Science a GoGo's Home Page Forums General Discussion General Science Discussion Forum This is a different take on what gravity is

Yes but it's also part of physics. I forgot to mention that F and d should be vectors, and their multiplication should be the dot product. It probably has the same effect as the cos factor you added.




Yes energy is used. The horse is converting its food energy into heat and perhaps some chemical energy, rather than work. But that's only because of the inefficient design of the horse. Imagine instead of a horse there's a big spring under tension, it's trying to pull the block but isn't strong enough. The spring continually applies a large force to the block but nothing moves. Here the spring is much more efficient than the horse because it doesn't consume energy when its not doing any work. You can leave it for years applying the same force and never have to recharge it.




Yea.


Try doing that in the absence of friction. You can just set it down and walk away. Come back next year and it'll still be spinning by itself.

There is some friction in the moon's orbit, caused by tides. So it will actually be losing energy. But this isn't required in principal so it doesn't matter.

I can't answer the question adequately, Bill, since it appears that to do so would require a working knowledge of tensor mathematics and Einstein's field equations. All I can tell you is that, as I understand it, the mass of the Earth defines the geometry of local spacetime. That's why no energy is expended.

Thanks for your patience in trying to explain these things to me, folks. Nimium vetus sum pro hoc stercore!


Let’s see if I have this straight in my mind.

1. The moon is orbiting the Earth without expenditure of energy because it is moving in a frictionless environment, where, according to Newtonian physics, it will continue to do so ad infinitum, unless energy is applied to stop or divert it.

2. The moon assumed its orbital position because the presence of the Earth’s mass distorts spacetime in such a way as to create the geodesic which the moon follows.

3. If point one is correct, the continued presence of the geodesic is not required to keep the moon in orbit.

4. The only way we can know that the Earth is still distorting spacetime is that other objects can assume Earth orbits, and there appears to be a “force” of gravity.

5. If the Earth could suddenly be removed from the scenario, the moon, according to Newton’s laws (and Kallog’s reasoning) would continue to orbit the spot where the Earth had been.

6. Presumably, when the Earth was removed, spacetime would return to its original configuration; suggesting that some energy must have been required to maintain the distortion.

Are you not overlooking the fact that the spring behaves as an elastic solid only for a finite period of time, albeit a long time? Eventually it will behave as a plastic solid. Its molecules will re-arrange themselves, with production and loss of heat, so over the long term the energy in the spring will be lost. Perhaps you would need to be holding one end of the spring to appreciate the energy exchange involved.

Someone please to splain to me what time has to do with gravity?

I see this term "spacetime" thrown about all the time like you folks understand it.

Care to couch an explanation in a way that even I can understand it?

And no, not with the bowling ball on a trampoline analogy, that's all that is, an analogy, you folks take that way too literally.

And when I say "in a way that even I can understand" I really mean in a way that shows me YOU understand it.

Bill, in reply to your points above

3. If point one is correct, the continued presence of the geodesic is not required to keep the moon in orbit.

- According to GR, the geodesic exists regardless of the Newtonian explanation - an explanation that's almost a perfect fit to reality. Almost. The difference is that Newton, using the knowledge at his disposal, would be unable to explain certain observations, and would find himself in error when provided with extremely accurate measuring tools. GR has proven to be as accurate as may be determined by today's methods of measurement, on larger scales where Newton's theories are shown to be approximations.

4. The only way we can know that the Earth is still distorting spacetime is that other objects can assume Earth orbits, and there appears to be a “force” of gravity.

- The only way we can be convinced of the validity of GR theory is to compare objective reality with its predictions. GR theory has superceded Newton in this respect, even though the latter is simpler, and suffices for most everyday purposes (including local space exploration).

5. If the Earth could suddenly be removed from the scenario, the moon, according to Newton’s laws (and Kallog’s reasoning) would continue to orbit the spot where the Earth had been.

- According to Newton, the Moon would immediately shoot of at a tangent. According to Einstein, there would be a delay of just over a second.

6. Presumably, when the Earth was removed, spacetime would return to its original configuration; suggesting that some energy must have been required to maintain the distortion.

