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Given that dark matter is explained by being a lot less matter than gravitational forces, how about the other way around? There is a lot of empty space.

Is it possible to construct a world where gravity wasn't a force but vice versa the empty space would push away? How would such world differ from ours?

From an amateur perspective it may just be a switch in signs.

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    $\begingroup$ One of the best ways to avoid your question becoming a ‘What If?’ is to refrain from asking, well, “What if …?” and the like. It is better if you ask “I want to do example, but am having these problems. How can I work around or through those problems?” You can also say “I think that something will work in a certain way. Am I correct or not?” $\endgroup$ – can-ned_food Oct 5 '17 at 7:50
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    $\begingroup$ You should clarify this: “being a lot less matter than gravitational forces” I think I know what you are attempting to say, but I could be incorrect; the same for other readers. $\endgroup$ – can-ned_food Oct 5 '17 at 7:57
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    $\begingroup$ You might want to hold off on accepting an answer for a while yet. It’s entirely possible people with better answers will appear. $\endgroup$ – Joe Bloggs Oct 5 '17 at 8:55
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    $\begingroup$ That's ridiculous - isn't it? Are you a patent clerk? $\endgroup$ – Grimm The Opiner Oct 5 '17 at 9:33
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    $\begingroup$ There is no gravity. The universe sucks. $\endgroup$ – Robert Columbia Oct 5 '17 at 11:48
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TL;DR: it is the same as gravity, unless you’re defining ‘empty space’ in a really odd way. If fact, I can’t think of any way to experimentally prove this is not the case.

I think the key to this question lies in the definition of ‘empty space pushing’

For starters let’s assume space is quantised into a series of infinitesimal points. This isn’t strictly true, but it’s easier to conceptualise. Let’s assume each point of space that doesn’t contain matter pushes away from itself with a force that diminishes with the square of distance. It’s pretty easy to show (if we assume the universe is big enough that we can ignore the boundaries) that the net force at any given point is 0: all the points of nothing negate each other.

Let’s introduce a speck of matter.

Suddenly there is a space in the universe that is not pushing away from itself, which leads to an imbalance where the net force near this speck of matter is directed towards the speck of matter. The magnitude of this ‘not pushing away’ force is exactly the same as the original force you removed, ie inversely proportional to the square of distance.

Wait. That looks suspiciously like gravity.

If we continue to add specks then we’ll see the specks, responding to the imbalance in pushing forces, clump together. This leads to a larger volume of ‘not pushing’ which leads to a larger imbalance.

The same is true even if you take an Einstinian view of gravity, just replace ‘empty space pushes away’ with ‘empty space deforms space near it upwards’ (a strange concept, perhaps ‘outwards’ is a better word, but hey).

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    $\begingroup$ I once read a book which proposed something along these lines. If I had it handy, I could cite it and mention why it proposed the idea as better than other ways of modelling gravity. Something to do with the shorter distances between masses in a curved and unbound space pushing those masses apart with less force than on the farther distances. $\endgroup$ – can-ned_food Oct 5 '17 at 10:42
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    $\begingroup$ How do we know this isn't the way gravity works IRL? $\endgroup$ – SIGSTACKFAULT Oct 5 '17 at 12:32
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    $\begingroup$ @Blacksilver: like I said: I can’t think of a way to prove this isn’t the case, but I guess the model of gravity being caused by matter is easier to think about than gravity being a consequence of matter stopping emptiness pushing us around. $\endgroup$ – Joe Bloggs Oct 5 '17 at 13:15
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    $\begingroup$ See the answer from @Ash as it brings up an interesting point. If the point of matter does not push, then we should assume less-dense matter pushes more and more-dense matter pushes less (less dense is closer to "nothing" and more dense is farther from it). In this case, denser air will accumulate at the outer reaches of the atmosphere and less-dense air would be pushed in farther, reversing the atmosphere. If the effect is pronounced enough, perhaps the entire planet would be reversed with an outer crust and a hollow atmosphere-filled core. $\endgroup$ – Loduwijk Oct 5 '17 at 13:19
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    $\begingroup$ Also, we cannot simply say that "Well, maybe hydrogen actually is heavier than lead per unit volume and we are measuring it backwards;" that does not work since we have other ways of measuring matter density than just measuring its weight. $\endgroup$ – Loduwijk Oct 5 '17 at 13:21
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Gravity causes matter to agglomerate. I can not see how empty space would push matter to agglomerate to something like a sun. Maybe matter would do that to be less pushed. But even then, there would be no planets orbiting a sun, there would be no galaxies, because they have no reason to rotate around a center. It is difficult to imagine a universe with a basic law missing.

I think all matter would be nothing more than single hydrogen molecules drifting around. If empty space pushes them, they will be dispersed homogenous (with small fluctuations) having a balance of same amount of empty space in every direction.

This universe would be an hydrogen air bubble.

