
10	Thar Desert (India, Pakistan)	450,000	175,000

HiLo48 (talk) 08:54, 4 June 2013 (UTC)

The Sahara as a desert has only existed since 1,600 BC, our article claims (with a "citation needed", if anyone comes across a good reference). The article also says the area cycles between wet and dry periods. So there will be not one but many periods in the past when the Sahara is not a desert, as well as many periods when it was.

A climate map for around the Sahara's last wet period would show vegetation somewhere around 5000-8000 BC. So far the closest I've found is this map [3] from the last glacial maximum, which shows the largest non-polar deserts in Argentina and Central Asia-Tibet-China. 184.147.118.213 (talk) 11:23, 4 June 2013 (UTC)

That map seems to show north Africa and the Arabian peninsula, central Australia, and other areas, covered by "Tropical extreme desert" Astronaut (talk) 13:30, 4 June 2013 (UTC)

Shouldn't this question be in the Science desk? My $0.02.... Kingsfold (Quack quack!) 20:02, 4 June 2013 (UTC)

"Sahara Forest? Don't you mean the Sahara Desert?" "Yes, now!"[4] ←Baseball Bugs ^{What's up, Doc?} carrots→ 22:09, 4 June 2013 (UTC)

The Sahara pump theory article has some information related to that question. Dauto (talk) 14:34, 5 June 2013 (UTC)

It would be nice to get more information on deserts in recent geological eras. I remember reading Florida was a desert some absurdly short time ago, which seems so at odds with the rich ecology of the Everglades - how far did that desert extend, though? Wnt (talk) 16:36, 5 June 2013 (UTC)

• If you look at a map of the deserts of the old world they strongly correlate with areas inhabited by wild goats and our various articles such as on the Gobi mention desertification caused by goats. See also various articles on early (goat, sheep and cattle) domestication and desertification. μηδείς (talk) 00:34, 6 June 2013 (UTC)