I agree, but the Army Corps of Engineers are *already* bypassing the delta entirely, by forcing the Mississippi to flow out through a single channel. If we're committed to that plan (and I think we shouldn't be), we could easily throw a lock across the current channel at the end of the "bird's foot" for ship passage, and run an osmosis power plant out there to mix the fresh water with salt right at the place where it's already mixing today.
You ain't seen silt until you've seen a real glacial runoff river.
But yeah, I have no doubt there are some serious and maybe impossible engineering challenges, I'm just making the point that from a basic physics perspective, the energy is there.
I was skeptical of the numbers, so I looked around to figure out how much energy we're talking about here. This link discussing desalinization is pretty useful... what we're talking about here is a desalinization plant run in reverse.
The short answer: 0.66 kcal (2760 joules) per liter of salt water converted to fresh water, so you'd get the same order of magnitude of energy *back* with an osmosis plant. The Mississippi river flow rate is 17 million liters per second at New Orleans, so the maximum possible energy output is 47 GW!
I don't see any obvious efficiency-loss factors here: it should be possible to do this pretty efficiently.
Another way of looking at the problem: the osmotic pressure difference between fresh water and seawater is 28 bar, which is equivalent to 280 meters of hydraulic head. That's roughly the same pressure gradient as is found across the Hoover Dam.
Now, the technical challenge of building miles and miles of carefully-folded osmotic membrane, and keeping it clean, is a bit daunting. But in theory, it should work!
Dammit, I can't both mod you up and ask a followup question.
In ordinary chemical reactions, which I'm more familiar with, the conservation of momentum problem is solved by having a third molecule participate in the collision. The 3-forming-2 collision allows energy and momentum to be conserved, but it does seriously limit the rate at which the reaction can proceed.
So is something like D + D + M --> 4He + M allowed, where M is an arbitrary atom? Or is it just too hard to get 3 nuclei at the same place at the same time?
No, I'm not. See my reply to init100 below for a discussion of the radiation pressure effect of short pulses. Short answer: a fast-moving mosquito would have a bigger impact.
Pulsed lasers are great for a lot of things, but their *radiation pressure effect* is still negligible.
For short pulses, what matters is the *impulse* provided by the beam:
delta-momentum = Force * time.
Impulse = 5400 N * 100 nanoseconds =.00054 kg m/s
If the 10-cm diameter piece of sheet metal being hit by the laser weighs 50 grams, a 100-ns pulse will change its velocity by 1 cm/s.
For comparison, if you gently whacked the target with a ping-pong paddle, you'd provide 100 times as much impulse, over a similar area and time duration.
Yeah, but for structures made of thin metal sheets, a fairly small pressure on the whole structure translates into much larger stresses on the material itself. For example, an aluminum soda can can't contain a pressure of anywhere near 40 ksi.
But yeah, my value is probably an underestimate: I was being conservative.
You can shoot down UAVs with this thing? WOOOOOW. Current UAVs are the short fat pimply kids of military aviation: they're slow and stupid, and you can shoot them down with conventional missiles, antiaircraft artillery, or a well-aimed fart.
This is why we only use them in asymmetric warfare situations, where the bad guys are armed with nothing but Ak-47s. They wouldn't last 30 seconds in the airspace of any competent superpower.
Designing a zillion dollar laser system to shoot them down is a pointless waste of money.
So say I run an insurance company. Someone comes to me and says they'll give me a pile of money on the condition I give them a huge payout after they return from the dead. I know that the odds that they'll be back someday are essentially nil. So basically, they want to give me free money. YES PLEASE!
As a general rule, you shouldn't be surprised that insurance companies will insure you against X, if X is impossible.
Vid is of a neighborhood substation exploding, and is worth watching.
I agree with your overall point that physical sabotage is probably much easier and more reliable than hacking, I'm just quibbling about the technique. Crowbars seem hazardous, and likely to take out a neighborhood but not an entire state. I assume major continental grid stations have better security than the substation down the block.
Someone here mentioned coordinated high-powered rifle attacks on major distribution switchyards. I don't know enough about how they're designed to comment on whether that would work, but it seems to me that a bullet in the windings of a 500 MVA transformer would have exciting consequences.
My guess is, we're seeing half of a contest pitting high-end defense technology vs the "stupid cheap easy" solution.
