How does gravity work?

How does science work? And what's all this about quantum mechanics?

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PauloL
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Re: How does gravity work?

Post by PauloL »

uwot wrote: Mon Aug 28, 2017 10:16 pm Well, all I really mean is that, on the intergalactic scale, the measurement of red-shift implies that there is a force (dark energy) which overcomes the force of gravity.
But red shift implies expansion. Doesn't gravity need to expand too for the red shift to be there? If only space expanded, why should there be a red shift? At least there must be some kind of interaction between space and gravity, don't think so?
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Re: How does gravity work?

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PauloL wrote: Mon Aug 28, 2017 10:51 pmBut red shift implies expansion. Doesn't gravity need to expand too for the red shift to be there? If only space expanded, why should there be a red shift?
The simplest explanation, at least in terms of established physics, is the Doppler effect (I'm sure you know what that is, but it's on page 8 on the blog).
PauloL wrote: Mon Aug 28, 2017 10:51 pmAt least there must be some kind of interaction between space and gravity, don't think so?
The model in the blog is premised on the idea that gravity is a product of the 'density' of space, so according to that model, yes. But there are many different ideas about how gravity works.
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PauloL
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Re: How does gravity work?

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uwot wrote: Mon Aug 28, 2017 11:10 pm The simplest explanation, at least in terms of established physics, is the Doppler effect (I'm sure you know what that is, but it's on page 8 on the blog).
Sure, red shit results from Doppler effect, but the question remains. If only space expanded, not gravity, there should be no red shift, or Doppler effect if you prefer.
The model in the blog is premised on the idea that gravity is a product of the 'density' of space, so according to that model, yes. But there are many different ideas about how gravity works.
But here, if expansion changes 'density' of space and gravity is a product of this, then expansion of space must alter gravity. Don't think so?
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Re: How does gravity work?

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PauloL wrote: Mon Aug 28, 2017 11:16 pm
uwot wrote: Mon Aug 28, 2017 11:10 pm The simplest explanation, at least in terms of established physics, is the Doppler effect (I'm sure you know what that is, but it's on page 8 on the blog).
Sure, red shit results from Doppler effect, but the question remains. If only space expanded, not gravity, there should be no red shift, or Doppler effect if you prefer.
Sorry, PaoloL, I don't see why that should be so. In the simplest explanation, Doppler is just an inescapable consequence of the relative motion of two objects. Waves are compressed if two objects are moving towards each other, like the Milky Way and Andromeda, the light from which is blue-shifted, and stretched when they are receding; as is the case with every galaxy outside the local group. The effect is the same if you attribute the expansion of space to the stretching of spacetime; it just means that any waves within it are stretched with it-a bit like running your finger down the fretboard. (In effect, that is entropy. Eventually the 'guitar string' is so long that, while it's still vibrating, the energy is so spread out that it is useless-heat death.)
PauloL wrote: Mon Aug 28, 2017 11:16 pm
The model in the blog is premised on the idea that gravity is a product of the 'density' of space, so according to that model, yes. But there are many different ideas about how gravity works.
But here, if expansion changes 'density' of space and gravity is a product of this, then expansion of space must alter gravity. Don't think so?
Well, whatever model is used, gravity decreases with distance, but I'm not sure if you are referring to some 'background' gravity that may be a result of the energy provided by quantum fluctuations in the 'vacuum', or some such mechanism.
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Re: How does gravity work?

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Universe does not "expand", it grows/condenses (from the another phase:)
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Re: How does gravity work?

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Cerveny wrote: Tue Aug 29, 2017 11:57 am Universe does not "expand", it grows/condenses (from the another phase:)
Who knows? One of the assumptions that most physicists make, is that the laws we observe on Earth are the same ones that apply across the universe. Given that the Doppler effect is easily demonstrable, and can be used to measure the rotation and angular momentum of planets we have sent probes to, it is not an unreasonable conjecture to suppose that the spectral shift we observe in galaxies outside the Milky Way, is caused by the same mechanism we have measured to high accuracy in our own solar system.
You can make up any story you like about regions of space we will never visit, but is there any experiment we can do on this planet that supports your claim?
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Hobbes' Choice
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Re: How does gravity work?

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uwot wrote: Mon Aug 28, 2017 6:14 pm
Hobbes' Choice wrote: Mon Aug 28, 2017 5:46 pmThose who insist on a distinct force of "gravity", are suggesting a teleology. My take on it is quite the opposite.
In fact your language demands that teleology. When you say "we don't understand the mechanism", you are implying that there is a distinct mechanism whose purpose is to make things fall.
Well, I wouldn't claim there is any purpose. What we know is, as you said, "Matter has a tendency to move towards other matter", although on cosmic scales, we rather think that matter has a tendency to move away from other matter. Still, on Earth at least, things fall to the ground. As far as I can tell your response is 'They just do.' Fair enough. Some physicists are instrumentalists and don't particularly care if there is a mechanism; Newton famously would frame any hypotheses. Others, including Einstein, think it might be worthwhile trying to work out whether there is.
Hobbes' Choice wrote: Mon Aug 28, 2017 5:46 pmI'm simply saying that the entire idea of "gravity" is a human interested narrative in which the universe is seen as a designed set of interactions that have a job to do.
Gravity is just the name given to the force that demonstrably exists between two or more bodies. If ever a cause of gravity is discovered, I don't imagine the name will change.
hypothesis non fingo is probably the greatest lie in history. His hypothesis was God.
Otherwise I think we are basically on the same page, except that your idea of a "single" cause is bogus. Causes are just human interested instruments in the maelstrom of the multiplicity of the necessity.
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Re: How does gravity work?

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My comments were generated by UWOT when they said that until the expansion of the universe was found to be accelerating, most people agreed with Einstein that gravity was unlimited.

