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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.

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    $\begingroup$ Possible duplicate: physics.stackexchange.com/q/65361 and links therein. $\endgroup$
    – HDE 226868
    Commented Apr 3, 2015 at 19:13
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    $\begingroup$ Hi all i can say is NOOO!!! The two dimensional idea is only a model to try and give you some idea of what space-time curvature is like. Space-time is actually 4-dimensional and it is impossible to visualize. Photons, and all matter follow the curves in space-time, regards $\endgroup$
    – user74893
    Commented Apr 3, 2015 at 19:15
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    $\begingroup$ I think there are two reasons you're getting downvotes. First, you're simply asking too much - you haven't given us one specific question to answer. Besides, you're asking a question that is out of your depth. You're asking about general relativity (and even quantum gravity) when it's clear that you have barely any background knowledge about either. I'm not saying it's wrong to ask those questions, but this site is not the place for them. $\endgroup$
    – David Z
    Commented Apr 4, 2015 at 7:34
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    $\begingroup$ Darshan J, I was going to be doing the exact same w/MIT but it wasn't going to effect my grades and I'd be getting extra work for nothing, so I didn't. I'm in the 9th grade! $\endgroup$
    – Damon Blevins
    Commented Apr 4, 2015 at 10:22
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    $\begingroup$ @DarshanJ Hi, Just to finish up on my comments, I study by myself and if I am told I don't have the background, I don't take it in any way personally because a. it's usually true :) and b. NOBODY but nobody in physics has all the background in all areas, that's what makes it interesting. You can ignore this comment if you like , but I would recommend if you really want to know about say, General Relativity, buy a book like at this link: amzn.to/1yLw2ij and read it and use it to ask questions here until you are happy you follow it. good luck with it, all the best $\endgroup$
    – user74893
    Commented Apr 4, 2015 at 17:44

3 Answers 3

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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.

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  • $\begingroup$ Yeah!,i would like to read the book that u have mentioned.And also it would be nice if u can provide me a graphical representation of what your trying to say using tick marks. $\endgroup$
    – DJphy
    Commented Apr 4, 2015 at 6:12
  • $\begingroup$ you are right its highly very difficult to visualize GR in four dimension. $\endgroup$
    – DJphy
    Commented Apr 4, 2015 at 7:05
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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.

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  • $\begingroup$ Then, is sound also effected by Gravity, just asking..That means anything that has energy is effected by Gravity?? $\endgroup$
    – DJphy
    Commented Apr 7, 2015 at 10:01
  • $\begingroup$ @DarshanJ Yes, sound is affected somewhat, more specifically, the atoms. Remember that sound is vibration. Ina sound wave, each atom travels for a short time then hits the next one, the pressure wave continues. This gives the atoms more velocity, increasing their energy, increasing gravity's effect on them until the collide again and impart their extra momentum to other particle. $\endgroup$
    – Jimmy360
    Commented Apr 7, 2015 at 11:18
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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

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