[Hayseed]

We bowed a flashlight beam thru sugar water, in 8th grade.  An example of the affects of a density gradient. 

[Kryptid]

I think he got lucky by applying his ideas to the Solar System. There his results, can by fudging, be made to seem plausible.

Please supply a reference for where "fudging" was used in order to make his predictions match reality.

The anomalous rotation of galaxies cannot be explained by Einstein's theory.  It's a complete failure.

That's not the same thing as anomalous orbital precession. At all.

which would increase its mass, thereby making it expand,

Then wait for it to cool down afterwards. Or use something that doesn't experience thermal expansion (like a laser beam).

We bowed a flashlight beam thru sugar water, in 8th grade.  An example of the affects of a density gradient. 

That's a different phenomenon.

[evan_au]

his results, can by fudging, be made to seem plausible

It is true that the size of Mecury's precession was known at the time of Einstein, and he calculated what the answer was before he published his theory of General Relativity (GR).

But the mathematical basis of GR has been extremely successful for over a century - even when Einstein himself didn't believe it was possible.

The equations of GR allow for:
-  An expanding universe, which Einstein himself didn't believe, and even tried to eliminate the possibility - until the reality was discovered by Hubble
- Black holes, which Einstein himself initially didn't believe. Penrose recently got the Nobel prize for showing that they were likely to form in supernova explosions, and now we have an image of a supermassive black hole in the center of a nearby galaxy
- Gravitational waves, which Einstein didn't think we would be able to detect - and now we have sound recordings of black holes colliding
- So it wasn't a matter of Einstein "fudging" the results - the mathematics produced results which Einstein himself did not believe.

There are many other experimental verifications, see: https://en.wikipedia.org/wiki/Tests_of_general_relativity

[lunar7]

How can it be measured.

One way is to measure (with a tape measure) the diameter of Earth, and then its circumference.  This isn't practical of course since you can't pass a tape measure through Earth, and it is hard to line up if held off to the side, but if you did, the circumference would definitely be less than the diameter * π.  This can only be explained with non-Euclidean spacetime, and is quantified precisely by Einstein's field equations.

The difference isn't much with Earth, but it is for more massive things like neutron stars.

Actually, I'm sure the size of the Earth can me measured during a lunar eclipse, when the Earth's shadow is cast on the Moon. Due to the vast distances it can be assumed the rays are parallel.

[Janus]

How can it be measured.

One way is to measure (with a tape measure) the diameter of Earth, and then its circumference.  This isn't practical of course since you can't pass a tape measure through Earth, and it is hard to line up if held off to the side, but if you did, the circumference would definitely be less than the diameter * π.  This can only be explained with non-Euclidean spacetime, and is quantified precisely by Einstein's field equations.

The difference isn't much with Earth, but it is for more massive things like neutron stars.

Actually, I'm sure the size of the Earth can me measured during a lunar eclipse, when the Earth's shadow is cast on the Moon. Due to the vast distances it can be assumed the rays are parallel.

Nowhere near to the accuracy needed to note the effects discussed.  In this case "nearly parallel" isn't good enough, especially when combined with the fact that the Sun isn't a perfect point source, which means the edge of the shadow it casts will be a bit fuzzy.

[CPT ArkAngel]

Gravitational mass bend space-time not space.

[acsinuk]

Light is electro-magnetic and as such could be bent if a voltage field attracted it or much more likely, if a magnetic field tried to magnetically spin the light beam .Cannot see that gravitation has anything to do with the deflection of light as the beam is totally mass-less?

[CPT ArkAngel]

Photons have an inertial mass and a delayed gravitational mass. Gravitational perturbations travel at the speed of light. What we call massive particles cannot reach the speed of light. For a photon, its gravitational mass is totally redshifted in its direction of motion. That's why light bends twice as much as massive particles, because it has no massive counter field in its direction of motion.

[Janus]

Cannot see that gravitation has anything to do with the deflection of light as the beam is totally mass-less?

