Gravity: Attractive or Repulsive Force?

[Whoeveryouare]

If we say that a body of mass blocks the effect of gravity emanating from the objects on the far side of that body, then would it be possible to treat gravity as a repulsive force? What experiment could we run to determine whether it is attractive or repulsive? I exect this is this an idiotic question, but I'm not quite sure why yet.

 

[arildno]

If we say that a body of mass blocks the effect of gravity emanating from the objects on the far side of that body,

It doesn't "block" any effect.

then would it be possible to treat gravity as a repulsive force?

No.

I exect this is this an idiotic question,

Yes.

but I'm not quite sure why yet.

Because gravity is an attractive force that cannot be blocked by anything.

 

[Hurkyl]

If we say that a body of mass blocks the effect of gravity emanating from the objects on the far side of that body,

What you are describing is one of the models for gravity -- the universe is bathed in a wash of particles that have no effect other than to push things by bouncing off of them. And so, between any two objects, there would be a deficit of the particles needed to keep them separated, thus resulting in an attractive force between them.

My understanding (although I don't have any direct knowledge) is that "results in an attractive force" is the extent of the resemblance between this model and what happens in the real world -- there is no semblance of hope that this model could reproduce what we actually see.

 

[Whoeveryouare]

Thanks. That's something like I had in mind. Could you suggest an experiment to settle this question?

 

[Hurkyl]

Thanks. That's something like I had in mind. Could you suggest an experiment to settle this question?

Your question isn't nearly ready for experiment -- you haven't worked out any quantitative predictions at all that could be tested.

Technically speaking, you haven't even worked out any qualitative predictions, since the argument that bodies will be pushed towards each other is heuristic at best. Note that air pressure works in pretty much exactly the fashion I described, but it doesn't push objects towards each other except in special circumstances.

Anyways, I did a google search for "push gravity" (be wary of doing this yourself -- you will find a lot of crackpottery), and it brought me to this wikipedia article, which I assume is reasonably accurate. Look at the "recent activity" and the "predictions and criticism" sections in particular.

 

[Whoeveryouare]

I was hoping you'd be able to spot a testable prediction, but no matter.

Thanks for the link. I don't like Fatio's theory because the corpuscle idea seems a non-starter to me, as it is for an attractive gravity imo, but I'm impressed he took it so far. There are some advantages to a repulsive theory that make it seem worth exporing further than him, it seems to me, so I will continue to explore it, hampered by knowing so little physics. It was the accelerated expansion of the universe that got me wondering.

 

[Hurkyl]

I was hoping you'd be able to spot a testable prediction, but no matter.

Your hypothesis is simply too vague to be able to make predictions. It takes a lot of work to transform a vague idea into a physical theory that could be tested, and it's much, much harder to produce a physical theory that actually agrees with the information we already know. It's not until you reach this point that you can even begin to seriously consider the searching for new experiments to test and validate your theory.

I don't like Fatio's theory because the corpuscle idea seems a non-starter to me ... There are some advantages ... seem worth exporing ... hampered by knowing so little physics.

You have neither the knowledge nor the experience to judge the scientific merit of these ideas! You need to take a moment to understand why you feel the way you do (you're probably making purely aesthetic judgements) or you'll risk wandering straight into la-la-land.

If you're serious, the most important thing for you to do is to learn more physics. If you do not, then you cannot even understand what has to be done to develop a physical theory, let alone learn know how to identify and work to overcome the obstacles on the path.

a non-starter to me, as it is for an attractive gravity imo

Gravity is attractive. Period. No ifs, ands, or buts. It doesn't matter whether gravity is a remote attractive force between objects (Newton), the result of objects traveling in straight lines through a non-Euclidean geometry (general relativity) or caused by something like push gravity -- if a theory doesn't predict that apples fall to the ground, then it cannot be an accurate theory of gravity.

 

[Hurkyl]

Oh, silly comment. Speaking as someone who also doesn't have the ability to judge scientific merit, the first thing I would try if I really wanted to develop a "push gravity" model is to start with general relativity, puzzle out a way to reinterpret the metric tensor as describing a swarm of particles, and then use laws of general relativity and differential geometry to figure out the mechanical laws satisfied by these particles. The net effect would be to devise a push gravity theory that is mathematically equivalent to GR, and so is exactly as accurate as GR in describing the universe.

