Archive for: January, 2011

One-Slide Explanation of Tides

I realize that this Bill O'Reilly quote is two weeks old, which in Internet time is a substantial fraction of the age of the Universe. And, the Internet being what it is, a top conservative commentator can't say something this butt-ignorant without having bloggers jump all over him within seconds. So, yes, I realize that I'm way, way behind the times, sort of like somebody getting all snarky to the dinosaurs because they didn't invest in programs tracking near-Earth asteroids. But, still, I think it bears repeating, to remind ourselves collectively the kind of people who are shaping the agenda of an entire political party in the USA right now.

Here's my one-slide explanation of how the tides work:


Click image for larger version

This slide does go along with some speaking, normally. Indeed, it is one (of 28) slides that I'll be using in the talk I'm giving in Second Life in about half an hour, all about interacting galaxies and whether or not they're connected to the phenomenon of active galactic nuclei. (Really, tides are relevant to this story!)

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Velikovsky: Worlds in Rectal Defilade

Perhaps the most extreme version of the persistent astronomy crackpot theory is Velikovsky's whole "Worlds in Collision" business. The basic idea behind this is that in Earth's recent past— that is, within the last several thousand years or so— there have been global catastrophes caused by objects in the Solar System moving around. Among other things, Velikovsky's ideas suggest that Venus was originally a comet (...yes...), that was ejected from Jupiter (...yes...), and migrated around the Solar System, having close passes with Earth and Mars, thereby causing the catastrophes that Earth supposedly had in the past. His most famous (infamous?) book is Worlds in Collision.

Right.

The various movements around the Solar System that Velikovsky needs, on the timescales he needs them, require massive violation of things like conservation of angular momentum, if the orbits are purely gravitational. His response to that was, well, electric forces must be responsible for planetary orbits! I've already pointed out how utterly unphysical and batty the whole "electric universe" thing is in my previous post. The Velikovsky business makes no sense, because the physical model of the solar system it needs to work makes no sense.

There's another basic reason why we can be pretty sure that Velikovsky is wrong. That is, there's absolutely no reason whatsoever to think that he might be right! His methodology for coming up with his model for the Solar System had absolutely nothing to do with science or scientific considerations. Rather, what he did was look at the myths of ancient cultures, and assumed that they were true— that is, when the myth said that there was a global catastrophe, there in fact was a global catastrophe. I would point out that this methodology, looking at ancient myths to determine natural history, is exactly the mythology used to come up with Young-Earth Creationism. As such, from a scientific point of view, there's absolutely no reason to pay any more attention to Velikovsky than there is to young-earth creationists. Trying to make a scientific refutation of it is like trying to explain the color blue in terms of musical theory.

So: Velikovsky's whole idea was based on non-scientific considerations, and as such isn't even worthy of debate on a scientific forum. Trying to shoehorn the physics to make it work requires resorting to the "electric universe" stuff that is crazy. And, as if that weren't enough, there's no evidence in the geologic record of the global catastrophes that Velikovsky was trying to "explain".

(So where do these ancient myths come from? If you've read the news for the last ten years, it's not very hard to imagine. Think of the people who lived through the east Asian tsunami in 2004— it's not very difficult to imagine somebody, especially somebody without the benefit of a world-wide media, believing that a global catastrophe had occurred after living through that! Flood myths are almost certainly so common in human cultures not because there ever was a global flood, but because there are floods, all the time, and sometimes they're really bad.)

Sadly, despite the fact that Velikovsky's ideas were ill-founded and have little or no connection to actual physics and astronomy, there remains a small fringe that think that the astronomy community is doing him a disservice by not taking him seriously. It's just like the electric universe business. Those of us in the "mainstream" are either deliberately hiding the truth, or are blinded by the dogma, and don't want to allow "outside" ideas to undermine the ideas that are the basis of our careers. Or, so they say. And, so, you can find books and websites out there saying that Velikovsky never got a fair hearing.

The truth is that Velikovsky has gotten way more attention and hearing than he ever deserved, at least as far as natural history is concerned. The fact that he wrote his stuff more than half a century ago, and I still feel some motivation to mention how wrong it is on this blog, indicates that his ideas have somehow garnered far more staying power than the ideas themselves would warrant.

