Erric
member
Reged: 10/26/09
Posts: 35
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From what little I know about gravity:
When a mass is present in the space-time it distorts it so that whilst it remains true that traveling through space causes you to travel through time, traveling through time now causes you to move (accelerate) through space. In other words just by existing, you are compelled to move through space.
Ok.
I don't understand how moving through time is moving me through space (if I wasn't planning on moving through space at all).
Also it seems the big lump of mass that's making the distortion in space-time is also making that space-time bigger.
So, does this mean I'm being "pushed" from normal space-time to the warped space-time? Like going from a high-pressure area to a low-pressure area?
Maybe I'm completely wrong, but I don't think there is any gravitons.
thanks, I'm just trying to learn the basics.
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StarmanDan
sage
Reged: 08/27/07
Posts: 392
Loc: China Spring, Texas
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By moving through time you are required to move through space and visa versa. Since it is impossible to stop moving, as viewed from an absolute space-time reference, you will always experience time. Gravity simply distorts the fabric of space-time and changes the way objects move through space and perceive time.
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"Starman" Dan Doyle
Texas Astronomical Society of Dallas
Central Texas Astronomical Society
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Shadowalker
Unpretentious Rocket Scientist
Reged: 11/23/04
Posts: 3560
Loc: Poplarville, MS, USA
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Quote:
Gravity simply distorts the fabric of space-time and changes the way objects move through space and perceive time.
Simply???? Maybe for you 
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Tom Nicolaides
http://www.first-light.org
My evil self is at that door, and I have no power to stop it
-- Dr. Edward Morbius
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imjeffp
Aluminum Falcon
Reged: 09/30/03
Posts: 4902
Loc: Cedar Park, Texas
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I still don't quite know if gravity is a cause or an effect.
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Erric
member
Reged: 10/26/09
Posts: 35
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OK thanks guys.
I think I understand this better. (so I try to)
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StarmanDan
sage
Reged: 08/27/07
Posts: 392
Loc: China Spring, Texas
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Quote:
Simply???? Maybe for you
Yea, well, when you're a time lord, these things become quite trivial and sometimes rather annoying! 
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"Starman" Dan Doyle
Texas Astronomical Society of Dallas
Central Texas Astronomical Society
8" LX200GPS w/ST80 guidescope, Canon 350D+DSI Pro
150mm f/8 Sky Watcher Refractor
10" f/4.5 Homemade Dob
RV-6 Criterion Dynascope
http://darcstar.wordpress.com
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HiggsBoson
Pooh-Bah
Reged: 02/21/07
Posts: 1118
Loc: Kal-li-fornia
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The current theory of gravity is called ‘The General Theory of Relativity’. In this theory the space and time are different aspect of the same stuff. The presence of mass distorts the geometry of space-time. Motion n space is not required for this distortion to occur. Motion in space has no meaning unless a reference is specified. For any given motion in space one can find a reference frame in which there is no motion.
If we choose a reference frame in which the object in question is not moving, then it has a velocity of c in the dimension of time. If it has motion relative to that frame then its speed in the time dimension is less than c. The interval in space-time is invariant over a change of reference frame.
I regret that this stuff is not easily explained in words of a language like English. The physics does not easily map into words.
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Michael
ATM: 6" F/9 Newtonian Travel Scope
ATM: 12.5" F/4.5 Real Soon Now...
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scopethis
professor emeritus
Reged: 05/30/08
Posts: 638
Loc: Kingman, Ks
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So what would space (the Universe) be like if all the mass (matter?) were removed? Would expansion still be happening? Is it correct to assume that if there was no mass that gravity would cease to exist?
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sailor70623
scholastic sledgehammer
Reged: 01/12/08
Posts: 944
Loc: Ok.
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Well with no mass there would be no gravity, but that doen't mean it doesn't exist. And while gravity is a very important piece of the relativity formula, you have to remember it also fits the Newtonian model too. Relativity just explanes why the Newtonian model seems to have flaws, while it is really the interdependene of gravity and time, as well as velocity, and energy. Relativity thus ties togeather so much of what was thought to be flaws in Newtonian physics to the reality we observe. However, even relativity does not tie it all togeather. Einstine and many since have been trying to find the theory to tie everything togeather. When we find it, it may open up new Universes to us, and hyper space travel. And many new theories that will require a NEW theory of everything. And so we will start all over again.
