So does anybody have a good,cheap,quick (pick two) primer on Quantum Physics? Something that can explain what we do know, along with the outstanding issues that we don't know?
Not offhand, but a couple of good places to look would be to check the various online bookstores for quantum mechanics textbooks to see what are recent/available and get good reviews, and course pages at various universities to see what textbooks they use and what online resources they have available. Expect to pay $100 or so for a good textbook on any given topic.
There are a number of online physics tutorials that should cover some of it, but I'm afraid I don't know where they are.
Thinks like wikipedia and everything2 have at least an overview, but detail will be quite spotty.
Then find out that by introducing the patch to fix one problem, you find another ( er... what exactly is dark matter? Anyone? Please? )
Some of it's ordinary matter that we can't see. This includes clouds of dust and gas that aren't illuminated by nearby stars, and "compact halo objects" like brown dwarfs, cometary bodies, and so forth that aren't tied to a given star.
Some of it (considerably more) is matter that doesn't interact via EM. Click through my user info to my other posts in this thread for a list of reasons why we think this exists in quantity. Neutrinos are certainly part of this mass - we know they have mass, and they only interact through the weak nuclear force. We know they're produced in great quantity by stars and that they were produced in even greater quantity during the nucleosynthesis period following the big bang. Neutrinos are energetic enough to fall into the "hot dark matter" category, which among other things means they aren't going to be gravitationally bound even by a galaxy.
We have considerable circumstantial evidence for at least one other type of particle that doesn't interact via EM - the Least-massive Supersymmetric Particle. The standard model of particle physics - which explains a very large variety of observations of the properties of atoms and of the other forces in the universe - is like a puzzle with one piece missing. We haven't found that piece, but we know it's there and we know roughly what shape it is. There are patterns of symmetry between the particles in the standard model, and one of these symmetries ("super-symmetry") predicts the existence of more particles with certain properties. Most of these will be unstable, but the lightest will be stable because it won't have any easy decay paths. This particle's mass isn't known, but we do know that it's quite large (otherwise we would have produced it in other experiments). This particle is a strong candidate for "cold dark matter", matter which moves slowly enough to clump around galaxies.
In summary, while we don't know everything about dark matter, there _is_ more evidence for it than just idle speculation (detailed in my other posts in this article), and we _do_ know some of its properties.
Does this help answer your "what exactly is dark matter" question?
I've seen a few references to a theory of a final "big rip," in which everything (even atoms) are torn apart by the expansionistic force.
A black hole is, for practical purposes, a single particle, defined only by the parameters an elementary particle has, and maybe not even all of those (I seem to recall lepton and quark numbers not having to be conserved). This suggests that they would not be torn apart by the kind of expansion you describe.
On the other hand, it sounds like you're describing a model where the rate of expansion increases to arbitrarily high values. In this scenario, no region outside the hole would be safe (even right next to the horizon).
I'd take such a model with a grain of salt until strong evidence came up to support it, though.
BZZZZT! Dark matter has never been observed directly. Observations have been made of motions (?) that could be explained by the presense of matter other than what we can see. Dark matter and energy are not the only possible explanations. The motions haven't been observed directly either, but infered from other things (red shift perhaps) and I think there are some other assumptions baked in there too.
We've known about the galactic rotation problem for a long time. Dark matter explains it very well (if you assume a more or less uniform halo of matter that we can't see, rotation numbers work out almost exactly right). By coincidence, this turns out to bring the universe a lot closer to the mass needed for a flat universe. By coincidence, models of the early universe require lots of non-EM-interactnig matter to be present in order for the nucleosynthesis period to produce the ratios of elements that we observe. Still circumstantial evidence, but when you have multiple observations pointing towards something, it starts looking more likely.
We've also seen lensing of distant galaxies due to non-luminous clumps of matter. If I recall correctly, we'd even managed to map the density distribution of these clumps, which ended up being consistent with a dark matter galactic halo.
Motions of nearby stars are observed by looking at Doppler shifts in spectral lines in them. A radar gun works on the same principle, so we're on pretty secure ground here.
Believe me, people are looking for gravitational explanations too. They just haven't found a satisfactory one yet.
The universe is expanding right near the minimum speed to prevent collapse. That seems like a strange coincidence, there must be some reason it's so close.
The universe we see is very close to being flat (balanced between open and closed). This isn't a "strange" coincindence - this is like being struck by lightning every day at the same time for a year. If the universe was even a little bit off from flatness, this discrepancy would amplify itself. So, there _is_ something that either keeps the universe flat or that forced the early universe to be insanely close to perfectly flat.
The simplest explanation found so far is that there's more matter present than we're seeing. We know from other sources (nucleosynthesis period and galactic rotation) that there's some additional matter already, which lends support to dark matter as a solution to the flatness problem.
Dark energy is a whole other story, but that's beyond the scope of this reply. Short version is that it's another variable that was ignored/set to zero early on and that we can now measure well enough that we can no longer pretend it doesn't exist.
Considering the ideas discussed and the nature of the theories, how do you determine who are the "real physicists" and who are the "quacks"?
Useful litmus tests for a Real Theory (or more accurately, model; it only gets to be a theory after being exhaustively tested):
Makes predictions that can be tested (at least in principle).
I.e., there are experimental regimes where this model and the "conventional" model produce different predictions, and we can at least in principle examine these experimental regimes directly or indirectly.
Does not cause more problems than it solves.
There is no shortage of proposed changes to gravity that attempt to explain the discrepancies in galactic rotation, but most of them either still leave problems with galactic rotation, _cause_ discrepancies with other observations, or both. That's one of the reasons why dark matter is still a popular explanation (there is other support for dark matter too).
Adding more parameteres that have to be set to specific values with no apparent physical cause also counts as "causing problems".
Is published in a well-respected and peer reviewed journal.
This says that other people in the field think it should be at least considered.
Is based on the work of other researchers, and is cited as a reference by other researchers.
This shows even more strongly that other people in the field think it should be considered as a possibility.
Useful litmus tests for a quack:
Includes no references in papers, or only references to their own past work.
Tests cosmological principles using tabletop equipment built on a shoestring budget.
Sometimes this can be done, but usually it can't. When it can be done, there will be examples in mainstream literature of similar experiments, so it's easy to check. A valid paper will cite them already.
Exhibits poor knowledge of how existing theories and models work.
Lots of the "theories" proposed by slashdot posters fall into this category. They explain a perceived flaw in the conventional models that are actually the result of a dumbed-down explanation.
Is self-educated/no formal training in the field they're claiming a breakthrough in.
Sometimes self-educated people can do great things. Usually self-educated researchers just don't understand the field as well as they think they do.
Claims that their ideas are being repressed by the scientific community.
Sometimes this happens. Far more often, though, they're being ignored for good reason.
