Wikipedia:Reference desk/Archives/Science/2018 January 18

Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is an archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

January 18

Coherent or highly monochromatic terahertz

Brain fart corrected in title, sorry

I was looking at a paper about imaging cancer with terahertz that is monochromatic, but I'm not sure how monochromatic:

Measurements were performed using a THz time-domain spectroscopy (THz-TDS) system based on a Ti:sapphire femtosecond oscillator (Synergy; Spectra-Physics) pumped by a 10-W diode laser, Verdi. The laser delivered 10-fs pulses at a wavelength of 800 nm with an 80-MHz repetition rate. The laser beam was separated by a polarizing beam-splitter and turned towards the emitter and the detector separately. The emitter was a p-InAs crystal that utilized the photo-Dember effect, and the laser beam was incident at 78° on the surface of the crystal for high THz intensity51. The THz beam that radiated from the emitter was gathered by a parabolic mirror and focused onto the sample holder by a pair of THz focusing lenses

My assumption is that this generates noncoherent light with a narrow range of frequencies. But I don't see how narrow.

The issue here is that they detect methylated DNA in cancer cells as a little bump, maybe a bit more than a tenth of the total signal present. Most of the signal is from a smooth curve of absorption by ice. The thing I'm getting at is that if they had a tighter range of frequencies, they might get a higher signal if they tune precisely right. I also don't know whether the absorption by ice might break up at a very fine scale, with bands where there is no absorption, if you had truly monochromatic terahertz waves to test with???

Obviously what I want to do is find something only the cancer absorbs and turn it up to MICROWAVE HIGH setting until every last tumor cooks through and through, while normal tissue with just a little methylated DNA gets toasty warm but not damaged. Wnt (talk) 01:24, 18 January 2018 (UTC)

Looking at graphs in this doi=10.1088/0022-3727/47/37/374002 paper, you only get about one full cycle of the THz radiation, and it drops off exponentially with time on the scale of 1ps. So it is actually hardly monochromatic, and would have a bandwidth of 100s of GHz. This would depend on the duration of the laser pulse. If you could pulse your laser every 2 ps perhaps you could get it as coherent as the pulse rate, but then you would have a near THz generator driving your laser anyway! On the topic of selecting DNA methylation, it is found in all kinds of cells, so perhaps it is not targetable. A vibration of a methyl group in a liquid environment may have a band, but it would not break down into fine lines due to the varying environment of the liquid. Graeme Bartlett (talk) 05:26, 18 January 2018 (UTC)

So is there any known way to make more monochromatic THz radiation? I would think this should be the hottest topic in weapons work right now (to DNA-identify people from space or displace specific binding factors and cause a different syndrome in every member of the political opposition), but all I want to kill this time is cancer. Wnt (talk) 14:55, 18 January 2018 (UTC)

You should read Terahertz_time-domain_spectroscopy. Ruslik_Zero 20:32, 18 January 2018 (UTC)

Canon de 75 modèle 1897: The Italian Response

I need help finding an artillery gun based on this information.

After taking notice of the success of the Famous French 75, Italy manufactured their own, indigenous design of the artillery piece. Although it had the same rate of fire as the Canon de 75 modèle 1897, it was more mechanically complex and was not as successful. Arima (talk) 04:42, 18 January 2018 (UTC)

Cannone da 75/27 modello 11 seems a possible candidate. The main italian field gun in WWI, the Cannone da 75/27 modello 06, was a licence-built Krupp design. Try List of artillery by country#Italy. Alansplodge (talk) 11:07, 18 January 2018 (UTC)

Cold desert at night

So if rock holds heat so well, why does the desert get freezing cold at night? Rock does hold heat well, right? Anna Frodesiak (talk) 09:12, 18 January 2018 (UTC)

Not as well as bodies of water. Also deserts have low precipitation, so often clear skies, thus the optimum for radiative losses.

