I find it disturbing that futurists tend to presume that the benefits of communication outweigh the risks. Okay, an Independence Day scenario is not likely. But how unlikely? 1 in 10 chance? 1 in 100? 1 in 1000? 1 in 10000? What is an acceptable risk?
We are already broadcasting more than enough radio noise to be easily seen by any hypothetical aliens who are hunting down and destroying other races, so attempting contact does not substantially increase the risk of this scenario.
Furthermore, due to the vast resources needed to cross between the stars, a campaign of interstellar conquest is fundamentally useless. Anything you could possibly hope to gain would be much more easily obtained by devoting the same resources to producing it in your own system.
In summary, attempting to commmunicate with aliens does not substantially increase the risk of conquest, which is for practical purposes zero in the first place.
This is useful because anyone who can hear our signals and generate signals for us to hear in return is almost certainly far more advanced than we are.
There is no proof of that. The whole hoping aliens are smarter than us is a bad assumption. If it took so long for life to evolve here, it would probably take just as long if not longer in other conditions.
If anyone responds to us, then life has already evolved, so that issue is not relevant. The relevant question is, "given that someone is there who _can_ respond, how advanced are they likely to be compared to us". For reasons outlined in my previous message, the answer is "much more advanced", as less advanced or non-existant people wouldn't be responding in the first place.
Your arguments instead influence the question of how likely it is that we get a response at all, but that was not under debate (different issue).
So far as the question of fuel capacity vs terminal velocity, two words: "Tau Zero", by Poul Anderson.
This craft used a Bussard ramscoop. There have been multiple messages in this thread explaining why they turn out not to work. For practical field configurations, drag greatly overwhelms fusion-produced thrust.
That's, of course, not the only kind of craft: ramscoop ideas have been around for a while, and while they're not exactly "production quality" ideas, there's nothing fundamentally killing them. We'd just have to figure out how to do fusion much better than we do now - which is not exactly new physics - it's new engineering.
Ramscoops are fundamentally killed by drag.
Consider a ramscoop to be a special case of a magnetic bottle. In a conventional magnetic bottle, matter leaks through "loss cones" at the pinch points of the mirrors. The tighter the constriction (i.e., the greater the field increase compared to the field in the middle of the bottle), the narrower the loss cone and the less material leaks out.
A ramscoop, on the other hand, works by passing material through the pinch point, hopefully with enough compression to fuse in the process. The problem is that, given the fact that the field is much larger than the coils used to generate it, the pinch point is very narrow indeed, and very little material gets through the loss cone (most is just deflected). So, you get lots of deflection and very little fusion to provide thrust. Bye bye dreams of ramjets.
Making the throat of the ramjet wider means less compression, and lack of fusion.
Using an active compression scheme (pulsed conical field coil, for instance) requires coils the size of the field - either heavy enough to make the ramscoop useless, or small enough to capture almost no fuel.
In summary, ramjets in practice don't seem to work very well.
Any relativistic craft launched would be sailcraft or other beam-driven craft of some kind, as that's the only way to provide enough energy to the craft.
Relativistic travel = huge cost (in 2003 dollars! note that economies of scale and necessity can help here. Automated antiproton factories, etc.: thank god for that tremendously huge fusion generator sitting next door pumping ungodly amounts of energy out all around us.)
Neither solar collectors nor antiproton factories are free, which means that the cost will always be significant. In practice, it will be colossally expensive to produce antiprotons for the forseeable future, for reasons outlined in detail in another post.
A phase-locked maser array would probably be the cheapest method for driving a relativistic craft, but has political problems outlined in my previous message.
Does antiproton synthesis require new physics, I thought it was largely an engineering problem?
Producing antiprotons efficiently requires new physics. Producing them _inefficiently_ is just a Small Matter of Engineering, and is already done on a semi-routine basis (though at quantities many orders of magnitude lower than would be needed for spacecraft fueling).
The problem is that when you create particle/antiparticle pairs, you get far more light ones than heavy ones. The fact that protons and antiprotons are composite particles doesn't help either (there's no particular reason to get a proton instead of a neutron or a bunch of mesons at those energies).
The numbers I've heard batted around for the best efficiency we can expect synthesizing antiprotons are in the 1e-4 to 1e-6 range. Considering that that's a factor of ten thousand to a million more than the vast rest energy stored in the antimatter in the first place, and you see why it gets expensive.
New physics could help by giving some magical way of turning energy into proton/antiproton pairs at near-perfect efficiency (i.e. getting mostly protons and antiprotons, not other junk or energy lost by other means). The key word here is "magic", in that there is no feasible mechanism for this selection that I know of.
Before anyone brings it up, yes, current antiproton generation proposals already consider tuning the source beam energy to the most favourable ranges for production.
Similarly inefficiencies in the production push up the economic cost, which makes it less practical, but by the time you come to address how to cross interstellar space from a practical perspective we'll presumably have mastered interplanetary travel, so power sources shouldn't be a major issue.