(1) the universe is not expanding at the speed of light (I think that it is less)
All parts of the universe that we can see are moving slower than light relative to us, because we can't see the parts that are moving faster (this is the "horizon" of the observable universe).
We can't determine anything for certain about the parts of the universe that have passed our observation horizon, but if the rest of the universe has the same distribution of velocities the observable part does, then the universe is infinite or near-infinite in extent and the parts past the horizon are moving faster than light relative to us.
As others have pointed out, this can be thought of as being due to motions of space itself, so there is no violation of relativity from relative motion greater than the speed of light. It just means that we can't observe anything past the horizon. The same thing happens at the event horizon of a black hole. One way of thinking of a black hole is to consider space itself to be "sucked in" at the speed of light at the horizon, which is why a photon fired from the horizon will never reach a distant observer despite travelling at C the whole time (ObDisclaimer about this being only one of many possible metrics for describing black holes, and one that can produce misleading conclusions under other circumstances, yadda yadda).
I am eagerly awaiting the next annoncement where someone again finds evidence to refute the dark matter claims. It seems like the science; "Dark Matter is like this" - "No, it can't be, actually it's like that".
The only articles I've seen that make statements like that are the commentaries on the commentaries on the dumbed-down press releases on the actual publications.
What's actually been happening is more along the lines of:
"There's a discrepency between galactic models and observations. What did we get wrong in the model/what needs to be added?"
"Maybe A? B? C? D?" "Let's try to test them and see."
"Not A, not D, but maybe B, maybe C." "What kind of B or C? B1? B2? B3?" "Let's try to test them and see."
"Our models only work if we have B _and_ C, and we've ruled out C1 and C2, but C3 still works."
"What kind of C3?"
"New observations show a new effect in addition to the old one. How do we explain it?"
"Maybe E? Maybe F?"
[Etc.]
This is a process of examining many possible explanations, and weeding out the ones that don't work until we have reasonable confidence that the ones left _do_ work.
We've gone from "galactic rotation doesn't match models based on stars alone, what could be causing this?" to "we know that there's about X amount of normal matter we aren't seeing, Y amount of abnormal matter that we aren't seeing, and that the properties of the abnormal matter fall somewhere in this range (that's wide but being narrowed)". There's surprisingly little backtracking. Tests that detect or fail to produce evidence for dark matter of various types all help to increase our understanding of what dark matter's actual properties are.
As for dark energy, if anything, it would be surprising if something like it _didn't_ exist. We already knew that a scalar field with similar properties was likely present in the early universe, and several models proposed universes where the _absence_ of the field was only a local effect. Even relativity contained a similar type of effect that was set to zero a priori as opposed to forced to zero through a mechanism inherent in the model.
We're still sifting through the myriad of possibilities, but we certainly are learning something each step of the way.
Not in this case. They just have to track what secret they use, possibly changing it regularly, as all they use it for is generating and then verifying a hash.
Not quite. It's used for generating and verifying an arbitrary identification string. In this case, you're right, the recipient doesn't need to know the secret (the hash is just a glorified request tracking number, because Slashdot can't reverse it easily either without lots of brute force). If it was intended to let the recipient confirm that they were indeed the IP being blocked, the recipient would need to know the secret to duplicate the hashing (not needed in this case, because they gave the IP later in the email).
Yeah, so, that's why you have a secret that gets added to the hash. You don't just do MD5(IP) since then anyone can do a brute force attack. MD5(secret + IP) would be much better.
The problem is that this secret must be transferred from one party to the other before it can be used. That makes it much less secretive. A discussion of the key exchange problem (of which this is a subset) is beyond the scope of this thread.
In the email quoted by the original poster, no secret value was cited. So if he hadn't blanked out the md5 (as the original AC suggested), anyone who wanted to would have his IP address right now.
Not that that's earth-shatteringly catastrophic, but my point still holds in the original context.
Something fairly nasty that they will do, is automatically debit your account and pay any company who runs through a check-by-phone type transfer. All that is needed is the information on the bottom of your check and no authorization whatsoever. It's happened to me 3 times now, each time for somewhere between $250-$500.
It occurs to me that the government sends you your tax return by cheque...
I'd be quite amused to see what happens to someone unwise enough to try this kind of fraud on them (not that it would be likely to work, but I doubt they'd like it any better if it failed).
Doesn't 100Mb flash using 180M transistors work out to 1.8 transistors/byte? I'm still just a student, but according to my intro to ECE class, even storing one bit takes more than 1.8 transistors...
Most modern flash memory uses multi-level storage, allowing several bits per cell (I'd known about 4 levels (2 bits), another poster mentioned 8 levels (3 bits)). Storage still only requires one transistor.
The way it works is that you have a FET with a floating gate. In "write" mode, you apply a high voltage to the non-floating gate to drive charge either in or out through the thin oxide layer separating the gate and the body. The charge on the floating gate (which is between the sense gate and the body) ends up effectively changing the transistor threshold voltage of the transistor. When the transistor is turned on by the sense gate, you get an amount of current that varies depending on the amount of charge on the floating gate.
Other types of flash memory exist. This is just one of the more common ones.
As for storing single bits, the standard SRAM cell has 6 FETs (two inverters, cross-coupled, and two readout FETs to connect the inverter outputs to a differential read/write bus). A DRAM cell, however, just has one transistor, which connects the read/write bus to a storage capacitor. Among other things, this means that DRAM reads are destructive (capacitor is discharged on to the read/write bus; this disturbance is amplified, driving the bus back to the rail voltage and re-charging the cell's storage capacitor).
A lower core clock can save you a lot... bot financial and in energy. Raising the clock rate on a chip will increase its energy usage exponentially.
[Rant]Why, oh _why_, do people keep horribly abusing the word "exponentially"?[/Rant]
Power goes up in direct proportion to the clock rate. This is a "linear" relation. If it was really "exponential", we'd be stuck running 10 MHz processors because anything else would melt.
For the really pedantic, the way to compute dynamic power dissipation is to figure out how much capacitance you have on nodes that are being switched, what fraction of the time they're being switched, the amount of energy required to switch a given amount of capacitance (depends on signal voltage), and the frequency, and multiply these together:
P = (1/2) * Vdd^2 * C_node * N_nodes * transitions_per_clock * f
The only thing that's _not_ linear is the power-vs-voltage relation, and that's _quadratic_. Anything "exponential" sucks a whole lot worse.
[ObDisclaimer about clock feedthrough, but that's linear with frequency and capacitance to.]
Memory is "large, fast, densely-packed - pick _one_".
Got ahead of myself.
What I'm trying to say is that any large SRAM array will be slower than a small SRAM array, and neither will have very high capacity. A DRAM array has high capacity, but is horribly slow. So-called "single transistor SRAM" is actually DRAM with a cache tacked on.
