"My basic objection to religion is not that it isn't true; I like plenty of things that aren't true. It's that religion grants its adherents malign, intoxicating and morally corrosive sensations. Destroying intellectual freedom is always evil, but only religion makes doing evil feel quite so good." - Philip Pullman
I haven't given up blogging - I'm just keeping very busy indeed. My mother even emailed me wondering what was wrong.
Tuesday, September 30, 2008
Tuesday, September 23, 2008
Drawing of the space habitat
It appears that a number of people are misunderstanding my description of the space habitat (on the order of the "invasion fleet being swallowed by a small dog" scale).
So here's a picture, approximately to scale:
(Edited to correct drawing the moon orbit too small.)
The large circle is the earth's orbit. The circle in the middle is the Sun. The little circle on the left, that's the Moon's orbit around Earth. (Earth itself is a teensy dot in the middle of that circle, about as big as the line marking the orbit of the Earth around the Sun. I drew the Earth in, but you probably can't see it in the small image). The relatively large circle on the right, that's the space habitat sitting at a Lagrange point (or rather, sitting with a Lagrange point at its centre). It is so phenomenally big it might not actually be stable here (I have not done the calculations to check).
(By comparison, the original Ringworld would be represented by the really big circle. Much larger.)
Now the habitat goes around the Sun in the same orbit as the Earth, offset by 60 degrees. It rotates about its own central axis once every 24 hours, giving it a day-night cycle. The ring would be tilted a little, so it doesn't shade itself most of the time - in fact the ring will precess as it goes. The artificial gravity induced by the rotation would be about 1g (it would, I think, vary somewhat between night and day, because at midnight you're orbiting a fair bit nearer to the Sun than you "should" for the center of mass of the habitat, and midday you're a bit further from the Sun).
So here's a picture, approximately to scale:
(Edited to correct drawing the moon orbit too small.)The large circle is the earth's orbit. The circle in the middle is the Sun. The little circle on the left, that's the Moon's orbit around Earth. (Earth itself is a teensy dot in the middle of that circle, about as big as the line marking the orbit of the Earth around the Sun. I drew the Earth in, but you probably can't see it in the small image). The relatively large circle on the right, that's the space habitat sitting at a Lagrange point (or rather, sitting with a Lagrange point at its centre). It is so phenomenally big it might not actually be stable here (I have not done the calculations to check).
(By comparison, the original Ringworld would be represented by the really big circle. Much larger.)
Now the habitat goes around the Sun in the same orbit as the Earth, offset by 60 degrees. It rotates about its own central axis once every 24 hours, giving it a day-night cycle. The ring would be tilted a little, so it doesn't shade itself most of the time - in fact the ring will precess as it goes. The artificial gravity induced by the rotation would be about 1g (it would, I think, vary somewhat between night and day, because at midnight you're orbiting a fair bit nearer to the Sun than you "should" for the center of mass of the habitat, and midday you're a bit further from the Sun).
Thursday, September 18, 2008
Large space habitats
I was thinking about space habitats. It was probably triggered indirectly by the fact that I'm reading* a SF story at present (The Algebraist, Iain M Banks), though there's no direct connection to my resulting train of thought.
*when I'm travelling, I read. There's lots of waiting about and sitting in planes and stuff (13-14 hours across the Pacific, for starters) and I rarely feel well enough in flight to concentrate on actual work. Even if I only spend a fraction of that time reading, I will get through several books on a trip.
It occurred to me that there will be a particular radius of "ring"-type space station where one rotation in 24 hours would produce artificial gravity of about 1g (this will obviously be large, even without doing calculations).
If you put such a habitat at an L4- or L5- point on the earth-sun system, you'll get earth-level radiation from the sun (without the nice van Allen belts to protect you, unfortunately), with earth "days" and earth "gravity".
So how big is it? Well, I did the calculations, and I get a radius of 1.85 million km. This thing is huge - the earth-moon system would fit comfortably inside it.