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    $\begingroup$ +1 for the point about objects not orbiting each other; my physics is rusty but I think you're correct. Trying to visualize whether proximity to another body would create a 'low pressure' side and push them together; without doing actual math, my memories of fluid dynamics say no. ...Also, I learned a new $5 word; now I need to go figure out when to use it properly. $\endgroup$ – brichins Oct 5 '17 at 13:58
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My gut says Quark Soup, i.e. a universe even remotely similar to our own simply cannot form without the gravitational constant we know and love. But my brain says no wait it would be exactly the same as what we have now except gravity would be a function of the super-vacuum instead of a function of mass so lets break it down:

"Space" pushes matter away, assuming a "push factor" equivalent to the modern constant "G" then cosmologically this would act almost like gravity but from the outside, matter would agglomerate into large clumps etc... but the "almost" has some killer implications:

For example consider the solar winds for a moment, most of the material in the winds is not massless photons but consists of massed particles like Protons and Neutrinos instead of accelerating away from their parent star these particles are now held back, radiation pressure will still expel them but they'll start to slow down immediately until they eventually stop and reverse course, you'll get a heavy radiation belt where such particles accumulate with vacuum and solar radiation both pushing them away to a matched degree.

On a planetary scale I think atmospheres become a serious issue, at a minimum they lose their pressure gradient since the particles furthest from the surface are now under higher gravity not those at sea level, it may well be that atmospheres as we know them are entirely impossible. An object that is pushed together from the outside in isn't going to behave the same way as a body that accretes slowly from the densest materials to the lightest. Worlds may in fact be inverted with the heavy material like rocks on the outside and the atmosphere held internally. Gravity at the bottom of an atmosphere (if there is one) may also be an issue, as in there may be none to speak of; gravity imposed from the outside will decrease with your distance from open space so at ground level gravity will not be what we would expect given our existing model.

That's my current thinking ask me in five minutes and I'll be the other way up again, this is a tricky one.

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  • $\begingroup$ I think some of your assumptions are off. Ex: From solar radiation, a particle ejected would not have all of space pushing it back towards the sun; every point of space would be pushing away from itself. Mostly they cancel each other out, but in the sun there would be less of a push because it's not empty, so the acceleration on the particle toward the sun would be the same as with conventional gravity. It would even decrease with distance. $\endgroup$ – Loduwijk Oct 5 '17 at 13:11
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    $\begingroup$ I think you might be onto something with the "density-distribution is reversed" though, and maybe not in just the atmosphere. If "nothing" pushes less against "something" then it should push even less yet against "heavier"-something, so maybe hollow worlds with atmosphere inside them? $\endgroup$ – Loduwijk Oct 5 '17 at 13:15
  • $\begingroup$ @Aaron "Gravity" would increase with distance from the sun, wouldn't it? Because the sun is a large body of "not space" and the vacuum would be getting "thicker" the further out you go, am I way off base on that somehow? $\endgroup$ – Ash Oct 5 '17 at 13:22
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    $\begingroup$ @Aaron Yeah I was wondering about hollow worlds I'm going to edit it in after all I think. $\endgroup$ – Ash Oct 5 '17 at 13:23
  • $\begingroup$ This reminds me of a book I read once where travelers went through a black hole, came out of a white hole somewhere else which pushes everything away, and the white hole was in the center of a reversed, hollow (except atmosphere) world. The white hole acted as the light and heat source and remained suspended in the same spot all the time in the sky. $\endgroup$ – Loduwijk Oct 5 '17 at 13:26
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If gravity would 'push' nothing would then stick together.

As gravity is a universal rule, not only the world you are talking of but the whole universe would then not be coherent.

What I could suggest is that a planet/world exist in which the gravity field is negative: It is better than changing a universal law.

For such planet you would need to put a system in such a way that the crust does not fly away then also forget about anything staying around including oxygen/air.

Maybe also: the impossible inhabitants would not be very egoist.

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In my imagination "the other way round" means that in your universe there would be as much matter as there is empty space in our universe - and as much empty space as there is matter in ours. In this case, the equivalent of planets would be little bubbles of empty space within an ocean of matter, which push the matter away from it.

Life would exist either within the matter between these space bubbles or on the surface of them where there is a sort of atmosphere. Energy would come in a way of geothermal energy produced by these bubbles travelling around within the matter and forcing it to stretch and compress. So matter would be hotter around these bubbles of emptiness than anywhere else.

There would be no stars of course, nor moons or solar systems, possibly not even galaxies, because when two of these bubbles moved towards each other, they would push the matter between them together until there would be no going closer, matter would become liquid from the pressure, and the bubbles would probably slide away from each other and find a new direction to travel in. It would be interesting to see of people who might live in these bubbles would find a way to travel to other bubbles.

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"If a tree falls in the forest and nobody's around to hear it, does it still make a sound?"

Notionally, you can say that this is already how gravity works without breaking anything of the model or of our observations. So, it might as well be deemed so.

That was easy!

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    $\begingroup$ No you can't the atmosphere wouldn't work right under the above model. $\endgroup$ – Ash Oct 5 '17 at 14:29
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This reminds me of old mechanical explanations of gravity, especially Le Sage's theory of gravity where empty space was filled with particles which pushed everything into each other. The problems were mostly thermodynamic concerns.

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