# The speed of gravity



## RJM Corbet

I thought this is worth a thread to itself?
Tie a weight to a rope and swing it in a circle around your head.
Make that rope very long.
Attach a beeper, that beeps at regular intervals, to the weight. 
Obviously you are going the hear the beep after the weight has emitted it, as you hear the starter's gun after you see the puff of smoke?
But the rope stays in a straight line.
I know this analogy is using sound, not light, but the same principle must apply to gravity/light.
_Metryq_ originally raised the issue, in the 'Dark Matter' and 'Something From Nothing' threads, with links _to Meta Research._
Another thing that gets me, but to which I'm sure there is a simple explanation, is why astronauts in the space station float around, free of gravity, when the earth's gravity is powerful enough to hold the space station, and even the moon, in orbit?


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## Ursa major

RJM Corbet said:


> Another thing that gets me, but to which I'm sure there is a simple explanation, is why astronauts in the space station float around, free of gravity, when the earth's gravity is powerful enough to hold the space station, and even the moon, in orbit?


To quote Wiki:


> an *orbit* is the gravitationally curved path of an object around a point in space


In effect, that object is falling, due to gravity, but as the path is curved, not straight down, the object will take a while** to hit the source of the gravity. Anyone inside that object is subject to the same gravitational pull.

If you are inside an object subject to the same gravity as you, you will be falling at the same rate and so will be able to float inside (assuming you're not restrained in some way). This is how weightlessness training works: the trainee is inside a plane that's diving towards the Earth and so does not experience gravity _with respect to the plane_. If the plane suddenly disappeared, the trainee would notice that they were hurtling towards the ground.



** - Sometimes a very long while.


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## Metryq

The astronauts floating around is called "free fall" because technically the space station and all of its contents are falling. You've probably seen the following diagram in physics textbooks:





The first three shots (A, B, C) of a hypothetical cannon did not have enough charge to put the projectile into orbit, but each successive shot carried the projectile farther. Shot D is the first with enough energy to go into orbit. The projectile is still falling towards the Earth, but because the surface curves away from the projectile, the resulting path is an _orbit_. 

The following shots (E, F) produce elliptical orbits with higher apogees (highest point above Earth).

NASA (and other aeronautics agencies) use cargo planes to simulate short spans of free fall in order to train future astronauts. They achieve this by flying parabolic flight paths in the sky—almost like a sine wave. Each drop produces short periods of free fall when everything in the plane "floats." The production crew that filmed the movie _Apollo 13_ rented time on NASA's "Vomit Comet" to get real free fall effects.

Technically, you're constantly "falling" too, only the ground gets in the way.

Your spinning weight at the end of a rope exercise is one way to simulate gravity in advanced space stations (centrifugal force).


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## Ursa major

RJM Corbet said:


> But the rope stays in a straight line.


Is this always true?

Is this _ever_ true? (I'm not sure it is, except in a thought experiment.)


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## RJM Corbet

Oh, I get it! The astronauts. Duhh! Thanks.
The rope?
On to the rope...


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## Metryq

Ursa major said:


> Is this always true?
> 
> Is this _ever_ true? (I'm not sure it is, except in a thought experiment.)



Let's not confuse Corbet with drag and other factors. (What is the wave propagation speed in a rope medium?) My high school physics teacher always reminded us that the class exercises were simplified. I'll always remember the demonstrations in trajectory: he brought in this little cannon and took everyone up to the football field.


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## RJM Corbet

He he he! Now the fun's starting ... its not a physics forum, its a writers forum -- a way of getting complicated ideas through to dummies like me, so we can understand and use the concepts as fiction writers ...
Gotta go -- Star Trek's on TV : )


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## TheEndIsNigh

However it does raise an interesting point.

What is the speed of gravitational attraction?

Can it be thought of as instantaneous or does it need time to take effect? 

In other words, is gravity faster than the speed of light?


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## Metryq

TheEndIsNigh said:


> In other words, is gravity faster than the speed of light?



That's been kicking around in some recent threads. Take a look at the articles in the Cosmology > Gravity section of the MetaResearch site.

