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Modified Gravity: Evidence from Cavendish Experiments?

Posted in The Universe and Stuff with tags , , , , , on January 15, 2019 by telescoper

A paper by Norbert Klein caught my eye as I tried to catch up on my arXiv reading after a couple of days away last week. It’s called Evidence for Modified Newtonian Dynamics from Cavendish-type gravitational constant experiments and the abstract reads:

Recent experimental results for the gravitational constant G from Cavendish-type experiments were analysed in the framework of MOND (Modified Newtonian Dynamics). The basic assumption for the analysis is that MOND corrections apply only to the component of the gravitational field which leads to an accelerated motion of the pendulum body according to Newtons second law. The analysis is based on numerical solutions of the MOND corrected differential equation for a linear pendulum at small acceleration magnitudes of the order of Milgroms fundamental acceleration parameter a0 = 10-10m s-2 for the case of a mixed gravitational and electromagnetic pendulum restoring force. The results from the pendulum simulations were employed to fit experimental data from recent Cavendish-type experiments with reported discrepancies between G values determined by different measurement methods for a similar experimental setup, namely time of swing, angular acceleration feedback, electrostatic servo and static deflection methods. The analysis revealed that the reported discrepancies can be explained by MOND corrections with one single fit parameter. The MOND corrected results were found to be consistent with a value of G = 6.6742 x 10-11 m3 kg-1 s-2 within a standard deviation of 14 ppm.

I have edited the abstract slightly for formatting and added the link to an explanation of MOND. You can find a PDF of the paper here.

I blogged about the discrepancies between different determinations of Newton’s Gravitational Constant G a few years ago here, where you can find this figure:

The claim that Modified Newtonian Dynamics can resolve these `discrepancies’  is very bold and I’m very skeptical of the arguments presented in this paper. It seems to me far more likely that the divergence in experimental measurements is due to systematics.  If anyone else has different views, however,  please feel free to share them through the comments box.

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Bernard Schutz wins the 2019 Eddington Medal

Posted in Cardiff, The Universe and Stuff with tags , , , on January 14, 2019 by telescoper

I wasn’t able to get to the Ordinary Meeting of the Royal Astronomical Society on Friday 11th January as I was otherwise engaged. In case you didn’t know, these meetings happen on the second Friday of every month and consist of short talks, longer set-piece prize lectures and Society business. The January meeting is when the annual awards are announced, so I missed the 2019 crop of medals and other prizes. When I got to the Athenaeum for dinner I was delighted to be informed that one of these – the prestigious Eddington Medal – had been awarded to my erstwhile Cardiff colleague Bernard Schutz (with whom I worked in the Data Innovation Research Institute and the School of Physics & Astronomy).

Here is a short video of the man himself talking about the work that led to this award:

The citation for Bernard’s award focuses on his invention of a method of measuring the Hubble constant using coalescing binary neutron stars. The idea was first published in September 1986 in a Letter to Nature. Here is the first paragraph:

I report here how gravitational wave observations can be used to determine the Hubble constant, H 0. The nearly monochromatic gravitational waves emitted by the decaying orbit of an ultra–compact, two–neutron–star binary system just before the stars coalesce are very likely to be detected by the kilometre–sized interferometric gravitational wave antennas now being designed1–4. The signal is easily identified and contains enough information to determine the absolute distance to the binary, independently of any assumptions about the masses of the stars. Ten events out to 100 Mpc may suffice to measure the Hubble constant to 3% accuracy.

In this paper, Bernard points out that a binary coalescence — such as the merger of two neutron stars — is a self calibrating `standard candle’, which means that it is possible to infer directly the distance without using the cosmic distance ladder. The key insight is that the rate at which the binary’s frequency changes is directly related to the amplitude of the gravitational waves it produces, i.e. how `loud’ the GW signal is. Just as the observed brightness of a star depends on both its intrinsic luminosity and how far away it is, the strength of the gravitational waves received at LIGO depends on both the intrinsic loudness of the source and how far away it is. By observing the waves with detectors like LIGO and Virgo, we can determine both the intrinsic loudness of the gravitational waves as well as their loudness at the Earth. This allows us to directly determine distance to the source.

It may have taken 31 years to get a measurement, but hopefully it won’t be long before there are enough detections to provide greater precision – and hopefully accuracy! – than the current methods can manage!

Congratulations to Bernard on his thoroughly well-deserved Eddington Medal!

