Tuesday, February 22, 2011

Gravitational waves remain still undetected

Both Sean Carroll and Lubos report that the LIGO has not detected gravitational waves from black holes with masses in the range 25-100 solar masses. This conforms with theoretical predictions. Earlier searches from Super Novae give also null result: in this case the searches are already at the boundaries of resolution so that one can start to worry.

The reduction of the spinning rate of Hulse-Taylor binary is consistent with the emission of gravitational waves with the predicted rate so that it seems that gravitons are emitted. One can however ask whether gravitational waves might remain undetected for some reason.

Massive gravitons is the first possibility. For a nice discussion see the article of Goldhaber and Nieto giving in their conclusions a table summarizing upper bounds on graviton mass coming from various arguments involving model dependent assumptions. The problem is that it is not at all clear what massive graviton means and whether a simple Yukawa like behavior (exponential damping) for Newtonian gravitational potential is consistent with the general coordinate invariance. In the case of massive photons one has similar problem with gauge invariance. One can of course naiively assume Yukawa like behavior for the Newtonian gravitational potential and derive lower bounds for the Compton wave length of gravitons. The bound is given by λc> 100 Mpc (parsec (pc) is about four light years).

Second bound comes from the pulsar timing measurements. The photons emitted by the pulsar are assume to surf in the sea of gravitational waves created by the pulsar. If gravitons are massive in Yukawa sense they arrive with velocities which are below light velocity, a dispersion of both graviton and photon arrival times is predicted. This gives a much weaker lower bound λc> 1 pc. Note that the distance of Hulse-Taylor binary is 6400 pc so that this upper bound for graviton mass could explain the possible absence of gravitational waves from Hulse-Taylor binary. There are also other bounds on graviton mass but all are plagued by model dependent assumptions.

Also in TGD framework one can imagine explanations for the possible absence of gravitational waves. I have discussed the possibility that gravitons are emitted as dark gravitons with gigantic value of hbar, which decay eventually to bunches of ordinary gravitons meaning that continous stream of gravitons is replaced with bursts which would not be interpreted in terms of gravitons but as noise (see this).

One of the breakthroughs of the last year was related to the twistor approach to TGD in zero energy ontology (ZEO).

  1. This approach leads to the vision that all building blocks (light-like wormhole throats) of physical particles -including also virtual particles and also string like objects- are massless. On mass shell particles are bound states of massless particles but virtual states do not satisfy bound state constraint and because negative energies are possible, also space-like virtual momenta are possible.

  2. Massive physical particles are identified as bound states of massless wormhole throats: since the three momenta can have different (as a special case opposite) directions, the bound states of light-like wormhole throats can be indeed massive.

  3. Masslessness of the fundamental objects saves from problems with gauge invariance and general coordinate invariance. It also makes it possible to apply twistor formalism, implies the absence of UV divergences, and yields an enormous simplification of generalized Feynman diagrammatics since mass shell constraints are satisfied at lines besides momentum conservation at vertices.

  4. A simple argument forces to conclude that all spin one and spin two particles- in particular graviton- identified in terms of multi-wormhole throat states must have arbitrary small but non-vanishing mass. The resulting physical IR cutoff guarantees the absence of IR divergences. This allows to preserve the exact Yangian symmetry of the M-matrix. One implication is that photon eats the TGD counterpart of the neutral Higgs and that only pseudoscalar counterpart of Higgs survives. The scalar counterparts of gluons suffer the same fate whereas their pseudoscalar partners would survive.

Is the massivation of gauge bosons and gravitons in this sense consistent with the Yukawa type behavior?

  1. The first thing to notice is that this massivation would be essentially a non-local quantal effect since both emitter and receiver both emit and receive light-like momenta. Therere the description of the massivation in terms of Yukawa potential and using ordinary QFT might well be impossible or be a good approximation at best.

  2. If the massive gauge bosons (gravitons) correspond to wormhole throat pair (pair of these) such that the three-momenta are light-like but in exactly opposite directions, no Yukawa type screening and velocity dispersion should take place.

  3. If the three momenta are not exactly opposite as is possible in quantum theory, Yukawa screening could take place since the classical cm velocity calculated from the total momentum for a massive particle is smaller than maximal signal velocity. The massivation of intermediate gauge bosons and the fact that Yukawa potential description works for them satisfactorily supports this interpretation.

  4. If the space-time sheets mediating gravitational interaction have gigantic values of gravitational Planck constant Compton length of graviton is scaled up dramatically so that screening would be absent but velocity dispersion would remain. This leaves open the possibility that gravitons from Hulse-Taylor binary could reveal the velocity dispersion if they are detected some day.

For details about large hbar gravitons see the chapter Quantum Astro-Physics of "Physics in Many-Sheeted Space-time". For the twistor approach to TGD see the chapter Yangian Symmetry, Twistors, and TGD of "Towards M-Matrix".


Ulla said...

Off topic, but

Anonymous said...

It's perfectly reasonable to wonder why GWs might not be detected, but you can't use recent reports of non-detection to support your stance. Gravitational-wave detectors aren't yet sensitive enough to actually detect GWs with any regularity. At current sensitivity, it's not surprising that there have been no detections. Wait a few years, and if the advanced configurations of the detectors still don't have a detection, then I would start seriously considering new theories of gravitation.

Matti Pitkänen said...

As a matter fact, I stated just the same in the beginning of posting concerning the sources studied.

In the case of other sources we are however already now at the limits. My purpose was to just mention briefly the notion of dark gravitons: in the case that gravitons are not detected this notion might explain the non-detection. The Allais effect could be actually seen as a support for quantal interference effect for dark gravitons

My intention was also to demonstrate that small mass for gravitons/photons does not mean loss of general coordinate invariance/gauge invariance in TGD framework and to discuss what massivation could actually mean physically. In GRT framework it is far from obvious whether the notion of massive graviton makes sense.