The fact that the Sun's magnetic field is large isn't what protects us from cosmic rays. The Sun's magnetic field encourages particles to orbit the Sun. That doesn't help us. What helps is when a dipole field gets closer to you - like when the Sun sloughs off a bunch of plasma that drifts near you. Hence a Forbush decrease. What protects us on Earth is the Earth's magnetic field, and the atmosphere.
Anyway, it's relatively easy to craft magnetic fields to any shape you want. So high magnetic field on the outside, zero magnetic field on the inside. We're really good at that. And 5 tesla (50,000 gauss) should be about enough. It has been studied.
The reason it's not ideal is because cosmic rays aren't all charged. Gamma rays make up a component of solar cosmic rays, and okay, there may (should) be a few neutrons from the Sun as well (though that part is really new and not well studied).
But magnetic shielding is very actively being looked at. It's just not an easy problem - we don't have very much experience with superconducting magnets in space, for instance.
Interestingly, one of the best things about this is that you don't really have to worry about the highest energy particles which will get through. Not only is the flux far, far lower, but they deposit less energy than lower energy particles which stop in your body. So it's pretty easy to figure out how high a magnetic field you need.
And smartass comment: magnetic fields don't drop like 1/r^2. Electric fields do. For a simple magnetic dipole, the field strength drops like 1/r^3. Different configurations drop differently, as well.
AFAIK it's always more efficient to produce the electricty closer to where you consume it.
Efficient in terms of power, yes. But efficient in terms of pollution? No.
Which would you rather have: 100 million individual pollution sources, or 1000? Which do you think would be easier to maintain for pollution controls? Which do you think would be easier to improve to reduce emissions?
And of course, if you're only polluting from power plants, you can relocate the power plants to avoid smog.
That's even worse. I have an even smaller window of movement to move in.
Huh? You have the same window of motion that an analog stick has. You're just using your wrist rather than a fingertip. It's the same as a mouse, and I'd imagine you can adjust the sensitivity, but I doubt you'd need to.
I push and nothing is stabilizing the controller but my one hand.
Your hand is stabilizing it, and your fingers are pushing the buttons. That should be fine.
I dunno. I can point a mouse and click it without moving pretty easily if I'm gripping it, rather than just holding it like a mouse.
I've got a feeling that given that one of the demos is a FPS, it probably works pretty well for an FPS. It's got to work better than a controller. Not much could be worse.
when I have to aim at only my part of the screen (which will be very small mind you)?
Don't think of it as a light gun. Think of it as an analog stick. Want to look up? Tilt up. Want to look left? Tilt left. You're not aiming. It's just an analog stick.
By the way, galaxies don't move away from us faster than the speed of light.
Actually, they do. Or will. Or are. Something like that.
Really. Not kidding. What's going on is exactly what the grandparent said. As the universe expands, galaxies far away from us are moving faster and faster, because space is expanding, and if it expands at a constant rate, then as the volume increases, the speed has to increase as well.
One would expect that it doesn't expand at a constant rate, but that the rate slows down over time, because you only had an initial impulse (the Big Bang) and a constant gravitational slowing of the expansion. But that doesn't appear to be what's happening. In fact, the rate seems to be accelerating.
What this means is that eventually, galaxies will move outside of our cosmic horizon - the distance to them will be increasing so rapidly that light simply cannot reach us. Effectively, they'll be travelling faster than the speed of light with reference to us.
That would mean that in our frame of reference, galaxies move faster than light
Well, kindof. They're effectively moving away from us faster than the speed of light. Or will. See above. This means that we don't see them at all, because the distance from that galaxy to us is increasing faster than the speed of light, which means the light can never reach us.
You can read here more about it. Note what it says there though - there's no real easy way to define "relative speed" in curved spacetime, although you could just calculate the rate of change in proper distance between two objects. If you would do that, for objects outside of our cosmic horizon, they're moving faster than the speed of light. They can't send messages to us, we can't send messages to them.
OK, so I take it it was just a crappy photo? Do they not photograph well?
No - what I'm trying to say is that the main benefits of E-ink you can't see in a photo - for several reasons.
For one, dim the lights, lengthen the exposure, and any LCD screen in the world will look gorgeous, and you'll be "oh my God, that's amazing". But then when you actually get the thing, and try to work on it outside, you'll be staring at the screen from half an inch away trying to read anything, because the display isn't bright enough.
Second, you really can't photograph the main benefits, because film (and CCDs) don't have the same response as your eye. Your eye is built for huge variations in light intensity. It's more than happy to see a dark closet right beside a sunlit window, and resolve structure in both. Cameras, however, have linear response, and they simply won't be able to see huge contrast. You could throw this display out on a park bench on a bright sunny day, and you can pick it up, and it's as clear to read as indoors with a light on. The high reflectivity also means you can read it in much less light (ten times less light!) than a passively lit LCD screen (like a Palm Pilot, or a first-generation GBA).
