To paraphrase the article a little more accurately than CNN, I hope.
There is a cluster of young bright stars, currently about 500 ly away from us. They analyse the known movements of cluster (and the Sun) and the likely rate of supernovae in the cluster over the last 5-10 million years. They conclude that there could very plausibly have been enough supernovae from that cluster to account for two things:
1. The "local bubble" a region of space about 500 ly in radius containing the Sun in which the usual interstellar gas is much hotter and thinner than usual.
2. The unusually high levels of a stable, but rare isotope of iron in seabed sediments laid down at certain times.
The rule out various mechanisms that might have stopped the iron from the supernovae reaching the Earth.
They look, much as an afterthought at the possible biological impacts of these supernovae. These are not strong, and I would not say that the paper really supports the idea that this is the trigger of any mass extinctions. The closest of the supernovae would, apparently significantly reduce ozone levels in the stratosphere (charged particles from the SN catalyse NO formation, which destroys ozone), and this would increase levels of UV-B at the surface, to which plankton and corals are especially susceptible, so there might have been some extinctions there, but that seems to be all.
Look. The calculation is easy. Moore's law says, taken very crudely, everything doubles every 18 months. So, to get from "32 bits is a bit tight" to "64 bits is a bit tight" needs 32*1.5 years, ie about another 50 years. If (and it's a big if) exponential growth continues at this rate, then in 2052, a mid-range laptop will have 2 Exabytes of main memory (2^32*512MB), 120EB of backing store, (enough for about 1million years of Mpeg-4 video, for instance) and about 8 ExaFlops of CPU power. Main memory bandwidth will presumably be about 4 EB/s.
From there, to fill up a 128 adress bits takes twice as long, of course (64*1.5 years) so it won't be until 2150 or so that the move to 256bit will become critical:-).
I read somewhere that they started out as radiators. Proto-insects, it is suggested, had raisable and lowerable fins on their backs as a way to control loss of excess heat. These evolved up to the size at which returns start to diminish (moving heat into the fin becomes too hard), which, by a fortunate coincidence, is just about big enough to be of some use in steering a descent when falling. This is valuable because it allows the proto-insect to (a) land the right way up and (b) pick a landing spot (on a leaf instead of the forest floor for instance). From there, they evolve to become bigger and more movable so as to steer better, then glide, then fly.
Electricity producing silicon solar-cells actually take more energy and generate more pollution during manufacture, than they will ever generate. The
uninformed non-tech green set never seems to understand this point.
I believe that this is no longer true. The latest generations of cells are much thinner and lighter (less silicon to refine and melt so less energy) and more efficient than earlier generations.
Hydrogen is not a primary source of energy. It's an energy transmission and storage system. As such, It has a lot of potential advantages over the current options -- long-distance power cables, tanks of gasoline, batteries, etc. but you still have to get energy from somewhere else to make it. The portability element makes some power generation options (off-shore wind and wave, desert solar, hydroelectric) more economic than they are at present when you have to build power lines, but oil and coal are not instantly obsolete.
It doesn't. The thing about Shor's algorithm is that, as initially written up, it factors an L bit number on a 3L q-bit quantum computer in polynomial time (O(L^3), I think). Obviously IBM have tweaked it a bit to get down from 12 (3*4) to 7 qbits, but even so, going from 4 bits to say 1024 would require 256 times as many qbits (the hard part) and 256^3= 16 million times as much time (not a big problem).
In contrast existing non-quantum techniques take O(e((log L)^(2/3)*(L)^(1/3))) time on a computer of fixed word size. To go from 4 bits to 1024 increases the run time by a factor of something like 10^18.
More to the point, on quantum computers, the race between prime finding, and so key pair generation, and factoring and so code-breaking is much less uneven.
There is an old cartoon, dating from a previous period of uncertainty in particle physics (before the quark theory) showing God adressing a crowd of angles. Caption "OK, they've got up to 1.1GeV. All those in favour of granting them a new particle raise one wing!"
See John Conway and Richard Guy's excellent book "The Book of Numbers" on p 14, where they define two systems of "illion" names for all powers of 1000. In their system, for instance,
four millinillitrillion and 14 is 4*10^{3000012} + 14 (American) and 4*10^{6000018} + 15 (British).
GAP is a powerful software system for computational abstract algebra and discrete mathematics, especially group theory. See http://www.gap-system.org for details (including mirrors) and download. It's distributed under a "copyleft" not too different from the GPL.
