OPTICS and GRAVITY

Discussion in 'Astronomy, Exobiology, & Cosmology' started by hansda, Sep 9, 2012.

  1. brucep Valued Senior Member

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    Since you don't understand your own references your posts are usually misinformed. Like your [non] analysis of Kevin Browns derivation. You are 'a crank', by my definition, because you argue from a soapbox of intellectual dishonesty. IE: you don't understand what you're complaining about. The comment you made that I reference in post #96 would be comical if it wasn't so pathetic. Once again:
    "What that means is this: in the room you're in, your ball doesn't fall down because the spacetime in that room is curved. That's what the equivalence principle tells us. So why does it fall down? Magic?"

    You said this to a PHD physicist. For me it's a 'big clue' that you're clueless about GR. You referencing the metric tensor is ridiculous since it's just for 'crank show and non-tell'. You're a nuisance. Some pretty knowledgeable people feel the need to correct your 'constant misinformation troll'. The good thing is everybody gets to read the PHD response.
     
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  3. hansda Valued Senior Member

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    Newtonian physics or Einsteinian physics, it is the same physical event hapenning in nature. Acceleration can be explained in both the models.


    If a massive particle travels along the path of light, where gravitational lensing is hapenning, it will also follow a curved trajectory like light. Will this 'curvature of trajectory for massive particle' and 'curvature of light/photon(massless particle) due gravitational lensing' be same?
     
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  5. hansda Valued Senior Member

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    I think 'M' for M-Theory also stands for magic or mystery.

    The fundamental mechanism may be 'conservation principle' of nature or 'equlibrium of nature'.

    I think optics also can lead us to understand gravity better.
     
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  7. Farsight Valued Senior Member

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    I explained why your precious PhD was wrong in post #93. But you refuse to accept it, you skate over the issue, and instead of offering a counter explanation or correction you're calling me names? Bruce, you're the troll. You're the nuisance.
     
  8. Farsight Valued Senior Member

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    No. Light is affected twice as much as matter.
     
  9. prometheus viva voce! Moderator

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    The distinction is indeed crucial, and you do not understand it. Remember the little discussion we had previously where you failed to grasp the fact that de Sitter and all of the FRW solutions to the EFE's have non zero curvature, yet can be written where the spatial sections are flat? Since you brought it up, in my viva I was asked a number of quite detailed questions about spacetime curvature and my examiners complimented my answers. For example, suppose you have an n + 1 dimensional spacetime. What happens if you put a surface that covers n - 1 spatial dimensions, and is pointlike in the remaining spatial dimension and the time dimension, and you are very far away from the surface in the spatial dimension? That question leads to a very nice result in the relationship between gravity and conformal field theory. In short, my authority card is there for a good reason.

    Przyk has already pointed out that you are talking about the components of the energy momentum tensor, not the metric tensor. I don't quite understand how you can argue with what I put previously. Einstein was talking about the components of the metric tensor (the energy momentum tensor has the same symmetry properties as the metric, of course, since they are related by the EFE's) and how it describes space. In this context he meant the four dimensional manifold that you insist is called spacetime. That context is generally clear to anyone remotely versed in GR, which is why physicists can afford to talk about space when meaning spacetime. The fact you make such a big deal out of it merely speaks of your inexperience with GR.

    let me take you back a moment:
    It is no such thing. A tensor is a multidimensional array that has specific transformation properties. Even a rank 2 tensor, that can be written in the same way as a matrix is most certainly not a matrix.

    Here we go again. You are saying that since the EFE's are called equations of motion, they tell you about motion the curvature is not fundamental. You are wrong, and it stems from your misunderstanding of the term "equations of motion." A mathematical way to explain this would be that one has an action, which in the case of GR is the Einstein Hilbert action, and varies it using the Euler Lagrang method. From this one gets the equations of motion for the system. Another way to say it is that the equations of motion tell you how a system evolves, but you need to be careful here about what you mean by "system." If your system is a car on an inclined plane then you will be figuring out how the car moves. The equations of motion of GR are for spacetime, so that tells you how spacetime evolves in time, not how "space moves through time." I don't believe for a second that you have any experience at all of time dependent solutions of the EFE's (they are quite complicated).

    If I were you I would go back to John Baez's website that you linked to earier and re read it. I explains exactly why the ball falls because of spacetime curvature. Do that, and avoid embarrassing posts like this one in the future.

    How exactly are you coming to this quantitative prediction. Just for the record, Hansada's question is too poorly posed to have a meaningful answer.
     
  10. hansda Valued Senior Member

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    Why?


    How?

    I guess you have guessed this from Newton's prediction. I think for a massive particle, the curvature may depend upon its mass also.
     
  11. hansda Valued Senior Member

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    Consider a particle Neutrino is travelling in the region of space, where gravitational lensing is hapenning. Here particle Neutrino also will bend like particle photon. So in this case, 'bending of particle Neutrino' will be 'greater than' or 'less than' or 'equal to' 'bending of particle photon'?
     
  12. brucep Valued Senior Member

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    4,098
    All you've explained is this science is 'way over your head'. What you wrote down on the subject of spacetime curvature is pathetic nonsense. The only one whose convinced you're right is you. Bad sign for you. Now pay attention to prometheus and you might learn something. I'm still waiting for you to clarify why you think the 'Kevin Brown' derivation is discarding 'something' on his say so. The counter explanation is found in GR and you need to learn what you're talking about or put a cork in the nonsense comments. Explain why the ball won't fall down because spacetime curvature holds it up. That's what you said. You're a 'verbose' crank in my estimation.
     