It's not Ringworld-huge, by a long, long shot - on that scale, it's miniscule. But still, very very large indeed. If it were a thin ring about 100 km wide (it's about 11.5 million km around, so 100 km wide is indeed "thin"), it would have a "land" area roughly eight times that of earth (assuming people live only on the inner surface of the ring). For the present I'm assuming you'd have a series of interconnected domes or similar on the inner surface, which allows you to bring air, water and other resources in stages.
Assuming, that is, that I have done all the calculations correctly.
Many of the problems associated with a ringworld habitat go away - you don't need to worry about the orbit being unstable for example, though I guess if the mass gets large enough there may be some issues with the stability of the Lagrange points. The amount of material required is far, far smaller than for a ringworld - and the relatively more obtainable amounts of material mean much less hyper-engineering is required.
I haven't done any engineering calculations to work out stresses and such, so I don't know whether you could build a lot of the base structure from simple rock, or if you really need to go to strong metals or even unobtainium.
A nice little thought experiment, anyway. I don't recall seeing anything like this in a story. I'm not sure if that's because I don't read around enough or just that nobody has used an idea like this - but I cannot have been the first person to work this out, so I am curious to know if it has been used in a story somewhere.
[Edit: there's some nifty ringworld artwork to be found.]
*when I'm travelling, I read. There's lots of waiting about and sitting in planes and stuff (13-14 hours across the Pacific, for starters) and I rarely feel well enough in flight to concentrate on actual work. Even if I only spend a fraction of that time reading, I will get through several books on a trip.
It occurred to me that there will be a particular radius of "ring"-type space station where one rotation in 24 hours would produce artificial gravity of about 1g (this will obviously be large, even without doing calculations).
If you put such a habitat at an L4- or L5- point on the earth-sun system, you'll get earth-level radiation from the sun (without the nice van Allen belts to protect you, unfortunately), with earth "days" and earth "gravity".
So how big is it? Well, I did the calculations, and I get a radius of 1.85 million km. This thing is huge - the earth-moon system would fit comfortably inside it.
It's not Ringworld-huge, by a long, long shot - on that scale, it's miniscule. But still, very very large indeed. If it were a thin ring about 100 km wide (it's about 11.5 million km around, so 100 km wide is indeed "thin"), it would have a "land" area roughly eight times that of earth (assuming people live only on the inner surface of the ring). For the present I'm assuming you'd have a series of interconnected domes or similar on the inner surface, which allows you to bring air, water and other resources in stages.
Assuming, that is, that I have done all the calculations correctly.
Many of the problems associated with a ringworld habitat go away - you don't need to worry about the orbit being unstable for example, though I guess if the mass gets large enough there may be some issues with the stability of the Lagrange points. The amount of material required is far, far smaller than for a ringworld - and the relatively more obtainable amounts of material mean much less hyper-engineering is required.
I haven't done any engineering calculations to work out stresses and such, so I don't know whether you could build a lot of the base structure from simple rock, or if you really need to go to strong metals or even unobtainium.
A nice little thought experiment, anyway. I don't recall seeing anything like this in a story. I'm not sure if that's because I don't read around enough or just that nobody has used an idea like this - but I cannot have been the first person to work this out, so I am curious to know if it has been used in a story somewhere.
[Edit: there's some nifty ringworld artwork to be found.]
Sunday, September 14, 2008
Living in interesting times
Regular readers will have noticed a dramatic reduction in my posting frequency over the last month. This has been largely due to an upcoming trip to the USA and some other things keeping me busy; I will be in the USA next week.
I will be busy for a time after I return as well, so the sporadic posting will continue for at least a little while.
I will be busy for a time after I return as well, so the sporadic posting will continue for at least a little while.
Saturday, September 13, 2008
TV Science Fiction on humanism and nonbelief
Characters who lack belief are common in SF, yet - unusually for television - they are usually presented in a very positive light. Let's consider a few popular shows.