Remember that this is essentially paper. It has roughly the same properties as paper. The display they're showing doesn't have terrific resolution, but that's not the main benefit of this device. The main benefit is the fact that it's readable in all lighting conditions that paper's readable in, which is a lot.
I know that right now, with the sun behind me in my office, I can read a piece of paper clear as day, with no effort. But my laptop's LCD is washed out a lot, and my eyes are glad when the sun goes behind a cloud.
You've always seen pretty pictures of LCDs, but you must know that they look like crap in sunlight, or if they're passively lit, they look like crap in low light. This is the one device that looks good in both.
Quoted "constrast ratio" for active screens is not the same as the actual viewed contrast ratio of the LCD. That's the contrast ratio of the emitted white sections over the emitted black sections. But that's not what the eye sees, because it sees "emitted+reflected". The true contrast ratio of an active LCD varies with lighting conditions. It can be very very high in dark rooms (100:1, 500:1, etc.), but will be very very low in any sort of lit room. Outside, it'll probably be near 1:1 - i.e., unviewable. Much lower than that 100:1, 500:1. More like 4:1, or lower, in normal viewing conditions.
The contrast ratio of an E-ink display is about 10:1. Moreover, the E-ink display has about a 40% reflectance (as opposed to a 4% reflectance for LCDs), which means it's much brighter too.
CRTs have the same problem. They quote a 3000:1 contrast ratio, but the black and white sections have virtually the same reflectivity, which means that that contrast ratio only applies when the light in the room is much less than the light emitted from the CRT.
If you want to compare passive and active displays, you have to do it equally. In the same viewing conditions. Most people I know don't work inside pitch black offices.
and simple lambertian demands limit the reflectivity of the white areas (its no mirror, you know?)
E-ink displays are slightly less white state reflective than newsprint, but not much (40% compared to 60%). They have a much, much higher reflectivity than LCD displays - about 10 times higher (LCDs are 4%). With that high reflectivity, it doesn't take a lot of light for an E-ink display to have a much higher contrast ratio than an active LCD.
What gives? Does the E-ink display really look so bad? Or is it just a bad photo for the dev kit?
There are a few advantages of E-ink displays over other displays, and unfortunately they're not going to really be visible in a picture. The first is contrast: the contrast can be made very, very good since the ink can be very dark, and the background very light. Much, much higher than LCDs.
The second is no backlighting. Now, this might not sound all that useful, because the first-generation GBA wasn't backlit, and that wasn't all that good, but E-ink's contrast is high enough that you don't need a backlight. Even just a small reading lamp is going to be easily enough to read by. This is the "easier on the eyes" part, and it's the one thing that current displays can't really compete with.
The third is battery life: since you don't need power to maintain the display, only to change it, the battery life is going to be measured in pages, not in time. For an e-book reader, this is perfect, because you can take as long as you want to read it. I wouldn't be surprised if a production e-book reader based on e-ink only turned on whenever you pushed a button.
There are other benefits (resolution's a biggie, but it doesn't look that great with this model, plus it's an image that's actually there, which means that it'll look good in all lighting and all angles) but I think those three are probably the biggest for the current generation.
The biggest limitation to E-ink right now is its refresh time (~ of order a second per page, or 1 fps) and its cost. But still, it's the only product which really has specifications which seriously compete with paper.
One of the other posts claimed that the termination shock is the only astrophysical shock we can study so we need to keep funding Voyager. It isn't entirely true that the termination shock between our heliosphere and the interstellar wind is the ONLY astrophysical shock we can study. A shock in a space plasma is a shock no matter where it is, and they all are pretty similar.
No. Absolutely not. The termination shock is huge. It's something like ~150-200 AU across the heliosphere. We have no idea what the structure of a shock like that is. I said it's the only astrophysical shock we can study, and I stand by that - it's the only astrophysical scale shock we can study. CMEs are far too small.
The fact that we're seeing things we completely didn't expect should tell you that. We do not understand the acceleration of particles at a shock. Coronal mass ejections happen in a few seconds. The termination shock has existed for millions of years. These are very different phenomena.
Cancelling the funding for Voyager right now is simply idiotic. We just found out that a lot of assumptions we had about the termination shock are wrong, and there's another probe heading there right now!, with more instruments! In terms of science per dollar, there is no better bet right now than funding Voyager.
Hey, wait! I'm right! The ACRs do not come from the shock itself. They didn't unroll at the termination shock - see Ed Stone's Science paper here. Quoth I:
However, in contradiction to many predictions, the intensity of anomalous cosmic ray (ACR) helium did not peak at the shock, indicating that the ACR source is not in the shock region local to Voyager 1.
It also might help if I hadn't screwed up "anomalous cosmic rays" and "termination shock particles". In my own defense, it's their freaking fault for using ACRs for cosmic rays that we understand that come from the termination shock, and TSPs for particles that don't actually come from the termination shock.
They are anomalous inasmuch as they have a different spectrum to the incredibly high energy cosmic rays that come from outside of the solar system.
Wait, I mixed up the ACRs and the TSPs, didn't I? Whoops.
