2011-01-16 09:27:21 8 Comments

I understand that people explain (in layman's terms at least) that the presence of mass "warps" space-time geometry, and this causes gravity. I have also of course heard the analogy of a blanket or trampoline bending under an object, which causes other objects to come together, but I always thought this was a hopelessly circular explanation because the blanket only bends because of "real" gravity pulling the object down and then pulling the other objects down the sloped blanket. In other words, to me, it seems that curved space wouldn't have any actual effect on objects unless there's already another force present.

So how is curved space-time itself actually capable of exerting a force (without some source of a fourth-dimensional force)?

I apologize for my ignorance in advance, and a purely mathematical explanation will probably go over my head, but if it's required I'll do my best to understand.

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## 12 comments

## @joseph f. johnson 2011-12-17 07:06:35

The other answers are more or less correct, but perhaps I can say something more to the point of the question, *How is curved spacetime itself actually capable of exerting a force?

No force whatsoever is needed.

Gravity is not a force. What is a force, anyway? Newton clarified for almost the first time in Science what a force is: First I will say it, then explain it: A force is something which makes the motion of a body deviate from uniform straightline motion.

Newton pointed out that bodies have a tendency, inertia, to continue in whatever direction they are already going, with whatever velocity they have at the moment. That means uniform, rectilineal motion: steady velocity, same direction. Newton actually knew this was what would be later called a geodesic, since « a straight line is the shortest distance between two points ».

Newton then went on to say that to overcome inertia, to overcome this tendency, requires a force: force is what makes a body depart from the geodesic it is (even momentarily) headed on (its direction and speed).

It was then Einstein (and partly Mach before him) who said this does not get to the essence of the question. For Einstein, any coordinate system had to be equally allowable, and in fact, space-time is curved (as already explained by other posters). A body or particle under the influence of gravity actually does travel in a geodesic....i.e., it does what a free particle does. I.e., it does what a particle

not under the influence of any forcedoes. So gravity is not a force.Newton did not realise that space-time could be curved and that then the geodesics would not appear to our sight to be straight lines

when projected into space alone. That ellipse you see in pictures of planetary orbits? It is not really there of course since the planet only reaches different points of the ellipseat different times...that ellipse is not what the planet really traverses in space-time, it is theprojectionof the path of the planet onto asliceof space, it is really only the shadow of the true path of the planet, and seems much more curved than the true path really is.( ¡ The curvature of space-time in the neighbourhood of the earth is really very small ! The path of the earth in space-time would even appear to be nearly straight to an imaginary Euclidean observer who, in a flat five-dimensional space larger than ours, was looking down on us in our slightly curved four dimensional space-time embedded in their world. It's $ct$, remember, so the curving around the ellipse gets distributed over an entire light-year, and appears to be nearly straight...and

isstraight when one takes into account the slight curvature of space-time.)Since every particle under the influence of gravity alone moves in a geodesic, it does not experience any force that would make it depart from its inertia and make it depart from this geodesic. So gravity is not a force, but electric forces still do exist. They could overcome the inertia of a charged body and make it deviate from the geodesic it is headed on: change its speed and direction (when speed and direction are measured in curved space-time).

Einstein (and me too) did not want to change the definition of force in this new situation, since after all electric forces are known to exist and are still forces in GR. So the old notion of force still retains its usefulness for things

other than gravity. To repeat: if a body is not moving in a geodesic in space-time, you go looking for a force that is overcoming its inertia....but since gravity and curvature of space-time do not make a body depart from a geodesic, neither of them is a force.See also http://www.einstein-online.info/elementary/generalRT/GeomGravity.html which avoids the trampoline fallacy and has a great image of the great circle.

## @Solomon Slow 2015-09-21 20:14:11

Gravity is not a forcein GR. Gravitywasa force in classical mechanics. Gravity is ________ in quantum theories (Sorry, I don't know enough to fill in the blank.) My point is that all of these realms aremodelsthat predict the motion of terrestrial and astronomical objects. Some models (e.g., GR) make better predictions than others (e.g., classical), but do any of them tell us what gravityreallyis?## @Ernesto Melo 2017-06-13 16:26:59

@james large, the answer is no. There isn't a complete theory of gravity. No one knows what causes.

