stupid physics questions

[zip accident]

Also, why is the Einstein cross not a ring?

http://en.wikipedia.org/wiki/Einstein_Cross

Answer? because the photons are polarised, and aren't affected by gravity at angles other than 0 degrees to the plane of vibration. Which royally stuffs bent space-time.

Or does someone have a reason why that doesn't work?

Okay, thought further about this while trying to get to sleep.

I think it works for two points of light with one lens, but with one lens the lens has to be exactly (or very nearly exactly) on the line between the origin and Earth to get four points of light. On the other hand, with two lenses, if they are both on the lines of polarisation, then you would get four points, which might be in a line, or in a sort of squiffy cross. The most common configuration by far would be two points of light, but there could be as many as you like, though they would be in at most two crossed lines.

"Almost exactly" means something like a small number of degrees (could be less than one, can't be more than five looking at the pictures) around the arc that would form the ring if the light wasn't polarised, such that if the mass falls between the lines which could be drawn between the right and left (or up vs down, depending on the orientation) limits, then it can act as a lens.

This was worked out by means of an imaginary illustration of the polarised light.

[pogo]

I think the cross arises for far simpler reasons. Whatever is doing the lensing in that case isn’t just a point mass but is somewhat larger. This could be doing two things. Distorting the gravity in a more complex way than the ring you would expect with a single point. And occluding it so only part of the light shows through. Both even, to some extent. Because the light source is incredibly bright and so incredibly distant we will probably never be able to see the much dimmer thing in the middle, so will never be able to confirm directly how it is working.

[zip accident]

I think the cross arises for far simpler reasons. Whatever is doing the lensing in that case isn’t just a point mass but is somewhat larger. This could be doing two things. Distorting the gravity in a more complex way than the ring you would expect with a single point. And occluding it so only part of the light shows through. Both even, to some extent. Because the light source is incredibly bright and so incredibly distant we will probably never be able to see the much dimmer thing in the middle, so will never be able to confirm directly how it is working.

There are too many for that. To occlude sections of a ring it would have to be very big indeed, much bigger than a galaxy, but in order not to have collapsed into a black hole, much much less massive. Since galaxies are essentially gas clouds, that's extremely improbable.

I think my polarisation hypotheisis is elegant, and supremely testable (how many crosses are there compared to lines of two dots (which would also be polarised in the same orientation (unless there's something polarising between us and the lens), so it's not a matter of just looking for random lined up galaxies)).

[pogo]

But light and similar radiation in space isn’t polarised. Stars and other bodies are spherically symmetric normally and will be emitting light in all directions with all orientations of polarity. Larger objects such as galaxies are made up of stars. Also gravity is independent of polarisation, in that it acts on light identically however it is polarised. There is no evidence light is polarised here, no connection between it and lensing. Finally there are many more examples of gravitational lenses, none of which are crosses. It’s the Einstein Cross, not an Einstein Cross. There’s only one an it’s just a coincidence it looks like that.

[zip accident]

But light and similar radiation in space isn’t polarised. Stars and other bodies are spherically symmetric normally and will be emitting light in all directions with all orientations of polarity. Larger objects such as galaxies are made up of stars.

Sure, most light isn't polarised, and most lensing therefore results in rings. If I'm right, then the polarity around the ring would be significant, the polarity of the ring as a whole would be average/random, but different points on the ring would have specific (four specific) polarities.

Also gravity is independent of polarisation, in that it acts on light identically however it is polarised.

I suspect that's an assumption, you can't do tests of it on Earth, the gravity field isn't strong enough.

There is no evidence light is polarised here, no connection between it and lensing.

"The" Cross is a quasar, quasars are (often or always?) polarised.

Finally there are many more examples of gravitational lenses, none of which are crosses. It’s the Einstein Cross, not an Einstein Cross. There’s only one an it’s just a coincidence it looks like that.

https://en.wikipedia.org/wiki/Einstein_Cross

This article is about a specific multiply-imaged quasar named "The Einstein Cross" . For the general concept of Einstein crosses, see gravitational lens.

That doesn't sound to me as if it's entirely unique. I entirely agree that rings ought to occur much more frequently. Second after rings should be pairs, and wadda ya know:

https://en.wikipedia.org/wiki/Twin_Quasar

[pogo]

There is no evidence that quasars emit polarised light. Gravity works identically on light no matter how it’s polarised. In fact it works identically on everything. That’s not an assumption it’s a law of physics, general relativity. And there is only one Einstein Cross, it is just a coincidence that it looks like that, a slightly skew cross. If polarisation were a factor you would expect to see many if not most of them aligned similarly. But that‘s the only one.