In fact the unrolling of the spectrum of the ACRs was critical evidence that we had reached the TS.
Yup, I think I mixed up the termination shock particles and the anomalous cosmic rays. I thought it was the TSPs that were seen to unroll, but the ACRs didn't, but now I think it's the other way around. It's in my notes, but they're at home.
Well, I can point you to the rapporteur talk when it goes up, but unfortunately, the conference was very poorly organized (it was in Pune, India - right by Mumbai, one day after the flooding - so that might explain some of it, although Pune wasn't really hit hard) and so I have no idea when it'll be up.
Also, a lot of it is very technical - although really, it's just demonstrating that we don't understand how wimpy shocks work, much less strong shocks. The anomalous cosmic rays were a good example of "who ordered these?!"
I actually got to see this data presented at a cosmic ray conference this summer. There are a few things you have to realize:
This is the only astronomical shock we are able to study closely
There are a lot of things we don't understand about shocks
Voyager 2 is still working, with better instruments, and will reach the termination shock early
We're seeing things we never, never expected
For instance, on the last bit, we expected to see cosmic rays from the termination shock, because shocks accelerate particles. We see them. But they don't appear to be coming from the shock. They're coming from somewhere else that we don't know. We see another set of cosmic rays (with a different spectrum) that we don't understand at all - we just call them "anomalous cosmic rays."
Also, inside the heliosphere, Voyager 1 kept crossing magnetic domains (so a needle on a compass would swing back and forth) periodically. It was expected after the shock that those domain switches would keep happening, much much faster. That didn't happen. In fact, the domain switches stopped. We don't understand why. That doesn't make a lot of sense.
This is our only probe and our only example of a large astronomical shock. It's full of information about how the Universe produces such violent outbursts like supernovae, or gamma ray bursts. We need to keep studying this.
Now granted, you'll still have to haul some fuel up the elevator
Much, much less than you'd think. In addition to allowing easy mechanical (no launch shock, either!) access to space, it also gives you a huge launch boost as well.
If you climb past geosynchronous orbit, you're moving faster than orbital speed at that point. The farther out you go, the faster you're going. In the baseline designs, if you go out to the edge of the ribbon, and let go, you'll reach Jupiter with no additional thrust. Just need to time it right.
Basically, for most trips in the inner solar system (which is... primarily where we'd want to go, anyway) you'd basically need no fuel. Some for maneuvering, I'd guess, though maybe you'd try to build some sort of ramscoop or solar sail and completely avoid carrying fuel altogether.
The hard part of the space elevator is NOT the climber
There are a lot of hard parts of the elevator's "baseline design." The climber is one of them. It's not easy to make a robot that can climb 62,000 miles reliably. The first thing you have to do is make a robot that climbs at all. Then you improve its reliability a whole, whole lot - by having the robot climb a whole, whole lot, find out what fails, and improve that piece.
Besides the cable, and the robot, you also have to worry about power delivery, deployment, ribbon design (not strength). Each of those is not an easy problem. You do need to solve all of them.
Unless the Earth suddenly reverses its rotation, it's pretty easy to find a place outside of hurricane lanes.
And, probably more importantly, the anchor will most likely not be fixed. So if some freak hurricane does show up, you can just move it. The elevator will be fine. It's in orbit, you're just dragging a really tiny portion of it.
I would think that one would want the CG to be slightly on the Earth side of L1 in order to maintain some cable tension at the Moon base. Otherwise, the cable would bob around and occasionally go slack at the end, which might not be too good if a car were being loaded onto the cable at the time, etc.
Depends what you mean by "slightly". A few km, maybe? Yes. But a few km out of 53,000 is piddling. A few thousand km? No way.
If the CG is significantly past L1, the cable will bob around more than it would at L1. The cable will be taut no matter what, as things aren't perfect (the elevator will likely be off-equator, and there are tidal considerations anyhow) but you still want it basically at L1.
There're simple rules in Larry Niven's "The Integral Trees" (worth reading) about orbital motion: "in takes you east, out takes you west" and "faster moves you inward, slower moves you outward". If you move the CG out from the Moon (closer to the Earth) it moves faster. It has to. The centripetal force increases - just look at the math.
As long as the body stays along the Earth-Moon line, F_c = F_e - F_m = Gm(M_e/r^2 - M_m/(d_m-r)^2. You're past the point where F_e = F_m, because the net force is pointing towards the Earth, otherwise it wouldn't be orbiting the Earth. Past that point, decreasing r (moving towards Earth) means increasing F_c (well, its absolute value). So it starts moving towards the earth and eastward. There's an opposing force at the base (tension), but it can't propagate infinitely fast, so the cable will start orbiting around L1. The amount that it orbits is going to be related to how far off of L1 it is.
The reason you might want it a little on the Earth side (a few km or so) is because the tension at the base can't restore the center of mass when it moves towards the Moon. When it moves towards the moon (out moves you west moves you slower) it'd be like trying to keep a kite up in dying wind without moving by tugging on it.