 

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LGBT+ STEMinar – Notes on my Keynote

Posted in Biographical, LGBTQ+, The Universe and Stuff with tags , , , on January 12, 2019 by telescoper

I’m in a hotel near King’s Cross having had my Full English and with an hour or so before I have to check out and trek to Heathrow for my flight back to Dublin.

First things first. I promised a few people yesterday at the LGBT STEMinar that I would post the slides I used in my Keynote talk yesterday so here you go:

And here are a few pictures of me in action. I got all these from Twitter so apologize for not giving due credit to the photographers. My timeline was very crowded yesterday!

What I tried to do in the talk was to discuss the theme of progress over the last thirty years, both in my area of research (cosmology, specifically the large-scale structure of the Universe) and in the area of LGBT+ rights.

I started with my time as a graduate student at Sussex. One of the first things I did during `Freshers Week’ at when I started there was to join the GaySoc (as it was called) and I gradually became more involved in it as time went on. Over the five years I was at Sussex, `Gaysoc’ became `Lesbian and Gay Soc’ but a move to recognize bisexual people in the title was voted down, by quite a large margin. Inclusivity was (and still isn’t) a given even among marginalized groups. Biphobia and transphobia are still very much around.

Initially I kept my sexual orientation separate from my academic life and wasn’t really all that open in the Department in which I worked. My decision to change that was largely because of things going on in the outside world that convinced me that there was a need to stand up and be counted.

One of these was the AIDS `panic’ exacerbated by the Thatcher Government’s awful advertising campaign, an example of which you can see above. It was a very frightening time to be gay, not only because of the fear of contracting AIDS oneself but also because of the hostility that arose as a reaction to the `gay plague’. I’m convinced that this campaign led directly to a great deal of the violence that was inflicted on gay people during this time, including myself.

The second thing that made me want to come out was the Local Government Act (1988), which included the now infamous Section 28 (above). This was the subject of the first political demonstrations I ever attended. We failed to stop it becoming law, which was what we had wanted to do, but one positive that came out of this was that it did galvanize a lot of people into action, and the law was eventually repealed.

Anyway, I just got fed up of hearing people making ill-informed generalisations during this time. Rather than make a big public statement about being gay, I just resolved to not let such comments pass. I think it only took a few intercessions in the tea room or Falmer Bar for it to become widely known in the Department that I was gay. That was how I came out in astrophysics, and thereafter almost everyone just seemed to know.

So that was the eighties. If somebody had told me then that in thirty years the United Kingdom would have legalized same-sex marriage I would just have laughed. That wasn’t even really being discussed by the LGBT+ community then.

Anyway, back to the talk. What I then tried to do – actually for most of the presentation – was to outline the progress that has been made over the last thirty years in cosmology. When I started in 1985 there was hardly any data. There were some small redshift surveys of the order of a thousand galaxies, but my thesis was supposed to be about the pattern of fluctuations in the cosmic microwave background and there were no relevant measurements back then. I had to rely on simulations, as I mentioned here a few days ago.

Over the years there has been tremendous progress, especially with the accumulation of data enabled by improvements in observational technology. Theory has moved on to the extent that we now have a standard model of cosmology that accounts for most of this data (at least in a broad-brush sense) with just six free parameters. That’s a great success.

This rapid progress has led some to suggest that cosmology is now basically over in the sense that we have done virtually everything that we’ll ever do. I disagree with this entirely. The standard model contains a number of assumptions (general relativity, cold dark matter, a cosmological constant, and so on) all of which should be questioned. In science every answer leads to new questions and all progress to new challenges. If we ever rest on our laurels the field will stagnate and die. Success should never lead to complacency.

So then in the talk I returned to LGBT+ rights. Some (straight) people have said to me that now that we have equal marriage then it’s basically all done, isn’t it? There’s now no discrimination. You can stop talking about LGBT+ matters and `just be a scientist’.

That, I’m afraid, is bollocks. We have equal marriage but, though welcome, by no means represents some sort of utopia. Society is still basically a patriarchy, configured in a way that is profoundly unfair to many groups of people, so there are still many challenges to be fought. Unless we keep pushing for a truly inclusive society there is a real danger that the rights we have won could easily be rolled back. This is no more over than cosmology is over. In fact, you could really say that it’s really just the start.

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The 2019 LGBT+STEMinar

Posted in Biographical, LGBTQ+, The Universe and Stuff with tags , on January 11, 2019 by telescoper

I’m just back in my hotel after an evening at the Royal Astronomical Society Club Dinner which I went to straight from today’s LGBT+STEMinar at the very nice new Institute of Physics building on Caledonian Road near King’s Cross. This is what it looked like when I arrived this morning:

I enjoyed the LGBT+ STEMinar very much indeed. There was a huge range of talks by a wonderfully diverse crowd of speakers, on topics ranging from nuclear waste, parasites in wood mice, glaciology, quantum optics, the evolution of island finches, bacterial pathogenesis, gamma-ray bursts and machine learning in astrophysics.