It's a delusion amongst the flat earth people that "Moonlight radiates cold". Indeed, moonlit rocks are often colder than those which aren't. However the reason is that days are hotter than nights, and so night temperatures depend on how fast things are cooling, not how well they're being heated. An area exposed to clear moonlight is the same area that's losing the most by radiation. Andy Dingley (talk) 09:29, 18 January 2018 (UTC)

Thank you, Andy. So this is about very clear air above allowing heat to escape, and about lack of moisture within the ground, which holds more heat than dry sand? (I've slept on both sun-baked earth and desert, and wow, that sand and air gets cold.) Anna Frodesiak (talk) 11:22, 18 January 2018 (UTC)

Yes. We have an article: Radiative cooling. "The exact process by which the earth loses heat is rather more complex than often portrayed..."

Here's a great free textbook on weather: Aviation Weather. I actually like the old version - I keep a link to it on my user-page - the AC 00-6A version - it still has the original cartoons from 1943's Meteorology for Pilots!

If you like math, you can use the Stefan-Boltzmann law to study why deserts get cold. Here's some science from Arizona State University - a world-reknowned center for excellence in both deserts and planetary science: Stefan-Boltzmann law for radiative cooling. In my old planetary science class, we were taught by a radioscientist, so he made sure we corrected for the non-zero cold sink of the cosmic microwave background radiation: on a clear night with no clouds, your desert ground is essentially losing heat to a cold-sink that's only a few kelvins! This is why it is possible to use a clear desert night to produce ice, even if the air temperature is above freezing! If you're really smart, you can make the physicists mad by pretending to violate the second law of thermodynamics, like this Nature letter demonstrates! Hey, I recognize that photograph - I've spent a lot of time on that roof looking skyward!

Nimur (talk) 17:18, 18 January 2018 (UTC)

The theoretical Volumetric heat capacity of Silica (Sand, Sandstone, Glass) is 1.5, of Water 4.18 and of Air 0.0013 J⋅cm³⋅K⁻¹ (Joule per cubic centimeter per temperature difference of 1 Kelvin). So Sand, which always contains some air is not a good material to store heat. Like a battery that can store 1 Kw/h compared to one that stores 4 Kw/h. The volumetric heat capacity of Sand is actually only around 0.8 tho there are many different "mixtures" of minerals called Sand and thus it can be a bit higher or lower. Additionally by definition deserts contain the least amounts of water. Thus a desert is physically less able to store the heat from sunlight and it gets hotter with less "energy efford" aswell as cold faster when the heat disperses. --Kharon (talk) 13:59, 18 January 2018 (UTC)

It has not been mentioned yet, but the ability to store heat (measured by the heat capacity) is not the same as the ability the regulate the environmental temperature. The specifics depend on a lot of complex phenomena (see convection), but one key variable is thermal conductivity.

For a quick example, in cold weather, it is less unpleasant to touch ice than a metal bar, even though the specific heat of ice is more than twice as high as that of aluminium (per kg, so that is even more per volume). (Solid-to-solid contact does not involve convection, and the driving variable is thermal effusivity.) Tigraan^{Click here to contact me} 10:34, 19 January 2018 (UTC)

Thank you, especially Nimur for the really good pdf. Weather has always mystified me and this is really interesting. And no, anything about maths goes way above my head. :) Anna Frodesiak (talk) 22:10, 18 January 2018 (UTC)

One reason for the "Moonlight radiates cold" adage is that moonlight indicates a lack of clouds, and clouds act like a blanket to hold in some of the radiant heat. —2606:A000:4C0C:E200:3C54:1374:B00A:7382 (talk) 05:40, 19 January 2018 (UTC)

If the daily temperature range's x tens of degrees in Atacama-grade air will it be at least 1°F less if there was more water vapor? (but not so much that condensation wets the ground at dawn or causes cloud) Sagittarian Milky Way (talk) 07:15, 19 January 2018 (UTC)

"Rock holds heat well" hides some complexity. What it's really about is that rock is a good insulator. This means it doesn't exchange heat quickly with its surroundings. This means hot rocks will stay hot for a long time (and, of course, the inverse). That's heat that isn't leaving the rock for its surroundings. Meanwhile, once the Sun goes down, atmospheric heat gets carried away via convection, which occurs rapidly. You probably don't live in an underground burrow, so you lose heat to the atmosphere. --47.157.122.192 (talk) 09:37, 19 January 2018 (UTC)

I can't find any good references at the moment, but one thing to consider is that "rock holds heat well" really only applies on a per-unit-mass basis. Rock is opaque, solid, and a poor conductor, so only the very surface gets heated by the sun, which in turn means it can loose that heat relatively rapidly once heating stops. In contrast, water is transparent, so solar heating affects a larger volume. Depending on the weather, it can also be physically mixed, transporting surface heat to a greater depth. This means that water bodies have a much larger effective heat capacity than rock, and so take longer to both heat up and cool down. Iapetus (talk) 10:55, 19 January 2018 (UTC)

Carbon fibre

How much weight can a carbon fibre sheet hold, in:

1) 1 inch thick sheet.