SCENE: PENTAGON STAFF ROOM Mil Contractor: "And so you see, with our latest satellite imaging systems, we can search and pinpoint the location of a human-sized target object within 10 days for a nation the size of the US or Russia." Dumb General: "Wow. We need to spend some billions on this." Smart General: "Pff. I bet you could do better by plain old "boots on the ground" spywork. You'd need a pretty big network of observers though..." Smart 5-star general: "Well, boys, let's find out."
at least, this is a good enough story that I *hope* it's what's going on...
Hi, I'm a physics professor. I say, take your notes on paper. Math is the most computer-incompatible writing system ever designed. You'll never ever be able to type equations fast enough to keep up with me on the blackboard.
And even if you manage to find a math entry system that's fast enough, it won't help you with the diagrams, graphs, and sketches.
Of course, I don't practice what I preach: my own lecture notes are in text files. But that's because to me, "block ramp friction mu=0.2, 1 kg 30deg 1m long, find final v. U=4.9 Wf=1.7 v=2.5" is a complete set of notes for a 20-minute segment of lecture.
Oh, also: write in pencil. I guarantee you that whenever you bring a pen, I will spend the entire lecture correcting minor mistakes by erasing with the heel of my hand, changing variable notations, and editing diagrams and drawings halfway through working a problem.
Step 1: buy out another airline, and start looking for ways to "eliminate redundancies". Step 2: force the workers to spend work time learning a byzantine new management system. Step 3: fire anyone who makes errors at work as a result. Step 4: Profit!
I tried feeding my dog gasoline, and I tried putting Purina in my gas tank. Now I've got to go see both the mechanic and the vet, but I'm not sure who should see which patient... This is a classic case of apples and oranges. You can't freely exchange food energy and fuel energy in today's society, so it's meaningless to compare their energy costs.
When you look at the calculation in detail, they work out the amount of farmland per dog (0.83 hectares), then convert the amount of energy used by an SUV into acres of land, by using THE INTENSITY OF SUNLIGHT on that land surface. So yeah, if we had solar-powered cars that worked at 100% efficiency, their calculation makes sense. Otherwise, it's rubbish.
Here's a better calculation: The U.S. has 1.5 hectares of farmland per capita. If every family of 4 owned one big dog, we'd be devoting 15% of our farmland to feeding pets. It's a noticeable chunk of our food resource, but it's not an SUV.
We are collecting data points like mad and its not looking good for extraterrestrial life.
This news is all about revising a term in the Drake Equationupward. That can't make ET life less likely.
As for spectra, the vast majority of planetary IDs give no information about the planets apart from their orbits and masses. And as far as I know, the few spectra we have are for Jupiters, not terrestrial planets.
So your dreams of bug-eyed-monsters are as alive as they ever were.
They're not measuring the side-to-side motion of the stars, that's impossible^H^H^H^Hvery difficult to measure, as your trig suggests.
They're measuring the Doppler shift of features in the star's optical spectrum, as it moves toward us and away. It's the world's most impressive police radar gun.
Right now the ratio between stars to planets in the milky way is about 1 billion to 1.
That's a ridiculous statistic. By that measure, the ratio between Diet Coke drinkers and humans is 3.5 billion to 1, because my wife and I are the only people in my group of friends who drink the stuff, and there are 7 billion people on the planet.
And yet somehow the Coca Cola company keeps making it, just for us...
A better statistic is the ratio of the number of planets discovered and the NUMBER OF STARS SEARCHED FOR PLANETS. As of 2003, this fraction was at least 10%, and given observational limits may prove to be as high as 100% -- it could well be that ALL sunlike stars have planets.
Slashdot Mirror
you would be bypassing the delta entirely.
I agree, but the Army Corps of Engineers are *already* bypassing the delta entirely, by forcing the Mississippi to flow out through a single channel. If we're committed to that plan (and I think we shouldn't be), we could easily throw a lock across the current channel at the end of the "bird's foot" for ship passage, and run an osmosis power plant out there to mix the fresh water with salt right at the place where it's already mixing today.
You ain't seen silt until you've seen a real glacial runoff river.
But yeah, I have no doubt there are some serious and maybe impossible engineering challenges, I'm just making the point that from a basic physics perspective, the energy is there.
I was skeptical of the numbers, so I looked around to figure out how much energy we're talking about here. This link discussing desalinization is pretty useful... what we're talking about here is a desalinization plant run in reverse.
