Gravitational Lensing : Eddington Experiment

Thats correct.

Do you mean to say that, "flat spacetime" does not correspond to any physical reality?

 

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No, I mean for the purposes of many applications, we can ignore the curvature of space and use an incorrect physics.

 

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This is a comment I'll add to what Physbang said. For most experiments conducted in the weak field, which is where we conduct all our experiments [the local proper Laboratory frame] the effects of the infinitesimal local spacetime curvature [gravity] can be ignored. For this case, the weak field, most all the relativistic math is the math of SR. The math of GR needs to be used when the effects of infinitesimal curvature need to accounted for even in the weak field. So the evaluation you're doing allows you to determine what mathematics are best to analyse the results of your experiment. Another example would be the folks, at the JPL, that use the Hoffman orbits to find the flight path for the experimental vehicles they've been sending out, with fantastic returns I might add, into our solar system. They don't need to use the math of GR because the evaluation using Einstein orbits won't add anything meaningful for calculating the paths they need to get the experiment in place. So they use celestial mechanics which is derived from Newton orbits. A great thing is these choices exist and the folks doing the analysis have the options to choose what works best for that specific analysis. For the most part the weak field covers the entire universe. We can always find a segment of our natural path that is flat spacetime. It's right where we're at at the moment. A definition for the strong field would be where the area of that segment gets smaller as we approach the local spacetime surrounding the neutron star and the black hole. You're asking some good questions.

 

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From the perspective of a worker I consider all the knowledge gained and included in the scientific literature as a tool for the working scientist to help him achieve his work goals. The more literature accumulated the more tools are in the scientists tool box. The great thing about completing a work project is you get to assess how these tools worked when you evaluate the results of the job. For the example of the JPL. The success of the projects confirm the usefulness of the tools used to complete the task. A philosophical analysis of why we use these tools serves no useful purpose once we've confirmed the usefulness of mathematical physics , or any other tool, used to successfully complete scientific work. I like to couch it that way since everybody has done some work with tools and should know what that entails. This brings me to wonder what tools are in the scientific illiterate cranks toolboxes? Haven't found any so far.

 

This is an interesting example for mathematical physics. The mathematical physics derivation is called " Parameterized Post-Newtonian Formalism" or simply Post Newtonian Parameters. Read about it. It's easy to understand what it's used for without being able to use them.
https://en.m.wikipedia.org/wiki/Parameterized_post-Newtonian_formalism

An interesting note for the use of this formalism. A while back, September 2002, this experiment was conducted using the PNF to evaluate the results. It was an attempt to measure the speed of gravity. The following link gives a basic description of the experiment and all the papers written in support and disagreement with what was actually measured. One of the dissenting papers was written by Professor Clifford Will who first derived the Post Newtonian formalism. Great experiment and the disagreement exists to this day. The post Newtonian Formalism is the Cadillac of weak field approximation.
http://www.phys.ufl.edu/~cmw/SpeedofGravity.html

 

Yeah, the PPN (as the folks I worked with call it) is essential for gravitational test since its creation. It's a great example of how to advance a science through empirical research, even through testing the fundamentals of the leading theory.

 

It is the PPN. My brain fart. Also Pad had to add transfer to my Hoffman orbits. First two mistakes I've ever made. Hahahaha. I was also interested to learn the first model was Eddington's. So were you involved in research on gravitational physics? Your comment was much appreciated.

 

Because you have to ask that question. That's why. You're only as genuine as you are intellectually honest. Not very genuine river.

 

I studied cosmology, but I essentially left academia post PHD. I looked at a lot of tests for GR and at cosmology as a test for GR & other theories.

 

My two favorite science subjects. Thanks for revealing that about yourself. How did you view the Jovian experiment? I learned a lot while I was following the discussion leading up to the experiment. As an amateur enthusiast of the tests of GR I was rooting for success. And I learned a lot following the argument after the paper detailing the experiment was published. Good example of how scientists communicate.

 

What do you mean by "incorrect physics" ?

 

You mean the speed of gravity thing? It's funny, but I don't remember it being a big deal at the time among the people I was around and I kinda forgot about it. Which is funny, since I know a guy who was working on gravitational tests within the solar system.

Very complicated and with a lot of potential error. Maybe we felt it wasn't worth a big fuss? Still a good example of how people who all agree on the same theory can disagree about what is a good test.

 

We know that the weak field approximation is wrong, it's a good approximation, but not as good as using the full machinery of GR.

But everything ins science involves choice, costs, and benefits. Scientists do not have the time to do every calculation and doing some calculations can produce accuracy that is below the threshold of what matters, especially when one is dealing with observations that have a degree of error to them. The accuracy of doing full calculations can fall below the amount of error one expects from the type of observations one is using. In this case, using an approximation that preserves a result within the boundaries of the expected error can be a good decision, especially when one is not trying to derive metaphysical principles or foundational physics but is trying to estimate the behavior of a physical system.