## @Mozibur Ullah 2018-08-16 15:15:37

Here's a simple way to think about it:

Newton's first law of motion says that in the absence of any force on a particle, the particle will move in a straight line.

Hence, if we see a particle move in a curved path - that is of it deviates from a curved path - we can say that there is a force on it.

Now, in GR, particles without any forces acting on it move on geodesics. This is the replacement for the notion of straight lines in a curved spacetime. Nevertheless we can detect the deviation from the usual notion of a straight line in flat space.

This deviation will be correlated with the force of gravity as experienced by this object in its local frame.

## @Abhimanyu Pallavi Sudhir 2013-06-21 13:35:32

It is straightforward to see how the

geometryof spacetime describes the force of gravity -- you just need to understand the geodesic equation, which in general relativity describes the paths of things subject to gravity and nothing else. This is the "spacetime affects matter" side of the theory.To understand why curvature in particular, as a property of the geometry, is important, you need to understand the "matter affects spacetime" side of general relativity. The postulate is that the Gravitational Lagrangian of the theory is equal to the scalar curvature -- this is called the "Einstein-Hilbert Action" --

$$S=\int{\left( {\lambda R + {{\mathcal{L}}_M}} \right)\sqrt { - g}\, d{x^4}} {\text{ }} $$

You set the variation in the action to zero, as with any classical theory, and solve for the equations of motion. The conventional way to do this goes something like this --

$$\int{\left( {\frac{{\delta \left( {\left( {{{\mathcal{L}}_M} + \lambda R} \right)\sqrt { - g} } \right)}}{{\delta {g_{\mu \nu }}}}} \right)\delta {g_{\mu \nu }}\,d{x^4}} = 0$$ $$ \sqrt { - g} \frac{{\delta {{\mathcal{L}}_M}}}{{\delta {g_{\mu \nu }}}} + \lambda \sqrt { - g} \frac{{\delta R}}{{\delta {g_{\mu \nu }}}} + \left( {{{\mathcal{L}}_M} + \lambda R} \right)\frac{{\delta \sqrt { - g} }}{{\delta {g_{\mu \nu }}}} = 0 $$ $$ \frac{{\delta R}}{{\delta {g_{\mu \nu }}}} + \frac{R}{{\sqrt { - g} }}\frac{{\delta \sqrt { - g} }}{{\delta {g_{\mu \nu }}}} = - \frac{1}{\lambda }\left( {\frac{1}{{\sqrt { - g} }}{{\mathcal{L}}_M}\frac{{\delta \sqrt { - g} }}{{\delta {g_{\mu \nu }}}} + \frac{{\delta {{\mathcal{L}}_M}}}{{\delta {g_{\mu \nu }}}}} \right)$$

$$ {R_{\mu \nu }} - \frac{1}{2}R{g_{\mu \nu }} = \frac{1}{{2\lambda }}{T_{\mu \nu }}$$

To fix the value of $\kappa=1/{2\lambda}$, we impose Newtonian gravity at low energies, for which we only consider the time-time component, which Newtonian gravity describes (I'll use $C$ for the gravitational constant, reserving $G$ for the trace of the Einstein tensor) --

$$\begin{gathered} {G_{00}} = \kappa c^4\rho \\ {R_{00}} = {G_{00}} - \frac{1}{2}Gg_{00} \\ \Rightarrow {R_{00}} \approx \kappa \left( {c^4\rho - \frac{1}{2}\frac{1}{{c^2}}c^4\rho c^2} \right) \approx \frac{1}{2}\kappa c^4\rho \\ \end{gathered} $$

Imposing Poisson's law from Newtonian gravity with $\partial^2\Phi$ approximating $\Gamma _{00,\alpha }^\alpha $,

$$ 4\pi C\rho \approx {\nabla ^2}\Phi \approx \Gamma _{00,\alpha }^\alpha \approx {R_{00}} \approx \frac{\kappa }{2}c^4\rho \\ \Rightarrow \kappa = \frac{{8\pi G}}{{c^4}} \\ $$

