Wikipedia:Reference desk/Archives/Science/2019 April 10

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April 10

Dark matter in subatomic particles

Can dark matter wander around inside protons and neutrons? Rich (talk) 02:39, 10 April 2019 (UTC)

No one really knows what dark matter is made of. But one of the leading candidates is weakly interacting massive particles, or WIMPs. If that's the right answer, then ... well I guess I still don't know, but protons and neutrons are made up of quarks interacting via the strong and electromagnetic forces. Since WIMPs interact only via the weak and gravitational forces, it seems at least plausible that they could pass through a proton or neutron without interacting, at least some of the time. --Trovatore (talk) 03:36, 10 April 2019 (UTC)

And some subatomic particles already are dark. Neutrons and neutrinos do not interact with visible light, and so appear transparent and are thus already "dark matter". Since neutrons decay, they are not likely part of galactic dark matter. Graeme Bartlett (talk) 06:04, 10 April 2019 (UTC)

Well, neutrons have no net charge, but they are not entirely electromagnetically inert. See neutron magnetic moment. As I understand it, neutrinos and WIMPs (if the latter exist) have no electromagnetic interaction whatsoever.

I believe neutrinos have been ruled out as a major contributor to dark matter (though you're right, technically they are dark matter, but the observed orbits of stars in galaxies suggest that there's much more than that). I don't remember the exact argument, but I think the words "virial theorem" appear in it somewhere. --Trovatore (talk) 08:16, 10 April 2019 (UTC)

The reason neutrinos are not a good candidate for dark matter is they are too light, at least the three active flavours. They travel at relativistic speeds so they don't form the dense regions in the early Universe that we see. However, if there are heavier sterile neutrinos, neutrinos may still explain dark matter. Dja1979 (talk) 15:08, 11 April 2019 (UTC)

• It should also be noted that much of what gets lumped into "dark matter" is better termed a "gravitational anomaly" rather than a form of matter. The best guess is that this anomaly is caused by some sort of matter, since as far as our theory currently tells us, you need matter to give you gravity, BUT, there also exists the quite real probability that our theory is incorrect, and that there is some other non-matter-based reason why our calculations cannot correctly predict the dynamics of large objects (like galaxies) in our universe. The thing with proposals like WIMPs and MACHOs and even the concept of "dark matter" is that there is no concrete evidence for any of it. It's all still speculation at this point. All we can say for certain is that our current calculations for how galaxies should move around do not match how that actually move around, and one proposed solution to that problem is "dark matter". When we ask "what could it be?" we need to be honest and say "it might not even be matter." The best guess we have, and the one that many (but not all) astrophysicists are putting their money behind is actual dark matter, but there are consistent theories that modify existing ideas about gravity without invoking dark matter. --Jayron32 12:41, 10 April 2019 (UTC)
  • I don't know; those alternatives all sound like reaching to me. The curves of orbital speed are pretty damn convincing. Sure, it could be explained by some unknown behavior of gravity, just as planetary motions could have been explained as epicycles. But I don't think it's the way to bet. --Trovatore (talk) 17:32, 10 April 2019 (UTC)
    • Yeah, but just to argue the other position, it is not unreasonable to assume that our equations of gravity need to be modified at different scales. Understanding that theories that to within all reasonable measurements work fine one one scale turn out to be wildly wrong on other scales is actually fairly common in these sorts of extreme size scale differences. After all, both general relativity and quantum mechanics were entirely un-obvious when working on human-scaled distances and times, and no one would have predicted their necessity until we started to notice the fact that existing Newtonian mechanics broke down at scales. It is not unreasonable to think that perhaps even general relativity breaks down at the galaxy-sized scale... After all, it gives the wrong answers. The last time science tried to invent an invisible substance to fix a broken theory we got luminiferous ether. History has shown that "our theory is wrong" is perhaps just as likely as "lets invent some undetectable stuff to explain these problems with the theory". --Jayron32 18:25, 10 April 2019 (UTC)
      • But it's a matter of explaining the precise form of the curves. Now, I haven't really looked into this in detail on my own, and it's not my field, so I could be off base, but it's my understanding that it all fits pretty well with WIMPs postulated as part of one of the fairly plausible supersymmetric extensions to the Standard Model. Sure, GR could need correction, but there's no similar reason to think that the correction needed would match the same way, and no second theoretical driver like supersymmetry theory. --Trovatore (talk) 19:31, 10 April 2019 (UTC)
        • There is a huge observational problem with modified gravity, and that is the existence of globular clusters that buck the rotation curve trend. WIMP models can explain these as the results of galactic collisions in which a remnant cluster is predominantly comprised of matter from a specific window of distances from the galactic center. The dark:light matter ratio varies with distance from the galactic center, so certain types of collisions will yield remnants with abnormal average ratios and therefore abnormal rotation curves. While a modified theory of gravity might explain the vast majority of rotation curves, you will still have these problematic ones. Either the theory is inconsistent, or you have to invoke unseen matter anyway. The unseen matter doesn't necessarily have to be WIMPs, of course, but this means that the modified gravity models don't actually eliminate dark matter, but simply confine it to a smaller region of space. Someguy1221 (talk) 20:31, 10 April 2019 (UTC)

