Orbits are impossible: a conundrum

siarad · Aug 25, 2004

Ah that old story. What makes anything follow the curve as it's not gravity which caused it.

asuscreative · Aug 25, 2004

i always find it easier to think of an object in orbit staying in orbit because its centripical force ,as a result of moving in a curve, is equal and oppisite to the force of gravity

CalliArcale · Aug 26, 2004

<blockquote>In reply to:<hr />What makes anything follow the curve as it's not gravity which caused it.<hr /></blockquote> But it is gravity that caused it. Whether gravity is curved spacetime or a force acting at a distance is immaterial; either way, gravity caused the deflection. I guess I'm not understanding what you're saying. This is entirely possible, as I have had only one can of Mountain Dew today. <img src="/images/icons/tongue.gif" /> <div class="Discussion_UserSignature"> "People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff." -- The Tenth Doctor, "Blink" </div>

siarad · Aug 26, 2004

If gravity caused the curve in space then it can't also be holding an object to it, that's a duality. I'm after why objects follow the curve. After all you can go straight across a curve by powered flight. This may be one of the problems of the three gravities. It's the fallacy of the dented rubber sheet where gravity is doing two things.

jitte · Aug 26, 2004

I'm not sure I follow where you get this idea from: "If gravity caused the curve in space then it can't also be holding an object to it" But in my understanding, the Cosmological Constant is a repelling force paralleling conventionally perceived gravity but acting in the opposite direction, a Universal Repelling Force originating in all Matter and projecting across space at the speed of light. ----------------

nacnud · Aug 26, 2004

The cosmological constant was a fudge factor added to general relativity, its not implicit but was added in to produce a steady state universe, remember it was before Edward Hubble and the big bang theory existed. Also when considering if gravity is a force or a result of curved spacetime you should remember that these terms should be used only to describe the effects of gravity not define what it actually is. When influenced by gravity a body will behave in a certain way. If you consider gravity to be a force and apply the maths to predict the motion of the body the result is very accurate but not exactly right. Considering gravity to act by curving spacetime you have to use different mathematics and this turn out to be closer to what actually happens. Neither method defines gravity but can be used to describe its effects.

Saiph · Aug 27, 2004

A horizontal firing is not the division between a parabolic and elliptical orbits. Only velocity is a factor (not orientation of launch). The object follows a curved path because, by newton, it's constantly deflected (as calli said) from a straight path. This deflection causes it to turn towards the earth. However, if the object moves fast enough (for a horizontal launch), gravity cannot curve it enough to make the object fall to the earth. The object falls, but misses. Einstein posed a different answer, that was compatible with relativity theory. That is, the object follows a straight path. However mass warps spacetime (in an effect known as gravity). In this new, curved geometry (as opposed to Newton's use of a flat one), the straight line, while straight, is curved back upon itself. So the gravity isn't "holding" anything. It's letting it go in a straight line. However that line through space has been altered. <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

siarad · Aug 28, 2004

<blockquote>In reply to:<hr />A horizontal firing is not the division between a parabolic and elliptical orbits. Only velocity is a factor (not orientation of launch).<hr /></blockquote> But can you prove it?

Saiph · Aug 29, 2004

Yeah, I can show you a verticle launch that results in both hyperbolic, and elliptical orbits. Saturn V launches are hyperbolic, Shuttle, satelite, ICBM's etc are all elliptical. Parrabolic orbits, in the true sense (not just a truncated elliptical, cuase it hits the ground) are harder to explain. <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

siarad · Aug 30, 2004

Not trying to put you on the spot. <img src="/images/icons/laugh.gif" /> However I'm talking of horizontal launches & unpowered during flight. I'm not able to do the maths but just day-dreamed er thought experimented, over it.

Saiph · Aug 30, 2004

Despite the rockets being "powered" most of thier trip is unpowered. As such the dynamics are relatively similar. Anyway, very good explainations have already been posted that are correct, and properly explain the point at hand. <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