You'd also want it a little there because when you load the elevator the center of gravity moves towards the Moon, and so you want it to be able to take that load. But you're not talking about having the CG of the elevator significantly off of L1.
Contradiction there. If you're orbiting at a larger radius, the velocity must increase if you're to have the same angular rate.
Yah, that's a typo. It should've said the velocity required to orbit at that period. See the math.
BTW, space elevators are not free bodys orbiting a planet/moon.
I can always replace the elevator with an equivalent body at the center of mass. The only thing that changes are the intraelevator forces. The center of mass of the elevator system - in the absence of other forces - will move exactly the same.
Since you're fixing the one end, you do have other forces. But since the cable's an extended body, those forces won't propagate infinitely quickly along the elevator. If you try to have an elevator with its center of mass past L1, it'll lean eastward. You'll try to pull it backwards, which will induce an oscillation in it.
You could set up a stable oscillation, but for this, the time-averaged center of mass of the object would be - guess where - L1.
Let me be a little more mathematical, just to explain it better.
L1 is often times described as "the location where the gravity of the Earth and the Moon balance each other". This isn't true. L1 is the location where F_{earth} - F_{moon} = m(v^2/r), where v = 2*pi*r/1 month. In other words, this is the position where the net force between the Earth and the Moon is the centripetal force required to orbit at the Moon's period. If you solve this, you get something like r = 324,000 km. The derivation is relatively simple, but tedious (since F_moon = G M m/(d_moon - r)^2)).
If you move closer to Earth, not only does F_earth increase, and F_moon decrease, but the velocity required to orbit at that location with that angular speed decreases. You can see that by replacing v above with 2*pi*r/T, and then the centripetal force is just 4*pi^2*m*r/T. Which means the centripetal force increases with increasing radius, and decreases with decreasing radius.
F_earth - F_moon increases with decreasing radius after the point where F_earth = F_moon, because the Earth is larger than the Moon. That point is closer to the Moon than L1, which is also obvious because F_earth is greater than F_moon at L1 - because the net force points towards the Earth, not the Moon.
So obviously, if you move closer to the Earth, you need to slow down, but you actually speed up. So you start orbiting faster than the Moon. From the lunar surface, the cable begins falling to the east (I believe).
The tension in the cable is just the opposite of the net force due to gravity - this is how the cable doesn't move. It's fixed by the position of the center of mass. It will increase as the mass moves towards Earth, but the cable will also fall.
Ahem. Yes it does. A cable from the moon to a mass at L1 will fall to the moon because the (lunar) weight of the cable will pull it down.
No, it won't. Let me get terminology a little clearer: when I said "mass at L1", I meant "center of mass at L1" (as I clarified above). If you maintain center of mass at L1, it isn't going to fall.
You can get center of mass exactly at L1 only with an infinite mass there. But obviously, you can get center of mass arbitrarily close to L1 with a large enough mass. But you'd probably end up having a few kilotons about 20,000 km past L1 or so, which is a good balance between length and mass requirement.
As you lengthen it (get closer to earth) the mass starts to get pulled toward the earth due to earth gravity.
And the center of mass moves farther out, and slows in its orbit, and falls.
Center of mass has to stay at L1. If it doesn't, it's not stationary. Which means it falls.
If you're talking about keeping the center of mass at L1, there's no real difference between putting more mass close to L1 or less mass far from L1.
No amount of mass there is a viable anchor because it's stable without a cable (so the cable would pull it down).
So is geostationary orbit. The idea is that the center of mass needs to sit at geostationary orbit for an Earth based space elevator, and at L1 for a lunar space elevator. The center of mass needs to sit at a stationary point in the system. For the Earth, we can ignore the Moon because the Earth spins very fast and much larger than the Moon, so the stationary orbit is close. The stationary orbit for the Moon is at Earth's orbit.
For the Moon, since it orbits and rotates at the same rate, the corotating threebody frame is the same as the corotating twobody frame. Which means you can also use stationary orbits in the threebody system - L1 and L2 are particularly useful here. The center of mass must be at L1, L2, L4, or L5 in order for the space elevator to remain fixed.
Obviously to be at exactly L1, the counterweight would have to be infinite mass, but if you're talking about a few thousand km past L1, it's not much of a difference.
Extend the cable closer to earth so our gravity gives a nice pull on the mass to hold the cable tight.
I don't think you understand the way space elevators work. Rotation keeps them taut. Not Earth's gravity. Earth's gravity helps in the L1 case, but it's not pulling the cable taut. It doesn't get "more taut" as you move the mass towards Earth.
Besides, what's wrong with a large mass?
The fact that you'd need to lift probably a kiloton or so of material in addition to the cable. More mass = more money.
You want the cable shorter because a longer cable is more fragile - plus there's no need to go farther than L1. Getting to L1 gets you anywhere.
I agree, if this can be done with Kevlar, someone should be trying it. Even if they just hang a mass on it as proof of concept.
