Wikipedia:Reference desk/Archives/Science/2015 November 13

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November 13

Why is the speed of light called the speed of light?

Because other things also travel at the speed of light like gravity waves. This indicates some kind of constant that applies to multiple things, not just light. ScienceApe (talk) 00:54, 13 November 2015 (UTC)

Notice that gravity waves are not gravitational waves.

And I would not take it as a fact that gravitational waves exist. However, even if several things have the same speed, the name is just a way of referring to it. In the same way that a plane can travel at the speed of sound, and there is no problem with the name "speed of sound." --Scicurious (talk) 00:57, 13 November 2015 (UTC)

The simple answer is the right one: we talk about the speed of light because we can see it. People have understood since ancient times that the speed of light was very fast if not infinite (Speed_of_light#Early_history), much faster than other things such as sound or material bodies. Amazingly enough, a pretty good measurement of the speed of light was made in 1675 (Rømer's determination of the speed of light) -- and you can be sure this was done with light rather than gravitational waves! Anything else that may travel at or near the speed of light is very difficult to detect and has only been discovered in modern times. For example, we still have very limited experimental tests of gravitational waves, and although they probably travel at the speed of light, we don't have a satisfactory measurement of this. --Amble (talk) 02:01, 13 November 2015 (UTC)

You're right that c is not really about light. It comes from the Lorentz transformation, which underpins relativity. Here's a good video from PBS Space Time on this topic. On top of that, saying "speed of light" when what you really mean is c is technically incorrect, because light only travels at that speed in a vacuum. Light travels more slowly when passing through media like air and water (this is why refraction exists). This imprecision confuses people who don't have a good background in physics, because they hear people saying "nothing can exceed the speed of light" and then later hear about how things like Cherenkov radiation are caused by things traveling faster than light. I prefer to just use c so there's no confusion over what is being discussed. As for why people use that term, it's because of the historical reasons outlined above. Maybe someday everyone will take my advice and use c instead! --71.119.131.184 (talk) 05:43, 13 November 2015 (UTC)

c has its own problems because it's used for wave propagation speeds in general (see wave equation and the Usenet physics FAQ). -- BenRG (talk) 08:37, 14 November 2015 (UTC)

If you want to be pedantic about it, c is a constant derived from the differential forms of Maxwell's equations. --Jasper Deng (talk) 06:07, 13 November 2015 (UTC)

You could say "speed of electromagnetic radiation", but that doesn't roll off the tongue so easily. ←Baseball Bugs ^{What's up, Doc?} carrots→ 01:42, 14 November 2015 (UTC)

• In two sentences, distilling the above: Historically, Roemer was the first to measure the speed of light in the 17th century, long before the nature of electromagnetism and the various invisible spectra were known. When Maxwell proved in the 19th century that light was electromagnetic radiation, all types of which propagated at the same speed, the existing term "the speed of light" was retained.

(Compare this to the fact that we still call chemical atoms atoms, even though we can split them.)

μηδείς (talk) 00:16, 15 November 2015 (UTC)

Common genetic material

What is the common genetic material between following pairs: two siblings (not twins), two humans (same gender), two humans (opposite gender), a human and a monkey?--Scicurious (talk) 00:55, 13 November 2015 (UTC)

Is this a quiz for us, or a quiz for you? And which monkey, and where did the humans buy their DNA? :)

We don't do your homework for you, though we’ll help you past the stuck point.

μηδείς (talk) 06:03, 13 November 2015 (UTC)

They will all have some DNA in common. The more closely related, the more will be the same. In the case of siblings, they may have several chromosomes in common (except for mutations, etc.). Chromosomes are collections of DNA. The siblings would also have the same mitochondrial DNA, inherited from their mother. The only difference genetically between males and females is that males have an X and Y sex chromosome, while females have 2 X chromosomes. However, many of the genes on the Y are also on the X, so it's not as much difference as you would think. StuRat (talk) 06:36, 13 November 2015 (UTC)

Vive la différence! --220 of ^Borg 16:13, 16 November 2015 (UTC)

Coefficient of relatedness, Human_genetic_clustering and Genetic_distance can help you answer some of these. SemanticMantis (talk) 14:45, 13 November 2015 (UTC)

Photography Camera History-1945

Hello. I am trying to answer the following questions for a short documentary I am producing about a 70 year old photographer who recently bicycled and photographed his way from Seattle to New York City: 1) What was the most common camera (professional or consumer) in 1945? 2) Approximately how many (or what percentage of) American households owned a camera in 1945?

In the United States, the most popular camera was almost certainly a Kodak Brownie of one type or another. The most popular 35mm format camera would probably have been a Rangefinder. You can read our history of SLR cameras to see some other models that would have been "top of the line" in that era. The reflex cameras did not really get popular until post-World War II, with models like the Nikon F.