Power plants will never be free to build or maintain, which means that energy will never be free. Likewise, the antimatter production facilities will have a finite maintenance lifetime over which cost is to be amortized. Thus, I don't expect antimatter to ever be cheap, or the cost of an antimatter-powered probe's fuel supply to be anything less than "colossal". It may still get built, but it won't be cheap for the forseeable future.
As far as I know serious scientists are still contemplating 1g acceleration for flight duration making it practical to visit planets within a few dozen lightyears
As cruise velocities would be much lower than C, and travel times in the decades, there is no reason to boost at C. As lower-thrust engines are usually easier to design, they'd be favoured. The only high-acceleration proposals I've heard about were unmanned sailcraft (a maser driven craft, in this case).
Several major assumptions go into assessing the feasibility of interstellar flight, one of the big unanswered ones being what is the galaxy made of. Until we can adequately explain what the bulk of the mass of the galaxy is, I suspect spectulating on whether we'll ever flip through it at a significant fraction of the speed of light is premature.
The "missing mass" doesn't seem to interact with things via the electromagnetic force. If it did, it would be opaque (in at least some bands) and be much easier to detect. As far as anyone can tell, dark matter would not impede spacecraft in any way except gravitationally (and that only on a galactic scale).
Yes there are practical problems building the technology we need, but I don't believe there are any obvious show stopping physics.
The show-stopping physics is the fact that the specific impulse required for relativistic flight is too high to attain with anything but a beamed-core antimatter drive. Anything else is limited to about 0.2c or lower if it carries its fuel with it. Beam-powered craft are another issue, as described in my previous message.
A beamed-core antimatter drive has a show-stopping cost issue, for reasons described above.
Whilst some entrepid spirits will try to travel would many peop
The same applies to space flight now. We can dream it, but we can't figure out how to do it. Some day, a bunch of different people will come up with a bunch of theories on "super-luminal" travel, then set out to prove their theories. One of them will be proven.
Why are you certain that one of them will be proven?
The universe is what it is, regardless of what we _want_ it to be. This may or may not include mechanisms for FTL travel, but we have seen no evidence of such phenomena occurring to date, and our models of the universe are self-consistent without them.
In the absence of observerations of FTL effects and of a theoretical mechanism by which it would occur, the most reasonable assumption is that it _doesn't_ occur.
If our universe is truly bound by the speed of light, wishing for FTL drives won't change a thing.
The wise thing to do is plan for STL, and continue learning all we can about the universe in the hopes that a loophole eventually shows up.
[ObPedant: Yes, I know about the various types of "space warp" drive proposed; however, these rely on negative energy density, which causes serious problems (does not appear to be consistent with our models of the universe). A few groups have been trying to demonstrate that negative energy density is possible. If they succeed, great, but until then the null assumption holds.]
It's been far too long since I read a non-fiction book with spaceships in it, but can't you (in theory) propel a spaceship by shining a very powerful light out of the back, using the photons themselves as the reaction mass? Then could you get nearer to c?
You can, but the problem is generating the light in the first place, and the fact that light has a lousy ratio of momentum to energy (it has to be very, very bright to generate significant thrust).
Most light sources that are bright enough to move a ship at any reasonable acceleration (e.g. fusion bombs wrapped in other matter or just shining on a shield block that can tolerate gamma rays) waste matter - the energy to mass ratio of a fusion bomb is much worse than that of the photons you're driving the ship with. This means you'd be better off just using a magnetic bottle to deflect the plasma resulting from the fusion explosion, and you'd still end up with specific impulse too low for relativistic flight.
A light source that doesn't ablate or otherwise lose mass has to be relatively dim (either a hot block of solid matter or a confined plasma ball), which means getting anywhere will take an extremely long time.
The forms of light propulsion that I've seen considered involve generating the light somewhere else (e.g. a laser array) and just reflecting it off the craft's sail. You still have a drive that's horribly inefficient energy-wise, but the energy source doesn't have to travel with the craft.
For reference, power to thrust is 3e8 W/N for a photon drive (energy to momentum ratio is C for photons).
Re. slower than light travel - if you get fast enough (i.e. a sizable fraction of c), then, even if it takes dacedes to get where you're going, time dilation will mean that far less time passes for the crew of a spacecraft - so, if you're going fast enough, a trip of 90 light-years, say, could be accomplished within the natural lifetime of the crew without FTL travel.
There turn out to be practical problems with this. Any craft that carries its own fuel with it - including the more practical breeds of antimatter drive - will be limited to a crusing speed of about 0.1-0.2C by the specific impulse of their fuel. The only thing that could approach speeds at which time dilation would be significant is a beamed core antimatter drive (that uses the charged particle shower from an antiproton annihilation as the reaction mass), but that requires unrealistic amounts of antimatter (positrons are easy to make, but antiproton synthesis is very inefficient, and will remain so unless new physics is discovered).