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Corornado PST
LB 16" & 12"
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llanitedave
Humble Megalomaniac
Reged: 09/26/05
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Loc: Amargosa Valley, NV, USA
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Take away all the mass, and just maybe there'll be another Big Bang to replace it.
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"Since the process of science generates more mysteries than it solves, I predict that we'll never learn everything: and we'll continue to generate new ignorance at the speed of knowledge."
"S.O.E." (Sauron's Other Eye), with 16" Royce conical mirror: A permanent work in progress.
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sailor70623
scholastic sledgehammer
Reged: 01/12/08
Posts: 944
Loc: Ok.
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Oh, we'll know what the Big Bang is/was when we figure out the Theory of everything. It may be a pulse of energy becoming matter with some of the matter turning back into energy. It may be the formation of a new bubble when two planes of two other universes collide. And so the theories go on and on. I think the bigger question is, what's out side of our universe?
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Corornado PST
LB 16" & 12"
Z 10"
LX50 8"
8" CPC
ETX127
102&90mm MAKs
80mm Richfield APO
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ETX60
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kraterkid
Post Laureate
Reged: 03/07/05
Posts: 4575
Loc: Jacumba, California
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Theoretically, even if you removed all the mass from the universe there would still be gravity. The energy at the first moment of the Big Bang produced gravitation and expanded the fabric of spacetime (inflation) even when there was no mass present.
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Rich
My CN Gallery
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ragebot
Carpal Tunnel
Reged: 08/26/05
Posts: 1672
Loc: Tallahassee, FL, USA
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Quote:
Take away all the mass, and just maybe there'll be another Big Bang to replace it.
I thought you needed a singularity to have a Big Bang, and by definition a singularity has mass; but I may be wrong about this.
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contrailmaker
sage
Reged: 01/02/09
Posts: 251
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Somebody mentioned that the Newtonian model has flaws. I prefer to say that it has limits. General relativity also has its limits.
As for the original post:
Time is not a spatial dimension so in a fixed reference frame, merely moving through time does not mean you are moving through space. An event in space-time (per General Relativity) is defined by the 3 physical dimensions plus time, therefore as you progress forward in time, you move in space-time.
cm
Edited by contrailmaker (10/30/09 05:17 PM)
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dreamregent
sage
Reged: 04/06/09
Posts: 457
Loc: Clearwater, FL
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Quote:
For any given motion in space one can find a reference frame in which there is no motion.
Can you expound on this further and provide an example?
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Building a f5.24 10" Dob
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HiggsBoson
Pooh-Bah
Reged: 02/21/07
Posts: 1118
Loc: Kal-li-fornia
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Quote:
So what would space (the Universe) be like if all the mass (matter?) were removed? Would expansion still be happening? Is it correct to assume that if there was no mass that gravity would cease to exist?
If we measure the recession velocities of galaxies and take an average we find the Hubble constant. But if we look at recession velocities of nearby galaxies alone and far off galaxies alone we find that the recession velocities of those far off is slower than we would expect if the expansion rate were constant over time. In fact they are receding slower and there fore it must be the case that the expansion is accelerating.
If we consider the universe to be matter dominated we find that the universe should be expanding more slowly than it is. We conclude that there is a second source of mass/energy that is adding to the expansion. We call this vacuum energy or dark energy. This energy is 70% of the total energy/mass density of the universe. From this we can answer your question. With the baryonic matter removed the expansion would be faster.
To your second question. If there were no matter, the law of gravity would exist but would not cause a change to the motion of objects because there aren’t any.
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Theoretically, even if you removed all the mass from the universe there would still be gravity. The energy at the first moment of the Big Bang produced gravitation and expanded the fabric of spacetime (inflation) even when there was no mass present.
I have not seen a source that suggest that there was no mass in the universe prior to inflation. I have seen suggestions that the universe may have been as small as 25 lbs but not zero. We have no information about what happened during the first Planck time but we believe that the universe had mass and existence during this time. Inflation did not start until 10^-36 seconds after the Big Bang event. Inflation took about 100 times 10^36 seconds ending at around 10^-34 seconds. So Inflation had a good long run. As I understand it most of the mass was created by conservation of energy when inflation stopped. All of that energy that had been released into vacuum had to go somewhere. The universe prior to inflation should have been a soup of quarks and photons, Both converting from one to another with neither being stable. Apparently high energy photons interact when they collide and create quarks which immediately decay into high energy photons. This does not happen in our cassegrain telescopes because visible photons lack to energy to produce a quark.