(The fact that this is possible is fantastic -- it means that simply postulating that spacetime is curved in a certain sense produces a force that agrees with our observations regarding gravity at low energies.) Giving us the Einstein-Field Equation,

$${G_{\mu \nu }} = \frac{{8\pi G}}{{c^4}}{T_{\mu \nu }}$$

## @Comp_Warrior 2013-06-22 01:20:10

This is not an explanation in "layman's terms"...

## @Comp_Warrior 2013-06-22 13:13:26

I just think the average person interested in OP's question would not have knowledge of Lagrangians, Tensors etc.

## @Dilaton 2013-06-22 14:46:22

@Comp_Warrior, the About says the site is for Academics, students, and researchers of physics and astronomy, so the average audience should not consist of laypeople and it is perfectly ok to give technical and advanced answers for the people who can stomach it. Even though it looks like this since quite some time, physics se is not meant to be a popular physics forum such as quora for example ...

## @Dilaton 2013-06-22 15:00:18

Btw the op says he does not mind technical answers, so why are you insisting on answers exclusively in layman terms?

## @Comp_Warrior 2013-06-22 15:52:19

I guess it is ok for technically adept audience, but the OP clearly mentions " a purely mathematical explanation will probably go over my head". I'm sure there are other questions on this site to which this answer would be more appropriate.

## @Dilaton 2013-06-22 22:20:27

@Comp_Warrier what dimension10 says, and the SE system is exactly desined that the op can accept the answer he likes best, maybe a popular one, whereas there can legitimately be other more technical answers too, that are liked by other members of the community. Answers to a question are not only meant to serve the op, but the whole community. So there is absolutely nothing wrong with a question getting answers of different level. It would be nice if you stop discouraging good technical posts which are perfectly legitimate.

## @Jerry Schirmer 2015-04-17 02:54:03

However, I would say that if you're going to go into the technical details of taking the variation of the Hilbert action, you shoudl be honest enough to work through all of the details, and the variation above is fudged to work out right, but doesn't follow the rules of either the standard variation, or the Palantini version -- the $\frac{\delta R}{\delta g^{ab}}$ bit is just sleighted away. Of course, it takes Landau-Lifschitz 15 pages to do it.

## @Jerry Schirmer 2015-04-17 02:59:35

This derivation is actually wrong, because $\frac{delta R}{\delta g^{ab}}$ is only equal to $R_{ab}$ up to boundary terms, which is enough to obtain the field equations, but will give you the wrong Hamiltonian, etc. And you have to take the variation inside of the integral.

## @heather 2016-08-03 16:52:42

A question was marked as a duplicate of a duplicate of this question, so I am posting my answer here.

Gravity is due to the curvature of spacetimeI believe it is true. That is what general relativity says, and general relativity has been confirmed in predictions ranging from the existence of black holes to the orbit of Mercury to the bending of light.

Relation between spacetime, curvature, mass and gravityYou say you are confused about how the curvature of spacetime and gravity are related. I am going to explain mainly that in my answer, starting with simpler examples, and moving to more complicated ones.

Okay, let's say you have a sheet of rubber. This is the classic example of spacetime. Let's say you take a bowling ball, and set it on the taut sheet of rubber. It has a large mass (compared to what else we'll be putting on the sheet), therefore the sheet curves a lot for the bowling ball. We now have an image in our head like the one below:

So mass leads to curvature. Then, take a baseball, say, and set it near the bowling ball. It rolls toward the bowling ball, right? This occurs because of the curvature of the sheet. So, then, curvature leads to gravity. So, if an object has large mass, it will curve spacetime dramatically, leading to strong gravity.

