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Does the law of universal gravity exist?

  1. May 14, 2013 #1
    Greetings everyone,

    I have a question that bothers me. What if two objects are traveling at 0.6C at opposite directions? Do no forces (gravitational mainly) apply to them at all? Is gravity only traveling at the speed of light or could this be challenged? It just seems to me weird that the gravity of objects 1 lightyear away take a year to occur while in the meantime one of the objects might have been destroyed.

    30c5bt4.jpg

    If these objects travel at such speed and direction the gravitational force will never reach them, thus this would render the law of universal gravitation false.
    I am no physics guy so if possible try to use something less academic.
    Thanks a lot for any answers.
     
    Last edited: May 14, 2013
  2. jcsd
  3. May 14, 2013 #2

    Nugatory

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    If you are sitting at rest in the middle of your picture watching one object moving to the left at .6c and the other moving to the right at .6c, an observer on one of the objects will not see the other one moving at .6c+.6c = 1.2c; the speed of one object relative to other will be some smaller number that's less than c. Google for "relativistic addition of velocity"... The formula is easy enough that you can calculate it for yourself.

    Thus, there's no problem with stuff happening at one object, like changes in gravitational or electrical fields, affecting the other.
     
  4. May 14, 2013 #3
    The gravitational influence from one would reach the other. Using Einstein's velocity addition formula. You'd see that in one objects reference frame the other would be traveling away at less than the speed of light.
     
  5. May 14, 2013 #4
    I could be wrong on this, but I'm pretty sure that like photons, gravitons, the hypothetical particles that carry the force of gravity, travel at the speed of light in all frames of reference. I think it's just like if you were to shine a flashlight behind a car going 25 m/s, the photons would still be moving at c relative to everything else, not c-25 m/s. So, to answer your question, yes, these objects would still attract each other gravitationally.
     
  6. May 14, 2013 #5

    pervect

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    Let's make both of the objects charged (oppositely) as well,

    Then we can say:

    1) The objects attract each other gravitationally
    2) The objects attract each other electrically
    3) Light signals can and do travel from one object to the other, the objects do not "outrun" light in any way.

    For point 3, you might want to see the relativistic velocity addition formula.

    It turns out that one can come up with stronger versions of your paradox where point #3 is not valid. In such cases points 1 and 2 still apply.

    I'm afraid without knowing more of your background, I can't say a lot more. There are two issues here - the issue of how the particles do and can exchange light, and the issue of how gravity and electrostatic forces work in situations (different from the one you described) where the particles can't send each other light signals.

    If you want to know more, you can start by picking one of the two points above, describing what you think is otherwise (or what source leads you to believe that things are otherwise) and then perhaps we can address the issues more specifically.
     
  7. May 15, 2013 #6

    Nugatory

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    Every change in our understanding of the universe builds on what we already know. Yes, people used to believe that the earth was flat, but when we learned that it was actually round (much more than 500 years ago - the Greeks knew this and even did a pretty decent job of calculating the diameter 2000 years ago) we didn't throw out everything we knew about surveying, laying out building foundations, building roads, all the things that we do assuming there's such a thing as" level ground". Instead, we understood that "flat earth" is a good theory across distances of meters or kilometers, but if you're going to work with longer distances you have to allow for curvature. Airline pilots worry about the curvature of the earth, but taxi drivers don't.

    Likewise, Einstein did not "overthrow" Newton or prove Newtonian gravity wrong. On the contrary, we would reject relativity out of hand if it did not agree with Newtonian mechanics everywhere that Newtonian mechanics is applicable. It has to be that way; we have innumerable experiments that support Newton, so there's no question of Newton's theory being wrong but just whether we can further improve on it.

    There's an implication here that many non-scientists miss: The first step in making a revolutionary scientific advance is to thoroughly understand the existing science. The lone genius slaving away in magnificent isolation, free of any preconceived notions, is a myth.
     
  8. May 15, 2013 #7
    I only half agree with this statement. I think that "thoroughly understanding the existing science" before you start to build your own theories and models is a prescription for coming up with essentially the same theories and models that you have been conditoined to learn, with perhaps a little personal twist of your own that really doesn't further the field much, if at all. Think about it, there must be some kind of mathematical/physical framework where GR and QM get along, yet no professional scientist has a clue as to what it is. The key point is essentially all of these researchers see the problem in the exactly same way, and experience the exact same block in finding a resolution. Why? You guessed it, they all "thoroughly understand the existing science" in exactly the same way, because they learned the exact same curriculum!

    That's not to say that you shouldn't thoroughly understand the existing science at some point, you're eventually going to need to in order to couch your model 1) in relation to existing models, and more importantly 2) in relation to observational data. I'm just saying that there is something to be said for for a little naive creativity and a sense to "follow your nose" early on in the model building process. Use the observational data to build those models; beware of letting existing dogma use you. The problem, however, is that there is no grading protocol for independent geniuses. It's hard to measure or grade how well you're progressing toward the TOE unless you actually come up with it and publish it, which you might eventually, but how long is that going to take?