In principle, some kind of sailcraft driven by a stationary laser or maser array could reach relativistic speeds, but the array would be very expensive to build and very large (we need to focus on a planet-sized sail at a range of many light-years). It would also work wonderfully as a weapon capable of melting cities to slag at a range of hundreds of AU (or even light-years, depending on configuration), so I suspect non-proliferation agreements would prevent it from being built in the first place.
In short, the only hope for relativistic travel at less than colossal cost is new physics.
There probably are planets out there with intelligent life -- maybe lots of them -- but they are so far away that it is impossible to have any contact with them. You can debate all you want about whether or not there's life out there, but you can't change the math.
"The math" also says two things:
We can most definitely contact systems within a few hundred light-years by radio. We'll need an array of phase-locked transceivers in space to do it, but it's not difficult or even horribly expensive to do. Contact by optical carrier depends on us building very large interferometric telescopes, which is a tougher engineering challenge but can also be done.
Communication occurs at the speed of light, so round-trip time to 90 light-years is 180 years, and one-way time is 90 years.
This is useful because anyone who can hear our signals and generate signals for us to hear in return is almost certainly far more advanced than we are.
Modern humans have existed for about 30,000 years. Human civilization has existed for on the order of 6000 years, depending on who you ask and what you call "civilization". If the lifespan of an alien technological race is longer than this - and it will be, especially once it decentralizes (makes colonies not on the same easily-bombed planet) - then, of the stretch of their civilization's existence where they can hear and respond to us, the segment where they are more advanced than us is much longer than the segment where they are less advanced than us. This makes it likely that _if_ we find someone to contact, they're in the "more advanced than us" stage.
This makes communication, even with a multiple-lifetime time lag, worth it.
This discussion overlooks the impact of any future technology that would confer either extreme longevity, or the ability to store and reconstruct a human mind-state/personality. In the first case, slower-than-light travel between the stars becomes feasible because we have the patience for it, and it doesn't take that large a chunk out of our lives. In the second case, we can be sent at the speed of light as data, with no subjective time elapsing en route, to be reconstructed at the other end.
In conclusion, communication is both possible and worthwhile even without FTL travel or exotic technologies.
The problem with putting a computer in a desk or cabinet is that it heats up due to the confined air pocket around the machine. Fans that just end up recycling hot air don't make the machine any cooler. One of my machines I've had to maintain has stability problems as a result of this.
Lastly, the air gap in the cabinet is not what's shielding the sound. It's shielded because the walls of the desk/cabinet are good at absorbing sound, and because you're farther away from it (less reaches you), and because vibrations in the desk can couple into the floor and other parts of the building you're in to sink energy instead of resonating in the room.
If anyone else is considering a similar desk mod, I'd suggest making cutouts in the back or side of the desk (something unobstructed) for a large exhaust fan and an air intake vent.
We'll probably never again be at a point where we say "What in the heck is out there?" We'll never again have Uncharted Territory. But rather we say "What in the heck will that look like up close."
This is only partly true. Many space objects are the next best thing to invisible. Barring a really concerted (and expensive) effort, we won't have maps of, say, the Kupier belt that are anywhere close to complete. Even closer to home, we only have records for the _big_ asteroids in the belt (and inner solar system).
Similarly, while we've found at least one white dwarf star in our local neighbourhood, others may very well exist that we aren't noticing - they're quite dim. Smaller objects, like gas giants ejected from systems during formation and drifting in interstellare space, or the myriad of objects in the Oort cloud, may not ever be found - unless an object emits a lot of light or is both large and quite close to a bright light source (like a star), it's lost in the void.
Think of our medium-term mapping situation as the equivalent of having the tourist brochures for the area we want to visit, and our current maps as being the blurb on the back of them. Still plenty to discover.
What makes the B-52 so effective is that it can carry tons of bombs to target. Its takes exponentially more energy to move that kind of tonnage faster (say supersonic speed).
Nitpick: It's a polynomial relation, not an exponential one. Force required goes up as te square of velocity, and power required as the cube, for cruising at a given altitude.
What actually happens is that there's a sharp reduction in range as speed goes up, due to the increased power requiremenets and the fact that you can carry a fixed amount of fuel.
Exponential relations are relatively rare, but everyone seems to like the word...
My own guess as to what will happen in the long term is that the US (and presumably other powers) will continue to maintain bases at strategically important locations near the areas they're interested in. Much easier way to maintain a fast response time. If we ever get cheap space transport, they might stock orbit with warheads that could be de-orbited on desired targets, but that's enough of a can of worms that I doubt it'll be the dominant approach to delivering strategic weapons.
So perhaps we can also get 7 and 9 quark particles.
In principle, we can get any number we please. The question is whether a particle with, say, 7 quarks would have lower energy than three separate particles (one baryon and two mesons). Depending on its internal energy structure, it would either be unstable, extremely unstable, or not bound as a particle at all (we know it's not stable because a) we don't see these around normally, and b) anything built from quarks and antiquarks decays [quickly if the Q and !Q have the same flavour, slowly if not]).
There has already been a search for tetraquark objects (same constituents but different energy structure than two mesons). I'm not sure how that turned out (mentioned, with paper citations, by another poster a few years back).
