Per the current theory, we feel gravitation as a consequence of distortions that bodies with mass create in the fabric of spacetime. To help the lay person understand it better, we illustrate it with a grid like picture with a stellar body creating a curve in the space time. Admittedly, it's only a 2-d simulation of a phenomenon that occurs in 3 dimensional space.

On another note, we hear that in space there's no direction. Or more accurately, absent a frame of reference, there's no sense of up and down or left and right.

To me, these two postulates make it hard to visualize how our Sun distorts the so-called spacetime.

Firstly, to imagine, even in a 3 dimensional space, a distortion that would create a pull is to think of space in terms of an absolute up and an absolute down and so forth. To aver that sun creates a curve in the fabric of spacetime implies an upward direction to which the slope (curve) is downward to. That being the case, how does it reconcile with the postulate that absent a frame of reference, there can be no up and down?

Secondly, wouldn't the shape of the distortion created by the Sun be the same for all the planets and asteroids - diminishing in the force it exerts as we go further away but the shape of the distortion being the same? If that were to be true then the shape of the orbits of all the planets must be the same? And yet we have minor planets like Pluto with a very distinct orbital path around the sun.

So, where do I err?

When you're looking at the gravitational field of the sun, it's not true that there's no direction singled out - there is - namely towards the sun.

twistor59 wrote:When you're looking at the gravitational field of the sun, it's not true that there's no direction singled out - there is - namely towards the sun.

That doesn't really solve my problem. The direction is towards the Sun and the shape (of the curve Sun creates in spacetime) is conical. But conical comes in two flavours and that is where my problem starts. Which way does the conical shape bulge out to? How is that direction of the bulge determined?

And why doesn't Pluto obey this consistent and absolute distortion like Earth does?

advaitya wrote:

twistor59 wrote:When you're looking at the gravitational field of the sun, it's not true that there's no direction singled out - there is - namely towards the sun.

That doesn't really solve my problem. The direction is towards the Sun and the shape (of the curve Sun creates in spacetime) is conical. But conical comes in two flavours and that is where my problem starts. Which way does the conical shape bulge out to? How is that direction of the bulge determined?

And why doesn't Pluto obey this consistent and absolute distortion like Earth does?

Imagine a spiderweb, but replace the spider with the sun. Each section of the web is one area of spacetime, which, towards the sun is stretched exponentially. In 3d, not just 2d like a spiderweb.
So if one would follow a straight line it wouldn't be straight but bend around the sun. Hence planets orbit the sun.

What do you mean with pluto? It's in orbit just like all other planets.
All planets have elliptical orbits btw, falling around the sun. It's like when you throw a ball upwards, it makes a curved trajectory back to earth.

Another difficulty you may have is that you're trying to imagine a distortion of space-time in terms of the rubber-sheet picture, which is something drawn in space only. It isn't meant to be taken too literally. I don't know how to visualize distortions in space-time - I wonder if anyone can. However we can clearly and unambiguously describe it using mathematics.

twistor59 wrote: rubber-sheet picture

Imagine multiple layers of rubber sheets from all possible directions in 3d. or a 3d spider web being stretched inwards towards a central point.

JoeB wrote:

advaitya wrote:

twistor59 wrote:When you're looking at the gravitational field of the sun, it's not true that there's no direction singled out - there is - namely towards the sun.

That doesn't really solve my problem. The direction is towards the Sun and the shape (of the curve Sun creates in spacetime) is conical. But conical comes in two flavours and that is where my problem starts. Which way does the conical shape bulge out to? How is that direction of the bulge determined?

And why doesn't Pluto obey this consistent and absolute distortion like Earth does?

Imagine a spiderweb, but replace the spider with the sun. Each section of the web is one area of spacetime, which, towards the sun is stretched exponentially. In 3d, not just 2d like a spiderweb.
So if one would follow a straight line it wouldn't be straight but bend around the sun. Hence planets orbit the sun.

What do you mean with pluto? It's in orbit just like all other planets.
All planets have elliptical orbits btw, falling around the sun. It's like when you throw a ball upwards, it makes a curved trajectory back to earth.

