Gravitational waves – and why they might not be from black holes

The discovery of gravitational waves – announced on February 11 – is an outstanding achievement. Personally, I think it is of greater importance even than finding the Higgs boson at CERN in 2012, which led to Nobel prizes for François Englert and Peter Higgs.

The existence of gravitational waves was predicted by Einstein’s theory of General Relativity. They were indirectly observed by two radioastronomers studying a binary pulsar discovered in 1974: the observations provided what was then by far the most rigorous test of Einstein’s theory. However, directly observing gravitational waves has been exceptionally difficult, requiring both exquisite precision and the ability to filter out all other confounding signals: hence the excitement that the same signal on 14 September 2015 appeared in two different detectors.

The discovery is also the best evidence yet of the actual existence of black holes. A couple of years ago I wrote an article on this blog about Why I don’t believe in black holes. The gravitational wave detections could therefore demonstrate that I was wrong, which is no big deal (especially since I ceased being a professional astronomer a long while back!). However, there is a small detail that indicates that the black hole interpretation might still be premature. According to the New Scientist, a gamma-ray burst appeared a mere half second later, and the chance of this being a random coincidence is about 0.2%. The problem is that such a burst is not supposed to happen with black hole mergers.

Artist's conception of a pair of black holes about to merge. Credit: NASA

Artist’s conception of a pair of black holes about to merge. Credit: NASA

Gamma-ray bursts are themselves spectacular but enigmatic events, lasting from a few milliseconds to no more than a several minutes. (For an introduction to gamma-ray bursts, see here.) The origins of many remain mysterious, but neutron stars and black holes are usually implicated.

So if gamma-ray bursts aren’t meant to happen with black hole mergers, what could be happening? It’s perfectly possible that the only problem is that current models aren’t sufficiently realistic, and that with better data and better models it will be shown that the gravitational wave detection on 14 September 2015 was indeed of a black hole merger. Alternatively, the detection of a gamma-ray burst at the same time may indicate that it wasn’t a black-hole merger at all: perhaps instead, the gravitational waves were produced by the merger of two ultradense, ultracompact objects that were denser than neutron stars but not actually black holes. As I understand it – and I’m happy to be proved wrong – this may be a heretical view but it hasn’t actually been disproven.

What is clear is that gravitational wave astronomy has arrived. New detectors are needed, so that the location of the events themselves can be measured more precisely, and so that the association with other events, such as gamma-ray bursts, can be clarified. It’s an exceptionally exciting new field – and by far the best way to ascertain whether or not black holes actually do exist.

Andy Beardmore, a former colleague of mine at Leicester University and now part of the research staff there, comments on Facebook: “Theorists can predict anything once given an incentive – more than one paper has since appeared saying that, of course, you can get EM waves [eg gamma rays] from a merging black hole binary…“. He also recommended an article from the New Yorker magazine that’s a particularly detailed description of the discovery (here).

Scientific hype? Surely not!

I was excited to find a letter of mine in this week’s New Scientist!

The BICEP2 station at the South Pole, which made the observations.

The BICEP2 station at the South Pole, which made the observations. (Photo from here)

Three weeks ago, there was massive fanfare about a major cosmological discovery. A telescope based near the South Pole had been surveying the cosmic microwave background, which is the fading echo of the Big Bang. Although the observations (described here) are impressive, it struck me that the interpretation being put on them was premature and over-hyped.

When the New Scientist reported the story, instead of giving a balanced account, they accentuated the hype further by trumpeting it as a confirmation of the multiverse (which it isn’t). I therefore wrote a letter “as a church minister with an astrophysics background”, which they published this week (and which I nearly missed!). Here’s the full letter (the published one is slightly shorter):

Dear New Scientist,

The potential confirmation of both gravitational waves and inflation is understandably exciting – but I have been very disappointed by the over-hyped coverage in the media generally. I had hoped that New Scientist would be able to give a sober assessment of the observations and their interpretation – instead, you have chosen to sensationalise it still further by talking it up as a confirmation of the multiverse (22 March, pp8-10, ‘Ripples of the multiverse’).