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I thought you needed a singularity to have a Big Bang, and by definition a singularity has mass; but I may be wrong about this.
A singularity is not required. We do not have physics to model the super force prior to inflation and hence we can not make strong statement about that situation.
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Somebody mentioned that the Newtonian model has flaws. I prefer to say that it has limits. General relativity also has its limits.
Generally I agree but…
In the case of Newtonian gravity there is no time dependence. This was wrong. The theory remains a useful approximation for a great number of problems.
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Michael
ATM: 6" F/9 Newtonian Travel Scope
ATM: 12.5" F/4.5 Real Soon Now...
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HiggsBoson
Pooh-Bah
Reged: 02/21/07
Posts: 1118
Loc: Kal-li-fornia
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Quote:
Quote:
For any given motion in space one can find a reference frame in which there is no motion.
Can you expound on this further and provide an example?
Consider two astronauts, in space suits 100 meters apart. They are approaching each other at 1 meter per second. I can choose to consider one moving and the other stationary. Or I can choose any other reference frame moving relative to the two. All are valid. There is no actual right answer as to which is moving.
For any problem I can always choose some point at random and consider it stationary. I can still do the physics and get a useful correct answer. I can compute when they will hit each other and how much energy will be released at that time.
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Michael
ATM: 6" F/9 Newtonian Travel Scope
ATM: 12.5" F/4.5 Real Soon Now...
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contrailmaker
sage
Reged: 01/02/09
Posts: 251
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I do not understand what you mean by time dependence in this case.
cm
Edited by contrailmaker (10/31/09 01:42 AM)
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HiggsBoson
Pooh-Bah
Reged: 02/21/07
Posts: 1118
Loc: Kal-li-fornia
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Newton’s Universal Law of Gravitation states that the Force between two bodies m1 and m2 is proportional to a Universal constant ‘G’, m1, m2 and inversely proportional to the square of the separation distance between them' ‘r’
F = G m1 m2 / r^2
Notice the absence of a ‘t’ in this equation. The force has no dependence upon time. This suggest that gravity somehow knows about a change in separation distance instantly! While this is of little important if I drop a brick on my foot but it suggest that if the sun disappeared that the Earth would feel the lack of gravity instantly. General Relativity predicts that the speed of gravitational awareness is ’c’. That it takes finite time for changes to be known at a distance. GR asserts that changes in the curvature in space-time propagate with ‘c’.
Hence, Newton’s has a great approximation that has provided wonderful service since 1687 but it is misleading in terms of actually understanding of the physical mechanism of gravity.
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Michael
ATM: 6" F/9 Newtonian Travel Scope
ATM: 12.5" F/4.5 Real Soon Now...
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dreamregent
sage
Reged: 04/06/09
Posts: 457
Loc: Clearwater, FL
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Quote:
Consider two astronauts, in space suits 100 meters apart. They are approaching each other at 1 meter per second. I can choose to consider one moving and the other stationary. Or I can choose any other reference frame moving relative to the two. All are valid. There is no actual right answer as to which is moving.
For any problem I can always choose some point at random and consider it stationary. I can still do the physics and get a useful correct answer. I can compute when they will hit each other and how much energy will be released at that time.
If they are both really moving in actuality, wouldn't your answer be wrong? I don't claim to be a physicist but I would think that the time it takes for them to collide and the amount of energy released at that time would vary depending on the rate at which they are moving until they collide. If one is stationary, the other would take 100 seconds to reach the stationary one. However, if they are moving directly toward each other at the same rate, it would take only 50 seconds. If you arbitrarily say one is stationary when he is not, how would you calculate the correct answer?
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Building a f5.24 10" Dob
in an octagonal wood tube
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Jarad
Post Laureate
Reged: 04/28/03
Posts: 4330
Loc: Atlanta, GA
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Quote:
If they are both really moving in actuality, wouldn't your answer be wrong?
His point is that there is no "in actuality". From the point of view of astronaut A, he is stationary and B is approaching at 1 m/s. From the point of view of B, he is stationary and A is approaching at 1 m/s. From the point of view of the point midway between them, they are both approaching from opposite directions at 0.5 m/s. From the point of view of someone on earth, A is orbiting at ~40,000,000 m/s, and B is orbiting at ~39,999,999 m/s. All 4 will correctly calculate that A nd B will crash into each other in 100 seconds, and correctly calculate where that impact will occur. All are equally correct.