- Despite having declared that (I think) it doesn't, it does seem reasonable doesn't it. I find myself relying on snippets of info that I've gleaned over recent years, and I'm very much the unqualified non-authority. This particular snippet might interest you:

Gravitational Faraday Effect Produced by a Ring Laser - http://www.phys.uconn.edu/~mallett/Mallett2006.pdf

Using the linearized Einstein gravitational field equations and the Maxwell field
equations it is shown that the plane of polarization of an electromagnetic wave
is rotated by the gravitational field created by the electromagnetic radiation of
a ring laser.

Marchimedes: "Someone please to splain to me what time has to do with gravity?"

I agree, it is difficult to understand. This knowledge has been derived from theoretical maths/physics that's beyond most of us, and we non-maths bods have to be content with the fact that the observations fit the theories (until they don't), even if we don't have a clue how the answers are found.

You could try getting your head round this:

http://www.astronomycafe.net/gravity/gravity.html

"Perhaps the most unusual thing about gravity we know about is that, unlike the other forces of nature, gravity is intimately related to space and time. In fact, space and time are viewed by physicists, and the mathematics of relativity theory, as qualities of the gravitational field of the cosmos that have no independent existence. Gravity does not exist like the frosting on a cake, embedded in some larger arena of space and time. Instead, the 'frosting' is everything, and matter is embedded and intimately and indivisibly connected to it. If you could turn off gravity, it is mathematically predicted that space and time would also vanish!"

Sorry, Marchi, I'm no professor. Maybe one will turn up to give you the kind of answer you're looking for.

Marchi, have a look at "The Road to Reality" by Roger Penrose. Much of what you seem to be having trouble with is covered there. OK, so he does go over the top with the maths, but when it comes to this sort of physics if you don't know the maths you know nothing.

I think you're confusing stored energy with continually spent energy. Stored energy can just stay there till it leaks away as you suggested. When it does leak away it's not being used to apply the force, it's just wasted as heat.

People and animals aren't very good at storing elastic energy in our muscles. We tend to consume energy just holding something still against a force. But that's only because we're not designed for that purpose.

Surely there is a difference. If the spring is under tension or compression, the molecules re-adjust in response to the "work" being done to maintain that position. without compression or tension, the energy leaks away, more slowly, as a result of natural deterioration of the metal.

I don't quite understand you, but you seem to be saying leaking potential energy is necessary to apply a force.

If it was necessary, surely some specific rate of energy loss would be required for a particular force. Yet different springs last different lengths of time. We can design springs to have arbitrarily long lives but all applying the same force.

.

My point was simply that the rate of dissipation of energy is greater when the spring is "trying" to do work, even if it is not succeeding within the meaning of the work function. Is this something that can just be ignored in order to make fact fit theory?

Yes it can. The theory only describes work, not other types of energy. If the spring is radiating light, then that can be ignored too.

Can you explain why something should use up energy when applying a static force?

Sorry, I don't do explanations; just ask naive questions. However, I accept that trying to explain something to someone else is possibly the best way of understanding it. I might stand a better chance of trying if you could first explain how something can apply any kind of force without expending energy.
I can see the possible answer: F=ma, so if a=0, m=0, so F=0; and I can equate that to moving objects. I can also see, to some extent, how that this fits with the work function, but I have problems accepting that this is the whole picture, especially where no movement is concerned.

Here are some lay-person's thoughts about gravitational potential energy. Some kicking about would be of value.

Consider a rock lying on the ground. If you pick up that rock, then let go of it, it will fall back to the ground. Your action in picking it up has involved a transfer of energy. Some energy from your muscles has been converted into gravitational potential energy as you raise the rock. If, when you have lifted the rock, you place it on a shelf, and let go of it, it will not fall, but it will still have the energy you transferred to it, still in the form of potential energy.

Obviously, if it is pushed off the shelf it will fall back to the ground. The argument here seems to be that the energy you put into the rock as you lift it equals the energy that would be necessary for gravity to bring it back to the Earth’s surface, so there is no net expenditure of energy. Gravity does not expend energy, therefore gravity is not a force.

Could this explain how gravity seems to work without any apparent energy source? A little thought about this situation must raise some doubts. For example, if the attraction of gravity is directly related to the amount of energy put into the rock you are lifting, why does gravitational attraction not increase with distance, as would be the case if you were stretching a spring? the higher you lift the rock the more potential energy you impart to it. What, then, about a spacecraft that travelled from Earth to the moon, why would it be attracted by the moon’s gravity? Not only would it not have been lifted from the surface of the moon, but it should have enough potential energy to whisk it straight back to Earth as soon as it stops moving away.

