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General Relativity

  1. Jul 30, 2004 #1
    I'm not sure what forum I am suppose to post GR questions so I am sorry if this is in the wrong spot. Ok my question is this. In classic phys. newton required mass to have gravity. I read that in GR einstein required stress energy. If light has not mass or should I say a photon but EM waves produce radiation pressure as a force then integrated over time would give you work and energy. So what I am asking does light or an EM wavelength cause a force, have energy, and most of all cause fluctuations in gravity. IE a large super star produces the force of gravity to a small planet beside it but also its light that is shined does too??? Is this possible or true.
  2. jcsd
  3. Jul 30, 2004 #2
    There is a forum called "Special & General Relativity." Its located at

    The correct way to say that is The source of gravity is active gravitational mass.
    Are you familiar with electromagnetics (EM)? If so then you know that if there is a frame of referance S in which there are charges at rest then in that frame there is an electric field. In this sense you can say "Charge is the source of the electric field." Change to a frame S' moving relative to S. In that frame the charges are moving. Charges in motion is called "current." Also in this frame, S', there is a magnetic field. In this sense you can say that "Current is the source of the magnetic field." Charge in one frame is current in another frame. There is a mathematical object called the "4-current" and its defined as J^u = (c*rho, j) where rho is charge density and j is current density. Lets say that "J^u is the mathematical quantity which plays the role of source in the equations of EM.

    There is something similar in GR. The mathematical object which plays the role of source is called the stress-energy-momentum tensor. Mass in one frame is stress and momentum in another frame.

    Light does have mass. It has no "rest mass" (which I prefer to call "proper mas")
    Sure. See - http://www.geocities.com/physics_world/grav_light.htm

  4. Jul 30, 2004 #3

    thanx and sorry bout the post in wrong spot i saw that right afterward.

    And yes i am taking EM now that how the though arrived my prof couldnt even answer the question (does light cause gravitational effects and he has his PHD lol)

    Ok i know that strong weak forces are much stronger than electro force and its stronger than gravity so the gravitational effects felt by a large star would be where in the midst of these forces?

    By the way, thanx for the link but i don't understand half the stuff in their how much physics and math do i need. I have had Phy I and II and Cal I and II and read up alot on SR and GR.
  5. Jul 30, 2004 #4
    another thing?

    Sorry to bother you i feel stupid about this but you say light has mass or at rest can you explain that, and when you say light do you mean a photon or a certain wavelength of Electromagnetics.
  6. Jul 30, 2004 #5
    I wouldn't laugh at someone because they have a PhD and didn't know that light generates a gravitational field. GR is not known by most physicists and the current trend of saying "Light has no mass" propagates that flawed assumption.
    what are strong weak forces?
    The gravitational field produced by light is incredibly small. I haven't put any numbers into that formula but it wouldn't surprise me if it was super-extreme-small.

  7. Jul 30, 2004 #6


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    The answer is yes.
  8. Jul 30, 2004 #7


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    Last edited by a moderator: Aug 2, 2004
  9. Jul 31, 2004 #8

    Thank you to both, but now I'm really confused.
  10. Jul 31, 2004 #9
    ok now really big wow

    Mr DW or can i call MR WAITE.

    Is that your site, like are you a prof. or what?
  11. Jul 31, 2004 #10

    can you explain this

    you say that a photon is massless but a group or bundle of EM waves can have a mass, does that mean a single photon doesnt but a group does? if so that is confusing.
  12. Jul 31, 2004 #11


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    Mass for a system of particles is its center of momentum frame energy. If the photons are moving in different directions there will be a center of momentum frame and the energy of all the particles according to that frame is the system's mass. When it comes to a single particle the mass is the length of the momentum four-vector which is zero for massless particles, but equal to the rest frame energy for massive ones.
  13. Jul 31, 2004 #12

    Both of you are extremely intelligent so I can't wait till I can have as much school as yall and maybe i'll be half as smart as you both.

    I am sophmore in college, I have had Cal 1,2 and phys I and II what else do i need to underestand the math for GR, ill be in cal III in a few weeks, like things such as tensors and stuff, don't get that stuff to well, or i haven't had good explanations I guess you could say, but all in all what kind of math do I need to understand the stuff on both of your web sites, and by the way thanx for both the links.

  14. Jul 31, 2004 #13
    The best way I can think of to address your question is for you to start reading this page

    and stop where you're even a bit confused. Then tell me what you don't understand and I'll help you step by step from there.

    Last edited by a moderator: Aug 2, 2004
  15. Jul 31, 2004 #14

    thanx pete alot I wanna learn this and I gonna get it with people like you and others on here.
  16. Jul 31, 2004 #15
    Pay close attention to Tom Mattson too. He's one sharp cookie!

  17. Jul 31, 2004 #16


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    I hate to get involved, but DW best represents orthodoxy despite his silly leotard.

    PMB, you are0
    hardly a relativsitic idiot, but you should try to stick to the orthodoxy.
  18. Jul 31, 2004 #17


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    Pedant mode on..

    Its somewhat misleading to say mass generates curvature. It is however true that the stress energy tensor generates curvature (kinda). There are mass/energy components, off diagonal shearing terms and pressure terms.

