# Gravity never zero

Discussion in 'Astronomy, Exobiology, & Cosmology' started by Ivan, Dec 18, 2011.

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1. ### AlexGLike nailing Jello to a treeValued Senior Member

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Bibbity, minimal and zero are different things.

3. ### Robittybob1BannedBanned

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I'll have to read up about the Planck limits on time, distance and mass again, for I thought they were the limits to minimal.

5. ### Robittybob1BannedBanned

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Gravity is so basic but so difficult to pin down.
Newton had a go and came up with the Law of Universal Gravitation
Einstein had a go and explained it as Curved Space Time.
And then there is Quantum Gravity with its graviton (and I'm about to look at that)

But when reading "Newton's law of universal gravitation" http://en.wikipedia.org/wiki/Law_of_universal_gravitation
There is plenty of room for error as the constant "G" used in the formula for gravitational attraction has a high level of uncertainty associated with it.

Gravitational constant
http://en.wikipedia.org/wiki/Gravitational_constant

"with relative standard uncertainty 1.2×10−4, or 1 part in 8300."

What this ultimate uncertainty means is beyond me as I am not great with maths.

7. ### RJBeeryNatural PhilosopherValued Senior Member

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AlexG: the map is not the territory. Just because the math suggests that gravity never goes to zero between two bodies, that does not preclude the possibility. Isn't that one of the consequences of quantum gravity? Are you suggesting that you are certain quantum gravity does not exist?

8. ### AlexGLike nailing Jello to a treeValued Senior Member

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The math doesn't 'suggest'. It states. And in every realm we know so far, it works, without deviation.

Gravity going to zero is NOT a feature of quantum gravity.

If you want to show that the math is wrong, you actually have to show it. Innuendo, implication and wishful thinking don't make it.

9. ### Robittybob1BannedBanned

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I read that the Universal Gravitational formula did not fully account for Mercury's precession and the amount of gravitational bending of light. So it has had it's draw backs.

10. ### TrippyALEA IACTA ESTStaff Member

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And yet GR becomes indistinguishable from it, within certain limits (for example, the orbit of any planets in our solar system that aren't mercury) so it can't be all bad.

11. ### Robittybob1BannedBanned

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Considering it was discovered 400 odd years ago, it was a major advancement in maths and science, but could it start to breakdown at the extremes? Supermassive Black Holes formed Early in the formation of the Universe (Billions of Solar Masses) maybe the clue as to gravitational attraction at large distances. Are these still sucking up galaxies.
"Early Black Holes Grew Big Eating Cold, Fast Food
Largest Cosmological Simulation To-Date Explains How Supermassive Black Holes Came Into Existence Shortly After the Big Bang"

http://www.cmu.edu/mcs/news/pressreleases/2011/12_7_MassiveBlack.html

12. ### RJBeeryNatural PhilosopherValued Senior Member

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Well, first of all, I'm not sure why you seem to be all frothy-mouthed about this. Secondly, the math states that gravity never goes to zero, which suggests that it never goes to zero in the "real world". Are we discussing the real world, or the math? Because that's what I meant by "the map is not the territory". Pointing out in the math where gravity goes to zero is irrelevant because it presumes that the math perfectly represents gravity's influence in the real world and we already know that this is not the case.

The rest of your comment is quite odd. As Robbity pointed out, reality's deviation from the math is quite measurable (with Mercury's precession or fact that galactic spiral arms seem to spin faster than expected). Lastly, I'm pretty sure quantum gravity would allow, in theory, for zero influence at a finite distance between masses (think about it: a gravitons are transferred at a given distance or they are not).

Not that any of this matters, really, just pointing out that you seem to be awfully adamant on subjects that probably do not deserve such confidence.

13. ### TrippyALEA IACTA ESTStaff Member

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Let's talk about the Graviton, shall we?

We know from experimentation that gravity propagates at c.
As far as we know, only massless particles can propagate at c, therefore the graviton must be massless.
As far as we know, the range of a force is governed by the mass of its carrying particle, and is determined by the uncertainty principle $\Delta E \Delta t \approx mc^2 \Delta t > \frac{\hbar}{2}$.
This is, again, supported by experimental evidence, and in turn tells us that the range of a massless force carrying particle is infinite.

So you see? It's not just speculation that makes us believe that the range of gravity is infinite, it's the conclusion that all of the available evidence leads us to.

14. ### RJBeeryNatural PhilosopherValued Senior Member

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Trippy, that's interesting (and probably correct). However, the discrete nature of the graviton along with its radial propagation from an originating mass would mean that there would certainly be pockets of space unaffected by the "field", and therefore could be considered "zero gravity"...but not at all times if there were any sort of continuous randomization in the direction of graviton emission.

15. ### TrippyALEA IACTA ESTStaff Member

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Again, experimental evidence says otherwise.

First, don't forget about wave-particle duality - the graviton is where precisley?

Second, the upper limit on graviton emission is what, precisely?

16. ### TrippyALEA IACTA ESTStaff Member

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Lemme put it another way - is there a space between magnetic field lines?

17. ### Robittybob1BannedBanned

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So are you saying gravity waves proves gravitons rather than a static field. If gravitons were emitted continuously, even if they were massless would they have energy? So do they stay like an attached ray extending out into space ?
Extending into space as a field 360 * 360 around a particle with mass or as a single line, in any random direction? Any ideas Trippy or RJ?