MACHO dark matter will soon either be confirmed or ruled out. The only window left open for this type of dark matter is in the mass range for supermassive black holes. It is predicted by certain models of inflation see here, as pointed out here, the first observations of gravitational waves are consistent with this idea. With more data about black hole mergers coming in, we'll soon know if supermassive black holes account for the dark matter or not. Count Iblis (talk) 01:50, 11 April 2019 (UTC)

Laptop batteries Wh and mAh

According to Lenovo, their internal 45 Wh battery lasts 9 h. Adding an external 14000 mAh (14 Ah) would add 6 h of work (that is, 2/3 of the internal one). Since W = V*A, wouldn't that imply that this laptop runs at like 2.1 V? This kind of sounds too low for me.

My train of thought is that 14000 mAh = 14 Ah. This is equivalent to a 30 Wh battery. That is, 30W h = 14 Ah * V V = 2.1

I know that battery running times is more akin to an art (marketing is an art), but wouldn't Lenovo, if inclined to do so, lie both in the running times of the internal and external battery by the same factor?

Anyway, I'm also puzzled by the fact that they don't state batteries sizes using the same units. Why not standardized unit across all their batteries (or across the whole industry)? Doroletho (talk) 16:22, 10 April 2019 (UTC)

Well, there are lies, damn lies, and battery ratings...

Cells are rated in mAh, but as you rightly note, batteries are far better judged in Wh. To make any further comment, I'd want to see a link to what Lenovo say, in more detail. As a wild guess, it's a 6 cell li-po pack (about 21V, typical for laptop batteries), and there's a decimal point slipped somewhere.

OTOH, having measured lots of 18650 li-pos (and why I no longer use them!) the "lying factor" for Chinese cells was typically at least 10× Andy Dingley (talk) 16:32, 10 April 2019 (UTC)

• Laptop: [1] "with up to 9 hours of battery life" and "Up to 9.37 hours*, 45Wh integrated" although they have the decency to add that "Battery life varies significantly with settings, usage, and other factors."
• Power bank: [2] USB-C Laptop Power Bank provides 48 Wh battery life (14000 mAh). I have to find a source for the 6h. But how come that 14000 mAh is 48 Wh? Doroletho (talk) 17:59, 10 April 2019 (UTC)

• Now I think the source for the 6h is not Lenovo, but some internet shop. Anyway, do the numbers make sense?Doroletho (talk) 18:02, 10 April 2019 (UTC)

• A watt is a volt-amp (see electric power) so 14000 mAh is 14 (W/V)h. That would imply a nominal 3.42 volts to produce 48 Wh. What is the voltage rating of the battery? --Jayron32 18:19, 10 April 2019 (UTC)