It is doable with Kevlar. Easily. The main problem is that you need a freaking big counterweight, since the Moon orbits so slowly. You go to the Earth-Moon L1 point, so it's only something like 50,000 km. But you'd need a counterweight on the order of multiple thousands of kilograms.
Still pretty doable, though it's not as trivial anymore.
Slashdot Mirror
Nope.
The fact that the Sun's magnetic field is large isn't what protects us from cosmic rays. The Sun's magnetic field encourages particles to orbit the Sun. That doesn't help us. What helps is when a dipole field gets closer to you - like when the Sun sloughs off a bunch of plasma that drifts near you. Hence a Forbush decrease. What protects us on Earth is the Earth's magnetic field, and the atmosphere.
Anyway, it's relatively easy to craft magnetic fields to any shape you want. So high magnetic field on the outside, zero magnetic field on the inside. We're really good at that. And 5 tesla (50,000 gauss) should be about enough. It has been studied.
The reason it's not ideal is because cosmic rays aren't all charged. Gamma rays make up a component of solar cosmic rays, and okay, there may (should) be a few neutrons from the Sun as well (though that part is really new and not well studied).
But magnetic shielding is very actively being looked at. It's just not an easy problem - we don't have very much experience with superconducting magnets in space, for instance.
Interestingly, one of the best things about this is that you don't really have to worry about the highest energy particles which will get through. Not only is the flux far, far lower, but they deposit less energy than lower energy particles which stop in your body. So it's pretty easy to figure out how high a magnetic field you need.
And smartass comment: magnetic fields don't drop like 1/r^2. Electric fields do. For a simple magnetic dipole, the field strength drops like 1/r^3. Different configurations drop differently, as well.
AFAIK it's always more efficient to produce the electricty closer to where you consume it.
Efficient in terms of power, yes. But efficient in terms of pollution? No.
Which would you rather have: 100 million individual pollution sources, or 1000? Which do you think would be easier to maintain for pollution controls? Which do you think would be easier to improve to reduce emissions?
And of course, if you're only polluting from power plants, you can relocate the power plants to avoid smog.
That's even worse. I have an even smaller window of movement to move in.
Huh? You have the same window of motion that an analog stick has. You're just using your wrist rather than a fingertip. It's the same as a mouse, and I'd imagine you can adjust the sensitivity, but I doubt you'd need to.
I push and nothing is stabilizing the controller but my one hand.
Your hand is stabilizing it, and your fingers are pushing the buttons. That should be fine.
I dunno. I can point a mouse and click it without moving pretty easily if I'm gripping it, rather than just holding it like a mouse.
I've got a feeling that given that one of the demos is a FPS, it probably works pretty well for an FPS. It's got to work better than a controller. Not much could be worse.
when I have to aim at only my part of the screen (which will be very small mind you)?
Don't think of it as a light gun. Think of it as an analog stick. Want to look up? Tilt up. Want to look left? Tilt left. You're not aiming. It's just an analog stick.
By the way, galaxies don't move away from us faster than the speed of light.
Actually, they do. Or will. Or are. Something like that.
Really. Not kidding. What's going on is exactly what the grandparent said. As the universe expands, galaxies far away from us are moving faster and faster, because space is expanding, and if it expands at a constant rate, then as the volume increases, the speed has to increase as well.
One would expect that it doesn't expand at a constant rate, but that the rate slows down over time, because you only had an initial impulse (the Big Bang) and a constant gravitational slowing of the expansion. But that doesn't appear to be what's happening. In fact, the rate seems to be accelerating.
What this means is that eventually, galaxies will move outside of our cosmic horizon - the distance to them will be increasing so rapidly that light simply cannot reach us. Effectively, they'll be travelling faster than the speed of light with reference to us.
That would mean that in our frame of reference, galaxies move faster than light
Well, kindof. They're effectively moving away from us faster than the speed of light. Or will. See above. This means that we don't see them at all, because the distance from that galaxy to us is increasing faster than the speed of light, which means the light can never reach us.
You can read here more about it. Note what it says there though - there's no real easy way to define "relative speed" in curved spacetime, although you could just calculate the rate of change in proper distance between two objects. If you would do that, for objects outside of our cosmic horizon, they're moving faster than the speed of light. They can't send messages to us, we can't send messages to them.
OK, so I take it it was just a crappy photo? Do they not photograph well?
No - what I'm trying to say is that the main benefits of E-ink you can't see in a photo - for several reasons.
For one, dim the lights, lengthen the exposure, and any LCD screen in the world will look gorgeous, and you'll be "oh my God, that's amazing". But then when you actually get the thing, and try to work on it outside, you'll be staring at the screen from half an inch away trying to read anything, because the display isn't bright enough.
Second, you really can't photograph the main benefits, because film (and CCDs) don't have the same response as your eye. Your eye is built for huge variations in light intensity. It's more than happy to see a dark closet right beside a sunlit window, and resolve structure in both. Cameras, however, have linear response, and they simply won't be able to see huge contrast. You could throw this display out on a park bench on a bright sunny day, and you can pick it up, and it's as clear to read as indoors with a light on. The high reflectivity also means you can read it in much less light (ten times less light!) than a passively lit LCD screen (like a Palm Pilot, or a first-generation GBA).