    The idea of institutes like Perimeter and advanced studies is great, and is a good "think tank" type model for exploring these deep basic research type areas. The only problem is that, by the time you get there, you're already indoctrinated with the standard dogma. What would be great is to create similar institutes or at least experimental collegiate programs whereby promising young scientists can utilize their naive creativity and nascent scientific skills to really "think outside the box." Don't hold your breath, though.
     
  9. May 15, 2013 #8

    pervect

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    Some mainstream reaction to Van Flandern's theories:

    Comment on “The Speed of Gravity”, Gerash et al, Physics Letters A Vol. 262, pp. 257-260(1999)
    http://www.gemarsh.com/archives/69

    "Aberration and the Speed of Gravity", Carlip,
    http://arxiv.org/abs/gr-qc/9909087

    In a nutshell, motion affects the apparent direction of light (this is called aberration). Theoretically it is expected that gravity waves will suffer a similar aberration, however we can't detect gravity waves yet (we're working on it), so we have no experimental data.

    If the topic of aberration isn't familiar to, the wiki article
    http://en.wikipedia.org/w/index.php?title=Aberration_of_light&oldid=554703282 isn't too bad.

    While aberration affects the direction of light, it does not, according to accepted mainstream theory, affect either the electrostatic force between two charges, nor the analogous gravitational "force" between two masses.

    So radiation (whether electromagnetic or gravitaional) is expecte to abberate, but the electrostatic and gravitational "central forces" are not expected to aberrate.

    This is fully in accordance with Maxwell's equations in the case of the electrostatic force.
     
  10. May 15, 2013 #9

    PeterDonis

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    You're assuming that this is the only way of thoroughly understanding the existing science. It isn't. Einstein didn't learn the existing science that way; yes, he went through the then-current version of the existing curriculum, but he didn't learn anything from it (IIRC he said after taking his final exam he couldn't even consider a physics problem for a year because the whole process had been so distasteful). He learned the existing science at his own pace and in his own way. But he did learn it; if he hadn't, if he had just started brainstorming without a superb understanding of the existing theories, I doubt he would have made the breakthroughs he did.

    Granted, that's just one example. But I'm not aware of *any* example of someone who came up with a viable new theory without a thorough understanding of the old theory--someone who just somehow brainstormed a viable new theory and only afterwards checked up to see how it differed from the old one. Do you have any such examples?
     
  11. May 15, 2013 #10

    Dale

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    I am also not aware of any such person. I doubt such a person exists.
     
  12. May 15, 2013 #11

    Dale

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    Every single major breakthrough of which I am aware contradicts this assertion, and I am not aware of any example which supports it. It sounds nice, but I think it is counter factual.
     
  13. May 15, 2013 #12
    DaleSpam:

    I think Dirac is trying to say that one needs to understand the current theories, but regard them as possibly being incomplete. And it's not to say that they are. I met an astrophysicist the other day at a local joint, and it amazed me by how little he refused to question anything he was talking about. Talked about it all as it as if it's all completely unfalsifiable. I think you and Donis are correct--you can't improve theories until you understand them--but I think Dirac is just saying that you should take a strong-minded approach to the theories and say, "Did Einstein or whomever perhaps miss something?" And I don't think there is anything wrong with that. You may never get anywhere--or be this great and revolutionary scientist--but there's nothing wrong with looking at the establishment (established science) with a questioning eye.
     
  14. May 15, 2013 #13

    Fredrik

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    It sounds like you have misunderstood a lot. The "establishment" already understands the limits of the best theories very well. (None of the best theories is perfect; this is known for sure). If an astrophysicist "refused to question anything", then maybe it was because you were questioning things that had been confirmed by countless experiments.
     
    Last edited: May 15, 2013
  15. May 15, 2013 #14

    Dale

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    All of the scientists I know are well aware of the incompleteness of current theories, and more importantly the incompleteness of the current experimental data. There may be exceptions, but the scientific community as a whole is aware.
     
  16. May 16, 2013 #15
    My point is not that it isn't essential to know the existing science in a field to make a breakthrough, of course its essential. I didn't mean to infer that. I just think that more independent thought should be encouraged in school, and its not, in my experience. What's rewarded is conformity to learning the existing models. I think the result of that is a scholastic model where you get everyone "corraled" down the same path instead of letting ideas spread out a bit in students where those ideas are still fresh. That's all.
     
    Last edited: May 16, 2013
  17. May 16, 2013 #16

    Dale

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    That seems reasonable to me.
     
  18. May 16, 2013 #17

    PeterDonis

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    I agree, but I'm not sure school can do otherwise. The whole premise of school is that a lot of people all receive the same instruction; otherwise it doesn't scale. But to encourage independent thought, everyone needs to learn in their own way, at their own pace, and whatever instruction they receive has to be individualized to them.

    In fact, the very idea of learning by receiving instruction doesn't seem very compatible with developing independent thought. Feynman once said, "What I cannot create, I do not understand." Learning by receiving instruction tends to deter people from creating things for themselves; they get used to being told things instead of figuring them out.

    So I think the solution, if there is one, won't be found by trying to get school to encourage independent thought. I think it will come from getting more people to understand that school is not a good way of encouraging independent thought, so that they look for other alternatives.
     
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