Wired (perhaps) isn't confused. You (perhaps) are.
on
Dark Matter Discovered
·
· Score: 4, Informative
Hi --
Distinguishing between baryonic matter -- stuff that bears any resemblance to everything around you, whether it is visible or not -- and other "dark" matter that does not fall into that category, is actually pretty commonplace in astrophysics. This seems like semantics, but turns out to be an important distinction.
The point is that the fraction of baryonic matter in the universe is, we think, reasonably well constrained (by both observations of light element abundances in conjunction with Big Bang nucleosynthesis models, and by measurements of fluctuations in the cosmic microwave background) to be only about 5% of the total mass/energy density. Yet there's an additional matter component (accounting for about 25% of the total density) that we know little about -- this is what most astronomers mean when they say "dark matter" these days.
This article says nothing at all about that 25%. It does, however, provide some clues towards a more complete accounting of the 5% that is "normal" (i.e. baryonic) matter. This is a very significant result, but the slashdot writeup and most of the comments to this article are completely distorting it.
The puzzle regarding the "normal" 5% was this: in the local universe (redshifts less than 2), only 10% or so of it is luminous matter, stars and galaxies and the like. More (40% or so) has been accounted for by studies of cool clouds of gas residing between stars, but this still left 50% in an unknown reservoir of baryons. Theory/simulation had suggested that one such reservoir might be the "warm/hot intergalactic medium" -- gas that is heated to millions of K.
The problem is that detecting low-density gas at that temperature is quite difficult, partly since most bound electrons have been lost. Only the more massive elements retain any electrons, and so can be visible in absorption in the FUV or X-rays.
What the paper discussed here (published today in Nature) does is to describe a plausible-looking detection of such filaments of "warm-hot" gas, through X-ray absorption. They use this detection to extrapolate a matter density of this WHIM component, and find that it could account for 30-50% of the baryonic mass, and so constitute the "missing" baryonic matter.
Note that this says nothing at all new about the 25% of truly "dark" non-baryonic matter.
One fairly large quibble is that the 30-50% number represents an extrapolation from just two absorbers, over a comparatively short distance, to infer the WHIM density in the whole universe. That's sort of a big jump, in case that part wasn't obvious. But you can't do this sort of analysis for very many sightlines -- you need a really bright emitting object on the other side of the WHIM clouds if you're going to see them, and such objects are few and far between -- so for right now that's what you get.
If you happen to be somewhere that has a subscription to Nature (most universities do), you can check out the two articles related to this in today's edition:
A few people have suggested launching something very similar to HST, with the new instrumentation that was supposed to go up in servicing mission 4. One such proposal is the "Hubble Origins Probe"; they had a poster at the last American Astronomical Society meeting, the abstract of which you can read here.
That abstract begins, "A no-new-technology HST-class observatory with COS and WFC3 as its core instruments..." (COS and WF3 are the Cosmic Origins Spectrograph and new Wide-Field Camera, respectively.)
Huh? Since when were "solid computer minds" trained to respond blindly to a single stimulus: "for X, do Y"? Show me a competent computer scientist and, generally, I will show you a person who can reason effectively (and yes, logically) about the best way to solve a given problem, or perform a certain function. The ability to reason through probable outcomes of particular actions, weighing the pluses and minuses of each, is important for any but the most mindless of jobs; it's also damn handy in poker.
Poker really is a fairly logical game. It's not about being a crazy ninja badass who raises T3o UTG because he had a feeling the cards were going to fall his way; it's not about staring your opponent down from underneath asinine wrap-around sunglasses, unless you're playing against incredibly weak and malleable opponents who might, conceivably, be more prone to buckle under your icy gaze than to laugh hysterically.
It's about taking all the information available to you and making an informed decision about what course of action will make you the most money. That's it. The issue is that "all the information available to you" really includes quite a lot: your cards, of course, but also how your opponents have acted on this and on the previous 500 hands you've played against them -- whether a bet from them means top pair or possibly also a draw or a bluff; how you've been doing for the past hour and thus how your opponents are likely to perceive you; whether the guy in seat 3 is drunk.
The fact that it's *difficult* to encode all these things as inputs to a deterministic evaluation does not mean the evaluation is pointless (i.e., that poker is fundamentally not amenable to logical analysis): just that it's hard. Most players use lots of things as proxies for this kind of logical analysis -- e.g., they simply characterization the old guy as a "rock," without explicitly considering the set of all hands he has raised/folded, every time they play a hand against him. But that characterization is just a distillation of lots of subtle analysis that they've learned to do over time.
It's true that at the higher levels, you may have to intentionally play "sub-optimally" for a given hand in order to deceive your opponents about your general playing tendencies; likewise they are often trying to deceive you, either on this hand specifically or more generally about the way they play. That is indeed difficult to quantify, but a) it's simply not an issue most of the time at any level below at least 10/20 (if it is, you're in the wrong game) and b) again, it's perfectly possible in principle to incorporate deception into a logical analysis of the game, even though in practice very few people do this.
I hear this kind of derision for "playing by the odds" all the time, as if there's fundamentally any other way to play. As if only a sucker would try to quantify their odds of winning mathematically, and then take action based on those odds. As if expert players rely on some other, more subtle sense of what to do.
All of this is largely silly. If by "playing by the odds," what you really mean is "assuming that your opponent's bets are 100% representative of their holdings, calculating your chances of improving to beat that hand, and slavishly following the course of action indicated by those chances," then of course you'll conclude that such a strategy has some problems. Namely, that your opponents' bets very seldom are completely representative of what they have, either because they are intentionally trying to mislead you, or because they are idiots, or both.
The thing is, almost no one "plays by the odds" using the above definition. Not inexperienced players, not intermediate players, not expert players, and not any bot worth mentioning either. A decent player uses all the information available to him -- your betting actions, his cards, the actions of other players, and every action he has ever seen you take -- to come to a decision about what course of action will make him the most money. Sometimes that means determining that you have the nut flush, he's drawing dead, and so he folds. Sometimes that means figuring you for overcards or middle pair, and so he puts in a raise with a worse hand because he thinks you will fold. Sometimes it's making a completely ludicrous check-raise river bluff that he believes will probably be called, because 50, 100, 200 hands from now you will be forced to pay off his next river check-raise -- and that one will be with a real hand. All of these decisions can be fundamentally reduced to a determination of how likely you are to hold a given hand, how likely you are to take a given action, how likely he is to make money. It's all "playing by the odds."
I see little reason in principle why computers cannot do the above analysis with a depth that surpasses most human players. Look, even casual poker players use "Poker Tracker," a program that is essentially a database of every hand you have ever played (provided by downloading the hand histories all the major online sites provide). It lets you see at a glance whether the player who just raised preflop raises one hand in 250, or raises 1/3 of his hands, whether he folds frequently to postflop aggression, etc. This kind of very simple analysis -- just a quick check whether the guy is loony or ridiculously tight or what have you -- is pretty trivially doable by a computer, just as it's pretty trivial to do yourself given a player history database. The threat of online poker bots is that they could in principle do all this at a level you probably can't -- they could quickly analyze the last 1,000 hands they've played against you, and instantly determine how likely your turn check-raise is to be a monster, a solid made hand, a semi-bluff, or a pure bluff.
That kind of analysis would be very, very difficult to counteract -- you would either have to resort to something approaching an "optimal" strategy against such a player, or attempt to adapt your playing style so rapidly and with so much alacrity that its attempts to exploit your play would backfire. Both are, like, hard.
I don't think any publicly-available bot does the kind of analysis I'm talking about here, but I have little doubt that such programs will appear eventually. When -- not if -- they do, inexperienced players are simply not going to play online poker unless they're very very dumb. (And if they are, their money will vanish pretty quickly anyway.) Right now, the beauty of online poker is that even the worst player believes he can win, and in the short run he's right -- any two cards can win this hand, or the next. And even in the moderately long term, it's not like there are tons of amazing players frequenting the
While radiation pressure is a very real effect in many astrophysical environs, it is not the dominant support mechanism for stellar interiors.