As a church minister with an astrophysics background, I am in the unusual position of having a number of friends who believe in 6-day creationism. I frequently find myself having to defend both the science of a multi-billion year universe, and the integrity of the scientific process. I argue that science is not inherently anti-God, even if some scientists are; and that scientific results are subject to rigorous scrutiny and not subject to spin and hype. You will therefore understand why I find the New Scientist’s treatment of this discovery most disappointing.

More sober discussion has been provided by Professor Peter Coles in his ‘In the dark’ blog, who regards confirmation at another frequency as a minimum requirement, and points out some oddities in the data. It would have been helpful if New Scientist had been able to include something similar. It is not long ago that great excitement was generated by the apparent discovery of […] neutrinos [going faster than the speed of light] – until it was discovered that there was an un-noticed calibration error, and the result was retracted. The difficulty of extracting the gravitational wave signal from the CMB data suggests that such a retraction could happen here.

Yours sincerely,
Rev Dr Rich Tweedy

Since then, there has already been a research paper that suggests the signals could be galactic in origin rather than cosmological (here). The observations merit close scrutiny from those qualified to do so – but the hype ultimately does no-one any favours.

Why I don’t believe in black holes

Hiding a heresy can be hard. I’ve been doing a few science-faith talk here – and somehow or other the shocking news slides out, that I don’t believe in black holes. This may seem strange as black holes frequently appear in the science news (for example here and here). So I thought I should explain why.

It’ll probably help to start by focussing on a prime example of a ‘black hole’ system.  The image below is an artist’s impression of what’s happening in Cygnus X-1: there’s a large, luminous, blue star that produces most of the visible light – but the x-rays largely come from an ultracompact object which is orbiting it. This object draws material off from the blue star, which swirls in a disk before eventually being dragged into the centre – some of which is then fired outwards via jets.

Cyg X-1: a black hole system in our own galaxy.

Cygnus X-1: a black hole system in our own galaxy?

The importance of this particular system is that the mass of the compact object can be measured (Kepler’s laws of planetary motion enable one to do this straightforwardly). It is about 15 times the mass of the Sun [ref]. This mass is essential to understanding what the object might or might not be: it means, for example, that it can’t be either a white dwarf or a neutron star – both of which would be much lighter. Let me explain.

A compact star is formed at the end of the life of a normal star. All normal stars, like the Sun, are giant nuclear fusion reactors: most burn hydrogen into helium, others (the red giant stars) burn helium into carbon, or carbon into other heavier elements; but whatever the fuel, it will eventually run out. At that point the core of the star will collapse in on itself. What happens next depends on how big it is.

The Sirius system: the bright main star, and the white dwarf

The Sirius system: the bright main star, and the faint white dwarf to the lower left

Most stars will end up as white dwarfs. The Sun certainly will, and the faint companion of Sirius (see opposite) is a nearby example. In this type of object, a star the mass of the Sun is compressed into an object the size of the Earth (which has one-millionth the volume), so that a thimble-full of material will weigh several tonnes.

However, because of the quantum properties of white dwarf material, there is a maximum mass for such a star: it’s 1.4 times the mass of the Sun. If it’s bigger than this, it will form a neutron star, an ultracompact object in which a star the mass of the Sun is compressed into a volume which is about ten miles across. The best known examples of these are the pulsars, such as the one in the Crab Nebula [here]. Astronomers know most about those which appear as pulsars, or those which are in binary star systems and whose effects can be seen in other ways. But there’s a maximum mass here, too: it’s about three times the mass of the Sun.

So what about a compact object like in Cygnus X-1 which, at 15 times the mass of the Sun, is comfortably bigger than these limits? The standard answer is that it becomes a black hole. But there is a hidden assumption: this is that nothing denser than neutron star material can exist, so if an ultracompact star is heavier than a neutron star, it must be a black hole. This assumption was excusable in the 1960s, when there really weren’t any other, denser forms of matter known – but not any longer.