I'd recommend reading any and all of the articles, but the ones you might find most pertinent are "The Speed of Gravity—What the Experiments Say" and "Primer on Lorentzian Relativity." I recommend the "Primer" because someone's bound to stand up and pontificate that "Einstein said nothing is faster than light." However, whether Einstein or Lorentz, the _experiments_ show light arrives from the sun with "aberration" (at an angle) while the force of gravity does not.

You might also wish to look at section 6 of the Animations page. (Section 4 illustrates aberration. The animation is SWF, or Flash, as it was made a while ago. If anyone wants a video conversion, I can make one.)


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## RJM Corbet

Metryq said:


> That's been kicking around in some recent threads. Take a look at the articles in the Cosmology > Gravity section of the MetaResearch site.
> 
> I'd recommend reading any and all of the articles, but the ones you might find most pertinent are "The Speed of Gravity—What the Experiments Say" and "Primer on Lorentzian Relativity." I recommend the "Primer" because someone's bound to stand up and pontificate that "Einstein said nothing is faster than light." However, whether Einstein or Lorentz, the _experiments_ show light arrives from the sun with "aberration" (at an angle) while the force of gravity does not.
> 
> You might also wish to look at section 6 of the Animations page. (Section 4 illustrates aberration. The animation is SWF, or Flash, as it was made a while ago. If anyone wants a video conversion, I can make one.)


 
As a left-brainer (or is it right -- or _no_ brainer) I find it difficult to follow the mathematical physics between the Einstein and Lorentz 'threads' -- but I am, through this forum, beginning to grasp the general idea -- which is that, in the Lorentz model, the speed of light is not the wall that it is in relativity. 
On the simplified level, if gravity and the speed of light were equivalent, that rope would keep kinking? The weight would not stay in orbit? Of course, when you think about it, it must be true. Even on the level of an Aristotlean _thought experiment._
Gravity must be as close as dammit to instantaneous, to work at all?
Which makes things very interesting, when you're thinking of faster-than-light propulsion.
And, boy, am I glad that stupid _Cydonia Face_ has been laid to rest ...


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## Metryq

RJM Corbet said:


> The weight would not stay in orbit?
> ...
> Gravity must be as close as dammit to instantaneous, to work at all?



The article explains that if gravity showed the same aberration as light from the sun (the same delay, the same angle when it arrived), then the planets would speed up, thus widening their orbits and eventually flying off into space.

Gravity is not instantaneous, but on the scale of the Solar system, the difference is indistinguishable. Van Flandern's model puts the speed of gravity at about 20 billion times the speed of light. So a trip to the nearest star system, Rigil Kentaurus (better known as Alpha Centauri) would take 0.0000068953356 seconds—if my thumb-fingered calculations are correct. You'd push the "engage" button on your nav computer, and the stars would blink into new positions. You wouldn't even be able to perceive the duration of the trip.

And if you royally screwed up and forgot to include an inertial damping field in your ship, you'd look like a new coat of paint on the back wall of the command bridge.


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## RJM Corbet

20 billion is quite good enough for me, thanks. Inertial damping field? Go on ...


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## Metryq

RJM Corbet said:


> Inertial damping field? Go on ...



A sci-fi concept: some kind of force field to neutralize the inertia of all mass within a ship. That means if the ship moves, you don't feel it. You don't get slammed into the walls on those S.O.B. turns, or plastered against your seat if the helmsman side-steps the clutch during warp maneuvers.

In rockets as we know them today, a fit human can take in excess of 10 Gs "eyes out." (That's lying on one's back—pilots say "eyes down" when seated, as in a fighter plane. In the latter case, 7 Gs will blackout even the toughest hombres.)

Some sci-fi writers describe inertial damping as a "stasis" field projected against every single particle of mass, but then everyone on board might be "frozen" in place. The "DC" (deceleration) stations shown in _Forbidden Planet_ looked a lot like (and probably inspired) the _Star Trek_ transporter—suggesting that the crew were either pinned in place (a stasis field), or converted into energy (and thus massless). I loved the bosun's barking, "D.C. stations! On the double. You wanna bounce through this one?!"

Of course, if one could nullify inertia, one could make the entire ship "massless." The Libby drive in Heinlein's _Methuselah's Children_ was such a technology. 