Just wanted to inform you that gandhi never said that!. something along those lines was said in us

Describing the stages of a winning strategy of nonviolent activism. There is no record of Gandhi saying this. A close variant of the quotation first appears in a 1918 US trade union address by Nicholas Klein:

And, my friends, in this story you have a history of this entire movement. First they ignore you. Then they ridicule you. And then they attack you and want to burn you. And then they build monuments to you. And that, is what is going to happen to the Amalgamated Clothing Workers of America.

http://en.wikiquote.org/wiki/Mohandas_Karamchand_Gandhi

*gulp* I'm sorry, I didn't know that!
*runs of to replace signature*

advaitya wrote:Secondly, wouldn't the shape of the distortion created by the Sun be the same for all the planets and asteroids - diminishing in the force it exerts as we go further away but the shape of the distortion being the same? If that were to be true then the shape of the orbits of all the planets must be the same? And yet we have minor planets like Pluto with a very distinct orbital path around the sun.

No. In GR, gravity isn't a force. Distortion (curvature) of spacetime is all there is. A planet, say, moving around the sun is not subjected to a force; it is following a geodesic in spacetime. This geodesic is analogous to uniform motion in a straight line in flat spacetime.

In general, it's impossible to visualize. You can calculate geodesics, but you have to do so by taking the measurements locally (that is, over a "volume" of spacetime small enough to be considered flat). You can also integrate over an interval, as long as you take everything into account. In general, though, that's all you can do. You cannot figure out some sort of overall picture in the general case, because the picture you get is going to depend on where the observer is and where it's going.

In the case of the solar system, if you completely ignore the contributions from anything but the sun, you can get a reasonable overall picture as an approximation. The rubber sheet thingies (which I think confuse a lot more than they elucidate) are an attempt at such. You can sort of see that a slice of the rubber at a certain "height" is a circular orbit. However, of course, it is possible to have a highly elliptical orbit, and for that orbit, you'd have to use a different shape of the rubber sheet.

This has made a lot of people very unhappy, but there isn't much that can be done about it. Any picture you get is relative to the observer. There is no absolute uniform picture, which is why it's called a theory of relativity. The only thing you can say overall is that the equations have to work everywhere and everywhen (as long as you aren't at a singularity).

I thought of this last night, when I saw a spirograph toy. You could make a picture for all possible settings of the spirograph, but it would just be solid black. The problem of GR is worse.

epepke wrote:

advaitya wrote:Secondly, wouldn't the shape of the distortion created by the Sun be the same for all the planets and asteroids - diminishing in the force it exerts as we go further away but the shape of the distortion being the same? If that were to be true then the shape of the orbits of all the planets must be the same? And yet we have minor planets like Pluto with a very distinct orbital path around the sun.

No. In GR, gravity isn't a force. Distortion (curvature) of spacetime is all there is. A planet, say, moving around the sun is not subjected to a force; it is following a geodesic in spacetime. This geodesic is analogous to uniform motion in a straight line in flat spacetime.

That seemed elegant to me until they invented the graviton and said it was a force carrier and now I'm confused.

John

z8000783 wrote:That seemed elegant to me until they invented the graviton and said it was a force carrier and now I'm confused.

John

That's OK. Everybody is. Confused, that is.

There are good reasons to believe that gravity should be quantized, because everything else seems to be. If it is, one could speak of gravitons. That doesn't mean that GR is wrong, however. Just as virtual photons can be seen as consistent with the idea that charges change the electric field, gravitons might one day be seen as consistent with the idea that energy/momentum (reducible to mass in certain simplified cases) change the spacetime fabric.

Maybe a complete understanding would have a way of seeing the electormagnetic, weak, and strong "forces" as pseudo-forces. We did this a long time ago for the centrifugal and Coriolis "forces" and much more recently for the gravitational "force."

My intuition (which, along with $3 will get you a cup of black water at Starbucks) is that there is some way of looking at all four of the traditional "forces" such that they can be considered forces in some contexts but as the absence of forces in another. That is, our basic concept of force will be seen to be illusory if you pick a particular way of looking at it. However, there are good reasons to think that this is easiest to do with gravity, which is in some ways the simplest. Nobody, so far, has been smart enough to do this for the other "forces."

I am heartened by the way that QED, which can be worked out on paper (though it's hard) has been supplemented by QCD using computers (on which I did some minor work with gluon-gluon interactions and the formation of neutrons). Some day in the future, there may be a unified view of the universe in which the force/not force and quantum/classical divisions will look like periods of confusion and misunderstanding, much as the most experienced people look at wave/particle duality today. It just hasn't happened yet.