Gravity:general question.

Discussion in 'Astronomy, Exobiology, & Cosmology' started by Snoopd0ug, Feb 18, 2011.

  1. Dywyddyr Penguinaciously duckalicious. Valued Senior Member

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    They have no rest mass (well actually we haven't experimentally confirmed that, but the absolute upper limit is extremely small).
    And photons travel in spacetime - which is "bent" by gravity. If the photon follows bent spacetime how else could it move other than in a curve?
    http://en.wikipedia.org/wiki/Gravitational_lens
     
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  3. Vern Registered Senior Member

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    I can't argue with that.

    I wonder if anyone has pondered why it is that gravity diminishes with the square of distance while volume away from a sphere increases with the cube of distance. If we think of little gravitons coming from a source, why wouldn't the quantity per unit volume decrease as the cube instead of square?

    I wonder about that.
     
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  5. NMSquirrel OCD ADHD THC IMO UR12 Valued Senior Member

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    i know current thinking does not associate gravity with magnetism,
    but i can't help but wonder if the two are related somehow..
    both attract things to it,
    both loose energy the farther you are away from it,

    have magnets been tested in space to see if it draws in objects with higher mass faster or slower than lesser masses?
    IOW what effect does magnetism have on mass, separated from gravity?
     
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  7. Dywyddyr Penguinaciously duckalicious. Valued Senior Member

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    One major difference is that magnets have two poles (let's leave monopoles out of it

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    ). And that like poles reject each other. So far gravity appears to be entirely attractive to everything. We haven't found any thing or any way that gravity repels.

    Do you know I can't remember seeing anything at all on magnets in space, but I'd say (and I'm sure someone will jump in and correct me) that the higher the mass the slower it will be pulled, since the attractive force from a magnet is a fixed value for that magnet. So we're back to F=MA.
     
  8. KilljoyKlown Whatever Valued Senior Member

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    Actually in another forum I used the same argument you are using and was told gravity is what's bending the photons and was then referred to a couple of web sites that convinced me photons do indeed have mass that can be acted on by gravity.

    I was trying to argue that the reason photons can't escape from a black hole is not because of the gravity, but because spacetime is so warped that light can only travel in a circle around the black hole once it crosses the event horizon. The response I got was that sense it can't be proved one way or the other, you can only assume gravity is the reason and not some theory about the curvature of spacetime.

    But if the truth be told I really haven't given up on the properties of curved spacetime if someone can supply some good arguments that I can use.
     
  9. Dywyddyr Penguinaciously duckalicious. Valued Senior Member

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    Maybe they're actually the same answer expressed in two different ways.
    After all the "bending of spacetime" argument also applies to planets and everything, no?
    And using that argument lets us continue to essentially consider a photon as massless.

    I've just Googled again and got a selection of both answers, but the NASA site (pfft wadda they know about space and stuff?) says:
    But then gets sort of "iffy" with
     
  10. KilljoyKlown Whatever Valued Senior Member

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    Yes but sometimes a point of view makes a great deal of difference in how you understand interactions. For instance isn't it said that the reason nothing with mass can ever reach the speed of light is that as it approaches the speed of light it gains mass exponentially so that no amount of energy will ever be able to accelerate it to the speed of light. By that reasoning I wouldn't expect a photon to have any mass. Yet gravity does bend light and gravity only acts on mass, which would tend to support photons have mass.
     
  11. Dywyddyr Penguinaciously duckalicious. Valued Senior Member

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    Gravity acts on spacetime too.

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  12. NMSquirrel OCD ADHD THC IMO UR12 Valued Senior Member

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    a unified theory?

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    force=Mass x velocity ?
     
  13. KilljoyKlown Whatever Valued Senior Member

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    If you say that gravity is a result of warped spacetime, then how is it that gravity can act on spacetime? Again that is a point of view question, but I think you can see where I'm coming from on this issue.

    It seems to me mass is acting on spacetime and the effect is gravity.
     
  14. NMSquirrel OCD ADHD THC IMO UR12 Valued Senior Member

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    looks like it..
    i have always wondered what (if any) effect spin has on gravity.
     