I particular enjoyed the talk by Niamh Kavanagh from the Tyndall Institute in Cork who handed out home-made filters to give herself a rainbow effect:

I was delighted and relieved that my keynote talk and the end of the day seemed to go down quite well, at least judging by the comments I found in my Twitter feed just now. Here’s a picture I found there!

I’ll post the slides from my talk and perhaps a few other comments about it tomorrow, after I’ve had a good night’s sleep. But I won’t delay in thanking the organisers, especially Angela Townsend, for making this such a special day.

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Cosmological Simulations down Memory Lane

Posted in Biographical, LGBTQ+, The Universe and Stuff with tags on January 8, 2019 by telescoper

On Friday I have to be in London to give a keynote talk at this year’s LGBT+ STEMinar, which is taking place at the new Institute of Physics Building near King’s Cross. I’ve been struggling to think what to say but a conversation this afternoon with some of our PhD students gave me an idea. I won’t spoil it for those going to the event by giving too much detail away, but it involves going over the past 30 years of cosmology and LGBT+ rights alongside each other, pointing out that in both areas there has been great progress but there is also still very much to do.

Anyway, in the course of this I had a look at my thesis (vintage 1988) and came up with the following pictures, in glorious monochrome:

You can click on them to make them bigger. When I started my graduate studies in 1985 my thesis was supposed to be about the statistical analysis of the cosmic microwave background. The problem was that way back then there weren’t any measurements, so I had to make simulations to test various analysis methods on. The above images are examples that ended up in a published paper.

You have no idea what a pain it was to make these images. I had very limited access to a graphics terminal so I had to send these to a special printer in the computer room  (which was behind closed doors and an airlock) and then wait (sometimes for days) for the operators to process the files and produce a the printout. If they came out wrong the process had to be repeated. It was all frustratingly slow as my programs were quite buggy, at least to begin with.

For those of you interested, these simulations were made using a (two-dimensional) Fast-Fourier Transform method, using a pseudo-random number generator to set up appropriate amplitudes and phases for the Fourier modes. The only even remotely clever bit was to find a way of generating Gaussian and non-Gaussian maps with the same two-point correlations.

In all it took me several months of work to complete the work that went into that paper (which was essentially a thesis chapter). When I look back on it I think if I’d been cleverer – and had a decent graphics screen like you find on a modern PC – I could have done it all in a couple of days!

And now, of course, we have real data as well as simulations!

My point is that things that seem very difficult at the time often look extremely easy in retrospect. And that’s not just the case in cosmology.

 

 

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The Winter Solstice 2018

Posted in The Universe and Stuff with tags , , , , , , on December 21, 2018 by telescoper

The winter solstice in the Northern hemisphere happens today, Friday 21st December 2018, at 22.23 Irish Time (22.23 UTC). Among other things, this means that today is the shortest day of the year. Days will get longer from now until the Summer Solstice next June.  In fact, the interval between sunrise and sunset tomorrow will be a whole second longer tomorrow than it is today. Yippee!

This does not mean that sunrise will happen earlier tomorrow than it did this morning, however. Actually, sunrise will carry on getting later until the new year. This is because there is a difference between mean solar time (measured by clocks) and apparent solar time (defined by the position of the Sun in the sky), so that a solar day does not always last exactly 24 hours. A description of apparent and mean time was given by Nevil Maskelyne in the Nautical Almanac for 1767:

Apparent Time is that deduced immediately from the Sun, whether from the Observation of his passing the Meridian, or from his observed Rising or Setting. This Time is different from that shewn by Clocks and Watches well regulated at Land, which is called equated or mean Time.

The discrepancy between mean time and apparent time arises because of the Earth’s axial tilt and the fact that it travels around the Sun in an elliptical orbit in which its orbital speed varies with time of year (being faster at perihelion than at aphelion).

In fact if you plot the position of the Sun in the sky at a fixed time each day from a fixed location on the Earth you get a thing called an analemma, which is a sort of figure-of-eight shape whose shape depends on the observer’s latitude. Here’s a photographic version taken in Edmonton, with photographs of the Sun’s position taken from the same position at the same time on different days over the course of a year:

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The winter solstice is the lowermost point on this curve and the summer solstice is at the top. The north–south component of the analemma is the Sun’s declination, and the east–west component is the so-called equation of time which quantifies the difference between mean solar time and apparent solar time. This curve can be used to calculate the earliest and/or latest sunrise and/or sunset.