You can check this: just ask where aT^4/3 (radiation pressure) is equal to the product of density*N_A*k*T/mu (gas pressure), with mu the mean molecular weight, Na and k atomic constants, and T the temperature. You'll get
density = 1.5 x 10^-23 T^3 g cm^-3
meaning that radiation pressure dominates gas pressure only for very high temperatures and low densities. This is the case in some outflows, for instance, but not in stellar interiors.
Any reasonable biometric -- which is to say, any biometric with a rat's chance in hell of actually being deployed to every ATM in Australia -- would employ so-called "liveness testing" to prevent the sort of attack you describe. The military is very, very interested in biometric technology -- do you honestly think it would never have occurred to them that someone might cut off your finger to fool a fingerprint detector?
In the context of a fingerprint scanner, you can check for a pulse; some properties of your skin are also different if blood flow has been cut off. With an iris scanner, you could at least check to make sure the pupil dilates when exposed to a flash of light, etc. I suspect -- though you're free to disagree, since I offer no proof -- that there are many, many other ways to do liveness testing, some of which are probably secret (since if they weren't, you could more easily figure out how to circumvent them).
It's maybe also worth noting that biometrics will probably not, in many cases, replace current methods of authentication -- rather, they can add an additional layer of security to a system, making it that much more difficult to compromise. There's a slogan about authentication methods that is much in fashion these days, which says they should be "something you have, something you know, and something you are." E.g., a card-swipe combined with a PIN, combined with a biometric. Not necessarily more convenient, but potentially more secure.
It's worth noting that a couple US airlines (particularly United) have reasonable Pacific networks -- in UA's case, that network is one of the few things the airline has going for it. You're incorrect in stating that the longest flights ex-USA are LAX-HNL; there are a number of flights to Pacific destinations that are quite a bit longer than that (and some to European ones, e.g. the SFO-LHR flights). United has daily nonstops SFO-SYD, which is a 14+ hour flight; they also have flights to Tokyo (SFO-NRT) that are likewise pretty lengthy (11-ish hours). Some of these flights (the SFO-NRT flights, and some of the HNL-Asia flights) are quite profitable, with reasonably high load factors and, just as importantly, massive cargo loads that can make them profitable to run even without a whole lot of passengers.
I'd be curious to see if Airbus is pitching the cargo capability of the 380, which is presumably quite large, heavily to UA and others, since that is part of what makes the 744s appealing on these routes. I suspect that the lack of response to the 380 from UA and other US airlines has as much to do with contractual obligations to Boeing as it does with believing the aircraft can't be profitable on these routes (or can be no more profitable than a loaded 744).
It's maybe also worth noting that some of these Pacific routes are highly coveted -- UA's landing rights at NRT, in particular -- because they are quite profitable (and allow access to inter-Asia traffic that is even more so). (Those landing rights were, after all, purchased from Pan Am in the latter stages of its demise, iirc.)
Please understand: I actually agree that the Shuttle is by this point a bloated and perhaps misguided enterprise. It's by no means an ideal vehicle for manned spaceflight.
But it's all we have, and all we will have for a while. I'm all for replacing it with something else, but in the meantime we need to keep it running.
FYI, you're correct that the NGST will not be serviceable... but that's a source of great concern to a lot of people right now, and I wouldn't be shocked if it changed. We don't want it to get out to L2 and then find out it's useless. Some folks are calling for NGST to be deployed to LEO, tested, and then somehow boosted to its final orbit; I've even heard mutterings about modifying the Shuttle to go to L2 (ha!). There are very, very compelling scientific reasons for placing NGST in a non-serviceable orbit, but it's a decision that wasn't taken lightly.
You say that "no significant science is being gained from continuing to send man into space." I humbly submit that you are wrong.:-)
I won't bore you with arguments about spin-off technology and so forth; I've never completely bought into them myself. I just want to tell you about a telescope.
For me, the Hubble Space Telescope is probably the best continuing example of why we need continuing manned spaceflight. You can argue that HST isn't worth the money, that the money would be better spent on Earth, etc., but I don't think you can argue that it doesn't return good scientific results -- if you do hold that view, I guess you can stop reading this now.
It's true that HST would have been largely useless without direct astronaut intervention early in its life. You remember those first photos, don't you? Those would've been all we had for our $2 billion or so initial investment, had HST not been serviceable. Those images of the Eagle Nebula (the "pillars of creation" that have become almost an icon)? Gone, along with countless less-heralded spectra and images and insights.
It's also true that HST should never have been screwed up in the first place, so maybe that's not a great argument for the Shuttle.:-) But there have been lots of less dramatic improvements to the instruments aboard HST over the years, continuing even today. Take, for example, the Cosmic Origins Spectrograph scheduled to be placed on HST in the next servicing mission -- I won't bore you with a catalog of all the scientific justifications for COS (you can find them if you want), but rest assured that the astronomical community, anyway, was excited by it. We need human intervention to install COS on HST, as we have needed it for countless other maintenance tasks and as we will need it again. (We also, incidentally, need the Shuttle to periodically reboost HST -- fortunately this was done in the last servicing mission, so we're fine till maybe 2007. But if the Shuttle isn't in operation again by then, HST will come down.)
There will probably be other missions like HST, missions that for whatever reason will require human intervention if they are to succeed. Maybe they will be faulty in some regard, and in need of repair; maybe they'll just need maintenance or upgrading or whatever. But they'll need something, every great once in a while.
You can argue that it isn't worth it, that the costs and risks of manned spaceflight outweigh the benefits; it's a perfectly legitimate argument, one I respect a great deal. I just want you to realize that there are scientific benefits, and that you (or some of us, anyway) will miss them if the capability for manned spaceflight disappears. Note that I'm not arguing that the Shuttle itself is a perfect launch vehicle, and I'm sure as hell not arguing that the ISS alone is a reason for sustaining human spaceflight.
There are other, less tangible benefits to human spaceflight; but they are appeals to the soul, not the mind, and it is for each of us to decide how much weight they can hold. That is a topic for another post; this one is long enough.
IAAAA (I am also an astronomer), and while I agree with almost everything you've said here, it's worth noting two things: one, that some of the things that make research-grade CCDs expensive and impractical are supreme non-issues for consumer-grade stuff, and two, I'd hope (though I am by no means certain) that film will remain in use as a complement to digital format, precisely because of what we've seen in the astronomy community. I'll explain what I mean by this below.
On the first point, as you're probably well aware: for serious work, we need CCDs with extremely low read-noise, dark current that is as near to zero as possible, and decent linearity over as much range as possible. The first two of these are non-issues for consumer use, and the third is (imho) actually an explicit anti-goal. Who cares about a few electrons of thermal noise popping around, when you have signals that are many orders of magnitude larger? And perfectly linearity means that your pixel saturates sooner (relative to a system like film where relative response decreases as counts increase), which is exactly the opposite of what you want if you're looking to get a large dynamic range.
As to the second point: personally, I miss the pictures you got with plates. Not the science, not the many hours of additional labor, etc, etc, just the pure artistic "feel" of those pictures. This is not to say that pictures created from CCD data can't be stunning -- I've made images from HST-WFPC2 data that I'm certainly proud of, and others have done far better -- but I think it's hard to deny that they are ineffably "different" than pictures that came from plates. Walking down the halls of the observatory here, you can pick out at a glance which of the many gallery-quality prints were made from film and which from CCD data; the same is generally true in looking at the amateur magazine galleries, though there the comparison is less fair because they're using consumer-grade equipment. I personally am happy that pictures made in the old style of certain objects exist. I am also happy that I never had to spend hours going over plates with a densitometer, trying to get some half-hearted measurement of counts and then trying to figure out the non-linear translation to actual photons, so I guess it's a fair trade.:-) Still, I hope that people continue to use film, because while it may not be "better," it is undeniably "different," and different in a way that I (sometimes) happen to like.
