Sunday, 21 August 2011

Relativity Homework

A busy weekend, so here is some weekend thoughts (and associated reading) for you. It's all to do with relativity and are linked with some blog posts.

Time does not flow: To paraphrase Shakespeare, All the world is a 4D manifold, and all its players are but worldlines. Basically, General Relativity tells us that the Universe is a 4D canvas and our paths are traced out. It's all there, past, present and future (from any individual's perspective). The Universe does not unfold as we progress into the future. It's all already written out (like a path on a map). Enter the philosophers and discussions on freewill, but taking relativity at face-value, there isn't any. Heck, it doesn't even tell us which direction our experience of time flows in, we set that with a decision on dt/dτ.

Energy is not conserved: In general, energy is not conserved in General Relativity. This one usually freaks out the students, but it caused Einstein a whole host of headaches as well. When the Universe expends, photons are redshifted. They "lose energy". Where does this energy go? You can do some groovy handwaving and say "into curvature", but the answer is "Don't worry about it, energy is not conserved".

Space does not expand: This is one I have been involved with for a long time, and, again, it freaks people out, especially with the wealth of popular science and textbooks that talk about expanding space. You have to be careful here, and a paper I wrote a number of years ago now, Expanding Space: the Root of all Evil?, discusses this in some detail. It comes as a shock to students of relativity to realise that, almost a hundred years after the discovery of the expanding Universe, that the meaning of the expansion is still a matter of debate. I'll close this with a quote from Steven Weinberg and Martin Rees, reported in New Scientist a few years ago;
Popular accounts, and even astronomers, talk about expanding space. But how is it possible for space, which is utterly empty, to expand? How can ‘nothing’ expand?
“Good question,” says Weinberg. “The answer is: space does not expand. Cosmologists sometimes talk about expanding space – but they should know better.” Rees agrees wholeheartedly. “Expanding space is a very unhelpful concept,” he says. “Think of the Universe in a Newtonian way – that is simply, in terms of galaxies exploding away from each other.”

Saturday, 20 August 2011

Has physics become cool again?

It's Friday, it's pouring with rain, and time for a grumpy post.

The BBC is running a story Has physics become cool again?. While the overall story is good news, that the number of people people doing A-Level physics in the UK is on the way up, it also high-lights what I see as a big problem, basically the media's portrayal of physicists.

Words like "nerd" and "geek" to generically describe scientists makes my blood boil, as does their portrayal as socially inadequate, Star Trek obsessed, comic-book reading dweebs. Case in point:

Apparently "smart is the new sexy".

And our own Beauty and the Geek had this  bunch
Not to mention Nerds FC. In fact, each year we get an email circulating around our physics department that Beauty and the Geek are looking for new contestants, but from physics they are only looking for geeks, not beauties.

Harmless fun you may say. But is it? We moan about falling numbers in science, and in physics, the number of women in science is a continual source of concern. We know that the teaching of science in school can be problematic, being too boring or too much work, but what message does the above send to kids?

(Pop psych warning) My feeling is that they say that to be a scientist, you must wear the geek/nerd badge. That you must be "uncool". No wonder young people would rather be lawyers or accountants (are there Big Bang Theory equivalents of these?). The portrayal of women in Beauty and the Geek is not particularly flattering either.

The BBC article continues with some mixed messages, with the "Cox Effect" (who can coolly stare from a mountain top in some remote location while make a deep comment about the Universe) inspiring a new generation of physicists. But there is also there is a comment from the excellent Professor Jim Al-Khalili (he of the fantastic Atom tv-series) apparently stating
"The geeks are on the march again!”
 A quick read of Al-Khalili's wikipedia page points out that he
"has been a supporter of Leeds United football club ever since the Revie days of the early seventies"
a football tragic and very un-geeky passtime. I wonder if he means other people are geeks?

In truth, physicists are people who do physics. A look at the lives of physicists reveals the same angsts, woes, joy and excitement as most other people. It doesn't take a lot of detective work to find out the very human activities of Schrodinger and Feynman (or even Einstein for that matter), as well as the tragedies in the lives of Boltzmann, Curie and Ehrenfest. They were people, doing physicsy things, but living peopley lives.

I'm not saying that there are not some nerdy and geeky people in science - there are. But there are nerdy and geeky people in all professions and walks of society, and there are very non-geeky and non-nerdy people in science (such as Cox and Al-Khalili). So, here's the new slogan:
and let's have a few more positive portrayals of physics in the media, doing the kind of things that we do best (like calculating the influence of evolving dark energy on the recent expansion history of the Universe), while hosting cool and sophisticated cocktail parties (well, perhaps at least a fun BBQ and a game of triv). Maybe this will have a positive influence on young people considering a career in science?