Using a more rapid calculational tool (Google), I found a table of the local mean times of sunrise and sunset for Dublin around the 2018 winter solstice. This shows that today is indeed the shortest day (with a time between sunrise and sunset of 7 hours 29 minutes and 59 seconds).  The table also shows that sunset already started occurring later in the day before the winter solstice, and sunrise will continue to happen later for a few days after the solstice, notwithstanding the fact that the interval between sunrise and sunset gets longer from today onwards.

I hope this clarifies the situation.

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The Open Journal of Astrophysics: Last Update for 2018

Posted in Maynooth, Open Access, The Universe and Stuff with tags , , , , , on December 20, 2018 by telescoper


As term staggers to its close tomorrow, two things this morning reminded me that I should post a final update for the The Open Journal of Astrophysics.  I’ve been regularly boring all my readers with a stream of stuff about the Open Journal of Astrophysics, but if it’s all new to you, try reading the short post about the background to the Open Journal project that you can find here.

The first item was a little piece in the latest edition of University News (which was delivered this morning) about the launch event for the journal here at Maynooth University way back in October.

I didn’t make it into the picture above as I had to leave the event before the end in order to give an undergraduate lecture, but the appearance of this piece reminds me to express once more my gratitude to Maynooth for providing such excellent support for this project.

The other event this morning was an email notification that the OJA has received yet another submission. We now have a number of papers in the pipeline, and interest seems to be picking up. I can’t say how many submissions will actually be published, but the papers we have received are working their way through the peer-review system and we’ll see what transpires.

On a related point, a few people have asked me why the Open Journal of Astrophysics is not in the Directory of Open Access Journals (DOAJ). The answer to that is simply that DOAJ requires a minimum of five papers published in a calendar year and because we had a long hiatus before the re-launch we don’t yet qualify. The way things are going we should meet the threshold for 2019 pretty quickly.

Though I stress that the Editorial Board of OJA is not going to compromise standards just to get listed on DOAJ.
we are hoping to increase the number of submissions with time (in a manageable way). As it happens, we do have funds available to promote the OJA as I think quite a large number of members of the astrophysics community still haven’t heard of it. This also makes it a little difficult to enlist referees who perhaps jump to the conclusion that this is a dodgy or predatory journal. There are many of these about, sadly, but I hope we can establish relatively easily the fact that we’re not of that ilk.

I would like here to repeat a small request I posted a while ago. Do any of you have any ideas for promoting The Open Journal of Astrophysics? We could advertise directly in journals of course, but I’m wondering if anyone out there in the interwebs has any more imaginative ideas? If you do please let me know through the comments box below.

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Sustainability and Irish Science

Posted in Politics, Science Politics, The Universe and Stuff with tags , , , , on December 19, 2018 by telescoper

There’s an interesting news item in the Education section of the Irish times about the appointment of Prof Séamus Davis to positions at both the University of Oxford and University College Cork, under a Science Foundation Ireland scheme intended to capitalize on Brexit (and the imminent loss of EU funding it implies) and the unhappy situation for science in the USA. This is the first appointment to one of the new Research Professorships, which allow the holders to be paid up to €250K.

While I support any investment in Irish science, and wish Prof. Davis every success in his new role, my reaction to the SFI scheme is very similar to my view of the Sêr Cymru (“Star Wales”) project which began a few years ago when I was working in Cardiff, with the aim of attracting `research leaders’ to Wales.

I am very skeptical about the likely success of `top-down’ moves like this. What Ireland really needs (and currently does not have) is a sustainable research base, so at very least I’d like to complementary  `bottom-up’ projects nurturing  researchers at PhD and PDRA level, perhaps through a greatly expanded system of national fellowships.  The trouble in Ireland is that there are so few opportunities for early career researchers that many have to go abroad to further their careers. There’s nothing wrong with Irish researchers choosing to work in another country, of course, but in an ideal world they would choose rather than be forced to do so by lack of opportunity and their loss would be offset by a other nationals choosing to come into Ireland. Unless this problem is fixed Ireland might end up with some leaders but nobody around to follow them.

The question I ask myself is, if one had to choose, what would be better in the long run for Irish science, one Professor on a salary of €250,000 or eight new postdoctoral fellowships (at roughly the same cost)?

