A good example of this would be the final problems in the code book: for DES the massively parallel approach worked well but for RSA they used a chunky Alpha.
Close, but not the whole answer. The sieving part of the computation was done in a massively parallel fashion but the group concerned had to do the linear algebra on a chunky Alpha because they didn't have an implementation for a cluster.
At least one implementation of the linear algebra runs on clusters. The underlying OS is unimportant in this regard, as it runs on both Windows and Linux/Unix boxes.
Roger Needham, the posthumous winner of a Winston award, was the head of MS Research in Cambridge. He was still actively working for the company until a few days before his death from cancer a month ago.
However, it should be obvious from the above that ammonia will only work at temperatures below -33.4C (liquid-ammonia based reactions have to be in liquid ammonia, after all). It's unlikely (if not impossible, but I don't discount life's ingenuity) that ammonia life would form and then regulate an atmosphere to be below its albedo temperature (planets cool, not heat, after all) so we would have to expect the planet's average temperature to be well below -33.4C in order to find ammonia-carbon based life.
:-)
Who said that the atmospheric pressure on a planet supporting ammonia-based life would have to be close to 1 bar?
Like most liquids, ammonia's boiling point can be raised substantially by increasing the pressure. Don't forget that domestic refrigerators used to use ammonia as a working fluid until safer alternatives were developed. It's quite easy to get ammonia liquid at "room" temperature.
Consider an ammonia-rich planet with a thick atmosphere. It could very easily have ammonia oceans at a temperature we would find comfortable. We could walk around on such a planet clothed only in lightweight gas-tight suits fitted with an oxygen supply in a backpack.
Second, the remarkable thing about water is that based on simple chemical rules it should not be a liquid at ordinary temperatures: ammonia, with a similar MW, is a gas. It is the strong hydrogen bonding between water molecules that gives it the high melting and boiling points, and the very wide range between them.
While that is true about water, it's also true about ammonia! There's quite strong hydrogen bonding in ammonia, which is why its boiling point and freezing point is so much higher than methane which genuinely doesn't have any hydrogen bonding. Methane has molecular weight of 16, ammonia of 17 and water of 18, so all these hydrides are quite similar in that respect. Their boiling points at atmospheric pressure are -161.6C, -33.4C and 100C respectively.
... does not react with oxygen, hydrogen, carbon or sulphur in the liquid state at ordinary pressures, and is easily formed in chemical reactions (which implies a small molecule).
I fail to see why a life-sustaining fluid must not react with oxygen at ordinary pressures. (I fail to see why it need not react with the others noted for that matter, but oxygen is the odd one out.) Oxygen is such a viciously reactive gas that it reacts with almost anything that isn't already heavily oxygenated. There is only free oxygen in the Earth's atmostphere because it has been generated by living organisms which have reacted water with CO_2 to produce useful stuff and a nasty toxic byproduct. Organisms capable of withstanding the corrosive atmosphere came much later and those which actually require free O_2 even later.
A biology that didn't use a hydrolysis reaction wouldn't produce a oxygenated atmosphere and ammonia would very probably serve well as a working fluid. An ammonia-water mixture would possibly be even more suitable.
And there isn't _anything_ that can go wrong at a fusion reactor. The absolute worst case is that the containment magnets could cut out - letting the superheated plasma hit and scar the chamber wall. This causes the operator to swear and fetch another cup of coffee before restarting the reaction.
Tell this to the people at JET. A few years ago the plasma accidentally touched the chamber wall. The tokamak was converted into a single-turn electromagnetic motor carrying a hell of a lot of current. The entire system, weighing hundreds of tons, leapt almost a foot into the air. The resultant thump when it landed was picked up on seismographs all over Europe. IIRC, it was equivalent to an earthquake measuring about 3 on the Richter scale.
The tritium-tritium reaction is a different beast though; that's potentially pretty clean, and pretty safe
Err, it's actually pretty dirty.
A much cleaner reaction is deuterium and helium-3 which generates primarily He-4, a proton and a gamma-ray. The charged particles slow down rapidly, giving up energy, and the gamma is absorbed in the shielding, again producing heat, and doesn't produce any significant induced radioactivity.