Remember that this is essentially paper. It has roughly the same properties as paper. The display they're showing doesn't have terrific resolution, but that's not the main benefit of this device. The main benefit is the fact that it's readable in all lighting conditions that paper's readable in, which is a lot.
I know that right now, with the sun behind me in my office, I can read a piece of paper clear as day, with no effort. But my laptop's LCD is washed out a lot, and my eyes are glad when the sun goes behind a cloud.
You've always seen pretty pictures of LCDs, but you must know that they look like crap in sunlight, or if they're passively lit, they look like crap in low light. This is the one device that looks good in both.
It does for that patient.
Sigh.
Quoted "constrast ratio" for active screens is not the same as the actual viewed contrast ratio of the LCD. That's the contrast ratio of the emitted white sections over the emitted black sections. But that's not what the eye sees, because it sees "emitted+reflected". The true contrast ratio of an active LCD varies with lighting conditions. It can be very very high in dark rooms (100:1, 500:1, etc.), but will be very very low in any sort of lit room. Outside, it'll probably be near 1:1 - i.e., unviewable. Much lower than that 100:1, 500:1. More like 4:1, or lower, in normal viewing conditions.
The contrast ratio of an E-ink display is about 10:1. Moreover, the E-ink display has about a 40% reflectance (as opposed to a 4% reflectance for LCDs), which means it's much brighter too.
CRTs have the same problem. They quote a 3000:1 contrast ratio, but the black and white sections have virtually the same reflectivity, which means that that contrast ratio only applies when the light in the room is much less than the light emitted from the CRT.
If you want to compare passive and active displays, you have to do it equally. In the same viewing conditions. Most people I know don't work inside pitch black offices.
and simple lambertian demands limit the reflectivity of the white areas (its no mirror, you know?)
E-ink displays are slightly less white state reflective than newsprint, but not much (40% compared to 60%). They have a much, much higher reflectivity than LCD displays - about 10 times higher (LCDs are 4%). With that high reflectivity, it doesn't take a lot of light for an E-ink display to have a much higher contrast ratio than an active LCD.
What gives? Does the E-ink display really look so bad? Or is it just a bad photo for the dev kit?
There are a few advantages of E-ink displays over other displays, and unfortunately they're not going to really be visible in a picture. The first is contrast: the contrast can be made very, very good since the ink can be very dark, and the background very light. Much, much higher than LCDs.
The second is no backlighting. Now, this might not sound all that useful, because the first-generation GBA wasn't backlit, and that wasn't all that good, but E-ink's contrast is high enough that you don't need a backlight. Even just a small reading lamp is going to be easily enough to read by. This is the "easier on the eyes" part, and it's the one thing that current displays can't really compete with.
The third is battery life: since you don't need power to maintain the display, only to change it, the battery life is going to be measured in pages, not in time. For an e-book reader, this is perfect, because you can take as long as you want to read it. I wouldn't be surprised if a production e-book reader based on e-ink only turned on whenever you pushed a button.
There are other benefits (resolution's a biggie, but it doesn't look that great with this model, plus it's an image that's actually there, which means that it'll look good in all lighting and all angles) but I think those three are probably the biggest for the current generation.
The biggest limitation to E-ink right now is its refresh time (~ of order a second per page, or 1 fps) and its cost. But still, it's the only product which really has specifications which seriously compete with paper.
One of the other posts claimed that the termination shock is the only astrophysical shock we can study so we need to keep funding Voyager. It isn't entirely true that the termination shock between our heliosphere and the interstellar wind is the ONLY astrophysical shock we can study. A shock in a space plasma is a shock no matter where it is, and they all are pretty similar.
No. Absolutely not. The termination shock is huge. It's something like ~150-200 AU across the heliosphere. We have no idea what the structure of a shock like that is. I said it's the only astrophysical shock we can study, and I stand by that - it's the only astrophysical scale shock we can study. CMEs are far too small.
The fact that we're seeing things we completely didn't expect should tell you that. We do not understand the acceleration of particles at a shock. Coronal mass ejections happen in a few seconds. The termination shock has existed for millions of years. These are very different phenomena.
Cancelling the funding for Voyager right now is simply idiotic. We just found out that a lot of assumptions we had about the termination shock are wrong, and there's another probe heading there right now!, with more instruments! In terms of science per dollar, there is no better bet right now than funding Voyager.
It also might help if I hadn't screwed up "anomalous cosmic rays" and "termination shock particles". In my own defense, it's their freaking fault for using ACRs for cosmic rays that we understand that come from the termination shock, and TSPs for particles that don't actually come from the termination shock.
They are anomalous inasmuch as they have a different spectrum to the incredibly high energy cosmic rays that come from outside of the solar system.
Wait, I mixed up the ACRs and the TSPs, didn't I? Whoops.