European camera manufacturers of that era were also very popular, although I don't see them in camera antique stores in the United States very much: Zeiss, Kamera-Werkstätten, and many other familiar brands mass-produced cameras in Europe, but I doubt they had enough market penetration in America to ever be "the most common."

Nimur (talk) 02:34, 13 November 2015 (UTC)

Not the most common but a serious photographer of the time may have used a Rolleiflex or similar. Rmhermen (talk) 04:19, 13 November 2015 (UTC)

Don't forget that the camera trade with Germany had been interrupted by the war. Even when the USA was still neutral, the British Blockade of Germany was very effective. Alansplodge (talk) 16:26, 13 November 2015 (UTC)

Many professionals at the time would have used much larger sheet, rather than roll, film. Either a monorail camera (for studio work), or a press camera or field camera (for outdoors) -- for example, Kodak Speed Graphics were common hand-held sheet film cameras through the 50s. These were all quite large cameras: probably in the range of 5 to 10 pounds. Both professionals and amateurs used smaller 6x6cm film TLRs, similar to the Rolleiflex. There were also an assortment of more compact folding cameras for 6x6cm and similar roll film formats that were popular in the 30s-50s. 35mm would still largely have been seen largely as an amateur's format. 50.1.86.192 (talk) 06:18, 15 November 2015 (UTC)

In effect, question 1 is really two different questions. Professionals such as press photographers used the big boxy cameras you're describing, while amateurs used the Brownie types, whose resolution was nowhere close to what a press photographer would use. Here's a random example:[1] ←Baseball Bugs ^{What's up, Doc?} carrots→ 10:17, 15 November 2015 (UTC)

Why are plastic toys/tools/devices cheaper than metal toys/tools/devices?

The cost of ABS plastic is a lot more expensive than all but precious metals, so why are they cheaper? ScienceApe (talk) 05:46, 13 November 2015 (UTC)

ABS plastic--link for convenience. μηδείς (talk) 06:00, 13 November 2015 (UTC)

Plastic parts are typically moulded to their finished size, whereas metal parts (a) need more expensive tooling (often), and (b) need to be finish machined in various ways. If you want a plastic part to equal the performance of a metal part then the cost often goes the other way, for example 'plastic' roadwheels cost about 40-100 times as much as metal ones, although they are lighter as well. The cost of raw materials is not a huge part of the endcost of most consumer items.Greglocock (talk) 06:03, 13 November 2015 (UTC)

Adam's apple and voice deepening

I've always thought that the Adam's apple helps to deepen a person's voice, and the article confirms this, but then when reading about Chondrolaryngoplasty I came across this: "While the effects of a trachea shave on voice are minimal..." Why doesn't removing the Adam's apple change a person's voice? 731Butai (talk) 07:43, 13 November 2015 (UTC)

The procedure reduces the size of the outer surface of the cartilage, but does not directly impact the length of the vocal cords since the interior anchor points are unchanged. BiologicalMe (talk) 13:31, 13 November 2015 (UTC)

Decelerating universe

I'm curious to know how we confirm the universe is accelerating its expansion. Since Hubble's law is just measuring the galaxies recession with respect to us, then I expect one should pay attention that measurements don't reflect real time but some point in the past due to the delay time light waves take to reach us. This delay time is also proportional to the distance between us and the galaxy in question. If we assume for example the universe is shrinking or just decelerating homogeneously such that the factor of deceleration is constant regardless the distance from us, then I believe we may reach equations that explain why we think the universe is still accelerating while it is indeed decelerating. I hope I can understand this point.--Almuhammedi (talk) 12:51, 13 November 2015 (UTC)

The standard calculation is done in a way that does take into account the time light takes to reach us. Physicists can make mistakes, but they are not going to miss anything so obvious. Looie496 (talk) 14:04, 13 November 2015 (UTC)

It appears that the question is implying that the rate of acceleration is decreasing. As such, it will eventually reach zero acceleration and then begin deceleration. The question assumes that is fact and then goes on to imply that since we are looking at acceleration of the distant past, the universe must be past that and in deceleration right now. If I am reading this correctly, the initial assumption is wrong. There is no evidence that the rate of acceleration is decreasing. If the questioner can provide evidence of that assumption, we can examine it. 209.149.114.132 (talk) 14:33, 13 November 2015 (UTC)

See Ultimate fate of the universe for some discussions of the various theories. The Future of an expanding universe discusses the OP's initial question in some detail. --Jayron32 14:48, 13 November 2015 (UTC)

is there any article discussing redshift calculations that take time into consideration?--Almuhammedi (talk) 19:18, 13 November 2015 (UTC)

By definition, redshift already takes time into consideration implicitly. Any cosmologist working in the past 100 years who didn't would be akin to navigator assuming the earth is flat. I'm not sure where you would get the assumption that time dilation was not something routinely taken into account. Heck, something as mundane as GPS requires calculations involving gravitational time dilation to work correctly. Everyone uses these principles always. --Jayron32 19:23, 13 November 2015 (UTC)