This is, of course, an overly simplistic example. It is 2-d, and it doesn't take into account other factors. Let us move to 3-d (keeping in mind the universe is accepted to be at 4-d, ignoring the holographic principle). The mass of a bowling ball now sucks in space around it, sort of like in the picture below:

And now, in this case, we can see (or understand) that more mass still leads to more curvature. The greater the mass, the more spacetime will "contract" around the object. So we still think that mass leads to curvature. Now, if we set an object near this massive object (like the moon next to Earth) it is "sucked in" sort of, by the curvature of spacetime, though of course the moon contracts spacetime around it as well. At this point, we can reasonably still conclude that in 3-d, mass leads to curvature which leads to gravity.

But, as I said earlier, the universe is generally thought of as 4-d. What does our picture look like when we add time? Well, the time dimension is contracted around a massive object. So let us picture our previous example but that the fabric of spacetime has a few clocks embedded in it occasionally. As the space stretches and contracts, so will the clocks (the "time") and so the time on those clocks will be "wrong" - it'll differ from the other clocks. And in this case, as the Earth contracts space and time around it, it changes the time and space (it curves spacetime) and so when another object enters our region of spacetime, it is "sucked in" still, but so is it's time. This is, of course, a very extreme example, but I hope this shows that we can conclude that mass leads to curvature which leads to gravity. And a black hole, is simply so much mass that it leads to so much curvature that the gravity is so strong that light cannot escape.

I hope this helps!

## @Mark Foskey 2015-04-17 02:45:23

A complete replacement of the brief answer I wrote some time ago:

More than one person has brought up the idea of a pair of ants walking on the surface of a sphere. Each ant is moving in what, for it, is a straight line, but the get closer together at an increasing rate until they collide. (Provided they're lined up right.)

This is an excellent metaphor, but it can be confusing because each ant is propelling itself, so it could stop if it wanted, and also they do have to be lined up right when they start or they won't collide. If you hold a rock still and then let go, it starts to move, which seems different from the ant picture.

All of these problems disappear if you realize that they don't call it space

timefor nothing. The surface of the balloon is two-dimensional in the ants-on-a-balloon analogy (and really the ants ought to be two-dimensional themselves, living embedded in the surface of the balloon just as we are embedded in spacetime). But it's wrong to think we are only throwing away one dimension to be able to visualize curved space. The right way to think of the balloon is that it has one dimension of space and one of time, so we're really throwing awaytwoout of the four dimensions.Each ant is racing headlong into its own future, and it can't stop or even slow down. And the ants can't miss each other, because the paths they follow are really the histories of their lives. The paths are called

world lines. Each point on a world line is a time and a place that the ant passed through. If two world lines cross, that means two ants were at the same place at the same time.This is still confusing, because the balloon is round. Which direction is time, and which direction is space? What happens when the ant goes all the way around the sphere? To make sense of these questions, you have to put a coordinate system on the sphere. For this toy universe, it actually makes sense to use latitude and longitude as coordinates. The south pole is kind of a big bang (take this with a lot of salt) and the north pole is the big crunch in the future (that definitely isn't going to happen in real life). The lines of latitude are the time coordinate, which means time progresses along the lines of longitude.

## @Timaeus 2015-01-05 07:08:22

Curvature affects motion by making the lines that are as straight as possible end up converging, just line how if you and your friends fly at constant altitude from the north pole, then no matter what directions you go (even if you and your friend head out in very different directions) then you start to converge on the south pole. This is a very good way to describe an effect that is determined by the path and not by the mass of the object taking the path. This is sometimes described as "spacetime tells matter how to move" but really this is just that the straightest possible lines converge when spacetime is curved the right way.

But something not mentioned enough is that while mass, energy, momentum, stress, and pressure are sources of curvature, they are not the only things that create curvature, curvature itself can create further and additional curvature. A gravitational wave can propagate or even spread in a vacuum of empty space devoid of all mass, energy, momentum, stress, and pressure.

The region outside a symmetric nonrotating static star is curved, even the parts far from any mass or energy or momentum or stress or pressure. The space remains curved because the existing curvature is exactly shaped so as to persist (or otherwise cause future curvature exactly like itself).