And then there's the Alcubierre drive, which is probably what _Star Trek's Enterprise_ uses. This is one of those voodoo physics ideas that could be described as surrounding the ship with a black hole in front and a white hole in back, thus pushing the ship along like a surfer on a wave of warped space. Or one might describe it as surrounding the ship with a bubble of exotic, warped space that is exempt from Einstein's restrictions. So the ship is essentially in normal space, but "greased" by a bubble of warped space. (Hence, the reason the crew of the _Enterprise_ does not suffer any time dilation effects.) "Isolated" this way, the ship is technically not moving at all.


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## Nik

Uh, unless Einstein's space-time metric stuff is waaay-wrong, gravity's influence travels at the speed of light, via gravitons...

This allows for arcana like 'frame dragging' which, IIRC, would be impossible if gravity worked faster...
Frame-dragging - Wikipedia, the free encyclopedia

How this would tie in with Modified Newtonian Dynamics, proposed to remove need for 'Dark Matter', I have no idea...


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## Metryq

This isn't just a model, Nik. Van Flandern and others have actually _measured_ the aberration in sunlight relative to gravitational force.

Once again, item 6 on the animations page distinguishes between gravitational _force_ and gravitational _waves_. "But the new physical interpretation of general relativity has no need to dodge the question."


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## skeptical

The basic principle says that : _No cause and effect relationship can be propogated at more than light speed._

For this reason, no matter, energy or information can travel faster than light.   Every time someone comes up with a theoretical method for FTL, the mathematical physicists discover that there is an impossibility in the way.  For example :  a warp drive would require more energy to warp space sufficiently than exists in the entire universe.  

A wormhole large enough to convey matter would require vast amounts of 'negative energy'.  Since we do not even know if negative energy *can* exist, let alone a method of making it, that seems a rather forlorn hope.

Of course, FTL is quite possible if no cause and effect relationship is involved.  For this reason, the universe can expand faster than light, if the parts that are moving apart faster than light cannot have any effect on each other - not even passing a single datum of information.


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## Ursa major

skeptical said:


> For this reason, no matter, energy or information can travel faster than light. Every time someone comes up with a theoretical method for FTL, the mathematical physicists discover that there is an impossibility in the way. For example : a warp drive would require more energy to warp space sufficiently than exists in the entire universe.



I know I'm quibbling but:
the amount of energy required seems to be a practical problem, not a theoretical one;
it rather assumes those mathematical physicists have discovered the size of the universe, as opposed to that of the observable universe, which I didn't think they had.


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## RJM Corbet

Please read Metryq's post again. The Lorentz Model does away with the speed of light as the untimate speed of everything. Gravity is estimated to work at 20 billion times the speed of light. Physics models are just _theories_. Even special relativity ...


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## Metryq

RJM Corbet said:


> The Lorentz Model does away with the speed of light as the untimate speed of everything.



The Lorentz model was published before Einstein's relativity papers, so there was no need to "do away" with the speed of light limit. That was something introduced in Einstein's math. The GPS article essentially states that no experiment to date can favor the Lorentz model or the Einstein model.



> We are, of course, free to question whether or not this mathematical theory retains a valid basis under the principles of causality. For those of us who answer "yes", LR is unnecessary, and inelegant because it depends on a preferred frame. For those of us who answer "no", LR is then the better descriptor of nature, requiring the sacrifice of symmetry (“covariance”) to retain causality.





> *Skeptical wrote:* Every time someone comes up with a theoretical method for FTL, the mathematical physicists discover that there is an impossibility in the way.



"Discover" or issue an official denial? Is any method of FTL any more or less "theoretical" than the edict against FTL?

There can be no "proof" in the real world the way there is in math. A mathematical model can suggest what we will find in unexplored regions of the real world, but eventually we will have to open the horse's mouth and count his teeth in person.


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## RJM Corbet

I'm quite surprised that you found nothing of much interest in Trevor Pitts paper? For anyone who hasn't followed this debate in the other threads, the title is 'Dark Matter, Antimatter, and Time-Symmetry' and the link is:
http://arxiv.org/html/physics/9812021v2


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## skeptical

RJ

That paper is interesting, but is entirely speculative.  As such, it is not truly science.   Science is based on empirical evidence, and speculation, though acceptable, is unconvincing without such evidence.

Before any such ideas can be regarded seriously , they must generate testable predictions, and pass those tests.


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## Nik

One nitty-picky from an earlier post...