[zip accident]

There is no evidence that quasars emit polarised light.

BL Lac objects do, and I'm pretty sure I'm reading Wikipedia correctly in saying they are quasars.

https://en.wikipedia.org/wiki/BL_Lac_object

This may also be relevant, though maybe not as they are not quasars:

Type II Seyfert galaxies

Type II Seyfert galaxies have the characteristic bright core, as well as appearing bright when viewed at infrared wavelengths.[41] Their spectra contain narrow lines associated with forbidden transitions, and broad lines associated with allowed strong dipole or intercombination transitions.[37] In some Type II Seyfert galaxies, analysis with a technique called spectro-polarimetry (spectroscopy of polarised light component) revealed obscured type I regions. In the case of NGC 1068, nuclear light reflected of a dust cloud was measured, which led scientists to believe in the presence of an obscuring dust torus around a bright continuum and broad emission line nucleus. When the galaxy is viewed from the side, the nucleus is indirectly observed through reflection by gas and dust above and below the torus. This reflection causes the polarisation.[42]

https://en.wikipedia.org/wiki/Seyfert_galaxy

Gravity works identically on light no matter how it’s polarised.

There is no way to test that on Earth, if you rotated a polarised source you couldn't be sure that errors in rotation weren't responsible for any change in direction of the beam, and the Earth's gravity is too small to have much effect. Unless you have a citation for an experiment, that is definitely an assumption.

And there is only one Einstein Cross, it is just a coincidence that it looks like that, a slightly skew cross. If polarisation were a factor you would expect to see many if not most of them aligned similarly. But that‘s the only one.

The first Einstein cross was found by J. Huchra in 1984, and two more were found by K. Ratnatunga et al. in that same year.

http://scienceworld.wolfram.com/physics ... Cross.html

[pogo]

You are not reading WP correctly, no. None of those are quasars. As for Wolfram it is far less reliable than WP, in particular its sparse and dated references, often to old textbooks the writer was obviously familiar with. That snippet looks particularly doubtful.

But your most glaring mistakes are in basic science, which is what relativity is. Since it’s discovery it has been confirmed time and time again, more than any other modern theory. GPS would not work without corrections for it. And relativity says that everything – matter and light in particular – are all affected by gravity the same way. If it wasn’t, over the vast distances of space, it would be very very obvious.

[gumboot]

And relativity says that everything – matter and light in particular – are all affected by gravity the same way. If it wasn’t, over the vast distances of space, it would be very very obvious.

I wonder if that statement is accurate.

Vast distances and all that, but most of what we can see we can see because there's nothing else in the way. As much as light travels a very long way to reach us, it also mostly travels past large masses at a substantial distance from them; and with an approximately (on a very large scale) even distribution of mass all around its path.

[pogo]

And relativity says that everything – matter and light in particular – are all affected by gravity the same way. If it wasn’t, over the vast distances of space, it would be very very obvious.

I wonder if that statement is accurate.

The thing is if different things were affected differently there would be evidence. Light of different wavelengths or different polarities coming from different directions, or spread out like a prism spreads out white light by bending different wavelengths by different amounts. We can detect particles as well as light, and they match the visible evidence.

It’s particularly pertinent here as gravitational lensing is one of the key predictions of general relativity. Predicted by Einstein in the 1930s but not observed for another 40 years. It’s only one of the many bits of evidence we have for general relativity.

[zip accident]

You are not reading WP correctly, no. None of those are quasars.

An optically violent variable quasar (often abbreviated as OVV quasar) is a type of highly variable quasar. It is a subtype of blazar that consists of a few rare, bright radio galaxies, whose visible light output can change by 50% in a day.[2]

They are similar in appearance to BL Lacs but generally have a stronger broad emission line, and tend to have higher red shift components.

https://en.wikipedia.org/wiki/OVV_quasar

You might be right about that, but that article is very short, no mention of polarisation either way.

This is more like it:

Blazars, like all AGN, are thought to be ultimately powered by material falling onto a supermassive black hole at the center of the host galaxy. Gas, dust and the occasional star are captured and spiral into this central black hole creating a hot accretion disk which generates enormous amounts of energy in the form of photons, electrons, positrons and other elementary particles. This region is quite small, approximately 10−3 parsecs in size.

There is also a larger opaque toroid extending several parsecs from the central black hole, containing a hot gas with embedded regions of higher density. These "clouds" can absorb and then re-emit energy from regions closer to the black hole. On Earth the clouds are detected as emission lines in the blazar spectrum.