So curvature allows and sometimes requires more and/or future curvature, just as a travelling electromagnetic wave allows and/or even requires there be more electromagnetic waves elsewhere and/or later. The vacuum allows curvature far from gravitational sources just as it allows electromagnetic waves far from electromagnetic sources. What electromagnetic sources allow is for electromagnetic fields to behave differently (namely to gain or lose energy as well as move in different ways and gain and lose momentum and stress). Similarly what gravitational sources do is allow curvature to react differently to itself than it otherwise would.

Imagine a flat region of space shaped like a ball, then imagine a funnel type curved space where two regions of surface area are farther apart than they would be if flat (like a higher dimensional version of a funnel and on a funnel surface two circles of a particular circumference are farther away as measured along the funnel then if two similarly sized circles were in a flat sheet). On its own, spacetime doesn't allow itself to connect those two kinds of regions together, but that mismatch is exactly the kind or not-lining-up that putting some mass or energy right there on the boundary fixes. So without mass those two regions can't line up, with mass they can. Just like an electromagnetic field can have a kink if there is a charge there.

So your curvature likes to propagate a certain way, and if you want it to deviate from that, you need mass, energy, momentum, stress, and/or pressure. And you'd need the right kind to get it to match up, the kind you want might be available, and might not even exist, so not all kinds of curvature will be allowed. But the point of a source is that it changes the balance between nearby curvature and not that affects future curvature. So there is a kind of balance, and there are things that can warp that a balance. Those things that warp that natural vacuum balance are called gravitational sources.

Having curved spacetime is something we observe. Having gravitational sources that can change the normal or usual way curvature evolves is something else entirely. We can make theories about how the sources evolve, and then the curvature is forced to co-evolve with it, and that's what gravity is about, about gravitational interactions (source and curvature together) changing how the curvature evolves changing the evolution that the curvature otherwise would have evolved a different way.

So there is nothing circular, curvature is observed, and on its own it interacts and affects itself in a particular way (that is also observed), but gravitational sources get to change that and by interacting with the gravitational sources (which we can do) we can ourselves make the curvature change in different ways than it otherwise would!

## @answerman 2013-02-10 00:09:18

As others mentioned, the main problem with the common visualization is, that it omits the time dimension. In the animation linked below the time-dimension is included to explain how General Relativity differs form Newton's model.

http://www.youtube.com/watch?v=DdC0QN6f3G4

## @gfk 2012-06-16 19:18:11

I think the problem for the layman is understanding why there is motion in spacetime and I think a sort of answer is that we already accept motion through time when we think of time and space as separate. Well we are in motion through spacetime where time and space are not separable and when we move through a region of spacetime that contains matter the shortest spacetime path between two events is the one that includes motion through the space bit as well as the time bit (ie not orthogonal to the space axes). That is experienced as falling under gravity.

## @Marek 2011-01-16 10:10:32

Luboš's answer is of course perfectly correct. I'll try to give you some examples why the straightest line is physically motivated (besides being mathematically exceptional as an extremal curve).

Image a 2-sphere (a surface of a ball). If an ant lives there and he just walks straight, it should be obvious that he'll come back where he came from with his trajectory being a circle. Imagine a second ant and suppose he'll start to walk from the same point as the first ant and at the same speed but into a different direction. He'll also produce circle and the two circles will cross at two points (you can imagine those circles as meridians and the crossing points as a north resp. south poles).

Now, from the ants' perspective who aren't aware that they are living in a curved space, this will seem that there is a force between them because their distance will be changing in time non-linearly (think about those meridians again). This is one of the effects of the curved space-time on movement on the particles (these are actually tidal forces). You might imagine that if the surface wasn't a sphere but instead was curved differently, the straight lines would also look different. E.g. for a trampoline you'll get ellipses (well, almost, they do not close completely, leading e.g. to the precession of the perihelion of the Mercury).