Planets orbiting further out orbit *slower*than those closer in.

And a WTF: IIRC, the Earth's mean orbital radius is increasing by an inch or so a year as a result of the solar tides. This is corresponding effect to that causing Moon to recede...


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## Metryq

Nik said:


> Planets orbiting further out orbit *slower*than those closer in.



Uh...no. Increase your orbital speed and your orbit will widen. It may take longer to travel the greater distance, but higher orbits are faster. 

Next time you're at the same altitude as your target and you're trying to dock, go ahead and fire the engines. Your craft will speed up, its orbit will widen, and your target will appear to drop and "race away" from you. In order to dock you must drop to a lower orbit. Your ship will appear to "catch up" with your target, but it will be higher than you. Depending on the speeds and such, you want to fire your thrusters when the target is still ahead of you and above. As you speed up, your ship will close with the target in a long, climbing diagonal.

Remember to bring your octant, in case the nav computer quits on you. Homework for next time: gravity assists—doesn't the outbound leg away from a planet eat up any speed gained in dropping toward it? What good are gravity assists anyway?


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## RJM Corbet

Yes, the KISS (keep it simple, stupid) Rule is often useful, reduce to basics instead of getting confused by increasing layers of complexity. A spinning disc is moving faster on the outer rim -- i.e. covering more distance than the inner rim.
Now, if one considers anti-particles, anti-gravity, anti-space/time -- they are not just running in reverse, like a cinema film played backwards, where a broken glass springs back up from the floor and comes together again. They are events happening in a separate (event space?) where they follow random event paths, as they would in _normal_ time/space -- so their movement cannot be be projected by simply reversing them. It isn't going to be as simple as that -- an anti-universe isn't going to be a simple reverse copy, like a photographic negative ...


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## Metryq

Excuse me, Nik was right. Higher orbits are slower because of the fall-off in gravity, but it does take more energy (accelerating your ship) to reach a higher orbit. A low Earth orbit is about 28,000 kph, while a geostationary orbit is about 11,000 kph.


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## RJM Corbet

Ok. I'm sure you've studied it. But is Jupiter moving faster than Earth -- pure velocity? Also, in the sense that, as you say, an object in orbit is really _falling_, is there a terminal velocity? And would that terminal velocity depend upon the gravity of the object to which it falls? So would the terminal velocity of an apple falling on Jupiter be greater than that of an apple falling on Earth?


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## Metryq

Same as with the LEO and GEO orbits—Earth is moving faster than Jupiter by a bit over 2x.

"Terminal velocity" is the maximum speed through a "fluid." In the case of falling through Earth's sky, terminal velocity for a skydiver (parallel to the ground) is about 195 kph. If the skydiver tucks in and drops head-first, the speed will increase.

Objects dropping through a vacuum do not have a "terminal velocity."


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## RJM Corbet

Uh Uh. An object falls to earth, through a vacuum, at 32 feet per second squared, until reaching a terminal velocity, of, I think, about 200 miles an hour. That is discounting wind resistance, as in the case of a skydiver 'arching' to slow down or diving to go faster -- using wind resistance. Terminal velocity is the maximum speed at which an object falls, through a vacuum, under pull of Earth gravity. Otherwise you're going to end up saying a heavier object falls faster than a lighter one.
Would terminal velocity be greater on Jupiter, which has a larger gravity than Earth, and less on, say, the moon?
Thanks for clearing me up on the orbit speeds, though. It's a good example of how a _thought experiment_ can go awry. It means that when the force that propels a body ceases, the body will continue to orbit, or 'fall' around the sun, at that speed a which it is travelling, regardless of its size, or mass, or distance from the sun?


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## skeptical

To RJ

Sorry buddy.

There is no such thing as 'terminal velocity' in a vacuum.  If an object is in orbit, the circular nature of the orbit is, by definition, accelerating.   If it is falling straight towards an object through vacuum, it will accelerate.

Terminal velocity is, as Metryq said, in a 'fluid'.  

Will terminal velocity be greater near Jupiter or near Earth?
I have not looked this up, but I suspect near Earth, if 'near' means within a few minutes of impact.  This is because the Earth's atmosphere is so much thinner.  An object falling towards the 'ground' at Jupiter will be pulled by a lot more gravity, but will be slowed by a lot more friction with atmosphere.