The main advantage of D+T is that it's easy to ignite (relatively speaking) and both reactants are readily available. The double electric charge of He-3 makes ignition harder and its relative scarcity doesn't help much. Although scarcity doesn't matter much for prototype reactors, it could become a serious limitation if D+He-3 becomes a commercial success. Proposals have been made for harvesting it from the solar wind or the atmospheres of Jupiter or Saturn, but you can be pretty sure that the start-up costs will be pretty large.
Conclusion - unless there is some sort of active cooling, nothing can cool down to less than temperature of the background radiation (3K).
Correct.
There is active cooling in this case, and it works the same as a domestic refrigorator. Both systems cool down because gases are expanded, thereby doing work. That energy has to come from somewhere and it comes from the heat content of the gas: it cools in other words.
At the center of nebulae like these is a star which is driving off the remnants of what was previously its outer layers. That is, its atmosphere is expanding. If the heat loss through expansion is greater than the heat input from the rest of the universe, the gas will cool.
That's probably one of the things that made me think that evolution of human has ended. I mean, what chance would the man carrying this gene have to reproduce if it wasn't for today's medicine?
Quite good, actually. And it's not just "the man", as the mutation is not on a sex-linked chromosome.
Some of the people afflicted with MEN2 die of thyroid cancer before they reach ten years. Others die of it in their fifties, long after they've had many chances to reproduce.
If/when a particular mutation has some features that allow it to dominate gene pool it is positive. If the mutation causes premature death, sterility etc the faulty genes do not enter the gene pool. This is negative.
This statement is almost always correct but in the last couple of years I've become aware of a counterexample.
Studies of Multiple Endocrine Neoplasia Type 2 syndrome (MEN2)
MEN2 is an uncommon dominantly inherited cancer predisposition syndrome comprising tumours of C-cells of the thyroid, the adrenal medulla, and sometimes the parathyroid. The syndrome is due to activating mutations in the RET gene; inactivating mutations of the same gene are associated with Hirschsprung disease.
Although the gene for MEN2 is dominantly inherited and almost invariably fatal, it continues in the gene pool because some sufferers live long enough to reproduce.
Otherwise, you have to say that height and energy are the same thing since E=m*g*h (i.e. a difference in gravitation potential, height, can be used as a source of energy but the thing that is height is not energy).
If you examine the case more carefully, you will find that the system "earth + separated massive body" has a greater mass (as measured by its external gravitational field, for instance) than the system "earth + adjacent massive body".
In the sense of "system of separated massive bodies", height most certainly does have mass, counterintuitive though it may seem.
Light does have momentum (part of the idea behind solar sails). However, light does not have mass(but you might be able to argue that since it has momentum it behaves like it has mass, whatever that means). Most people think that this cannot be since p=m*v and if it has no m, it can't have any p. Well, relativity shows that E=p*c for light so if the light has energy, it can have momentum. In this way, momentum is more of a fundamental quantity than mass or velocity and cannot, in general, be separated into a product of the two
Light has momentum: true.
Light doesn't have mass: false.
Light has mass because light has energy. Mass and energy are the same quantity expressed in different units. The conversion factor from mass to energy is c-squared.
A much bigger problem is that Linux filesystems have a capacity limit of 2TB.
Many servers now have the physical capacity of over 2TB on a filesystem storage device.
Unfortunately this is still a very significant limitation.
This problem is much more commonly encountered than file size limitations
An interesting observation, but not one I've ever made. I'm much more likely to want to store over 2Gb of data in a single file than to want a 2Tb file system. Indeed, I don't have 2TB of disk to make into a file system, but I create large files relatively often.
Acceleration is rate of change in velocity. delta-v is absolute change in velocity.
You can have very low acceleration and very high delta-v if you wait long enough. For example, a solar sail may accelerate at micro-g but still be travelling like a bat out of hell after a couple of years.
If you are going to accuse people of pedantry, and especially if you do so in an offensive manner, please be very careful to ensure you are correct before posting.
Damn! I misread the subject and thought "ethernet over audio".
Now if it had been running wireless network protocols over an ultrasonic link, that would have been a geek's delight.
Before the nit-pickers start, I'm well aware that IP over an analogue modem link could be regarded as an audio network. It's the wireless bit that's more interesting to me.
Running it over a sonic link (i.e. under 10kHz) would likely be both painful and slow.
This is on the basis that the breaks of 56 bit DES and 512 bit RSA came at arround the same time and used roughly equivalent amounts of processing. In fact there is a slight discontinuity since only half of the RSA calculation could be farmed out. The farming stage results in a heck of a big matrix that you have to invert which was done on a CM5 I seem to recall.