The drawback is that the particles would probably have really weird mechanical properties, so the people who are attracted to push gravity for aesthetic reasons would probably reject the above for those same aesthetic reasons.

(Surely someone has looked into this already. Someone has probably already even succeeded. But I wouldn't know how to go about looking for it)

 

[jtbell]

The "push gravity" idea is most commonly associated with Le Sage. There's a Wikipedia page which seems to be pretty good:

http://en.wikipedia.org/wiki/Le_Sage's_theory_of_gravitation

 

[Phrak]

It was not an idiotic question.

In order to obtain an inverse square law of attraction, the momentum of the 'pushing flux' must be attenuated in proportion to the mass density of each element within the attracting bodies.

Also, the amount of attenuated momentum of the flux from any given direction must be very small in comparison to its total momentum, or the gravitational force of attraction will deviate from the inverse square law; it will become progressively weaker than expected for increasing masses.

 

[Whoeveryouare]

Hurkyl

I understand why you feel so strongly that I shouldn't be bothering myself with these issues, not being a physicst and all, but I think you underestimate how simple some of them are in philosophy, where all one has to do is study principles. It doesn't take a genius to see some difficulties with the corpuscular theory, nor much effort to see that many physicists have difficulties with it also. I think your suggestion for my education shows that you're very clever, but I'm managing okay for the moment thanks.

 

[ZapperZ]

Hurkyl

I understand why you feel so strongly that I shouldn't be bothering myself with these issues, not being a physicst and all, but I think you underestimate how simple some of them are in philosophy, where all one has to do is study principles. It doesn't take a genius to see some difficulties with the corpuscular theory, nor much effort to see that many physicists have difficulties with it also. I think your suggestion for my education shows that you're very clever, but I'm managing okay for the moment thanks.

Please note that we are dealing with physics, not philosophy. This means that any proposed model must have a quantitative, not just qualitative, prediction that either conforms to already-established principles, or conforms to experimental observations. Lacking such aspects will reduce what you are proposing as an unverified personal theory based on ignorance, and that violates the https://www.physicsforums.com/showthread.php?t=5374" that you had agreed to.

Zz.

 

[Whoeveryouare]

I don't get this. I didn't come here to start an argument, and I certainly had no intention of proposing an unverified personal theory based on ignorance that violates the PF Guidelines that I have agreed to. This is clear from my posts. I went to great pains not to propose anything. How did it come to this?

I came here because try as I might I cannot think of a thought experiment that would decide whether gravity, whatever it is, is in effect attractive or repulsive. I thought perhaps there was a well known experiment in physics that settled the issue, or that someone could suggest one. I'm not proposing a theory for which it's one or the other, I'm asking for the benefit of your expertise on a general question.

The answer I seem to have been given is that in physics we don't know which it is, but as yet have no repulsive theory that works.

I suppose I dismissed Fatio's corpuscular theory rather casually for a layman, as if I understood the theory and the physics behind it, which clearly I don't, which would have seemed ignorant and been rather annoying. But the problems for such theories arise in all sorts of contexts in philosophy, for Russell's neutral monism, for example, and almost as soon as I started reading the Wiki article about this theory I felt qualified to dismiss this aspect of it. I took note first, however, that many physicists do the same.

Where did I go wrong?

 

[M Grandin]

In youth I happened to read (in a book aimed for young guys) about a question of why timber logs floating in water gathered in "heaps" and not continued float freely. The answer was, that the logs covered each other from wind and waves, resulting in approaching each other until close together.

Later as adult I thought this might be used as explanation of Newton gravity. Some simple calculus resulted in something resembling Newton's inverse square law. Ackording to what is discussed here. I understood others must have thought of the same thing - but hadn't seen anything written about it. My presumptions, like someone also mentioned here, was a kind of beams that only very mildly were attenuated by matter. Not collissions, just passing through almost unaffected. But it resulted in formula where gravity is proportional to mass and inversely to square of distance.

But I never continued these calculus (for instance about power necessary and resulting heating). The main reason for my not continuing this, what that it only explained the weak force gravity. To be interesting, it should also be able explaining strong forces like electromagnetism - and why not even common mechanical forces?