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## RJM Corbet

I don't want to overdo the subject, but an object falling from the sky towards Earth -- like a lead weight chucked out of a plane -- as opposed to an object in orbit -- does not accelerate indefinitely, it reaches a terminal velocity, regardless of fluid resistance -- and one assumes that velocity will be greater if the gravity is greater. I bought it up because it is a direct way to measure gravity? I know there are scientific papers out there, and great minds working on this stuff.
Another thing to which there is no doubt a simple answer is that, if heat does not travel through a vacuum, how does the sun's heat reach the Earth? It is obviously transmitted as some sort of radiation that becomes heat when it reaches the earth.
If I sound like a dumb amateur that's exactly what I am. The scientific papers are full of complicated math, and Wikipaedia often just confuses more than enlightens ...


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## Ursa major

The terminal velocity in a vacuum would be, in theory**, a significant fraction*** of _c_. Until then, the object would be accelerating, provided it didn't hit the attractor or any fluid (e.g. atmosphere) surrounding it.






** - I can't visualise this in reality, particularly given the inverse square law.

*** - I can't be bothered to do the maths, but as the object's velocity approaches _c_, its mass increases, which should increase the attraction; but as momentum is conserved, increasing mass must act to reduce velocity.


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## skeptical

There is no terminal velocity in vacuum.   Of course, when the object being accelerated by a gravitational attractor is a long way from that attractor, acceleration is minimal, due to the inverse square law.  Acceleration only becomes strong when it gets quite close.   Which is why the final velocity, just before it hits the attractor, is usually only a very small fraction of light speed.


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## RJM Corbet

Well then, I will take your word for it! A lead weight, falling towards a planet, through a vacuum, would accelerate indefinitely, with the velocity/mass thing happening as the velocity approaches c? Regardless of the mass (and hence gravitational attraction) of the body towards which it is falling -- not orbiting -- falling directly -- like a skydiver? It's not what I was taught in school, but that was a _long_ time ago. An _orbiting_ body is _not_ accelerating. From what I understand here, you push it up to a certain speed, then stop pushing and, because there is no air resistance in the vacuum of space, it will continue to move at that speed indefinitely -- unless it hits something?


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## Ursa major

RJM Corbet said:


> Well then, I will take your word for it! A lead weight, falling towards a planet, through a vacuum, will accelerate indefinitely, with the velocity/mass thing happening as the velocity approaches c? Regardless of the mass (and hence gravitational attraction) of the body towards which it is falling -- not orbiting -- falling directly -- like a skydiver? It's not what I was taught in school, but that was a _long_ time ago.


We've mentioned _c_ only because that _is_ a limit, albeit one that won't really be approached when dealing with gravity-induced motion.

(I assume your objection to something travelling straight at an attractor is based on the objects' motions before gravity has a measurable effect on them, meaning that their approach won't be described by a straight line. If so, gravity itself is not the source of the curved path; its interaction with the objects' previous vectors is.)


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## chrispenycate

Any decent Gravitic drive applies the propulsive force to every molecule, to every particle  in the ship, so there is no need for an inertial compensator, as, unlike a reaction drive, the engines are not pushing the ship, but distorting the continuum. I have had discussions about this on the Weber "Honorverse" forum, and nobody's managed to shoot me down yet.

But, with a 100% efficient drive, the amount of energy required to accelerate something up to light speed is exactly equal to the total mass of the vessel. To accelerate a ship up to several billion times this speed would obviously require an external energy source, whether it be 'surfing the gravity wave' or taking vacuum apart into its constituent nothingnesses. And wit exceptionally high efficiency, too. 
Obviously, deccelerating gives all the same problems, but all this is only true if you need to have velocity, ie. have to exist in the intermediate points between source and destination. 



Metryq said:


> The article explains that if gravity showed the same aberration as light from the sun (the same delay, the same angle when it arrived), then the planets would speed up, thus widening their orbits and eventually flying off into space.
> 
> Gravity is not instantaneous, but on the scale of the Solar system, the difference is indistinguishable. Van Flandern's model puts the speed of gravity at about 20 billion times the speed of light. So a trip to the nearest star system, Rigil Kentaurus (better known as Alpha Centauri) would take 0.0000068953356 seconds—if my thumb-fingered calculations are correct. You'd push the "engage" button on your nav computer, and the stars would blink into new positions. You wouldn't even be able to perceive the duration of the trip.
> 
> And if you royally screwed up and forgot to include an inertial damping field in your ship, you'd look like a new coat of paint on the back wall of the command bridge.