In fact the unrolling of the spectrum of the ACRs was critical evidence that we had reached the TS.
Yup, I think I mixed up the termination shock particles and the anomalous cosmic rays. I thought it was the TSPs that were seen to unroll, but the ACRs didn't, but now I think it's the other way around. It's in my notes, but they're at home.
That'd be a really, really cheap levee. Funding for the Voyager probes is in the single millions.
Well, I can point you to the rapporteur talk when it goes up, but unfortunately, the conference was very poorly organized (it was in Pune, India - right by Mumbai, one day after the flooding - so that might explain some of it, although Pune wasn't really hit hard) and so I have no idea when it'll be up.
Also, a lot of it is very technical - although really, it's just demonstrating that we don't understand how wimpy shocks work, much less strong shocks. The anomalous cosmic rays were a good example of "who ordered these?!"
For instance, on the last bit, we expected to see cosmic rays from the termination shock, because shocks accelerate particles. We see them. But they don't appear to be coming from the shock. They're coming from somewhere else that we don't know. We see another set of cosmic rays (with a different spectrum) that we don't understand at all - we just call them "anomalous cosmic rays."
Also, inside the heliosphere, Voyager 1 kept crossing magnetic domains (so a needle on a compass would swing back and forth) periodically. It was expected after the shock that those domain switches would keep happening, much much faster. That didn't happen. In fact, the domain switches stopped. We don't understand why. That doesn't make a lot of sense.
This is our only probe and our only example of a large astronomical shock. It's full of information about how the Universe produces such violent outbursts like supernovae, or gamma ray bursts. We need to keep studying this.
Now granted, you'll still have to haul some fuel up the elevator
Much, much less than you'd think. In addition to allowing easy mechanical (no launch shock, either!) access to space, it also gives you a huge launch boost as well.
If you climb past geosynchronous orbit, you're moving faster than orbital speed at that point. The farther out you go, the faster you're going. In the baseline designs, if you go out to the edge of the ribbon, and let go, you'll reach Jupiter with no additional thrust. Just need to time it right.
Basically, for most trips in the inner solar system (which is... primarily where we'd want to go, anyway) you'd basically need no fuel. Some for maneuvering, I'd guess, though maybe you'd try to build some sort of ramscoop or solar sail and completely avoid carrying fuel altogether.
The hard part of the space elevator is NOT the climber
There are a lot of hard parts of the elevator's "baseline design." The climber is one of them. It's not easy to make a robot that can climb 62,000 miles reliably. The first thing you have to do is make a robot that climbs at all. Then you improve its reliability a whole, whole lot - by having the robot climb a whole, whole lot, find out what fails, and improve that piece.
Besides the cable, and the robot, you also have to worry about power delivery, deployment, ribbon design (not strength). Each of those is not an easy problem. You do need to solve all of them.
Unless the Earth suddenly reverses its rotation, it's pretty easy to find a place outside of hurricane lanes.
And, probably more importantly, the anchor will most likely not be fixed. So if some freak hurricane does show up, you can just move it. The elevator will be fine. It's in orbit, you're just dragging a really tiny portion of it.
I would think that one would want the CG to be slightly on the Earth side of L1 in order to maintain some cable tension at the Moon base.
Otherwise, the cable would bob around and occasionally go slack at the end, which might not be too good if a car were being loaded onto the cable at the time, etc.
Depends what you mean by "slightly". A few km, maybe? Yes. But a few km out of 53,000 is piddling. A few thousand km? No way.
If the CG is significantly past L1, the cable will bob around more than it would at L1. The cable will be taut no matter what, as things aren't perfect (the elevator will likely be off-equator, and there are tidal considerations anyhow) but you still want it basically at L1.
There're simple rules in Larry Niven's "The Integral Trees" (worth reading) about orbital motion: "in takes you east, out takes you west" and "faster moves you inward, slower moves you outward". If you move the CG out from the Moon (closer to the Earth) it moves faster. It has to. The centripetal force increases - just look at the math.
As long as the body stays along the Earth-Moon line, F_c = F_e - F_m = Gm(M_e/r^2 - M_m/(d_m-r)^2. You're past the point where F_e = F_m, because the net force is pointing towards the Earth, otherwise it wouldn't be orbiting the Earth. Past that point, decreasing r (moving towards Earth) means increasing F_c (well, its absolute value). So it starts moving towards the earth and eastward. There's an opposing force at the base (tension), but it can't propagate infinitely fast, so the cable will start orbiting around L1. The amount that it orbits is going to be related to how far off of L1 it is.
The reason you might want it a little on the Earth side (a few km or so) is because the tension at the base can't restore the center of mass when it moves towards the Moon. When it moves towards the moon (out moves you west moves you slower) it'd be like trying to keep a kite up in dying wind without moving by tugging on it.
You'd also want it a little there because when you load the elevator the center of gravity moves towards the Moon, and so you want it to be able to take that load. But you're not talking about having the CG of the elevator significantly off of L1.
Contradiction there. If you're orbiting at a larger radius, the velocity must increase if you're to have the same angular rate.