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How I know "plasma cosmology" is wrong

In my previous post, I showed direct statistical evidence that the Arp notion of non-cosmological redshifts for quasars is wrong. That was just the tip of the iceberg, though. Non-cosmological redshifts are a crank theory in astronomy that a scary fringe element keeps whinging on about. However, there's this other crank theory that no actual respectable astronomer subscribes to, yet that seems to keep sucking in interested members of the public. That is so-called plasma cosmology (which also has an even more extreme (!!) version known as the "electric universe"). The non-cosmological redshifts for quasars model may have been a respectable alternate model in the first years or first decade after Maarten Schmidt's identification of the then-amazingly high redshift of quasar 3C273 (that paper was in Nature, so you won't actually get to see it, sigh). In contrast, the whole plasma cosmology paradigm was never reasonable, and is certainly not reasonable now.

The basic idea of plasma cosmology is that electromagnetic forces in the bulk motions of astronomical objects are far more important than mainstream astronomy admits. Now, to be sure, mainstream astronomy places tremendous importance on electromagnetic forces. There's all kind of crazy stuff going on on the Earth's magnetosphere, as a result of the plasma from the Sun interacting with the magnetic fields of the Earth. Magnetic fields are responsible for initially collimating jets in active galactic nuclei that are observed shooting out over hundreds of thousands of light-years. So, the assertion you sometimes see that astronomers don't train their grad students about electromagnetic forces and that astronomers don't take into account those forces is an assertion that's wildly wrong. However, plasma cosmology also asserts that electromagnetic forces between plasma flowing through the solar system and through the Universe and the magnetic fields of objects (or even the objects themselves, as they'll often decide, for instance, that comets must have a substantial electric charge) make significant contributions to the motion of objects that mainstream astronomy is able to explain entirely through gravity.

Unfortunately, rhetoric being what it is, it's very easy to find sites on the web (and books) that promote the notion of plasma cosmology, and after reading them it's easy for the interested but uninformed layman to be convinced. It helps that it feeds into the whole "few brave pioneers fighting the oppression of the mainstream dogma" story that seems to be so popular in (at least) American culture. How do you know whether to believe my assertion in the first paragraph above that plasma cosmology is all bunk, or a much more elegant assertion that people like me are just part of the entrenched mainstream refusing to listen to somebody with a new idea that challenges the underpinning of our whole careers? The problem is that when actual real astronomers such as myself are confronted with plasma cosmology, we have a hard time doing anything other than shaking our heads sadly, because it's so amazingly wrong, so patently silly if you know anything, that it's difficult even to know how to begin saying that it's wrong.

I'm going to try to take down plasma cosmology on two points. The first is a general point, the second is a specific point. As far as I can tell, plasma cosmology is motivated by people who just want to be different, or by people who have aesthetic or conceptual problems with things such as dark matter and cosmological distances. However, let's go ahead and give it the benefit of the doubt (way too much benefit, but bear with me) of saying that it's an idea inspired by trying to explain something that may not be satisfactorily explained by mainstream science. An example of something like this is MOND, or "MOdified Newtonian Dynamics". Standard Newtonian gravity can't explain the observed rotation speeds of galaxies. The right answer is that there is dark matter in those galaxies; we know this is the right answer because there is a whole lot of other evidence for dark matter. However, MOND was introduced as a way of modifying Newtonian gravity, rather than by introducing a new component to galaxies, to explain the flaw.

Here's the thing, though. Even if the "standard" explanation has a flaw, when you introduce an alternate explanation to address that flaw, your alternate explanation must explain everything the standard explanation already explains. (Strictly speaking, it doesn't have to initially explain everything. For instance, Copernicus' model of the heliocentric Solar System initially didn't produce as accurate predictions for planet positions as the old Ptolemaic geocentric model did. However, your new model must at least get close, and there must be ways to improve your model to explain what the old model explained.) Given the wide range of observations that standard gravity-based expanding-Universe cosmology explains, there's really no need for a gigantic rethink of all of it such as plasma cosmology offers. If we are to do that gigantic rethink, there has got to be a compelling observational reason beyond somebody's aesthetic sensibilities. (For instance, Quantum Mechanics was a gigantic rethink of our understanding of the fundamental nature of reality. However, not only did it explain some troubling problems about the light emitted by hot objects, it went on to propose a whole bunch of other experiments that couldn't have been explained without it. That's how successful paradigm-changing theories work.)

Given that we're able to explain all the orbits in the solar system with a straightforward application of gravity, where's the problem that plasma cosmology is supposed to solve? Likewise, with the whole Universe, we explain a wide range of observations with Big Bang cosmology. If we are to even bother spending ten minutes thinking about plasma cosmology, we must first know: does it even show promise to explain everything, and what does it offer that the Big Bang does not?

In other words, plasma cosmology is a waste of time.