> A lead weight, falling towards a planet, through a vacuum, will accelerate indefinitely, with the velocity/mass thing happening as the velocity approaches c? Regardless of the mass (and hence gravitational attraction) of the body towards which it is falling -- not orbiting -- falling directly -- like a skydiver? It's not what I was taught in school, but that was a long time ago.


Your lead weight, falling from infinity, will hit something long before light speed limitations cut in; remember, the gravitational attraction falls off as the square of the distance. But if you did happen to start with enough velocity that the extra gravitational acceleration would bring it into the critical zone (unlikely, as g is in cm/sec, and you just wouldn't have many seconds at that speed to make any difference in) then time contraction would make up the difference. If there were some way to generate a gravitational force other than a mass, this might change, but as it stands every mass in space, from a single hydrogen atom to the heart of a galaxy, has a maximum velocity it can generate by gravitational attraction alone, and this is its escape velocity– the velocity you need to attain to reverse the process. If this is in the region of light speed, you are almost certainly working with a black hole, and physics gets very non-intuititive in such regions.


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## RJM Corbet

Sorry Ursa, and now Crispen -- I was editing my post while you were replying to it. Let's ignore skydivers, who have adopted 'terminal velocity' into their own jargon -- for now. Yes, what I'm getting at is whether or not the gravity of the body towards which an object is falling -- straight down towards it, not orbiting it -- whether or not the gravity of the body affects the rate of descent -- taking away atmosphere resistance?


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## Ursa major

RJM Corbet said:


> whether or not the gravity of the body affects the rate of descent -- taking away atmosphere resistance?


The body accelerates; hence the phrase, acceleration due to gravity.


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## Chaoticheart

RJM Corbet said:


> Sorry Ursa, and now Crispen -- I was editing my post while you were replying to it. Let's ignore skydivers, who have adopted 'terminal velocity' into their own jargon -- for now. Yes, what I'm getting at is whether or not the gravity of the body towards which an object is falling -- straight down towards it, not orbiting it -- whether or not the gravity of the body affects the rate of descent -- taking away atmosphere resistance?



Assuming I haven't misinterpreted the question.  The short answer is yes. Different bodies, have gravitational fields of varying strength, thus the gravitational acceleration (the acceleration on an object as a result of gravity) also varies. 

e.g. At the same time on Jupiter and Earth two completely identical objects are held 500m above the ground. The two objects are then dropped. Because the gravitational acceleration on Jupiter is 25 m/s/s whereas Earths is a meager 9.8 m/s/s, the object falling on Jupiter will accelerate faster and thus land faster. (Values are approximate.)

Note: When dealing with objects at a point in space gravitational acceleration must be calculated using the following forumula:







where:
_M_ is the mass of the attracting object,





 is the unit vector from center of mass of the attracting object to the center of mass of the object being accelerated.
_r_ is the distance between the two objects._
G_ is the gravitational constant of the universe.

edit: Damn you and your doing the logical thing by giving a brief answer, Ursa!


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## chrispenycate

Of course gravity will alter the velocity of any body in its vicinity; that's what gravity is. A force, practically Newtonian. As such, its effect is the change of velocity, whether said velocity be aligned with the field, transversal or oblique.

Getting rid of skydivers, sky surfers and mountains, let us hypothesise a planet in interstellar space, smooth as a billiard ball (that coating used to be its atmosphere, frozen out in the somewhat cool conditions) It is slightly more dense than Earth, bringing its surface gravity up to a nice regular ten metres per second per second, and simplifying calculations. Drop a ball bearing onto it from rest at a few hundred kilometres distance – almost negligible on planetary scales – and we can predict it's distance, ever decreasing, and velocity, ever increasing, relative to the planetary surface, until it strikes same. The maximum speed it achieves is a bit less than escape velocity.  However, at no point has the increase in speed diminished, unlike Earth, where atmospheric friction would be siphoning off energy and converting it into heat, ultimately vaporising the object (what happens when it hits the surface is another question. Not too big a crater, I hope, as I'm going to need that smooth surface for later orbital and slingshot examples). The potential energy of position has been exchanged for kinetic energy of motion, and if the frozen gas surface were perfectly elastic the ball bearing would bounce back up to exactly the same height before coming to rest, and starting the operation over, like a superball from a skyscraper window, but the superball only makes it because its speed is low enough that atmospheric friction is minimal.