So much for the explanation of how curved space-time (discussion above was just about space; if you introduce special relativity into the picture, you'll get also new effects of mixing of space and time as usual). But how does the space-time know it should be curved in the first place? Well, it's because it obeys Einstein's equations (why does it obey these equations is a separate question though). These equations describe precisely how matter affects space-time. They are of course compatible with Newtonian gravity in low-velocity, small-mass regime, so e.g. for a Sun you'll obtain that trampoline curvature and the planets (which will also produce little dents, catching moons, for example; but forget about those for a moment because they are not that important for the movement of the planet around the Sun) will follow straight lines, moving in ellipses (again, almost ellipses).

## @Zac 2011-01-16 12:08:22

Thanks heaps guys, it's starting to make some sense. So that makes sense to me with moving objects, but I still don't quite understand how it causes objects to accelerate. For example, with your analogy, what if the ants were stationary on the ball? When we lift something off the ground and let go, it accelerates toward the earth. Is this just because that's the "straighest" line through the curved spacetime around the earth? Why must it always be "moving" through a straight line, and what does it mean in terms of curved spacetime for something to be stationary?

## @Marek 2011-01-16 14:33:12

@Zac: good questions. First, geodesics contain more information than just shape of the path (which we call trajectory). They also contain information about how

fastthe particle will move along it. This is what will give you acceleration towards the Earth for free falling object but it has to be computed from the equations (actually, in this case conservation of energy is enough to derive that acceleration).## @Marek 2011-01-16 14:37:20

@Zac: as to why particle move through straight paths: this has its origin in quantum mechanics actually. One can compute that classical particles will move through trajectories that extremize (that is maximize, minimize or make stationary) the action. This is what reproduces standard Newton's law for motion in flat space-time and will give you straight (in the context of the given geometry) line in non-flat space-times. Also, one can include forces (like for charged particles in EM field) and this will modify the trajectory. There is surely a lot more to discuss about this topic.

## @wsc 2011-01-16 18:28:00

PeriheliON, not to be confused with parahelium. :)

## @wsc 2011-01-16 18:33:55

Also @Zac, for something to be stationary in spaceTIME means that it only exists for a single instant in time! Even something that stays stationary in space for all time moves on a curve in spacetime. (think about what an x vs. t plot looks like for a stationary object)

## @Marek 2011-01-16 19:34:49

@wsc: it's really called perihelium in my language (Slovak) so I never imagined it might be something different in English. Anyway, thank you :)

## @Marek 2011-01-16 19:41:34

@wsc: let me correct you on the

stationarypart though. The word has two different meanings. You used the physical meaning (as in not moving) but here the mathematical meaning was implied. In that sense stationary point is a synonym for an inflection point, as in one of the three types of extrema. Generalized to curves, this means that there are curves around the extremal curve of both shorter and longer length but the first variation still vanishes.## @wsc 2011-01-16 20:07:24

@Marek: nor did I realize it was different in Slovak; always nice to learn! Anyway, I was using that meaning of stationary since that was what @Zac was using: his question seemed to me to be: 'Sure you have geodesics on curved manifolds, but why do the ants have to

move?' Which is a very good question, you just have to remember that time is a coordinate too.## @Marek 2011-01-17 00:15:33

@wsc: I see, I didn't understand that part.

## @Viney Hill 2016-07-28 15:41:20

Still didn't get why a object just appeared out of nowhere few kms above the earth would fall it should stay there if no force is acting as is and it wasn't moving either.

## @Adam Hughes 2017-02-07 17:57:22

I agree, I'm not finding any obviously convincing laymen explanations for why two static objects would start moving when they have no initial momentum. Perhaps the answer cannot be explained in laymen's terms?

## @Zac 2017-08-04 16:06:29

@AdamHughes: So the crucial difference is that spacetime includes curved

timeas well. No object can remain truly stationary in spacetime because that would require being stationary in time. An object that magically appears above Earth may begin stationary in the space dimension of spacetime, but it nevertheless continues to "move" through the time dimension. It must follow the geodesic (straightest/shortest possible path) from that point, and the geodesic from the point above Earth points through time but also towards Earth because of its effect on the curvature of spacetime.## @Ajinkya Naik 2019-04-25 03:35:33

@Zac So an object moving at the velocity of light experiences no time and infinite length contraction hence it is stationary both in space and time does that imply that it shouldnt be affected by Gravity? (on the contrary we all know that it is affected)

## @Sklivvz 2011-01-16 11:48:26

What Einstein's equation tell us, at a basic level, is that the curvature of space-time and stress-energy are the same thing.

