Archive for gravitational waves

Lessons from LIGO

Posted in The Universe and Stuff with tags , , , on February 13, 2016 by telescoper

At the end of a very exciting week I had the pleasure last night of toasting LIGO and the future of gravitational wave astronomy with champagne at the RAS Club in London. Two members of the LIGO collaboration were there, Alberto Vecchio and Mike Cruise (both from Birmingham); Alberto had delivered a very nice talk earlier in the day summarising the LIGO discovery while Mike made a short speech at the club.

This morning I found this very nice video produced by California Institute of Technology (CalTech) which discusses the history of the LIGO experiment:

It has taken over 40 years of determination and hard work to get this far. You can see pictures of some of the protagonists from Thursday’s press conference, such as Kip Thorne, when they were much younger. I bet there were times during the past four decades when they must have doubted that they would ever get there, but they kept the faith and now can enjoy the well-deserved celebrations. They certainly will all be glad they stuck with gravitational waves now, and all must be mighty proud!

Mike Cruise made two points in his speech that I think are worth repeating here. One is that we think of the LIGO discovery is a triumph of physics. It is that, of course. But the LIGO consortium of over a thousand people comprises not only physicists, but also various kinds of engineers, designers, technicians and software specialists. Moreover the membership of LIGO is international. It’s wonderful that people from all over the world can join forces, blend their skils and expertise, and achieve something remarkable. There’s a lesson right there for those who would seek to lead us into small-minded isolationism.

The other point was that the LIGO discovery provides a powerful testament for university research. LIGO was a high-risk experiment that took decades to yield a result. It’s impossible to imagine any commercial company undertaking such an endeavour, so this could only have happened in an institution (or, more correctly, a network of institutions) committed to “blue skies” science. This is research done for its own sake, not to create a short-term profit but to enrich our understanding of the Universe. Asking  profound questions and trying to answer them is one of the things that makes us human. It’s a pity we are so obsessed with wealth and property that we need to be reminded of this, but clearly we do.

The current system of Research Assessment in the UK requires university research to generate “impact” outside the world of academia in a relatively short timescale. That pressure is completely at odds with experiments like LIGO. Who would start an experiment now that would take 40 years to deliver?  I’ve said it time and time again to my bosses at the University of Sussex that if you’re serious about supporting physics you have to play a long game because it requires substantial initial investment and generates results only very slowly.  I worry what future lies in store for physics if the fixation on market-driven research continues much longer.

Finally, I couldn’t resist making a comment about another modern fixation – bibliometrics. The LIGO discovery paper in Physical Review Letters has 1,004 authors. By any standard this is an extraordinarily significant article, but because it has over a thousand authors it stands to be entirely excluded by the Times Higher when they compile the next World University Rankings.  Whatever the science community or the general public thinks about the discovery of gravitational waves, the bean-counters deem it worthless. We need to take a stand against this sort of nonsense.

 

 

 

 

LIGO at the Royal Astronomical Society

Posted in Biographical, The Universe and Stuff with tags , , on February 12, 2016 by telescoper

image

My monthly trip to London for the Royal Astronomical Society Meeting allowed me not only to get out of the office for the day but also to attend a nice talk by Alberto Vecchio about yesterday’s amazing results.

I hear that we will be having champagne at the club later on to celebrate. In the meantime here’s a little Haiku I wrote on the theme:

Two black holes collide
A billion years ago.
LIGO feels the strain.

LIGO: Live Reaction Blog

Posted in The Universe and Stuff with tags , , , , on February 11, 2016 by telescoper

So the eagerly awaited press conference happened this afternoon. It started in unequivocal fashion.

“We detected gravitational gravitational waves. We did it!”

As rumoured, the signal corresponds to the coalescence of two black holes, of masses 29 and 36 times the mass of the Sun.

The signal arrived in September 2015, very shortly after Advanced LIGO was switched on. There’s synchronicity for you! The LIGO collaboration have done wondrous things getting their sensitivity down to such a level that they can measure such a tiny effect, but there still has to be an event producing a signal to measure. Collisions of two such massive black holes are probably extremely rare so it’s a bit of good fortune that one happened just at the right time. Actually it was during an engineering test!

Here are the key results:

 

LIGO

 

Excellent signal to noise! I’m convinced! Many congratulations to everyone involved in LIGO! This has been a heroic effort that has taken many years of hard slog. They deserve the highest praise, as do the funding agencies who have been prepared to cover the costs of this experiment over such a long time. Physics of this kind is a slow burner, but it delivers spectacularly in the end!

You can find the paper here, although the server seems to be struggling to cope! One part of the rumour was wrong, however, the result is not in Nature, but in Physical Review Letters. There will no doubt be many more!

And right on cue here is the first batch of science papers!

No prizes for guessing where the 2016 Nobel Prize for Physics is heading, but in a collaboration of over 1000 people across the world which few will receive the award?

So, as usual, I had a day filled with lectures, workshops and other meetings so I was thinking I would miss the press conference entirely, but in the end I couldn’t resist interrupting a meeting with the Head of the Department of Mathematics to watch the live stream…

P.S. A quick shout out the UK teams involved in this work, including many old friends in the Gravitational Physics Group at Cardiff University (see BBC News item here) and Jim Hough and Sheila Rowan from Glasgow. If any of them are reading this, enjoy your trip to Stockholm!

