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In our current understanding of physics, is there any way to generate gravity apart from creating a concentration of mass?

The only things I could find about this subject were Alcubierre Drives and Kugelblitz.

Alcubierre drive would be an hypothetical machine that bends space-time and Kugelblitz is an artificial black hole that could only be made by concentrating enormous amounts of energy in a point smaller than an electron. Enough energy to destroy earth, in fact.

And even then, I don't know what would be considered "gravitational force", since I heard that acceleration and gravity are basically the same.

Rexcirus
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Fulano
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    FYI: In the framework of General Relativity, there is no "gravitational force." Newton's laws gave us a conundrum: Why was gravitational mass always proportional to inertial mass? General relativity answers, It's because they are the same thing. The force you feel standing on the ground is not gravity pulling you down. It's the ground pushing up against your own inertia, preventing you from falling (a.k.a., shifting your world line away from any geodesic.) It's the same as if you were standing on the floor of an accelerating space ship, and the ship's motor was pushing up against... – Solomon Slow Oct 16 '23 at 00:35
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    ...your own inertia, preventing you from feeling weightless (a.k.a., shifting your world line away from any geodesic.) The "shift" in the rocket case goes in a different spacetime direction from the shift you experience standing on the planet, but either way, the force you feel is the force of contact between yourself and some physical object that's pushing you in a direction that your inertia resists. It's never a mysterious "action at a distance" force pulling you in that direction. – Solomon Slow Oct 16 '23 at 00:35
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    According to current physics, gravitational force is generated by mass. So the way to generate gravity, is to get lots of mass in one place. For example, Earth's gravity is generated by a concentration of about 10^24 Kg of rock within a few thousand kilometers. – Mitchell Porter Oct 16 '23 at 01:42
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    Voting to re-open. A perfectly clear question with a straightforward answer. – gandalf61 Oct 16 '23 at 05:29
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    SolomonSlow and MitchellPorter, both your answers deserve to be, well, answers! – Marius Ladegård Meyer Oct 16 '23 at 07:05
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    @MariusLadegårdMeyer, My comment could be the answer to a question, but I did not think it answered the actual question that OP asked. The main question seems to be (pardon my paraphrasing) "Could we ever learn to usefully 'bend' spacetime by means other than concentrating a lot of mass/energy in one place?" I can't answer that. I only meant to call the OP's attention to the fact that the language of their question mixes ideas from general relativity with ideas from Newtonian mechanics. – Solomon Slow Oct 16 '23 at 17:45
  • In your comprehension of physics, what might that constitute? – Robbie Goodwin Oct 16 '23 at 22:49
  • How much? Any accumulation of energy modifies the gravitational field (that's the $T^\mu_\nu$ on the RHS of the Einstein field equations). Pump light into a cavity (two opposing mirrors in the simplest case) and the light contained (to use your word) "generates" gravity. – tobi_s Oct 17 '23 at 04:33

3 Answers3

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How impractical are you ready to get?

At least somewhat practical? Ordinary mass is the only way, like our planet.

Ready to spend a star worth of resources? You could use energy in a form of light or speed of matter as a dominating source of gravity. Like a particle accelerator but scaled up to a star size.

Ready to spend a galaxy worth of resources? You could use gravitation waves themself as a dominating source of gravity, by colliding black holes just the right way. We are not sure about details of this one.

Think that even that is not enough? you could spend even more resources to control the flow of dark matter in a way that would make it somehow concentrate more. We are not sure that this is even possible.

Still want more exotic way? You could in theory discover how the dark energy works, it looks like it has something to do with the gravity, but we have no idea how it works.

Surprised Seagull
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In some sense, black holes can be viewed as a gravitational field (gravitational forces in space) without any particles that make up ordinary matter or energy that explain these forces.

How is this possible? Well, the Schwarzschild vacuum solution is a curvature of space which is self-sustaining - the curvature of space creates stress-energy which in turn maintains this curvature. So there is a "concentration of energy" so in one sense the answer to your question is "no", but on the other hand there are no usual forms of energy (no particles with mass and movement, no photons, and so on) so in that sense the answer to your question is "yes" - we have gravitational forces without a "concentration of mass or [traditional forms of] energy".

For a better explanation, see If black holes are just empty vacuum of space inside, then what causes the curvature?

Nadav Har'El
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depending on what you mean by "gravity", you could just use acceleration.

rotating space craft are a great example of this. it would be quite simple and practical to tether 2 copies of pretty much any space craft at the right distance, then spin them up so that they rotate at the correct rate to ensure 1kg of mass exerts 1kg of force towards the "bottom" of the craft. most designs call for quite reasonable tether lengths of around 100m, but longer tethers would reduce the side effects such as the view out the window rotating and thrown objects following unexpected paths thru the coordinate system of the craft.

teldon james turner
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