• It's a USB-C power bank, so, it should be 5V.Doroletho (talk) 18:44, 10 April 2019 (UTC)

Battery ratings are as trustworthy as gas mileage ratings on a car. If you installed the battery in a spherical cow, maybe you could get reproducible results. In reality, there is such a diversity of the way that batteries are stored and used there is a wide variability in the usage rate of batteries. Hypothetically, installing the same batteries in the same device which was used under the same conditions should produce reproducible data, but once you start messing with any of that, all bets are off... --Jayron32 16:38, 10 April 2019 (UTC)

You say Lenovo's internal 45 Wh battery lasts 9 h. This means that the average consumption throughout that 9 hours till discharge is 5 watts. At times the consumption may be much more than 5W, but at other times it might drop very low. A laptop battery is typically 18-19V. Let's say 20. This means that the average current drain is 1/4 Amp or .25 Amp, since Volts x Amps = Watts and 20 x .25 = 5. The 14 Ah battery, if connected alone, should result in a run time to complete discharge of 14/.25 = 56 hours. When this battery is connected in parallel with the internal one, a run time of 60 hours seems correct (since the internal battery can contribute only 4 instead of 9 since it discharges faster and actually becomes a drain on the big battery). So, I suspect the 6 hours of extra work may be a typo for 60. Akld guy (talk) 23:20, 10 April 2019 (UTC)

When you see "14,000 mAH external battery", those batteries are always rated as if they contained a single 3.7 volt lithium cell, so it would be about 50WH, roughly the same total capacity as the internal 45WH battery, but there are inefficencies introduced through voltage conversions and stuff like that. Laptop batteries are multi cell and are typically 11 volts: 3 cells in series, possibly with multiple strings in parallel. They vary though. The charger input for older big laptops is usually 19 volts so there is some step-down inside the laptop. Lately there is USB Power Delivery with different voltages (not just the traditional USB 5 volts) but that is still pretty new. 173.228.123.166 (talk) 00:07, 11 April 2019 (UTC)

Why do filling stations sell by volume (and not by mass)?

Filling stations in many countries prominently display the price of their products per liter (or other local volume unit). Well, I would rather they sell by the joule (i.e. by amount of standard enthalpy of reaction), but I get why it is not the case. However, why do they not sell by mass instead of volume?

The reason I ask is that mass is a better proxy for enthalpy. In Europe, both diesel fuel and gasoline are used in general-purpose cars; both have a very similar lower heating value which means that 1kg of either fuel contains about as much energy. However, diesel is ~10% more dense than gasoline; thus determining which is cheaper per joule is complicated when you have prices per liter. (Yes, I am aware that you should not fill a gasoline car with diesel or vice versa.)

Since the historical fuel is coal, which was surely sold by mass, I guess there is a practical reason in how the sale is measured, but I cannot see it. Are flow meters really cheaper/more reliable that weighing machines or something? Tigraan^{Click here to contact me} 16:59, 10 April 2019 (UTC)

Citation: Digital coal scales 75kg. Alansplodge (talk) 14:28, 12 April 2019 (UTC)

For one reason, fuel tanks are sized by volume, and fuel efficiency is measured in distance/volume, etc. —2606:A000:1126:28D:F935:C7E2:FE1:E49 (talk) 17:06, 10 April 2019 (UTC)

At least in the US, the indicated amount of fuel must be adjusted for the expansion and contraction due to the temperature of the fuel. So the indicated volume is actually an analog for mass. For further information see NIST Handbook 44 Jc3s5h (talk) 17:13, 10 April 2019 (UTC)

• Yes, flow meters are much easier to certify as accurate long-term than mass measuring was. Also the standards for this became established in the 1920s, when this was far more the case, and the case to change always has to be stronger than one simply to choose, ab initio. Andy Dingley (talk) 17:29, 10 April 2019 (UTC)
• Gasoline should have a fairly consistent density, which means that it doesn't matter whether volume or mass is used for measurement, it's a simple conversion between the two. Generally, liquids are sold by volume, and solids by mass. --Jayron32 14:18, 12 April 2019 (UTC)