Close, but no cigar. Much more than half of the RSA-155 factorization was farmed out. The General Number Field Sieve algorithm falls naturally into several phases. The first phase, polynomial selection, was run over several weeks on a number of computers and used 100 MIPS-years. The sieving phase, which accounted for about 90% of the computation, was spread over close to three hundred machines and took 15 weeks. The sieving used about 8400 MIPS-years.
The final three stages were not widely distributed, though the last was run on four different machines concurrently. I don't have details of the filtering stage but, based on personal experience, I suspect it was done on one or two machines and used less than a week of cpu time.
The linear algebra was actually done on the Cray C916 (not a CM5) at SARA in Amsterdam. It took 224 cpu-hours and 2Gb of RAM. The last stage, extracting a square root in an algebraic number field, was run on four 300MHz SGI processors and each job ran for around 40 hours.
Incidentally, the matrix stage doesn't require a single supercomputer. A parallelized version is being developed which runs nicely on a Beowulf-type cluster. I'll be starting the matrix for a 506-bit GNFS factorization on just such a cluster later this week.
Even if it were, Mars' atmosphere is much thinner and has a much lower pressure than Earth's. Meteors entering Mars' atmosphere stand a much better chance at reaching the surface than they would on Earth. Combined with the ambient atmospheric temperature of the planet's atmosphere, even the density wouldn't prevent this.
True but, I claim, unimportant. Anything big enough to make a significant change to the Martian surface is big enough to get through the Earth's atmosphere and have interesting side effects when it lands here. A 10km body impacting the earth hardly notices the atmosphere.
While it may not be possible to find the "rest frame" of the Universe, it is certainly possible to show that it has no overall angular momentum.
Are you sure about this claim?
It's been a while since I last consulted my Misner, Thorne and Wheeler but I'm fairly sure that there are solutions to Einstein's field equations that describe a rotating universe.
An observer in such a universe could make a choice: either he could observe the distant stars not moving with respect to his field of view but feel a "centrifugal force" (thereby showing he's not in an inertial reference frame) or he could be in a frame with no centrifugal force but which requires the stars to move past his field of view.
First, chirality is not the same as "specific" shapes". Carbon dioxide is not chiral, but it has a specific shape. In the ground state it is a linear chain 0=C=0. Methane is not chiral; in its ground state it is tetrahedral.
Second, it's not at all obvious that chirality is required for life.
Third, silicon can form tetrahedal molecules. Silane, SiH_4 is an example.
Fourth, chirality is a property of the molecule as a whole, not of any particular atom in the molecule. If you want an example, consider hexahelicene. The molecule is chiral and yet the chirality can not be located at any one atom. It consists of six benzene rings joined together in a broken hexagon. A picture of this molecule can be found here.
The silicon based life argument is betting a bit old. Silicon has similar properties to carbon but it forms very weak bonds and very short chains. The bonds silicon makes aren't going to stand up to much meaning you need extraordinarily extreme conditions for that type of life to survive but with those sort of extremes the sort of processes you could catalyze the formation of life are likely too extreme for said life to even form
No disrespect, but this argument is also getting a bit old. True, pure Si-Si chains are short and weak. However, put some oxygen in there and you can get some very long, complex, stable and interesting molecules. Ever heard of silicones?
I'm certainly not claiming that silicone (with an E) lifeforms must exist but neither am I dismissing the possibility out of hand.
Slashdot Mirror
Anyone know where I can find an Assyriologist to help me read my tablet? Such people seem to be very few and far between.
Egyptologists, OTOH, are relatively easy to find and self-education books on Middle Egyptian are readily available.
Paul
Close, but not the whole answer. The sieving part of the computation was done in a massively parallel fashion but the group concerned had to do the linear algebra on a chunky Alpha because they didn't have an implementation for a cluster.
At least one implementation of the linear algebra runs on clusters. The underlying OS is unimportant in this regard, as it runs on both Windows and Linux/Unix boxes.
Paul
Paul
Very few, actually. Parts of scientists, on what's left of the other hand ...
Paul
Roger Needham, the posthumous winner of a Winston award, was the head of MS Research in Cambridge. He was still actively working for the company until a few days before his death from cancer a month ago.
Paul
Who said that the atmospheric pressure on a planet supporting ammonia-based life would have to be close to 1 bar?
