r/AskPhysics • u/Downtown_Opinion_439 • 2d ago
If gravity bends space-time, then do objects shrink or change shape when experiencing gravity?
Objects take up space and time. If gravity bends space-time, why don't objects shrink under its effect? Most explanations I've read suggest that gravity affects only empty space. Isn't occupied space also space?
I suppose if you try to measure the shrinkage with a ruler, the ruler itself would shrink too, so maybe we can't detect it. Other than that, I don't know.
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u/Photon6626 2d ago
Yes but typically the solid materials that make up planets and other things prevent it because other forces between the molecules that make up the object overpower the forces from gravity(electricity and magnetism). But liquids change shape, e.g. tides on earth. I think planets and moons with liquid cores can change shape if they have an orbit that is elliptical enough(Enceladus maybe?). This can provide enough heat via friction to keep the cores liquid. Also in extreme conditions like near black holes it can stretch solid materials in a process known as spaghettification.
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u/stevevdvkpe 2d ago
Gravity does compress matter, in that the inward force of gravity works against the outward forces (mostly electrostatic repulsion) that cause matter to maintain its volume. But gravity from a normal amount and density of matter doesn't bend space and time all that much so the effects aren't especially noticeable. The effect of gravity is mainly to create pressure, not to shrink objects.
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u/ampere03 2d ago
During the evolution of large stars the final stage is a catastrophic implosion due to the positive nuclear binding energy past iron resulting in a supernova remnant- neutron star/black hole which is tiny in comparison to the original star.
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u/Reality-Isnt 2d ago
The old pedagogical approach is to picture an initially perfectly round ball of non-interacting coffee grounds freefalling in a gravitational field and observe what happens over time. The ball of coffee grounds distorts into a volume preserving ellipsoid shape with its minor axis perpendicular to the direction of fall, and the major axis pointing in the direction of fall. That gives you an idea of how the field can affect an object.
So, if we take a solid ball of cohesive material, the gravitational field will try to distort it into the ellipsoid shape. If there is a powerful gravitational gradient, the field will induce significant stresses and strains within the material, and depending on the strength of the material, potentially deforming it.
This is all explained with a rather complicated, mathematical object called the Riemann tensor also knowns as the tidal tensor, which can be used to calculate the stresses, strains, and deformation of an object in a gravitational field.
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u/Downtown_Opinion_439 2d ago
So matter does get distorted. It’s just usually hard to notice is what I’m concluding.
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u/Reality-Isnt 2d ago
if it’s a small object in a weak field the internal forces binding the object together will prevent distortion. Near or within a black hole (depending on mass of black hole), the internal forces will bevovercome and the object will spaghettify
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u/Presence_Academic 2d ago
The effect of gravity on space is tiny, not detectable in normal circumstances. Time is affected much more, but still quite little.
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u/nicuramar 2d ago
Although it’s important to remember that one dimension on its own can’t curve, it’s always between dimensions. The curvature in space-space sections and time-space sections are of similar “strength”, it’s just that our time coordinate, being multiplied by c, “progresses” much faster.
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u/nicuramar 2d ago
Gravity doesn’t bend spacetime. Rather, energy and momentum flux curve spacetime, and the resulting curvature is what we experience as gravity.
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u/Downtown_Opinion_439 2d ago
Does curving spacetime cause matter in that space time to also be curved?
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u/nicuramar 2d ago
For that question I’m a bit out of my depth, actually. It does impose a force on the matter, which is called the tidal force.
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u/Infinite_Research_52 👻Top 10²⁷²⁰⁰⁰ Commenter 2d ago
What is curved matter?
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u/Downtown_Opinion_439 2d ago
Like if a canvas were stretched, then the drawing on that canvas would also be stretched. So if space-time were stretched then the objects in space-time would also be stretched.
But as I think about it, I don’t think that’s the case. The canvas stretches but the objects on the canvas stays the same size. That seems more like the current depiction of gravity.
So it seems like gravity only affects empty space and not all space. Idk if I make sense.
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u/Unable-Primary1954 2d ago edited 2d ago
Yes.
But purely spatial deformations are incredibly tiny on Earth (though it is like this that we observe gravitational waves going through the Earth).
Tidal forces correspond to deformations that involve space and time directions. It is tiny on Earth, but measurable. (things get multiplied by c, and c is big)
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u/ChocolateValuable221 2d ago
Time changes shape... not space... gravity isn't actually bent space as much as it is just bent time. Light doesn't go the shortest distance it goes the shortest time.
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u/california_snowhare 2d ago
Yes?
It's why planets are round and not some other shape and have internal densities higher than that of the same matter measured in laboratories. Gravity is trying to push it all to the center and the cumulative effect all that matter trying to squeeze together is to shape it into a sphere and compress it slight bit.
That is an indirect effect though - not gravity acting directly on the atoms and molecules to change their shapes/sizes.
In small objects gravity is just normally too weak to NOTICEABLY compress matter or reshape it directly. Gravity is very, very, very, very weak compared with the electromagnetic forces in matter that normally control its size and shape.
It takes a LOT of force to noticeably distort matter and the direct (not cumulative) effect is so small that even on the surface of a neutron star the tidal gradient across a single atom is too small to measurably distort it because the EM forces on it are something like 24 orders of magnitude larger than the gravitational forces.
That said, a neutron star or stellar mass black hole could tidally rip apart a steel ball bearing only 2 cm across if it passed in free fall near the surface of a neutron star/the event horizon of a stellar mass black hole.
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u/EighthGreen 2d ago
It sounds as if you conceive of objects in space to be painted, so to speak, on space. Well, they're not. If they were, they wouldn't even be able to move.
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u/Skindiacus Graduate 2d ago
Objects in strong non-homogenous gravitational fields can undergo spaghettification.