Star Trek
The characters in the various series are almost universally humanist, and the Federation is almost relentlessly humanist. Star Trek is famous for the sense of hope it conveys about the future, and I think that's largely connected to the humanist sentiment that runs through the entire franchise. In the original series the only main character with any apparent religious sensibilities is McCoy, who does refer to God, and does seem to have a religious background and appears to maintain some level of faith (though he does not appear to be observant of religion). The only main character with any real involvement in even quasi-religious ceremony is Spock, but that ceremony is disconnected from supernatural beliefs. In TNG, the main characters are if anything, more humanist. Worf, while raised by humans, appears to have had enough steeping in Klingon culture to have some degree of acceptance of Klingon religion (he does make reference to Sto'Vo'Kor, for example) and practices some Klingon rituals. None of the humans is particularly religious. Deep Space Nine presents a strongly faith-based culture (the Bajorans), but it is made clear in the show that the beings they base their religion on are not supernatural, simply very powerful aliens.
Firefly
The captain of the Firefly, Mal Reynolds, repeatedly discusses his lack of belief, and is consequently presented as an atheist. One of the other main characters, Book, is a "Shepherd", a kind of priest, but we almost immediately discover very strong indications that he is much more than a simple priest. In one 'episode' (in both senses) River fixes Book's bible, by removing or changing all the parts that make no sense - the book ends up in tatters. The main "spiritual" character is Inara, her religious sensibilities are more Eastern, and several times she is seen to minister to Book; to my recollection, one episode ends with what can only be described as a kind of benediction - Book kneels before her in misery while she places her hand on his head in a metaphorical blessing. Religion may be somewhat important in the wider culture (it features in several episodes, but is as frequently a source of hatred and manipulation as much as comfort), but it is not important in the lives of most of the most of the crew. A sense that people will be good or evil with or without religion clearly comes through. Firefly is somewhat more independent and libertarian in sentiment than the other shows, but many of its characters have a strong humanist bent.
Battlestar Galactica
This show is unusual in that it's an overtly religious society, though many characters are not particularly religious (and a few are openly doubtful). Doubters are not regarded as "evil". Religious and nonreligious characters generally seem broadly accepting of each other. The main religion of the humans is polytheistic, that of the Cylons is monotheistic. Several humans become much more religious over time, but one is of dubious sanity (starting out sane and atheist and becoming apparently insane and somehwat religious); in each case the increase in belief is associated with apparent evidence consistent with that belief (even though some of the resulting beliefs are contradictory).
Stargate
Stargate is another overtly "humanist" program, but it is much more explicit in its treatment of religion. There are three main "enemies" in the series - the Goa'uld, the Replicators and the Ori. The first and last are gods to their followers - false gods, but gods with great powers nonetheless, so it is little surprise that they have great followings. The heroes aim to convince their followers that those they worship are not gods, and ultimately to defeat the false gods. None of the main characters are religious (though Te'alc is initially a believer, he throws off his religion). The Ori in particular, mirror the worst aspects of fundamentalist, dominionist religiosity.
Dr Who
The Doctor himself is an avowed lover of humanity, and the show is unremittingly humanist. Religious themes do come into the show sometimes, but the explanation is generally more on the natural side than the supernatural.
All of these programs deal in some way with "constructed families"; Firefly is probably the most explicit of these, but in each of them, "family" is something you make, not somethng you are. 'Traditional' families are not a major aspect of any of the shows (on DS9, Chief O'Brien has a 'traditional' family ... and marriage problems); one parent families are common. Yet love and loyalty to friends and colleagues run very strongly in all of the programs. All four show quite clearly that morality and religious belief are largely orthogonal. Stargate is perhaps the strongest in its anti-faith message (it takes no clear position that all religions are false - but all the religions that have a substantive place on the show clearly are false, dangerous and evil). People who lack overt belief are common, and almost all of them are moral, heroic, loyal, loving... and most of all, human.
