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Interesting paper from back in July (how did I miss this?), co-authored by Giorgio Immirzi, one of the foremost experts on GR and quantum gravity: https://arxiv.org/abs/2207.04279 This is just the latest paper within an increasingly large body of work that indicates that ‘dark matter’ as a separate phenomenon may be entirely superfluous. The basic idea here is that, under certain specific circumstances, even in the weak-field and low velocity regime, there may be non-negligible GR effects that aren’t found in Newtonian gravity. Hence, sometimes Newtonian gravity is not a valid approximation to GR in the weak field domain - which is the very assumption from which the idea of dark matter arises in the first place. What’s more, it turns out that under certain circumstances not even weak-field approximations of GR (such as GEM for example) are valid - specifically, this appears to be the case for rotating systems. In other words, within this paradigm, dark matter would neither be some new exotic form of particulate matter, nor is it a modification of the laws of gravity; it’s quite simply an artefact of us not having applied standard GR correctly, because the assumption “weak field”=“Newtonian” does not always hold. Small selection of other papers along the same lines: https://arxiv.org/abs/2207.09736 https://link.springer.com/article/10.1140/epjc/s10052-021-08967-3 https://www.worldscientific.com/doi/abs/10.1142/S0218271808012140 https://arxiv.org/abs/astro-ph/0610370 https://academic.oup.com/mnras/article-abstract/496/2/2107/5850386?redirectedFrom=fulltext https://arxiv.org/abs/1509.09288 https://arxiv.org/abs/2112.04116 https://arxiv.org/abs/2109.13515 https://arxiv.org/abs/2102.11282

I came across this by chance the other day, and felt that this is a particularly good explanation of the meaning of “metric” in the context of GR, so I thought I’d share it here for whoever is interested: Some nice visualisations in there. Also, the other GR-related videos on this particular channel are quite good. No particular point for discussion, I’m simply sharing this for all those who wish to learn more about this subject without having to dive too deeply into the abstract maths. Particular emphasis on comments from time stamp 19:40 onwards (on coordinate vs proper measurements), as that’s were all those many common misconceptions arise. EDIT: Apologies, I’m just after realising that there’s an open thread for posting YouTube science content: Moderators, feel free to merge my comment there, if necessary.

I would be comfortable enough starting from that seed for the purposes of a discussion. I would even go as far as to say that there are hints of an anthropological basis for the emergence of a notion of the self --or intensification of it-- from changes in patterns of human behaviour, but that this self is not a necessary part of the animal condition. Hunter-gatherers seem to have been more at peace with their mortality, even though they practiced burials, and seem to have been aware of this ending of existence. And tribal conflicts emerged only when they had to fight for scarce resourses. I'm aware that I'm identifying burial culture and awareness of death with awareness of self, but I think it's a reliable-enough yardstick for it. When you settle, on the other hand, you tend to identify yourself with things, people, tools, etc, around you. You create this concept of home. This is my landscape, the landscape of my forefathers. This is my game, and my staples, and so on. It's by virtue of the recurrence of your experiences, repetition, that you try to make sense of these "correlations in your experience" --for lack of a better term-- onto a self, which is nothing but a placeholder, that holds all of that experience together. Language, of course, has a powerful role to play in all of this, giving names to things, and people, and generally facilitating all of these initially loose notions to stick. It would be very interesting to know if/how people who are constantly on the run, barely trying to survive one more day, with faces and landscapes being forgotten in a matter of weeks, would be able to develop a notion of "I" in a similar way than we do. My feeling is that they wouldn't. They would be far too busy with the "something is happening" aspect of things. I don't know about Brentano and Husserl a great deal, TBH.

Well baby steps are always best ,especially in the icy conditions we have had the past week.