However, let me also take down one of the specific pieces of the model that underpins plasma cosmology. That's actually very difficult to do— not because the model is robust, but because it's so ill-defined! If you go to plasmacosmology.net and follow the "technical" links, you get a bunch of text about various different "core concepts". If you don't know a lot about physics and astronomy, I can see where it looks like they've put together a well thought-out framework here, and that it's criminal for mainstream astronomers not to address this. The problem is, if you're a mainstream astronomer like me, and you try to figure out exactly what it is that their model here is doing, often you can't. What you've got, really, is a lot of nice sounding technical jargon that ultimately doesn't make clear what it is that they're really saying. In short, where's the math? If you're going to make quantitative predictions about where things are going, we need to know the equations that go along with your nice words.

Here's one of the things they say about the Solar System that's at odds with what mainstream science knows:

Because the sun is seen to emit roughly equal quantities of ions and electrons, the solar wind is considered electrically neutral in mainstream circles. This is wrong. In reality it is a huge bipolar electric current, and the terms solar wind and solar radiation result from the fact that the mainstream refuses to acknowledge electricity in space.

OK.... First of all, the mainstream does acknowledge electricity in space. But, never mind that. The term "solar radiation" results from the fact that the Sun is radiating. We see light coming off of the Sun. We also, via satellites, observe a stream of charged particles (of both signs, mixed together) coming off of the Sun. It seems exceedingly bizarre to assert that the term "solar radiation" comes out of some sort of global willful blindness, when it's just a very straightforward identification of the fact that the Sun is not completely dark, and is thus, er, radiating.

But, OK, what I really wanted to object to was "a huge bipolar electric current". What exactly does this mean? To me, if it's bipolar, it would mean that on the North pole (say) the particles flowing off of the Sun are mostly positive, and on the South pole they're mostly negative. This would, indeed, be a bipolar current. The problem is, if it's really bipolar like this, then the particles flowing along the equator— you know, the plane where most of the planets and comets are all orbiting, so where you'd need things happening to have an effect— would be neutral in bulk. (That is, there is an even mix of positive and negative particles.) Thus, you're not going to get any net interactions of that current with the magnetic fields of planets or anything else that will produce bulk motions. (You will get all the fun stuff like the Van Allen belts and aurora... but, of course, mainstream astronomy already describes all of that!)

So what are you guys really trying to say here?

I do have one guess, based on something written further down:

This behaviour derives from Ampére's Law or the Biot-Savart force law which states that currents in the same direction attract while currents in the opposite direction repel. They do so inversely in relation to the distance between them. This results in a far larger ranging force of interaction than the gravitational force between two masses. Gravitational force is only attractive and varies inversely with the square of the distance.

Except for one crucial omission, this statement is correct. It is true that if you calculate the attractive force between two long parallel currents, it only goes as 1/r, whereas gravity goes as 1/r2. This means that the strength of gravity drops off faster with distance than the magnetic attraction of the two currents, so even if gravity dominates, eventually you will reach a point where the strength of gravity drops below the magnetic strength. So, it seems, you really ought to be taking all this current stuff seriously.

Here's the problem though. The result that the magnetic attraction between two parallel currents drops off as 1/r only applies to infinitely long parallel currents. Practically speaking, that means that the length of each current (the length of the wire carrying the current, for example) must be a substantially bigger than the distance between the two currents. In other words, for this 1/r law to be relevant in the Solar System, there would have to be some current associated with (say) the Earth, perpendicular to the plane of the Solar System, whose length is at least several times the distance between the Earth and the Sun. The Sun would likewise have to have a current that long associated with it.

And that's just batty.

The mistake here is a common mistake, actually. It involves taking a legitimate result from legitimate equations, and applying it where it does not apply. This is why, in physics, you shouldn't just do algebra blindly. You should understand what you're doing. Even if you understand the vector algebra that leads to the derivation of the 1/r force law, you need to understand why you used the equations you did, and why you made the simplifying assumptions that you did, in deriving that law. And, in understanding that you need to understand the limitations on when you can apply your result.

If you (somehow) manage to have two short parallel lengths of current all going in one direction, then the strength of the force between them drops off as 1/r2, just like gravity, once the distance between the two currents is large compared to their length. But, you can't have this, as all the charge from that current has to go somewhere. So, in practice, if you have a small bit of current, you have to have a loop. The force between two loops of current drops off faster than 1/r2. In other words, even if it's significant at smallish distances, eventually it will become insignificant compared to gravity.