  13. brucep Valued Senior Member

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    4,098
    I'd like to see the Farsight proof.
     
  14. Farsight Valued Senior Member

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    Because of the wave nature of matter. You can make an electron and a positron out of a photon via pair production. And you can diffract an electron. The electron has a definite wave nature. Now take a look at atomic orbital on wikipedia and pay careful attention to this: The electrons do not orbit the nucleus in the sense of a planet orbiting the sun, but instead exist as standing waves. When we think of standing waves, we often think of a Fabry-Perot cavity where the light goes back and forth. Think instead of a standing wave going round a square cavity. Draw a square to represent the cavity then another square inside it at 45 degrees to represent the light path. Then rotate your drawing by 45 degrees. Look at the light path. Light curves in a gravitational field. If this was in a gravitational field, only the horizontals would be curved down towards the ground. The verticals aren't affected.

    I didn't guess, Einstein predicted it. His first attempt matched the Newtonian prediction, which is OK for matter. But then he realised that light is deflected twice as much. See for example Doubling the Deflection on mathspages:

    "In the completed theory of 1915 the predicted frequency shift for light in a gravitational field is the same as Einstein had predicted in 1911, but the amount of deflection which a ray of light is predicted to undergo when passing by a gravitating body is twice as much as he had predicted in 1911".
     
  15. hansda Valued Senior Member

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    Here you have considered wave nature of electron and its diffraction. You did not consider the particle nature of electron(with mass) in a gravitational field. It is only curvature of light in a gravitational field.

    Einstein predicted curvature of light in a gravitational field. Did he calculated 'curvature of a massive particle' in a gravitational field?

    The equation dphi=4M/R, do not contain any mass value of the particle. Is it that their deflection in a gravitational field is independant of their mass?
     
  16. Farsight Valued Senior Member

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    That's right. Modern physics employs quantum field theory, the electron is comprised of "field", or wavefunction if you prefer. Fields and waves are closely related, especially when you start talking about standing waves. For example the evanescent waves is a standing wave, and is also called the near field.

    Yes, he predicted the perihelion advance of Mercury.

    Yes. Things get more complicated when you start talking about say two planets, but when you limit yourself to a planet and test particles, their deflection is independent of their mass.
     
  17. hansda Valued Senior Member

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    So it can be said that, 'bending of massive test particles' and 'bending of massless photon particles' are same in any gravitational field. This bending in a gravitational field can be quantified as '4M/R', which is independant of their mass.
     
  18. Farsight Valued Senior Member

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    No. Photons are affected twice as much as massive particles. Have a read of this Deflection and Delay of Light article:

    "...during the war Einstein changed his prediction to 1.75 arcseconds, which is twice the Newtonian deflection..."

    The Newtonian affect on massive particles is right enough, we can check it out by dropping a brick. Einstein worked out that light is affected twice as much. However see Newtonian gravitational deflection of light revisited for an interesting read about Newton.
     
  19. prometheus viva voce! Moderator

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    So are we to assume you don't actually know how deflection angle depends on mass in general relativity?
     
  20. hansda Valued Senior Member

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    Here you are comparing Newtonian model with Einsteinian model.

    If both massive particles and massless particles can be compared in Einsteinian model, i think this link http://en.wikipedia.org/wiki/Schwarzschild_geodesics better describes this case of bending in a gravitational field. Following is a quote from the link:

    "In general relativity, the geodesics of the Schwarzschild metric describe the motion of particles of infinitesimal mass in the gravitational field of a central fixed mass M. The Schwarzschild geodesics have been pivotal in the validation of the Einstein's theory of general relativity. For example, they provide quantitative predictions of the anomalous precession of the planets in the Solar System, and of the deflection of light by gravity.




    The Schwarzschild geodesics pertain only to the motion of particles of infinitesimal mass m, i.e., particles that do not themselves contribute to the gravitational field. However, they are highly accurate provided that m is many-fold smaller than the central mass M, e.g., for planets orbiting their sun."



    From the above it can be said that, bending of massive particles and massless particles are same in any gravitational field.
     
  21. Farsight Valued Senior Member

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    3,475
    Yes. Because if I drop a cannonball and a bullet, they both hit the ground at the same time. And if I fire them at 1000m/s, then ignoring wind resistance, they still hit the ground at the same time.

    hansda: don't take my word for it, ask around and do some research. Light is deflected twice as much as matter. If you find something that proves I'm wrong about that, I will gladly eat humble pie and give you my apologies.
     
  22. prometheus viva voce! Moderator

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    This is only true because the mass of the object you're dropping is negligibly small compared to the mass of the planet. It is not true in general.

    This promise might have more weight if anyone believed you would actually follow through on it. At what point are you going to start eating humble pie over your horrendous misunderstanding of the curvature of de Sitter, or the FRW solutions?
     
  23. Farsight Valued Senior Member

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    Yes I know that, see above where I said things get more complicated when you start talking about two planets. We had a conversation about this with RJ a while back.

    I don't misunderstand them. But you can look back at your post #90 where you said "how can you possibly object to anyone using space and spacetime interchangeably" . And then you can look at my post #93 where I higlighted "Note: not the curvature of space, but of spacetime. The distinction is crucial.". And you can eat humble pie over your horrendous misunderstanding of that.
     

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