2) 1 feet long stand, filled round stick or non-filled round stick.

Also,

I require a calculating method or so, to calculate weight vs. height, length, filled/non-filled carbon fibre base.

123.108.244.156 (talk) 15:45, 18 January 2018 (UTC)

There are enough varieties in construction of such materials that you would need to be much more specific if you want quantitative results.

Typically, material properties can be found from whomever sells such materials. For example, here is one vendor listing, technical data sheets from PlastiComp. You can use standard-form equations from our article on flexural modulus to model the material. Beware that fibers and textiles are not always well-modeled by simple equations! In such a case, you'll need to track down data from an empirical test.

Nimur (talk) 16:41, 18 January 2018 (UTC)

Isn't pure carbon fibre better than mixed with plastic? What is used in formula 1 Cars? What material would you say is solid and light? 123.108.244.156 (talk) 16:55, 18 January 2018 (UTC)

Have you read Carbon fibres and Carbon fibre reinforced polymer? Also do you know what fibre means? Nil Einne (talk) 17:08, 18 January 2018 (UTC)

MPG - casues of significant improvement?

I have recently noticed a significant improvement in the mpg I get from my Fiat Punto Mk2 1.2l 8 valve. In the past I have usually got about 40-45 mpg, but the last two fills I have got 50 mpg. The types of journeys and loads have not changed. What could cause such an improvement? DuncanHill (talk) 19:40, 18 January 2018 (UTC)

Setting aside that fascination of such minutia is possibly a sign of a mental disorder ; -) It may be that DH lives in the northern hemisphere where it is currently winter. Cold air is denser. Thus the compression ratio is slightly raised thus increasing engine efficiency. 10-20 % seems a lot however. Is this metered fuel from the pump or what appears on the gauge or a mixture of both? As you may be aware, gasoline/petrol and diesel have large coefficient of heat expansion. This is why aircraft are fuelled by weight and not volume. Therefore, volumetric gauges can easily be out by 10-15% over the course of a year. RH can affect MPG as well. Oh, and of course, have you recently inflated the tyres/tires ? That can reduce the rolling friction too. So that dispenses with the most obvious. Back to you to add more info. Aspro (talk) 20:25, 18 January 2018 (UTC)

Metered from the pump, Northern Hemisphere, no change to tyre pressures. Nothing mentally disordered about noticing that you're saving money! DuncanHill (talk) 20:54, 18 January 2018 (UTC)

Perhaps you are just driving more efficiently since you have become interested in mpg. Dmcq (talk) 23:36, 18 January 2018 (UTC)

• Another possibility, seasonal change of boutique fuel: [1] , [2] , [3] , etc., etc., etc... (no article?) —2606:A000:4C0C:E200:3C54:1374:B00A:7382 (talk) 00:23, 19 January 2018 (UTC)

I don't think we have "boutique fuels" in the UK. DuncanHill (talk) 00:26, 19 January 2018 (UTC)

I haven't done an exhaustive search, but its possible that your fuel provider changes blend seasonally; cf:[4] —2606:A000:4C0C:E200:3C54:1374:B00A:7382 (talk) 01:29, 19 January 2018 (UTC)

If there's a way to get 11-20% better fuel efficiency from a car (similar to my own), I'd really like to know about it! The only possible factor I can think of in this instance, Duncan, is that it's been somewhat windy in the UK of late – is it possible that by lucky chance your recent journeys have experienced more favourable wind assistance (and less wind resistance) than average? {The poster formerly known as 87.81.230.195} 90.200.41.3 (talk) 00:45, 19 January 2018 (UTC)