Jarad
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astrotrf
professor emeritus
Reged: 09/30/07
Posts: 709
Loc: Rodeo, NM
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Quote:
If you arbitrarily say one is stationary when he is not, how would you calculate the correct answer?
As Jarad points out, there is no such thing as knowing whether something is or is not stationary. All motion is *relative* (hence, "relativity") as it must be measured with respect to some other reference, and *that* reference may or may not be stationary with respect to some *other* reference, etc. It's turtles all the way down ...
What this means is that if I shut you in a box out in the middle of deep space, far away from any gravitational influences, with no outside forces acting on you to change your rate or direction of motion (i. e., you aren't being accelerated), there is no experiment you can perform to detect whether or not you are moving.
In fact, if I next cut a window in the box, and you cannot see any objects out that window, you might reasonably begin to ask what the word "moving" really means.
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Terry (astrotrf)
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cavefrog
sage
Reged: 11/11/08
Posts: 449
Loc: loozyanna
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It's turtles all the way down ...
Love it! lol. I just came across this statement recently. good placing for it.
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I am only an egg
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dreamregent
sage
Reged: 04/06/09
Posts: 457
Loc: Clearwater, FL
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Ok, I understand you have varying ways of looking at it and reaching an answer by arbitrarily assigning a point to be stationary. However, has anything in the universe ever been shown to be stationary relative to the rest of the universe in it's entirety?
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Building a f5.24 10" Dob
in an octagonal wood tube
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Mike K
professor emeritus
Reged: 04/01/07
Posts: 626
Loc: Central Texas
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Quote:
...has anything in the universe ever been shown to be stationary relative to the rest of the universe in it's entirety?
What does it mean to be stationary relative to the rest of the universe? The universe is made up of innumerable particles, all moving relative to each other.
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Clear skies,
Mike K.
30°31" N 97°44" W, LP: Red
Observe: Once or twice a week back yard, once a month under dark skies
Favorites: Globulars, planets, face-on spirals
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dreamregent
sage
Reged: 04/06/09
Posts: 457
Loc: Clearwater, FL
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Quote:
What does it mean to be stationary relative to the rest of the universe? The universe is made up of innumerable particles, all moving relative to each other.
The concept of "stationary" sounds more and more like an imaginary construct. That's ok, though...there is an imaginary construct in math, too. I suppose if it suits our purposes, it's all good.
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Building a f5.24 10" Dob
in an octagonal wood tube
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Jarad
Post Laureate
Reged: 04/28/03
Posts: 4330
Loc: Atlanta, GA
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In Relativity, the concept of "stationary" has been replaced by the concept of "rest frames". A rest frame is a frame of reference that is not undergoing any accelleration. A space ship in orbit is a rest frame. A ship lifting off is not. A "stationary" point on the surface of the earth is not, either (we are experiencing accelleration due to gravity, and due to the earth's rotation).
Jarad
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astrotrf
professor emeritus
Reged: 09/30/07
Posts: 709
Loc: Rodeo, NM
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Quote:
Ok, I understand you have varying ways of looking at it and reaching an answer by arbitrarily assigning a point to be stationary. However, has anything in the universe ever been shown to be stationary relative to the rest of the universe in it's entirety?
Well ...
In a certain sense, the cosmic background radiation is at rest relative to the Universe as a whole, to the extent that the question has a clearly-definable meaning.
But it is still important to remember that there is nothing physically special about such a reference frame.
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Terry (astrotrf)
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hwhall
member
Reged: 09/22/08
Posts: 189
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Quote:
A rest frame is a frame of reference that is not undergoing any accelleration. A space ship in orbit is a rest frame... A "stationary" point on the surface of the earth is not, either (we are experiencing accelleration due to gravity, and due to the earth's rotation).
Jarad
I'm not sure that an orbiting craft is a rest frame. It is also under gravitational acceleration. The craft is actually in "free fall" within earth's gravity field. The fact that the direction of the craft's velocity vector is constantly changing (as it circles the earth) indicates an acceleration is at work.
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HiggsBoson
Pooh-Bah
Reged: 02/21/07
Posts: 1118
Loc: Kal-li-fornia
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Under Newtonian Mechanics an object in orbit is being accelerated by gravity. This force keeps it falling round the gravitation body.