For decades now, memory frequency scaling has lagged that of the microprocessor. Although there has been some great strides recently, latency is still rearing its ugly head. External DRAM is too electrically distant to remain at the heart of any high-performance system.
Once we get processor and memory combined, we'll see performance increasing by several orders of magnatude.
This idea has been around for what is almost certainly longer than either of us have been alive. It turns out that there are problems.
The main problem is that no matter how much memory a system has, we find ways to use it. In the time I've been using computers, memory size has gone up four orders of _magnitude_, and I'm sure the greybeards listening will top that. The processor sitting in your machine right now has more on-die memory (the cache) than, say, an early XT had, but the tasks you're running have a memory footprint too large to fit. This is the price for being able to _do_ more than you could do on that old XT.
Another problem is with the structure of memory itself. You've heard of "fast, cheap, good - pick two"? Memory is "large, fast, densely-packed - pick _one_". The reason why integrated logic/DRAM processes tend to do one or the other badly is that DRAM and logic have to optimize transistor characteristics for exactly opposite things (high "on" current for logic, low leakage current for DRAM). Among other things, this means that DRAM is either slow or very power-hungry. SRAM is bulky no matter what you do - it's the cost of playing, when you have six transistors instead of one. Any kind of large RAM array is slow no matter what you do - you have to propagate signals across a huge structure instead of a smaller one.
The solution to date has been a hierarchical cache system, where small, fast, on-die memory is accessed whenever possible, and when that overflows, larger, moderately fast, on-die memory, and when that fails, DRAM. This works amazingly well, giving you almost all of the benefits of fully on-die memory for problems that fit in cache. Problems that don't fit in cache won't fit in on-die memory, so going with an on-die implementation doesn't help for them.
Progress in improving memory response times is made in two ways. The first is to use a better cache indexing algorithm that is less suceptible to pathalogical situations. In the simpler indexing schemes, you can end up with situations where a short repeating access pattern can hammer on the same small set of cache blocks, causing cache misses even when there's plenty of space elsewhere. Higher associativity and tricks like victim caches reduce this problem. Techniques like a "preferred" block in a set reduce the time penalty for high associativity, and techniques like content-addressable memory reduce the power penalty. This is still a field of active research - build a better cache, and you get closer to a system that _acts_ as if it has all memory on-die.
The second way of improving memory subsystem performance is to use memory speculation. This involves either figuring out (or even guessing) what memory locations are going to be needed and preemptively fetching their contents, or taking a guess at the value that will be returned by a memory fetch before the real result comes in. In both cases, you're masking most of the latency of the memory access, while paying a price for failed speculations (either in higher memory _bandwidth_ required, or in power for speculated threads that have to be squashed). Build a better address and data speculation engine, and you'll again approach performance of an impossible all-on-die-and-fast system.
In summary, it turns out that putting all of the memory of a general-purpose system isn't practical now and won't be as long as requirements for memory keep increasing. However, caches already give you performance approaching this for problems tha are small enough to _fit_ in on-die memory, and cache technology is constantly being improved. This is where effort should be (and is) going.
Let me clarify. Your conflating earth's magnetic shielding with shielding using matter. In the case of earth (the one example I was using), a smaller magnetic field than present will always be worse--and that's what we are talking about here, earth and its magnetic field.
He is using high-density shielding as an analogy to illustrate why a simplistic evaluation does not always give the correct result. Nowhere did he say that it was _directly_ analogous to the magnetic field question, and from what I can see from the first post he was quite careful _not_ to take a position on the original article's claims at all. The claim was that it couldn't be dismissed out of hand without analysis.
In summary, you are picking a nit that isn't there:).
Re:I guess light travels more slowly than I rememb
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· Score: 2, Interesting
Score one for a Robert L. Forward fan. Not the best in plot and character development but nifty science.
I actually doubt that the Forward scheme for sail decelleration will be used. The problem is that you need an array with an aperture size large enough to hit the primary sail at destination range, instead of just 1 LY or so (distance at the end of the boost phase). This makes it a lot more expensive to build.
You also end up having to use a truly huge primary sail (so that it can focus on the secondary sail at about 1 LY range at the end of the decelleration phase), and keep it perfectly aligned optically during decelleration.
A maser-driven craft with an active-antenna mesh that could do phase-shifting as the primary sail might be able to do this, but primary sail size and maser array size become prohibitive.
Fast-flyby probes are much easier to construct and boost, so I think they're more likely to be implemented if a sail scheme is used at all.
I guess light travels more slowly than I remember.
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· Score: 3, Insightful
to find another planet. 150,000,000 years to get to it. Don't forget that we are seeing things as they used to be! discovering other planets is only has good as our ability to get there, which is nil. Not to mention that they probably arn't even there anymore.
You do realize that with a detection range of a few dozen to a few hundred light-years, we'll be seeing planets as they were at most a few dozen to a few hundred years ago, not hundreds of millions of years, right?
A laser boosted sail-probe could reach a nearby star system ( 10 LY) within one human lifetime. It would be impractial to send one big enough to carry humans, but an automated flyby survey would definitely be feasible.
I've heard of the inferometry plan before - it's basically a fleet of 7 - 11 satellites flying in near-perfect line abreast formation. That coupled with a lot of image processing gives the effect of a radio telescope with a dish the size of the formation.
Close. A radio interferometric telescope works like this, because we can record and timestamp radio signals with timing precision much finer than their period (typically nanosecond-range and longer). An optical interferometric telescope has to actually bring all of the gathered light to one place and do interference directly, as our electronics aren't good enough to do direct signal sampling, and won't be any time soon (timing precision needed is on the order of femtoseconds for near-IR, and still tens to hundreds of femtoseconds for thermal IR).
This requires _extremely_ good station-keeping for the telescopes, but this is a manageable problem (especially since you don't have to worry about as many vibration sources as you do for earth-based interferometric telescopes).
Googling for "astronomy" and "optical interferometer" will get you links for the interferometric telescopes that have been built to date. Interesting stuff.
Is it possible that advanced civilizations might produce something similar to superconductors but on an EM basis? In other words, the outer shell of their Dyson spheres might mop up the rest of the emissions so that there is nothing for us to detect?
The problem is that no matter what you do, power that is produced inside the sphere ends up as heat eventually. If it flows out through the outermost sphere, the sphere radiates in a detectable band. If you somehow prevent radiation from the outermost sphere, the inside of the system heats up (and keeps heating up, as the central star keeps pumping out energy).
In practice, there is no known way to keep the sphere from radiating in a steady-state system. You could dump all of your emissions as a narrow beam in one direction, but that would only make you _less_ detectable, not invisible. You could keep a pet black hole and dump thermal emissions into it, but a black hole with an event horizon large enough to be useful for this task would weigh as much as the galaxy.
So, if a dyson existed, it would be visible, unless currently-known physics is grossly incorrect.
(And if you're proposing changing physics, it's easier just to propose a change that removes the need for dark matter:)).