That's why you can trivially make an electromagnet and pick up paper clips with it, easily overcoming Earth's gravity. However, once you move that electromagnet (say) a meter away from the paperclip (unless you've really gone nuts with your current), the Earth's gravity overcomes it and you no longer pick up the paperclip.

As far as I can tell, the plasma cosmology people are basing all of their objections on a (probably unconscious) desire to be the Justified Iconoclast, latching on with their friends to a Truth that the mainstream refuses to see. And, indeed, this is a very attractive notion, and I think this is part of why intelligent and interested members of the public get sucked in by it. The problem is, their justifications fall apart under even a little bit of scrutiny. Please, please, pay no attention to plasma cosmology. It's a persistent but extremely off-base crackpottery that plagues astronomy.

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One of Astronomy's pet crackpot theories: non-cosmological quasar redshifts

In the standard Big Bang theory of cosmology— a theory that explains a wide range of observations— distant objects show a shift in the observed wavelengths of features in their spectrum as a result of the expansion of the Universe. In between the time when the light is emitted by a distant object, and the time that we see that light, the Universe expands. The wavelength of the light goes up by a relative factor that has the same value as the relative expansion in the size of the Universe. This effect is called cosmological redshift. Because the Universe has always been expanding, we can use this to measure distance to an object, and to measure how far back in time we're looking (i.e. how much time it took the light to reach us). The more redshift we see, the more the Universe expanded, so thus the more time the Universe was expanding while the light was on its way to us, and thus the longer the trip to us was.

Astronomy has long had a handful of fringe scientists who argue that at least some of the redshifts we see are non-cosmological in origin. In particular, Halton Arp, most famous for a catalog of galaxies with disturbed morphologies (as a result of interactions), argues that quasars aren't really cosmologically distant objects at all, but are rather objects ejected from nearby galaxies, showing their redshifts as the result of an extreme Doppler shift due to their high ejection velocities. He based this originally on anecdotal observations of quasars with much higher redshifts seemingly correlated with much more nearby galaxies on the sky. For a long time, it was hard to test this correlation quantatively, for the selection effects were huge. By and large, we targeted interesting objects, but there were other interesting objects in the field. So, of all the quasars known, there was an observational bias that there would be more known near other objects that were observed for other reasons.

Various other things are claimed together with quasars supposedly being ejected from galaxies. In particular, there are claims of periodic redshifts— that is, that quasars are preferentially observed at certain redshifts, or at certain redshifts relative to the redshift of the galaxy that supposedly ejected them.

With the advent of large-scale sky surveys such as the Sloan Digital Sky Survey (SDSS), it has become possible to statistically test these predictions. Of course, the vast majority of astronomers haven't bothered, because we have extremely good models of quasars as cosmological objects that explain a wide range of observations about them, meaning that there's really no need to pay attention to the crank fringe asserting that there must be something wrong with the mainstream model. However, this (rational) response does feed into the natural tendency of many people to be attracted to conspiracy theories, who then assert that the "dogma" of mainstream science is "ignoring the evidence" for these decidedly non-conventional models of quasars. So, a few people have used the data in the SDSS to look for correlations of quasars and foreground galaxies, or to look for evidence of periodic redshifts in quasars. The result, of course, is that there is no evidence to support these theory, and indeed that the large statistics afforded by these surveys support the cosmological model.

In other words, if you want it summed up in fewer words: Arp is wrong. The evidence does not back up his arguments.

(He will disagree, and if you go to his website you can see the paranoia on the nicely designed, sparse front page. However, even if he is right about being ignored by the mainstream of science, that is because the mainstream of science has good reason to ignore him.)

Su Min Tang and Shuang Nan Zhang did a careful statistical analysis of SDSS data to look for the effects of periodic redshifts in quasars, and for correlations between quasars and galaxies. In other words, they took the predictions of Arp and his followers seriously, at least for purposes of performing the analysis. I've already stated the result above: no effects observed. Here is one of their "money" plots, Figure 7 from that paper:

Tang & Zhang, 2005, Figure 7

The circles here are the data from the sky survey. The various lines are the results of simulations, with the error bars on the lines showing the scatter in the simulations. The solid line is the only one that's consistent with the data throughout the whole range. The various dashed lines are simulations that would result of quasars were ejected from galaxies at various different velocities.

Reference to Arp's work is also part of the larger net-crank alternate astronomy theory, "plasma cosmology" (and the even more cranky, if that were possible, "electric universe" notion, as well as modern day followers of Velikovsky). That this lynchpin has been completely debunked should hopefully help you conclude that plasma cosmology isn't anything that should be taken seriously. I hope to address more of that in later posts.