Like most liquids, ammonia's boiling point can be raised substantially by increasing the pressure. Don't forget that domestic refrigerators used to use ammonia as a working fluid until safer alternatives were developed. It's quite easy to get ammonia liquid at "room" temperature.
Consider an ammonia-rich planet with a thick atmosphere. It could very easily have ammonia oceans at a temperature we would find comfortable. We could walk around on such a planet clothed only in lightweight gas-tight suits fitted with an oxygen supply in a backpack.
Paul
While that is true about water, it's also true about ammonia! There's quite strong hydrogen bonding in ammonia, which is why its boiling point and freezing point is so much higher than methane which genuinely doesn't have any hydrogen bonding. Methane has molecular weight of 16, ammonia of 17 and water of 18, so all these hydrides are quite similar in that respect. Their boiling points at atmospheric pressure are -161.6C, -33.4C and 100C respectively.
I fail to see why a life-sustaining fluid must not react with oxygen at ordinary pressures. (I fail to see why it need not react with the others noted for that matter, but oxygen is the odd one out.) Oxygen is such a viciously reactive gas that it reacts with almost anything that isn't already heavily oxygenated. There is only free oxygen in the Earth's atmostphere because it has been generated by living organisms which have reacted water with CO_2 to produce useful stuff and a nasty toxic byproduct. Organisms capable of withstanding the corrosive atmosphere came much later and those which actually require free O_2 even later.
A biology that didn't use a hydrolysis reaction wouldn't produce a oxygenated atmosphere and ammonia would very probably serve well as a working fluid. An ammonia-water mixture would possibly be even more suitable.
Paul
Tell this to the people at JET. A few years ago the plasma accidentally touched the chamber wall. The tokamak was converted into a single-turn electromagnetic motor carrying a hell of a lot of current. The entire system, weighing hundreds of tons, leapt almost a foot into the air. The resultant thump when it landed was picked up on seismographs all over Europe. IIRC, it was equivalent to an earthquake measuring about 3 on the Richter scale.
Paul
Err, it's actually pretty dirty.
A much cleaner reaction is deuterium and helium-3 which generates primarily He-4, a proton and a gamma-ray. The charged particles slow down rapidly, giving up energy, and the gamma is absorbed in the shielding, again producing heat, and doesn't produce any significant induced radioactivity.
The main advantage of D+T is that it's easy to ignite (relatively speaking) and both reactants are readily available. The double electric charge of He-3 makes ignition harder and its relative scarcity doesn't help much. Although scarcity doesn't matter much for prototype reactors, it could become a serious limitation if D+He-3 becomes a commercial success. Proposals have been made for harvesting it from the solar wind or the atmospheres of Jupiter or Saturn, but you can be pretty sure that the start-up costs will be pretty large.
Paul
Correct.
There is active cooling in this case, and it works the same as a domestic refrigorator. Both systems cool down because gases are expanded, thereby doing work. That energy has to come from somewhere and it comes from the heat content of the gas: it cools in other words.
At the center of nebulae like these is a star which is driving off the remnants of what was previously its outer layers. That is, its atmosphere is expanding. If the heat loss through expansion is greater than the heat input from the rest of the universe, the gas will cool.
Paul
Quite good, actually. And it's not just "the man", as the mutation is not on a sex-linked chromosome.
Some of the people afflicted with MEN2 die of thyroid cancer before they reach ten years. Others die of it in their fifties, long after they've had many chances to reproduce.
Paul
This statement is almost always correct but in the last couple of years I've become aware of a counterexample.
I quote from a web page about MEN2:
Studies of Multiple Endocrine Neoplasia Type 2 syndrome (MEN2)
MEN2 is an uncommon dominantly inherited cancer predisposition syndrome comprising tumours of C-cells of the thyroid, the adrenal medulla, and sometimes the parathyroid. The syndrome is due to activating mutations in the RET gene; inactivating mutations of the same gene are associated with Hirschsprung disease.
Although the gene for MEN2 is dominantly inherited and almost invariably fatal, it continues in the gene pool because some sufferers live long enough to reproduce.
Paul
If you examine the case more carefully, you will find that the system "earth + separated massive body" has a greater mass (as measured by its external gravitational field, for instance) than the system "earth + adjacent massive body".
In the sense of "system of separated massive bodies", height most certainly does have mass, counterintuitive though it may seem.
