People often illustrate space-time curvature with an heavy ball curving a sheet but the heavy ball is curving the sheet because that heavy ball is pulled by Earth’s gravity but in space there is no gravity without space time bending,so while there is no gravity how is the space-time bent wherever there is mass?
In: Physics
> in space there is no gravity
This might be where you’re tripping up. I don’t know where you got the idea that there is no gravity in space…there is gravity EVERYWHERE. All matter exhibits gravity on everything. You’re being pulled toward Saturn, and Andromeda, and quasars billions of light-years away, and even toward me. You don’t notice it because the Earth’s gravity is much, much stronger.
The illustration of the ball on a sheet is grossly misleading. We reside in a four dimensional space, three spatial and one temporal (time). All of these are bent, but the temporal one is affected a little more, which happens to give rise to the time dilation effect of general relativity (not to be confused with time dilation in special relativity).
The thing with the curvature of space-time is that this IS what we call gravity. This is one of the important conclusions of Einsteins general relativity. Mass bends space. Mass does not exert an instantaneous force on another object. We can use the description of gravitational force to explain most phenomenon you can come across, but it’s not completely accurate.
This curvature can be calculated with the Einstein equations (Einstein tensors for curvature if anyone is interested) which consist of materia information. This means, if there is no materia, there is no curvature. This curvature is remarkable around large celestial bodies, like Earth. Space around the planet (and inside even) is curved which gives rise to the moons orbit around us, and us staying on Earth.
While a sheet deforms downward when you put a ball in it due to the force of gravity on sheet and ball, that’s merely an analogy.
In General Relativity, gravity is the warping of space-time by anything with mass; this warping occurs [in three dimensions](https://i.stack.imgur.com/iGT6a.jpg). The point of the ball-and-sheet stuff is to show why that warping would cause things to slide together or orbit in a way that’s easy to grasp.
The ball/ sheet example is a just a visual aid to help in picturing what is happening, not a literal recreation. The important part is that the presence of the ball is warping the sheet, just as how in reality, the presence of a planet warps sapacetime around it.
The reason the ball warps the sheet is different to the reason a planet warps spacetime, but for the purposes of the illustration that doesn’t matter, the key part is that you can see the sheet being warped by the presence of the ball.
Gravity itself is inherently and proportionally related to mass, not a separate force from it. It’s only effective when an object has a big enough mass to actually affect exert force over a wide radius.
Anyway, if an object is moving at any sort of significant ratio of the speed of light, the energy of the system is increased…. and since mass and energy directly proportional, the mass of the object actually increases as well. I know there’s more to explain, but it’s been awhile since I’ve thought about, let alone studied, relativity and astrophysics.
That’s what gravity is in general relativity. That curvature of space-time is gravity. Space-time tells matter how to move and matter tells space-time how to curve.
Mass bending space time IS what gravity is. The example show with a ball on the sheet represents that phenomena in a method that humans can visualize.
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