 

The interesting thing was everybody agreed that the measurement was good but there was a large error bar. Similar to the error bar for frame dragging measurement during Gravity Probe B. The disagreement was over how the PPN modeled was being interpreted for this measurement. Proffessor Kopeikin's team interpreted the second order measurement as the speed of gravity while the dissenters interpret it as a second order measurement of the local speed of light. Thanks for your comments.

 

Very informative post. Thanks.

 

I believe he means a less precise or less accurate physics.

 

We've discussed a specific example associated with accuracy when we look at the precision of predictions derived from the PPN mathematical physics. The key word for what Physbang is talking about is usefulness. Even though Newtons theory has been falsified at some level that can be associated with accuracy it's still accurate enough to use it to build a super accurate weak field approximation. Usefulness for doing physics. The employed physicists job. The wiki page describing the PPN is a good place to find out why scientists build, derive, such tools. There's many such tools. Many are being derived to assist physicists in quantum gravity research and cosmological research. Then they're the great experimental models continuing to test the accuracy of GR and quantum mechanics. I'm impressed. Very impressed. At the work they're doing.

 

Ëxcellent summaries and rundowns brucy baby!
In the mean time here's some more..........
http://csep10.phys.utk.edu/astr162/lect/galaxies/gravlens.html

Gravitational Lenses

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HUBBLE VIEWS DISTANT GALAXIES THROUGH A COSMIC LENS


This NASA Hubble Space Telescope image of the rich galaxy cluster, Abell 2218, is a spectacular example of gravitational lensing. The arc-like pattern spread across the picture like a spider web is an illusion caused by the gravitational field of the cluster.
The cluster is so massive and compact that light rays passing through it are deflected by its enormous gravitational field, much as an optical lens bends light to form an image. The process magnifies, brightens and distorts images of objects that lie far beyond the cluster. This provides a powerful "zoom lens" for viewing galaxies that are so far away they could not normally be observed with the largest available telescopes.

Hubble's high resolution reveals numerous arcs which are difficult to detect with ground-based telescopes because they appear to be so thin. The arcs are the distorted images of a very distant galaxy population extending 5-10 times farther than the lensing cluster. This population existed when the universe was just one quarter of its present age. The arcs provide a direct glimpse of how star forming regions are distributed in remote galaxies, and other clues to the early evoution of galaxies.

Hubble also reveals multiple imaging, a rarer lensing event that happens when the distortion is large enough to produce more than one image of the same galaxy. Abell 2218 has an unprecedented total of seven multiple systems.

The abundance of lensing features in Abell 2218 has been used to make a detailed map of the distribution of matter in the cluster's center. From this, distances can be calculated for a sample of 120 faint arclets found on the Hubble image. These arclets represent galaxies that are 50 times fainter than objects that can be seen with ground-based telescopes.

Studies of remote galaxies viewed through well-studied lenses like Abell 2218 promise to reveal the nature of normal galaxies at much earlier epochs than was previously possible. The technique is a powerful combination of Hubble's superlative capabilities and the "natural" focusing properties of massive clusters like Abell 2218.

The image was taken with the Wide Field Planetary Camera 2.

Credits: W.Couch (University of New South Wales), R. Ellis (Cambridge University), and NASA
PHOTO FILE NO.: STScI-PF95-14



Here is another example of a gravitational lens called the Einstein Cross . In this image, a single object appears as four objects. A very distant quasar is thought to be positioned behind a massive galaxy. The gravitational effect of the galaxy has created multiple images through a lensing effect of gravitation on the light from the quasar.

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The stars in the foreground galaxy also seem to be acting as gravitational lenses, making the images change their relative brightnesses in these two photographs taken 3 years apart. Credit: Geraint Lewis and Michael Irwin, William Hershel Telescope.

 

PHD = Pizza Hut Delivery, but I like Papa john's...

And you forgot the Jovian Experiment so soon ? By the way what was the field of your specialization for PhD..(err PHD).

 

Basically speaking GR is just the GR maths only for all practical purpose...

We still do not know what is the demarcation between weak field and strong field...and the funny part is we do not know if GR is ok in strong field ? Most of the calculations even at galactic level are done using Newtonian / Keplerian Gravity (so called weak filed), and we do not know if GR is valid near Neutron star or at the center of BH (so called strong field)....thats GR for us.

Can anyone answer that light (photon) closer to the periphery of a massive Galaxy gets bent (Thats mainstream lensing) but the outermost star (or object) is comfortably wandering around, it does not get pulled inside. If the light is bent (or curved) than imagine what would happen to star inward...but it does not happen.