Isn't everything inadequate for estimating anything for the simple reason that people might be wrong?
That's why multiple completely different methods with different theoretical underpinnings are being used. It is far less likely that all are wrong than that any single approach is.
(it would be two co-orbiting, which would have different particles)...different properties. That's what I get for slashdotting at silly-o'clock in the morning.
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Think of this as a merged baryon and meson.
No. They specifically said that the merged version would have different properties.
"Merged" != "Bound".
A bound pair would not be a distinct particle (it would be two co-orbiting, which would have different particles). A pentaquark can be thought of has a merger of the two particles, containing the constituents of both but having its own energy structure.
List of the constituents is the same. Hence, my example.
Re:Color?
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I wonder what the color makeup of the pentaquark is. As I undestand it, for mesons it's red/anti-red, blue/anti-blue, or green/anti-green. For baryons it's red/blue/green or anti-red/anti-blue/anti-green. What do you do for five?
At that frazzle point the guy would have either contacted the local gdb guru,just rewritten the damn thing, or documented it as a known issue and blame it on hardware.
This isn't always possible. A bug that can't be reliably duplicated is difficult to pin down in a debugger, and a bug that you have no idea of the root cause of can't just be removed with a code rewrite - you don't know which block of code to rewrite.
The hardest part of debugging is identifying where the problem is. Once you know that, you can usually backtrack and fix it.
Something that stays that elusive for that long falls into the "add firewall code and pray something trips it" category.
Hmmm... energy can't be created. What did the Big Bang do, then?
In the Einsteinian physics world, energy can only be created by the destruction of matter. Which certainly was going on at the Big Bang.
Actually, the creation of matter was happening at the big bang. Matter as we know it condensed out of the high-energy primordial soup as it cooled off.
If I understand correctly, the idea is that the universe had zero net energy at formation, and so was allowed to spontaneously appear out of nothing. The energy of its contents (positive) was perfectly balanced by its gravitational potential energy (negative).
How inflation and other scalar fields (e.g. "dark energy") mesh with this is something you'd have to ask an astrophysicist about.
Sorry, but I call all those suns much more concentrated than the big bang; true, you had all energy in one place...but then again, you had all places in one place:)
Fact remeains, that there is much more structure to be observed in the current universe than when it was a singlularity.
This turns out not to be correct. The entropy of a system relates to the number of possible states that system can take, or alternatively the number of degrees of freedom in that system. If all matter and energy is in one location, the number of degrees of freedom is a lot more limited.
For a more detailed explanation, ask your local physics professor.
How does a gigantic power plant "fall" into *anyone's* hands?
Any rationalization that depends upon the alleged inability to minaturize inherently nanoscopic processes is intellectually bankrupt from the start.
And now it becomes apparent that you've been blowing smoke without any knowledge of how nuclear plants work at all.
The minimum size of a nuclear plant is determined by the distance neutrons must travel before being a) thermalized and b) captured. Both of these are many, many orders of magnitude larger than "nanoscopic".
Fission plants need not be large. What evidence do you have that a fission plant could not operate in a 10x8x12ft toolshed? These things power many sizes of submarines, remember.
A fission plant capable of producing useful quantities of weapons-grade material is big.
expensive
I'm glad to have convinced you.
Read again. A coal-fired plant is expensive to build too. Re. your arguments, a rogue state won't bother with insurance, so your vaprous $0.20/kWh figure is doubly irrelevant. A small country that wants to produce nuclear weapons won't be able to afford a large number of full-scale nuclear plants, and has to spend quite a bit of effort building them, which makes the detection problem easier.
On the contrary, I have an EM spectrum chart that the Exploratorium sold me for a few dollars that has everything anyone needs to know about shielding up to 938 Mev. You can use water or lead, but mountain soil is usually the easiest.
If you bury a nuclear reactor in the bottom of a mineshaft, you're going to have one heck of a time cooling it.
If you do anything less than bury it at the bottom of a mine shaft, you get gamma radiation shining through. _Most_ of it is attenuated - this is why we bother with shielding at all - but not all of it. This is why there is a network of gamma-detecting satellites flying above your head.
How exactly is my supporting nuclear power in Canada proliferating nuclear weapons, again?
It is because proliferation risks are continiously devolving from nations to small groups of people. To rely upon something that you don't want everyone to have is contradictory policy.
How so? Anyone can already look up how to build a nuclear plant, so it's not a knowledge concern. I seriously doubt that some rogue group is going to steal a million-tonne building, so the plant itself doesn't help them. How do power plants here help nuclear weapons development there, if neither knowledge nor facilities are problem?
A "small group of people" can't build a weapon-supplying reactor.
Canada could dismantle all of its nuclear plants and India and Pakistan and North Korea would still be making bombs.
ObDisclaimer: I do not consider the above to be "rogue states", but name them as they are the most likely to be cited on a list of people with nuclear weapons that may be cause for concern for the lister.
Slashdot Mirror
And this is exactly why companies should stay the hell away from Linux - the pirate mentality that goes with it.