Science Fiction, and in particular, TV science fiction - because of it reaching a large and regular audience, has a small but significant influence on our society. Because it is forward looking, the influence is generally strongly progressive, and in the way it presents its major characters, generally presents atheism in an extremely positive way, far out of keeping with the common depiction of atheists in other programmes (who are often presented as cold, abrasive, misanthropic or amoral, when they're presented at all).
--
Book: What are we up to, sweetheart?
River: Fixing your Bible.
Book: I, um... What?
[River is working on a mangled bible. Passages have been crossed out or corrected. Loose pages lie about.]
River: Bible's broken. Contradictions, false logistics... doesn't make sense.
Book: No, no. You - you can't...
River: So we'll integrate non-progressional evolution theory with God's creation of Eden. Eleven inherent metaphoric parallels already there. Eleven. Important number. Prime number. One goes into the house of eleven eleven times, but always comes out one. [She looks him in the eye.] Noah's ark is a problem.
Book: Really?
River: We'll have to call it "early quantum state phenomenon". Only way to fit [laughing quietly] 5,000 species of mammals on the same boat... [She rips more pages out.]
Book: River, you don't... fix the Bible.
River: [Speaking gently.] It's broken. It doesn't make sense.
Book: It's not about... making sense. It's about believing in something. And letting that belief be real enough to change your life. It's about faith. You don't fix faith, River. It fixes you.
(Book tries to pull some of the ripped out pages from River's hand, but they tear.)
Book: You hang on to those then.
Firefly: Jaynestown
Star Trek
The characters in the various series are almost universally humanist, and the Federation is almost relentlessly humanist. Star Trek is famous for the sense of hope it conveys about the future, and I think that's largely connected to the humanist sentiment that runs through the entire franchise. In the original series the only main character with any apparent religious sensibilities is McCoy, who does refer to God, and does seem to have a religious background and appears to maintain some level of faith (though he does not appear to be observant of religion). The only main character with any real involvement in even quasi-religious ceremony is Spock, but that ceremony is disconnected from supernatural beliefs. In TNG, the main characters are if anything, more humanist. Worf, while raised by humans, appears to have had enough steeping in Klingon culture to have some degree of acceptance of Klingon religion (he does make reference to Sto'Vo'Kor, for example) and practices some Klingon rituals. None of the humans is particularly religious. Deep Space Nine presents a strongly faith-based culture (the Bajorans), but it is made clear in the show that the beings they base their religion on are not supernatural, simply very powerful aliens.
Firefly
The captain of the Firefly, Mal Reynolds, repeatedly discusses his lack of belief, and is consequently presented as an atheist. One of the other main characters, Book, is a "Shepherd", a kind of priest, but we almost immediately discover very strong indications that he is much more than a simple priest. In one 'episode' (in both senses) River fixes Book's bible, by removing or changing all the parts that make no sense - the book ends up in tatters. The main "spiritual" character is Inara, her religious sensibilities are more Eastern, and several times she is seen to minister to Book; to my recollection, one episode ends with what can only be described as a kind of benediction - Book kneels before her in misery while she places her hand on his head in a metaphorical blessing. Religion may be somewhat important in the wider culture (it features in several episodes, but is as frequently a source of hatred and manipulation as much as comfort), but it is not important in the lives of most of the most of the crew. A sense that people will be good or evil with or without religion clearly comes through. Firefly is somewhat more independent and libertarian in sentiment than the other shows, but many of its characters have a strong humanist bent.
Battlestar Galactica
This show is unusual in that it's an overtly religious society, though many characters are not particularly religious (and a few are openly doubtful). Doubters are not regarded as "evil". Religious and nonreligious characters generally seem broadly accepting of each other. The main religion of the humans is polytheistic, that of the Cylons is monotheistic. Several humans become much more religious over time, but one is of dubious sanity (starting out sane and atheist and becoming apparently insane and somehwat religious); in each case the increase in belief is associated with apparent evidence consistent with that belief (even though some of the resulting beliefs are contradictory).