Let's suppose the Universe expands 1 meter right now. Doesn't this affect gravity? If so, space must interact with gravity, otherwise, if only the current bodies existing in Universe were responsible for gravity, why would it change when adding space? But it does change, that's why we can observe red shift (or Doppler effect if you prefer).

My question is: how does space affect gravity between bodies in the Universe?

I don't know if I make myself clear, this is a bit confusing, but let me know.
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Re: How does gravity work?

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Hobbes' Choice wrote: Wed Aug 30, 2017 9:00 pm hypothesis non fingo is probably the greatest lie in history.
I don't think you mean that.
Hobbes' Choice wrote: Wed Aug 30, 2017 9:00 pmHis hypothesis was God.
Well god, in Newton's view, was the cause of everything, and while he would step in from time to time, to make adjustments to planetary orbits, generally god was out of the picture. If god was responsible for gravity, Newton couldn't work out how he did it.
Hobbes' Choice wrote: Wed Aug 30, 2017 9:00 pmOtherwise I think we are basically on the same page, except that your idea of a "single" cause is bogus.
I get Hume's critique of causality, but 'cause', in the sense that Hume meant, is not the same as mechanism. You are free to believe that the planets are kept in place by magic, if it pleases you; personally, I think 'How?' is a legitimate question.
Hobbes' Choice wrote: Wed Aug 30, 2017 9:00 pmCauses are just human interested instruments in the maelstrom of the multiplicity of the necessity.




What?
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Re: How does gravity work?

Post by uwot »

PauloL wrote: Wed Aug 30, 2017 9:35 pmMy comments were generated by UWOT when they said that until the expansion of the universe was found to be accelerating, most people agreed with Einstein that gravity was unlimited.
That's not quite what I said. The point I was making is not that gravity is limited, rather that there appears to be another force which, at huge distances, becomes stronger than the force of gravity. 'Dark energy' is pushing galaxies apart, more than gravity is keeping them together.
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Re: How does gravity work?

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uwot wrote: Thu Aug 31, 2017 2:02 am
Hobbes' Choice wrote: Wed Aug 30, 2017 9:00 pm hypothesis non fingo is probably the greatest lie in history.
I don't think you mean that.
Hobbes' Choice wrote: Wed Aug 30, 2017 9:00 pmHis hypothesis was God.
Well god, in Newton's view, was the cause of everything, and while he would step in from time to time, to make adjustments to planetary orbits, generally god was out of the picture. If god was responsible for gravity, Newton couldn't work out how he did it.
Hobbes' Choice wrote: Wed Aug 30, 2017 9:00 pmOtherwise I think we are basically on the same page, except that your idea of a "single" cause is bogus.
I get Hume's critique of causality, but 'cause', in the sense that Hume meant, is not the same as mechanism. You are free to believe that the planets are kept in place by magic, if it pleases you; personally, I think 'How?' is a legitimate question.
Hobbes' Choice wrote: Wed Aug 30, 2017 9:00 pmCauses are just human interested instruments in the maelstrom of the multiplicity of the necessity.
What?
My objection to your idea of cause is not to suggest there is no such thing, but that it can be so easily reduced to a SINGLE "magic" cause as you seem to think.
All science can do is describe what it seems and nominates the phenomena that we can observe. Causality is not a simple chain.
Read again my last sentence I think you can get it.
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Hobbes' Choice
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Re: How does gravity work?

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uwot wrote: Thu Aug 31, 2017 2:13 am
PauloL wrote: Wed Aug 30, 2017 9:35 pmMy comments were generated by UWOT when they said that until the expansion of the universe was found to be accelerating, most people agreed with Einstein that gravity was unlimited.
That's not quite what I said. The point I was making is not that gravity is limited, rather that there appears to be another force which, at huge distances, becomes stronger than the force of gravity. 'Dark energy' is pushing galaxies apart, more than gravity is keeping them together.
oooooh, spooky magical forces!!!
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Re: How does gravity work?

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Hobbes' Choice wrote: Thu Aug 31, 2017 11:37 amMy objection to your idea of cause is not to suggest there is no such thing, but that it can be so easily reduced to a SINGLE "magic" cause as you seem to think.
It is fairly obvious that the observable behaviour of matter is the result of all the different forces which act on it.
Hobbes' Choice wrote: Thu Aug 31, 2017 11:37 amAll science can do is describe what it seems and nominates the phenomena that we can observe.
Quite. All we know for certain is that forces exist; we know this by observing the behaviour of objects in any particular field of force. What is less certain is how any given force is mediated. It is conceivable that two or more mechanisms are responsible for a single force, but until the evidence compels us to do otherwise, it is reasonable to start by trying to discover one.
Hobbes' Choice wrote: Thu Aug 31, 2017 11:37 amCausality is not a simple chain.
Again; causality is not the same as mechanism. As above, a variety of mechanisms may be responsible for the behaviour we observe; it doesn't follow that there is more than one mechanism responsible for any of the individual forces we measure, and which together account for the behaviour we observe.
Hobbes' Choice wrote: Thu Aug 31, 2017 11:39 amoooooh, spooky magical forces!!!
No more than any other force.
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Re: How does gravity work?

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uwot wrote: Thu Aug 31, 2017 2:13 am 'Dark energy' is pushing galaxies apart, more than gravity is keeping them together.
What do you call 'Dark energy'?
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Re: How does gravity work?

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PauloL wrote: Thu Aug 31, 2017 1:15 pmWhat do you call 'Dark energy'?
It's just the name given to whatever is making the expansion of the universe accelerate. No one knows what it actually is, but my guess is on page 27 of my blog. Or the last page before the 'How does gravity work?' section. (I'll have to put page numbers in.)
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