If you want to use GAP for research or teaching and can't download it (we've had people whose bandwidth is too low, and people whose countries do not allow arbitrary internet downloads for political/religious reasons) let us know (mail one of the addresses on the Web site) and we can usually manage to send a CD.
... is that unless the FBI are playing a very deep game, then they cannot crack PGP directly. Of course if the NSA had made a major beakthorugh in factpring, they probably wouldn't have told the FBI, but I guess it's still something...
Yes, really. There is a cloud of (very diffuse) dust and gas there, and, provided you look over large enough distance scales, it makes sense to talk of sound waves and the speed of sound in this gas. Over small scales, this breaks down because individual atoms can don't collide enough.
A wave of more gas (also very diffuse) is hitting this cloud, faster than the speed of sound in the cloud, and pushing this "shock wave" in front of it.
In a sense you're right, but you need more context. It sometimes happens in science that an observed phenomenon (in this case the moon) proves hard to explain in a satisfactory way (whatever that means) at all. In such a case, it becomes interesting whenever anyone comes up with any plausible explanation that seems to work. This is an example of this. It is hard to come up with any explanation which gives us such a large moon with no iron core and so on.
Another example is string theory. It is observable that quantum mechanics works extremely well at low energies and that general relativity works extremely well at large scales. Unfortunately, they don't work well together at high energies and small scales. ANY theory which manages to match them up and looks like it might have some predictive value eventually is interesting.
"Hi, we think we have detected someone who might be able to receive this message. Here are 90 years of transmissions from our encyclopedias, archives, libraries, etc., with lots of redundancy, various frequencies etc. etc."
90 years later, if all goes well, you start receiving replies like
"Hey, good to talk, we've decoded your language primers. Here are our encyclopedias etc."
Then a few months later
"Based on what you've sent so far, we'd like to hear more about fly fishing, barbecue cookery and string theory (or whatever). We're also starting to skip the basic physics in our encyclopedias where it matches up with what you're telling us you already know."
If you haven't already sent the requested info, you slip it in when the question arrives.
It's not exactly a conversation, but if both sides are willing, you can learn a lot about one another in a couple of centuries.
Since you ask, computational pure maths. Everytime I can double my data sizes, I can solve the next problem. I admit that 1GB or so would be enough for other purposes (I also do a lot of compiling and a lot of RAM really helps there).
OK. I'll bite. I haven't bought a P4 yet, but I might.
Firstly, for the application I'm interested in the P4 really screams! How do I know this? Because (an old version of) the application IS one of the SPECCPU2000 benchamarks (254.gap) and the P4 does really, really well on that one benchmark.
Secondly, the CPU cost is just not a big part of total system cost. RAM, disks, case, etc all add up.
Thirdly, I have reports of stability problems for high-memory Athlon systems under Linux. They're a bit old now, and possibly obsolete, but I'd want to be sure.
The big issue against the P4 is memory costs and lack of a dual processor platform, although this last is due to change soon. I like at least 2GB of RAM, ideally 4, and that does tend to point at SDRAM as the only affordable technology. Even DDR SDRAM may be a problem if the mobos don't have enough channels/slots. So, I need to know what damage SDRAM does to the P4 in my favourite benchamrk.
I think you did miss something, although the article almost misses it too. This is not, actually cBN, but cBC2N and cBC4N. That is, the compound is a mixture of carbon, boron and nitrogen, in the cubic lattice, not just BN (cBN) or just C (diamond).
The figure is 200 GeV/nucleon. Since a gold nucleus has about 250 nucleons this is about 50 TeV/nucleus.
This sort of accelerator explores a different physical regime from something thje LHC or the Tevatron, which use single higher-energy particles. It
Re:how a supernova explodes
on
Star In A Jar
·
· Score: 2
Excellent account. I thought there was one more detail which brightened up the explosion quite a bit. When the core collapses into neutronium, and cools by radiating off a very bright neutrino flash the rest of the star, which is still mostly hydrogen and helium, implodes and heats dramatically. This causes a fusion explosion in the remaining hydrogen, above the collapsed core and that is the source of most of what we see.
One more thing -- this is a type II supernova. A type I (more common, I think) occurs when a white dwarf in a close binary system acretes enough mass to do the catastrophic collapse into neutronium bit.
Your link to the other article seems to be dead, but I imagine it was the one about TB/cm^3 storage using femto-second lasers and spectral hole burning. Someone commented then that fs lasers are huge power hungry monsters. A day ro so after reading that I passed a poster display from some of our physics grad students, who now have a suitcase sized, battery powered, fully portable femto-second laser.
