Workshop + Conference @ The Fields Institute

"The year of the 100th anniversary of the formulation of Einstein's General Relativity will be remembered as the least productive one for the general-relativity community."

This is what I've heard more than once during the time spent at The Fields Institute in Toronto these weeks :-)
The Institute is organizing a long Focus Program to celebrate the centenary of one of Einstein's greatest achievements, the formulation of a relativistic theory of gravitational interactions, General Relativity for short.

The reason for the pessimist statement above is that this is only one of the numerous events that have been organized around the world to celebrate this important discovery. Now that the term ended, scientists are more free to travel to conferences and workshops and, to save time and money, it's very common to go directly from one conference to another on the week after. This does not leave too much time to sit down and do "actual" work.

Last week I've attended a very interesting workshop on Perturbation Theory which was organized here at the Field Institute as part of the Focus Program. This week instead the Institute is hosting a conference on Black Holes which ends today. Speakers at the conference included Clifford Will, Saul Teukolsky, Robert Wald, Eric Poisson, Gary Horowitz and William Unruh, to mention a few. Personally, I found the workshop much more stimulating and useful than the conference, but it's certainly suggestive to take the opportunity to discuss with some of the fathers of modern General Relativity.

This coming summer will be very busy with other events, especially in July and August, so I tend to agree that year 2015 will not be the most fruitful one in terms of actual work done.

On the other hand, do not underestimate the importance for the community to gather together and discuss open problems and (possibly) crazy ideas! Peer discussion and new collaborations are at the core of the scientific process and they are a crucial part of the duties of any scientist.  For this reason I am positive that the discussions and the collaborations that originate during these meetings will certainly contribute to solve open problems in the field and, who knows, perhaps the next breakthrough is just around the corner in 2016!

Doing a short tour of Toronto with Helvi Witek. Helvi was Ph.D. student in Lisbon when I moved there 4 years ago and she is now postdoc at Cambridge University working on numerical simulations of black-hole systems.

The Unbearable Baldness of Black Holes

One of the most awe-inspiring properties of black holes is their absolute simplicity, or as John Wheeler famously put it, "black holes have no hair". As their progenitor collapses, its memory is forever lost, and all that remains is a quiescient, bald black hole. In a new article in Physical Review Letters, a team of scientists (that only by chance includes me...) has shown that black holes can nevertheless "grow hair" in the presence of matter, connecting them to the rest of the host galaxy.

Black holes are almost xeroxed copies of one another, differing at most in mass and rotation. These objects are described by a solution discovered by Roy Kerr in 1963. Remarkably, Kerr black holes are ubiquitous in almost any other theory of gravity, to the extend that the "Kerr hypothesis" is the current paradigm in astrophysics. 

First time I saw this picture was in one of Stephen Hawking's popular-science books, probably 'Brief History of Time'. It is supposed to describe the 'baldness' that this post refers too, am I the only one finding it a bit pathetic? :)

We have shown that in simple and attractive extensions of Einstein's theory (known as scalar-tensor gravity) black holes may not be described by the Kerr metric, as was previously thought. The crucial ingredient is the matter surrounding astrophysical black holes, typically in the form of accretion disks. The presence of matter triggers an instability that forces the bald Kerr black hole to develop a new charge -- a "scalar hair" -- connecting it to the matter around it and possibly to the entire galaxy. 

This hair growth is accompanied by a peculiar emission of gravitational waves, potentially by upcoming laser interferometers, which may test the Kerr hypothesis and probe the very foundations of gravity.

Read what real outreach journalists wrote on this on:

NewScientist

Huffington Post

Portuguese newspaper Público

De devulgationi eloquentia

Our gravity group in Lisbon have just published a series of short movies where many group members describe some general aspect of Einstein's theory and beyond.

Have a look at Videoteca do Gravitão, muito boa!

Check out all cool resources at our group's outreach webpage

NRHEP Network First Meeting

The first meeting of the Numerical Relativity and High Energy Physics Network have taken place at Aveiro University, from 9-13 July 2012. This meeting brought together researchers from Brazil, Italy, Portugal, Spain and the USA, to discuss the state of the art in relativistic gravity and its application in various contexts, ranging from astrophysics to particle physics, using both anaytical and numerical techniques.

The organizers produced this nice movie. Visit  the workshop website for more info!

Everything you always wanted to know about GR tests in Astrophysics but were afraid to ask

While I was looking for a refreshing reading on GR tests in Astrophysics, I stumbled upon this very nice review by Dimitrios Psaltis.
He's one of the most renowned experts on GR tests with compact objects (the one always invited at conferences to give review talks on the topic, if you know what I mean), and this paper gives a very nice overview of what constraints can be put on GR and alternative theories with astrophysical observations. It dates 2008 but nonetheless there are only a handful of things that need to be updated. For example, there will be no IXO (that was dropped by NASA, then became Athena, and was officially dropped by ESA yesterday, shame on them. Ok, I stop complaining about it. No I don't).

Anyways, if you are interested on this topic, you can read the review. And you should.

It starts by explaining in layman's terms (ok, maybe in young PhD-understandable terms) why it is important to test GR over alternative theories, and what are the differences between them.
An important thing that is pointed out is that our most successful tests of GR in Astrophysics have only shown that it works very well in the limit of very weak gravitational fields (low curvature of the spacetime). Even double neutron star observations like the one on this fantastic double pulsar, that follows GR predictions with an uncertainty of 0.05%, probe a spacetime whose curvature is not very different from the one probed in solar system tests. Wikipedia classifies, incorrectly, those tests as "Strong field tests". (But keep in mind that even weak fields can be used to test alternative theories, as a former post in this blog showed).

Is there a way to probe strong gravitational fields? Yes. We make observations of accreting black holes and neutron stars (the most compact objects present in the Universe), for example, and we measure (possibly relativistic) oscillations, broadened spectral lines, orbital decays and more that can be used to measure the physical properties of the compact object, model-dependently, and test the predictions of GR.

The constraints that we can put today on GR and alternatives are weak, but still significant. Some sporadic observations of particularly interesting sources might improve greatly our knowledge even with current instruments. For example, the recent discovery of a very heavy neutron star permitted to put unprecedented constraints on the equation of state of these objects, and hence to have better estimates on the gravitational pull at their surface. The importance for nuclear physicists of the constraints on neutron star equations of state is even greater, but we'll talk about it in the future.

Let's not talk about canceled missions (I cannot even pronounce her name without dropping a tear, poor Greek goddess) that would have had the features to give nice measures where now we struggle with upper limits, it's too painful. But let us hope that smaller but powerful missions like LOFT have better chance (If you are interested, we host a science meeting in Toulouse in September). Stay tuned.

A classic, but always good basis to start with

Uyuni, Bolivia. Taken from here