In order for this law to be respected it is clear that the stress-energy of a test particle cannot be constant in a space-time with changing curvature.

So, if you can choose a coordinate set in which the stress energy tensor is represented by the mass-energy of the particle, then the practical effect you can observe is changing energy and momenta of the test particle.

When you therefore observe the test particle, you will see it as having changing energy and momenta, and therefore derive a force driving these changes. This is what we call gravity.

However, general relativity gives a much deeper picture of gravity as a description of the curvature of space-time, so, in a way, gravity is an

observed effectof the curvature of space-time, or, if you like, anobserved effectof the distribution of mass and energy.## @Nat 2018-09-26 10:01:11

Part of this answer has been quoted in a new question.

## @David Z 2011-01-16 11:28:06

There are actually two different parts of general relativity. They're often stated as

Point #1 is actually straightforward to explain: objects simply travel on the straightest possible paths through spacetime, called geodesics. The paths only seem curved because of the warping of spacetime. If you're a physicist, then you would want to know that that fact can be derived from the principle of extremal action (with all the requisite mathematical details), but if you don't want to wade through the math, hopefully it at least makes sense that objects move on "straight" lines. There is no actual force involved when a massive (or even a massless) object's trajectory curves in response to gravity, because it doesn't take any force to keep something moving on a straight line. (I can definitely expand on this point if you want)

Now, I mentioned that spacetime needs to be warped in order for objects' trajectories to appear curved to us despite them actually being "straight." So the essence of point #2 is, why is spacetime warped in the first place? Physics doesn't have a good answer to that. Technically, we don't have an answer to point #1 either, but the "straight line" argument at least makes it seem plausible; unfortunately, there's no equivalent plausibility argument for why spacetime warps itself around matter. (Perhaps someday we will come up with one) All we can do right now is produce equations that describe

howspacetime behaves around matter, namely the Einstein equations which can be written $G_{\mu\nu} = 8\pi T_{\mu\nu}$ among other ways.## @Jerry Schirmer 2015-04-17 02:48:57

I never got why Wheeler wasn't as famous as Feynman. He had that same magical way of reducing things down to really clear, simple statements that made complicated things seem obvious.

## @Luboš Motl 2011-01-16 09:38:38

the trampoline analogy needs an extra source of gravity - because this is what the laymen, the recipients of the explanation, intuitively understand - but the real general relativity doesn't need any extra "external" gravity.

Instead, general relativity says that the space is getting curved by Einstein's equations, $$G=T$$ where the left-hand side are numbers describing the curvature at a given point and the right hand side is the density of matter and momentum. I omit indices and constants haha. So general relativity says how the spacetime is curved under the influence of matter.

The second part of the story is that general relativity also says how matter moves in external geometry. It moves along "geodesics", lines that are as straight as you can get. $$\delta S_{action\,ie\,proper\,length} = 0$$ This actually means that the objects move along the predicted, seemingly curved trajectories. These trajectories are actually as straight in the curved spacetime as you can get.

Imagine that there is a hemisphere replacing a disk in the trampoline. So there exists a (nearly) straight line on the hemisphere - namely the equator near the junction with the rest of the trampoline. Note that the equator on the Earth is a maximum circle - so it is one of the straightest lines you can draw on the surface of Earth. The same is true for all actual trajectories that objects choose in spacetime of general relativity.

So in the hemisphere-above-trampoline example, particles can orbit around the equator of the attached hemisphere, just like planets, because it is the straightest and most natural line they can choose. I don't use any external gravity to explain the real gravity; instead, I use the principle that particles choose the most natural - the straightest - line they can find in the curved spacetime.

Best wishes Lubos