LIGO Newsflash

Posted in The Universe and Stuff with tags , on February 5, 2016 by telescoper

This morning I heard the same rumour from two distinct (and possibly independent) sources. That’s not enough to prove that the rumour is true, but perhaps enough to make it  repeating here.

The rumour is that, on Thursday 11th February in Washington DC at 10.40am 10.30am local time (15.40 15.30GMT), the Laser Interferometry Gravitational Wave Observatory (LIGO) will announce the direct experimental detection of gravitational waves.

If true this is immensely exciting, but I reiterate that it is, for the time being at least, only a rumour.

I will add more as soon as I get it. Please feel free to provide updates through the comments. Likewise if you have information to the contrary…

 

UPDATE: 9th February 2016. An official announcement of the forthcoming announcement has now been announced. It will take place at 10.30 local time in Washington (15.30 GMT), although it is believed the first ten minutes will involve a couple of songs by the popular vocal artist Beyoncé.

 

On the Theory of Gravitational Wave Rumour Sources

Posted in Astrohype, The Universe and Stuff, Uncategorized with tags , , on January 12, 2016 by telescoper

There has been a great deal of excitement almost nowhere in the astrophysics community since it was announced recently that rumours of the detection of gravitational waves had yet again begun to circulate, so I thought I would add here a brief discussion of the theoretical background to these phenomena.

The standard theoretical model of such rumours is that they are  produced from time to time during the lifetime of a supermassive science project after periods of relative quiescence. It is thought that they are associated with a perceived lack of publicity which might threaten funding and lead to financial collapse of the project. This stimulates a temporary emission of hype produced by vigorous gossip-mongering which acts to inflate the external profile of the project, resisting external pressures and restoring equilibrium. This general phenomenon is not restricted to gravitational wave detection, but also occurs across many other branches of Big Science, especially cosmology and particle physics.

However, observations of the latest outburst suggest support for a rival theory, in which rumours are produced not by the project itself but by some other body or bodies in orbit around it or even perhaps entirely independent of it. Although there is evidence in favour of this theory, it is relatively new and many questions remain to be answered. In particular it is not known what the effect of rumours produced in this way might be on the long-term evolution of the project or on the source itself.

Lawrence Krauss is 61.

 

 

Lisa Pathfinder – better late than never!

Posted in Science Politics, The Universe and Stuff with tags , , , , , , , on December 3, 2015 by telescoper

Determined to post about something positive after yesterday’s act of collective idiocy by Parliament I find myself given a golden opportunity by today’s successful launch of the Lisa Pathfinder experiment by the European Space Agency.

As space missions go, LISA Pathfinder seems quite a modest one: it is basically a pair of identical 46 mm gold–platinum cubes separated by 38 cm. The idea is to put these test masses in free fall and measure their relative positions as accurately as possible.

After a false start yesterday, LISA Pathfinder was successfully launched in the early hours of this morning and is now en route to the First Lagrangian Point of the Earth-Sun system, about 1.5 million miles from Earth, at the location marked L1 in the diagram:

Lagrange_saddle

The contours show the “effective potential” of the Earth-Sun system, which takes into account the effect of rotation as well as gravity. The five Lagrangian points are the places at which tis effective potential is locally flat, i.e. where its spatial gradient vanishes. Any physics student will know that when the gradient of the potential is zero there is no force on a test particle. What this means is that an object placed exactly at any of the 5 Lagrangian points stays in the same position relative to the Earth and Sun as the system rotates. Put a spacecraft at one of these points, therefore, and it stays put when viewed in a frame rotating around the Sun  at the same speed as the Earth.

It’s not quite as simple as this because, as you will observe the Lagrangian points are not stable: L1, L2 and L3 are saddle-points; a  stable point would be a local minimum. However, around the first three there are stable orbits so in effect a test mass displaced from L1, say, oscillates around it without doing anything too drastic. L4 and L5 can be stable or unstable, in a general system but are stable for the case of the Solar System, hence the tendency of asteroids (the Trojans) to accumulate at these locations.

You may remember that WMAP, Planck and Herschel were all parked in orbits around L2. A spacecraft positioned exactly at L2 is permanently screened from the Sun by the Earth. That might be very useful if you want to do long-wavelength observations that require very cool detectors, but not if you want to use the Sun as a source of power. In any case, as I explained above, spacecraft are not generally located exactly at L2 but in orbit around it. Planck in fact had solar cells on the base of the satellite that provided power but also formed a shield as they always faced the Sun as the satellite rotated and moved in its orbit to map the sky. The choice of L1 for LISA Pathfinder was made on the basis of spacecraft design considerations as it will operate in a very different manner from Planck.

The reason for doing eLISA is to demonstrate the technological feasibility of a much more ambitious planned gravitational wave detector in space originally called LISA, but now called eLISA. The displacement of test masses caused by gravitational waves is tiny so in order for eLisa it is necessary (a) to screen out every effect other than gravity, e.g. electromagnetic interactions due to residual charges, to great precision and (b) to measure relative positions to great accuracy. That’s why it was decided to fly a cheaper technology demonstrator mission, to prove the idea is feasible.