I'll finish on a footnote; in a couple of weeks, I will be attending Speed Meet a Geek in the city. When invited to participate, I said I was not interested due to the geek label (which, apparently, is a term of endearment rather than derision), and now my invite says I will be at Speed Meet a Scientist. However, it's still being advertised as Speed Meet a Geek to the kids; Sigh, I'll be sure to wear my polyester trousers, brush up on my Star Trek trivia, and bring some Spiderman comics along......

Sunday, 14 August 2011

Dark matter may be an illusion

Again, with teaching, PhD reviewing, public talks etc, things have been very busy, but should calm down a little in the next few weeks. So, again, a brief post this week. An as I am feeling grumpy, I'm going to comment on

Is dark matter an illusion created by the gravitational polarization of the quantum vacuum? (original paper here).

We are all aware that we don't know what dark matter is, but there are a couple of things we do know; 1) we know that we need something to explain the way we observe the Universe 2) that something may be something material (like a dark matter particle, or left over black holes) or even a modification to our laws of physics, such as modified Newtonian Dynamics or (cue weird music) leaky dimensions, or other such stuff.

Now, I will say this again. We know this. IMHO, most astronomers don't worry too much about this and they go with the simplest assumption, that dark matter is just that, matter which is dark. Most papers report very simple numbers, such as a Mass-to-Light ratio, or if there are more direct probes, such as gravitational lensing, you may get the shape of the dark matter.

But if someone said tomorrow that Dark Matter is definitely not material, but is something else, something weird, most people would say "OK" and get on understanding their results within this framework. I don't think people will be really shocked.

So most astronomers don't really bat an eye-lid at papers like this one. They are quite common, maybe dark matter is this, or dark matter is that, or unicorns, or whatever. But they often claim to explain everything, but have very few (if any) predictions that can truly differentiate the new idea from plain old vanilla dark matter. And so on we carry, quoting ML and other simple quantities.

To be clear, I am not attacking this current paper. The author, at the end, states that;
"In conclusion, we have revealed the first indications that what we call dark matter may be consequence of the gravitational repulsion between matter and antimatter and the corresponding gravitational polarization of the quantum vacuum by the existing baryonic matter. Of course, this is not a claim, just possibility. A lot of work would be needed before such a claim would be eventually possible."
Basically, the paper is one of a myriad of speculations of what dark matter might be.

No, my reason for posting is what happens when this stuff gets into the media. Of course, the speculation often gets firmed up in the press release. And then the discussion on blogs etc begin. Just check out SlashDot! The first comment is
"I hope so. Dark matter is the ugliest kludge to the standard model ever."
and so on and on. Apparently,
 "Average Slashdotters have been more skeptical of they dark matter theory than physicists, from what I've seen."
This is the kind of comment that really gets to me. No, what is actually happening is that average slashdotters like ideas with lost of mysterious sounding, but actually extremely skeptical, ideas, than the simplest working model.

 It's actually the opposite of Occam's razor, the wish to employ the most complex and whizz-bang over the straight-forward. Of course, this actually happens in science, when we have to let go of old ideas and adopt new ones, but things like quantum mechanics were not adopted because they were whacky and out-there, but because the weight of observational evidence crushed classical ideas.

Currently, all observations are consistent with dark matter being just that. When we have the observational evidence that does not agree with this simplest of hypotheses, then it will be discarded. And this will not happen until the alternatives can tell us precisely what tests we need to do to single out their theories as the best. Today is not the day that this is going to happen.

Anyway, looking over the comments, it's clear that many slashdotters actually understand this. But to the others, this is for you;

Friday, 5 August 2011

They might be giants: a mind-blowing sense of stellar scale

Well, after saying that I am not much of a star person, my article on the sizes of stars was just published in The Conversation. It's titled They might be giants: a mind-blowing sense of stellar scale and covers the size of stars from red dwarfs to hypergiants. The mini-cusps appear in the article as well :)

For a quick review, I fully recommend this video on YouTube;
It makes my head hurt a little every time I watch it.

A New Candidate Magnetar Located Outside the Galactic Plane

Magnetars!! I'm not a star person (they are useful kinematic tracers and stellar evolution [being on the Red Giant Branch] allows us to isolate stars in specific systems), but with ARC Postdoctoral Fellow, Sean Farrell, I'm an author on a paper on the detection of candidate magnetar outside the galactic plane.
The study uses mainly X-ray observations, plus a lack of an optical source (from the PAndAS survey; my contribution to the paper) to identify one of these weird objects. Basically, these are neutron stars which huge magnetic fields, and this results in some dramatic bursts of energy.

