OneOnOne1162

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No it is not how all neutrons are formed. Just as a single counter example that you will already be aware of - Positron Emission (as in PET scan)

 

This is when a proton decays into neutron, a positron and an electron neutrino:

 

[latex] p \rightarrow n + v_e + e^+ [/latex]

 

This is however a poor example in that it is very very similar to the initial example (the absence of an electron IS the existence of a positron) - but it is one that crops up in everyday life through the medical application.

 

I am not sure if we truly understand the part of baryogenesis in which the bulk of the universe's neutron and protons (and the antiparticles) are formed a fraction of a second after the big bang - we definitely do not understand where all the antiparticles went to. We do think that the reason we have so many neutrons (which should otherwise have rapidly decayed) is that they can form deuterons (a proton and a neutron - ie a deuterium nucleus) which are stable in the prevalent conditions at the time

Alright, thanks for the answers.

 

They combine by reverse beta decay aka electron capture

 

[latex]p + e^- \rightarrow n + v_e[/latex]

 

It is energetically favourable for electrons and protons to combine to neutrons only due to the massive pressure due to the intense gravitational force - a "gas" of neutrons is higher "density" (ie more are packed in to a given volume) than the plasma of protons and electrons.

 

This is because the electrons will resist being put too close together due to the exclusion principle - that two half spin fermion particles cannot share the same quantum space. Neutrons also resist due to the fermionic exclusion principle - but at the energy levels considered the wavelength of the neutron is much much shorter than that of the electron which very simplistically means that the space required for a neutron to be happy is much much smaller than the space for an electron to be happy.

 

At even higher gravitational pressures the neutrons get forced together too much and it is hypothesised that they might split into their component quarks which again can be packed even tighter. Again simplifying; This does not continue - at higher gravitational pressure the matter is so dense that the mass is packed within a radius that is lower than the schwartzchild radius - and from this point you have a black hole; we cannot know what is inside a black hole as there is a one way barrier at the event horizon

That (and swansont's answer) answers most of my questions. Though I was wondering if you know the answer to my last question too: Is this how all neutrons were originally formed?

In laboratory free neutrons are created by bombarding f.e. Deuterium by high energy alpha particle.

[math]D^+ + 2.22 MeV \rightarrow p^+ + n^0[/math]

(because Deuterium has the lowest energy needed to split it)

 

There are some other unstable isotopes that after bombarding them with high energy particles are releasing free neutrons.

F.e. Uranium-235 while fission release ~3 new free neutrons, after bombarding it by 1 free neutron.

 

Neutron is unstable particle, with half-life ~10 minutes, mean-life ~15 minutes. So soon after creation, they decay back to:

[math]n^0 \rightarrow p^+ + e^- + \bar{v}_e + 0.782 MeV[/math]

Well, but those are atoms, right? Doesn't that mean that they simply release a neutron that was already present in their atomic structure?

Let me preface this by saying I'm not a physicist, or physics student.

Now that we have that out of the way, I've been playing a game called Elite Dangerous and seeing a neutron star there reminded me that while I've looked up plenty on black holes, I'd never really looked into neutron stars very much. So I was looking up the composition of a neutron star, etc. and I read (on this website http://science.nationalgeographic.com/science/space/solar-system/neutron-stars/) thata neutron star's birth involves " A neutron star's almost incomprehensible density causes protons and electrons to combine into neutrons." Now, is this correct? How do protons and electrons combine into neutrons? Why don't they normally bind into neutrons while inside of an atom together? Just lack of pressure condensing them? And finally, is this how all neutrons ever are formed?

 

 

Hi!

 

Is that you in your pic? You're gorgeous.

 

Now......on topic: Funny you should ask, as I am currently working on a project for the summer school Anthro class I teach.

 

I have thus far found this website here to be the best, hands down, as far as answering your question.

 

This is a fascinating topic, btw, and dovetails with my personal discipline of study.

 

Thanks!

 

https://genographic.nationalgeographic.com/human-journey/

Alright, that's very useful. Thanks.

 

 

It's probably much shorter than even femtoseconds. Reactions with diprotons may have been observed in the lab, but not to the extent that a lifetime could be measured. And the reaction has to occur on the scale of the diproton lifetime. Possibly faster than 10^21 seconds.

