JRC,
Thanks for the link. I visited and was interesting. In your statement, "Modified Newtonian Dynamics, which instead of requiring some new kind of matter (dark matter) to account for rotation velocities in galaxies as GR does,..." may not need involving GR. Given the Newtonian relationship between orbital velocity, radius of orbit and mass within the orbit, viz.
v2 = GM/r
whenever the v is higher than the observed M at an orbital radius, r, Newton's law says we should start looking for a higher, M. It was in this way that Leverrier discovered some of the outer planets.
That is the primary evidence for dark matter. There are now other evidence from astronomical refraction of light (also called gravitational lensing) and requirements for a 'clumping agent' to fit our cosmological model. In my research I think the requirement for matter which does not shine (called dark matter) is overwhelming. If I may go further to say, dark matter is even right there in abundance in your living room as you read this.
My grouse therefore with the research by Dr Pavel Kroupa and others is why search for dark matter presence or absence so far away in other galaxies using expensive telescopes when dark matter is in their laboratory and can be deduced from simpler experiments?
To put it simply, if you are told that some form of matter is so abundant with a ratio 9:1 over baryonic matter in our galaxy; that our Solar system going by its rotation curve is in the very middle of the galactic zone where it is most abundant; that the dark matter particles can interact gravitationally with normal matter and so can be gravitationally captured and bound by the gravity of celestial bodies; that the dark matter particles were present in our galaxy when our planet was congealing and forming out of the galactic gas clouds; that the dark matter can transmit light waves, just like other matter; can you not think of some light experiment to rule out or confirm this dark matter possibility in your laboratory, instead of staring at other galaxies?
In the words of Dr Kroupa, "Because tidal dwarf galaxies cannot have much dark matter (they are too small to capture it, even if it were to exist", he has to modify Newtonian gravity. This he called the "fourth failure of Newton". If the Earth and solar planets can capture dark matter, check here and here, how then can it be claimed that a dwarf galaxy cannot capture and retain dark matter? Newton has not failed. It is Dr Kroupa who has.
Tom,
To the question, "does a photon have inertia?" You said, No. Keeping Einstein's question in mind, does a photon have energy content? I presume your answer would be Yes. If that is the case, Einstein shouldn't then have asked the question in the first place, 'Does the inertia of a body depend upon its energy-content?' since there is no compulsory relationship between energy content and inertia. That is, according to your view, a particle can have energy content without necessarily having inertia. As I pointed out earlier, the paper I linked claimed that Einstein changed his mind a year later in 1906 because of the absurdities associated with that line of thinking. I don't know if you were able to get access to the paper? I will search the web if I can get a free version.
Lyle wayne Moss,
I understand your frustration. Keep hope alive. Perhaps, before 2020 according to Peter J, we should reach some closure.
Regards,
Akinbo