LISA Pathfinder won’t make any science discoveries but hopefully it will pave the way towards eLISA.

It has to be said that LISA Pathfinder has had a fairly troubled history. I just had a quick look at some papers I have dating back to the time when I was Chair of PPARC Astronomy Advisory. Among them I found the categorical statement that

LISA Pathfinder will be launched in 2009.

Hmm. Not quite. It’s obviously running quite a long way behind schedule and no doubt considerably over its initial budget but it’s good to see it under way at last. There will be a lot of sighs of relief that LISA Pathfinder has finally made it into space! Now let’s see if it can do what it is supposed to do!

 

 

 

BICEP2 bites the dust.. or does it?

Posted in Bad Statistics, Open Access, Science Politics, The Universe and Stuff with tags , , , , , , , , on September 22, 2014 by telescoper

Well, it’s come about three weeks later than I suggested – you should know that you can never trust anything you read in a blog – but the long-awaited Planck analysis of polarized dust emission from our Galaxy has now hit the arXiv. Here is the abstract, which you can click on to make it larger:

PlanckvBICEP2

My twitter feed was already alive with reactions to the paper when I woke up at 6am, so I’m already a bit late on the story, but I couldn’t resist a quick comment or two.

The bottom line is of course that the polarized emission from Galactic dust is much larger in the BICEP2 field than had been anticipated in the BICEP2 analysis of their data (now published  in Physical Review Letters after being refereed). Indeed, as the abstract states, the actual dust contamination in the BICEP2 field is subject to considerable statistical and systematic uncertainties, but seems to be around the same level as BICEP2’s claimed detection. In other words the Planck analysis shows that the BICEP2 result is completely consistent with what is now known about polarized dust emission.  To put it bluntly, the Planck analysis shows that the claim that primordial gravitational waves had been detected was premature, to say the least. I remind you that the original  BICEP2 result was spun as a ‘7σ’ detection of a primordial polarization signal associated with gravitational waves. This level of confidence is now known to have been false.  I’m going to resist (for the time being) another rant about p-values

Although it is consistent with being entirely dust, the Planck analysis does not entirely kill off the idea that there might be a primordial contribution to the BICEP2 measurement, which could be of similar amplitude to the dust signal. However, identifying and extracting that signal will require the much more sophisticated joint analysis alluded to in the final sentence of the abstract above. Planck and BICEP2 have differing strengths and weaknesses and a joint analysis will benefit from considerable complementarity. Planck has wider spectral coverage, and has mapped the entire sky; BICEP2 is more sensitive, but works at only one frequency and covers only a relatively small field of view. Between them they may be able to identify an excess source of polarization over and above the foreground, so it is not impossible that there may a gravitational wave component may be isolated. That will be a tough job, however, and there’s by no means any guarantee that it will work. We will just have to wait and see.

In the mean time let’s see how big an effect this paper has on my poll:

 

 

Note also that the abstract states:

We show that even in the faintest dust-emitting regions there are no “clean” windows where primordial CMB B-mode polarization could be measured without subtraction of dust emission.

It is as I always thought. Our Galaxy is a rather grubby place to live. Even the windows are filthy. It’s far too dusty for fussy cosmologists, who need to have everything just so, but probably fine for astrophysicists who generally like mucking about and getting their hands dirty…

This discussion suggests that a confident detection of B-modes from primordial gravitational waves (if there is one to detect) may have to wait for a sensitive all-sky experiment, which would have to be done in space. On the other hand, Planck has identified some regions which appear to be significantly less contaminated than the BICEP2 field (which is outlined in black):

Quieter dust

Could it be possible to direct some of the ongoing ground- or balloon-based CMB polarization experiments towards the cleaner (dark blue area in the right-hand panel) just south of the BICEP2 field?

From a theorist’s perspective, I think this result means that all the models of the early Universe that we thought were dead because they couldn’t produce the high level of primordial gravitational waves detected by BICEP2 have no come back to life, and those that came to life to explain the BICEP2 result may soon be read the last rites if the signal turns out to be predominantly dust.

Another important thing that remains to be seen is the extent to which the extraordinary media hype surrounding the announcement back in March will affect the credibility of the BICEP2 team itself and indeed the cosmological community as a whole. On the one hand, there’s nothing wrong with what has happened from a scientific point of view: results get scrutinized, tested, and sometimes refuted.  To that extent all this episode demonstrates is that science works.  On the other hand most of this stuff usually goes on behind the scenes as far as the public are concerned. The BICEP2 team decided to announce their results by press conference before they had been subjected to proper peer review. I’m sure they made that decision because they were confident in their results, but it now looks like it may have backfired rather badly. I think the public needs to understand more about how science functions as a process, often very messily, but how much of this mess should be out in the open?

 

UPDATE: Here’s a piece by Jonathan Amos on the BBC Website about the story.

ANOTHER UPDATE: Here’s the Physics World take on the story.

ANOTHER OTHER UPDATE: A National Geographic story

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