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## Vertigo

> An _orbiting_ body is _not_ accelerating. From what I understand here, you push it up to a certain speed, then stop pushing and, because there is no air resistance in the vacuum of space, it will continue to move at that speed indefinitely -- unless it hits something?


 
RJM - don't get speed and velocity mixed up. Speed is just that, with no consideration for direction. Velocity is speed and direction. A body in orbit maintains the same *speed*; with no air resistance there is nothing to slow it down. However its *velocity* is constantly changing; otherwise it would just be shooting off on a tangent. That change of velocity is brought about by the force of the Earth's gravity being applied to it and constantly accelerating it towards the surface (which it doesn't hit because its tangential velocity is always moving it parallel to the surface). So instead of flying of on a tangent it follows a circular (or elliptic) orbit. Of course if your speed isn't sufficiently high then the acceleration and the change in velocity required for a stable orbit would not balance and the orbit would decay.

Terminal velocity is purely an effect of resistance to motion (air resistance in this case). It is only true that a lead weight and a feather, released simultaneously, would hit the ground simultaneously *if, and only if,* they are both in vacuum. In air the lead weight will hit much sooner because it has a much higher terminal velocity and that is because the ratio of its surface area (that air resistance works on ) to its mass is *much *smaller than the feather, which has a very high surface area for its mass. If you were to take your lead weight and hammer it out really thin so that it has the same mass to surface area as the feather then they would indeed hit the ground together even in air.


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## RJM Corbet

Vertigo said:


> ... In air the lead weight will hit much sooner because it has a much higher terminal velocity and that is because the ratio of its surface area (that air resistance works on ) to its mass is *much *smaller than the feather ...


 
Oh _please_, Vertigo! Even _I_ know that. I thought we had agreed to leave skydivers out of this? Chaotic has answered the question. Yes: the greater the gravity, the greater the acceleration? So if you're swinging a bucket full of sand round your head on the end of a rope, the rope has to be strong enough not to break and send the bucket through your neighbour's window? Obviously. But take it further. I'm hearing that gravity is _curved,_ but the rope is always going to remain straight? Is gravity curved, or does gravity cause time/space to curve? Not the same thing. And the rubber sheet gravity thing assumes that Einstein's SR is final. It has worked in practice, so far, but seems to be getting a little dodgy these days? If the bucket is far enough away from you, like the sun, the bucket you see is going to be where it was 8 minutes ago. But at no time will the rope have curved or kinked? I'm sorry, it's a very primitive argument (and I'm not implying that the sun goes round the earth, by the way) but _gravity_ seems to have a lot of explaining itself to do. Now you have the _Higgs_ field, adding mass to pure energy -- acting very much as a medium for pure _energy_, wouldn't you say, as Newton's _aether _was doing for _light? _A purely hypothetical field, so far? I'm learning here, stuff I should know already before even opening my mouth -- but I don't see how the 'speed of gravity' = _c. _Sorry. And I really like the gravity bubble faster than light intergalactic space drive concept. Where else would I have got it but here?


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## Nik

There's a core problem in that gravity wave emissions can be predicted for stuff like binary pulsars, and the timing changes match the predictions to scary precision but, so far, IIRC, all attempts to detect gravity waves have failed...

This means that QED/QCD must stand aside from Gravity until some-one figures out how gravity *really* works and, if gravitons exist, how they fit into the particle zoo...

Hence the Big Hunt for the Higg's Boson: If that comes up negative, then a lot of theorists must get drunk and wheel out their wilder notions...

FWIW, the question was asked how heat travels through a vacuum. As electromagnetic photons, is the answer. If you use a prism to split sunlight, you can measure some radiant energy just beyond the red end of visible spectrum. This is infra-red (IR). Our Sun, a G0, gives off a lot of energy in infra-red. IIRC, Red Dwarf stars give off most of their energy in IR. Brown Dwarfs may only glow very faintly, and only in IR. NASA's WISE mission Wide-field Infrared Survey Explorer - Wikipedia, the free encyclopedia has already found lots of interest in our neighbourhood...


