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I am just 16 and curious to learn about Theory of Relativity. Can any one explain it simple enough for me to understand? I read that it is bending of time-space or space-time that causes gravity. How can bending of time takes place?

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    $\begingroup$ Hi Jayadev, and welcome to Physics SE. The question you've asked here is almost an exact duplicate of this question, which has many high-quality answers. $\endgroup$ – tok3rat0r Jul 2 '15 at 16:56
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When we use the terms "bending" or "warping" with respect to spacetime and gravity, you have to keep in mind that these words are not being used in a literal way. Since the majority of concepts in General Relativity are far beyond what our experiences allow us to comprehend, we have come up with a few ways of picturing these concepts in our minds, none of which are very accurate, but it helps us relate to it all.

Gravity doesn't literally bend spacetime. What it actually does is modify the spacetime interval. This modification can cause straight paths to appear to bend and time durations to alter to an outside observer. Because one of our convenient ways of thinking about spacetime is as one interwoven fabric where the border between time and space is a bit fuzzy, we say that gravity can "bend" or "warp" spacetime and alter the shape of this fabric/surface/whatever.

So to answer your question, time does not literally "bend". A massive object modifies the proper time interval around it such that an outside observer would see objects near the mass experience less time and spacetime intervals would have their spatial components modified accordingly. But that is a lot to say. It's much easier for us to simply say that gravity is spacetime being warped.

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  • $\begingroup$ so you're saying that if we imagine a 2D space, instead of deforming into an additional dimension like this i.imgur.com/XyckRFe.png it actually deforms into itself like this i.imgur.com/0gVFNtW.png is that right ? $\endgroup$ – Gamnamno Aug 4 '17 at 5:40
  • $\begingroup$ @Gamnamno Yes, that's a better example of what it might be like. To be technically correct (the best kind of correct), the former image you linked is that of a gravitational potential energy well, not of spacetime curvature. $\endgroup$ – Jim Aug 4 '17 at 12:49
  • $\begingroup$ The reason it's so commonly believed to be an image of warping is because at some point, someone used this to show how warped space alters trajectories. On Earth's surface, since we have near constant gravity, the shape of a potential well is, by definition, perfect to do this demo. It also correlates great for how more mass warps space more. This leads people to interpret the energy well as an actual representation of warped space. And as Aristotle once said "The least initial deviation from the truth is multiplied later a thousandfold". $\endgroup$ – Jim Aug 4 '17 at 12:49
  • $\begingroup$ in that case, can you answer this ? physics.stackexchange.com/questions/350288/… $\endgroup$ – Gamnamno Aug 4 '17 at 12:57
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I'll go into a bit more detail below but, basically, Einstein discovered that space and time can't be separated. They are part of the same thing, and gravity is actually produced be bending and warping this "spacetime," like a heavy ball sitting on a sheet of elastic.

Classical (or Newtonian) Physics

Time can be considered as a dimension, just like distances. In this way, our universe is four dimensional: three for space and one for time. Before Einstein came along, people thought that these dimensions were fixed: anyone who looked at something would measure the same distances, see the same things happening at the same time.

Special Relativity

Einstein realised that this is not true. Two events that appear simultaneous to one observer may seem to happen at different times to a different observer: simultaneity is not preserved.

He came up with this idea by looking at light and realising that light always travels at the same speed, no matter how fast you are traveling. Even if you chase a photon, it always moves away from you at the same speed ($c$) from your point of view.

The only way that this can be made to work is if there is a link between time and space. We can't consider them to be separate things anymore; time and space get mixed together depending on your frame of reference.

This is why physicists talk about spacetime. Just like the x, y & z directions only depend on where you put your axis so it doesn't make sense to consider e.g. the x direction separately from the y & z directions, so the time dimension cannot be considered apart from spacial dimensions since different observers put their axis in different places. The image below shows this: while one observer thinks that $z$ is a spacial direction and $ct$ is a time direction, another observer (one who is moving fast with respect to the first observer) thinks that it's $z'$ and $ct'$ instead.

Different reference frames

General Relativity

Einstein then went a step further. He realised that considering time and space together as spacetime allowed him to think about gravity. Einstein imagined spacetime being bent and warped by heavy objects. This way, other object always move on straight lines but those lines look curved to us because spacetime itself is bent.

For example, the moon looks like it's moving in a circle around the earth, but in spacetime it's actually moving in a straight line. These straight lines are called geodesics.

The image below shows this. Here, the earth's mass is bending spacetime and the moon is moving along a geodesic in this bent space, just like a water spins around a plug. It's important to realise that this picture is a 3D picture, but spacetime is actually 4D. We can't show 4D pictures easily because our brains aren't built for it, but fortunately maths works just fine with 4 dimensions.

Moon orbit geodesic

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