 

The initial rate is going to be related to the time I mentioned. If it's a billion years for a fusion reaction, the rate is about 20 million reactions per second for 1 mole (i.e.1 gram). But that only works under the conditions of our sun, because it depends on N^2, and has two dependencies on temperature — the speed, which dictates how often collisions occur, and the fraction of collisions that will have enough energy to allow the reaction to occur.

I think that answers my question pretty fully now, thanks.

Though a side question: Is there any sort of graph which shows how the rate would change depending on those last factors you mentioned?

But was that the original question?

 

Several billion years is the mean time it takes for nuclei to have a chance to come to the conditions permitting the fusion. Once they're at the proper distance, the beta decay and other operations must be snappy. Everything has to be quick, faster than the protons need to come apart if the fusion doesn't proceed.

It was not in fact the original question. I was personally looking for more of an answer in femtoseconds or something. You know, the time it takes for two hydrogen atoms to fuse together from the moment it becomes possible for them to do so.

Or alternatively what the timetable would be for 1 gram of hydrogen to become fully fused. Alternatively the equivalent of "half-life" would also be a fine answer (since it may in fact be the case that the rate slows down).

This may be kind of an odd question, but provided that two hydrogen atoms are in a position where they are about to fuse, how long does this process actually take?

As an extra question, how long would it take to fuse every atom in 1 gram of hydrogen, assuming you had conditions that allowed for this to happen?

 

 

It's probable that the earliest "humans" emerged from beings with a fair amount of technology - fire, stone tools, shelter construction, probably crude nets and rudimentary gathering/carrying apparatus - and the social organization to match.

 

It's hard to imagine where else the kind of physiological adaptations that require such a long childhood could have obtained the necessary evolutionary pressure from.

So what about previous species of human then?

Strays kind of away from my central question, but you have made me curious.

"What do these caste systems generally entail?

 

They vary. Typically in modern hunter/gatherer societies the castes are comprised of members who have specialized skills - hunters, warriors, weavers, laborers, fishers, etc. They can and do overlap, so these don't exactly resemble the caste system we see in agrarian societies like recently in India. They tend to be rigidly hereditary (and at times racist) but there is some overlap in skills and roles between groups. The relative importance of each caste also varies. In the modern Watta culture, for example, hunters were considered a low caste while in the San they were among the most privileged. As agriculture took root, some castes -especially the warrior and religious castes- began to take and wield the kind of power and authority we come today to know as class.

 

"And does that mean there's a leader caste who makes decisions for the group?"

 

The difference between caste and class can be a bit complex and subtle, but in a class system there is mobility between classes while in caste systems there is comparatively little social movement. Both systems can be hereditary, but the caste system is more rigid in this regard. Oddly, though modern hunter-gatherer groups have a more hereditary style (castes), they don't show as much reliance on hereditary power and authority as do agrarian cultures, which are almost all run by warrior/religious classes - themselves often hereditary. This may reflect the egalitarianism of a hunter-gatherer, which seems at odds with the overall caste system. Perhaps it is better to view it as pragmatism rather than egalitarianism. To be sure, it is much more common for the warrior or religious castes to be in control of a hunter-gatherer group than, say, a fisher caste but power and control in the hunter-gatherers is more tenuous and can differ from group to group. It's been a long time since I've read up on this but I think the reasoning is that because hunter-gatherer societies are dependent on highly variable resources no one kind of caste can claim authority all the time or in all places.

 

I should point out that everything we know about the structure of hunter-gatherer societies comes from studying modern ones. With very few exceptions, these modern hunter-gatherers did not live without interaction with or influence from modern agrarian or industrial societies. One of the constant problems those who study these groups face is to try to disentangle those influences that, in many cases, are superimposed on the hunter-gatherers. This superimposition can change how their societies function, how they are structured and how they change. Always we need to be careful extending these findings to ancient human groups. It is best to let the direct evidence (via fossils and artifacts) speak for themselves.

That last thing is something I was wondering about. This seems fairly complex for the earliest human groups. I would've expected something more like the system of other genetically close species of ape.

