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Gravity?

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Re: Gravity?
Post by Randomiser   » Thu Jan 21, 2016 6:57 am

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The immediately important parts of Newton's work for Charis would be his 3 laws of motion which underlie all classical mechanics, plus calculus with algebra as a necessary precursor. The astronomical parts of Principia, if that is what Dr Maklyn first got, will be of little immediate use in disproving the Ptolomaic theory of the Universe on Safehold because all the observations on which it was based refer to our solar system and therefore are uncheckable and may as well be a fairy tale, as far as everyone outside the Inner Circle is concerned, unless and until corresponding observations can be made in Safehold's system.
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Re: Gravity?
Post by Keith_w   » Thu Jan 21, 2016 7:57 am

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I am still trying to get my head around the statement "The speed of Gravity". I am pretty sure that gravity is an effect of mass, and therefore has no speed in and of itself. Any "speed" it may have is related to the speed of the mass itself. However, I don't want to start a religious argument...
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Re: Gravity?
Post by phillies   » Thu Jan 21, 2016 12:12 pm

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Keith_w wrote:I am still trying to get my head around the statement "The speed of Gravity". I am pretty sure that gravity is an effect of mass, and therefore has no speed in and of itself. Any "speed" it may have is related to the speed of the mass itself. However, I don't want to start a religious argument...


If a moving giant planet passes nearby the earth, so we are drawn up into the sky, in precisely which direction to we fall up, if the giant planet is accelerating. The current direction to the planet? The direction in which we see the planet, that being where it was a while ago thanks to speed of light delay. Yes, it matters that the planet is accelerating.

That's the speed of gravity.
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Re: Gravity?
Post by Silverwall   » Thu Jan 21, 2016 3:04 pm

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Keith_w wrote:I am still trying to get my head around the statement "The speed of Gravity". I am pretty sure that gravity is an effect of mass, and therefore has no speed in and of itself. Any "speed" it may have is related to the speed of the mass itself. However, I don't want to start a religious argument...


What people mean by the speed of gravity is this:

If the sun suddenly vanished which would we notice here on earth first? the lack of light or the sudden planetary jolt as our orbit changed?

All current know physics says that if the sun suddenly vanished we would not notice the change in it's gravity until 8 minutes later once the pulse of no gravity had crossed space to us aka at the same time we noticed that someone had turned out the lights. You can see the same effect in a dropped extened slinky spring where the base coil does not move until the top has contracted onto it. See https://www.youtube.com/watch?v=rCw5JXD18y4 for a visual of this effect.

AKA the speed of gravity = the speed of light. T
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Re: Gravity?
Post by Keith_w   » Thu Jan 21, 2016 6:33 pm

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phillies wrote:
Keith_w wrote:I am still trying to get my head around the statement "The speed of Gravity". I am pretty sure that gravity is an effect of mass, and therefore has no speed in and of itself. Any "speed" it may have is related to the speed of the mass itself. However, I don't want to start a religious argument...


If a moving giant planet passes nearby the earth, so we are drawn up into the sky, in precisely which direction to we fall up, if the giant planet is accelerating. The current direction to the planet? The direction in which we see the planet, that being where it was a while ago thanks to speed of light delay. Yes, it matters that the planet is accelerating.

That's the speed of gravity.


You would, along with the remnants of the planet upon which we are standing, be pulled along in the wake of your hypothetical giant planet.
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A common mistake people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools.
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Re: Gravity?
Post by Keith_w   » Thu Jan 21, 2016 7:04 pm

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Silverwall wrote:
Keith_w wrote:I am still trying to get my head around the statement "The speed of Gravity". I am pretty sure that gravity is an effect of mass, and therefore has no speed in and of itself. Any "speed" it may have is related to the speed of the mass itself. However, I don't want to start a religious argument...


What people mean by the speed of gravity is this:

If the sun suddenly vanished which would we notice here on earth first? the lack of light or the sudden planetary jolt as our orbit changed?

All current know physics says that if the sun suddenly vanished we would not notice the change in it's gravity until 8 minutes later once the pulse of no gravity had crossed space to us aka at the same time we noticed that someone had turned out the lights. You can see the same effect in a dropped extened slinky spring where the base coil does not move until the top has contracted onto it. See https://www.youtube.com/watch?v=rCw5JXD18y4 for a visual of this effect.

AKA the speed of gravity = the speed of light. T


Actually, it is the speed of light in a vacuum, since the speed of light varies with the medium through which it is travelling. More succinctly, the speed of gravity is c, which is the fastest anything can go.