• Depends on your definition of "fairly". Typical gasolines can vary in density by around 10% between the coldest and hottest environments in which they are likely to be sold. Considering the quantity that's sold, that could amount to a fairly substantial over- or undercharging. Someguy1221 (talk) 19:08, 12 April 2019 (UTC)

While we're mincing over details, fuel efficiency can vary with respect to temperature, and in a modern fuel-injected vehicle with modern tires and other modern equipment and trim, the specific fuel consumption, as a percentage, might change more than the volumetric expansion of gasoline, given the same temperature change. Here's Why is the fuel economy of an automobile worse in the winter than in the summer?, by Harold Schock, professor of mechanical engineering and the director of the Automotive Research Experiment Station at Michigan State University. When it's winter weather and very cold out, your fuel is more dense; so if you pay by the dollar, you're get more kilograms of fuel for the same price-per-gallon; but if your fuel efficiency falls by 50% (as the article claims), you're spending twice as much fuel per mile, so you're paying for twice as much fuel to do the same trip as under summer conditions. The thermal volumetric expansion of the fuel, after all things are considered, becomes pretty irrelevant.

Among the items that affect fuel efficiency, and vary with temperature, are fuel density, outside air density; tire performance, traction, and wheel slip; lubricant performance; safe driving speed and regime-of-operation for the engine; aerodynamic drag changes; ... add on to this that SAE tells us our internal combustion engines are still less than 50% efficient, in terms of net carnot efficiency, ... and again, your fuel's thermal expansion is low on the list of your real concerns.

Nimur (talk) 20:32, 12 April 2019 (UTC)

• It's not "overcharging" to charge the same amount for a lesser quantity of the same product, if that's a fair measure according to the agreed terms of the local weights and measures legislation. If they say, "In this country, we sell petrol by the litre" then it's recognised that you get "a better deal" when it's cold.

As an engineer, I've measured fuel consumption by weight (when testing engine performance), as that's (in terms of moles) "what really matters", no matter the weather. It's really awkward! You can measure volume flow with a direct and continuous measuring technique, but for weight measurement it involves a pair of vessels with weighing attachments and valves to switch between filling one and emptying the other. Really not practicable for a store-front petrol pump.

I've also worked on energy billing systems for utilities (electricity and gas). For retail sales, it's most important that these are simple. So homeowners get a meter with a number on the front of it, and the cost billed is based on some simple, stable multiple of that, plus a calendar charge. Go into commercial billing though and it goes crazy. So many adjustment factors! Adjustment tweaks for gas quality, for gas density (temperature). It's so complicated that it can't actually be worked out reliably (the billing errors are enormous – if you're a big energy buyer, talk to my day job and save a couple of million a year). So yes, petrol is sold by the litre. Because it might not be accurate, but it's precise, it's honest and it's robust against dishonest manipulation of the pump's meter. Andy Dingley (talk) 20:53, 12 April 2019 (UTC)

Even my residential natural-gas bill first reports the meter-reading in "units" (almsot surely this is actually CCF: 100 cubic feet). Then the bill scales that value by a therm factor (currently 1.121). The gas cost and distribution charges are based on therms. DMacks (talk) 14:30, 13 April 2019 (UTC)

But the therm rating for gas is, as I put it, 'long term stable'. The idea is that it's an approximate value across the monthly or quarterly billing period, so that the final bill is that simple multiple of the number on the meter. It's largely based on the composition of the gas, which may itself be a blend to achieve a particular them rating (i.e. the gas is adjusted to give the number, rather than the number being determined by measuring the gas on the day).

What domestic metering mostly ignores are the rapid fluctuations, such as either the spot price of gas changing on the markets, or the demand for gas changing during brief cold weather and the cost of providing a reliable supply going up. Andy Dingley (talk) 16:41, 13 April 2019 (UTC)