In the past, I have always agreed with the statement that digital cameras would be unable to match the dynamic range of film for a long time. However, I was intrigued by the following comments in the posted review: "Few photographers will find the 1Ds wanting with regard to available dynamic range. I judge it to be about one to two stops better than transparency film, and roughly comparable to colour negative film - but of course much less noisy / grainy."
Now, I am tempted not to take this at face value, because there are good reasons why CCDs should essentially never have the dynamic range possible with film. (Essentially: film responds to light non-linearly, such that x photons hitting your camera does not equal the same amount of "brightness" on your image independent of how many previous photons have been registered. CCDs basiclaly are linear in response -- x photons equals x number of counts, modulo factors of gain, etc. -- up to the point where the number of photons registered is a significant fraction (like say 1/2) of the maximum well depth. Note that film is in this way more like your eye: an object that is twice as luminous does not look twice as bright to your eye, and you can simulaneously see things with your eyes that are many orders of magnitude apart in true brightness. To go even more off-topic in this comment: this is basically the reason why the most common stellar magnitude scale is defined logarithmically, where a difference of one magnitude corresponds to a factor of about 2.5 in brightness; it's an historical relic of the fact that when Hipparchos looked out at the stars, he called the brightest ones "1st magnitude" and some of the faintest ones "6th magnitude"... and the latter turn out to be about 100 times dimmer than the former. Whew.)
Having said that, though, I don't actually have one of these things, and he doesn't really post any objective backup for his statements about dynamic range, so it's hard to prove or disprove them. He probably does know a hell of a lot more about photography than I do, so I'm sort of tempted to believe that they dynamic range issue is ceasing to be a problem, even if only by careful post-processing and choice of exposure. fwiw.
Looks to me like you must have made an error in your calculations.
Escape velocity from the Sun at a given radius,r, is just sqrt(2*G*M_sun/r). Plugging in (G=6.67e-8 in cgs units; M=2e33 g; r=93 AU = 93 *(1.496e13 cm)), I get v_escape of about 4.4e5 cm/s, or 4.4 km/s. (About 15,800 km/hr, or 9800 mi/hr, safely less than Voyager's velocity.)
It's a pleasure to hear from someone with real expertise in the area; thanks for the post. However, given that the vast majority of Slashdot readers aren't going to read both your paper and the Drake et al. preprint, you might consider posting a bit more detailed critique of their analysis. Their paper actually struck me as fairly reasonable, over-the-top press releases notwithstanding. While I agree that extraordinary claims require extraordinary proof, which they haven't really yet provided, there are some issues raised in their work that bear discussing.
In particular, the lack of pulsation isn't quite the only thing pointing to something odd going on (as I'm sure you're aware, but some people might not be). They find a fairly good spectral fit to a 60 ev (700,000-ish K) blackbody, which yields a radius less than the 10-12 km or so allowed by current NS theories; while I haven't really gone over their paper in detail, they claim to have ruled out two-component blackbodies, at least at any level that would contribute appreciably to the flux, and power-law sources at high confidence. And while there remains some question as to the distance (the Walter (2001) measurements of 60 pc versus Kaplan et al (2002)'s 140 pc or so), I think their arguments in support of the larger distance (e.g., the larger distance is more in agreement with neutral Hydrogen column measurements plus standard physical density estimates) are reasonably compelling, albeit prone to criticism.
I'd be curious to hear your thoughts on this -- i.e., do you think a much-lower temperature blackbody (or "hot spot" model) is not truly excluded by the data on this line of sight? Because the lack of pulsation here is just one part of the puzzle.:-)
indeed. I think "One day..." has the best ending of any book, ever.
(For any random people who have happened upon this thread, the last couple lines are something like: "There were 3,653 days just like that in Ivan Denisovich's sentence. The extra three were for leap years." Wow.):-)
Re:Is Hubble So useful? Adaptive optics is cheaper
on
Happy Birthday Hubble
·
· Score: 2
If all you cared about was image resolution, then sure, AO would be great. But you're neglecting a much more fundamental limitation of graound-based astronomy, which is that the Earth's atmosphere is quite opaque over large regions of wavelength space -- that is, you just can't observe in, say, the far ultraviolet from the ground, because you can't see through the atmosphere.
Of course, this is an argument for space-based missions generally, not HST specifically. But still, Hubble is a great orbiting platform with unbelievably good pointing (amazing what you can do with a few billion dollars), whose general operational procedures / problems are well understood. Contrast this with a new satellite system, built from the ground up.
Note, also, that great things are still in the cards for Hubble -- eg the Cosmic Origins Spectrograph, which will be installed onboard in a couple years. So it's not just decades-old technology that has since been superseded by ground-based stuff...
Did you notice the little bit in the SciAm article about increasing page charges? This seems to be the "solution" some publishers are considering to the backlash against high subscription fees, their rationale being that it's better to charge the authors than the readers.
It's easy to see that they'll have a lot of support for this, from a lot of people -- vastly more people might have an occasional interest in reading, say, the Astrophysical Journal than have an interest in writing an article for it. And the publishers are able to defend an increase in page charges by saying it's paid for by researchers' grants, not by the researchers themselves.
I'm a co-author on a paper which will be submitted to the ApJ in the next few weeks; the page charges will be several thousand dollars. We haven't really bothered about it too much, because there's no real way to avoid paying it, and besides the grant money is there. It should also be mentioned that this paper is a somewhat extreme case -- around 20 pages, with a number of color figures (which are, I think, $600 for the first and $150 thereafter, but which are also unfortunately necessary). ApJ charges around $130 a page, IIRC, so you can do the math.
Does this strike you as absurd? It does me. $3000 is more than a trip to a great conference costs, more than the cost of supporting an observing run, more than a lot of things. It's only bearable because I happen to be doing space-based astronomy, where grants are big enough to support these kinds of outlays. But the problem is that a lot of research doesn't need big grants, or shouldn't -- I know plenty of people who do pure analytical theory which doesn't even require applying for supercomputer time. Admittedly, faculty at many institutions have to apply for grant money to pay their summer salary, so it's not totally indefensible, but still : a thousand bucks can take a pretty healthy chunk out of many grants. Some journals allow "hardship exemptions," whereby page charges are waived, but I don't know how easy/difficult it is to get them.
I've often suspected that a major driver behind page charges is their action as a "gatekeeper": ApJ probably doesn't get a lot of cranks submitting wacko stuff, b/c who the hell would be willing to pay a thousand bucks of their own money to see their article printed there? But I think page charges have the unpleasant tendency to constrain good research as well. Other things do this -- ie, there is already a tendency to work in areas where you know the money is easy to come by -- but that doesn't make it defensible.
As with all interesting things, there are no easy answers here. I don't think the ApJ (to keep using the same example) is an evil institution -- it's a publication of the American Astronomical Society, which is a non-profit organization that does many good things. And I'd be very surprised if the AAS didn't derive an appreciable fraction of its operating budget from ApJ-related charges; making the journal charge less overall would probably mean fewer activities funded by the AAS. It's also worth noting that astronomy/astrophysics journals (ApJ, AJ, A and A) are perhaps unusual in that they have no ads, so that's not a potential source of income. Note, also, that currently issues of ApJ more than three years old are available online without a subscription -- but see aforementioned bits about how much we pay for this privilege.