Yah, that's a typo. It should've said the velocity required to orbit at that period. See the math.
BTW, space elevators are not free bodys orbiting a planet/moon.
I can always replace the elevator with an equivalent body at the center of mass. The only thing that changes are the intraelevator forces. The center of mass of the elevator system - in the absence of other forces - will move exactly the same.
Since you're fixing the one end, you do have other forces. But since the cable's an extended body, those forces won't propagate infinitely quickly along the elevator. If you try to have an elevator with its center of mass past L1, it'll lean eastward. You'll try to pull it backwards, which will induce an oscillation in it.
You could set up a stable oscillation, but for this, the time-averaged center of mass of the object would be - guess where - L1.
Let me be a little more mathematical, just to explain it better.
L1 is often times described as "the location where the gravity of the Earth and the Moon balance each other". This isn't true. L1 is the location where F_{earth} - F_{moon} = m(v^2/r), where v = 2*pi*r/1 month. In other words, this is the position where the net force between the Earth and the Moon is the centripetal force required to orbit at the Moon's period. If you solve this, you get something like r = 324,000 km. The derivation is relatively simple, but tedious (since F_moon = G M m/(d_moon - r)^2)).
If you move closer to Earth, not only does F_earth increase, and F_moon decrease, but the velocity required to orbit at that location with that angular speed decreases. You can see that by replacing v above with 2*pi*r/T, and then the centripetal force is just 4*pi^2*m*r/T. Which means the centripetal force increases with increasing radius, and decreases with decreasing radius.
F_earth - F_moon increases with decreasing radius after the point where F_earth = F_moon, because the Earth is larger than the Moon. That point is closer to the Moon than L1, which is also obvious because F_earth is greater than F_moon at L1 - because the net force points towards the Earth, not the Moon.
So obviously, if you move closer to the Earth, you need to slow down, but you actually speed up. So you start orbiting faster than the Moon. From the lunar surface, the cable begins falling to the east (I believe).
The tension in the cable is just the opposite of the net force due to gravity - this is how the cable doesn't move. It's fixed by the position of the center of mass. It will increase as the mass moves towards Earth, but the cable will also fall.
Ahem. Yes it does. A cable from the moon to a mass at L1 will fall to the moon because the (lunar) weight of the cable will pull it down.
No, it won't. Let me get terminology a little clearer: when I said "mass at L1", I meant "center of mass at L1" (as I clarified above). If you maintain center of mass at L1, it isn't going to fall.
You can get center of mass exactly at L1 only with an infinite mass there. But obviously, you can get center of mass arbitrarily close to L1 with a large enough mass. But you'd probably end up having a few kilotons about 20,000 km past L1 or so, which is a good balance between length and mass requirement.
As you lengthen it (get closer to earth) the mass starts to get pulled toward the earth due to earth gravity.
And the center of mass moves farther out, and slows in its orbit, and falls.
Center of mass has to stay at L1. If it doesn't, it's not stationary. Which means it falls.
If you're talking about keeping the center of mass at L1, there's no real difference between putting more mass close to L1 or less mass far from L1.
No amount of mass there is a viable anchor because it's stable without a cable (so the cable would pull it down).
So is geostationary orbit. The idea is that the center of mass needs to sit at geostationary orbit for an Earth based space elevator, and at L1 for a lunar space elevator. The center of mass needs to sit at a stationary point in the system. For the Earth, we can ignore the Moon because the Earth spins very fast and much larger than the Moon, so the stationary orbit is close. The stationary orbit for the Moon is at Earth's orbit.
For the Moon, since it orbits and rotates at the same rate, the corotating threebody frame is the same as the corotating twobody frame. Which means you can also use stationary orbits in the threebody system - L1 and L2 are particularly useful here. The center of mass must be at L1, L2, L4, or L5 in order for the space elevator to remain fixed.
Obviously to be at exactly L1, the counterweight would have to be infinite mass, but if you're talking about a few thousand km past L1, it's not much of a difference.
Extend the cable closer to earth so our gravity gives a nice pull on the mass to hold the cable tight.
I don't think you understand the way space elevators work. Rotation keeps them taut. Not Earth's gravity. Earth's gravity helps in the L1 case, but it's not pulling the cable taut. It doesn't get "more taut" as you move the mass towards Earth.
Besides, what's wrong with a large mass?
The fact that you'd need to lift probably a kiloton or so of material in addition to the cable. More mass = more money.
You want the cable shorter because a longer cable is more fragile - plus there's no need to go farther than L1. Getting to L1 gets you anywhere.
I agree, if this can be done with Kevlar, someone should be trying it. Even if they just hang a mass on it as proof of concept.
It is doable with Kevlar. Easily. The main problem is that you need a freaking big counterweight, since the Moon orbits so slowly. You go to the Earth-Moon L1 point, so it's only something like 50,000 km. But you'd need a counterweight on the order of multiple thousands of kilograms.
Still pretty doable, though it's not as trivial anymore.