The other response mentioned gravity. We are currently doing experiments with gravity. Perhaps in the future we'll figure out how to contain it?
Gravity does not appear to be something that can be contained, even in principle. One of the particle physics professors in the audience can give you a more detailed explanation of why than I can. Short version is that it's just the nature of the kind of field we think gravity is.
A shell of negative mass around the system (of magnitude equal to the system's mass) would remove its gravitational effects, but negative mass is almost certainly not compatible with the universe as we know it. This falls into the "changing physics" category:).
To sum up, while it's possible to make dyson spheres that are _difficult_ to detect, they'd still be detectable, especially if present in great enough number to explain the baryonic component of dark matter (non-baryonic component can't be explained by dyson spheres, detectable or not).
I was thinking about the retroreflector a bit, and then I suddenly realised you could also put it on the laser installation and personell. That should be good enough to rereflect away any laser light bounced back to the laser and personell. Thus making the entire scheme useless in killing a laser system.
The laser's still bright enough to kill the missile with or without the retroreflective coating (reflection isn't perfect). That means it's bright enough to harm itself, with or without a reflective or retroreflective coating on the installation.
Putting a reflective or retroreflective coating on the laser installation turns out to have other drawbacks too, though it's probably still tolerable if your laser is in an area you control.
You know, the "other kenetic weapons" thing always bothered me about films like starwars. Why didn't they equip all those fancy robots with things like rocket launchers to take out the Jedi? I mean, load of good your lightsabre would do you then.
Because it wouldn't have made as fun a movie;). FWIW, the d20 RPG version of Star Wars shows much the same effect - while a Jedi can send blaster bolts bouncing away, you can really ruin one's day with a flame thrower or a grenade. FWIW also, the "Heavy Gear" RPG seems to have one of the saner approaches to weapons technology I've seen yet - most of their weapons are kinetic, and their robots run on diesel-type fuel.
In any case, and back to the question at hand, wouldn't a painted missle be invisible if it was reflective, as the "paint" would just be reflected laser?
It's a lot easier to track a missile if it's glowing brilliantly with reflected laser light than if you have to pick its reflected sunlight out of a cluttered background (or even hazy sky). Use a pulsed laser and it can be thousands of times brighter than anything else in the scene while the laser is on, while keeping average power reasonable. Your guidance system looks for something flashing very brightly very briefly, and locks on to it.
I don't mean to sound crass, but wouldn't the Military have thought of things like, "Well, they'll just put mirrors on the missles if we attack them with light, Sir".
They've thought of it, they just consider it an acceptable inefficiency. Especially for missiles bought off the shelf, you aren't going to get much back-scatter, and they'll absorb enough energy to go "boom" satisfactorily. Even a highly-reflective missile would be destroyed by a powerful enough laser. The problem is that it means a more expensive laser. For anti-guerrilla applications where you're trying to protect a small number of known targets and are willing to spend lots of money to do it, it works fine.
It just won't be terribly cost-effective for other scenarios.
And missiles or other devices custom-designed to foil the system could damage either the expensive laser system or the personnel running it for a cost that's probably less than the cost of the laser.
Would the reflective material hold up well to such pressures as take-off and super-sonic speeds?
It wouldn't have to. You'd coat it with a more durable covering that's transparent (or at least "transparent enough") to the laser's light for the retroreflector to send an uncomfortable amount of energy back to the sender before the covering degrades, the missile is destroyed, or what-have-you.
For a missile that's just shiny enough to resist being damaged a bit longer, sure, it'll work. Give it a polished aluminum or chrome steel skin. Just watch out for heating near the front of the missile if you're going at high speed (but these missiles don't).
Would mirrors on the outside disrupt internal guidance systems?
No. A self-guided missile's optical window has an unobstructed view of the target. The coating would be on the missile. It would only provide visual clutter if the missile could see itself, which it shouldn't be able to. If it can, then it already has clutter there, so no further loss.
What about something as simple as a cloudy sky?
Unless clouds are a lot more solid than I'm used to, the missile won't care about them. The laser trying to shoot it down will:). If the missile is retro-reflecting the light back to the laser, it'll be attenuated by haze and so forth, but if the laser can get enough energy to the missile to damage it, chances are the beam path is clear enough for the missile to send enough back to be unhealthy too.
If you're talking about the "paint and shoot" scheme, you'd probably still have more than enough light from the missile for the shell to track for intercept. Only concern is the beam itself being visible due to scatter
aperture of less than a metre. Even in near-IR, that gives it a divergence of about one part in a million, meaning a kilometre away the spot is going to be a metre or two wide.
Don't mind me, I just tend to mess up math past my bedtime...
At 1.0e-6 divergence, a 1m beam won't diverge significantly over 1 km. However, it's still a 1m beam.
Try to make it narrower, and it diverges. Widen it enough to not diverge much, and it's big enough that you won't be able to do fine targetting on the missile.
My point holds, even if my first try at the math didn't:).
Someone will probably chime in now suggesting a 1m aperture that focuses to a much narrower beam waist, but there is no way in heck you're getting optics that good (_adaptive_ optics that good) for a weapon that has to operate on the field, in real-time.
Missles can't be completely covered in any material. They require propulsion and lots of other things. Depends how accurate this laser is, that's the real "depends".
Diffraction limits the degree to which the laser can be focused. The fixed-site version of THEL (the big, stationary version) has an aperture of less than a metre. Even in near-IR, that gives it a divergence of about one part in a million, meaning a kilometre away the spot is going to be a metre or two wide. Farther away, bigger area painted. Put retro-reflective material on any reasonable fraction of the missile's surface, and you'll get enough retro-reflected light to be decidedly unhealthy for anything near the THEL system. The missile will certainly still be destroyed, but THEL's a lot more expensive than the missile.
You don't need anything complicated for the reflector. Stamp a cube pattern into the missile's surface and plate it with aluminum (or gold, if you want the extra fraction of a percent IR reflectivity), and coat it with something IR-transparent to avoid wrecking the missile's aerodynamics. Poof; laser-hostile missile.
Laser weapons are lousy on the battlefield for a number of reasons. Mainly, the problem is that they're very power-hungry even for relatively efficient versions, they need to have massive overkill in order to be able to harm reflective targets, they need to have even more massive overkill in order to be able to harm a target despite having a beam larger than the target at any significant range, and it's easy to retro-reflect them and ruin the day of any poorly-shielded operator of the laser weapon.
Guns (and other kinetic weapons) are very efficient to deploy and harder to shield against.
The best really effective anti-missile system would probably be a system that used a laser just as a designator to paint the missile, and a gun firing "guided bullet" type shells that adjusted their course to intercept the painted missile (targetting is the main problem with trying to shoot down a missile with kinetic weapons). A laser only works if you don't mind the massive overhead (e.g. if you have a lot more resources in the field than your opponent).