But maybe also strong forces could be explained in corresponding way - using these "imaginary" beams. For instance a mechanical construction could be regarded as a set of "wave guides" where forces are result of reflected power from beams of this kind inside
the construction? (If these beams also may have reflecting qualities - just loosely thinking). The possibilities are unlimited.

 

[jtbell]

The answer I seem to have been given is that in physics we don't know which it is, but as yet have no repulsive theory that works.

Physics is all about "what works." If a theory does not make predictions that agree with experiment, or fails to explain certain well-verified aspects of the phenomenon that it is supposed to explain, then it's not going to make much headway among physicists.

For example, general relativity predicts a form of time dilation in connection with gravitation, which has been verified by experiment. As far as I know, Le Sage type gravitation theories don't address this at all.

 

[Whoeveryouare]

Okay. But I was not supporting Le Sage's theory, and assume it would be a hotter topic if it worked. I still have the impression (with MGrandin) that there is nothing that finally rules out the possibility of a repulsive theory which would account for the data equally well as an attractive one.

Mind you, I suppose that if gravity is the curvature of spacetime then perhaps attraction and repulsion are not the right concepts in the first place. Would it be only for Newton's theory that this question arises?

 

[Hurkyl]

I came here because try as I might I cannot think of a thought experiment that would decide whether gravity, whatever it is, is in effect attractive or repulsive.

Gravity is attractive: apples fall to the ground rather than fly up into the sky. (At least it's attractive in this and similar examples)

The answer I seem to have been given is that in physics we don't know which it is, but as yet have no repulsive theory that works.

We know that gravity is geometric in the strongest sense possible in science -- general relativity agrees quite accurately with observation, and more so than its competitors.

Where did I go wrong?

It's not your conclusions, but the reasoning behind them. Science "won" in the field of natural philosophy, providing both a systematic method for developing theories and an objective standard for evaluating those theories. It doesn't really matter how much a given theory adheres to or violates whatever principles you hold dear -- in the end, all that matters is whether or not it let's you make accurate predictions about "reality".

Philosophy is a much harder subject than you give it credit for -- it is generally exceedingly difficult to evaluate your ideas and tell if you're on the right track. Sure, you can come up with all sorts of criteria to evaluate your musings, but you've just shifted the problem, because it's difficult to evaluate your criteria to tell if they tell you anything useful, and so forth.

 

[Whoeveryouare]

Gravity is attractive: apples fall to the ground rather than fly up into the sky. (At least it's attractive in this and similar examples).

You don't need me to point out that the fact that apples fall to the ground doesn't settle the question here. They might have been pushed.

It's not your conclusions, but the reasoning behind them. Science "won" in the field of natural philosophy, providing both a systematic method for developing theories and an objective standard for evaluating those theories. It doesn't really matter how much a given theory adheres to or violates whatever principles you hold dear -- in the end, all that matters is whether or not it let's you make accurate predictions about "reality".

Other than the idea that science 'won in the field of natural philosophy', which I don't understand, let's take all this for granted. What follows for gravity? Why do you assume I'm trying to hold onto a principle I hold dear rather than just asking a question about physics? You forgot to mention what is wrong with my reasoning.

Philosophy is a much harder subject than you give it credit for -- it is generally exceedingly difficult to evaluate your ideas and tell if you're on the right track.

It would depend on the idea. Often they're incredibly easy to evaluate.

Can we start again? I seem to have given you the impression that I'm an idiot.

 

[ZapperZ]

If I set up a classical attractive force in my Hamiltonian, I can solve for the equation of motion for practically ALL of the dynamics that I see.

If I set up a classical "repulsive" force in my Hamiltonian, I can solve for the equation of motion, but it agrees with NONE of the dynamics that I see.

Which part of this do you have a problem with?

Zz.

 

[Whoeveryouare]

Why should I have a problem with this? It's the best answer I've been given.

Is this a secure proof that gravity is attractive, or could it be an artefact of the way the mathematics is set up?

 

[ZapperZ]

Why should I have a problem with this? It's the best answer I've been given.

Is this a secure proof that gravity is attractive, or could it be an artefact of the way the mathematics is set up?

There are no "proofs" in physics. There are valid description. I've shown you how that description works. Now show me why it doesn't that some requires you to reinvent it.