Stargate
Stargate is another overtly "humanist" program, but it is much more explicit in its treatment of religion. There are three main "enemies" in the series - the Goa'uld, the Replicators and the Ori. The first and last are gods to their followers - false gods, but gods with great powers nonetheless, so it is little surprise that they have great followings. The heroes aim to convince their followers that those they worship are not gods, and ultimately to defeat the false gods. None of the main characters are religious (though Te'alc is initially a believer, he throws off his religion). The Ori in particular, mirror the worst aspects of fundamentalist, dominionist religiosity.
Dr Who
The Doctor himself is an avowed lover of humanity, and the show is unremittingly humanist. Religious themes do come into the show sometimes, but the explanation is generally more on the natural side than the supernatural.
All of these programs deal in some way with "constructed families"; Firefly is probably the most explicit of these, but in each of them, "family" is something you make, not somethng you are. 'Traditional' families are not a major aspect of any of the shows (on DS9, Chief O'Brien has a 'traditional' family ... and marriage problems); one parent families are common. Yet love and loyalty to friends and colleagues run very strongly in all of the programs. All four show quite clearly that morality and religious belief are largely orthogonal. Stargate is perhaps the strongest in its anti-faith message (it takes no clear position that all religions are false - but all the religions that have a substantive place on the show clearly are false, dangerous and evil). People who lack overt belief are common, and almost all of them are moral, heroic, loyal, loving... and most of all, human.
Science Fiction, and in particular, TV science fiction - because of it reaching a large and regular audience, has a small but significant influence on our society. Because it is forward looking, the influence is generally strongly progressive, and in the way it presents its major characters, generally presents atheism in an extremely positive way, far out of keeping with the common depiction of atheists in other programmes (who are often presented as cold, abrasive, misanthropic or amoral, when they're presented at all).
--
Book: What are we up to, sweetheart?
River: Fixing your Bible.
Book: I, um... What?
[River is working on a mangled bible. Passages have been crossed out or corrected. Loose pages lie about.]
River: Bible's broken. Contradictions, false logistics... doesn't make sense.
Book: No, no. You - you can't...
River: So we'll integrate non-progressional evolution theory with God's creation of Eden. Eleven inherent metaphoric parallels already there. Eleven. Important number. Prime number. One goes into the house of eleven eleven times, but always comes out one. [She looks him in the eye.] Noah's ark is a problem.
Book: Really?
River: We'll have to call it "early quantum state phenomenon". Only way to fit [laughing quietly] 5,000 species of mammals on the same boat... [She rips more pages out.]
Book: River, you don't... fix the Bible.
River: [Speaking gently.] It's broken. It doesn't make sense.
Book: It's not about... making sense. It's about believing in something. And letting that belief be real enough to change your life. It's about faith. You don't fix faith, River. It fixes you.
(Book tries to pull some of the ripped out pages from River's hand, but they tear.)
Book: You hang on to those then.
Firefly: Jaynestown
Friday, September 12, 2008
Two spins, one story
In looking for some links to a completely different story, I came across two different headlines for exactly the same piece of news:
NSW students 'above average in all areas' (which goes on to say "over 90 per cent at, or over, the national minimum benchmarks")
Story quotes the State Education Minister:"It's just such a tribute to our teachers, to our principals and of course to our kids themselves. They've done brilliantly."
vs
10pc of students fail to meet minimum literacy standards
and quotes the Federal Education Minister:"While it was very pleasing that 90 per cent of students met minimum standards, governments have to focus on making sure every Australian child succeeds at school."
NSW students 'above average in all areas' (which goes on to say "over 90 per cent at, or over, the national minimum benchmarks")
Story quotes the State Education Minister:"It's just such a tribute to our teachers, to our principals and of course to our kids themselves. They've done brilliantly."
vs
10pc of students fail to meet minimum literacy standards
and quotes the Federal Education Minister:"While it was very pleasing that 90 per cent of students met minimum standards, governments have to focus on making sure every Australian child succeeds at school."