The point of my (absurdly long) diatribe is this: if researchers are able to "convince" publishers to supply online versions of everything for free with no negative repurcussions, great. But if the publishers recoup some of their lost subscription charges by increasing page charges, well, maybe that ain't so great. I don't know what page charges are for biosciences journals are these days, and I don't know enough about the culture of research in that field to know whether dramatic increases in those charges would have a seriously detrimental effect. If the current charges are low, would these folks be willing to accept charges similar to those "enjoyed" in the astro community? Would they be willing to accept charges running into the several thousand dollar range? I don't know, but I suspect we may find out.
I don't know that they expect to get hugely improved spatial resolution (this is just a guess, as I tend to sort of phase out talks about 100-meter class telescopes) -- it's still something to shoot for, though, since there's really no substitute for aperture in terms of light-gathering power. (You can use adaptive optics till the cows come home, but if you don't have a big enough aperture, you'll be integrating for a month.)
You raise a good point, though. I am curious about their plans for AO on such a large scope -- I've always understood this as a fundamental limitation, independent of the technology involved. (Err, you sound like you're almost certainly aware of this, but : Each point on your secondary corresponds to multiple points on the sky, but the mirror can only be in one place at a time. thus, you can only correct for one of the real-sky positions which maps to a given spot on your secondary. This isn't a big deal if you have a small scope, but when you talk about going over 20m, I really don't see how you can get around this, since atmospheric variations occur on scales smaller than this.)
Suppose I'm sitting on my porch, taking a long exposure of something many miles away. (Err, I live on a mountaintop, and it's dark. Or something like that.) And suppose that while I'm taking the picture, I pace nervously back and forth across my porch, holding the shutter open all the while, but keeping my camera pointed at that distant object the whole time. No problem, right? The object will appear to change slightly as I view it from different angles, but if the distance between me and it is much much greater than the width of my porch, the effect is negligible.
Same sort of deal here. Chandra can remain pointed to reasonably high accuracy on something as it moves in its orbit. Sure, it's moving (and so is the Earth, and so is the distant object, etc.), but as long as the telescope remains pointed at the object in question, you're fine.
There are actually several ways to come up with some estimate of how much mass is dark -- ie, "missing," though I hate that term.
The key thing is that, for a given system, you come up with some way to determine the total mass, and then compare that with the luminous mass (ie, the stuff you can see) -- that's really all it comes down to. The trick, of course, is in figuring out a way to get that estimate of the total mass.
There are several possible solutions. If you're looking at some systems (like, say, a cluster of galaxies), you can look at the "velocity dispersions" of the component galaxies, and from those (and using the assumption that the cluster has more-or-less virialized) get an estimate of the total mass that is "pulling" on each galaxy. You can also use gravitational lensing, as suggested by another poster. With clusters, you actually have another (very spiffy) option, which is to look at the X-ray emission from the hot gas between the galaxies in the cluster -- the temperature of the gas, which relates to the X-ray luminosity, is also related to the mass of the cluster.) The results with all of these methods suggest that a great deal of the mass in large clusters of galaxies is non-luminous.
In our own galaxy (and others), you can do a similar sort of analysis and come up with the same sort of result (that much of the mass must be dark). (Here, the usual procedure is to form a "rotation curve," which shows the velocity as a function of radius from galactic center; if you do this, you find that the radial velocity "flattens out" and stays that way for much longer than we'd expect unless there is a substantial "dark" mass in a halo surrounding the galaxy.) It's a little dangerous to make comparisons like these, since it's possible that the source of "extra" mass may be different on large (ie cluster) and small (ie galaxy) scales, but the basic idea is the same: that a big, big chunk of the matter in the Universe is probably not luminous.
Sure, the Shuttle is expensive as a launch vehicle. But only part of that is the cost of the actual launch itself : a hidden, but very real (and rather large) cost to those who might use it is the additional quality assurance program you have to go through to certify any reasonably-sized payload to fly on the Shuttle. Because the Shuttle carries humans -- so if your payload screws up in a catastrophic way, people might very possibly die -- you have to prove that your payload is safe in a much more rigorous fashion than with unmanned launch vehicles; this doesn't just mean proving "my payload won't explode," you have to guarantee that no screw can pop loose and rupture something in the cargo bay -- which entails lots of nasty things like x-raying components and looking for signs of metal stress -- plus lots of other stuff. Thus, development cost for a mission designed to fly on the Shuttle is higher by at least a factor of 3 in most cases than one designed to fly on an unmanned vehicle.
Some applications don't care about this, and I expect you're correct in thinking that if the actual launch cost went down, that might lower the barrier to some companies, and of course there are some things where you actually need the Shuttle, but there will still be many, many applications for which the Shuttle is not a cost-effective launch vehicle. It's a two-edged sword: the Shuttle is great because it's a manned vehicle, but it's also terrible for the same reason, and there's no good way around that.
I don't know if I'm feeding the troll, or what, but hold on just a second. You state nothing but generalization, ie, "People forget that..." or "Both branches religiously attempt to..." in a manner which is, IMHO, patently absurd. Disagree with specific theories --fine, challenge their assertions --fine; make blanket statements about the motivations and methodologies of the practioners -- bullshit. Astrophysics is nothing more or less than the application of ordinary everyday physics to astronomical phenomena; casting aside for the moment certain special cases, you can't argue one is flawed without casting aside the other as well. It may surprise you to note (maybe not, I dunno) that the vast majority of astronomers are not cosmologists; that most study the births and deaths of stars or the generation of magnetic fields or the composition of the interstellar medium or some such other random topic -- the point is that in very few cases does astrophysics involve fundamentally new physics, and when it does, there are usually many people outside the astrophysics community engaged in an attempt to understand such physics.
Even the "example" you quote, that of "how the Universe came into existence," about which you say, "the fact of the matter is that we know nothing..." is a poor one. Like all scientists, astrophysicists have formulated a theory (namely, inflationary Big Bang) which explains certain observational facts, makes certain predictions, etc.; furthermore (and quite significantly), it is generic in the sense that the results it predicts are not sensitive to a large number of parameters. I could yak on about this ad nauseum, but briefly, observational corroboration includes the existence of a Hubble flow (velocity is proportional to distance), the existence AND detailed properties of the cosmic microwave background (to wit, its isotropy to one part in 10 ^ 5 or so, the fact that it has the spectrum of a perfect 2.7 K blackbody, and the manner of the very small-scale anisotropies), and the ratio of light element abundances. Show me another theory that predicts all these things, in a natural way (ie, without invoking 27 free parameters to tune as you wish), and I'll listen to you; but hell, do that and you've probably won a Nobel anyway, so why bother with me? Furthermore, the physics of the Universe before nonlinearity should actually be very well described by "normal" physics -- and if it's not, that's a fundamental problem, independent of the application; the kicker, though, is that we have no solid evidence that the physics here doesn't work.
To sum up, my NSHO is that you are a) unclear about what exactly astrophysics entails, is, and relates to, and b) are unable to back up your broad claims of illegitimacy with fact. Your one anecdotal evidence of the evils of astrophysics, regarding the greenhouse effect on Venus, is so incidental as to be laughable: I know jack about the atmosphere of Venus, or about greenhouse gases in general, or really about planetary science in general, but then again I don't really have to; the vast majority of topics in astrophysics have absolutely zero to do with it. My totally biased opinion is that Carl Sagan probably has given more thought to this than I have, or than you have, or even (perish the thought) than Gunnar Heinsohn, whoever the hell he is. Your argument is like suggesting that if a chemist can't predict the bonding properties of C60, all of chemistry is flawed.
What I find interesting is the way you've clearly been using LaTeX waaaaay too much -- \Omega has become almost subconscious for me, too.:-)
BTW -- and I could be remember things incorrectly here, but I don't think so -- the issue of dark-matter bias (how well the visible matter traces the dark matter) doesn't really affect these particular results too much, at least as long as you assume the bias isn't changing dramatically over time. (And as long as you take whatever bias you pick as a prior for all possible values of Omega, as opposed to mushing it around as you see fit.) You're right that there's degeneracy, but I think most of that comes from the lambda, Omega business, and the distributions of clusters expected are not totally indistinguishable, just close.
the questions you pose don't have straightforward answers -- at least not ones that appear straightforward to me. But you can come up with some broad estimates.