From the speed of gravity article in Wikipedia:
The speed of gravitational waves in the general theory of relativity is equal to the speed of light in vacuum, c.[1] Within the theory of special relativity, the constant c is not exclusively about light; instead it is the highest possible speed for any interaction in nature. Formally, c is a conversion factor for changing the unit of time to the unit of space.[2] This makes it the only speed which does not depend either on the motion of an observer or a source of light and/or gravity. Thus, the speed of "light" is also the speed of gravitational waves and any other massless particle. Such particles include the gluon (carrier of the strong force), the photons that make up light, and the theoretical gravitons which make up the associated field particles of gravity (however a theory of the graviton requires a theory of quantum gravity)The speed of gravitational waves in the general theory of relativity is equal to the speed of light in vacuum, c.[1] Within the theory of special relativity, the constant c is not exclusively about light; instead it is the highest possible speed for any interaction in nature. Formally, c is a conversion factor for changing the unit of time to the unit of space.[2] This makes it the only speed which does not depend either on the motion of an observer or a source of light and/or gravity. Thus, the speed of "light" is also the speed of gravitational waves and any other massless particle. Such particles include the gluon (carrier of the strong force), the photons that make up light, and the theoretical gravitons which make up the associated field particles of gravity (however a theory of the graviton requires a theory of quantum gravity)The speed of gravitational waves in the general theory of relativity is equal to the speed of light in vacuum, c.[1] Within the theory of special relativity, the constant c is not exclusively about light; instead it is the highest possible speed for any interaction in nature. Formally, c is a conversion factor for changing the unit of time to the unit of space.[2] This makes it the only speed which does not depend either on the motion of an observer or a source of light and/or gravity. Thus, the speed of "light" is also the speed of gravitational waves and any other massless particle. Such particles include the gluon (carrier of the strong force), the photons that make up light, and the theoretical gravitons which make up the associated field particles of gravity (however a theory of the graviton requires a theory of quantum gravity).

https://en.wikipedia.org/wiki/Speed_of_gravity
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Re: Gravity?
Post by Hildum   » Thu Jan 21, 2016 8:13 pm

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If you really want something mind blowing, consider that photons from the cosmic background radiation coming to our detector on earth 14.x billion years later do not experience time at all - they simply come into existence, cross the entire distance, and interact with the detector at the same "time."

In essence photons, gravitons, neutrinos, and the like are true four dimensional objects as compared to the three dimensional objects (matter) we interact with all the time.
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Re: Gravity?
Post by DDHvi   » Mon Jan 25, 2016 7:17 pm

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Hildum wrote:If you really want something mind blowing, consider that photons from the cosmic background radiation coming to our detector on earth 14.x billion years later do not experience time at all - they simply come into existence, cross the entire distance, and interact with the detector at the same "time."

In essence photons, gravitons, neutrinos, and the like are true four dimensional objects as compared to the three dimensional objects (matter) we interact with all the time.


I'd not considered the four dimensional aspect uasing our current theory, but it fits.

While happily using any theory that fits known facts, I try to add, at least in my mind, the phrase "assuming the theory is correct and complete." Since Goedel's proof of incompleteness can be applied to all formulae, and a statement is a verbal formula, we should keep an eye out for contrasting experimental evidence - always. Newton's work was useful for centuries, and still is enough for most things, but . . ..
;)
Douglas Hvistendahl
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Dumb mistakes are very irritating.
Smart mistakes go on forever
Unless you test your assumptions!
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Re: Gravity?
Post by Louis R   » Fri Feb 05, 2016 11:46 pm

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FYI, here are a couple of recent papers reviewing the current state of affairs in General Relativity:

http://arxiv.org/pdf/1504.04623v1
http://arxiv.org/pdf/1512.03818.pdf

DDHvi wrote:
Hildum wrote:If you really want something mind blowing, consider that photons from the cosmic background radiation coming to our detector on earth 14.x billion years later do not experience time at all - they simply come into existence, cross the entire distance, and interact with the detector at the same "time."

In essence photons, gravitons, neutrinos, and the like are true four dimensional objects as compared to the three dimensional objects (matter) we interact with all the time.


I'd not considered the four dimensional aspect uasing our current theory, but it fits.

While happily using any theory that fits known facts, I try to add, at least in my mind, the phrase "assuming the theory is correct and complete." Since Goedel's proof of incompleteness can be applied to all formulae, and a statement is a verbal formula, we should keep an eye out for contrasting experimental evidence - always. Newton's work was useful for centuries, and still is enough for most things, but . . ..
;)
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Re: Gravity?
Post by phillies   » Sat Feb 06, 2016 12:34 am

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Keith_w wrote:
phillies wrote:If a moving giant planet passes nearby the earth, so we are drawn up into the sky, in precisely which direction to we fall up, if the giant planet is accelerating. The current direction to the planet? The direction in which we see the planet, that being where it was a while ago thanks to speed of light delay. Yes, it matters that the planet is accelerating.

That's the speed of gravity.


You would, along with the remnants of the planet upon which we are standing, be pulled along in the wake of your hypothetical giant planet.


It's not quite that simple. Angular momentum and energy conservation mean that the solutions are orbits, not 'drags along'. One solution is that we gain considerable angular momentum and are ejected from the solar system along our own orbit. Another outcome is that we lose our angular moment, and up falling into the Sun.

On a different note "c is the largest speed allowed" refers to speed as measured by a local observer. An observer a large distance away, further from the Sun that the photon, can infer that the photon is moving faster than the canonical c, as measured from her distant location. Infer? The travel time is less than expected. This effect has been observed experimentally.
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