This, if I recall my college philosophy courses well enough, was one of the major objections to Descartes' cogito. Descartes posited that some "mischievous imp" could deceive us about any aspect of the physical world (hello, The Matrix) and we could only be certain of our own existence in that given moment as a self that thinks. His later critics pointed out that even the self, the "I," could be illusory and therefore there was only warrant to say that consciousness happens. The only epistemically confident statement would then be "there is thinking, therefore something exists." Pretty hard to argue with that. 😀 Starting from that seed, the discipline of phenomenology proceeds, trying to understand experiences as they present to consciousness and how consciousness is directed at apparent objects. Fellows like Brentano and Husserl got the ball rolling with the idea that consciousness is always about something, a quality that is known in philosophy as intentionality.

Parents, neighbors, teachers, peers, ... will.

He was not joking. You don't know how other people, animals or plants feel and interpret what they see, hear and receive stimuli through their organs. You just think that others see the same thing as you, which may be true, or may be just an illusion.

In the US: Drinking age: 21 Age of consent: Per the federal government it is 18. Unless the older person is 21, then the age of consent is seventeen. But if the older person is 20 then the age of consent is 16. This progression goes down to an age of consent of 12 if the older person is 16. Unless you are in California where two people who are 17 do not have the right to consent. This changes by state. In France a 15 year old can consent to sex with an 80 year old. In New Zealand you'd have to be 16. Marriage age: 18. Unless you are in Nebraska then it is 19. Or Mississippi where it is 21. Unless you have parental consent, then it is younger. Voting age: Varies by country from 16 to 21. Retirement age (with benefits): Between the ages of 62 and 70. Running for political office: In the states it varies from 18 to 30. For president you must be 35. As you can see, the age at which you can do any of those things depends on where you live. There is no standard age of minority or majority. And even in locations where there is a standard age of minority and majority, the right to drink, fornicate, marry, vote and run for office do not all follow those standard ages. Even though one is considered an adult wrt fornicating, you may still have to be older to drink, marry, vote and run for office. In the US you are generally considered an adult at 18, but you still can't buy a beer, run for certain offices, or maybe even marry without your parents approving. That and the fact that anyone impacted by it probably doesn't even know it is happening, and can't do anything about it if they do know.

I am not sure whether I understood the instructions, but I am licking coffee from the whiteboard now.

There is an instantaneous acceleration treatment by applying the four acceleration equations. A specific equation that describes this is \[\alpha=\gamma^3 a\frac{1}{(1-u^2/c^2}\frac{du}{dT}\] where \[\alpha\] is the proper acceleration for objects with mass The large T is specifying coordinate time to be more obvious. u here is the instantaneous velocity. you can further simply that equation by applying motion on the Minkoskii hyperbolic curve the above equation leads to which simplifies to \[g^4/c^2\] \[x'^2-ct^2=x^2\]. the equation above works for both forms of acceleration via change in velocity or direction. This equation has been used in Born rigidity examination as well. An interesting consequence of relativity is the observer effects. Place an observer at a static location your classic rest frame. The train has length so he's going to observe different parts of the train at different angles. Even if we only consider the observer along the x axis on top of the train he will observe a different length front to rear. The approximate point of simultaneity of signals received from the front and rear would be the center of the train. The only way to preserve that simultaneity from any two equidistance points either in the x+ or x- direction the length contraction must occur in a symmetric fashion from that observer point of view. In a linear acceleration case that isn't too hard if you allow some mechanism that the entirety of the bus gains speed. however once the train starts to turn your going to lose simultaneity from that same location. At least I don't know of any solution where you won't. treating simultaneity in terms of signals received by an Observer