A rough estimate of interstellar gas density is on the order of 1 particle per cm^3 -- a bit lower (0.1) between clouds, a bit higher (20) in diffuse clouds, much higher (10^3 - 10^6) in molecular clouds.
At these densities, it turns out that the likely effect of galaxy - galaxy collisions may be to strip out a large portion of the gas in both galaxies. Certainly the large-scale effects are enormous, and you can see them in our own Milky Way -- the galactic disk is "warped" upwards by as much as 4Kpc (12,000 light years) at large (20 kpc) distances from the nucleus, and this is thought to be a result of a tidal interaction long ago with the Large/Small Magellanic Clouds. Also, one of the most popular theories for how elliptical galaxies (or at least some elliptical galaxies) form is via collisions between spirals -- ellipticals have very little gas and dust. This theory is borne out somewhat by the fact that the concentration of ellipticals is much higher in rich clusters of galaxies than it is in the "field" -- as the density goes up, you would expect more collisions, hence more formation of ellipticals.
And hey, while we're at it, the process of gas stripping is a fundamental issue in the study of clusters of galaxies. As galaxies in a rich cluster move through the (very hot) intracluster medium, a shock develops and basically pushes a bunch of the gas out -- for a relatively simple physical analysis of this situation, see for instance Shore's book on Astrophysical Hydrodynamics.
But to get back to the original issue: if we were sitting on Earth when the MW collided with Andromeda, what would it be like? The answer is that I don't really know -- my hunch is that the local (in both space and time) effects would not be all that great; life around the Sun would probably get along just fine. But I don't know, because I'm too lazy to work out the problem.:-) (I'm sure this is in the literature somewhere, if you're truly dedicated -- try the Astronomy and Astrophysics data abstract service,.) Certainly the very long-term effects would be enormous, though.
Yes, they're pretty, but that doesn't mean they're scientifically useless. I've harped about this before, but the point is that there's a lot to be had from pictures like these; I don't know the details of this particular image, but in general, you can often get a sort of "poor man's spectroscopy" using narrow-band pictures. By taking images centered around some wavelength of particular interest (say the [OIII] doublet at 5007 angstroms, and hbeta at 4861), and doing some appropriate calibrations (subtracting the "continuum" level from the images, and calibrating to some absolute flux units), you can even get maps of the temperature or density of these objects. (Err, okay, so this doesn't really apply to this particular picture, but you get the idea -- I've used this technique successfully with HST WFPC2 imagery of planetary nebulae before.) In the particular case of colliding galaxies, there's also a lot to be said for "fuzzier" science, looking qualitatively at what happens in this kind of situation and trying to generalize it. You might look at the morphology of the spiral arms, or where you see the most star formation (by looking in Halpha) -- is the region of most star formation highly correlated with some aspects of the collision? Can you maybe infer something about the gas dynamics in this system, just by looking at enough pictures like this? The answer is yes, though of course there are limits.
End result: certainly I agree with you that media other than imaging have their place -- spectroscopy is the way to go for a lot of things. And other wavelength bands (as you say, IR, UV, x-ray, etc.) are important, too -- but don't knock the visual band!:-) And "nice pictures" and nice science aren't necessarily mutually exclusive.
Slashdot Mirror
Hi --
Distinguishing between baryonic matter -- stuff that bears any resemblance to everything around you, whether it is visible or not -- and other "dark" matter that does not fall into that category, is actually pretty commonplace in astrophysics. This seems like semantics, but turns out to be an important distinction.
The point is that the fraction of baryonic matter in the universe is, we think, reasonably well constrained (by both observations of light element abundances in conjunction with Big Bang nucleosynthesis models, and by measurements of fluctuations in the cosmic microwave background) to be only about 5% of the total mass/energy density. Yet there's an additional matter component (accounting for about 25% of the total density) that we know little about -- this is what most astronomers mean when they say "dark matter" these days.
This article says nothing at all about that 25%. It does, however, provide some clues towards a more complete accounting of the 5% that is "normal" (i.e. baryonic) matter. This is a very significant result, but the slashdot writeup and most of the comments to this article are completely distorting it.
The puzzle regarding the "normal" 5% was this: in the local universe (redshifts less than 2), only 10% or so of it is luminous matter, stars and galaxies and the like. More (40% or so) has been accounted for by studies of cool clouds of gas residing between stars, but this still left 50% in an unknown reservoir of baryons. Theory/simulation had suggested that one such reservoir might be the "warm/hot intergalactic medium" -- gas that is heated to millions of K.
The problem is that detecting low-density gas at that temperature is quite difficult, partly since most bound electrons have been lost. Only the more massive elements retain any electrons, and so can be visible in absorption in the FUV or X-rays.
What the paper discussed here (published today in Nature) does is to describe a plausible-looking detection of such filaments of "warm-hot" gas, through X-ray absorption. They use this detection to extrapolate a matter density of this WHIM component, and find that it could account for 30-50% of the baryonic mass, and so constitute the "missing" baryonic matter.
Note that this says nothing at all new about the 25% of truly "dark" non-baryonic matter.
One fairly large quibble is that the 30-50% number represents an extrapolation from just two absorbers, over a comparatively short distance, to infer the WHIM density in the whole universe. That's sort of a big jump, in case that part wasn't obvious. But you can't do this sort of analysis for very many sightlines -- you need a really bright emitting object on the other side of the WHIM clouds if you're going to see them, and such objects are few and far between -- so for right now that's what you get.
If you happen to be somewhere that has a subscription to Nature (most universities do), you can check out the two articles related to this in today's edition:
There's a "news and views" article by Mike Shull that's a nice summary of the issues involved. And there's the full research article by Nicastro et al.
Hope that clears at least a few things up. If I have time later tonight, I'll try to come back and respond to some of your other points.
cheers.
A few people have suggested launching something very similar to HST, with the new instrumentation that was supposed to go up in servicing mission 4. One such proposal is the "Hubble Origins Probe"; they had a poster at the last American Astronomical Society meeting, the abstract of which you can read here.
..." (COS and WF3 are the Cosmic Origins Spectrograph and new Wide-Field Camera, respectively.)
That abstract begins, "A no-new-technology HST-class observatory with COS and WFC3 as its core instruments
There's also a brief article about this at New Scientist.
I'm not crazy about this idea, for a bunch of reasons, but it is under active investigation.
Huh? Since when were "solid computer minds" trained to respond blindly to a single stimulus: "for X, do Y"? Show me a competent computer scientist and, generally, I will show you a person who can reason effectively (and yes, logically) about the best way to solve a given problem, or perform a certain function. The ability to reason through probable outcomes of particular actions, weighing the pluses and minuses of each, is important for any but the most mindless of jobs; it's also damn handy in poker.
Poker really is a fairly logical game. It's not about being a crazy ninja badass who raises T3o UTG because he had a feeling the cards were going to fall his way; it's not about staring your opponent down from underneath asinine wrap-around sunglasses, unless you're playing against incredibly weak and malleable opponents who might, conceivably, be more prone to buckle under your icy gaze than to laugh hysterically.
It's about taking all the information available to you and making an informed decision about what course of action will make you the most money. That's it. The issue is that "all the information available to you" really includes quite a lot: your cards, of course, but also how your opponents have acted on this and on the previous 500 hands you've played against them -- whether a bet from them means top pair or possibly also a draw or a bluff; how you've been doing for the past hour and thus how your opponents are likely to perceive you; whether the guy in seat 3 is drunk.