I doubt the ZnO lasers will get down to femto-second pulse lengths very soon though, it is rather a specialized trick that they use to get pulses that short.
My very own favourtie analogy, but as I understood this article it was saying that quantum cosmologists are moving on from this viewpoint, or perhaps circumventing it. In the moments from the Planck time to the inflationary epoch (roughly the first 10^-35 of a second) the structure of the universe is somewhat malleable (eg by inflation) but not as completely unknowable (without a full theory of quantum gravity) as it was before the Planck time. Some versions of inflation, for instance, would make our universe just a small region where inflation happened to stop, in a much bigger universe where inflation may be a permanent condition.
The acceleration is described as towards the Sun, not opposed to the motion of the spacecraft. Assuming those to directions are not so close together that they can't be distinguished, that would pretty much rule out drag.
I think the science instruments would also have detected a change in the velocity of the surrounding medium. It's precisely to detect the heliopause that they are being run.
Robinson himself admits that he was grossly over-optimistic in his timescale for terraforming Mars. Even with all the techniques he suggests, it would take several times longer. An interesting alternative I once heard about is the "world-house" (cf green house) a plastic skin holding down a km or so of breathable atmosphere near the surface, supported on pylons. Surprisingly, the physics can be made to work out without ridiculously strong materials, and the skin can be built progressively.
I think you're being rather pessimistic. If we get a few years warning, say 10^8 seconds, plausible even now, and almost guaranteed with a quite affordable SpaceGuard type project, then a velocity change now of even a meter per second would be enough to change a hit to a near-miss.
Now suppose we could use nukes to split off a 100m cube of rock and shove it away. To get the desired 1 m^s-1 for a 1km^3 asteroid (a big one) we need about 1000 m^s-1 for the other chunk, representing an energy of 2.5 * 10^15 J, or the equivalent of about 30g of matter, which, if I recall correctly is about 3 Megatons.
Now, any such system is going to be hugely inefficient, but if even 1% of the energy of some nukes can be used to split off such a "chunk "of rock, we only need a handful of standard warheads.
Getting them in-place and dug in within a few months of detection would be hard, but not necessarily impossible, and 10 years and a few billion $ would build the infrastructure to make it fairly routine if necessary.
Slashdot Mirror
To paraphrase the article a little more accurately than CNN, I hope.
There is a cluster of young bright stars, currently about 500 ly away from us. They analyse the known movements of cluster (and the Sun) and the likely rate of supernovae in the cluster over the last 5-10 million years. They conclude that there could very plausibly have been enough supernovae from that cluster to account for two things:
1. The "local bubble" a region of space about 500 ly in radius containing the Sun in which the usual interstellar gas is much hotter and thinner than usual.
2. The unusually high levels of a stable, but rare
isotope of iron in seabed sediments laid down at certain times.
The rule out various mechanisms that might have stopped the iron from the supernovae reaching the Earth.
They look, much as an afterthought at the possible biological impacts of these supernovae. These are not strong, and I would not say that the paper
really supports the idea that this is the trigger
of any mass extinctions. The closest of the supernovae would, apparently significantly reduce
ozone levels in the stratosphere (charged particles from the SN catalyse NO formation, which
destroys ozone), and this would increase levels of
UV-B at the surface, to which plankton and corals
are especially susceptible, so there might have been some extinctions there, but that seems to be all.
Look. The calculation is easy. Moore's law says, taken very crudely, everything doubles every 18 months. So, to get from "32 bits is a bit tight" to "64 bits is a bit tight" needs 32*1.5 years, ie about another 50 years. If (and it's a big if) exponential growth continues at this rate, then in 2052, a mid-range laptop will have 2 Exabytes of main memory (2^32*512MB), 120EB of backing store, (enough for about 1million years of Mpeg-4 video, for instance) and about 8 ExaFlops of CPU power. Main memory bandwidth will presumably be about 4 EB/s.
:-).
From there, to fill up a 128 adress bits takes twice as long, of course (64*1.5 years) so it won't be until 2150 or so that the move to 256bit will become critical
The original article is in Physics Review Letters and seems sound.
I read somewhere that they started out as radiators. Proto-insects, it is suggested, had raisable and lowerable fins on their backs as a way to control loss of excess heat. These evolved up to the size at which returns start to diminish (moving heat into the fin becomes too hard), which, by a fortunate coincidence, is just about big enough to be of some use in steering a descent when falling. This is valuable because it allows the proto-insect to (a) land the right way up and (b) pick a landing spot (on a leaf instead of the forest floor for instance). From there, they evolve to become bigger and more movable so as to steer better, then glide, then fly.