Of course the idea of bringing in `research leaders’ is that they will manage to bring in funds from elsewhere, especially the European Union. This may indeed happen and indeed some may already have money in the bag when they move in. The problem with the strategy, though,  is that it’s not very easy to persuade such leaders to leave their current institutions, especially in experimental sciences, if they’ve already spend years acquiring the funding needed to equip their laboratories. This is not just a question of moving people, which is relatively easy, but can involve trying to replace lots of expensive and delicate equipment. The financial inducements needed to fund the relocation of a major research group and fight off counter-offers from its present host are likely to be so expensive that the benefit gained from doing this takes years to accrue, even they succeed. And EU grants are exceptionally competitive..

It’s a big shame that Ireland does not take research funding as seriously as it should, especially in fundamental science. Brexit could well turn out to be very damaging for the Irish economy, but science is one area where in which there are enormous opportunities if only there was the political will to seize them.

 

 

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Fourier, Hamilton and Ptolemy

Posted in History, Poetry, The Universe and Stuff with tags , , , , , , , on December 17, 2018 by telescoper

As we stagger into the last week of term I find myself with just two lectures to give in my second-year module on Vector Calculus and Fourier Series. I didn’t want to present the two topics mentioned in the title as disconnected, so I linked them in a lecture in which I used the divergence theorem of vector calculus to derive the heat equation, the solution of which led Joseph Fourier to devise his series in Mémoire sur la propagation de la chaleur dans les corps solides (1807), a truly remarkable work for its time that inspired so many subsequent developments.

Fourier’s work was so influential and widely admired that it inspired a famous Irish mathematician William Rowan Hamilton to write the following poem:

Hamilton-for Fourier

The serious thing that strikes me is not the quality of the verse, but how many scientists of the 19th Century, Hamilton included, saw their scientific interrogation of Nature as a manifestation of the human condition just as the romantic poets saw their artistic contemplation and how many poets of the time were also interested in science.

Anyway I was looking for nice demonstrations of Fourier series to help my class get to grips with them when I remembered this little video recommended to me some time ago by esteemed Professor George Ellis. It’s a nice illustration of the principles of Fourier series, by which any periodic function can be decomposed into a series of sine and cosine functions.

This reminds me of a point I’ve made a few times in popular talks about Astronomy. It’s a common view that Kepler’s laws of planetary motion according to which which the planets move in elliptical motion around the Sun, is a completely different formulation from the previous Ptolemaic system which involved epicycles and deferents and which is generally held to have been much more complicated.

The video demonstrates however that epicycles and deferents can be viewed as the elements used in the construction of a Fourier series. Since elliptical orbits are periodic, it is perfectly valid to present them in the form a Fourier series. Therefore, in a sense, there’s nothing so very wrong with epicycles. I admit, however, that a closed-form expression for such an orbit is considerably more compact and elegant than a Fourier representation, and also encapsulates a deeper level of physical understanding.

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On Probability and Cosmology

Posted in The Universe and Stuff with tags , , , on December 12, 2018 by telescoper

I just noticed a potentially interesting paper by Martin Sahlén on the arXiv. I haven’t actually read it yet, so don’t know if I agree with it, but thought I’d point it out here for those interested in cosmology and things Bayesian.

Here is the abstract:

Modern scientific cosmology pushes the boundaries of knowledge and the knowable. This is prompting questions on the nature of scientific knowledge. A central issue is what defines a ‘good’ model. When addressing global properties of the Universe or its initial state this becomes a particularly pressing issue. How to assess the probability of the Universe as a whole is empirically ambiguous, since we can examine only part of a single realisation of the system under investigation: at some point, data will run out. We review the basics of applying Bayesian statistical explanation to the Universe as a whole. We argue that a conventional Bayesian approach to model inference generally fails in such circumstances, and cannot resolve, e.g., the so-called ‘measure problem’ in inflationary cosmology. Implicit and non-empirical valuations inevitably enter model assessment in these cases. This undermines the possibility to perform Bayesian model comparison. One must therefore either stay silent, or pursue a more general form of systematic and rational model assessment. We outline a generalised axiological Bayesian model inference framework, based on mathematical lattices. This extends inference based on empirical data (evidence) to additionally consider the properties of model structure (elegance) and model possibility space (beneficence). We propose this as a natural and theoretically well-motivated framework for introducing an explicit, rational approach to theoretical model prejudice and inference beyond data.

You can download a PDF of the paper here.

As usual, comments are welcome below. I’ll add my thoughts later, after I’ve had the chance to read the article!

 

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