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Lately I've been contemplating this analogy to better explain the phenomena we call gravity to my children. Since I'm not a physicist or a mathematician, I'd like some input on the validity of my analogy.

Consider that a helium filled balloon will move from a high atmospheric pressure to a lower one. The medium in which the balloon seeks equilibrium is our atmosphere, a.k.a. air.

Just for the analogy, if we consider spacetime as a similar medium to air, we can explain that the presence of matter in this medium will create a low-pressure situation in the spacetime in which that matter exists. Therefore, it's easy to see how gravity is really a buoyancy effect on matter in the medium of spacetime. It seems like a loop in causality, since the presence of matter is what causes the low-pressure in spacetime which then causes nearby matter to be pushed by spacetime via buoyancy towards the center of the low-pressure depression.

This may seem trivial but I've always had a problem with gravity being described as a pulling force. I like to think of it as the result of spacetime density when matter is present.

To extend the analogy, I think the idea of spacetime having varying density can also explain how what we call gravity waves exist. Waves in air (sound waves) are the compression and decompression of air moving out in all directions from its point of origin. Gravity waves have this same effect on spacetime. Spacetime is compressed and decompressed in a wave pattern that travels out from the point of origin. So, shouldn't these gravity waves be called spacetime waves?

Does this compute?

Eric Lord
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  • You thinking is correct with one exception. The quantity you are looking for is not the "density" of spacetime, but the speed of time. With this correction the logic of the first part of your question stands: matter makes time go slower and is attracted to the regions of slower time. – safesphere Sep 23 '17 at 06:45
  • Another thing to be careful of is that buoyancy is a purely statistical phenomena. Gravity is much more fundamental – Señor O Sep 23 '17 at 06:54
  • Victor Strom, separating time from spacetime seems incorrect. Matter causing time to go slower is a side effect of the spacetime where the matter resides being stretched into a curve. Besides, "time going slower" relative to what? ;-) – Eric Lord Sep 23 '17 at 07:57
  • Senor O, buoyancy is just the only word I could think of to convey the idea that matter prefers areas of lower density spacetime. The irony that matter causes these areas of lower density in spacetime is not lost on me. ;-) – Eric Lord Sep 23 '17 at 08:03
  • @EricLord: Please don't forget to put @ before the name. Otherwise there is no notification. Time is slower in stronger gravity (e.g. closer to a star) relative to weaker gravity (e.g. farther from the star). While space is also curved, the OP was not looking for a full mathematical identity. So the time only analogy works fine for his intuitive explanation. Furthermore, this effect may depend on the choice of coordinates. (Sorry, I don't have the details.) – safesphere Sep 23 '17 at 08:34

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The problem is that you are thinking of spacetime as a physical object. In effect you have reinvented the idea of the aether. Apart from the fact there is no experimental evidence for this, an aether would pick out a preferred reference frame and contradict the basic principles of relativity.

Incidentally, and on the slightly nit picking front, be careful to distinguish between gravity waves and gravitational waves. The latter are the oscillations in the spacetime geometry recently detected by LIGO.