Slashdot Mirror
So does anybody have a good,cheap,quick (pick two) primer on Quantum Physics? Something that can explain what we do know, along with the outstanding issues that we don't know?
:).
Not offhand, but a couple of good places to look would be to check the various online bookstores for quantum mechanics textbooks to see what are recent/available and get good reviews, and course pages at various universities to see what textbooks they use and what online resources they have available. Expect to pay $100 or so for a good textbook on any given topic.
There are a number of online physics tutorials that should cover some of it, but I'm afraid I don't know where they are.
Thinks like wikipedia and everything2 have at least an overview, but detail will be quite spotty.
Good luck
Then find out that by introducing the patch to fix one problem, you find another ( er... what exactly is dark matter? Anyone? Please? )
Some of it's ordinary matter that we can't see. This includes clouds of dust and gas that aren't illuminated by nearby stars, and "compact halo objects" like brown dwarfs, cometary bodies, and so forth that aren't tied to a given star.
Some of it (considerably more) is matter that doesn't interact via EM. Click through my user info to my other posts in this thread for a list of reasons why we think this exists in quantity. Neutrinos are certainly part of this mass - we know they have mass, and they only interact through the weak nuclear force. We know they're produced in great quantity by stars and that they were produced in even greater quantity during the nucleosynthesis period following the big bang. Neutrinos are energetic enough to fall into the "hot dark matter" category, which among other things means they aren't going to be gravitationally bound even by a galaxy.
We have considerable circumstantial evidence for at least one other type of particle that doesn't interact via EM - the Least-massive Supersymmetric Particle. The standard model of particle physics - which explains a very large variety of observations of the properties of atoms and of the other forces in the universe - is like a puzzle with one piece missing. We haven't found that piece, but we know it's there and we know roughly what shape it is. There are patterns of symmetry between the particles in the standard model, and one of these symmetries ("super-symmetry") predicts the existence of more particles with certain properties. Most of these will be unstable, but the lightest will be stable because it won't have any easy decay paths. This particle's mass isn't known, but we do know that it's quite large (otherwise we would have produced it in other experiments). This particle is a strong candidate for "cold dark matter", matter which moves slowly enough to clump around galaxies.
In summary, while we don't know everything about dark matter, there _is_ more evidence for it than just idle speculation (detailed in my other posts in this article), and we _do_ know some of its properties.
Does this help answer your "what exactly is dark matter" question?
I've seen a few references to a theory of a final "big rip," in which everything (even atoms) are torn apart by the expansionistic force.
A black hole is, for practical purposes, a single particle, defined only by the parameters an elementary particle has, and maybe not even all of those (I seem to recall lepton and quark numbers not having to be conserved). This suggests that they would not be torn apart by the kind of expansion you describe.
On the other hand, it sounds like you're describing a model where the rate of expansion increases to arbitrarily high values. In this scenario, no region outside the hole would be safe (even right next to the horizon).
I'd take such a model with a grain of salt until strong evidence came up to support it, though.
BZZZZT! Dark matter has never been observed directly. Observations have been made of motions (?) that could be explained by the presense of matter other than what we can see. Dark matter and energy are not the only possible explanations. The motions haven't been observed directly either, but infered from other things (red shift perhaps) and I think there are some other assumptions baked in there too.
We've known about the galactic rotation problem for a long time. Dark matter explains it very well (if you assume a more or less uniform halo of matter that we can't see, rotation numbers work out almost exactly right). By coincidence, this turns out to bring the universe a lot closer to the mass needed for a flat universe. By coincidence, models of the early universe require lots of non-EM-interactnig matter to be present in order for the nucleosynthesis period to produce the ratios of elements that we observe. Still circumstantial evidence, but when you have multiple observations pointing towards something, it starts looking more likely.
We've also seen lensing of distant galaxies due to non-luminous clumps of matter. If I recall correctly, we'd even managed to map the density distribution of these clumps, which ended up being consistent with a dark matter galactic halo.
Motions of nearby stars are observed by looking at Doppler shifts in spectral lines in them. A radar gun works on the same principle, so we're on pretty secure ground here.
Believe me, people are looking for gravitational explanations too. They just haven't found a satisfactory one yet.
The universe is expanding right near the minimum speed to prevent collapse. That seems like a strange coincidence, there must be some reason it's so close.
The universe we see is very close to being flat (balanced between open and closed). This isn't a "strange" coincindence - this is like being struck by lightning every day at the same time for a year. If the universe was even a little bit off from flatness, this discrepancy would amplify itself. So, there _is_ something that either keeps the universe flat or that forced the early universe to be insanely close to perfectly flat.
The simplest explanation found so far is that there's more matter present than we're seeing. We know from other sources (nucleosynthesis period and galactic rotation) that there's some additional matter already, which lends support to dark matter as a solution to the flatness problem.
Dark energy is a whole other story, but that's beyond the scope of this reply. Short version is that it's another variable that was ignored/set to zero early on and that we can now measure well enough that we can no longer pretend it doesn't exist.
Useful litmus tests for a Real Theory (or more accurately, model; it only gets to be a theory after being exhaustively tested):
I.e., there are experimental regimes where this model and the "conventional" model produce different predictions, and we can at least in principle examine these experimental regimes directly or indirectly.
There is no shortage of proposed changes to gravity that attempt to explain the discrepancies in galactic rotation, but most of them either still leave problems with galactic rotation, _cause_ discrepancies with other observations, or both. That's one of the reasons why dark matter is still a popular explanation (there is other support for dark matter too).
Adding more parameteres that have to be set to specific values with no apparent physical cause also counts as "causing problems".
This says that other people in the field think it should be at least considered.
This shows even more strongly that other people in the field think it should be considered as a possibility.
Useful litmus tests for a quack:
Sometimes this can be done, but usually it can't. When it can be done, there will be examples in mainstream literature of similar experiments, so it's easy to check. A valid paper will cite them already.
Lots of the "theories" proposed by slashdot posters fall into this category. They explain a perceived flaw in the conventional models that are actually the result of a dumbed-down explanation.
Sometimes self-educated people can do great things. Usually self-educated researchers just don't understand the field as well as they think they do.
Sometimes this happens. Far more often, though, they're being ignored for good reason.
I hope this helps.
(1) the universe is not expanding at the speed of light (I think that it is less)
All parts of the universe that we can see are moving slower than light relative to us, because we can't see the parts that are moving faster (this is the "horizon" of the observable universe).
We can't determine anything for certain about the parts of the universe that have passed our observation horizon, but if the rest of the universe has the same distribution of velocities the observable part does, then the universe is infinite or near-infinite in extent and the parts past the horizon are moving faster than light relative to us.