I believe that this is no longer true. The latest generations of cells are much thinner and lighter (less silicon to refine and melt so less energy) and more efficient than earlier generations.
There are hazards, but the gas is not basically explosive. The main hazards are:
1. A slow leak, resulting in soem hydrogen being mixed with the air, followed by a spark which could set off a very violent explosion indeed
2. A fracture of the bottle, which must be under fairly high pressure.
Hydrogen is not a primary source of energy. It's an energy transmission and storage system. As such, It has a lot of potential advantages over the current options -- long-distance power cables, tanks of gasoline, batteries, etc. but you still have to get energy from somewhere else to make it. The portability element makes some power generation options (off-shore wind and wave, desert solar, hydroelectric) more economic than they are at present when you have to build power lines, but oil and coal are not instantly obsolete.
It doesn't. The thing about Shor's algorithm is that, as initially written up, it factors an L bit number on a 3L q-bit quantum computer in polynomial time (O(L^3), I think). Obviously IBM have tweaked it a bit to get down from 12 (3*4) to 7 qbits, but even so, going from 4 bits to say 1024 would require 256 times as many qbits (the hard part) and 256^3= 16 million times as much time (not a big problem).
In contrast existing non-quantum techniques take O(e((log L)^(2/3)*(L)^(1/3))) time on a computer of fixed word size. To go from 4 bits to 1024 increases the run time by a factor of something like 10^18.
More to the point, on quantum computers, the race between prime finding, and so key pair generation, and factoring and so code-breaking is much less uneven.
There is an old cartoon, dating from a previous period of uncertainty in particle physics (before the quark theory) showing God adressing a crowd of angles. Caption "OK, they've got up to 1.1GeV. All those in favour of granting them a new particle raise one wing!"
See John Conway and Richard Guy's excellent book "The Book of Numbers" on p 14, where they define two systems of "illion" names for all powers of 1000. In their system, for instance,
four millinillitrillion and 14 is 4*10^{3000012} + 14 (American) and 4*10^{6000018} + 15 (British).
GAP is a powerful software system for computational abstract algebra and discrete mathematics, especially group theory. See http://www.gap-system.org for details (including mirrors) and download. It's distributed under a "copyleft" not too different from the GPL.
If you want to use GAP for research or teaching and can't download it (we've had people whose bandwidth is too low, and people whose countries do not allow arbitrary internet downloads for political/religious reasons) let us know (mail one of the addresses on the Web site) and we can usually manage to send a CD.
Steve Linton
... is that unless the FBI are playing a very deep game, then they cannot crack PGP directly. Of course if the NSA had made a major beakthorugh in factpring, they probably wouldn't have told the FBI, but I guess it's still something...
Yes, really. There is a cloud of (very diffuse) dust and gas there, and, provided you look over large enough distance scales, it makes sense to talk of sound waves and the speed of sound in this gas. Over small scales, this breaks down because individual atoms can don't collide enough.
A wave of more gas (also very diffuse) is hitting this cloud, faster than the speed of sound in the cloud, and pushing this "shock wave" in front of it.
In a sense you're right, but you need more context. It sometimes happens in science that an observed phenomenon (in this case the moon) proves hard to explain in a satisfactory way (whatever that means) at all. In such a case, it becomes interesting whenever anyone comes up with any plausible explanation that seems to work. This is an example of this. It is hard to come up with any explanation which gives us such a large moon with no iron core and so on.
Another example is string theory. It is observable that quantum mechanics works extremely well at low energies and that general relativity works extremely well at large scales. Unfortunately, they don't work well together at high energies and small scales. ANY theory which manages to match them up and looks like it might have some predictive value eventually is interesting.
Of course, this isn't what you do. You say.
"Hi, we think we have detected someone who might be able to receive this message. Here are 90 years of transmissions from our encyclopedias, archives, libraries, etc., with lots of redundancy, various frequencies etc. etc."
90 years later, if all goes well, you start receiving replies like
"Hey, good to talk, we've decoded your language primers. Here are our encyclopedias etc."
Then a few months later
"Based on what you've sent so far, we'd like to hear more about fly fishing, barbecue cookery and string theory (or whatever). We're also starting to skip the basic physics in our encyclopedias where it matches up with what you're telling us you already know."
If you haven't already sent the requested info, you slip it in when the question arrives.
It's not exactly a conversation, but if both sides are willing, you can learn a lot about one another in a couple of centuries.