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Online talk tomorrow morning: "Observational Evidence for Black Holes"

Tomorrow morning, I'll be giving a public lecture entitled Observational Evidence for Black Holes. This is part of a regular series of talks sponsored by MICA, Saturday mornings at 10:00 AM pacific time (1:00 PM Eastern, 18:00 UT). They're open to anybody.

These talks are in Second Life. A basic Second Life account— everything you need to attend the talk— is free. Go to the Second Life page I just linked in order to sign up. Once you've downloaded the Second Life viewer, and have created an account and logged in to Second Life, you can follow the link on our Upcoming Public Events page to find the talk.

Here's my blurb for tomorrow's talk:

Black holes are a theoretical prediction of Einstein's Relativity. But do they really exist? The answer is a nuanced "yes." We have observational evidence for two sorts of black holes. In our Galaxy, we observe black holes that are several times the mass of the Sun. At the core of almost every big Galaxy, we find a supermassive black hole that's a million or more times the mass of the Sun. In this talk, I'll give an overview of the evidence that these objects are in fact black holes. I'll also point out that the observational definition of "black hole", meaning those things that we know exist, isn't exactly the same as the definition of the objects predicted by Relativity, although most astronomers suspect and assume that what we observe are in fact the things that Relativity predicts.

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Maybe galaxy mergers don't cause nuclear activity!

Jan 06 2011 Published by under Astronomy & Physics

Knock me over with a feather. I really didn't expect this. I learned about this first over at Phil Plait's blog, but given the title of my blog I think I really ought to address it.

First, some background. My obsession with interacting galaxies goes back to my first year of grad school, when, in 1991, I started working with Tom Soifer. This wasn't too long after the publication of the IRAS Bright Galaxy Sample— if you follow the link, you'll see that Soifer is the first author on that article. A lot of follow-up work was done (and is still being done!) on the class of galaxies that was established in this sample. In particular, there are a class of galaxies that are particularly luminous at infrared wavelengths. There are things going on hidden behind dust that prevents them from always being particularly bright at optical wavelengths. Many of the galaxies, especially the brighter ones, in this sample show evidence of vigorous star formation. Additionally, many of these galaxies show evidence of being in the process of a massive collision, or being the recent result of a major merger.

The association between galaxy mergers and strong nuclear starbursts remains robust. A search at ADS on appropriate keywords can bring up a lot of recent papers, and perusing them shows that the observational evidence and theoretical reasoning that leads us to believe that major mergers of two big galaxies triggers a vigorous burst of star formation near the core of the merger.

Stars are formed from gas. A compact nuclear starburst associated with mergers means that gas is getting funnelled down to the centers of galaxies when they're merging. You'd also expect this theoretically, as gas clouds in the two galaxies run into each other and dissipate bulk orbital energy in the fluid interactions. What's more, we have known for more than a decade now that all, or at least most, big galaxies have a black hole at their core. Finally, we know that the phenomenon of Active Galactic Nuclei (AGN) is fuelled by accretion on to a black hole. If you put all of these things together, you would expect AGN activity to be correlated with galaxy mergers. To turn on an AGN, you have to feed the black hole at the center of a galaxy, which means getting all the gas down to the center. We know that mergers can send a lot of gas towards the center. Thus, you'd expect a correlation.

It is true that if you're talking about a "compact" nuclear starburst, it's not nearly as compact as the accretion disk around a black hole. The compact nuclear starburst will be of a size that's hundreds, or even a thousand or so, parsecs... whereas the size of the accretion disk is much, much less than a single parsec. So, just because you can get the gas down far enough to make a compact nuclear starburst, it may well be that you can't get the gas the rest of the way down to fuel an AGN as part of a merger, and that other processes are needed. It's so easy to say that now, with this new result (arXiv preprint of the paper) showing that AGN activity is not correlated with the morphological signatures of a major merger. If I am to be completely honest, before today I wouldn't have pointed out the scale difference between an AGN accretion disk and the (much larger) size of a compact nuclear starburst. Indeed, in popular talks I've given about interacting galaxies, I've repeated what the general consensus (or at least one popular model) was, that mergers can trigger AGN.

It will be interesting to see if this result holds. I do feel a small twinge of envy that I wasn't the one (or one of the ones) doing this statistical survey, as in a previous life it was something that I had on my list of research topics that would be interesting to try to confirm. But, clearly, it was a topic of interest to a lot of people, and it's great that we've got a result now... even if the result isn't the one I expected or hoped for. I look forward to the further unfolding of this story.

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