The short answer: 0.66 kcal (2760 joules) per liter of salt water converted to fresh water, so you'd get the same order of magnitude of energy *back* with an osmosis plant. The Mississippi river flow rate is 17 million liters per second at New Orleans, so the maximum possible energy output is 47 GW!
I don't see any obvious efficiency-loss factors here: it should be possible to do this pretty efficiently.
Another way of looking at the problem: the osmotic pressure difference between fresh water and seawater is 28 bar, which is equivalent to 280 meters of hydraulic head. That's roughly the same pressure gradient as is found across the Hoover Dam.
Now, the technical challenge of building miles and miles of carefully-folded osmotic membrane, and keeping it clean, is a bit daunting. But in theory, it should work!
Dammit, I can't both mod you up and ask a followup question.
In ordinary chemical reactions, which I'm more familiar with, the conservation of momentum problem is solved by having a third molecule participate in the collision. The 3-forming-2 collision allows energy and momentum to be conserved, but it does seriously limit the rate at which the reaction can proceed.
So is something like
D + D + M --> 4He + M
allowed, where M is an arbitrary atom? Or is it just too hard to get 3 nuclei at the same place at the same time?
No, I'm not. See my reply to init100 below for a discussion of the radiation pressure effect of short pulses. Short answer: a fast-moving mosquito would have a bigger impact.
Pulsed lasers are great for a lot of things, but their *radiation pressure effect* is still negligible.
Keep in mind my estimate of 100 psi to do damage is very conservative.
I thought of this, but didn't post it.
For short pulses, what matters is the *impulse* provided by the beam:
delta-momentum = Force * time.
Impulse = 5400 N * 100 nanoseconds = .00054 kg m/s
If the 10-cm diameter piece of sheet metal being hit by the laser weighs 50 grams, a 100-ns pulse will change its velocity by 1 cm/s.
For comparison, if you gently whacked the target with a ping-pong paddle, you'd provide 100 times as much impulse, over a similar area and time duration.
Yeah, but for structures made of thin metal sheets, a fairly small pressure on the whole structure translates into much larger stresses on the material itself. For example, an aluminum soda can can't contain a pressure of anywhere near 40 ksi.
But yeah, my value is probably an underestimate: I was being conservative.
You can shoot down UAVs with this thing? WOOOOOW. Current UAVs are the short fat pimply kids of military aviation: they're slow and stupid, and you can shoot them down with conventional missiles, antiaircraft artillery, or a well-aimed fart.
This is why we only use them in asymmetric warfare situations, where the bad guys are armed with nothing but Ak-47s. They wouldn't last 30 seconds in the airspace of any competent superpower.
Designing a zillion dollar laser system to shoot them down is a pointless waste of money.
Homework assignment! To rip apart sheet metal, you'll need a pressure in the ballpark of 100 PSI.
Calculate the laser power needed to create this much radiation pressure with a 10-cm diameter beam.
Answer:
100 PSI = 69 N/cm^2
A = pi r^2 = 78 cm^2
F = P * A = 5400 N
F = dp/dt = 2 I/c
I = F c / 2 = 5400 * 3e8 / 2 = 800 GW
This amounts to 1/4 of total U.S. electricity consumption. Utterly impractical.
So say I run an insurance company. Someone comes to me and says they'll give me a pile of money on the condition I give them a huge payout after they return from the dead. I know that the odds that they'll be back someday are essentially nil. So basically, they want to give me free money. YES PLEASE!
As a general rule, you shouldn't be surprised that insurance companies will insure you against X, if X is impossible.
Tag this one !news.
Since when is a gazillion-dollar company considered "under fire" because one dude with no legal status is annoyed at them?
By that logic, "McDonald's has come under fire this week for serving goodmanj a batch of stale fries last time he went there."
Vid is of a neighborhood substation exploding, and is worth watching.
I agree with your overall point that physical sabotage is probably much easier and more reliable than hacking, I'm just quibbling about the technique. Crowbars seem hazardous, and likely to take out a neighborhood but not an entire state. I assume major continental grid stations have better security than the substation down the block.
Someone here mentioned coordinated high-powered rifle attacks on major distribution switchyards. I don't know enough about how they're designed to comment on whether that would work, but it seems to me that a bullet in the windings of a 500 MVA transformer would have exciting consequences.