As these were "prehistoric" there isn't much to go on. Still, we can glean some things from the artifacts and fossils left for us. Prior to the invention of agriculture, our species is likely to have existed much as recent hunter-gatherers societies did (do). That means that we lived in relatively small groups, perhaps up to 100 or so individuals but likely much smaller. In addition, these small kin-groups were likely part of much larger supergroups - perhaps of kin or anyway groups that they did not live with year round but who they were used to interacting with, trading with, warring with, from time to time. We know that there was constant contact with others because our species has a relatively low genetic diversity, indicating not only bottlenecks (one, thought to be about 75k years ago may have resulted in our species population dropping to the low 10s of thousands) but it also indicates extensive gene flow between groups. We can even map these out and get estimates for when and how human populations moved and interbred. This indicates that we were highly mobile both geographically and socially, even then.

 

The evidence shows we lived in hunter/gatherer societies where group size was highly proscribed by resource availability. For all its attendant problems, the invention of agriculture allowed for much more intensive living as well as greater stratification and diversification of roles in the society. The rise of cities was mirrored by the rise of the ruling class and the appearance on non-food producing/procuring professions. Hunter/gatherers do have some stratification and diversity in them, but the rigors and difficulties of such a lifestyle serves to reduce their breadth - the difficulties inherent in living a hunter/gatherer lifestyle meant they could not afford to have too many specialized skills or roles. We don't know how their communities were stratified but modern hunter/gatherer cultures are built on something like a caste system - sometimes hereditary though often not. Modern hunter/gatherers tend to be more egalitarian both in the structure of their communities and the outcomes of their interactions and there is no reason to think early humans were any different. This egalitarianism and role limitation may simply reflect a solution to survival with limited resources; they couldn't afford to mess about with the kinds of functions intensive, complex societies face.

 

There are many studies on the ethnology of modern hunter/gatherers that would be of interest to you. These groups were studied partly (largely) because they represent the style of life our ancestors led.

What do these caste systems generally entail? And does that mean there's a leader caste who makes decisions for the group?

The commonly encountered minimum number for a successful breeding population of humans to have a good chance of maintaining itself indefinitely is about 500.

 

The generally accepted number for the maximum size a human population can reach and still maintain a reputation based and consensus system of decision-making is about 150 (that's the number a mammal with a human sized brain can track socially).

 

So realistically, with a safety margin, we're talking about dozens of groups of people of 20 to 100 each, numbering in the thousands all together, encountering each other frequently and routinely under standard circumstances they have set up in advance. Traveling to these encounters, even, at certain times.

 

It's a bit misleading to think of pre-agricultural humans as having no fixed residences. Most were nomadic to a degree

 

(exceptions like the Pacific Northwest salmon fishers are much studied, and they all appear to be coastal populations with resources that came to them by water - but that may have been how the entire species got started, so we can't assume they are "atypical" somehow in an evolutionary sense)

 

but the traveling was often between fairly fixed and traditional locations in a well-marked and bounded home range. They probably had winter camps and summer camps, rainy season camps and dry season camps, etc.

So you're saying that culturally it was sort of like a "high school" where there weren't so much rules among the individuals participating in the group or a larger governing structure but rather cliques of people where one or multiple may have a "pecking order" amongst themselves and that interact based on reputation, rumour and whatnot? And I mean the students of a high school, not taking into account the organized structures of the teachers, etc.

Iirc, there's a good map in the book Guns, Germs, and Steel by Jared Diamond. The PBS website for his show may have some good maps as well.

Any links to any of these maps?

There's still debate about even some of the major features. Multiple exchanges with Africa, in and out rather than simply outmigration, seem to be gaining in importance, for example.

Well, but there must be a currently more-or-less accepted migration pattern, no?

I was wondering about what the organization of "groups" of humans was in prehistoric times. Here, of course, I'm talking about before we started using agriculture and before we started settling in one place.

What was the typical size of a group of humans? Why was it this size? How was it lead? How were the jobs divided? What was the hierarchy? How was the hierarchy maintained or changed? How was it organized? All of that good stuff.

So, any thoughts?

Simply links to for example documentaries about this are also appreciated.