As others have pointed out, this can be thought of as being due to motions of space itself, so there is no violation of relativity from relative motion greater than the speed of light. It just means that we can't observe anything past the horizon. The same thing happens at the event horizon of a black hole. One way of thinking of a black hole is to consider space itself to be "sucked in" at the speed of light at the horizon, which is why a photon fired from the horizon will never reach a distant observer despite travelling at C the whole time (ObDisclaimer about this being only one of many possible metrics for describing black holes, and one that can produce misleading conclusions under other circumstances, yadda yadda).
I am eagerly awaiting the next annoncement where someone again finds evidence to refute the dark matter claims. It seems like the science; "Dark Matter is like this" - "No, it can't be, actually it's like that".
The only articles I've seen that make statements like that are the commentaries on the commentaries on the dumbed-down press releases on the actual publications.
What's actually been happening is more along the lines of:
"There's a discrepency between galactic models and observations. What did we get wrong in the model/what needs to be added?"
"Maybe A? B? C? D?" "Let's try to test them and see."
"Not A, not D, but maybe B, maybe C." "What kind of B or C? B1? B2? B3?" "Let's try to test them and see."
"Our models only work if we have B _and_ C, and we've ruled out C1 and C2, but C3 still works."
"What kind of C3?"
"New observations show a new effect in addition to the old one. How do we explain it?"
"Maybe E? Maybe F?"
[Etc.]
This is a process of examining many possible explanations, and weeding out the ones that don't work until we have reasonable confidence that the ones left _do_ work.
We've gone from "galactic rotation doesn't match models based on stars alone, what could be causing this?" to "we know that there's about X amount of normal matter we aren't seeing, Y amount of abnormal matter that we aren't seeing, and that the properties of the abnormal matter fall somewhere in this range (that's wide but being narrowed)". There's surprisingly little backtracking. Tests that detect or fail to produce evidence for dark matter of various types all help to increase our understanding of what dark matter's actual properties are.
As for dark energy, if anything, it would be surprising if something like it _didn't_ exist. We already knew that a scalar field with similar properties was likely present in the early universe, and several models proposed universes where the _absence_ of the field was only a local effect. Even relativity contained a similar type of effect that was set to zero a priori as opposed to forced to zero through a mechanism inherent in the model.
We're still sifting through the myriad of possibilities, but we certainly are learning something each step of the way.
Well then that's a flawed implementation.
:). In this case, it was a straight md5, so blocking out the hash was a justified action.
Which doesn't change the validity of my point
I'm not here to argue about how a _better_ system would be constructed, just about this specific action in response to this specific message.
Not in this case. They just have to track what secret they use, possibly changing it regularly, as all they use it for is generating and then verifying a hash.
Not quite. It's used for generating and verifying an arbitrary identification string. In this case, you're right, the recipient doesn't need to know the secret (the hash is just a glorified request tracking number, because Slashdot can't reverse it easily either without lots of brute force). If it was intended to let the recipient confirm that they were indeed the IP being blocked, the recipient would need to know the secret to duplicate the hashing (not needed in this case, because they gave the IP later in the email).
Yeah, so, that's why you have a secret that gets added to the hash. You don't just do MD5(IP) since then anyone can do a brute force attack. MD5(secret + IP) would be much better.
The problem is that this secret must be transferred from one party to the other before it can be used. That makes it much less secretive. A discussion of the key exchange problem (of which this is a subset) is beyond the scope of this thread.
In the email quoted by the original poster, no secret value was cited. So if he hadn't blanked out the md5 (as the original AC suggested), anyone who wanted to would have his IP address right now.
Not that that's earth-shatteringly catastrophic, but my point still holds in the original context.
Whew! good thing you "x"-ed out your MD5'd IP and Subnet.
/etc/shadow).
Given the MD5 hashes, you could find the IP within a few hours with a brute-force search. IPv4 space isn't *that* big.
Subnet would be within milliseconds, as there are only 32 normally-sane values, and only about 10 are actually used.
One-way hashes don't help much if the space of possible keys is small (that's why we have
Something fairly nasty that they will do, is automatically debit your account and pay any company who runs through a check-by-phone type transfer. All that is needed is the information on the bottom of your check and no authorization whatsoever. It's happened to me 3 times now, each time for somewhere between $250-$500.
It occurs to me that the government sends you your tax return by cheque...
I'd be quite amused to see what happens to someone unwise enough to try this kind of fraud on them (not that it would be likely to work, but I doubt they'd like it any better if it failed).
Doesn't 100Mb flash using 180M transistors work out to 1.8 transistors/byte? I'm still just a student, but according to my intro to ECE class, even storing one bit takes more than 1.8 transistors...
Most modern flash memory uses multi-level storage, allowing several bits per cell (I'd known about 4 levels (2 bits), another poster mentioned 8 levels (3 bits)). Storage still only requires one transistor.
The way it works is that you have a FET with a floating gate. In "write" mode, you apply a high voltage to the non-floating gate to drive charge either in or out through the thin oxide layer separating the gate and the body. The charge on the floating gate (which is between the sense gate and the body) ends up effectively changing the transistor threshold voltage of the transistor. When the transistor is turned on by the sense gate, you get an amount of current that varies depending on the amount of charge on the floating gate.
Other types of flash memory exist. This is just one of the more common ones.
As for storing single bits, the standard SRAM cell has 6 FETs (two inverters, cross-coupled, and two readout FETs to connect the inverter outputs to a differential read/write bus). A DRAM cell, however, just has one transistor, which connects the read/write bus to a storage capacitor. Among other things, this means that DRAM reads are destructive (capacitor is discharged on to the read/write bus; this disturbance is amplified, driving the bus back to the rail voltage and re-charging the cell's storage capacitor).
A lower core clock can save you a lot... bot financial and in energy. Raising the clock rate on a chip will increase its energy usage exponentially.
[Rant]Why, oh _why_, do people keep horribly abusing the word "exponentially"?[/Rant]
Power goes up in direct proportion to the clock rate. This is a "linear" relation. If it was really "exponential", we'd be stuck running 10 MHz processors because anything else would melt.
For the really pedantic, the way to compute dynamic power dissipation is to figure out how much capacitance you have on nodes that are being switched, what fraction of the time they're being switched, the amount of energy required to switch a given amount of capacitance (depends on signal voltage), and the frequency, and multiply these together:
P = (1/2) * Vdd^2 * C_node * N_nodes * transitions_per_clock * f
The only thing that's _not_ linear is the power-vs-voltage relation, and that's _quadratic_. Anything "exponential" sucks a whole lot worse.
[ObDisclaimer about clock feedthrough, but that's linear with frequency and capacitance to.]
Memory is "large, fast, densely-packed - pick _one_".
Got ahead of myself.
What I'm trying to say is that any large SRAM array will be slower than a small SRAM array, and neither will have very high capacity. A DRAM array has high capacity, but is horribly slow. So-called "single transistor SRAM" is actually DRAM with a cache tacked on.