Wednesday, September 3, 2008
A Sherlock moment
This morning I travelled to the university with my partner and as she pulls up, I look out the window at the car in the adjacent space and had a Sherlock moment (it happens now and then). As I stepped out of the car, I said:
"The driver of that car wears a lot of large rings on their right hand".
"What? How on earth can you know that?"
"Look at the car door. See the pattern of scratches on the duco where the handle is? Lots and lots of little gouges made by something sharp ... and its all up and down near the handle, not concentrated in one particular part, like it would be if it was just one finger. They must have rings on several fingers. And they're *big* - see way up here? The rings are also scratching way up here, *above* where the handle indentation starts, so either the rings are really big, or the person is very tall and has large hands to boot. Now see where the key fits - you really can't unlock it with your right hand and open it with your left (I cross my hands to demonstrate), so those are right hand scratches. Now see the area around the lock? Rings on the left hand, too, but maybe fewer - there are no scratches under the lock, so the little finger is probably ring free, or only has a small ring. I'm guessing a woman with a *lot* of rings - and either she is really tall, or those rings are *huge*, probably set stones."
If we'd had a bit more time I probably could have figured out a half dozen other things out from just looking at the car. A half-decent sleep, and I just start to notice stuff like this.
Does that happen to everyone and they just keep it to themselves, or is it just me?
"The driver of that car wears a lot of large rings on their right hand".
"What? How on earth can you know that?"
"Look at the car door. See the pattern of scratches on the duco where the handle is? Lots and lots of little gouges made by something sharp ... and its all up and down near the handle, not concentrated in one particular part, like it would be if it was just one finger. They must have rings on several fingers. And they're *big* - see way up here? The rings are also scratching way up here, *above* where the handle indentation starts, so either the rings are really big, or the person is very tall and has large hands to boot. Now see where the key fits - you really can't unlock it with your right hand and open it with your left (I cross my hands to demonstrate), so those are right hand scratches. Now see the area around the lock? Rings on the left hand, too, but maybe fewer - there are no scratches under the lock, so the little finger is probably ring free, or only has a small ring. I'm guessing a woman with a *lot* of rings - and either she is really tall, or those rings are *huge*, probably set stones."
If we'd had a bit more time I probably could have figured out a half dozen other things out from just looking at the car. A half-decent sleep, and I just start to notice stuff like this.
Does that happen to everyone and they just keep it to themselves, or is it just me?
Monday, September 1, 2008
Life expectancy may not be what you expect
Odd Nectar makes some good points against ID.
Along the way, he says the following (LE is "life expectancy"):
"If we look back at the Greco-Roman days, LE was about 25 years. Now that's design at its best, don't you think? I suppose if I were an illiterate desert farmer circa 100 b.c.e. having a staring contest with death at 20 years of age..."
The implication being that if your life expectancy is 25 (actually, 25 is could even be a little high), and you're 20, you expect to live only a few more years ("a staring contest with death"). In fact, most people would assume you expect to live five more years.
Here's a simple two part experiment that may help with the ideas. (You can actually do this experiment if you like, but it will take a while. Or you can simulate it on a computer if you know how). Or, if you're in a hurry, I'll just tell you the answers (for a fair die) in a little while.
I) roll a six-sided die, counting the number of rolls until you get a '1' (including the roll on which you do get a 1). Repeat this many times (say, until you get 90 ones - it should be about 540 rolls, give or take). Average the counts for each set of rolls until a '1' appeared
II) roll a die 4 times. If you didn't get a '1' in those rolls, start counting how many additional rolls you need until you get a 1 (if you did get a '1' in those initial 4 rolls, forget that one and start over). Repeat this many times. [Actually, you can use the information from the experiment in part (I): if the count of rolls was 4 or less, throw it away, and if it was greater than 4, subtract 4 from the count.] Average the counts you keep.