Paul
Light has momentum: true.
Light doesn't have mass: false.
Light has mass because light has energy. Mass and energy are the same quantity expressed in different units. The conversion factor from mass to energy is c-squared.
What light doesn't have is rest mass.
Paul
Many servers now have the physical capacity of over 2TB on a filesystem storage device.
Unfortunately this is still a very significant limitation.
This problem is much more commonly encountered than file size limitations
An interesting observation, but not one I've ever made. I'm much more likely to want to store over 2Gb of data in a single file than to want a 2Tb file system. Indeed, I don't have 2TB of disk to make into a file system, but I create large files relatively often.
Paul
That's acceleration in English, pedantic asshole.
No it isn't.
Acceleration is rate of change in velocity. delta-v is absolute change in velocity.
You can have very low acceleration and very high delta-v if you wait long enough. For example, a solar sail may accelerate at micro-g but still be travelling like a bat out of hell after a couple of years.
If you are going to accuse people of pedantry, and especially if you do so in an offensive manner, please be very careful to ensure you are correct before posting.
Paul
Now if it had been running wireless network protocols over an ultrasonic link, that would have been a geek's delight.
Before the nit-pickers start, I'm well aware that IP over an analogue modem link could be regarded as an audio network. It's the wireless bit that's more interesting to me.
Running it over a sonic link (i.e. under 10kHz) would likely be both painful and slow.
Paul
Close, but no cigar. Much more than half of the RSA-155 factorization was farmed out. The General Number Field Sieve algorithm falls naturally into several phases. The first phase, polynomial selection, was run over several weeks on a number of computers and used 100 MIPS-years. The sieving phase, which accounted for about 90% of the computation, was spread over close to three hundred machines and took 15 weeks. The sieving used about 8400 MIPS-years.
The final three stages were not widely distributed, though the last was run on four different machines concurrently. I don't have details of the filtering stage but, based on personal experience, I suspect it was done on one or two machines and used less than a week of cpu time.
The linear algebra was actually done on the Cray C916 (not a CM5) at SARA in Amsterdam. It took 224 cpu-hours and 2Gb of RAM. The last stage, extracting a square root in an algebraic number field, was run on four 300MHz SGI processors and each job ran for around 40 hours.
Incidentally, the matrix stage doesn't require a single supercomputer. A parallelized version is being developed which runs nicely on a Beowulf-type cluster. I'll be starting the matrix for a 506-bit GNFS factorization on just such a cluster later this week.
Paul
True but, I claim, unimportant. Anything big enough to make a significant change to the Martian surface is big enough to get through the Earth's atmosphere and have interesting side effects when it lands here. A 10km body impacting the earth hardly notices the atmosphere.
Paul
Funny, I could have sworn Ultrix ran on Vaxen as well.
Paul
Are you sure about this claim?
It's been a while since I last consulted my Misner, Thorne and Wheeler but I'm fairly sure that there are solutions to Einstein's field equations that describe a rotating universe.
An observer in such a universe could make a choice: either he could observe the distant stars not moving with respect to his field of view but feel a "centrifugal force" (thereby showing he's not in an inertial reference frame) or he could be in a frame with no centrifugal force but which requires the stars to move past his field of view.
Paul
Gravity is relativity. That is, general relativity is Einstein's theory of gravity.
Paul
Belgium, man, Belgium!
Apologies for the bad language. I just couldn't help myself.
Paul
First, chirality is not the same as "specific" shapes". Carbon dioxide is not chiral, but it has a specific shape. In the ground state it is a linear chain 0=C=0. Methane is not chiral; in its ground state it is tetrahedral.
Second, it's not at all obvious that chirality is required for life.
Third, silicon can form tetrahedal molecules. Silane, SiH_4 is an example.
Fourth, chirality is a property of the molecule as a whole, not of any particular atom in the molecule. If you want an example, consider hexahelicene. The molecule is chiral and yet the chirality can not be located at any one atom. It consists of six benzene rings joined together in a broken hexagon. A picture of this molecule can be found here.
Paul
No disrespect, but this argument is also getting a bit old. True, pure Si-Si chains are short and weak. However, put some oxygen in there and you can get some very long, complex, stable and interesting molecules. Ever heard of silicones?
I'm certainly not claiming that silicone (with an E) lifeforms must exist but neither am I dismissing the possibility out of hand.
Paul