  15. James R Just this guy, you know? Staff Member

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    KilljoyKlown:

    There's always room for new discoveries in science. Whenever a scientist says "X is the case", they always mean "As far as the available evidence goes, it appears most probable that X is the case."

    A couple of other points in passing: sub-atomic particles and neutrinos are matter. Neutrinos have mass - that is confirmed. Photons do not have (rest) mass.

    Nobody is sure what generates the property we call mass, though there are a few unconfirmed theories out there about that.

    None of this changes the answer to the original question "What causes gravity?". Answer: mass and energy (in some respects mass is energy).

    The reason we don't have an answer that question yet is that we don't have a tested theory of quantum gravity yet, and we need that to properly analyse black holes.

    If you want to avoid general relativity as much as possible, you can under some limited circumstances analyse photons as if they have mass. But that approach doesn't really work very well. For example, if you try to calculate the bending of light by massive objects it gives an answer that is a factor of two too small. The best way to deal with light and gravity is to use general relativity with massless photons and curved spacetime.

    It's actually worse than that. Light can only move towards the centre of the hole once it crosses the event horizon. Travelling in a circle at constant radius is not an option inside the event horizon.
     
  16. tashja Registered Senior Member

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    I've read somewhere that gravity couples not to mass (rest mass), but to energy(stress energy tensor) of any kind. To which rest mass is just one contribution. Is that wrong?
     
  17. James R Just this guy, you know? Staff Member

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    Saquist:

    Matter and energy determine gravity, as far as we know. There is nothing else. Dark matter is a (weird) form of matter. Dark energy is a (weird) form of energy.

    When did he appoint you as his spokesman. He can tell me himself what he's looking for. He doesn't actually need you.
     
  18. James R Just this guy, you know? Staff Member

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    No. That is correct.
     
  19. KilljoyKlown Whatever Valued Senior Member

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    How is it possible to know anything about what happens on the other side of an event horizon? I know they always picture a black hole as a point of mass in a very deep gravity well and I assume the event horizon is somewhere at the top of the gravity well picture? But I've heard that everything on the other side of the event horizon is considered the black hole and there is no real concept of a solid surface any distance beyond the event horizon. Is this correct or can you paint a better picture for me?
     
  20. James R Just this guy, you know? Staff Member

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    KilljoyKlown:

    In general relativity, the black hole spacetime geometry is described mathematically. Outside the hole, the geometry is the same as for any other massive body (e.g. the Sun or the Earth), and we assume the same geometry continues inside the event horizon. The Schwarzschild radius of the hole is the boundary of the event horizon, and is the radius beyond which the escape velocity from the hole becomes faster than the speed of light.

    However, in terms of the spacetime geometry of a black hole, the event horizon is nothing special. If you were actually in a spaceship crossing the event horizon of a black hole, you wouldn't be aware of the exact point at which you crossed the horizon. There's no sudden change in spacetime at the horizon. It is only a convention to define the size of a black hole by its event horizon. We have to define the size somehow because there's "empty space" everywhere inside and outside of the hole, except right at the centre.

    While the event horizon is not special from the point of view of anything falling into the hole, it is special from the point of view of an observer a long way outside the hole. Such an observer can't see anything inside the horizon (because no light can escape) - hence the term "black hole". And nothing that crosses the horizon can ever emerge again from the hole (ignoring Hawking radiation, which is a bit of a cheat).

    Not sure how much this helps...
     
  21. Dywyddyr Penguinaciously duckalicious. Valued Senior Member

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    Yes. Which means that, given that the force is a fixed value then a larger mass will experience a smaller acceleration than a small mass.
    A magnet pulls a light object faster than it does a heavy one.
     
  22. Dywyddyr Penguinaciously duckalicious. Valued Senior Member

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    Yes. I worded that incredibly badly.

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    Gravity itself is the warping of spacetime due to mass. (Or can be considered as such).
     
  23. KilljoyKlown Whatever Valued Senior Member

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    I'm also not sure how much it helps but it is the best description I've heard to date, and I also like to ignore Hawking radiation. This might be a little off subject, but can you tell me why we only have 4 states of matter when we have neutron stars and black holes that should be considered separate states of matter?
     

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