Is there anyone out there who has a map of the world (as it was thousands of years ago) where the various waves of human migrations are marked with arrows with the arrows also having dates? I'm talking here of course about the migrations that occured while we were still hunter-gatherers.

If possible it would also be very interesting if someone could tell me what the pain push and pull factors for humans were at that time.

Beer isn't a very efficient process for making alcohol.

Efficiency isn't always the most important criterion.

Being able to brew hydrogen from cow dung might be useful in some circumstances.

If you wanted a hydrogen balloon to get you off your deserted island it might be a very useful thing to know about.

 

But if you plan to do fusion, there are more practical ways to get hydrogen, and I doubt you could get much deuterium.

What makes the process so energy intensive/inefficient? Could that be solved?

Paul Allen, the billionaire, recently announced whose biological research he would be funding with many millions of dollars in a new venture: the criterion was the best responses to the question "What is the dark matter of biology?"

 

One of the four best responses was the question of the evolution of form - how and why organisms come to have the shapes and conformations they do, over evolutionary time. It's a dark matter problem - large, dominant, important, and largely mysterious at the moment.

 

Nobody knows, really.

Well, but there must be some prevailing ideas, etc.

I mean for example the tail of the peacock we can be fairly sure was caused by sexual selection and female-choice.

At a guess, the amount of biomass required to make usable amounts would be large and extracting it pure would be energy intensive.

Well, but what about producing it as a byproduct of a larger "system." You know, as a hypothetical you grow some plant to feed cattle and then you harvest hydrogen from it while it grows or something.

As John said, organisms can produce hydrogen but it is quite inefficient compared to other methods.

How come it's so inefficient?

There are certainly bugs that produce hydrogen.

https://en.wikipedia.org/wiki/Biohydrogen#Bacterial_biohydrogen

 

And there are related bugs that produce isotopic enrichment

http://www.ncbi.nlm.nih.gov/pubmed/667738

 

Why do you want to know?

I was just curious whether it'd be possible to have biological organisms synthesize hydrogen or deuturium for nuclear fusion.

I was wondering if there is any biological organism out there which does or can produce either hydrogen or deuterium. If so, which one? And if possible to answer, how? On that last one I wouldn't mind being directed to a place where I can be informed more extensively like a book or documentary if it's too much to explain in short, though a summary is preferable.

 

What you said at the end is essentially correct, although there is no "maximum point" so much as a limit that you can continuously approach but never reach.

 

You know the CoverFlow effect that's used sometimes for slideshows, and especially for music libraries, where the whatever the current image you are viewing is displaying normally front and center, and then off to either side the images is a visible series of other images that you can scroll through, and as you get further from the current image in either direction, the images get more and more scrunched together? So a bit like this:

 

 

That's how I conceptualize frames of reference, where each image is one frame, and there are an infinite number of images extending out in either direction, but since they get more and more scrunched together, they never quite reach the edges of the frame. The central image is your rest frame. The "further" an image is from your frame of reference (i.e. the larger the velocity relative to your own rest frame), the more scrunched it appears to be, and the closer it seems to be to the images adjacent to it. If you scroll over and make that image your rest frame, however, the image on either side will appear more spread out, as it looks in the center of the above image.

 

A change in frames is really a change in velocity. So to get from one image to the next, you would need to accelerate until you reached whatever speed that image represents. Now, if you were to look at someone who is currently in a frame way at one end of this image (i.e. moving with a high velocity relative to you), they would appear length contracted and time dilated, and if they were to accelerate, they would continue moving further and further along the images. But each change from one image to the next would constitute a smaller and smaller "distance" along the line as each image (from your perspective) gets more and more scrunched together. And, of course, moving along the images in the line, they can never leave the slideshow because the images don't extend beyond the edge of the screen. They just continually get closer and closer to it extending on to infinity.

 

For a practical example of how this relates to the question, we'll say that a gun accelerates a bullet to the point that it is five "frames" away from the shooter (remembering that a frame = a relative velocity), If you are firing from the center image, the bullet appears to be going a certain amount faster than the gun in the rest frame. If someone in one of the frames at the edge fires the gun, the bullet will move the same five frames, but since the frames at the edges, from your view, look like they are closer together, the relative difference doesn't look as great as it would if you were viewing it from the frame of the gun.