For decades now, memory frequency scaling has lagged that of the microprocessor. Although there has been some great strides recently, latency is still rearing its ugly head. External DRAM is too electrically distant to remain at the heart of any high-performance system.
Once we get processor and memory combined, we'll see performance increasing by several orders of magnatude.
This idea has been around for what is almost certainly longer than either of us have been alive. It turns out that there are problems.
The main problem is that no matter how much memory a system has, we find ways to use it. In the time I've been using computers, memory size has gone up four orders of _magnitude_, and I'm sure the greybeards listening will top that. The processor sitting in your machine right now has more on-die memory (the cache) than, say, an early XT had, but the tasks you're running have a memory footprint too large to fit. This is the price for being able to _do_ more than you could do on that old XT.
Another problem is with the structure of memory itself. You've heard of "fast, cheap, good - pick two"? Memory is "large, fast, densely-packed - pick _one_". The reason why integrated logic/DRAM processes tend to do one or the other badly is that DRAM and logic have to optimize transistor characteristics for exactly opposite things (high "on" current for logic, low leakage current for DRAM). Among other things, this means that DRAM is either slow or very power-hungry. SRAM is bulky no matter what you do - it's the cost of playing, when you have six transistors instead of one. Any kind of large RAM array is slow no matter what you do - you have to propagate signals across a huge structure instead of a smaller one.
The solution to date has been a hierarchical cache system, where small, fast, on-die memory is accessed whenever possible, and when that overflows, larger, moderately fast, on-die memory, and when that fails, DRAM. This works amazingly well, giving you almost all of the benefits of fully on-die memory for problems that fit in cache. Problems that don't fit in cache won't fit in on-die memory, so going with an on-die implementation doesn't help for them.
Progress in improving memory response times is made in two ways. The first is to use a better cache indexing algorithm that is less suceptible to pathalogical situations. In the simpler indexing schemes, you can end up with situations where a short repeating access pattern can hammer on the same small set of cache blocks, causing cache misses even when there's plenty of space elsewhere. Higher associativity and tricks like victim caches reduce this problem. Techniques like a "preferred" block in a set reduce the time penalty for high associativity, and techniques like content-addressable memory reduce the power penalty. This is still a field of active research - build a better cache, and you get closer to a system that _acts_ as if it has all memory on-die.
The second way of improving memory subsystem performance is to use memory speculation. This involves either figuring out (or even guessing) what memory locations are going to be needed and preemptively fetching their contents, or taking a guess at the value that will be returned by a memory fetch before the real result comes in. In both cases, you're masking most of the latency of the memory access, while paying a price for failed speculations (either in higher memory _bandwidth_ required, or in power for speculated threads that have to be squashed). Build a better address and data speculation engine, and you'll again approach performance of an impossible all-on-die-and-fast system.
In summary, it turns out that putting all of the memory of a general-purpose system isn't practical now and won't be as long as requirements for memory keep increasing. However, caches already give you performance approaching this for problems tha are small enough to _fit_ in on-die memory, and cache technology is constantly being improved. This is where effort should be (and is) going.
Let me clarify. Your conflating earth's magnetic shielding with shielding using matter. In the case of earth (the one example I was using), a smaller magnetic field than present will always be worse--and that's what we are talking about here, earth and its magnetic field.
:).
He is using high-density shielding as an analogy to illustrate why a simplistic evaluation does not always give the correct result. Nowhere did he say that it was _directly_ analogous to the magnetic field question, and from what I can see from the first post he was quite careful _not_ to take a position on the original article's claims at all. The claim was that it couldn't be dismissed out of hand without analysis.
In summary, you are picking a nit that isn't there
Score one for a Robert L. Forward fan. Not the best in plot and character development but nifty science.
I actually doubt that the Forward scheme for sail decelleration will be used. The problem is that you need an array with an aperture size large enough to hit the primary sail at destination range, instead of just 1 LY or so (distance at the end of the boost phase). This makes it a lot more expensive to build.
You also end up having to use a truly huge primary sail (so that it can focus on the secondary sail at about 1 LY range at the end of the decelleration phase), and keep it perfectly aligned optically during decelleration.
A maser-driven craft with an active-antenna mesh that could do phase-shifting as the primary sail might be able to do this, but primary sail size and maser array size become prohibitive.
Fast-flyby probes are much easier to construct and boost, so I think they're more likely to be implemented if a sail scheme is used at all.
to find another planet. 150,000,000 years to get to it. Don't forget that we are seeing things as they used to be! discovering other planets is only has good as our ability to get there, which is nil. Not to mention that they probably arn't even there anymore.
You do realize that with a detection range of a few dozen to a few hundred light-years, we'll be seeing planets as they were at most a few dozen to a few hundred years ago, not hundreds of millions of years, right?
A laser boosted sail-probe could reach a nearby star system ( 10 LY) within one human lifetime. It would be impractial to send one big enough to carry humans, but an automated flyby survey would definitely be feasible.
I've heard of the inferometry plan before - it's basically a fleet of 7 - 11 satellites flying in near-perfect line abreast formation. That coupled with a lot of image processing gives the effect of a radio telescope with a dish the size of the formation.
Close. A radio interferometric telescope works like this, because we can record and timestamp radio signals with timing precision much finer than their period (typically nanosecond-range and longer). An optical interferometric telescope has to actually bring all of the gathered light to one place and do interference directly, as our electronics aren't good enough to do direct signal sampling, and won't be any time soon (timing precision needed is on the order of femtoseconds for near-IR, and still tens to hundreds of femtoseconds for thermal IR).
This requires _extremely_ good station-keeping for the telescopes, but this is a manageable problem (especially since you don't have to worry about as many vibration sources as you do for earth-based interferometric telescopes).
Googling for "astronomy" and "optical interferometer" will get you links for the interferometric telescopes that have been built to date. Interesting stuff.
Is it possible that advanced civilizations might produce something similar to superconductors but on an EM basis? In other words, the outer shell of their Dyson spheres might mop up the rest of the emissions so that there is nothing for us to detect?
:)).
:).
The problem is that no matter what you do, power that is produced inside the sphere ends up as heat eventually. If it flows out through the outermost sphere, the sphere radiates in a detectable band. If you somehow prevent radiation from the outermost sphere, the inside of the system heats up (and keeps heating up, as the central star keeps pumping out energy).
In practice, there is no known way to keep the sphere from radiating in a steady-state system. You could dump all of your emissions as a narrow beam in one direction, but that would only make you _less_ detectable, not invisible. You could keep a pet black hole and dump thermal emissions into it, but a black hole with an event horizon large enough to be useful for this task would weigh as much as the galaxy.
So, if a dyson existed, it would be visible, unless currently-known physics is grossly incorrect.
(And if you're proposing changing physics, it's easier just to propose a change that removes the need for dark matter
The other response mentioned gravity. We are currently doing experiments with gravity. Perhaps in the future we'll figure out how to contain it?
