The detection of gravitational waves, predictions, MOND and my visit to the Weizmann Institut in Rehovot, Israel

The announcement on Feb.11th, 2016, that gravitational waves have been detected is a sensation and it is indeed rather incredible to imagine that space-time is constantly wobbling with and around us all the time because of some cosmic events, as is expected to be the case in Einstein’s theory of general relativity.

Imagine a wave comes though and everything gets distorted. Obviously, we will not measure a change,  since also the ruler is distorted. So the way LIGO works is to use two 4 km long rulers or measuring arms angled to each other, and to use overlapping light waves from both arms to seek the tiniest of tiniest relative changes between the two lengths. This is possible because gravitational waves are polarized.

This way and with the truly most incredibly developed hyper-sensitive length-measurement technology, the LIGO team can measure changes in relative length between the two arms that amount to 1/10000 of the diameter of a proton, or 10^-19 m.

In the announced case, two heavy stellar-mass black holes (with masses of about 29 and 36 Solar masses) coalesced about 1.3×10^9 yr ago to an about 36 Solar mass black hole plus about 3 Solar masses in radiated gravitational wave energy, leading to the detection of gravitational waves on Earth.

 

What is the source of these waves?

There are two possibilities.  The rumors that a signal with its properties had been detected by AdLIGO was already available by October 2015 as reported on The Reference Frame by Lubos Motl.

Individual massive star binaries: very fine-tuned solutions?

On Dec. 15th, 2015, Amaro-Seoane & Chen placed predictions on the likely to-be-found-by-AdLIGO  events on the arXiv arguing for massive black holes and that these circularise before coalescence due to gravitational wave emission.

One group (Marchant et al.) at Bonn University placed a paper onto the arXiv preprint server on Januray 14th, 2016, predicting essentially the particular waves which were then reported on Feb. 11th, 2016, by the LIGO team.

On Feb. 15th another group (Beczynski et al.) came up with a similar prediction.

Both of these latter contribution demonstrate that the two massive black holes orbiting each other may arise from one stellar binary system in which both stars were very massive and that this system evolved through stellar-wind-driven mass loss of both stars followed by their individual supernova explosions, to form a binary black hole system which is sufficiently tight to merge within much less than a Hubble time through the radiation of gravitational waves. From the above description it emerges that this is a highly fine-tuned problem to work out as the source of the very first observed gravitational wave emission. This scenario does have interesting consequences, namely that it leads to aligned spins of the black holes and that the kicks the black holes receive must be smaller than typically 400 km/s as emphasized by Belczynski et al.

Rather common events:  star clusters as engines for making them

But, there is another process which actually makes such black-hole merging events common, to the degree of AdLIGO (the now operating advanced LIGO observatory) observing 31 plus minus 7 such events per year.

The process begins with the birth of a massive star cluster somewhere in the universe. This massive star cluster, being typical in every respect (e.g. weighing 10^4 Mun, having a 1pc radius, with a normal stellar population), has its share of very massive stars which explode, one after another, as type II supernovae. Some of these leave a stellar-mass black hole in the cluster, which consequently and over a time of roughly 3-50 Myr builds-up a population of such black holes. These, being more massive than the stars in the cluster, sink to the centre of the cluster forming, by about 100Myr, a core of black-holes. There they meet and interact stellar-dynamically and they pair up through three-body dynamical encounters: one takes away the energy leaving two black holes in a binary. Such a binary may become tighter (i.e. it shrinks) with time because of the constant perturbations by the other cluster members. The black-hole binary “hardens” over time, until a final strong encounter with another black hole in the cluster center hardens it strongly, in which case the recoil energy may fling it out of the cluster. Independently of whether it is ejected out of its cluster, some such hard black-hole binaries may be so tight and eccentric, that their orbit shrinks due to the radiation of gravitation waves at peri-center. The binary shrinks further and circularizes, until it merges, as was observed by AdLIGO.

Because star clusters are observed everywhere in the Universe in and around galaxies, them being the building block of galaxies,  these events become common and not special. The calculations of the process described above have been published in 2010 by a Bonn-University team led by Sambaran Banerjee et al. They perform detailed stellar-dynamical computations of the above processes such that we can estimate the rate of binary black hole mergers at a given time produced by a star cluster. We can then sum up all such events from all star clusters in the Universe (since we know how many star clusters there are per galaxy approximately) to come up for the first time with such a prediction, which appears to have been nicely verified now with the AdLIGO announcement. The above mentioned rate (31±7 events per year) predicted in 2010, may be somewhat larger if less-massive star clusters are incorporated into the calculations. Low-metallicity stars leave more massive black holes, essentially because their weaker winds sweep away a smaller fraction of the star’s initial mass, and so modern stellar-evolution theory readily accounts for black holes more massive than 30 Solar masses in low-metallicity clusters which are abundant. The most massive of these black holes are most likely to dynamically interact near the star-cluster core, producing massive black-hole–black-hole binaries.