Do you really want to be within crowbar-tossing distance of this?
http://video.google.com/videoplay?docid=-2674646408572574875#
Hijacking the power grid and forcing entire states to pay ransom or suffer brownouts? Such a thing has never happened before!
http://en.wikipedia.org/wiki/Death_Star_(Business)
My guess is, we're seeing half of a contest pitting high-end defense technology vs the "stupid cheap easy" solution.
SCENE: PENTAGON STAFF ROOM
Mil Contractor: "And so you see, with our latest satellite imaging systems, we can search and pinpoint the location of a human-sized target object within 10 days for a nation the size of the US or Russia."
Dumb General: "Wow. We need to spend some billions on this."
Smart General: "Pff. I bet you could do better by plain old "boots on the ground" spywork. You'd need a pretty big network of observers though..."
Smart 5-star general: "Well, boys, let's find out."
at least, this is a good enough story that I *hope* it's what's going on...
Tonight, on Slashdot: Minor scientists beg for NASA research cash by overhyping their research interests. Film at 11.
Great story, but [Citation needed].
Yeah, but Murphy's Law says the prof will be especially scatterbrained on the day you bring a pen.
Hi, I'm a physics professor. I say, take your notes on paper. Math is the most computer-incompatible writing system ever designed. You'll never ever be able to type equations fast enough to keep up with me on the blackboard.
And even if you manage to find a math entry system that's fast enough, it won't help you with the diagrams, graphs, and sketches.
Of course, I don't practice what I preach: my own lecture notes are in text files. But that's because to me, "block ramp friction mu=0.2, 1 kg 30deg 1m long, find final v. U=4.9 Wf=1.7 v=2.5" is a complete set of notes for a 20-minute segment of lecture.
Oh, also: write in pencil. I guarantee you that whenever you bring a pen, I will spend the entire lecture correcting minor mistakes by erasing with the heel of my hand, changing variable notations, and editing diagrams and drawings halfway through working a problem.
Step 1: buy out another airline, and start looking for ways to "eliminate redundancies".
Step 2: force the workers to spend work time learning a byzantine new management system.
Step 3: fire anyone who makes errors at work as a result.
Step 4: Profit!
I tried feeding my dog gasoline, and I tried putting Purina in my gas tank. Now I've got to go see both the mechanic and the vet, but I'm not sure who should see which patient... This is a classic case of apples and oranges. You can't freely exchange food energy and fuel energy in today's society, so it's meaningless to compare their energy costs.
When you look at the calculation in detail, they work out the amount of farmland per dog (0.83 hectares), then convert the amount of energy used by an SUV into acres of land, by using THE INTENSITY OF SUNLIGHT on that land surface. So yeah, if we had solar-powered cars that worked at 100% efficiency, their calculation makes sense. Otherwise, it's rubbish.
Here's a better calculation: The U.S. has 1.5 hectares of farmland per capita. If every family of 4 owned one big dog, we'd be devoting 15% of our farmland to feeding pets. It's a noticeable chunk of our food resource, but it's not an SUV.
We are collecting data points like mad and its not looking good for extraterrestrial life.
This news is all about revising a term in the Drake Equation upward. That can't make ET life less likely.
As for spectra, the vast majority of planetary IDs give no information about the planets apart from their orbits and masses. And as far as I know, the few spectra we have are for Jupiters, not terrestrial planets.
So your dreams of bug-eyed-monsters are as alive as they ever were.
They're not measuring the side-to-side motion of the stars, that's impossible^H^H^H^Hvery difficult to measure, as your trig suggests.
They're measuring the Doppler shift of features in the star's optical spectrum, as it moves toward us and away. It's the world's most impressive police radar gun.
Right now the ratio between stars to planets in the milky way is about 1 billion to 1.
That's a ridiculous statistic. By that measure, the ratio between Diet Coke drinkers and humans is 3.5 billion to 1, because my wife and I are the only people in my group of friends who drink the stuff, and there are 7 billion people on the planet.
And yet somehow the Coca Cola company keeps making it, just for us...
A better statistic is the ratio of the number of planets discovered and the NUMBER OF STARS SEARCHED FOR PLANETS. As of 2003, this fraction was at least 10%, and given observational limits may prove to be as high as 100% -- it could well be that ALL sunlike stars have planets.
http://arxiv.org/abs/astro-ph/0306524