Or, he drives downhill -- both ways. —2606:A000:4C0C:E200:3C54:1374:B00A:7382 (talk) 01:14, 19 January 2018 (UTC)

Gasoline is denser at cold temperatures, so you get more kgs per gallon. Add to that that maybe you took an extra motorway trip or avoided some traffic jams (maybe roads were quieter because of holidays), and 10-20% is certainly possible, especeially if you have a 30-40 L tank. — Preceding unsigned comment added by 78.0.222.210 (talk) 02:23, 19 January 2018 (UTC)

@Aspro: Cold air is denser. Thus the compression ratio is slightly raised thus increasing engine efficiency. - huh? The compression ratio is a geometric factor, and while it may be affected by a few ppm by metal contraction in cold weather, it has little to do with air density. Tigraan^{Click here to contact me} 10:22, 19 January 2018 (UTC)

Gasoline is indeed denser in winter, but it does not explain everything. The CTE is about 0.1% per °C, so assuming a 30°C swing between summer and winter temperature, that explains 3%. Significant, but certainly not "10-15%" (that would require a 100-150°C swing, which no place on Earth has ever seen). Tigraan^{Click here to contact me} 10:28, 19 January 2018 (UTC)

Thanks for all the suggestions. I haven't noticed similar improvements in previous winters. Indeed, if memory serves, cold damp weather in the past has led to an increase in consumption which I have put down to it being a bugger to start and misfiring until it warms up (if it passes the next MOT I might buy it new coils and HT leads as a treat). Hmm. I think my last couple of long-distance trips were smoother than usual - there's a new bit of dual carriageway over the moors, that would help. And it's possible my driving habits have improved. It's just rather disconcerting to get such a major improvement on an old car - was worried it might be a sign of impending doom! DuncanHill (talk) 14:36, 19 January 2018 (UTC)

The way to mitigate miss firing is to treat yourself to a 'sports' coil and new HT now ! For Italian's are cheap skates and just deliver cars at the lowest possible price. Then spray the HT over with 'damp start' to seal out the moister. Cost-wise this will pay for the outlay. Another is to use a sump heater such as [5]. They use so little paraffin/kerosene per week that the running cost is negligible. Throw an old woollen blanket over the bonnet/hood, as the engine is a big block of metal and acts as a thermal store, so capitalize on it. Finally, don't forget the battery/accumulator. Short runs in cold weather may lead them to sulfate up – requiring a not cheap replacement. Make sure the terminals are clean and there is good electrical contact. Silicon grease is better than Vaseline in this respect and a hydrometer will indicate the condition of your battery so you can take proactive active measures before problems surface. Also, water thermostats. It should not open until the engine is up to running temperature. But if it is tired, it may not be closing properly to start with – so replace it. Some people when they start off on a cold winters morn, scrap off the ice on their widows whilst have their cars running on full choke and with the heater on full blast. Again, it is chilling the engine before it has chance to warm up and doesn’t 'pull' properly until several miles down the road. Aspro (talk) 19:41, 19 January 2018 (UTC)

Sealed battery (haven't seen an unsealed one for years). No choke (haven't seen one of those for years either). And I doubt the police would look kindly on someone leaving lit paraffin heaters under cars in the street (and where would I put it once I had driven off?), and I'm sure my insurers would have kittens! I do know not to run the heater until it's warmed up. A blanket over the bonnet would either blow away or get nicked. DuncanHill (talk) 17:35, 20 January 2018 (UTC)

I must admit seeing lorry drivers light heaters under their fuel tanks gives me a feeling of, ah um, trepidation. Dmcq (talk) 19:07, 22 January 2018 (UTC)

I hate to say it, but the most probable explanation is that one or two of your measurements were off. Wnt (talk) 12:51, 20 January 2018 (UTC)

Mileage taken from trip meter, fuel volume taken from the pump. And I can see that the gauge is going down less for a given journey now. DuncanHill (talk) 17:35, 20 January 2018 (UTC)

Could some intermittent mechanical problem you were used to have gone away? I know practically nothing about cars, but if this one has fuel injectors, could it have had a plugged fuel injector that spontaneously started passing fuel again? You mentioned something about misfiring. Wnt (talk) 17:50, 22 January 2018 (UTC)

Physics Question (one more)

Are the standard sizes and masses of a proton, neutron, electron etc. are naturally selected and why do they not change over time?