You are intermixing 2 different physics models here; Newtonian and Relativity.
Under Newton, gravity is an attraction between masses, using the Newtonian definition for mass.
With Relativity, gravity is coupled to the Energy momentum tensor, of which the Newtonian concept of "mass" is only a single component.
Classical Newtonian mass is what Relativity would call rest or Proper mass.
Thus when it is said that light is massless, we means it has no proper mass.   However, it still has energy, momentum* etc, which are other components of the energy-momentum tensor through gravity couples.

So under Newton, gravity only relies on the "mass", while under Relativity energy content also plays a role.

The point is that you can't use the gravity model and definition of "mass" from one theory and expect it to make sense if you apply it to a theory that uses a different model.

* and while the Newtonian definition of momentum is p= mv, momentum of a photon is related to its energy by p = E/c

[gem]

HI All

There are two ways of looking at it. One way is the way that lunar7 has described (that the path of light is deflected because the space around a massive body is distorted. This is the explanation by general relativity). The other is that light is attracted to the gravitational field of the massive body and is therefore pulled towards it. Both explanations are equally correct.

Janus states

So under Newton, gravity only relies on the "mass", while under Relativity energy content also plays a role.

The point is that you can't use the gravity model and definition of "mass" from one theory and expect it to make sense if you apply it to a theory that uses a different model.

* and while the Newtonian definition of momentum is p= mv, momentum of a photon is related to its energy by p = E/c

So if we use Kryptids excellent point of gravity pulling towards I believe Janus the aspect that links  the two definitions is Energy as you state, ie the energy potential of a gravitational field as is demonstrated in the Harvard tower experiment.

https://en.wikipedia.org/wiki/Pound%E2%80%93Rebka_experiment
 
for example I believe you can link the time dilation delta for the height of the tower (bottom - top or vice versa) to the gravitational potential accounting for the red/blue shift of light .

And gives exactly the same as using the velocity time dilation equation if you dropped and object from said height and plugged in velocity reached (ignoring friction) over that distance in that acceleration field into velocity time dilation equation.
So I believe you could calculate some aspects of what's under consideration using things as simple as suvat equations and they wouldn't  be contradictory.   

[evan_au]

I think (Einstein) got lucky by applying his ideas to the Solar System. There his results, can by fudging, be made to seem plausible.

I would add that the accelerating expansion of the universe was only discovered in the 1990s, so Einstein was totally unaware of it
- It applies to objects far outside our Solar System (and would not be measurable on the scale of our galaxy, let alone our Solar System)
- And yet, it seems that Einstein's mathematical model of General Relativity is the best model we have for it, with the expanding set of measurements we are continually adding

I don't think the mathematical model was "lucky" or "fudged" - it just followed the evidence
- The part where Einstein did find out he had fudged the result was in picking a value for the cosmological constant to fit in with the common view at the time (stretching all the way back to Newton, and probably to Greek philosophers) that the universe as a whole (outside the Earth & Planets) was static and unchanging.

So, when you say (in another thread) that "I don’t understand physics: does anyone understand physics these days?", are you accepting the expansion of the universe, or are you staying with the ancient philosophers?
- And are you accepting the accelerating expansion of the universe, or are you staying with Hubble?

[Janus]

I don't think the mathematical model was "lucky" or "fudged" - it just followed the evidence
- The part where Einstein did find out he had fudged the result was in picking a value for the cosmological constant to fit in with the common view at the time (stretching all the way back to Newton, and probably to Greek philosophers) that the universe as a whole (outside the Earth & Planets) was static and unchanging.

A couple of points.  Einstein tended to approach things a bit differently that other theorists.  While they would start from the experimental evidence and work back to a theory to explain it, he tended to start from basic concepts and assumptions, follow them to their logical conclusion and see if it matched the evidence.  Pretty much the opposite of "fudging".

The other thing to consider with his cosmological constant was that, at the time, he developed GR, it still had not been established that the universe extended past the Milky way.  So not only did he not know of  the expansion, but he was considering a much smaller "universe".