This is using Linksys software on the Linksys hardware that it was provided with. How is it copyright violation?
I find it disturbing that futurists tend to presume that the benefits of communication outweigh the risks. Okay, an Independence Day scenario is not likely. But how unlikely? 1 in 10 chance? 1 in 100? 1 in 1000? 1 in 10000? What is an acceptable risk?
We are already broadcasting more than enough radio noise to be easily seen by any hypothetical aliens who are hunting down and destroying other races, so attempting contact does not substantially increase the risk of this scenario.
Furthermore, due to the vast resources needed to cross between the stars, a campaign of interstellar conquest is fundamentally useless. Anything you could possibly hope to gain would be much more easily obtained by devoting the same resources to producing it in your own system.
In summary, attempting to commmunicate with aliens does not substantially increase the risk of conquest, which is for practical purposes zero in the first place.
This is useful because anyone who can hear our signals and generate signals for us to hear in return is almost certainly far more advanced than we are.
There is no proof of that. The whole hoping aliens are smarter than us is a bad assumption. If it took so long for life to evolve here, it would probably take just as long if not longer in other conditions.
If anyone responds to us, then life has already evolved, so that issue is not relevant. The relevant question is, "given that someone is there who _can_ respond, how advanced are they likely to be compared to us". For reasons outlined in my previous message, the answer is "much more advanced", as less advanced or non-existant people wouldn't be responding in the first place.
Your arguments instead influence the question of how likely it is that we get a response at all, but that was not under debate (different issue).
So far as the question of fuel capacity vs terminal velocity, two words: "Tau Zero", by Poul Anderson.
This craft used a Bussard ramscoop. There have been multiple messages in this thread explaining why they turn out not to work. For practical field configurations, drag greatly overwhelms fusion-produced thrust.
That's, of course, not the only kind of craft: ramscoop ideas have been around for a while, and while they're not exactly "production quality" ideas, there's nothing fundamentally killing them. We'd just have to figure out how to do fusion much better than we do now - which is not exactly new physics - it's new engineering.
Ramscoops are fundamentally killed by drag.
Consider a ramscoop to be a special case of a magnetic bottle. In a conventional magnetic bottle, matter leaks through "loss cones" at the pinch points of the mirrors. The tighter the constriction (i.e., the greater the field increase compared to the field in the middle of the bottle), the narrower the loss cone and the less material leaks out.
A ramscoop, on the other hand, works by passing material through the pinch point, hopefully with enough compression to fuse in the process. The problem is that, given the fact that the field is much larger than the coils used to generate it, the pinch point is very narrow indeed, and very little material gets through the loss cone (most is just deflected). So, you get lots of deflection and very little fusion to provide thrust. Bye bye dreams of ramjets.
Making the throat of the ramjet wider means less compression, and lack of fusion.
Using an active compression scheme (pulsed conical field coil, for instance) requires coils the size of the field - either heavy enough to make the ramscoop useless, or small enough to capture almost no fuel.
In summary, ramjets in practice don't seem to work very well.
Any relativistic craft launched would be sailcraft or other beam-driven craft of some kind, as that's the only way to provide enough energy to the craft.
Relativistic travel = huge cost (in 2003 dollars! note that economies of scale and necessity can help here. Automated antiproton factories, etc.: thank god for that tremendously huge fusion generator sitting next door pumping ungodly amounts of energy out all around us.)
Neither solar collectors nor antiproton factories are free, which means that the cost will always be significant. In practice, it will be colossally expensive to produce antiprotons for the forseeable future, for reasons outlined in detail in another post.
A phase-locked maser array would probably be the cheapest method for driving a relativistic craft, but has political problems outlined in my previous message.
Does antiproton synthesis require new physics, I thought it was largely an engineering problem?
Producing antiprotons efficiently requires new physics. Producing them _inefficiently_ is just a Small Matter of Engineering, and is already done on a semi-routine basis (though at quantities many orders of magnitude lower than would be needed for spacecraft fueling).
The problem is that when you create particle/antiparticle pairs, you get far more light ones than heavy ones. The fact that protons and antiprotons are composite particles doesn't help either (there's no particular reason to get a proton instead of a neutron or a bunch of mesons at those energies).
The numbers I've heard batted around for the best efficiency we can expect synthesizing antiprotons are in the 1e-4 to 1e-6 range. Considering that that's a factor of ten thousand to a million more than the vast rest energy stored in the antimatter in the first place, and you see why it gets expensive.
New physics could help by giving some magical way of turning energy into proton/antiproton pairs at near-perfect efficiency (i.e. getting mostly protons and antiprotons, not other junk or energy lost by other means). The key word here is "magic", in that there is no feasible mechanism for this selection that I know of.
Before anyone brings it up, yes, current antiproton generation proposals already consider tuning the source beam energy to the most favourable ranges for production.
Similarly inefficiencies in the production push up the economic cost, which makes it less practical, but by the time you come to address how to cross interstellar space from a practical perspective we'll presumably have mastered interplanetary travel, so power sources shouldn't be a major issue.