John Rennie
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  • -1: Special relativity is based on the Lorentz transformations developed in his theory of aether. Therefore the concept of aether does not contradict relativity. – safesphere Sep 23 '17 at 06:29
  • John Rennie, I wasn't aware of the distinction between gravity waves and gravitational waves. To correct my mistake, I was referring to gravitational waves in my analogy. – Eric Lord Sep 23 '17 at 07:35
  • John Rennie, I don't think of spacetime as a physical object. I think I was pretty clear that it's a medium in which mater/energy exist. I don't think General Relativity comes into play in my analogy. As general relativity is about the constant of the speed of causality regardless of the velocity of the point of reference. I guess that does play into spacetime overall but it's a concept that might dilute my analogy with regard to the medium of spacetime having a buoyancy effect on matter causing matter to coalesce. – Eric Lord Sep 23 '17 at 07:41
  • @EricLord: the gravity/gravitational wave is a common one. Even we old timers are guilty of this from time to time :-) Anyhow, when you say I don't think General Relativity comes into play in my analogy you're effectively saying let's ignore this well tested and successful theory that contradicts my idea and that isn't going to get you very far. The idea that spacetime has a pressure like a fluid and this pressure causes matter to move from regions of higher to lower pressure simply doesn't work. – John Rennie Sep 23 '17 at 07:45
  • John Rennie, thx for the reply. This is just an analogy I'm formulating as a tool for better understanding. "Pressure" is the wrong word. I chose it poorly. "Density" might be a better word. I'd love to discuss the reasons why it "simply doesn't work". – Eric Lord Sep 23 '17 at 08:08
  • John Rennie, Please don't misunderstand. I'm not excluding General Relativity. I guess I'm including it as a fundamental property of spacetime. So, when I say the "medium of spacetime", General Relativity is a inextricable component. I'm new in these discussions. Some latitude to misspeak is appreciated. – Eric Lord Sep 23 '17 at 08:11
  • John Rennie, I guess my point is that gravity is not matter "attracting" other matter. The "attraction" of matter to other matter is a ruse... The real force originates from spacetime, not matter. – Eric Lord Sep 23 '17 at 08:22
  • @EricLord: if gravity behaved like a pressure then the force on an object would be proportional to its volume. That means objects with the same mass but different densities would accelerate at different rates. Experiment shows this isn't the case. – John Rennie Sep 23 '17 at 13:45
  • John Rennie, most of the volume of an object is made up of the gaps between atomic particles. You're suggesting that spacetime would care about the surface area of an object. I'm suggesting that spacetime doesn't care about the surface area of the object, only that object's individual particles of matter. Consider spaggetification. – Eric Lord Sep 23 '17 at 22:57
  • @EricLord: most of the volume of an object is made up of the gaps between atomic particles - That isn't true. Atoms are not mostly empty space. – John Rennie Sep 24 '17 at 04:05
  • John Rennie, Let me rephrase: Volume != Mass. Mass is what curves spacetime. – Eric Lord Sep 24 '17 at 05:45
  • @EricLord: right, but you're trying to describe gravity as if it was due to the pressure of some kind of fluid. I'm pointing out this analogy isn't a good one. – John Rennie Sep 24 '17 at 05:49
  • @EricLord: if you want to continue we should take this to the chat room – John Rennie Sep 24 '17 at 05:50
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Well not to discourage from thinking about creative ideas, I would point a flaw with your reasoning.

First, what would make the matter to cause lower pressure of the "medium of space"?

Secondly, suppose even if your mechanism is true, then the force of gravity would be equal to

(Normal pressure of the "medium of space") minus (Reduced pressure of the "medium of space")

The reduced pressure of the "medium of space" can reduce till it disappears altogether, means it would drop to zero.

That means, there would be an upper limit on the force of gravity when the reduced pressure becomes zero (supposing it does drop to zero, say at an event horizon of a black hole).

That upper limit would be the normal pressure of the "medium of space".

As the force of gravity approaches infinite near an event horizon, that does not fit well with your idea unless the normal pressure of the "medium of space" is itself infinite. That would crush matter anywhere in space.

Keep thinking though.You know what to tell your children as you know their age and grasping capability.

kpv
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  • I don't think of it as pressure exerted on matter like air pressure on the surface of a helium balloon or water pressure on a submarine. I don't think spacetime would have a crushing effect on matter. It's more about density than pressure. Perhaps I chose my terminology poorly? – Eric Lord Sep 23 '17 at 07:51
  • You might not win the terminology battle but only because the discussion seems to hang on the mechanism in which the shape of spacetime exerts influence on a moving mass; that mechanism has not been explained, only expressed. How does a mass 'know' what the geodesic path is? There must be some linkage somewhat like how the Higgs field gives particles mass. Until then we are left with analogies of varying degrees of 'leakyness'. I think your question is intriguing, as I still have not heard how the waves detected by LIGO are explained without spacetime having some true density. – Kelly S. French Jan 31 '19 at 18:10