As I understand it, under General Relativity the concept of a gravational force has been eliminated. The object is unaccelerated if it is moving along a geodetic path in space-time. Hence light and free moving object follow the curvature of space.
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Michael
ATM: 6" F/9 Newtonian Travel Scope
ATM: 12.5" F/4.5 Real Soon Now...
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hwhall
member
Reged: 09/22/08
Posts: 189
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> The object is unaccelerated if it is moving along a
> geodetic path in space-time.
Would that include an object sitting on the surface of the earth as well (ignoring planetary motion about the sun that is)?
It was my recollection, and my physics courses are somewhat behind me in the timestream <G>, that a rest frame is such that object motion within the frame does not give rise to fictional forces (e.g., coriolis). If that is so... for the craft in orbit, when one tossed an object from it radially (toward the earth or away - down or up the gravity well) it would appear to be affected by an accelerating or decelerating force (for it would not go straight down or up, it would begin leading or lagging the craft's radial position, curving away from the direction it was tossed).
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hwhall
member
Reged: 09/22/08
Posts: 189
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Quote:
What this means is that if I shut you in a box out in the middle of deep space, far away from any gravitational influences, with no outside forces acting on you to change your rate or direction of motion (i. e., you aren't being accelerated), there is no experiment you can perform to detect whether or not you are moving.
If the box is rotating, there is an experiment that will reveal that fact. An object tossed outward from near the center of the box will appear to move along a curved path instead of straight.
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ragebot
Carpal Tunnel
Reged: 08/26/05
Posts: 1672
Loc: Tallahassee, FL, USA
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Quote:
SNIP
Hence, Newton’s has a great approximation that has provided wonderful service since 1687 but it is misleading in terms of actually understanding of the physical mechanism of gravity.
I tend to go easy on Newton, mostly because there was really no experiment/observation of gravity bending light till after 1900. Newton's formula described what he was seeing.
I still wonder about how well we are able to bring time into the picture. Fast speeds seem to confuse time, especially when you get close to the speed of c.
While it makes sense to me to say gravity is propagated at the speed of light I have to wonder how the force of gravity might vary between two galaxies light years apart moving quickly towards or away from each other, compared to two galaxies with a constant distance orbit around each other.
If gravity is propagated at the speed of light and two objects move apart would the force of gravity change just based on the change in distance, or would the speed the object was moving need to be factored into a formula? 
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Meade ETX 90, Meade AR5, Orion ED80, Atlas GT, 8 in Newt, Coronado DS SM40, Garrett 10.5X70, Sigma SD10, SD14, Canon 1D2, Xti, Nikon CP4500, C-14
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contrailmaker
sage
Reged: 01/02/09
Posts: 251
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I concurr with your asessment of Newton here. It is kind of silly to say that Newton was wrong by not including time in his equations. There simply is no need at the scales for which he was writing the equations. He was writing equations to explain everyday observable phenomena at a time when the speed of light was assumed to be instantaneous. We forget that the nature of light was poorly understood at that time.
The answer to your last question is yes, the speed needs to be factored in.
cm
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HiggsBoson
Pooh-Bah
Reged: 02/21/07
Posts: 1118
Loc: Kal-li-fornia
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Please do not take the word wrong as implying criticism of Newton. This is not intended. I use the word with regard to theory when the concept prevents understanding of the phenomenon. My understanding of gravity became much clearer when I realized that a time dependence was needed.
If one considers MOND it postulates that the law of gravity changes at large scales. This postulate is either right or wrong. It is not a matter of how close it is. I do not intend to talk anything away from Newton. He like others before him did an incredible job with the range of experimentation available to him. The issue to me is how we evaluate new theories. If they are based upon a postulate that is not true, I do not give them credit for being close.
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Michael
ATM: 6" F/9 Newtonian Travel Scope
ATM: 12.5" F/4.5 Real Soon Now...
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Jarad
Post Laureate
Reged: 04/28/03
Posts: 4330
Loc: Atlanta, GA
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Quote:
I'm not sure that an orbiting craft is a rest frame. It is also under gravitational acceleration. The craft is actually in "free fall" within earth's gravity field. The fact that the direction of the craft's velocity vector is constantly changing (as it circles the earth) indicates an acceleration is at work.