Since you ask, computational pure maths. Everytime I can double my data sizes, I can solve the next problem. I admit that 1GB or so would be enough for other purposes (I also do a lot of compiling and a lot of RAM really helps there).
OK. I'll bite. I haven't bought a P4 yet, but I might.
Firstly, for the application I'm interested in the P4 really screams! How do I know this? Because (an old version of) the application IS one of the SPECCPU2000 benchamarks (254.gap) and the P4 does really, really well on that one benchmark.
Secondly, the CPU cost is just not a big part of total system cost. RAM, disks, case, etc all add up.
Thirdly, I have reports of stability problems for high-memory Athlon systems under Linux. They're a bit old now, and possibly obsolete, but I'd want to be sure.
The big issue against the P4 is memory costs and lack of a dual processor platform, although this last is due to change soon. I like at least 2GB of RAM, ideally 4, and that does tend to point at SDRAM as the only affordable technology. Even DDR SDRAM may be a problem if the mobos don't have enough channels/slots. So, I need to know what damage SDRAM does to the P4 in my favourite benchamrk.
I think you did miss something, although the article almost misses it too. This is not, actually cBN, but cBC2N and cBC4N. That is, the compound is a mixture of carbon, boron and nitrogen, in the cubic lattice, not just BN (cBN) or just C (diamond).
The figure is 200 GeV/nucleon. Since a gold nucleus has about 250 nucleons this is about 50 TeV/nucleus.
This sort of accelerator explores a different physical regime from something thje LHC or the Tevatron, which use single higher-energy particles. It
Excellent account. I thought there was one more detail which brightened up the explosion quite a bit. When the core collapses into neutronium, and cools by radiating off a very bright neutrino flash the rest of the star, which is still mostly hydrogen and helium, implodes and heats dramatically. This causes a fusion explosion in the remaining hydrogen, above the collapsed core and that is the source of most of what we see.
One more thing -- this is a type II supernova. A type I (more common, I think) occurs when a white dwarf in a close binary system acretes enough mass to do the catastrophic collapse into neutronium bit.
Your link to the other article seems to be dead, but I imagine it was the one about TB/cm^3 storage using femto-second lasers and spectral hole burning. Someone commented then that fs lasers are huge power hungry monsters. A day ro so after reading that I passed a poster display from some of our physics grad students, who now have a suitcase sized, battery powered, fully portable femto-second laser.
I doubt the ZnO lasers will get down to femto-second pulse lengths very soon though, it is rather a specialized trick that they use to get pulses that short.
My very own favourtie analogy, but as I understood this article it was saying that quantum cosmologists are moving on from this viewpoint, or perhaps circumventing it. In the moments from the Planck time to the inflationary epoch (roughly the first 10^-35 of a second) the structure of the universe is somewhat malleable (eg by inflation) but not as completely unknowable (without a full theory of quantum gravity) as it was before the Planck time. Some versions of inflation, for instance, would make our universe just a small region where inflation happened to stop, in a much bigger universe where inflation may be a permanent condition.
At least I think that's what they're saying.
The acceleration is described as towards the Sun, not opposed to the motion of the spacecraft. Assuming those to directions are not so close together that they can't be distinguished, that would pretty much rule out drag.
I think the science instruments would also have detected a change in the velocity of the surrounding medium. It's precisely to detect the heliopause that they are being run.
Robinson himself admits that he was grossly over-optimistic in his timescale for terraforming Mars. Even with all the techniques he suggests, it would take several times longer. An interesting alternative I once heard about is the "world-house" (cf green house) a plastic skin holding down a km or so of breathable atmosphere near the surface, supported on pylons. Surprisingly, the physics can be made to work out without ridiculously strong materials, and the skin can be built progressively.
I think you're being rather pessimistic. If we get a few years warning, say 10^8 seconds, plausible even now, and almost guaranteed with a quite affordable SpaceGuard type project, then a velocity change now of even a meter per second would be enough to change a hit to a near-miss.
Now suppose we could use nukes to split off a 100m cube of rock and shove it away. To get the desired 1 m^s-1 for a 1km^3 asteroid (a big one) we need about 1000 m^s-1 for the other chunk, representing an energy of 2.5 * 10^15 J, or the equivalent of about 30g of matter, which, if I recall correctly is about 3 Megatons.
Now, any such system is going to be hugely inefficient, but if even 1% of the energy of some nukes can be used to split off such a "chunk "of rock, we only need a handful of standard warheads.
Getting them in-place and dug in within a few months of detection would be hard, but not necessarily impossible, and 10 years and a few billion $ would build the infrastructure to make it fairly routine if necessary.