Power plants will never be free to build or maintain, which means that energy will never be free. Likewise, the antimatter production facilities will have a finite maintenance lifetime over which cost is to be amortized. Thus, I don't expect antimatter to ever be cheap, or the cost of an antimatter-powered probe's fuel supply to be anything less than "colossal". It may still get built, but it won't be cheap for the forseeable future.
As far as I know serious scientists are still contemplating 1g acceleration for flight duration making it practical to visit planets within a few dozen lightyears
As cruise velocities would be much lower than C, and travel times in the decades, there is no reason to boost at C. As lower-thrust engines are usually easier to design, they'd be favoured. The only high-acceleration proposals I've heard about were unmanned sailcraft (a maser driven craft, in this case).
Several major assumptions go into assessing the feasibility of interstellar flight, one of the big unanswered ones being what is the galaxy made of. Until we can adequately explain what the bulk of the mass of the galaxy is, I suspect spectulating on whether we'll ever flip through it at a significant fraction of the speed of light is premature.
The "missing mass" doesn't seem to interact with things via the electromagnetic force. If it did, it would be opaque (in at least some bands) and be much easier to detect. As far as anyone can tell, dark matter would not impede spacecraft in any way except gravitationally (and that only on a galactic scale).
Yes there are practical problems building the technology we need, but I don't believe there are any obvious show stopping physics.
The show-stopping physics is the fact that the specific impulse required for relativistic flight is too high to attain with anything but a beamed-core antimatter drive. Anything else is limited to about 0.2c or lower if it carries its fuel with it. Beam-powered craft are another issue, as described in my previous message.
A beamed-core antimatter drive has a show-stopping cost issue, for reasons described above.
Whilst some entrepid spirits will try to travel would many peop
The same applies to space flight now. We can dream it, but we can't figure out how to do it. Some day, a bunch of different people will come up with a bunch of theories on "super-luminal" travel, then set out to prove their theories. One of them will be proven.
Why are you certain that one of them will be proven?
The universe is what it is, regardless of what we _want_ it to be. This may or may not include mechanisms for FTL travel, but we have seen no evidence of such phenomena occurring to date, and our models of the universe are self-consistent without them.
In the absence of observerations of FTL effects and of a theoretical mechanism by which it would occur, the most reasonable assumption is that it _doesn't_ occur.
If our universe is truly bound by the speed of light, wishing for FTL drives won't change a thing.
The wise thing to do is plan for STL, and continue learning all we can about the universe in the hopes that a loophole eventually shows up.
[ObPedant: Yes, I know about the various types of "space warp" drive proposed; however, these rely on negative energy density, which causes serious problems (does not appear to be consistent with our models of the universe). A few groups have been trying to demonstrate that negative energy density is possible. If they succeed, great, but until then the null assumption holds.]
It's been far too long since I read a non-fiction book with spaceships in it, but can't you (in theory) propel a spaceship by shining a very powerful light out of the back, using the photons themselves as the reaction mass? Then could you get nearer to c?
You can, but the problem is generating the light in the first place, and the fact that light has a lousy ratio of momentum to energy (it has to be very, very bright to generate significant thrust).
Most light sources that are bright enough to move a ship at any reasonable acceleration (e.g. fusion bombs wrapped in other matter or just shining on a shield block that can tolerate gamma rays) waste matter - the energy to mass ratio of a fusion bomb is much worse than that of the photons you're driving the ship with. This means you'd be better off just using a magnetic bottle to deflect the plasma resulting from the fusion explosion, and you'd still end up with specific impulse too low for relativistic flight.
A light source that doesn't ablate or otherwise lose mass has to be relatively dim (either a hot block of solid matter or a confined plasma ball), which means getting anywhere will take an extremely long time.
The forms of light propulsion that I've seen considered involve generating the light somewhere else (e.g. a laser array) and just reflecting it off the craft's sail. You still have a drive that's horribly inefficient energy-wise, but the energy source doesn't have to travel with the craft.
For reference, power to thrust is 3e8 W/N for a photon drive (energy to momentum ratio is C for photons).
Re. slower than light travel - if you get fast enough (i.e. a sizable fraction of c), then, even if it takes dacedes to get where you're going, time dilation will mean that far less time passes for the crew of a spacecraft - so, if you're going fast enough, a trip of 90 light-years, say, could be accomplished within the natural lifetime of the crew without FTL travel.
There turn out to be practical problems with this. Any craft that carries its own fuel with it - including the more practical breeds of antimatter drive - will be limited to a crusing speed of about 0.1-0.2C by the specific impulse of their fuel. The only thing that could approach speeds at which time dilation would be significant is a beamed core antimatter drive (that uses the charged particle shower from an antiproton annihilation as the reaction mass), but that requires unrealistic amounts of antimatter (positrons are easy to make, but antiproton synthesis is very inefficient, and will remain so unless new physics is discovered).