Perpendicular to the accretion disk, a pair of relativistic jets carries a highly energetic plasma away from the AGN. The jet is collimated by a combination of intense magnetic fields and powerful winds from the accretion disk and toroid. Inside the jet, high energy photons and particles interact with each other and the strong magnetic field. These relativistic jets can extend as far as many tens of kiloparsecs from the central black hole.

All of these regions can produce a variety of observed energy, mostly in the form of a nonthermal spectrum ranging from very low frequency radio to extremely energetic gamma rays, with a high polarization (typically a few percent) at some frequencies.

https://en.wikipedia.org/wiki/Blazar

But your most glaring mistakes are in basic science, which is what relativity is.

Science is experiments, not dogma.

... everything – matter and light in particular – are all affected by gravity the same way. If it wasn’t, over the vast distances of space, it would be very very obvious.

This is a singular phenomenon, that occurs rarely in exceptional circumstances. Someone might have looked at it, but they equally might not have. You'd have to be looking at the polarisation of light after gravitational lensing to have any chance of spotting it experimentally.

It’s particularly pertinent here as gravitational lensing is one of the key predictions of general relativity. Predicted by Einstein in the 1930s but not observed for another 40 years. It’s only one of the many bits of evidence we have for general relativity.

History

Henry Cavendish in 1784 (in an unpublished manuscript) and Johann Georg von Soldner in 1801 (published in 1804) had pointed out that Newtonian gravity predicts that starlight will bend around a massive object.[6] The same value as Soldner's was calculated by Einstein in 1911 based on the equivalence principle alone. However, Einstein noted in 1915 in the process of completing general relativity, that his (and thus Soldner's) 1911-result is only half of the correct value. Einstein became the first to calculate the correct value for light bending.[7]

https://en.wikipedia.org/wiki/Gravitational_lens

[pogo]

You believe what you want. Mainstream physics believes and accepts relativity, special and general, as the only thing that explains the universe on a large scale. It is fairly advanced: I did not encounter it until well into a Maths degree. If you have not studied it you have little hope of understanding it as it has a largely mathematical basis.

A lot of work was done before Einstein discovered relativity, but it is almost always wrong, as people were coming at it from the wrong direction of Newtonian mechanics, which is simply unable to describe astronomical phenomena. Even when they hit upon the right answer it was often by accident, or by numerical reasoning without understanding or being able to justify the answer. Relativity ended that by putting it on a strong footing.

[gumboot]

And relativity says that everything – matter and light in particular – are all affected by gravity the same way. If it wasn’t, over the vast distances of space, it would be very very obvious.

I wonder if that statement is accurate.

The thing is if different things were affected differently there would be evidence. Light of different wavelengths or different polarities coming from different directions, or spread out like a prism spreads out white light by bending different wavelengths by different amounts. We can detect particles as well as light, and they match the visible evidence.

It's not uncommon for observations to fail to match our understanding. http://news.ucdavis.edu/search/news_det ... so?id=8364

That's not to say that our understanding is incorrect; but you can't say it's obviously correct because it matches observations, if you have to dismiss observations or offer ad-hoc theories about why they're weird.


Even so, I'd focus on trying to figure out an arrangement of galaxies that might split an image in four instead of two. Has nobody simulated this to figure out possibilities?

[pogo]

Mentioned here:

https://en.wikipedia.org/wiki/Quantum_foam

Worth quoting in full

The MAGIC (Major Atmospheric Gamma-ray Imaging Cherenkov) telescopes have detected that among gamma-ray photons arriving from the blazar Markarian 501, some photons at different energy levels arrived at different times, suggesting that some of the photons had moved more slowly and thus contradicting the theory of general relativity's notion of the speed of light being constant, a discrepancy which could be explained by the irregularity of quantum foam. More recent experiments were however unable to confirm the supposed variation on the speed of light due to graininess of space. Other experiments involving the polarization of light from distant gamma ray bursts have also produced contradictory results. More Earth-based experiments are ongoing or proposed.

And in science results are not generally accepted until they have been independently confirmed. Plus there could be any number of explanations for this, other than contradicting the theory of general relativity. Relativity and QM are the two pillars on which all of our science is built today. It takes more than a few disputed measurements to invalidate them.

[gumboot]

That's totally missing the point. I'm not trying to invalidate relativity or quantum mechanics. I'm just saying that it'l bullshit to say that they must be right because the data agrees. The data is all over the place and demands supplemental explanations over and over until somebody reframes the whole thing; which is how relativity came about in the first place.