Zz.

 

[Whoeveryouare]

Why would I bother reinventing something that doesn't need reinventing? Wouldn't that be waste of time?

At any rate, what you say suggests that although your observation about Hamiltonians is interesting it does not answer my question.

I'll abandon this discussion because my question has been answered to my satisfaction and I'm sure you'll agree it's becoming rather silly.

 

[ZapperZ]

Why would I bother reinventing something that doesn't need reinventing? Surely it would be waste of time. Why have I been branded an opponent of physics when I'm a fan of it?

At any rate, what you say suggests that although your observation about Hamiltonians is interesting it does not answer my question.

Let's recap this, shall we?

You asked:

If we say that a body of mass blocks the effect of gravity emanating from the objects on the far side of that body, then would it be possible to treat gravity as a repulsive force? What experiment could we run to determine whether it is attractive or repulsive? I exect this is this an idiotic question, but I'm not quite sure why yet.

Are you saying that this question has STILL not been answered, after ALL of the posts in this thread?

Zz.

 

[russ_watters]

Why would I bother reinventing something that doesn't need reinventing? Surely it would be waste of time.

Perhpas you should answer that question for us: you started the thread!

Why have I been branded an opponent of physics when I'm a fan of it?

You asked a question challenging one of the most successful physics theories of all time, with no real basis but idle speculation to back the challenge*. That would seem to make you an opponent of physics. As you said: why would you bother reinventing something that doesn't need reinventing?

What I dislike about threads like this is that you've come to us with an idea that you can't/won't develop yourself and doesn't seem to us to have any value and you expect us to develop it for you. Why would we bother doing that?

*That's even a little strong: you haven't really even speculated about what improvement in our understanding of the universe could come of our pursuit of this idea. Would an inventor set out to invent something that he has no reason to believe is useful? Would you get in your car and drive somewhere that you have no reason to go to? Why bother?

 

[Whoeveryouare]

Are you saying that this question has STILL not been answered, after ALL of the posts in this thread?Zz.

What I am saying, as you can quite easily verify, is that my question has been answered. I thought I made this clear.

 

[Whoeveryouare]

Perhaps you should answer that question for us: you started the thread! You asked a question challenging one of the most successful physics theories of all time, with no real basis but idle speculation to back the challenge. That would seem to make you an opponent of physics. As you said: why would you bother reinventing something that doesn't need reinventing?

What I dislike about threads like this is that you've come to us with an idea that you can't/won't develop yourself and doesn't seem to us to have any value and you expect us to develop it for you. Why would we bother doing that?

Just for a moment try relaxing and seeing this from my point of view. I ask a simple question about gravity and next thing I know I'm receiving insults from moderators. I'm not trying to develop a new theory of gravity, I'm interested in symmetry. I had a follow up question about symmetry but can't ask it because everyone is so defensive that they assume I'm defending some crackpot theory. I'm not. I couldn't care less whether gravity is attractive or repulsive, I'm interested in why we're so sure it's attractive. I asked a simple question and expected a simple answer.

It may be rather a chore, but perhaps you could explain to me why this question caused so much trouble. What I have I said that would lead anyone to think I was proposing or defending a theory? I asked only whether we could rule out theories for which gravity is repulsive and how we would do it. Hell, even when I clearly state that I think my question has been answered I get a reply asking me why I think it hasn't been.

Be as honest as you like. If I said something stupid point it out.

 

[ZapperZ]

What I am saying, as you can quite easily verify, is that my question has been answered. I thought I made this clear.

Then this thread is done, no?

Zz.

 

[Whoeveryouare]

Yes, or that was what I suggested. Unless you have time to help me figure out what went wrong.

 

[jtbell]

It may be rather a chore, but perhaps you could explain to me why this question caused so much trouble.

The problem is that forums like this attract true crackpots and cranks like flies, and their discussions often start off very much like yours did, as possibly innocent questions from newcomers to physics. Only after the discussion has progressed for a bit does it become clear that they actually have an agenda to promote and they aren't really interested in learning physics.

 

[Whoeveryouare]

Fair enough. And I suppose my last question was rather disingenuous, since I do understand this problem and had already reached the same diagnosis. In some ways email is a terrible way to communicate.