The question concerns this video:
It says that the Gravitational Lens effect is an illusion, meaning it's not caused by gravity but by change in density of a plasma atmosphere. It claims further that this gravitational lens effect is not present with other massive objects that aren't made of, or surrounded by, plasma.
Does this mean general relativity is invalid, somewhat invalid, or still luckily valid? What is the merit of claim that bending could be caused by plasma and not by gravity?
Did you notice the title of the video?
Former NASA Physicist Disputes Einstein’s Relativity Theory
At the moment main stream physics, which is what we discuss here, does not accept all the various theories trying to unseat General Relativity . Usually these people have an obsession with Einstein or other prominent physicist as personalities . Look at the humorous crackpot index .
On the subject, the lensing has been established from images of galaxies lensed by clusters of galaxies. Plasma is a phenomenon around hot stars and have nothing to do with observations ( except that in trying to measure the gravitational lensing by the sun would need carefully taken care of).
Sorry to say it does not sound a serious proposal.
Is it true that gravitational lensing only occurs for objects made of plasma?
Most of space contains a plasma, or at least some ionisation, at some density or other, making the claim hard to dispute at that level. Also, gases and/or plasmas will tend to exist with higher densities in deeper gravity wells.
What is the merit of claim that bending could be caused by plasma and not by gravity?
In general it is possible for light paths to be changed by interaction with matter (a glass lens being an obvious example). In principle similar distortion could happen with a rarefied plasma over large distances. A density variation at the right scale, and with a simple shape would be required.
I expect however that the claim in the link is just a conjecture/challenge, and not mathematically or scientifically robust (although I don't personally have the knowledge and skills to assess that). I suspect that the lensing seen in an Einstein ring would be very hard to duplicate using realistic values for plasma density near the lensing object.
To me, there are plenty of warning signs that this is a crackpot claim from the link. Unfortunately the world is full of them. Often such people cannot be corrected by scientific evidence, in fact they will self-promote that there is "controversy" when engaged in conversation, or complain of being "suppressed by the establishment" when ignored. I am not saying those thing apply to the author (I have not checked other publications), but I would be wary of the "a NASA physicist" label - it is likely correct on some technical label (he worked for NASA and has a qualification in physics) but is not particularly meaningful.
It doesn't help the lay audience (NB I include myself here) when real scientific controversies and useful scepticism can follow a similar pattern. Popular proponents of badly-thought-through or outlier theories often manage to pick up on the form of this and produce "science-y" work, making it harder to filter out good from bad.
Update: I have struggled through a few pages of http://www.extinctionshift.com and stand by this - the work appears to be random picking of equations plus handwaving arguments about how they connect. Nothing is properly analysed.
Does this mean general relativity is invalid?
No, even if somehow the claim had merit, there is plenty of evidence for GR outside of gravitational lensing.
For example: Time-dilation effects of general relativity can be measured directly by atomic clocks. They have got so accurate that we can actually measure the time dilation of a few metres height difference at ground level. The GPS system needs to account for this effect.
However
It is well known that General Relativity and the Standard Model of quantum mechanics, although our best current models in physics, cannot be the last word. They are not compatible, and where they overlap (typically when considering cosmology or black holes) there is a need for a combined or replacement theory. That doesn't make the theories "wrong" any more than Newtonian physics was "wrong", but it does mean we can expect some extensions or a deeper theory to emerge at some point.
While it's a redshift rather than an angular deflection, the Pound-Rebka experiment confirming the interaction between light and gravity did not involve any plasma anywhere.
You could presumably repeat the experiment using photons directed at an angle, rather than upwards, though the angular deflection would be so small it only be possible to infer.
No, plasma cannot be responsible for lensing. That would be wavelength dependent. Different wavelengths would be refracted more than others. As see when light passes through a prism. In gravitational lensing, there is no wavelength dependence. All wavelengths are affected to the same degree. Dowdye was trying to fit science into his religious beliefs. There is no reason to take him seriously.
Yes if it were true, then the sun and every star you see would have lensing. None of them do.
I believe Dr Ed Dowdye is correct and Einstein is wrong. My blog and video on the subject: https://www.funk.co.nz/blog/science/plasma-lensing
The merits are enormous and I think this is the strongest evidence we need to revisit what is red shift? And rename them Plasma Lensing or similar.
Yes, it would be silly of us to imagine that those huge halos of plasma of up to twice the mass of the galaxy itself discovered in 2012, surrounding every galaxy, might have something to do with it. As it would be silly to assume plasma has nothing to do with redshift.
http://www.newtonphysics.on.ca/chromosphere/index.html
http://www.newtonphysics.on.ca/hubble/index.html
The simple fact is that plasma does not behave like solids, liquids or gasses. So to assume that a universe 99% plasma would behave like solids, liquids and gasses would be in error. One can not use the kinematics of ordinary gasses to describe a plasma state, yet this is precisely what is being done when one talks of it as being dust and gas that they observe in space. Such thinking leads to errors.