The fact that it's *difficult* to encode all these things as inputs to a deterministic evaluation does not mean the evaluation is pointless (i.e., that poker is fundamentally not amenable to logical analysis): just that it's hard. Most players use lots of things as proxies for this kind of logical analysis -- e.g., they simply characterization the old guy as a "rock," without explicitly considering the set of all hands he has raised/folded, every time they play a hand against him. But that characterization is just a distillation of lots of subtle analysis that they've learned to do over time.
It's true that at the higher levels, you may have to intentionally play "sub-optimally" for a given hand in order to deceive your opponents about your general playing tendencies; likewise they are often trying to deceive you, either on this hand specifically or more generally about the way they play. That is indeed difficult to quantify, but a) it's simply not an issue most of the time at any level below at least 10/20 (if it is, you're in the wrong game) and b) again, it's perfectly possible in principle to incorporate deception into a logical analysis of the game, even though in practice very few people do this.
Bah, this is too long. Whatever.
have a nice day.
I hear this kind of derision for "playing by the odds" all the time, as if there's fundamentally any other way to play. As if only a sucker would try to quantify their odds of winning mathematically, and then take action based on those odds. As if expert players rely on some other, more subtle sense of what to do.
All of this is largely silly. If by "playing by the odds," what you really mean is "assuming that your opponent's bets are 100% representative of their holdings, calculating your chances of improving to beat that hand, and slavishly following the course of action indicated by those chances," then of course you'll conclude that such a strategy has some problems. Namely, that your opponents' bets very seldom are completely representative of what they have, either because they are intentionally trying to mislead you, or because they are idiots, or both.
The thing is, almost no one "plays by the odds" using the above definition. Not inexperienced players, not intermediate players, not expert players, and not any bot worth mentioning either. A decent player uses all the information available to him -- your betting actions, his cards, the actions of other players, and every action he has ever seen you take -- to come to a decision about what course of action will make him the most money. Sometimes that means determining that you have the nut flush, he's drawing dead, and so he folds. Sometimes that means figuring you for overcards or middle pair, and so he puts in a raise with a worse hand because he thinks you will fold. Sometimes it's making a completely ludicrous check-raise river bluff that he believes will probably be called, because 50, 100, 200 hands from now you will be forced to pay off his next river check-raise -- and that one will be with a real hand. All of these decisions can be fundamentally reduced to a determination of how likely you are to hold a given hand, how likely you are to take a given action, how likely he is to make money. It's all "playing by the odds."
I see little reason in principle why computers cannot do the above analysis with a depth that surpasses most human players. Look, even casual poker players use "Poker Tracker," a program that is essentially a database of every hand you have ever played (provided by downloading the hand histories all the major online sites provide). It lets you see at a glance whether the player who just raised preflop raises one hand in 250, or raises 1/3 of his hands, whether he folds frequently to postflop aggression, etc. This kind of very simple analysis -- just a quick check whether the guy is loony or ridiculously tight or what have you -- is pretty trivially doable by a computer, just as it's pretty trivial to do yourself given a player history database. The threat of online poker bots is that they could in principle do all this at a level you probably can't -- they could quickly analyze the last 1,000 hands they've played against you, and instantly determine how likely your turn check-raise is to be a monster, a solid made hand, a semi-bluff, or a pure bluff.
That kind of analysis would be very, very difficult to counteract -- you would either have to resort to something approaching an "optimal" strategy against such a player, or attempt to adapt your playing style so rapidly and with so much alacrity that its attempts to exploit your play would backfire. Both are, like, hard.
I don't think any publicly-available bot does the kind of analysis I'm talking about here, but I have little doubt that such programs will appear eventually. When -- not if -- they do, inexperienced players are simply not going to play online poker unless they're very very dumb. (And if they are, their money will vanish pretty quickly anyway.) Right now, the beauty of online poker is that even the worst player believes he can win, and in the short run he's right -- any two cards can win this hand, or the next. And even in the moderately long term, it's not like there are tons of amazing players frequenting the
You can check this: just ask where aT^4/3 (radiation pressure) is equal to the product of density*N_A*k*T/mu (gas pressure), with mu the mean molecular weight, Na and k atomic constants, and T the temperature. You'll get
density = 1.5 x 10^-23 T^3 g cm^-3
meaning that radiation pressure dominates gas pressure only for very high temperatures and low densities. This is the case in some outflows, for instance, but not in stellar interiors.
The article, though, is still bollocks. :-)
In the context of a fingerprint scanner, you can check for a pulse; some properties of your skin are also different if blood flow has been cut off. With an iris scanner, you could at least check to make sure the pupil dilates when exposed to a flash of light, etc. I suspect -- though you're free to disagree, since I offer no proof -- that there are many, many other ways to do liveness testing, some of which are probably secret (since if they weren't, you could more easily figure out how to circumvent them).
It's maybe also worth noting that biometrics will probably not, in many cases, replace current methods of authentication -- rather, they can add an additional layer of security to a system, making it that much more difficult to compromise. There's a slogan about authentication methods that is much in fashion these days, which says they should be "something you have, something you know, and something you are." E.g., a card-swipe combined with a PIN, combined with a biometric. Not necessarily more convenient, but potentially more secure.
I'd be curious to see if Airbus is pitching the cargo capability of the 380, which is presumably quite large, heavily to UA and others, since that is part of what makes the 744s appealing on these routes. I suspect that the lack of response to the 380 from UA and other US airlines has as much to do with contractual obligations to Boeing as it does with believing the aircraft can't be profitable on these routes (or can be no more profitable than a loaded 744).
It's maybe also worth noting that some of these Pacific routes are highly coveted -- UA's landing rights at NRT, in particular -- because they are quite profitable (and allow access to inter-Asia traffic that is even more so). (Those landing rights were, after all, purchased from Pan Am in the latter stages of its demise, iirc.)
But it's all we have, and all we will have for a while. I'm all for replacing it with something else, but in the meantime we need to keep it running.
FYI, you're correct that the NGST will not be serviceable... but that's a source of great concern to a lot of people right now, and I wouldn't be shocked if it changed. We don't want it to get out to L2 and then find out it's useless. Some folks are calling for NGST to be deployed to LEO, tested, and then somehow boosted to its final orbit; I've even heard mutterings about modifying the Shuttle to go to L2 (ha!). There are very, very compelling scientific reasons for placing NGST in a non-serviceable orbit, but it's a decision that wasn't taken lightly.
I won't bore you with arguments about spin-off technology and so forth; I've never completely bought into them myself. I just want to tell you about a telescope.
For me, the Hubble Space Telescope is probably the best continuing example of why we need continuing manned spaceflight. You can argue that HST isn't worth the money, that the money would be better spent on Earth, etc., but I don't think you can argue that it doesn't return good scientific results -- if you do hold that view, I guess you can stop reading this now.
It's true that HST would have been largely useless without direct astronaut intervention early in its life. You remember those first photos, don't you? Those would've been all we had for our $2 billion or so initial investment, had HST not been serviceable. Those images of the Eagle Nebula (the "pillars of creation" that have become almost an icon)? Gone, along with countless less-heralded spectra and images and insights.
It's also true that HST should never have been screwed up in the first place, so maybe that's not a great argument for the Shuttle.
There will probably be other missions like HST, missions that for whatever reason will require human intervention if they are to succeed. Maybe they will be faulty in some regard, and in need of repair; maybe they'll just need maintenance or upgrading or whatever. But they'll need something, every great once in a while.
You can argue that it isn't worth it, that the costs and risks of manned spaceflight outweigh the benefits; it's a perfectly legitimate argument, one I respect a great deal. I just want you to realize that there are scientific benefits, and that you (or some of us, anyway) will miss them if the capability for manned spaceflight disappears. Note that I'm not arguing that the Shuttle itself is a perfect launch vehicle, and I'm sure as hell not arguing that the ISS alone is a reason for sustaining human spaceflight.