The question we're interested in is "How much larger is the average in experiment I than in experiment II?"
What you you guess?
A lot of people would guess 4. (It's the same as the reasoning in the life expectancy example I quoted above.)
Well, actually, the averages are much closer. If you roll a great many times, and your die is fair, you should get 6 for both!
(I just did this experiment using Excel to simulate the die roll - 540 times for experiment I and reused the 272 of them that exceeded 4 for experiment II - the results were about 5.8 and 5.4, which is not quite as close to six as it should be, but at least we can clearly see that the two numbers don't differ by anything like 4.
[Why is this related to life expectancy? Well, assume we have some creature that has a 1/6 chance of death each year (it dies when it rolls a '1') - so its life expectancy is six years. When it reaches 4 years of age, what's its remaining life expectancy? ... in this case, the answer still six!]
Human life expectancy is not that much like the die roll experiment (even if we put a lot more sides on the die), because the probability of death isn't constant at all ages. However, the basic ideas carry over.
Actually, in ancient times, at age 20, your remaining life expectancy then may even have been more than an addtional 25 years!
At birth, the average life span may have been 25, but the average adult was far older than 25.
What made life expectancy so low? Well, higher death rates, obviously, but the higher death rates didn't impact all ages equally. Most of the increase in death rates was for the youngest ages - especially for newborns. If you could survive past about 5 years of age, death rates were much lower - your chances of making it to adulthood were pretty good, and once you were an adult, your life expectancy was reasonable (not great by today's standards, but it was a lot more than a handful of years).
To simplify things dramatically, imagine there's a 50% chance of dying in your first month, and a life expectancy at birth of 25. What's your life expectancy if you survive that first month?
Well, it's 50 (less maybe a few weeks). The overall average in this case will be the average of the lifespan of those who die in the first month - almost 0 - and those who don't. If those who don't die near birth average 50 years, that makes the overall average lifespan (0 + 50)/2 = 25.
Infant mortality rates were extremely high. I don't know the figures for ancient times, but 50% within the first few years is probably reasonably close.
So your expected lifespan at birth was 25, but your expected lifespan conditional on getting past the most dangerous early part was much higher.
Most of the increase in our lifespan during the 19th and 20th centuries was caused by dramatically reduced infant mortality. A large number of dead infants has a huge impact on the average lifespan, so when you improve infant survival, you greatly increase average lifespan. Of course, survival at all ages improved a lot, but it was the infant mortality where the greatest improvements were realised (and these are also the ages where that survival has the greatest impact on average lifespan).
And what was the dramatic increase in lifespan caused by? Mostly better sanitation, clean drinking water, fewer foodborne diseases (with improvements in handling, storage and so on) and other basic disease prevention measures. Inventions like antibiotics in the mid-20th century were amazing, saving/prolonging millions of lives - but basic sanitation and clean water was even more dramatic, particularly among the young - the biggest improvements in expected lifespan happened before the advent of antibiotics.
So our 20 year old Roman-era desert farmer was not staring death in the face, waiting out his last handful of years ... a 20 year old faced many dangers, but their life was much less risky than the at-birth expectancy figure might make you think, unless you're also thinking about the fact that surviving the first few years was the really hard part.
Along the way, he says the following (LE is "life expectancy"):
"If we look back at the Greco-Roman days, LE was about 25 years. Now that's design at its best, don't you think? I suppose if I were an illiterate desert farmer circa 100 b.c.e. having a staring contest with death at 20 years of age..."
The implication being that if your life expectancy is 25 (actually, 25 is could even be a little high), and you're 20, you expect to live only a few more years ("a staring contest with death"). In fact, most people would assume you expect to live five more years.
Here's a simple two part experiment that may help with the ideas. (You can actually do this experiment if you like, but it will take a while. Or you can simulate it on a computer if you know how). Or, if you're in a hurry, I'll just tell you the answers (for a fair die) in a little while.