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## RJM Corbet

Nik said:


> There's a core problem in that gravity wave emissions can be predicted for stuff like binary pulsars, and the timing changes match the predictions to scary precision but, so far, IIRC, all attempts to detect gravity waves have failed...
> 
> This means that QED/QCD must stand aside from Gravity until some-one figures out how gravity *really* works and, if gravitons exist, how they fit into the particle zoo...
> 
> Hence the Big Hunt for the Higg's Boson: If that comes up negative, then a lot of theorists must get drunk and wheel out their wilder notions...
> 
> FWIW, the question was asked how heat travels through a vacuum. As electromagnetic photons, is the answer. If you use a prism to split sunlight, you can measure some radiant energy just beyond the red end of visible spectrum. This is infra-red (IR). Our Sun, a G0, gives off a lot of energy in infra-red. IIRC, Red Dwarf stars give off most of their energy in IR. Brown Dwarfs may only glow very faintly, and only in IR. NASA's WISE mission Wide-field Infrared Survey Explorer - Wikipedia, the free encyclopedia has already found lots of interest in our neighbourhood...


 
Thank you. The Earth's atmosphere reflects enough, absorbs enough radiation. 'How fragile we are?'
Gravity, as explained by Special Relativity, is the curvature of time/space caused by mass?
Enough mass, and you get the old bottomless 'black hole' -- pure gravity that has swallowed up the mass that originally created it. There, as Crispen observed, intuition must be abandoned at the 'event horizon'.
So gravity is not curved -- gravity _is_ the curvature, and everything -- light, time, space, mass, electromagnetic energy -- slides down the slippery slope to be forever lost, pulled downwards by _what? _
I know physicists have answers to these questions, and I am certainly not disputing the conclusions of people who do mathematics in 11 dimensions, and devote entire lives to the subject. Of course not. I'm being entirely sincere. But sometimes it takes a child to question the emperor's new clothes?
The _Higgs,_ by imparting mass, is also obviously imparting gravity?


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## Nik

A more subtle issue: Mass or Mass-Energy provides for inertia, which apparently simple feature of the world as we know it rapidly drives hapless theorists to strong drink...

FWIW, the Higgs, by imparting mass, may *only* impart mass, so that gravity and its transfer medium or wavicle could be a separate issue...

By the way, do you browse PhysOrg.com - Science News, Technology, Physics, Nanotechnology, Space Science, Earth Science, Medicine or Science 2.0 - ® The world's best scientists, the Internet's smartest readers. ??


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## Vertigo

Nik said:


> A more subtle issue: Mass or Mass-Energy provides for inertia, which apparently simple feature of the world as we know it rapidly drives hapless theorists to strong drink...


 
I have been wondering about that inertia issue, Nik. As I understand it inertia is not tied in to gravity, which seems like a bit of a problem with gravity based starship drives. It's all very well having some sort of anti-gravity so it would seem to be easy to accelerate away from a large gravity well, but then you still have to overcome inertia to get your spaceship moving which strikes me as taking us almost back to square one. Now my head hurts!


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## RJM Corbet

Nik said:


> A more subtle issue: Mass or Mass-Energy provides for inertia, which apparently simple feature of the world as we know it rapidly drives hapless theorists to strong drink...
> 
> FWIW, the Higgs, by imparting mass, may *only* impart mass, so that gravity and its transfer medium or wavicle could be a separate issue...
> 
> By the way, do you browse PhysOrg.com - Science News, Technology, Physics, Nanotechnology, Space Science, Earth Science, Medicine or Science 2.0 - ® The world's best scientists, the Internet's smartest readers. ??


 
No, but I will. Cool links. Thank you


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## RJM Corbet

Frame dragging has now been detected and measured exactly in line with Einstein's prediction. General relativity seems to have just received a new boost, as powerful as the one provided by the measurement of light rays bending during that eclipse? Here's the link:

www.nytimes.com/2011/05/05/science/space/05gravity.html?_r=2&hpw

(From G. Hancock's site. Thanks Vertigo)


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