There are other, less tangible benefits to human spaceflight; but they are appeals to the soul, not the mind, and it is for each of us to decide how much weight they can hold. That is a topic for another post; this one is long enough.
Just my 2 cents. (and yes, IAAA.)
On the first point, as you're probably well aware: for serious work, we need CCDs with extremely low read-noise, dark current that is as near to zero as possible, and decent linearity over as much range as possible. The first two of these are non-issues for consumer use, and the third is (imho) actually an explicit anti-goal. Who cares about a few electrons of thermal noise popping around, when you have signals that are many orders of magnitude larger? And perfectly linearity means that your pixel saturates sooner (relative to a system like film where relative response decreases as counts increase), which is exactly the opposite of what you want if you're looking to get a large dynamic range.
As to the second point: personally, I miss the pictures you got with plates. Not the science, not the many hours of additional labor, etc, etc, just the pure artistic "feel" of those pictures. This is not to say that pictures created from CCD data can't be stunning -- I've made images from HST-WFPC2 data that I'm certainly proud of, and others have done far better -- but I think it's hard to deny that they are ineffably "different" than pictures that came from plates. Walking down the halls of the observatory here, you can pick out at a glance which of the many gallery-quality prints were made from film and which from CCD data; the same is generally true in looking at the amateur magazine galleries, though there the comparison is less fair because they're using consumer-grade equipment. I personally am happy that pictures made in the old style of certain objects exist. I am also happy that I never had to spend hours going over plates with a densitometer, trying to get some half-hearted measurement of counts and then trying to figure out the non-linear translation to actual photons, so I guess it's a fair trade. :-) Still, I hope that people continue to use film, because while it may not be "better," it is undeniably "different," and different in a way that I (sometimes) happen to like.
Just my 2 cents. :-)
Now, I am tempted not to take this at face value, because there are good reasons why CCDs should essentially never have the dynamic range possible with film. (Essentially: film responds to light non-linearly, such that x photons hitting your camera does not equal the same amount of "brightness" on your image independent of how many previous photons have been registered. CCDs basiclaly are linear in response -- x photons equals x number of counts, modulo factors of gain, etc. -- up to the point where the number of photons registered is a significant fraction (like say 1/2) of the maximum well depth. Note that film is in this way more like your eye: an object that is twice as luminous does not look twice as bright to your eye, and you can simulaneously see things with your eyes that are many orders of magnitude apart in true brightness. To go even more off-topic in this comment: this is basically the reason why the most common stellar magnitude scale is defined logarithmically, where a difference of one magnitude corresponds to a factor of about 2.5 in brightness; it's an historical relic of the fact that when Hipparchos looked out at the stars, he called the brightest ones "1st magnitude" and some of the faintest ones "6th magnitude" ... and the latter turn out to be about 100 times dimmer than the former. Whew.)
Having said that, though, I don't actually have one of these things, and he doesn't really post any objective backup for his statements about dynamic range, so it's hard to prove or disprove them. He probably does know a hell of a lot more about photography than I do, so I'm sort of tempted to believe that they dynamic range issue is ceasing to be a problem, even if only by careful post-processing and choice of exposure. fwiw.
Escape velocity from the Sun at a given radius,r, is just sqrt(2*G*M_sun/r). Plugging in (G=6.67e-8 in cgs units; M=2e33 g; r=93 AU = 93 *(1.496e13 cm)), I get v_escape of about 4.4e5 cm/s, or 4.4 km/s. (About 15,800 km/hr, or 9800 mi/hr, safely less than Voyager's velocity.)
It was an interesting thought, though. :-)
In particular, the lack of pulsation isn't quite the only thing pointing to something odd going on (as I'm sure you're aware, but some people might not be). They find a fairly good spectral fit to a 60 ev (700,000-ish K) blackbody, which yields a radius less than the 10-12 km or so allowed by current NS theories; while I haven't really gone over their paper in detail, they claim to have ruled out two-component blackbodies, at least at any level that would contribute appreciably to the flux, and power-law sources at high confidence. And while there remains some question as to the distance (the Walter (2001) measurements of 60 pc versus Kaplan et al (2002)'s 140 pc or so), I think their arguments in support of the larger distance (e.g., the larger distance is more in agreement with neutral Hydrogen column measurements plus standard physical density estimates) are reasonably compelling, albeit prone to criticism.
I'd be curious to hear your thoughts on this -- i.e., do you think a much-lower temperature blackbody (or "hot spot" model) is not truly excluded by the data on this line of sight? Because the lack of pulsation here is just one part of the puzzle.
Cheers.
(For any random people who have happened upon this thread, the last couple lines are something like: "There were 3,653 days just like that in Ivan Denisovich's sentence. The extra three were for leap years." Wow.)
Of course, this is an argument for space-based missions generally, not HST specifically. But still, Hubble is a great orbiting platform with unbelievably good pointing (amazing what you can do with a few billion dollars), whose general operational procedures / problems are well understood. Contrast this with a new satellite system, built from the ground up.
Note, also, that great things are still in the cards for Hubble -- eg the Cosmic Origins Spectrograph, which will be installed onboard in a couple years. So it's not just decades-old technology that has since been superseded by ground-based stuff...
It's easy to see that they'll have a lot of support for this, from a lot of people -- vastly more people might have an occasional interest in reading, say, the Astrophysical Journal than have an interest in writing an article for it. And the publishers are able to defend an increase in page charges by saying it's paid for by researchers' grants, not by the researchers themselves.
I'm a co-author on a paper which will be submitted to the ApJ in the next few weeks; the page charges will be several thousand dollars. We haven't really bothered about it too much, because there's no real way to avoid paying it, and besides the grant money is there. It should also be mentioned that this paper is a somewhat extreme case -- around 20 pages, with a number of color figures (which are, I think, $600 for the first and $150 thereafter, but which are also unfortunately necessary). ApJ charges around $130 a page, IIRC, so you can do the math.
Does this strike you as absurd? It does me. $3000 is more than a trip to a great conference costs, more than the cost of supporting an observing run, more than a lot of things. It's only bearable because I happen to be doing space-based astronomy, where grants are big enough to support these kinds of outlays. But the problem is that a lot of research doesn't need big grants, or shouldn't -- I know plenty of people who do pure analytical theory which doesn't even require applying for supercomputer time. Admittedly, faculty at many institutions have to apply for grant money to pay their summer salary, so it's not totally indefensible, but still : a thousand bucks can take a pretty healthy chunk out of many grants. Some journals allow "hardship exemptions," whereby page charges are waived, but I don't know how easy/difficult it is to get them.
I've often suspected that a major driver behind page charges is their action as a "gatekeeper": ApJ probably doesn't get a lot of cranks submitting wacko stuff, b/c who the hell would be willing to pay a thousand bucks of their own money to see their article printed there? But I think page charges have the unpleasant tendency to constrain good research as well. Other things do this -- ie, there is already a tendency to work in areas where you know the money is easy to come by -- but that doesn't make it defensible.
As with all interesting things, there are no easy answers here. I don't think the ApJ (to keep using the same example) is an evil institution -- it's a publication of the American Astronomical Society, which is a non-profit organization that does many good things. And I'd be very surprised if the AAS didn't derive an appreciable fraction of its operating budget from ApJ-related charges; making the journal charge less overall would probably mean fewer activities funded by the AAS. It's also worth noting that astronomy/astrophysics journals (ApJ, AJ, A and A) are perhaps unusual in that they have no ads, so that's not a potential source of income. Note, also, that currently issues of ApJ more than three years old are available online without a subscription -- but see aforementioned bits about how much we pay for this privilege.