I) roll a six-sided die, counting the number of rolls until you get a '1' (including the roll on which you do get a 1). Repeat this many times (say, until you get 90 ones - it should be about 540 rolls, give or take). Average the counts for each set of rolls until a '1' appeared
II) roll a die 4 times. If you didn't get a '1' in those rolls, start counting how many additional rolls you need until you get a 1 (if you did get a '1' in those initial 4 rolls, forget that one and start over). Repeat this many times. [Actually, you can use the information from the experiment in part (I): if the count of rolls was 4 or less, throw it away, and if it was greater than 4, subtract 4 from the count.] Average the counts you keep.
The question we're interested in is "How much larger is the average in experiment I than in experiment II?"
What you you guess?
A lot of people would guess 4. (It's the same as the reasoning in the life expectancy example I quoted above.)
Well, actually, the averages are much closer. If you roll a great many times, and your die is fair, you should get 6 for both!
(I just did this experiment using Excel to simulate the die roll - 540 times for experiment I and reused the 272 of them that exceeded 4 for experiment II - the results were about 5.8 and 5.4, which is not quite as close to six as it should be, but at least we can clearly see that the two numbers don't differ by anything like 4.
[Why is this related to life expectancy? Well, assume we have some creature that has a 1/6 chance of death each year (it dies when it rolls a '1') - so its life expectancy is six years. When it reaches 4 years of age, what's its remaining life expectancy? ... in this case, the answer still six!]
Human life expectancy is not that much like the die roll experiment (even if we put a lot more sides on the die), because the probability of death isn't constant at all ages. However, the basic ideas carry over.
Actually, in ancient times, at age 20, your remaining life expectancy then may even have been more than an addtional 25 years!
At birth, the average life span may have been 25, but the average adult was far older than 25.
What made life expectancy so low? Well, higher death rates, obviously, but the higher death rates didn't impact all ages equally. Most of the increase in death rates was for the youngest ages - especially for newborns. If you could survive past about 5 years of age, death rates were much lower - your chances of making it to adulthood were pretty good, and once you were an adult, your life expectancy was reasonable (not great by today's standards, but it was a lot more than a handful of years).
To simplify things dramatically, imagine there's a 50% chance of dying in your first month, and a life expectancy at birth of 25. What's your life expectancy if you survive that first month?
Well, it's 50 (less maybe a few weeks). The overall average in this case will be the average of the lifespan of those who die in the first month - almost 0 - and those who don't. If those who don't die near birth average 50 years, that makes the overall average lifespan (0 + 50)/2 = 25.
Infant mortality rates were extremely high. I don't know the figures for ancient times, but 50% within the first few years is probably reasonably close.
So your expected lifespan at birth was 25, but your expected lifespan conditional on getting past the most dangerous early part was much higher.
Most of the increase in our lifespan during the 19th and 20th centuries was caused by dramatically reduced infant mortality. A large number of dead infants has a huge impact on the average lifespan, so when you improve infant survival, you greatly increase average lifespan. Of course, survival at all ages improved a lot, but it was the infant mortality where the greatest improvements were realised (and these are also the ages where that survival has the greatest impact on average lifespan).
And what was the dramatic increase in lifespan caused by? Mostly better sanitation, clean drinking water, fewer foodborne diseases (with improvements in handling, storage and so on) and other basic disease prevention measures. Inventions like antibiotics in the mid-20th century were amazing, saving/prolonging millions of lives - but basic sanitation and clean water was even more dramatic, particularly among the young - the biggest improvements in expected lifespan happened before the advent of antibiotics.
So our 20 year old Roman-era desert farmer was not staring death in the face, waiting out his last handful of years ... a 20 year old faced many dangers, but their life was much less risky than the at-birth expectancy figure might make you think, unless you're also thinking about the fact that surviving the first few years was the really hard part.
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