I think changing the mindset is crucial no matter what we decide to do. So much of the trash generated is convenience-oriented packaging that has to go from manufacturer to distributor to supplier to consumer, then sit on our shelves for a short amount of time. Nobody but the consumer is interested in making products last in the package once you've gotten it home and opened it (see any Ziplock bags inside a cereal box?). I reuse all good plastic and glass containers, over and over if I can. I have a whole collection downstairs that I use to store other stuff, mix a small amount of paint or oil in, fill with water to use as a weight, or any other job where you need a jug or can with a cap or lid. And I keep hoping the laws will change and I can start taking my old containers to fill them up with bulk rice or flour or sugar. Didn't I see a headline a while back about a bacteria that eats plastic and poops out something better for the environment? I'd seriously be willing to get rid of my back yard and turn it into a mulch for little plastic piranha bugs. I'm so sick of wasting resources on a lawn in a high altitude desert. I agree, there's no single fix. It has to be a compound effort. I'm sure the US campaign from the 60s was accompanied by stiff fines for littering, but the message got to me. My family was big on camping when I was growing up, and we always packed out our trash. Keep America Beautiful resonated with me, and as an adult, if there's no trash can I stuff my refuse in a pocket and throw it away later. I know if I toss something on the ground, I'm creating both an eyesore and extra work for someone. Taking care of my own litter taught me not to take my problems and make them other people's problems.

There could easily be a huge market for it. Hydrogen would be close to a drop-in replacement for methane in domestic heating (you can put up to 25% into the supply today without even changing burners). Also I believe hydrogen is thought to be a good candidate for heavy duty truck transport, for which batteries would be very large and heavy. We already have hydrogen buses in some places. It needs something to kick-start it though - probably government. I don't think, myself, that trying to totally decarbonise shipping is a top priority. Switch to lower carbon liquid fuels in the medium term while you go for other sectors with more impact on total emissions.

All this talk of hydrogen production relies on there being a huge surplus of renewable power some time in the future. If they use it to make hydrogen, they are likely to use that at night, or when the wind doesn't blow, for electrical generation. I don't think that it's likely there will be a big surplus, over and above that. There would need to be a sure-fire market for hydrogen, at a high price, before people would invest in renewable hardware to produce it. You could create that market, by banning marine diesel, but I can't see that happening.

This should help. Exactly. That is what Erina quoted from Tesla So charging efficiency. I have attached two diagrams firstly the charging diagram of an ordinary common or garden 12 volt car battery. As can be seen the charger or alternator produces a (laughingly) constant voltage and the current is initially high for a discharged battery but gradually drops off as the battery voltage rises so the difference between the charging voltage and the battery voltage falls until the charger is just receiving a 'trickle charge'. This trickle charge can safely go on indefinitely. Processes within the battery dissipate the very small amount of energy safely as heat. Also since the charger voltage is only just higher than the battery voltage it cannot add further charge to the battery. Also shown is the dashed constant current line that sophisticated battery 'fast' chargers operate. It is not safe to try to pump the high initial charging current into the battery so when the finish point is reached this type of charger switches itself of and perhaps disconnects the battery. Lithium ion batteries can also be charged in this manner but constant voltage charging is slow especially when the amount of charge to be added is considered. So they use constant current charging and sophisticated controls which stop the process at the point A on the curve. Now an EV battery is much higher voltage than the old type - the Kia is over 300 volts - so the safety implications of overcharging are much more serious. Also there will be statistical variation in the absolute capacity from battery to battery. So the manufacturer rates the battery below this point and I have shown the statistical safety margin above this on the curve. But note the curve also turns over compared to a straight line 100% efficient charging, just as the 12 volt battery did. I have also drawn line BC to show this 100% efficient charging line. Now the further up the curve you go the further below 100% is the charging efficiency. So where the manufacturer places their battery rating on the curve determines the charging efficiency. The Kia is a cheaper car than the Tesla so I expect that they can less afford to offer a generous rating and are pushing their batteries harder. This would imply that their charging efficiency suffers comapred to the Tesla.