The point of my (absurdly long) diatribe is this: if researchers are able to "convince" publishers to supply online versions of everything for free with no negative repurcussions, great. But if the publishers recoup some of their lost subscription charges by increasing page charges, well, maybe that ain't so great. I don't know what page charges are for biosciences journals are these days, and I don't know enough about the culture of research in that field to know whether dramatic increases in those charges would have a seriously detrimental effect. If the current charges are low, would these folks be willing to accept charges similar to those "enjoyed" in the astro community? Would they be willing to accept charges running into the several thousand dollar range? I don't know, but I suspect we may find out.
I think this is the funniest damn post I've seen here in a while. pity I don't have any mod points right now. :-)
You raise a good point, though. I am curious about their plans for AO on such a large scope -- I've always understood this as a fundamental limitation, independent of the technology involved. (Err, you sound like you're almost certainly aware of this, but : Each point on your secondary corresponds to multiple points on the sky, but the mirror can only be in one place at a time. thus, you can only correct for one of the real-sky positions which maps to a given spot on your secondary. This isn't a big deal if you have a small scope, but when you talk about going over 20m, I really don't see how you can get around this, since atmospheric variations occur on scales smaller than this.)
Same sort of deal here. Chandra can remain pointed to reasonably high accuracy on something as it moves in its orbit. Sure, it's moving (and so is the Earth, and so is the distant object, etc.), but as long as the telescope remains pointed at the object in question, you're fine.
The key thing is that, for a given system, you come up with some way to determine the total mass, and then compare that with the luminous mass (ie, the stuff you can see) -- that's really all it comes down to. The trick, of course, is in figuring out a way to get that estimate of the total mass.
There are several possible solutions. If you're looking at some systems (like, say, a cluster of galaxies), you can look at the "velocity dispersions" of the component galaxies, and from those (and using the assumption that the cluster has more-or-less virialized) get an estimate of the total mass that is "pulling" on each galaxy. You can also use gravitational lensing, as suggested by another poster. With clusters, you actually have another (very spiffy) option, which is to look at the X-ray emission from the hot gas between the galaxies in the cluster -- the temperature of the gas, which relates to the X-ray luminosity, is also related to the mass of the cluster.) The results with all of these methods suggest that a great deal of the mass in large clusters of galaxies is non-luminous.
In our own galaxy (and others), you can do a similar sort of analysis and come up with the same sort of result (that much of the mass must be dark). (Here, the usual procedure is to form a "rotation curve," which shows the velocity as a function of radius from galactic center; if you do this, you find that the radial velocity "flattens out" and stays that way for much longer than we'd expect unless there is a substantial "dark" mass in a halo surrounding the galaxy.) It's a little dangerous to make comparisons like these, since it's possible that the source of "extra" mass may be different on large (ie cluster) and small (ie galaxy) scales, but the basic idea is the same: that a big, big chunk of the matter in the Universe is probably not luminous.
A large answer to a short question. :-)
Some applications don't care about this, and I expect you're correct in thinking that if the actual launch cost went down, that might lower the barrier to some companies, and of course there are some things where you actually need the Shuttle, but there will still be many, many applications for which the Shuttle is not a cost-effective launch vehicle. It's a two-edged sword: the Shuttle is great because it's a manned vehicle, but it's also terrible for the same reason, and there's no good way around that.
Just my 2 cents. :-)
Even the "example" you quote, that of "how the Universe came into existence," about which you say, "the fact of the matter is that we know nothing..." is a poor one. Like all scientists, astrophysicists have formulated a theory (namely, inflationary Big Bang) which explains certain observational facts, makes certain predictions, etc.; furthermore (and quite significantly), it is generic in the sense that the results it predicts are not sensitive to a large number of parameters. I could yak on about this ad nauseum, but briefly, observational corroboration includes the existence of a Hubble flow (velocity is proportional to distance), the existence AND detailed properties of the cosmic microwave background (to wit, its isotropy to one part in 10 ^ 5 or so, the fact that it has the spectrum of a perfect 2.7 K blackbody, and the manner of the very small-scale anisotropies), and the ratio of light element abundances. Show me another theory that predicts all these things, in a natural way (ie, without invoking 27 free parameters to tune as you wish), and I'll listen to you; but hell, do that and you've probably won a Nobel anyway, so why bother with me? Furthermore, the physics of the Universe before nonlinearity should actually be very well described by "normal" physics -- and if it's not, that's a fundamental problem, independent of the application; the kicker, though, is that we have no solid evidence that the physics here doesn't work.
To sum up, my NSHO is that you are a) unclear about what exactly astrophysics entails, is, and relates to, and b) are unable to back up your broad claims of illegitimacy with fact. Your one anecdotal evidence of the evils of astrophysics, regarding the greenhouse effect on Venus, is so incidental as to be laughable: I know jack about the atmosphere of Venus, or about greenhouse gases in general, or really about planetary science in general, but then again I don't really have to; the vast majority of topics in astrophysics have absolutely zero to do with it. My totally biased opinion is that Carl Sagan probably has given more thought to this than I have, or than you have, or even (perish the thought) than Gunnar Heinsohn, whoever the hell he is. Your argument is like suggesting that if a chemist can't predict the bonding properties of C60, all of chemistry is flawed.
Have a nice day. :-)
BTW -- and I could be remember things incorrectly here, but I don't think so -- the issue of dark-matter bias (how well the visible matter traces the dark matter) doesn't really affect these particular results too much, at least as long as you assume the bias isn't changing dramatically over time. (And as long as you take whatever bias you pick as a prior for all possible values of Omega, as opposed to mushing it around as you see fit.) You're right that there's degeneracy, but I think most of that comes from the lambda, Omega business, and the distributions of clusters expected are not totally indistinguishable, just close.
the questions you pose don't have straightforward answers -- at least not ones that appear straightforward to me. But you can come up with some broad estimates.
A rough estimate of interstellar gas density is on the order of 1 particle per cm^3 -- a bit lower (0.1) between clouds, a bit higher (20) in diffuse clouds, much higher (10^3 - 10^6) in molecular clouds.
At these densities, it turns out that the likely effect of galaxy - galaxy collisions may be to strip out a large portion of the gas in both galaxies. Certainly the large-scale effects are enormous, and you can see them in our own Milky Way -- the galactic disk is "warped" upwards by as much as 4Kpc (12,000 light years) at large (20 kpc) distances from the nucleus, and this is thought to be a result of a tidal interaction long ago with the Large/Small Magellanic Clouds. Also, one of the most popular theories for how elliptical galaxies (or at least some elliptical galaxies) form is via collisions between spirals -- ellipticals have very little gas and dust. This theory is borne out somewhat by the fact that the concentration of ellipticals is much higher in rich clusters of galaxies than it is in the "field" -- as the density goes up, you would expect more collisions, hence more formation of ellipticals.
And hey, while we're at it, the process of gas stripping is a fundamental issue in the study of clusters of galaxies. As galaxies in a rich cluster move through the (very hot) intracluster medium, a shock develops and basically pushes a bunch of the gas out -- for a relatively simple physical analysis of this situation, see for instance Shore's book on Astrophysical Hydrodynamics.
But to get back to the original issue: if we were sitting on Earth when the MW collided with Andromeda, what would it be like? The answer is that I don't really know -- my hunch is that the local (in both space and time) effects would not be all that great; life around the Sun would probably get along just fine. But I don't know, because I'm too lazy to work out the problem. :-) (I'm sure this is in the literature somewhere, if you're truly dedicated -- try the Astronomy and Astrophysics data abstract service,.) Certainly the very long-term effects would be enormous, though.
Hope that helped clear things up...
End result: certainly I agree with you that media other than imaging have their place -- spectroscopy is the way to go for a lot of things. And other wavelength bands (as you say, IR, UV, x-ray, etc.) are important, too -- but don't knock the visual band! :-) And "nice pictures" and nice science aren't necessarily mutually exclusive.