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Nonrelativistic deflection 
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#1
Jul2514, 12:09 AM

PF Gold
P: 636

Hi,
I am trying to understand the difference between GR and classical deflection of light by the Sun.I found this R = 2.5c (7.5*10^10 cm) point of impact, x distance of light from R in lightseconds, r (=√x^{2} +R^{2}) distance of light from center of sun and then distance sunearth = 200c (http://www.wolframalpha.com/input/?i...+from+0+to+200) = 2.4998 and distance starsun ∞ (http://www.wolframalpha.com/input/?i...+infinity+to+0) = 2.5 (R/c) the non relativistic pull should then be [itex]\frac{GM}{R^2} = 23600 *2*2.5 \frac{2*R}{c} → \frac{2 GM}{Rc} ≈ 118 000 cm/s^2 [/itex] Can you check if it is right and clarify a couple of points:  do observed light beams actually 'graze' the Sun? what is the usual distance of the observed beams? can you give me a link where to find details of recent observations? I read that deflection has been adjusted to 1.67'',but, at what distance from the center of the sun?  In order to get the global pull by the sun we must multiply g_{sun} (the force at x=0 (R)) by the integral and by 2 because the pull is exerted from ∞ to 200. This seems the obvious classical procedure, but BvU (I am not sure about mfb) considers this factor as the relativitic γfactor, which would give 4 GM. what is correct? Your help is greatly appreciated 


#2
Jul2514, 02:23 AM

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Please provide a link to the thread in question.
What do you mean by "classical"? In Newtonian mechanics  light is not deflected by the Sun because light has no mass. The deflection of starlight by the Sun, or by other bodies, is well documented. As for the grazing path: as the Sun traverses the sky it sometimes blocks the stars. It folows that there must be a time when the star appears just beyond the limb of the Sun as viewed from the Earth ... so the answer is "yes" light rays can graze the Sun. Accessible source: http://cosmictimes.gsfc.nasa.gov/tea...starlight.html The link between GR and Newtonian gravitation is better understood from the maths: http://preposterousuniverse.com/grnotes/grtinypdf.pdf http://csep10.phys.utk.edu/astr162/l...y/gravity.html 


#3
Jul2514, 02:51 AM

PF Gold
P: 636

In 1803 Soldner gave a first extimate of the deflection, was at the time anything but Newtonian physics? Are you saying that he quoted thread treats the issue wrt to GR, does it? It seems it is integrating the Newtonian acceleration that says (5.6) the that the formula is modified by the factor [itex]\frac{1+\gamma}{2}[/itex] where γ is 1 in GR and 0 in Newtonian gravity. I read that half bending comes from curvature of space and 1/2 from curvature of time. If it is so, it is quite simple If you click on the quote you go to the thread do you think I should consider R = 7*10^10cm? 


#4
Jul2614, 11:58 PM

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Nonrelativistic deflection
Well OK.
By Newton's second law, the acceleration of gravity does not depend on the mass of the accelerating object  which would imply that light would be deflected by massive objects. iirc the "no mass" objection comes from trying to work out the gravity of the light  and considering the 3rd law. There is also a problem with there being a force of gravity where there is no mass, since the force depends on the mass of both objects. If you ignore these, you can do calculations for the deflection of light by gravity from Newtonian physics and then compare with experiment. Note: GR has to confirm the Newtonian results. The analysis in the thread is extremely simplistic  I'd have done it as a central force problem for a small test mass travelling at the speed of light in the Newtonian framework. I don't know of any recent "grazing light" observations, but "gravitational lensing" is standard in astrophysics these days and uses the same physics. My undertstanding is that the Newtonian trajectory close to the Sun is flatter than the GR trajectory, but there is close agreement for greater distances. It is not clear to me what you are trying to understand. You should be aware that GR replaces Newtonian gravitation because there are some things that don't work in Newton. Therefore, trying to use your understanding of Newton to understand GR won't get you far. You wll also need to improve your maths. 


#5
Jul2714, 01:15 AM

PF Gold
P: 636

I am asking  if the analysis in that thread is nonrelativistic.I thought so , as the result is 2GM... instead of 4GM (as in GR)  I cannot understand why BvU says that 2* GM is what Einstein predicted, as he predicted 2*2GM.  I'd like someone to confirm that the GR applies the same laws and parameter of Newtonian gravitation and simply multiplies it by a factor of 2 (1+γ(=1)/2) (did you read the article I quoted?) and lastly I would like to find an article that describes in detail a recent observation of deflection with precise figures. I have other minor queries , if you have time Thanks. 