Similarly, nature has the ability of random selection too, therefore, we can't neglect completely the chances of varying masses of subatomic particles instead of standard as a proton is approx. 1850 times’ massive/heavier than an electron so it means the chances of the diversity in the masses of subatomic particles in the random selection are more than the chances of the standard masses of subatomic particles. Please disregard if not interested 50.66.1.32 (talk) 21:04, 18 January 2018 (UTC)EEK

Relevant articles include Proton-to-electron mass ratio, Time-variation of fundamental constants, and perhaps Fine-tuned Universe. -- ToE 22:12, 18 January 2018 (UTC)

Natural selection in the usual scientific sense requires the populations of entities being selected producing similar but not identical progeny with slightly varying characteristics due to occasional errors in the functioning of a genetic code. Since particular kinds of subatomic particles do not have a genetic code (or any analogue to it) and do not reproduce their own kind (though they can give rise to different kinds by decaying or combining) the concept is not applicable. The OP's second sentence ("Similarly [. . .] particles.") does not appear to be meaningful English in any of its clauses. {The poster formerly known as 87.81.230.195} 90.200.41.3 (talk) 00:41, 19 January 2018 (UTC)

The anthropic principle, while not selection, is a selection bias relevant to this question. -- ToE 02:36, 19 January 2018 (UTC)

Another possibility is that Big Bang(s) that reach 2018* tech are unlikely to end before Big Bang-causing tech makes Big Bang(s) that reach 2018* tech and are unlikely to end before Big Bang-causing tech makes Big Bang(s) that reach 2018* tech and are unlikely to end before Big Bang-causing tech makes Big Bang(s) that.. Thus if humanity doesn't want reality itself to be something a pimply alien made one science class or a quantum of hyperdrive exhaust it can help by not inventing universe-causing technology or non-human(s) that might invent it. *increment number by 1 each year. Sagittarian Milky Way (talk) 08:31, 19 January 2018 (UTC)

"Turn that off now Zxyygjqck it's time for bed"; "Aw, can I have one more billion years ma - it's just getting interesting" Gandalf61 (talk) 09:57, 19 January 2018 (UTC)

The anthropic principle seems like it may be baked in to the idea of "collapsing the state-vector", at least under some face-value versions of the Copenhagen interpretation. An electron does not pass through one slit or the other unless/until you observe it. Does this mean that the positions of all the stars in the cosmos were a blur until the First Man looked up and saw them? For that matter, were they a blur until the Last Man saw them, and the other Men before him were part of that collapse of probabilities? Is there something special about some past, present, or future consciousness of person that makes it better positioned to be the one to perceive a state that allows Him/Her/Itself to exist? What if the seventh seal is the distinction between the past and the future? Wnt (talk) 20:43, 19 January 2018 (UTC)

If you put Schrodinger's Cat box in an anechoic room, leave a microphone that can hear cat heartbeats at its distance from the box in the room before you leave and have it send the reading to you over wifi to a phone with a bushel over it do you think the screen is both cat organ noise and no cat organ noise till a human looks?Sagittarian Milky Way (talk) 22:50, 19 January 2018 (UTC)

Well, inside the box, there's no cat's meow without a live cat. Outside the box, it doesn't matter if you look by light or by a wi-fi transmission ... I think. Wnt (talk) 01:27, 20 January 2018 (UTC)

Lee Smolin's Cosmological natural selection (last version of our article before it was merged, perhaps more specifically informative) from his The Life of the Cosmos may be close to what the OP has in mind.John Z (talk) 01:02, 21 January 2018 (UTC)

• Why doesn't square green become anxious vanilla as the clock ticks below the Tropic of Cancer? See category error and The Fallacy of Fine Tuning. μηδείς (talk) 02:30, 21 January 2018 (UTC)

As others have pointed out, there is no evidence that the masses are changing. Somewhere in the last couple of months I saw someone say that the particles are defined by their mathematical values of properties. The proton is the particle that has this particular mass, etc. You don't have protons that have different masses - those would be different particles. Bubba73 ^{You talkin' to me?} 16:33, 25 January 2018 (UTC)