[acsinuk]

Einstein also did not know that gravitation could not balance the universe; or that dark matter and dark energy had to be invented to try to explain the WMAP results. 
But if the universe is magnetised and polarised then because of the magnoflux spin effect most of the forces that operate inside  galaxies can be explained.

[charles1948]

We bowed a flashlight beam thru sugar water, in 8th grade.  An example of the affects of a density gradient.

Yes.  Though I'm not quite sure why "sugar water", whatever that it is, was employed in the experiment

Wouldn't a glass of plain water, exhibit the same effect?  I'm thinking of the well-known phenomenon of a drinking-straw placed in a glass of water. 

The straw then looks bent.  This is a purely optical effect.  Caused by refraction of light through the different "density gradients", as you put it, of water and air.

Is the "bending"  caused by any enormous differences in the mass of the three substances involved  -  water, glass, and air.  Surely not. The mass of a glass of water is minuscule.  The air even less.

Yet somehow, they can cause a beam of photons to exhibit a startling deviation, and cause a straight straw appear bent.

I think simple observations such as this, may suggest that caution needs to be exerted, when interpreting "lensing" effects observed among distant galaxies.

[Bored chemist]

Yes.  Though I'm not quite sure why "sugar water", whatever that it is, was employed in the experiment

So, you fundamentally fail to understand the whole thing...

Wouldn't it have been better to stop at that point, and ask?

[charles1948]

I could look up what sugar-water is, but decline to do, as it doesn't seem of any real relevance.

The point I was making is this: refraction of light through the lens-shaped dust clouds which surround distant galaxies, could explain the image displacement effects which we observe from Earth.
 
How can you confute this reasonable suggestion.  When even a glass of water, observed at close quarters, can make a straw look bent, when it actually isn't.  I rest my case.

[Bored chemist]

it doesn't seem of any real relevance.

It doesn't "seem" that way to you, but you don't know what you are talking about.
So your approach is to carry on with your own special brand of ignorance.

Why would you do that?

How can you confute this reasonable suggestion. 

Chromatic aberration.

Also, dust doesn't diffract light, it scatters it.
Did you really think  that science hadn't already thought of that?

[charles1948]

You accuse me for being an ignorant person.  That's a charge I never heard before.

I probably have a wider range of knowledge, across all fields, than anyone else on this forum.

 But mere knowledge, by itself, is not enough to gain insight into truth.  Only constant questioning can do that.

That's why I ask questions.   Sometimes the answers on here provide new insights.  For which I'm grateful.

[Bored chemist]

You accuse me for being an ignorant person. 

It's not an accusation; it's an observation.

YThat's a charge I never heard before.

I find that hard to believe.

That's why I ask questions.

No, even when it's obvious that you would gain from asking a question, you choose not to.

I could look up what sugar-water is, but decline to do, as it doesn't seem of any real relevance.

Because you are ignorant of the importance of the answer.

But mere knowledge, by itself, is not enough to gain insight into truth. 

The lack of knowledge isn't much of a path to truth either, is it?

[Janus]

I could look up what sugar-water is, but decline to do, as it doesn't seem of any real relevance.

The point I was making is this: refraction of light through the lens-shaped dust clouds which surround distant galaxies, could explain the image displacement effects which we observe from Earth.
 
How can you confute this reasonable suggestion.  When even a glass of water, observed at close quarters, can make a straw look bent, when it actually isn't.  I rest my case.

And if you look really close at that pencil, you will not a fuzzy rainbow effect at its edges.  This is the chromatic aberration that Bored Chemist brought up.   When light is refracted or even scattered, different frequencies of light have their paths altered by different amounts (This is why a prism can break white light up into is component colors.)

The fact that light passing by galaxies shows no sign of this effect indicates that refraction or scattering cannot be the cause.
And to reiterate Bored Chemist's remark:   Why do you think that scientists wouldn't have ruled out already known effect before coming to the to the conclusion they did?
If you thought of it with your limited grasp of science, why wouldn't had they?
What is it that makes you think that the professional scientists that study these things are so incompetent that hey couldn't find their backside with both hands?

Why do you think that you can see things they can't?

The only reason I can come up with is misplaced hubris on your part.