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April 2

Does a quantum vacuum slow down light a little?

Light travels at maximum speed in a perfect vacuum, but a quantum vacuum contains "zero-point energy" or "vacuum fluctuations". Does it slow down light compared to true vacuum, or why not? 193.253.244.40 (talk) 15:12, 2 April 2019 (UTC)[reply]

Exactly which speed are you interested in? There are many speeds that are relevant to the wave-nature of a dynamical system.

When physicists are discussing quantum-mechanically-correct dynamics of light, they need to be a little bit more precise. To quote our article on light, the best we can do is to construct "something that can be described sometimes with mathematics appropriate to... a macroscopic metaphor."

Nimur (talk) 18:48, 2 April 2019 (UTC)[reply]

I would guess phase velocity is probably the most appropriate one to the OP's question, which sounds like a generalization of the fact that light's phase velocity in matter is usually less than c (though it can also be greater than c, for example for extreme ultraviolet in some materials).

But I wonder whether the question makes sense at all. How exactly would you determine what light would do in a "pure" vacuum, given that QM really doesn't allow such a thing to exist? Maybe there's some way to make sense of the question, but it isn't obvious to me. --Trovatore (talk) 20:27, 2 April 2019 (UTC)[reply]

The best answer I can construct: if you define the propagation velocity in terms of electric and magnetic constants (vacuum permittivity and vacuum permeability), then the question is reduced to the equivalent, and more precisely worded question: are there cases where these values ε and μ are other-than-nominal values for ε₀ μ₀ ? The answer is, "yes, such cases exist, even in classical settings..."; and if we consider quantum-mechanical settings, we can precisely formalize the consequences and make predictions: for example, one hypothesis might propose that μ depends on electric field magnitude, even in the absence of any material. If that hypothesis is true, it would imply a measurable effect, and we could construct an experiment to test it.

Our article on vacuum state has a section on the consequences, cites a few research papers, and summarizes that such interesting effects would be very small and difficult to measure experimentally. If I understand the article correctly, it would probably easier to measure a different observable consequence (like birefringence or polarization or anisotropy) - something other than a direct measure of an altered speed-of-light, but that would be mathematically equivalent to the same causal effect. Nimur (talk) 23:21, 2 April 2019 (UTC)[reply]

See Scharnhorst effect. Count Iblis (talk) 09:37, 3 April 2019 (UTC)[reply]

@Trovatore: "How exactly would you determine what light would do in a "pure" vacuum, given that QM really doesn't allow such a thing to exist?" -- What about an analogy with the "effective charge" of an electron? (The electron in a "pure" vacuum is supposed to have a "naked charge" that quantum fluctuations turn into its "observed charge" computed through renormalization, right?) So, could light have a "naked speed" modified into an "observed speed"?
@Nimur: Well, vacuum state doesn't mention "velocity" or "speed" or even "light" anywhere, and its "Non-linear permittivity" section is exclusively technical. If it has potential consequences for lightspeed, maybe it should have some non-technical lines about it?
@Count Iblis: thanks, that's my Q exactly. On the side, I found a similar untested effect at https://phys.org/news/2013-03-ephemeral-vacuum-particles-speed-of-light-fluctuations.html -- I guess that's how far this topic is currently probed? 193.253.244.40 (talk) 12:47, 8 April 2019 (UTC)[reply]

With all due respect - how in the heck does one understand advanced physics without getting "technical"? We can do our best to simplify some of the more basic parts of physics, but you're asking about details that are excruciatingly complicated. Normally, a highly-qualified specialist spends about ten or more full-time years of their life studying this material: it's hardly something we can summarize briefly in an encyclopedia article without sacrificing a little bit of accuracy! Nimur (talk) 17:46, 8 April 2019 (UTC)[reply]

Railway building in Japan

Do the JRTT (https://www.jrtt.go.jp/11English/English-index.html) in Japan do all construction works in house or do they employ construction companies to do the physical work?82.132.221.234 (talk) 16:32, 2 April 2019 (UTC)[reply]

See Japan Railway Construction, Transport and Technology Agency and its predecessor, Japan Railway Construction Public Corporation. Alansplodge (talk) 19:38, 4 April 2019 (UTC)[reply]

What are the small swarming flying insects I sometimes encounter in the woods?

Northeast US. If they bite it must be painless. They follow you wherever you walk and if you get bugged enough and try to lose them they'll catch up to you if you stop running inside their sensor range. If you leave their detection range they don't come back even if they've had enough time to catch up. Are they attracted to CO2? Sagittarian Milky Way (talk) 21:28, 2 April 2019 (UTC)[reply]

Midges and gnats. "Midge" is not a proper taxon, but rather a group name for the various tiny swarming flies that behave as you describe. A few of them bite and are disease vectors. Gnats are more rigorously defined, but there are still a lot of different varieties. Matt Deres (talk) 22:19, 2 April 2019 (UTC)[reply]

I always assumed they were attracted to the water vapor people give off. They seem to go for my eyes, presumably to get a drink. SinisterLefty (talk) 13:21, 3 April 2019 (UTC)[reply]

According to this midges are attracted by "CO2, body heat and particular scents we make". As a regular visitor to the Scottish Highlands and victim of Culicoides impunctatus I would be delighted to meet these non- or painless biters. AndrewWTaylor (talk) 16:25, 3 April 2019 (UTC)[reply]

In the Northeastern U.S., especially swampy land in New England, black flies are common (sometimes locally called "mayflies" because they are very common in May, but are unrelated to actual mayflies). They were a little larger than gnats but smaller than mosquitoes. Also unlike gnats, which were more prevalent in later summer (June-July), black flies would feed on blood; unlike mosquitoes their bites were rarely itchy and they tended to prefer to feed in ears and noses (thinner skin maybe?) It was not uncommon for us as kids to have tiny little scabs around and in our ears when playing outside, from all of the black fly bites. There are also no-see-ums which are smaller than gnats, and lead to itchy rashes, unlike black flies which usually just leave tiny little scabs in your ears and nose. --Jayron 32 16:45, 3 April 2019 (UTC)[reply]

Not sure how "northeast" you are, but here is information on the two species of black fly endemic to New Hampshire; only one of the two (the one out in May I remember from my youth) actually bites. The later season black fly species does not; these may have been what I called "gnats" as a kid. Language is fuzzy, remember, and people will use whatever word makes most sense to them even if it isn't strictly in line with taxonomy. --Jayron 32 16:50, 3 April 2019 (UTC)[reply]

This very old source [1] suggests they release an anaesthetic anticoagulant which could partly explain the painless bit although I would be cautious about correct identification of the species for such an old source even more so since it's in Japan. Of course, as with most things like that, YMMV [2]. For that matter, how much you get bit [3].

As an interesting aside, the NZ sandflies which per the article Jayron32 linked are black flies Austrosimulium are known for their painful and itchy bites in humans [4] [5] [6] [7] [8]. No idea how penguins fare, but even if it's painless bringing one with you to take the bites is bad idea for several reasons .....

Nil Einne (talk) 09:35, 4 April 2019 (UTC)[reply]