The observed rate of wave detections will test these predictions. One important aspect has been raised by Belczynski et al. above, namely that this dynamical star-cluster process predicts the black-hole spins to not be aligned, while the above stellar-binary-process does. So a given gravitational wave detection can be used to assess the particular channel of production of the pre-black-hole merger event.

 

Gravitational theories (and dark matter?):

MOND: Does the existence of gravitational waves, as predicted by the theory of general relativity, pose a problem for MOND? This is an important question to study now, since the detected signals constrain gravitational theories (a theory which does not allow gravitational waves to propagate is of course ruled out now). The detection of gravitational waves does not prove Einstein’s theory to be right, since there may be another theory which leads to the same effect.   But the detection is certainly consistent with this theory. The analysis of the signals implies that the gravitational waves are propagating with a speed which is indistinguishable form the speed of light and this constraints the mass of the graviton to be less than 2.1×10^−58 kg or 1.2×10^−22 eV/c2.

One possible interpretation of MOND is that it is a consequence of gravity being mediated by a massive graviton. Sascha Trippe at Seoul National University discusses this implications in his 2015 paper stating10^−69 kg or 10^−33 eV c−2 as being the mass of the graviton.  So this is consistent with the AdLIGO limits.

Also, the detection of gravitational waves does not prove the existence of dark matter at all, in the sense that someone may want to argue that since Einstein’s general theory predicted the waves, their verification now shows that this theory is right, and since this theory implies cold or warm dark matter particles in the standard LCDM or LWDM model of cosmology (which nearly everyone says is right but some of us _ know is ruled out by astronomical data), then dark matter must exist. This would be a false deduction.

The existence and the observed properties of gravitational waves however place important constraints on the theories of gravity which yield the classical MOND limit. Mordehai Milgrom already published a study of this issue in 2014 in PhRvD. Further research is required to test the various formulations in detail, given the observed gravitational waves and their properties.

 

The Weizmann Institute and my impending visit there:

I am visiting the Particle Physics and Astrophysics group at the Weizmann Institute in Rehovot this coming week (06.-14.03.2016), having kindly been invited by Mordehai Milgrom together with Francoise Combes and Benoit Famaey. Undoubtedly, apart from a planned sight-seeing tour through the incredibly deeply historic and beautiful lands of Israel on one day, we will of course be discussing gravitatonal wave propagation in a Milgromian Universe, as well as the most recent computational results already now obtained on various problems researched in Strasbourg and Bonn with the Phantom of Ramses computer code (the PoR code, the first PoR workshop).

 

Caveat (not to be taken seriously)

So far so good.   But there is one caveat I’d like to very carefully mention finally.

Natural science must be reproducible!  As much as we might be excited and thrilled, this is at present not given by the AdLIGO claims. Here, one team reports the detection of a transient signal with their own two observing devices. No-one can ever go back and check if the seen signal actually occurred. We have every reason to believe that the detection is true, but an independently working team would verify or independently observe such events, preferably with their own detectors. Undoubtedly this will happen, when the additional other gravitational wave observatories hopefully being comissioned soon in other countries will begin to listen to the Universe. But is is essential that independent verification be ensured. That AdLIGO is rumored to have been detecting a substantial number of additional events indeed emphasizes that the detections are occurring and that the events are common, as predicted.

 

The future

Apart from verifying by direct detection that gravitational waves exist, this is a gound-breaking event because physicists now have build new devices to probe the very fabric of space time itself. Once we have full-scale gravitational wave observatories the view we will obtain of the whole Universe is surely going to be something none of us can barely imagine today. In the past, comparable revolutions have occurred. Galileo Galilei’s first-time ever observation of heavenly objects with the first primitive telescope completely changed our world view for ever. Then, 400 years ago, no-one would have even imagined the incredibly powerfull optical observatories operating today and peering right to the beginning of time. The first-ever detection of radio waves from the heavens with the first primitive radio receivers is of a similar scale of events by leading us to the detection of the cosmic microwave background emission, which essentially is an image of the beginning of time if its physical interpretation is correct. When the first radio antennae were put up, no-one would have imagined that we will one day be able to image Solar system scales in distant galaxies, let alone view the Beginnings, as is being done routinely today. Assuming our open inquisitive, equal-human-rights, rational and non-religeous-argument based civilisation still exists, what will we be seeing with gravitational wave observatories in 100 years time?…

 

The Dark Matter Crisis by Pavel Kroupa and Marcel Pawlowski. A listing of contents of all contributions is available here.

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Author: Prof. Dr. Pavel Kroupa

I am a Czech-Australian teaching and researching at the University of Bonn on dynamics and stellar populations. After studying physics at The University of Western Australia, Perth, I obtained my PhD from Cambridge University, UK, as an Isaac Newton Scholar at Trinity College. After spending eight years in Heidelberg I habilitated at the University of Kiel, Germany. I then took up a Heisenberg Fellowship and later accepted the position as a professor at Bonn University in 2004. I was awarded a Leverhulme Trust Visiting Professorship (2007, Sheffield, UK) and a Swinburne Visiting Professorship (2007, Melbourne, Australia). In 2013 I received the Silver Commemorative Medal of the Senate of the Czech Republic, and I took-up an affiliation with the Charles University in Prague in 2016. Pure innovative science can only truly thrive in non-hierarchical societies in which competition for resources is not extreme. Therefore I see the need for the German academic system to modernise (away from its hierarchies) and warn of academic systems that are based on an extreme competition for resources (USA), as these stifle the experimentation with new ideas.