In principle, some kind of sailcraft driven by a stationary laser or maser array could reach relativistic speeds, but the array would be very expensive to build and very large (we need to focus on a planet-sized sail at a range of many light-years). It would also work wonderfully as a weapon capable of melting cities to slag at a range of hundreds of AU (or even light-years, depending on configuration), so I suspect non-proliferation agreements would prevent it from being built in the first place.
In short, the only hope for relativistic travel at less than colossal cost is new physics.
"The math" also says two things:
Communication occurs at the speed of light, so round-trip time to 90 light-years is 180 years, and one-way time is 90 years.
Modern humans have existed for about 30,000 years. Human civilization has existed for on the order of 6000 years, depending on who you ask and what you call "civilization". If the lifespan of an alien technological race is longer than this - and it will be, especially once it decentralizes (makes colonies not on the same easily-bombed planet) - then, of the stretch of their civilization's existence where they can hear and respond to us, the segment where they are more advanced than us is much longer than the segment where they are less advanced than us. This makes it likely that _if_ we find someone to contact, they're in the "more advanced than us" stage.
This makes communication, even with a multiple-lifetime time lag, worth it.
This discussion overlooks the impact of any future technology that would confer either extreme longevity, or the ability to store and reconstruct a human mind-state/personality. In the first case, slower-than-light travel between the stars becomes feasible because we have the patience for it, and it doesn't take that large a chunk out of our lives. In the second case, we can be sent at the speed of light as data, with no subjective time elapsing en route, to be reconstructed at the other end.
In conclusion, communication is both possible and worthwhile even without FTL travel or exotic technologies.
The problem with putting a computer in a desk or cabinet is that it heats up due to the confined air pocket around the machine. Fans that just end up recycling hot air don't make the machine any cooler. One of my machines I've had to maintain has stability problems as a result of this.
Lastly, the air gap in the cabinet is not what's shielding the sound. It's shielded because the walls of the desk/cabinet are good at absorbing sound, and because you're farther away from it (less reaches you), and because vibrations in the desk can couple into the floor and other parts of the building you're in to sink energy instead of resonating in the room.
If anyone else is considering a similar desk mod, I'd suggest making cutouts in the back or side of the desk (something unobstructed) for a large exhaust fan and an air intake vent.
We'll probably never again be at a point where we say "What in the heck is out there?" We'll never again have Uncharted Territory. But rather we say "What in the heck will that look like up close."
This is only partly true. Many space objects are the next best thing to invisible. Barring a really concerted (and expensive) effort, we won't have maps of, say, the Kupier belt that are anywhere close to complete. Even closer to home, we only have records for the _big_ asteroids in the belt (and inner solar system).
Similarly, while we've found at least one white dwarf star in our local neighbourhood, others may very well exist that we aren't noticing - they're quite dim. Smaller objects, like gas giants ejected from systems during formation and drifting in interstellare space, or the myriad of objects in the Oort cloud, may not ever be found - unless an object emits a lot of light or is both large and quite close to a bright light source (like a star), it's lost in the void.
Think of our medium-term mapping situation as the equivalent of having the tourist brochures for the area we want to visit, and our current maps as being the blurb on the back of them. Still plenty to discover.
What makes the B-52 so effective is that it can carry tons of bombs to target. Its takes exponentially more energy to move that kind of tonnage faster (say supersonic speed).
Nitpick: It's a polynomial relation, not an exponential one. Force required goes up as te square of velocity, and power required as the cube, for cruising at a given altitude.
What actually happens is that there's a sharp reduction in range as speed goes up, due to the increased power requiremenets and the fact that you can carry a fixed amount of fuel.
Exponential relations are relatively rare, but everyone seems to like the word...
My own guess as to what will happen in the long term is that the US (and presumably other powers) will continue to maintain bases at strategically important locations near the areas they're interested in. Much easier way to maintain a fast response time. If we ever get cheap space transport, they might stock orbit with warheads that could be de-orbited on desired targets, but that's enough of a can of worms that I doubt it'll be the dominant approach to delivering strategic weapons.
On the other hand, that's just a guess.
So perhaps we can also get 7 and 9 quark particles.
In principle, we can get any number we please. The question is whether a particle with, say, 7 quarks would have lower energy than three separate particles (one baryon and two mesons). Depending on its internal energy structure, it would either be unstable, extremely unstable, or not bound as a particle at all (we know it's not stable because a) we don't see these around normally, and b) anything built from quarks and antiquarks decays [quickly if the Q and !Q have the same flavour, slowly if not]).
There has already been a search for tetraquark objects (same constituents but different energy structure than two mesons). I'm not sure how that turned out (mentioned, with paper citations, by another poster a few years back).
Isn't everything inadequate for estimating anything for the simple reason that people might be wrong?
That's why multiple completely different methods with different theoretical underpinnings are being used. It is far less likely that all are wrong than that any single approach is.
(it would be two co-orbiting, which would have different particles) ...different properties. That's what I get for slashdotting at silly-o'clock in the morning.
Think of this as a merged baryon and meson.
No. They specifically said that the merged version would have different properties.