Ok, thanks for explaining.--Almuhammedi (talk) 19:57, 13 November 2015 (UTC)

Also, although common, it's actually kind of inaccurate to say that we're seeing distant things "as they were X number of years ago". The theory of relativity says that no two points in spacetime can exchange information faster than c, so when we look at the stars we really are seeing them as they are in our "now" (relativity of simultaneity). One of the core predictions of relativity (and one of the hardest for us humans to accept) is that there is no universal standard of time; time is part of spacetime, and different observers have different clocks. --71.119.131.184 (talk) 00:02, 14 November 2015 (UTC)

It's not unreasonable to describe the past light cone as a kind of "now", especially because the spacetime interval to all those points is 0. But it's not the "now" of relativity of simultaneity. Relativity of simultaneity is about how any plane of simultaneity is as good as any other (and they're all physically meaningless). The past light cone is a cone, not a plane, and it's unique and physically meaningful. -- BenRG (talk) 00:57, 14 November 2015 (UTC)

It's reasonable to suppose that the expansion "could be" reversing, but if so, there's apparently no evidence for it, so from the scientific viewpoint it's nothing more than an idea. ←Baseball Bugs ^{What's up, Doc?} carrots→ 01:41, 14 November 2015 (UTC)

Facial recognition

Is there any way to get an objective, non-political verdict on whether the person shown in [2] etc. is actually al-Baghdadi or not? There are reports that this is debunked, but some people like Rand Paul don't believe this (and I can't blame them, when whoever it is supposed to be is kept secret "to protect him")

I understand there is computer facial recognition -- is there any public way to get a near top quality result from that? And is computer facial recognition as good as human facial recognition, or would a human jury work better? Are there people recognized, objectively, as good at recognizing faces who rent out their talents for this kind of thing? (I have no intuition to fall back on here ... it seems like some people do things, like identify kids from the back of milk cartons, or even identify teenage concentration camp guards when they are in their 70s, which seem like nothing short of a paranormal ability to me) Wnt (talk) 20:57, 13 November 2015 (UTC)

Here [3] is an open source package that includes fairly sophisticated facial recognition routines. So if you have access to a confirmed photo of a person, you have at least one free public method that can try to tell you if a test photo is of that same person.

Humans are really good at facial recognition (millions of years of evolution, etc., our human-centric article as at Face_perception). We're so good at it that we often see faces where there aren't any - Pareidolia affects humans, but also computer routines! But even though some algorithms are better than some people at certain specific ID tasks, most people are better than most algorithms when a person's face has a few odd bits. See e.g. here [4] [5] for popular coverage, and here [6] [7] for scholarly works about how people can make their faces far harder for automated systems to detect and recognize. The important bit is that these countermeasures don't work on humans! You can still recognize your good friend if he grows his bangs out and puts on some blotchy makeup, but most current automated systems cannot. My understanding is not that the algorithms are currently significantly better than humans at general facial recognition, but that they can tirelessly scan zillions images and videos. SemanticMantis (talk) 21:38, 13 November 2015 (UTC)

Heart pounding

Why is it that pounding, fast heart beats as a result of fear or intense exercise doesn't cause cardiac arrests in people who are healthy? You would think that if the heart beat that forcefully and fast, it could damage it or not pump blood properly. 2A02:C7D:B8FF:7E00:1471:5287:B808:35C8 (talk) 23:29, 13 November 2015 (UTC)

Have a look at Heart_rate#Maximum_heart_rate and cardiac arrest for starters. SemanticMantis (talk) 23:38, 13 November 2015 (UTC)

To elaborate: it can. Why are you assuming it doesn't? The heart rate just has to be pretty extreme before you get problems, at least in a healthy person. The heart is very adaptable, and heart rate, as well as other aspects of the cardiovascular system, are regulated by various feedback mechanisms. Since hearts are pretty important for animals that have them, there's a strong selective pressure for them to be resilient. --71.119.131.184 (talk) 01:37, 14 November 2015 (UTC)

Because you very rarely hear of people having cardiac arrest due to extreme heart rates when the person has no existing conditions. 176.24.24.18 (talk) 10:54, 14 November 2015 (UTC)

What looks like a wedge in the sphenoid bone?

Sphenoid bone is one of the skull bones. The name sphenoid is greek and means "wedge". I would like to know two things about that: 1) What is the meaning of the "wedge" in this context? (I'm not English native speaker and that's why I'm asking it. According to the dictionaries there are a lot of meanings for this word) 2. What looks like a "wedge" in this bone? I saw this bone and it doesn't match (I think) to any definition of this word. 23:49, 13 November 2015 (UTC) — Preceding unsigned comment added by 78.111.186.49 (talk)

The caption for the Gray's Anatomy illustration in Sphenoid bone calls it "wedge-shaped". Wiktionary's wedge entry has an illustration which may help in your understanding. ←Baseball Bugs ^{What's up, Doc?} carrots→ 01:37, 14 November 2015 (UTC)