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I know that we are surrounded by dark matter, does the dark matter affects space time ???and I know the reason for gravity according to explanations in general relativity. But all the explanation about GR is explained in two dimension, where space is imagined as a fabric and placing an object creates a dip and hence pulling objects near-by and this is gravity.

But why does photons of light get fallen into this dip, by the way photons have zero mass.Some people explain that photons have energy hence they are also effected by Gravity, but gravity is all about mass that can cause a distortion in space-time.All that i need to know here is why photons of light are affected by Gravity.

Please help me, i want to know more about space-time.

DJphy
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In the two-dimensional rubber sheet visualization, it is wrong to think that things fall towards the massive object because they are "rolling down the hill" of the curved spacetime. There is no perpendicular gravity pulling things down into the well. What happens is that you are moving along your world line at a constant velocity, "into the future at one second per second." But your world line is a geodesic, which means that locally it is the straightest it can be given the curvature of spacetime. And the nature of the spacetime curvature is that geodesics curve towards the world line of the massive object.

A visualization that I use to think about geodesics is to imagine little tick marks sticking out to either side. The distances from the left endpoint of a tick mark to its fore and aft neighbors should be the same as the distances of the right endpoint. In your world line, a tick mark would be a flash that would appear all around you to a distance of (say) one light millisecond. In your frame of reference it would be simultaneous. If time were slower on one side, then your world line would have to bend to keep the flashes "equidistant" on both sides. This is incomplete and approximate, but it may help you to think about it.

My favorite book to read on this is Relativity Visualized, by Lewis Carroll Epstein. It is ideal for a lay-person's view that is intuitive and basically accurate.

Mark Foskey
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Yes, photons are affected by those curves. They also curve space-time due to something called a stress-energy tensor. Remember that the energy of a photon is given by $$E = hf$$, so photons do have energy. This energy lets them be affected by gravity.

Jimmy360
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The simplified answer for this is that gravity basically means that you are in elevator which is accelerating: even if you are standing on the ground.

So if you take that constant gravity field is pretty much the same as a constant acceleration, then if a beam of light bends downward in an accelerating elevator, it must bend downward if it is in an constant gravity field.

An excellent overview of general relatvity can be found on you tube: https://www.youtube.com/watch?v=JRZgW1YjCKk

user2196351
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