For example, there is a substance called Bose-Einstein condensate, which has been manufactured in laboratories [ref]. Unlike the material that makes up white dwarfs and neutron stars, this substance does not have a maximum mass. Consequently, a small number of theorists have considered this material as the basis for an alternative to black holes, which they call gravastars.

Particle physicists get excited about a substance called quark-gluon plasma, which has been produced at particle accelerator laboratories [ref]. It’s believed that this state of matter existed for a few microseconds after the Big Bang, before the universe cooled enough to form protons and neutrons. One Indian physicist, Abhas Mitra – a self-proclaimed heretic who is a bit too gifted to be easily ignored [ref] – has developed a theory that quark-gluon plasma balls form instead of black holes [ref].

In both cases, the main problem is that it would be exceptionally difficult to be able to show observationally that a compact star is a gravastar or a quark-gluon plasma ball as opposed to a black hole (or vice versa). But perhaps the burden of proof is the wrong way round: given the existence of such material, shouldn’t it be necessary to demonstrate that black holes do exist, rather than to assume that they do because objects exist which are larger than neutron stars?

So that’s why I’m a heretic about black holes!

Update July 2017 – I was gratified to read a New Scientist feature article from July 12 which presents the argument that black holes might not exist…

My scientific past

Saguaro cacti near Tucson

Saguaro cacti near Tucson

I’ve just completed a couple of weeks doing talks on science and faith. This included a Lent talk to the church, but I’ve also been doing some talks in the Chantry – the local secondary school in Martley (having been kindly invited by the RE teacher, Mel Palmer).

I’ve started each of the talks with a brief autobiographical account of my involvement in the astronomical world. This is the background for why I strongly believe that Christians who speak about science should at least be scientifically literate. I therefore thought I’d give a brief summary here of my life as an astronomer, with a look at one of the projects I undertook.

I spent just over three years in Tucson, Arizona in the early 1990s, working as a postdoc at the University. I was massively fortunate to have this opportunity – one that I perhaps didn’t quite appreciate at the time as much as I should have done! While there I had about one hundred nights observing, mostly on Kitt Peak, which was then a leading observatory (but has since been overtaken by much bigger ones on higher mountains and in more remote places).

Kitt Peak National Observatory - the telescope I used most is the one in the open dome.

Kitt Peak National Observatory – the telescope I used most is the one in the open dome.

The Burrell-Schmidt telescope - a small, old scope which had fast optics, which was ideal for what I wanted to do.

The Burrell-Schmidt telescope – a small, old scope which had fast optics, which was ideal for what I wanted to do.

The Ring Nebula as viewed by the Hubble Space Telescope. Note the star at the centre, which is becoming a white dwarf.

The Ring Nebula as viewed by the Hubble Space Telescope. Note the star at the centre, which is becoming a white dwarf. Credit: NASA and The Hubble Heritage Team (STScI/AURA)

My main research interest was in white dwarfs and old planetary nebulae. When a star like the sun runs out of its nuclear fuel (thus, after it has burned all the hydrogen in its core, and then burned all of the helium), the core of the star collapses to become a white dwarf. Meanwhile, the outer layers lift off into space, for a while becoming a planetary nebula, one of the most beautiful sights in the sky. One example is the Ring Nebula (shown on the right), which is visible with binoculars, but is spectacular when seen with the Hubble Space Telescope; the star at the centre is becoming a white dwarf.

The closest planetary nebula Sh 2-216, viewed with the Burrell-Schmidt in 1995, filtering for light from hydrogen (technically H-alpha]). Displayed to reveal the brighter stuff.

The closest planetary nebula Sh 2-216, viewed with the Burrell-Schmidt in 1995, filtering for light from hydrogen (technically H-alpha]). Displayed to reveal the brighter stuff – compare with the image below, which is displayed to show the fainter parts. The arrow marks the white dwarf.

Eventually, as the nebula expands, it becomes buffeted by the ambient wisps and clumps of interstellar gas, into which it eventually dissipates. The focus of my research was on those nebulae which were at this stage, of interacting with this interstellar material.