 

 

That's a little more muddled than I might have liked, and I probably need to work on explaining the metaphor a bit more simply. I know it isn't the easiest one in the world to grasp because it requires thinking of velocities as a locations along a line with distance representing relative speed (and in a more strictly accurate analogy, the "line" would extend in every direction instead of just left and right), but this is the one that I use in my own mind and that makes it easiest to hold a lot of the concepts in my mind. Hopefully it will be of some help.

 

 

Edit: Heh, now I just had an idea for a little interactive tool that uses the coverflow set up, but with a series of continuously running animations that you can accelerate between by scrolling and that display proper time dilation and length contraction depending on your relative distance from them along the "velocity line." If I stuck clocks on them, you should even be able to pull off your own twin paradox experiment.

 

Hm...

This is actually the first time I've ever heard of CoverFlow. >.<

But what I appear to kind of get from this is that the amount of acceleration the bullet gets compared to the at-rest observer would be a tiny fraction, but somehow the space it has to travel for the moving observer is smaller by essentially the same amount so that it wouldn't ever reach lightspeed to an outside observer but the speed it did "gain" would basically be enough to propel it through the reduced amount of space of the moving observer at a speed that would seem to be as fast as a gun normally fires to him. Am I way of, or?

H. heidelbergensis is not much different than H neanderthalensis, except that (as you point out) there for some secondary characters such as a sloping forehead, pronounced brow and smaller chin in the Neanderthals. Neanderthals had more robust skeletons and musculature and had bigger brains than either heidelbergensis or sapiens. H. sapiens differs from both species by being taller and more gracile. We also have different secondary characters that are much more pronounced with comparison tot he Neanderthals but also differ with heidelbergensis such as a flatter face, more robust chin, almost no brow and a high vaulting cranium (though with a lower capacity than the Neanderthals).

 

What accounts for these differences? Since no other human species exists today we can only guess. Perhaps our smaller, more gracile frame is an adaptation to a more migratory lifestyle. It does seem that our species moved around a great deal more than the Neanderthals. Differences in the chin (and thus the jaw) may reflect differences in diet -there is some evidence for this. Differences in the shape of the face and skull may reflect differences in the size and function of the different brains. Though Neanderthals had bigger brains it may be that their fore brain function wasn't as important to them, thus though overall their brains were bigger, their fore brains may have been smaller and this resulted in a more sloping forehead - we do not have any Neanderthal brains to know for sure. It may well be that the Neanderthal brain was better suited for a sense of smell and thus their facial structures were adapted for that (a large prognathous face may have allowed for more acute sense of smell). On the other hand, some or all of these characters may be the result of sexual selection - it may be that a certain face or head shapes were preferred by either males or females (or both). It's also possible that some of these differences may be mere spandrels and have no adaptive purpose.

Very interesting, thank you.

I was wondering about what factors caused humans to have traits and morphology different from Homo neanderthalensis and Homo heidelbergensis (our possibly closest ancestor with Homo neanderthalensis).

 

For example neanderthals have a more sloped forehead, a broader nose, are smaller, have a barrelled chest, etc. So what different conditions caused them to develop each one of these traits or what conditions caused us to develop the traits we have?

 

To give an example of the sort of anwser I might want, If my question had been about the morphology of birds with a particularly long and glorious tail like the male peacock I might expect this response:

"This is sexual dimorphism caused by female choice. Where females are the ones choosing male mates and male mates have to find ways to stand out and attract wandering females to them. This probably became the case because the resources the bird feeds on were spread equally around their habitat as opposed to being concentrated in specific areas (in which case male-male competition would've happened instead of female choice and they might've grown more muscular but without the tail)."

 

That's the sort of thing I'm looking for, except for course about the differences between Homo sapiens, Homo neanderthalensis and Homo heidelbergensis.

So nobody has any idea? It doesn't need to be all of them at once. If you know only how one particular characteristic came to be, feel free to explain that one.

Now replace the gun and bullet with two flashlamps. One aimed forward, the other backwards...

Well, I've already that particular example in the past. But light doesn't have mass, a bullet does.