Co-author, Bryan Gaensler, and I gave some talks on "Music and the Cosmos" over the last could of years, and he recounts the story of SGR 1806-20; and I'm surprised that more people don't really know about it. Basically, this magnetar is 50,000 light-years away, and 50,000 years ago, it had a major energy burst, releasing, in one tenth of a second, the same amount of energy as the Sun outputs in 100,000 years (thank you wikipedia).

This outburst hit the Earth's atmosphere on December 27th, 2004. To directly quote wikipedia
The gamma rays struck the ionosphere and created more ionization which briefly expanded the ionosphere.
Remember, this is an outburst on a star on the other side of the Galaxy!! Pretty cool. On this new candidate magentar, well done Sean!!!

A New Candidate Magnetar Located Outside the Galactic Plane

Joseph R. Callingham (1), Sean A. Farrell (1,2), Bryan M. Gaensler (1,2), Geraint F. Lewis (1), Matthew J. Middleton (3) ((1) The University of Sydney, Australia, (2) ARC Centre of Excellence for All-sky Astrophysics (CAASTRO), Australia, (3) University of Durham, UK)
In this paper we present detailed analysis of the transient X-ray source 2XMMi J003833.3+402133 detected by XMM-Newton in January 2008 during a survey of M31. This source has previously been identified as a black hole X-ray binary in M31, but here we argue that the X-ray spectra and timing data are inconsistent with this conclusion. We instead argue that 2XMMi J003833.3+402133 may be a new addition to the rare class of magnetars. The X-ray spectrum is well fitted by either a steep power law plus a blackbody model or a double blackbody model. Prior observations with XMM-Newton, Chandra, Swift and ROSAT spanning 1991 to 2007, as well as an additional Swift observation in 2011, all failed to detect this source. No counterpart was detected in deep optical imaging with the Canada France Hawaii Telescope down to a 3sigma lower limit of g = 26.5 mag. The transient behaviour, X-ray spectrum, and lack of an optical counterpart are all consistent with a magnetar. The derived luminosity and black body emitting radius at the distance of M31 argue against an extragalactic location, implying that it is located within the Milky Way but 22 deg out of the plane. The high Galactic latitude could be explained if 2XMMi J003833.3+402133 were an old magnetar, or if its progenitor was a runaway star that traveled away from the plane prior to going supernova.

Wednesday, 3 August 2011

Gravitational Microlensing of a Reverberating Quasar Broad Line Region - I. Method and Qualitative Results

PhD student, Hugh Garsden, postdoc, Nick Bate, and I just had a paper accepted for publication in MNRAS. As you can see by the title, we combine two separate astrophysical techniques to probe the inner regions of quasars, namely gravitational microlesing.

I don't have much time this week (too much teaching and paperwork to do), but will expand on these in the near future. In summary, microlensing accounts for the gravitational lensing due to the myriad of compact objects, be they stars, planets or black holes, as they pass through the line of sight. Unlike the relatively boring microlensing within the galactic halo, this microlensing results on beautiful and complex magnification maps;
Reverberation mapping happens when a flare from the central engine of a quasar, typically thought to be an accretion disk orbiting a supermassive black hole, propagates through surrounding clouds of high velocity material, the Broad Line Region. I pinched the below picture from Brendon Brewer's recent paper on using Bayesian methods to measure the black hole mass from reverberation mapping;
In combining these two effects, we now have a source which varies its structure over time, with the image of the broad line region changing as the flare travel through, and various regions begin selectively magnified by the microlensing. Here's an example of the BLR sources superimposed on the magnification map;
You should look at the paper to see what the result is when they are combined; it's pretty interesting! But for now, I just want to say, Well Done Hugh and Nick!!

 Gravitational Microlensing of a Reverberating Quasar Broad Line Region - I. Method and Qualitative Results

H. Garsden, N. F. Bate, G. F. Lewis
The kinematics and morphology of the broad emission line region (BELR) of quasars are the subject of significant debate. The two leading methods for constraining BELR properties are microlensing and reverberation mapping. Here we combine these two methods with a study of the microlensing behaviour of the BELR in Q2237+0305, as a change in continuum emission (a "flare") passes through it. Beginning with some generic models of the BELR - sphere, bicones, disk - we slice in velocity and time to produce brightness profiles of the BELR over the duration of the flare. These are numerically microlensed to determine whether microlensing of reverberation mapping provides new information about the properties of BELRs. We describe our method and show images of the models as they are flaring, and the unlensed and lensed spectra that are produced. Qualitative results and a discussion of the spectra are given in this paper, highlighting some effects that could be observed. Our conclusion is that the influence of microlensing, while not strong, can produce significant observable effects that will help in differentiating the properties of BELRs.