CalliArcale · Aug 31, 2004

<blockquote>In reply to:<hr />If gravity caused the curve in space then it can't also be holding an object to it, that's a duality. I'm after why objects follow the curve. After all you can go straight across a curve by powered flight. This may be one of the problems of the three gravities. It's the fallacy of the dented rubber sheet where gravity is doing two things.<hr /></blockquote> Okay, by "holding an object to it" you mean like a rock staying stuck to the Earth and not floating away, right? It's not a duality -- it's exactly the same thing happening in both situations. Gravity is producing a downward acceleration. If an object is propelled forward by a cannon at, say, a 90 degree angle to the line between the cannon and the Earth's center of mass, that downward acceleration will result in a trajectory that curves gently down. If the object is simply set on the ground, the force of gravity is still acting on it. But you don't see the object accelerating downwards, because the ground is in the way. Instead, what you get is weight -- the force the object's mass applies on the ground if it has gravity acting upon it. You'd get exactly the same weight if there was no gravity and you attached a magical massless rocket motor to the object and applied thrust equivalent to what we on Earth regard as the object's weight. Weight is really just a measure of force, and the force is usually gravity. (It doesn't have to be, though, which is why you can be weightless within a gravity field, such as on board the Vomit Comet.) So the object sitting on the ground (held to the Earth) and the object being deflected into a curved trajectory are both experiencing exactly the same force. It's not a duality. It's the same thing in both cases. BTW, the "dented rubber sheet" is entirely separate from Newtonian physics. It's an entirely different way of looking at it. <div class="Discussion_UserSignature"> "People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff." -- The Tenth Doctor, "Blink" </div>

unclefred · Aug 31, 2004

Dear Saiph. you said "Einstein posed a different answer, that was compatible with relativity theory. That is, the object follows a straight path. However mass warps spacetime (in an effect known as gravity). In this new, curved geometry (as opposed to Newton's use of a flat one), the straight line, while straight, is curved back upon itself. So the gravity isn't "holding" anything. It's letting it go in a straight line. However that line through space has been altered. " While I don't particularly disagree, I question what you mean by your explanation. You say that an object will follow a straight path. What do you mean by that? Consider that my spaceship is going to pass by some mass and my path will be appropriated affected. Before I get there I put a ball directly ahead of me and another directly behind me. Both are on the exact same path as me. Just to make sure that we don't collide I push the ball ahead of me just slightly and slow down the ball behind me just slightly. What happens? Since we are just following straight lines and we are on identical paths, we should just follow each other. No so. Our paths will diverge and we will come out in 3 different places going 3 different directions. Thus the "straight line in curved space" concept leads one to all sorts of wrong conclusions because our normal definition of "straight line" does not apply.

Saiph · Sep 2, 2004

actually, if you place them ahead, and behind (so they aren't moving relative to you) they will stay in the same place. The three objects will stay on the same path, and go the same speed. Only if their velocity is a bit different, will they end up going different directions. <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

unclefred · Sep 2, 2004

That is exactly what I said. Glad that you agree with me. If everything is traveling along the same line to begin with and, as I have heard many times and again in this thread, space is warped such that things travel in straight lines. What does that mean? The answer is obvious, unless I am missing something: the definition of a straight line must depend on your speed. If that were not true then how else can the objects sometime follow you and sometimes diverge, as you just agreed is the case.

Saiph · Sep 3, 2004

:::shrugs::: GR is beyond me at the moment. There's probably a neat and tidy way of dealing with the velocity issue, most likely to do with energy conservation, and treating the curved geometry as a well (like those coin wells at grocery stores). Most likely if you go fast enough for a region, GR might not be able to curve your path fast enough, and as such you head "out" until it can (you slow due to loss of KE to PE). <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

newtonian · Sep 5, 2004

Truly stable orbits are impossible for humans. Saiph, you all - Newton was corrrect, so was Einstein- Einstein simply added more detail. Yes, we are still learning the statutes of the heavens, including gravity: (Job 38:33) 33 Have you come to know the statutes of the heavens, Or could you put its authority in the earth? Newton believed the Bible and learned something of the 'nothing' which holds earth in place (orbit): (Job 26:7) He is stretching out the north over the empty place, Hanging the earth upon nothing; Now, how stable are the orbits of man-made satellites compared with the earth and moon?

spacester · Sep 6, 2004

Very interesting question. I think the three objects are still all following straight-line paths in spite of differing trajectories. As they pass through space-time, the time coordinates are not the same, and the rate of change in position is changing as well. This transaction in energy exchange is taking place in a *non-inertial frame of reference* (right?), plus the deflection maneuver takes place within several gravity fields. Somehow, that adds up to three different straight lines. <div class="Discussion_UserSignature"> </div>

Saiph · Sep 7, 2004

Ummm, paul, they're entirely possible for humans. You just can't have them to close to an atmosphere enshrouded planet (you know, drag an' stuff). Geosynchronous satellites are good examples. They really shouldn't go anywhere. <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

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