PS: With Einstein's theory of curved Space Time if the degree of displacement of the Space Time due to a mass is less than 1 Planck Length I wonder if you can say Space Time has moved at all in other words it is as good as zero.

Alex at what distance from a Neutron would the curvature of Space time be less than 1 planck length? Who's good at maths?

Last edited: Dec 20, 2011
18. ### OnlyMeValued Senior Member

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I don't believe this is an accurate statement.

We have not detected any gravity waves, yet. They remain hypothetical or theoretical, without either experiemntal or experiential confirmation, again yet.

The limitation of the speed of gravity to c, remains an assumption of relativity. As far as I am aware, we have no way to actually test this. To model the orbits of moons, planets, even galaxies (within galactic clusters), we must assume the instantaneous position of the gravitating masses involved, not their time of light delayed positions. This suggests an instantaneous action....

Newton explained this by defining gravity, as an instantaneous attraction between two masses. Einstein, uncomfortable with instantaneous action at a distance, described it as a curvature of space.., a gravitational field, whose focal point in any inertial system, is always the instantaneous position of the gravitating mass.

It is only changes to gravitational interaction, that is limited to the speed of light restrictions of relativity. Within the context of experience there is only one situation where it would even be possible to detect, a changing gravitational field. That would be associated with an accelerating mass. Unfortunately, all such masses we are exposed to, are far too small and moving to slow to measure the changing influence with any degreee of certainty.

I do believe that changes to a gravitational filed propagate at or less than the speed of light. We just have no, observational evidence, of any gravitational source, which is either created or destroyed/dissipated, within a time scale that, any change in the gravitational field can be measured.

In theory, a binary system composed of two black holes, perhaps even neutron stars, or the equivalent, should produce detectable gravity waves. We are looking, but as of yet have not confirmed, this.

This is accurate! Everything within our ability to observe and measure, suggests that gravitational interactions between gravitationally significant masses, has no limitation defined by the distance separating the two masses.

Given that we cannot assume that we are at the center of the universe, this could also imply that there are parts of the universe, beyond the sphere of our ability to observe, for which we are not yet subject to any gravitational interaction. Assuming that gravity does propagate at the speed of light, any gravitationally significant part of the universe that lies further from us, than the age of the universe, in light years, would also lie beyond the point in space where gravity "from there to here", would have had time to travel.

19. ### RJBeeryNatural PhilosopherValued Senior Member

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I understand what you're saying but if a continuous gravity field exerted a continuous influence then the very concept of quantum carrier particles is meaningless. Put another way, the field is only continuous when we're not discussing gravitons, OR when we're discussing scales such that we can treat it as "essentially continuous". We cannot do that here, because we're talking about any arbitrarily distant point away from a mass, which allows us to choose an arbitrarily small scale of influence. Now, if you want to argue that we have not yet proven the existence of gravitons I cannot disagree; my point to AlexG was that he was speaking in an overly-authoritative manner on a subject that many Physicists still feel has some ambiguity.

Disclaimer: if I'm inadvertently defending someone's pet theory by pointing out that there are potentially areas in the Universe free from gravitational influence from a given mass, I am not necessarily endorsing that theory.

20. ### Robittybob1BannedBanned

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I like the idea of a graviton but it is just the mechanism of it's action I don't seemed to have comprehend yet.
They must spread out from all mass, and in a way the strength obeys an inverse square relationship. What happens at the quantum level and at extreme distances in particular is fascinating to me.

21. ### TrippyALEA IACTA ESTStaff Member

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There's the work of Kopeikin et al which suggests that gravity propagates at a velocity within 20% of c (they made the measurements using Jupiter and it's effects on a quasar (there is, it seems, some controversey over this is to whether they actually measured c instead).

There's also the Hulse-Taylor binary systerm, which suggests that it propagates at c to within 1%.

Because static fields are not subjected to abberation, it's also true of an electrostatic field.

And lorentz invariance means that what is seen by a moving body emitting a static field, and a body moving through a static field, and since a stationary object emitting a static field sees no abberation, no abberation is seen by a moving object in a static field, and we come to the conclusion regarding instantaneous position.

Yes we do, the work of Kopeikin, and the measurements of the Hulse-Taylor binary system (which, incidentally, also gives us evidence for gravitational waves, even though we have yet to measure them directly).

We have, by measuring their rate of orbital decay.

No it isn't, because the wave-particle dual nature of particles means that if a particles wave function predicts it has a probability of being anywhere in a given volume of space, it will behave as if it is everywhere in that given volume.

The symetrical structure of Benzene alone proves this.

Again, you're forgetting about the dual nature of gravitons. They can behave as a wave and a particle. A wave can exist everywhere simultaneously, and a particle can behave as if it is everywhere.

22. ### TrippyALEA IACTA ESTStaff Member

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Again, discussion of whether or not they actually exist is irrelevant to a discussion of what properties they must have if they do.

23. ### wlminexBannedBanned

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. . . "Whether or not they (sic, gravitons) actually exist??" . . . if they don't exist . . . a discussion of their imaginary properties is a moot and ludicrous point!