#6
Jul2714, 02:04 AM

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#7
Jul2714, 02:43 AM

PF Gold
P: 636

that file is too heavy for me, what about the simpler one I quoted? it uses tensor calculus or whatever, (derived formula: 5.5) but just to plug in there the Newtonian value (2*2GM/RC^{2}) , which then is multiplied by 1+γ/2 (5.6), where γ is 1 in Gr and 0 in nonGR, producing 4GM/RC^2 *1/2 = 2GM/RC^2, did I get it wrong? another thing I'd like to know is why, once you get the force 118000128000 cm/s^2 (GM/Rc), you divide that value by C and get GM/Rc^2 , to determine the angle, shouldn't it be divided by C/R since light is deflected in its orbit ,which has radius R and not C ? I look forward to a link about real experiments, the Eddington data are not reliable. In the meanwhile, do you know how close can a photon graze the Sun without being altered?, does extreme heat affect light? Thanks Mr R, your help has been invaluable. 


#8
Jul2714, 03:08 AM

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Okey, I started like this. I was curious about the light deflection so I did what you did. Let me calculate one for you and see if that helps clear some things 


#9
Jul2714, 03:36 AM

P: 74

I hope this helps. My last post said Sin. make it tan and I used the change of velocity here to get the angle.



#10
Jul2714, 03:56 AM

PF Gold
P: 636

I am referring to any calculation, I just worked out an equation to find the total F acting on the photon, and I found GM/Rc, like anybody else, using the laws I would use for an asteroid or an electron. I meant to go on by the same laws , finding what angle corresponds to 128000 cm/s^{2}, if the orbit is 7c and speed is C. I do not understand the change of speed of light, never heard about it, can you explain? 


#11
Jul2714, 04:27 AM

P: 74

I see, you want the F that when divided by will get you the angle. I think you mixed up some stuff in your calculation because your "Force" (GM/Rc) is actually the velocity component that deviated from the straight path the photon was travelling. and you divide that by the straight path (C) velocity to get the angle. Velocity change? Since in this quasiNewtonian mumbo jumbo we considered the photons to be some sort of capsules that has mass. Then it has a variable velocity. Plus: what do you mean by c? as in orbit is 7c? 


#12
Jul2714, 04:44 AM

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P: 636

We have F_{g} = GM/R^{2}, which is the acceleration a pointmass gets in 1 sec at distance R from the sun, right? If you integrate this value to find the acceleration it gets in 200 sec, whatever units you get , it's always acceleration (F_{g}) isnt' it? we merge the formula together but the gist is F_{g} in 1 sec = 27500, Fg in 200 sec = F_{g} * (R/C = 2.5) = 128000. I am not an expert in units, but the logic is that, right? I made my example using c as unit, so R is 7c = 7(.5)*10^{10} cm and the orbit of the photon grazing the sun as radius 7c. In Newtonian orbits , both for planets and electrons, we find the required acceleration to make an objet bend its trajectory by 1 radiant multiplying momentum/velocity by the ratio velocity/radius of the orbit (v*v/r= v^{2}/r) I was trying to apply the same formulas I use for other orbits. Is it a total mess? 


#13
Jul2714, 05:01 AM

P: 74

I am really sorry for not being able to help. Where are the experts when we need them?! 


#14
Jul2714, 07:46 AM

PF Gold
P: 636

But now I am beginning to understand why you talk about velocity v/c , is that so? so you find acceleration per second 27500, integrate and get velocity 128 000? then you divide it by C and you get what? what are now the units of v/C ? it should be a dimensionless number for what I know, why should it be an angle? Thanks a lot, you have been very helpful, perhaps an expert would not have understood me at all! P.S it's August, most people are on holidays/vacation, I suppose 


#15
Jul2714, 08:08 AM

P: 74

Just forget your numbers. Do it again and calculate the numbers at the last moment. 


#16
Jul2814, 02:55 AM

PF Gold
P: 636

the earth (v=10^{6}cm/s) gets roughly F = 1 c/s^{2}. We integrate over 200 s and we get v=200, what does that mean? what is here [itex]tan(o/a)=tan(v/c)≈v/c=θ[/itex] if 200/10^6 =θ, what is that? 


#17
Aug314, 11:36 PM

PF Gold
P: 636

For a small angle it is sinθ ≈ tanθ , and not ≈θ ? In this case v/c = 0.000004233, tan^{1} = θ =0.0002425 and sin θ = 0.000004223 can you also explain what is θ for the deflection of the earth by the Sun in 200 sec: tanθ = 200/10^{6} = 0.0002 Is this angle small enough to consider tan^{1} Thanks 


#18
Aug414, 12:44 AM

Mentor
P: 3,975

##sin(x)=x\frac{1}{6}x^3+....## ##tan(x)=x+\frac{1}{3}x^3+...## If ##x## is small than ##x^3## will be even smaller and the terms further to the right will be yet smaller than that. So you can approximate either function to just ##x## any time that ##x## is small enough that an error of ##x^3## is acceptable. 


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