Gravity does not appear to be something that can be contained, even in principle. One of the particle physics professors in the audience can give you a more detailed explanation of why than I can. Short version is that it's just the nature of the kind of field we think gravity is.
A shell of negative mass around the system (of magnitude equal to the system's mass) would remove its gravitational effects, but negative mass is almost certainly not compatible with the universe as we know it. This falls into the "changing physics" category
To sum up, while it's possible to make dyson spheres that are _difficult_ to detect, they'd still be detectable, especially if present in great enough number to explain the baryonic component of dark matter (non-baryonic component can't be explained by dyson spheres, detectable or not).
I hope this helps.
I was thinking about the retroreflector a bit, and then I suddenly realised you could also put it on the laser installation and personell. That should be good enough to rereflect away any laser light bounced back to the laser and personell. Thus making the entire scheme useless in killing a laser system.
The laser's still bright enough to kill the missile with or without the retroreflective coating (reflection isn't perfect). That means it's bright enough to harm itself, with or without a reflective or retroreflective coating on the installation.
Putting a reflective or retroreflective coating on the laser installation turns out to have other drawbacks too, though it's probably still tolerable if your laser is in an area you control.
You know, the "other kenetic weapons" thing always bothered me about films like starwars. Why didn't they equip all those fancy robots with things like rocket launchers to take out the Jedi? I mean, load of good your lightsabre would do you then.
;). FWIW, the d20 RPG version of Star Wars shows much the same effect - while a Jedi can send blaster bolts bouncing away, you can really ruin one's day with a flame thrower or a grenade. FWIW also, the "Heavy Gear" RPG seems to have one of the saner approaches to weapons technology I've seen yet - most of their weapons are kinetic, and their robots run on diesel-type fuel.
:). If the missile is retro-reflecting the light back to the laser, it'll be attenuated by haze and so forth, but if the laser can get enough energy to the missile to damage it, chances are the beam path is clear enough for the missile to send enough back to be unhealthy too.
Because it wouldn't have made as fun a movie
In any case, and back to the question at hand, wouldn't a painted missle be invisible if it was reflective, as the "paint" would just be reflected laser?
It's a lot easier to track a missile if it's glowing brilliantly with reflected laser light than if you have to pick its reflected sunlight out of a cluttered background (or even hazy sky). Use a pulsed laser and it can be thousands of times brighter than anything else in the scene while the laser is on, while keeping average power reasonable. Your guidance system looks for something flashing very brightly very briefly, and locks on to it.
I don't mean to sound crass, but wouldn't the Military have thought of things like, "Well, they'll just put mirrors on the missles if we attack them with light, Sir".
They've thought of it, they just consider it an acceptable inefficiency. Especially for missiles bought off the shelf, you aren't going to get much back-scatter, and they'll absorb enough energy to go "boom" satisfactorily. Even a highly-reflective missile would be destroyed by a powerful enough laser. The problem is that it means a more expensive laser. For anti-guerrilla applications where you're trying to protect a small number of known targets and are willing to spend lots of money to do it, it works fine.
It just won't be terribly cost-effective for other scenarios.
And missiles or other devices custom-designed to foil the system could damage either the expensive laser system or the personnel running it for a cost that's probably less than the cost of the laser.
Would the reflective material hold up well to such pressures as take-off and super-sonic speeds?
It wouldn't have to. You'd coat it with a more durable covering that's transparent (or at least "transparent enough") to the laser's light for the retroreflector to send an uncomfortable amount of energy back to the sender before the covering degrades, the missile is destroyed, or what-have-you.
For a missile that's just shiny enough to resist being damaged a bit longer, sure, it'll work. Give it a polished aluminum or chrome steel skin. Just watch out for heating near the front of the missile if you're going at high speed (but these missiles don't).
Would mirrors on the outside disrupt internal guidance systems?
No. A self-guided missile's optical window has an unobstructed view of the target. The coating would be on the missile. It would only provide visual clutter if the missile could see itself, which it shouldn't be able to. If it can, then it already has clutter there, so no further loss.
What about something as simple as a cloudy sky?
Unless clouds are a lot more solid than I'm used to, the missile won't care about them. The laser trying to shoot it down will
If you're talking about the "paint and shoot" scheme, you'd probably still have more than enough light from the missile for the shell to track for intercept. Only concern is the beam itself being visible due to scatter
aperture of less than a metre. Even in near-IR, that gives it a divergence of about one part in a million, meaning a kilometre away the spot is going to be a metre or two wide.
:).
Don't mind me, I just tend to mess up math past my bedtime...
At 1.0e-6 divergence, a 1m beam won't diverge significantly over 1 km. However, it's still a 1m beam.
Try to make it narrower, and it diverges. Widen it enough to not diverge much, and it's big enough that you won't be able to do fine targetting on the missile.
My point holds, even if my first try at the math didn't
Someone will probably chime in now suggesting a 1m aperture that focuses to a much narrower beam waist, but there is no way in heck you're getting optics that good (_adaptive_ optics that good) for a weapon that has to operate on the field, in real-time.
Missles can't be completely covered in any material. They require propulsion and lots of other things. Depends how accurate this laser is, that's the real "depends".
Diffraction limits the degree to which the laser can be focused. The fixed-site version of THEL (the big, stationary version) has an aperture of less than a metre. Even in near-IR, that gives it a divergence of about one part in a million, meaning a kilometre away the spot is going to be a metre or two wide. Farther away, bigger area painted. Put retro-reflective material on any reasonable fraction of the missile's surface, and you'll get enough retro-reflected light to be decidedly unhealthy for anything near the THEL system. The missile will certainly still be destroyed, but THEL's a lot more expensive than the missile.
You don't need anything complicated for the reflector. Stamp a cube pattern into the missile's surface and plate it with aluminum (or gold, if you want the extra fraction of a percent IR reflectivity), and coat it with something IR-transparent to avoid wrecking the missile's aerodynamics. Poof; laser-hostile missile.
Laser weapons are lousy on the battlefield for a number of reasons. Mainly, the problem is that they're very power-hungry even for relatively efficient versions, they need to have massive overkill in order to be able to harm reflective targets, they need to have even more massive overkill in order to be able to harm a target despite having a beam larger than the target at any significant range, and it's easy to retro-reflect them and ruin the day of any poorly-shielded operator of the laser weapon.
Guns (and other kinetic weapons) are very efficient to deploy and harder to shield against.
The best really effective anti-missile system would probably be a system that used a laser just as a designator to paint the missile, and a gun firing "guided bullet" type shells that adjusted their course to intercept the painted missile (targetting is the main problem with trying to shoot down a missile with kinetic weapons). A laser only works if you don't mind the massive overhead (e.g. if you have a lot more resources in the field than your opponent).