Like @studiot, I'm having some trouble following what you are saying. You can't "negotiate" with electrochemistry or physics. Do you mean "optimising", by reducing losses or something? Also there is nothing fundamental about any of the formulae in this thread so far. All we have covered is the arithmetic to work out a % efficiency, given certain measured inputs and outputs. I think you need to be a bit careful what is meant by efficiency in the context of charging and discharging a battery. I see that @studiot has used it to mean the efficiency with which a given electricity supply, as input to a charging system, results in actual charge entering the battery. So that's the efficiency of the charging process. However the figure of 80-90% I was quoting is the efficiency with which the battery itself stores charge, i.e. the amount of charge you get back out for the amount you put in. This is the charge/discharge efficiency. What is the formula you are referring to?

I'm not sure what all these numbers tell us. The energy content is the same regardless of the fuel used. There is a certain efficiency limitation for the production of hydrogen, depending on how it is produced. Those numbers together tell us how much energy would be needed to fuel all these ships. The amount of hydrogen that one part of one company currently says it can produce is not really very relevant. So much depends on how much money the energy industry feels it can make out of supplying it and a lot of that will depend on what other (higher intrinsic value) applications there may also be for the fuel. In the case of hydrogen there are potential applications in domestic heating and as truck fuel. But in any case, I'm not sure that hydrogen is the right fuel to focus on, at least in the medium term. Hydrogen is currently not the most favoured future option for marine bunker fuel. Here is an article about the options from Bureau Veritas: https://marine-offshore.bureauveritas.com/insight/future-marine-fuels-pathways-decarbonization Decarbonisation will be a process, taking decades and most likely involving a number of intermediate steps. You will see that less carbon-rich hydrocarbons, and/or liquid biofuels, are likely to come into use first. Longer term, hydrogen is one possibility certainly, though not on any scale by 2030, while ammonia may well be preferred to hydrogen, due to the easier storage on board (you can liquefy it). Though to make the ammonia you presumably need the hydrogen anyway, so eventually a lot of it will no doubt be needed, one way or another. (You can make "blue" hydrogen from natural gas, if you capture and store the CO2 generated as a byproduct.) If you are really interested in this subject, there is information available on the internet from organisations such as the IMO and CIMAC (a forum for marine engine designers and builders that I used to attend, when I was still working for Shell).

I'm sorry I don't understand this comment. Heat is not necessarily a bad thing. The capacity of an electric vehicle battery decreases markedly with lowering of temperature, (or raising it above optimum which is in the 20oC to 23oC range) The car manufacturer will specify the capacity at this temperature. At 5oC the capacity will be 25% - 30 % lower and it can be as little as 50% lower in freezing conditions. Also the capacity depends upon the equipment used to charge the battery. The Kia Nero has a 64kW-hr rated battery. But it needs a 3 phase supply to achieve this capacity. Ordinary UK mains can only reach 80% of this value ie 51 kW-hrs, I recently charged a Kia from completely flat to this level - It took 28 hours charging at 10 amps on the UK 240 volt mains. This equates to 67 kW-hrs. So the charging efficiency was 51/67 x 100 = 76% Other types of lithium-ion (there are several types) batteries have different characteristics.

My understanding is that a modern Li ion battery has a charge/discharge efficiency of 80-90%. 75%, let alone 67%, seems low to me.

For all those mega container-ships? Yes, it might be better than diesel, eventually, but all it does in the longer term is reduce the particulate and CO2 emissions from shipping all that unnecessary crap around the world in all those giant containers. The unnecessary crap is still going to be manufactured - according to whatever the regulations, or lack thereof, happen to be in the country of origin, by whatever labour practices obtain there, packaged in miles and miles and miles and miles of plastic wrap, and whatever other packaging happens to be deemed marketable by the manufacturers, and taken from places of cheap labour and dirty water to places that used to be more prosperous when its citizens had jobs in manufacturing that no longer exists, but are not buying the unnecaesary crap on credit, which is driving their entire entire nations into a pit of debt, in ships that disrupt migration routes of sea-life and birds and drived whales mad enough to commit suicide. A nose-frontwards approach might be to make necessary goods and food products close to where they'll be used.