The research which probably gave me the most satisfaction was on the closest of all the planetary nebulae, which has the unglamorous name of Sh 2-216. I first became interested in it when I was studying white dwarfs at Leicester University – and was intrigued that the nebula has been so buffeted by the surrounding interstellar material that the white dwarf is no longer at the centre.

The nebula is very faint indeed, but spans about 1.6 degrees – far larger than any other then known. The telescope I used was ideal for this project, because it had a wide enough field-of-view for the task. I still had to produce a mosaic of images to cover it, but it was very doable.

It’s an indication of how much technology has improved in the 18 years since then that this nebula is now within the range of amateur observers, for whom it is a rewarding challenge. For example, this image is much prettier!

The closest planetary nebula Sh 2-216, viewed with the Burrell-Schmidt in 1995, filtering for the light of ionised nitrogen (technically [N II]). Overexposed to reveal the faint stuff.

The closest planetary nebula Sh 2-216, viewed with the Burrell-Schmidt in 1995, filtering for the light of ionised nitrogen (technically [N II]). The region to the left has been overexposed to reveal the faint stuff, but is shown in the previous image.

The problem with this research field is that the observations are easy but the theoretical modelling is spectacularly difficult – and it’s not quite as glitzy as quasars and distant galaxies! However an unexpected breakthrough came in 2008 as a result of the work of a team of radio astronomers, who were doing the Canadian Galactic Plane survey. They were very surprised to see it all – indeed, they only detected one other planetary nebula anywhere else in the sky – but the reason it was detectable was because of the interaction between the nebula and its surroundings. Ryan Ransom used the data to measure quantities such as the strength of the ambient magnetic field in the surroundings. However, he also emphasised the difficulties of deducing much more – largely due to the difficulties of the theoretical modelling. Nevertheless, the radio data is an important new angle that significantly improves the understanding of the nebula and its environment.

References: Optical; Radio.

The Martian golf-cart

A small motorised contraption, the size of a golf cart, has been trundling across the Martian desert for the last seven years. It may no longer capture the  news headlines as it did shortly after it first bounced onto the red planet in January 2004, but it is an extraordinarily successful mission that has far exceeded the expectations of NASA, who had planned for a 90-day lifetime. It’s a project I’ve followed avidly since it arrived. A couple of weeks ago, it reached the edge of the 14-mile wide crater Endeavour, and has begun a new science investigation that could last several years.

Prior to 2004, Mars had been the graveyard of space science missions – the most recent being the ill-fated British Beagle 2 expedition. Nevertheless, NASA had optimistically planned to land two rovers which would explore the terrain and begin looking for signs of water in the planet’s geological history. The first landed in the giant Gusev Crater; the second, named Opportunity, arrived on a plateau on the opposite side of the planet. They used an innovative landing technique, inflating giant airbags that bounced onto the surface before deflating and unfurling, allowing the rovers to roll off.

The Mars rover Opportunity leaves the crater where it first arrived in January 2004 – with the airbag platform it landed in. (Copyright NASA)

The ‘blueberries’ on Mars which enabled the NASA team to deduce water had flowed in the planet’s geological past. The pale circle was made by the rover’s rock abrasion tool to prepare a rock for geological analysis. (Copyright NASA)

Opportunity scored a bull’s-eye when it landed, as it rolled into the middle of a small crater. This meant that it could immediately start exploring the exposed rock in its environment without travelling far. Before long, NASA focussed on some small, blueberry-sized spherules. They found that these were rich in the iron-based chemical, haematite. On Earth, they are rare, but are found in desert conditions where there is evaporating water. It was the first and most dramatic confirmation that water had once existed on Mars, and paves the way for future expeditions that could explore the possibility that life might once have existed there, too.

During these initial three months, dust gradually accumulated on the solar panels, which was expected to block out the Sun’s radiation. However, after a while it became apparent that some of the dust would disappear – perhaps being blown off by winds in the thin Martian atmosphere – and the possibility of a longer expedition arose. Thus, a rover designed to travel half a mile began to range far beyond its expected capabilities.

Two and a half years later, it arrived at the half-mile wide Victoria crater, without doubt its most spectacular destination, where it remained for another two years. The aim was to be able to probe the geology of the area at far deeper levels than ever before.