6 thoughts on “The detection of gravitational waves, predictions, MOND and my visit to the Weizmann Institut in Rehovot, Israel”

  1. I have the greatest respect for the work of you and your colleages because you are not afraid to be skeptical of standard cosmological theories if they are shown to fall short. In that spirit, I submit that some skepticism of LIGO’s acclaimed discovery should be retained, and also of its purported validation of Einstein’s ideas about gravity waves and black holes. Einstein in fact had no firm commitment to the actual existence of gravity waves or black holes. 

    Anyway, some food for critical thought may be munched at http://www.oneindia.com/international/playing-devils-advocate-on-the-discovery-of-gravitational-2018782.html, and some interesting discussion at
    http://fqxi.org/community/forum/topic/2597.

    I am not a scientist, but would certainly value some analysis on the basis of facts and with a commitment to independent thinking.

  2. Thank you, Daniel, for your comments and for providing the links. I agree with the comment by Raja to http://www.oneindia.com/international/playing-devils-advocate-on-the-discovery-of-gravitational-2018782.html which is the first link you provide: “the LIGO Detectors are an EXTRAORDINARY piece of instrumentation, probably the most complex/sensitive instrument ever built by humankind”. I do not doubt that AdLIGO detected gravitational waves, and the detectors designed and build in the USA to achieve these are as described by Raja, and this team and project absolutely deserves the greatest honours, respect and success for doing so. Repeating myself, the rumours are that AdLIGO is detecting many such events, and that these events are measured in both detectors, so there is essentially no doubt that they have the waves and that this is one of the greatest scientific discoveries of all times! But, from a purely philosophical point of view, science requires independent verification, and so this must ultimately be achieved also in this case. And here is now a potential problem: if the very high-level civilisations which can build such complex observatories detoriate, for example through internal decomposition as is now proceeding in Europe through the rapidly increasing influence of a particular type of religion, then humanity will be increasingly constrained in progressing with such endeavours.

  3. Thank you for your response. Do you have any particular comments on the specific doubts raised in the article, such as the millisecond rather than microsecond delay in detection/travel of the putative gravity wave from one LIGO instrument to the other, along with a drop in amplitude of the signal in the same order? There were other points as well that I think deserve some attention.

  4. I do not now see any reason, given the data and the differences between the two detectors, to doubt the reality of the signal. In particular, the delay is reasonable if the signal came form the right direction. But let us wait and see how observational gravitational wave astrophysics develops. The prospects are extremely exciting indeed. It is important to keep a critical view though.

  5. “Does the existence of gravitational waves, as predicted by the general theory of relativity, pose a problem for MOND?” Relativistic MOND might originate from 2 main possibilities for modifying general relativity theory: (1) the equivalence principle is true and deviations from Einsteinian geodesics are caused by interactions among alternate universes or some other unknown cause, or (2) the equivalence principle is false.
    The empirical successes of Milgrom’s MOdified Newtonian Dynamics (MOND) suggest that there is a serious problem with Newtonian-Einsteinian gravitational theory. Could dark matter have positive gravitational mass-energy and zero inertial mass-energy? Einstein wrote (“The Meaning of Relativity”, 5th edition, page 57) that “A little reflection will show that the law of the equality of the inert and the gravitational mass is equivalent to the assertion that the acceleration imparted to a body by a gravitational field is independent of the nature of the body. For Newton’s equation of motion in a gravitational field, written out in full, is (Inert mass) * (Acceleration) = (Intensity of the gravitational field) * (Gravitational mass). It is only when there is numerical equality between the inert and gravitational mass that the acceleration is independent of the nature of the body.” Is it necessarily true that IT IS ONLY WHEN THERE IS NUMERICAL EQUALITY between inertial mass-energy and gravitational mass-energy that THE ACCELERATION IS INDEPENDENT OF THE NATURE OF THE BODY? There is a possibility that the acceleration is independent of the nature of the body but THERE IS A SYSTEMATIC DEVIATION between the inertial mass-energy and gravitational mass-energy. Consider Einstein’s field equations: R(mu,nu) + (-1/2) * g(mu,nu) * R = – κ * T(mu,nu) – Λ * g(mu,nu) — what might be wrong? Consider the possible correction R(mu,nu) + (-1/2 + dark-matter-compensation-constant) * g(mu,nu) * R = – κ * (T(mu,nu) / equivalence-principle-failure-factor) – Λ * g(mu,nu), where equivalence-principle-failure-factor = (1 – (T(mu,nu)/T(max))^2)^(1/2) — if dark-matter-compensation-constant = 0 and T(max) = +∞ then Einstein’s field equations are recovered. Empirical evidence shows that black holes exist. The event horizons of black holes might be figments of the imagination caused by believing in Einstein’s equivalence principle.

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