Being in "free fall" is actually what makes it a rest frame. Sitting on the surface of the earth is not a rest frame - the ground is pushing up on you at 1G, accellerating you upward. If you place an unsupported ball in the air next to you, it will not stay stationary relative to you - it will fall. If you are in orbit, and you place an unsupported ball in the air next to you, it will sit stationary relative to you (you are both in the same rest frame) until some force acts to accellerate you or the ball (at which time one or the other of you is no longer in a rest frame - you are experiencing accelleration).
In addition to gravity, there are also other forces acting on us here on the surface of the earth, including the Coriolis force (easy to measure with a pendulum).
That's one of the reasons that we wanted to have a permanent International Space Station - there are some experiments that cannot be performed on the surface of the earth because it is not a rest frame. Sadly, most of those cannot be performed there because it has not been completed to the level originally planned, but some have been.
Jarad
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Glaucus
member
Reged: 07/12/09
Posts: 45
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An orbiting object is not an inertial frame of reference relative to the planet itself.
If the orbiting object is the inertial frame, then the planet is not. The one is accelerated with respect to the other. So they can't both be inertial. If both are accelerated in the same way, they can both be inertial frames of reference.
A frame of reference is a coordinate system. If your origin is the location of a satellite and you want to consider the movement of the planet you are actually orbiting then Newton's laws of motion won't hold. You will observe fictitious forces.
This is why it was so difficult for the geocentric model to work out mathematically. You will notice the Coriolis effect and the basic laws of Newton won't work.
Edited by Glaucus (11/02/09 08:00 PM)
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HiggsBoson
Pooh-Bah
Reged: 02/21/07
Posts: 1118
Loc: Kal-li-fornia
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Quote:
Under Newtonian Mechanics an object in orbit is being accelerated by gravity. This force keeps it falling round the gravitation body.
As I understand it, under General Relativity the concept of a gravational force has been eliminated. The object is unaccelerated if it is moving along a geodetic path in space-time. Hence light and free moving object follow the curvature of space.
As stated earlier, the two theories define being accelerated differently.
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Michael
ATM: 6" F/9 Newtonian Travel Scope
ATM: 12.5" F/4.5 Real Soon Now...
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Glaucus
member
Reged: 07/12/09
Posts: 45
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In general relativity gravity is not a force.
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hwhall
member
Reged: 09/22/08
Posts: 189
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Quote:
Being in "free fall" is actually what makes it a rest frame. Sitting on the surface of the earth is not a rest frame... - Jarad
Hi, Jarad.
I thought that the earth surface would not really be a rest frame...
I what you mean about "free fall", but what I wanted to convey was the notion that if the satellite velocity vector was constantly changing, was that not an indication of an applied force and acceleration...but I think I see that the 'change in direction' is an artifact of looking at the satellite in a frame outside the satellite.
Still, an object in earth orbit is a rotating frame. That it is rotating can be discerned by experiment within the frame. Can a rotating frame truly be a 'rest frame'?
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Jarad
Post Laureate
Reged: 04/28/03
Posts: 4330
Loc: Atlanta, GA
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Quote:
Still, an object in earth orbit is a rotating frame. That it is rotating can be discerned by experiment within the frame. Can a rotating frame truly be a 'rest frame'?
Not necessarily. If the spacecraft is rotating to keep the same side facing the earth as it orbits, then it is a rotating frame. But if it does not rotate as it orbits, it is not a rotating frame. For example, the Hubble Space Telescope does not rotate - if it did, the "frame" of the CCD camera would have star trails like a shot of the sky from a fixed tripod on earth. It keeps it orientation relative to the stars, and is a non-rotating rest frame.
Quote:
An orbiting object is not an inertial frame of reference relative to the planet itself.
There is a difference between inertial frames and rest frames. Two frames can both be rest frames, but not at rest relative to each other. You could also have non-rest frames that are at rest relative to each other (for example, a car on the highway going parallel to a train on a track at the same speed).
Jarad
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Edited by Jarad (11/03/09 03:23 PM)
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hwhall
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Reged: 09/22/08
Posts: 189
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Quote:
Not necessarily. If the spacecraft is rotating to keep the same side facing the earth as it orbits, then it is a rotating frame. But if it does not rotate as it orbits, it is not a rotating frame. For example, the Hubble Space Telescope does not rotate
Doh!  I should have remembered that.
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