"Merged" != "Bound".
A bound pair would not be a distinct particle (it would be two co-orbiting, which would have different particles). A pentaquark can be thought of has a merger of the two particles, containing the constituents of both but having its own energy structure.
List of the constituents is the same. Hence, my example.
I wonder what the color makeup of the pentaquark is. As I undestand it, for mesons it's red/anti-red, blue/anti-blue, or green/anti-green. For baryons it's red/blue/green or anti-red/anti-blue/anti-green. What do you do for five?
RGB plus R!R or G!G or B!B.
Think of this as a merged baryon and meson.
At that frazzle point the guy would have either contacted the local gdb guru,just rewritten the damn thing, or documented it as a known issue and blame it on hardware.
This isn't always possible. A bug that can't be reliably duplicated is difficult to pin down in a debugger, and a bug that you have no idea of the root cause of can't just be removed with a code rewrite - you don't know which block of code to rewrite.
The hardest part of debugging is identifying where the problem is. Once you know that, you can usually backtrack and fix it.
Something that stays that elusive for that long falls into the "add firewall code and pray something trips it" category.
Hmmm... energy can't be created. What did the Big Bang do, then?
In the Einsteinian physics world, energy can only be created by the destruction of matter. Which certainly was going on at the Big Bang.
Actually, the creation of matter was happening at the big bang. Matter as we know it condensed out of the high-energy primordial soup as it cooled off.
If I understand correctly, the idea is that the universe had zero net energy at formation, and so was allowed to spontaneously appear out of nothing. The energy of its contents (positive) was perfectly balanced by its gravitational potential energy (negative).
How inflation and other scalar fields (e.g. "dark energy") mesh with this is something you'd have to ask an astrophysicist about.
Sorry, but I call all those suns much more concentrated than the big bang; true, you had all energy in one place...but then again, you had all places in one place :)
Fact remeains, that there is much more structure to be observed in the current universe than when it was a singlularity.
This turns out not to be correct. The entropy of a system relates to the number of possible states that system can take, or alternatively the number of degrees of freedom in that system. If all matter and energy is in one location, the number of degrees of freedom is a lot more limited.
For a more detailed explanation, ask your local physics professor.
How does a gigantic power plant "fall" into *anyone's* hands?
Any rationalization that depends upon the alleged inability to minaturize inherently nanoscopic processes is intellectually bankrupt from the start.
And now it becomes apparent that you've been blowing smoke without any knowledge of how nuclear plants work at all.
The minimum size of a nuclear plant is determined by the distance neutrons must travel before being a) thermalized and b) captured. Both of these are many, many orders of magnitude larger than "nanoscopic".
Goodbye, troll.
A "small group of people" can't build a weapon-supplying reactor.
Let's hope you're right. The more the civilized world relies on nuclear power, the easier it becomes for it to fall into the wrong hands.
You keep saying this, yet keep failing to demonstrate _how_ it would occur.
How does a gigantic power plant "fall" into *anyone's* hands?
The technical know-how has *already* been widely disseminated, so all that's *left* are the physical structures themselves.
Fission plants need not be large. What evidence do you have that a fission plant could not operate in a 10x8x12ft toolshed? These things power many sizes of submarines, remember.
A fission plant capable of producing useful quantities of weapons-grade material is big.
expensive
I'm glad to have convinced you.
Read again. A coal-fired plant is expensive to build too. Re. your arguments, a rogue state won't bother with insurance, so your vaprous $0.20/kWh figure is doubly irrelevant. A small country that wants to produce nuclear weapons won't be able to afford a large number of full-scale nuclear plants, and has to spend quite a bit of effort building them, which makes the detection problem easier.
On the contrary, I have an EM spectrum chart that the Exploratorium sold me for a few dollars that has everything anyone needs to know about shielding up to 938 Mev. You can use water or lead, but mountain soil is usually the easiest.
If you bury a nuclear reactor in the bottom of a mineshaft, you're going to have one heck of a time cooling it.
If you do anything less than bury it at the bottom of a mine shaft, you get gamma radiation shining through. _Most_ of it is attenuated - this is why we bother with shielding at all - but not all of it. This is why there is a network of gamma-detecting satellites flying above your head.
How exactly is my supporting nuclear power in Canada proliferating nuclear weapons, again?
It is because proliferation risks are continiously devolving from nations to small groups of people. To rely upon something that you don't want everyone to have is contradictory policy.
How so? Anyone can already look up how to build a nuclear plant, so it's not a knowledge concern. I seriously doubt that some rogue group is going to steal a million-tonne building, so the plant itself doesn't help them. How do power plants here help nuclear weapons development there, if neither knowledge nor facilities are problem?
A "small group of people" can't build a weapon-supplying reactor.
Canada could dismantle all of its nuclear plants and India and Pakistan and North Korea would still be making bombs.
ObDisclaimer: I do not consider the above to be "rogue states", but name them as they are the most likely to be cited on a list of people with nuclear weapons that may be cause for concern for the lister.