Returning to the rock, if, having picked it up you altered your position so that you were holding your rock over an open well; when you released it you would be very surprised if it did not fall to the bottom of the well, in spite of the fact that you transferred to it only enough energy to take it as far as the ground surface.

There are more thoughts about, but I need to get them into some sort of order before posting them.

Indeed. I gain a great deal from forums like this for exactly that reason.


I can see the possible answer: F=ma, so if a=0, m=0, so F=0;
..
especially where no movement is concerned. [/quote]

With no movement that equation doesn't help much. Remember F is the total force, so if it's not accelerating then there must be some other force counteracting the applied one.

Doubtful bit of logic there but I think I see what you mean. All the energy originally came from your muscles, and it was just held by the gravitational field for a while, then released to some other form when the rock fell off.


The higher you lift it, also the further it can drop, so the more energy is released. I suppose that's where the saying "the higher they climb the harder they fall" comes from :P For a spring the relationship between extension and force is different, and can be customized by designing the spring differently. But still, whatever energy you put in is available to be extracted again.


Because the effect of the moon's gravity becomes greater than the earth's only when you're up close. It's a bit like having a magnet on the ceiling - throw a paperclip up and it usually falls down, but if it gets close enough to the magnet then it can be held up indefinitely. It still has potential energy ready to be released by falling down, but to release that energy you first have to input some energy.

Magnets, moons and springs are all just devices for storing energy not supplying it. You can't get anything out of them without putting something in.



Yes but the rock already had some extra potential energy before you picked it up, just by being on the ground above the bottom of the well. It's that energy that's released.

Not my logic. Einstein's, according to various P S books. Perhaps it looks better the other way round. Gravity is not a force, therefore it does not expend energy. Seems much the same to me though.



Suddenly gravity becomes a force? The moon attracts the spacecraft without the craft having been lifted off the moon?
Where does the energy come from?



That's where the "other thoughts" were leading. It seems as though everything has sufficient gravitational potential energy to bring it into contact with everything else in the Universe, but where does the energy come from? How's this for an idea?


Gravity is the curvature of spacetime, and it appears that the degree of curvature is directly related to both the mass and density of the body causing the curvature. For example, a body of the mass and density of the sun will cause relatively gentle curvature over a large area. If this mass were compressed to the size of the Earth, the curvature of spacetime around it would be much more severe.

Given a situation in which an enormous mass, such as the total mass of the Universe, is compressed into an unthinkably small “speck”, with a diameter no greater than the Planck distance, we might just be forgiven for referring to the resulting curvature of spacetime as “infinite”. (:P) This, we are told, approximates to the state of the Universe at the instant of the Big Bang. If this is the case, it follows that every particle of matter and energy in the Universe, at the start of its life – or of this cycle of its life – occupied the same point in spacetime. The energy, whatever its source, that caused this infinitesimal, primordial speck to expand, transforming itself into billions of light years of spacetime would also have caused the curvature of spacetime to expand as well, and to “soften”, but, it would always remain curved, thus it would always tend to return to its original condition, like the rock falling back to Earth once the restraining force has been removed. This would mean that the energy which drives gravitational attraction is the potential energy imparted to every particle in the Universe by the Big Bang. Thus, there is sufficient potential energy within the Universe to bring every particle back to an infinitesimally small speck.

It's not necessary to use any modern physics to decide if gravitating objects should be continually spewing out energy. I don't know about that stuff so I can't comment.


It all makes sense if you compare it to the magnet on the ceiling example. If you want to say gravity isn't a force then it'll still work out but not in a way I can explain.

Of course if gravity isn't a force then throw away F=mg and F=G m1 m2 / r^2, and even F=ma because an accelerating, falling object has no force applied to it. I'm not being sarcastic, but just pointing out that it's not going to be easy to use GR to sort out this problem which can also be solved classically.



I was just about to say the same thing :P



Yea. Tho it may not be easy to release that potential energy because the things also have kinetic energy which, if it is enough, has to be dissipated before they can stop seperating and start coming back together.

I agree, this is a large part of my problem, but then I read things like: "Today we understand gravity through Einstein’s theory rather than Newton’s, and we know that no such force exists.....Nothing is pulling us downwards." (David Deutsch. “The Fabric of Reality”), and I think: all these chunks of matter are being diverted/moved about by something that is distorting spacetime, but "nothing" is doing it.