Panorama of Victoria Crater, which Opportunity explored between 2006-8. (Click to enlarge) (Copyright NASA)

It has not been without alarms. On one occasion it was stuck on the side of a crater, on a slope that was almost too steep for its wheels. Then later it became trapped in a sand dune for several weeks before some clever routines enabled it to inch slowly out.

Opportunitty’s path from Eagle crater to Endeavour Crater – 20 miles in 7 years. (Click to enlarge) (Copyright NASA)

Nevertheless, the rover that was planned to drive half a mile in ninety days has now completed 20 miles in seven and a half years, an extraordinary accomplishment that will enable NASA to plan much more ambitious follow-up missions. As it sits on the edge of Endurance crater, NASA scientists can now plan geological projects in the years ahead, which would never have been thought possible when this golf buggy first arrived on the planet.

Check out this video of the journey to Endeavour Crater.

Thinking for radio

One of the most interesting modules I’ve done while in Durham is the current one on “Preaching and Apologetics”. For those not versed in Christian jargon, apologetics is the art of defending Christianity to a secular audience. One of the lecturers is David Wilkinson, the Principal of the college of which Cranmer Hall is a part, who is one of the foremost experts in this field – especially where science is concerned.

One well-known slot for this is Radio 4’s “Thought for the Day”. Not everyone is keen on it – militant atheists hate there being any slot for religion on radio, and some Christians don’t like it either because the faith content is often bland and safe. Nevertheless, it is an opportunity to give relevant Christian commentary on current news items. For one of David Wilkinson’s Thoughts, giving a Christian perspective on the giant physics experiment, the  Large Hadron Collider, click here.

As an assignment, we all had a go at composing, and reading aloud, a Thought for the Day. This was a remarkably powerful experience: Andy Grant, an ex-soldier, told us about four servicemen, severely wounded in the Afghan conflict, who are walking to the North Pole; Tom Hiney gave a moving reflection on Mohamed Bouazizi, the former stall holder in Tunisia who, by setting fire to himself, sparked the current conflagration across North Africa; Matt ‘Woodie’ Woodcock spoke of his dread about going to Auschwitz this weekend, to witness the site of the Nazi atrocities.

Mine was about a small black rock that fell from the sky… here it is:

The black rock from space… ©NASA

[Earlier this week] a small piece of black rock hit the news. It doesn’t look particularly special… Yet scientists are claiming that this particular lump may help to explain the origin of life on earth.

A camp-site on the ice-sheet near where the rock was found: makes you shiver just looking at it! ©NASA

It’s a piece of rock that has been on quite a journey. It was picked up in Antarctica about 200 miles from the South Pole. This is an area where scientists go looking for meteorites, those rocks that fall from the sky. Antarctica may seem a strange place to look for them, but dark coloured rocks are easy to pick out from the endless white ice-sheets.

This particular meteorite has an unusual chemistry with an abundant amount of ammonia – which is what has got the scientists excited. It’s incredibly difficult to try to understand the origins of life on earth – not least because we can’t go back four billion years to make observations and conduct experiments. However, ammonia appears to have been a crucial ingredient – but the problem is that it was in incredibly short supply. This discovery shows that meteorites, showering the Earth from outer space, could have provided enough ammonia to help seed the origins of life. Although it’s just one piece in the jig-saw, it’s an important one nonetheless.

That’s why this apparently mundane lump of black rock has an unusual significance. It belongs to the origin myths of our time. Like people across the ages, we humans have a great fascination with stories about where we come from. And yet – if all we are is explained by chemistry, does that really answer our innate desire to understand our origins?

As a Christian I believe that our origins are not just explained by chemistry, but that over everything there is a God who set all the scientific processes in motion. The heated debate about creation or evolution is in many ways a barren one: however life emerged, whatever mechanism was used, the big story for Christians is that God did it. I’m fascinated by the black lump discovered in Antarctica, and how it may be a part of understanding the way life began on Earth – but for me, the main thing is that God did it.