    Its these pressure terms that are hard to think about with classical reasoning, and is a solid and testable prediction of GR.

    The reason I say 'kinda', is that the equations need to be self consistent and aren't really a 'pure' equality (thus I wonder sometimes if its perfectly true to say one side generates the other). Intrinsically in the definition of stress energy, there are units of area. What meaning is there for an area, if we haven't figured out what the geometry is yet?

    So proffessional physicists when they are solving Einstein's field equations, must always double check everything to make sure that indeed everything is self consistent. Only then, in such a specific case, is it true to say the equality is valid (and it usually is).
  19. Jul 31, 2004 #18
    Who said that it did? Not I that's for sure. I hold that mass is the source of gravity in the same sense as charge is the source of an EM field.

  20. Aug 1, 2004 #19
    stress energy tensor

    What constitutes a stress energy tensor? What is the equation I guess i should ask?

    I have a question not along gravity lines but about relativity.

    Ok if DW is stationary on earth in our reference frame and Pmb_phy is flying at .99 C away in this reference pt pmb's clock will slow down correct and he will not age in a year what DW will.
    Now can we not say that it is not pmb that is moving at C it is earth with DW on it . Would he now not age slower than pmb phy who is stationary?
  21. Aug 1, 2004 #20
    sorry, but one more thing

    Sorry to post again but can someone who knows alot, I guess all of you do. Answer my question about what kind of math past calc I and II and III and Phys I and II i would need to understand the math and symbolism in GR, that's what I can't get past all the sub and superscrips everywhere.

    Cheers Woody
  22. Aug 1, 2004 #21
    Hi Again jcsd

    You didn't get back to me yet on my question. What was it that I posted that gave you the impreession that I wasn't sticking to the "orthodoxy".

    You can PM me if you'd like to take this offline.

  23. Aug 5, 2004 #22


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    The stress energy tensor expresses the density of energy and momentum in space.

    Energy and momentum in relativity (both special and general) are combined into what's called a 4-vector. If you are curious about relativity, I'd highly recommend about reading about 4-vectors before you get into anything more advanced. They'll take you a l long way. space+time are one example of a 4 vector in relativity, energy+momentum are another. We don't talk about space and time separately - we talk about them as a unit, a 4-vector. The same holds true for energy and momentum.

    The stress-energy tensor is a siginficantly more advanced concept than a 4 vector. The tricky part is defining a region of space, actually.

    What is actually done is to define the direction of time with a four vector, and call everything else that's at right angles to that time vector, space. This will probably confuse the heck out of you, for which I apologize. But remember all that Lorentz contraction business? Volume is most definitely NOT an invariant, it depends on the observer. If I construct a box that's 1meter by 1meter by 1 meter in my reference frame, someone who is moving looks at it and says - hey - that box isn't a cube, it's been squished! That's a problem.

    The rest is relatively easy. The stress energy tensor just maps one vector into another. You can think of it as being a matrix, it takes in the time vector I just talked about, and spits out the energy/momentum 4 vector density that an observer with that time vector would see.
  24. Aug 5, 2004 #23


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    All those sub and superscripts are tensors.

    To really understand tensors you'll need to understand what a vector space is. This is a part of linear algebra, but unfortunately not all linear algebra courses are created equal :-(.

    If you already understand the concept of a vector and a dual vector, then you might get something out of


    Rather than tackle general realtivity directly, you would be better off working up to it. I'd say that the best approach would be starting with a course in special relativity, then linear algebra (you've already mentioned the calculus), then relativistic electrodynamics (which if taken at the right level will get you into tensors and relativity in a context that's easier to learn than gravity), and then you'll be about ready to tackle General Relativity.
  25. Aug 5, 2004 #24
    thanx alot perfect, i have worked through a few books on SR and worked with alot of the math that was in it, it wasn't too bad but they may be because the stuff wasn't advanced enough it was just Lorentz's all the lenght and time, and that stuff, but i have begun a tensor analysis book that i dl'd from that physics napster things whichi must say is awesome you all should check it out. and the author is doing well teacher me tensors but i need to look into the four space stuff first I assume. You say electrodynamics, I am not totally sure what that encompasses, i have had phys II E and M, is that what you mean.

    thanx perfect.
  26. Aug 5, 2004 #25


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    Special relativity actually _is_ pretty easy, but if the treatment you got didn't include 4-vectors, you'll probably want to read more. I believe that Taylor & Wheeler's "Spacetime Physics" is still considered to be a good modern SR book. Perhaps others can comment if this is still considered to be good, I'm probably outdated.

    As far as Electrodynamics goes, when I posted I was thinking of Jackson's "Classical Electrodynamics". It's a book on dynamics and electromagnetism - hence, electrodynamics. Difficult as this book (it's graduate level) it's still easier than dealing with gravity. But as I think, I think there must be better, easier books out there to learn from than Jackson, something that's more of an undergraduate level but still deals with some of the same material. The idea is that electromagnetism is a lot easier to deal with than gravity, and by considering how electric and magnetic fields transform in relativity theory, (i.e. the Faraday tensor), you'll be in a position to have something to fall back on when you start to tackle the trickier issues involved with gravity.
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