2
$\begingroup$

This question already has an answer here:

Let me preface this by saying that I'm a high school student so my knowledge of GR might be seriously flawed.

As far as I'm concerned, Einstein's theory of GR states that matter curves the fabric of space-time so instead of there being an imaginary "tug," objects are simply falling in.

My question is, how do the objects "fall" in because in real-life simulations of the curvature of space-time, the force that's causing objects to "fall" in is Earth's gravity. In other parts of space, what causes objects to "fall" in. Does the Newtonian "tug" of gravity still exist within Einstein's theory because, without it, I can't seem to understand why curving space-time would cause stuff to "fall" in?

$\endgroup$

marked as duplicate by John Rennie general-relativity Feb 6 at 8:06

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • $\begingroup$ I'm sorry to be dissapointing but you're right in your introduction paragraph. The blanket metaphor is more an example to explain what happens, rather than reality. But it's good you're interested in the stuff haha. $\endgroup$ – FGSUZ Feb 6 at 1:27
  • $\begingroup$ This is a complicated question, and I think the confusions you have about it have to do with the fact that the idea of a curved space is something that is hard to express on the high school level, especially if the teacher is a "traditional" levers/inclined planes/ropes and pulleys-style instructor. There is a LOT of stuff that has been written about it that is available on-line, can you tell us what resources you have tried so far to help? Book titles, wiki articles, etc.? $\endgroup$ – niels nielsen Feb 6 at 2:21
  • $\begingroup$ I was just watching some youtube videos about Einstein's theories and the video talked about how mass curves the fabric of space-time. The video then went on to say that because space is curved, objects fall in and that only makes sense if Newtonian Gravity is a real thing. $\endgroup$ – Sidhant Rajadnya Feb 6 at 2:41
  • 2
    $\begingroup$ And that's the major problem with that rubber sheet / blanket model: it seems to imply that there's some kind of meta-gravity pulling the massive bodies down & causing test particles to travel down into the depressions. But that's completely wrong. $\endgroup$ – PM 2Ring Feb 6 at 7:24
  • $\begingroup$ I always hated the blanket analogy. And don't even see what is good for. Without a Newtonian view object on the blanket won't fall. I admit I was proud of understanding at least that it doesn't make sense $\endgroup$ – Alchimista Feb 6 at 12:17
1
$\begingroup$

Don't think of "falling in". Instead, think of "moving in a straight line". I assume you've learned Newton's la of inertia, so it's clear and intuitive to you that if a puck moves on a frictionless horizontal plane it will just carry on moving on a straight line at constant velocity.

Now to think of GR, people tell you to think about a curved surface, rather than a plane. But the important thing to realise is that this is just an analogy. The real "curvature" isn't like that, but is abstract.

What you should do, is imagine this situation from above. In the Newtonian case, the puck just continues in a straight line. In the GR case, the puck appears to move in "non-straight" line. Ignore the fact that in the GR the surface is "curved" - it isn't really bulging in real-space, that's just an analogy to make the trajectory from above look curved. The point of it is that now the puck moves in a "line" that appears curved to us. No forces are acting, the puck is still moving inertially along the line of inertial movement.

One way to say it is to treat the inertial-movement line as the definition of a straight line. So you keep the Newtonian idea that inertial bodies move along a "straight line", only now this line appears curved in 3d-space. This is like how a circumfrance-line of a sphere is a "straight line" along it, i.e. it's the shortest way to connect two points along it, even though the line is actually curved in 3d-space.

$\endgroup$
  • $\begingroup$ Oh ok. So basically the geometry of moving in a "straight line" changes when you switch to 3-dimensions right? $\endgroup$ – Sidhant Rajadnya Feb 6 at 7:06
  • $\begingroup$ @PM2Ring D'oh. Thanks. $\endgroup$ – PhysicsTeacher Feb 6 at 10:12
  • $\begingroup$ @SidhantRajadnya Well, the point is the change from curved to non-curved space. Consider plane's trajectory from point A to B on the 2d surface of Earth. If projected onto a 2d map, it still won't look like a "straight line". The shortest line between two points on a curved space just doesn't look like a straight line when you try to cram it into a flat space, regardless of the number of dimensions. $\endgroup$ – PhysicsTeacher Feb 6 at 10:28
2
$\begingroup$

The most important of General Relativity, which is generally not told by the popular science educators (where I am assuming you heard about General Relativity), is that objects always follow the shortest path through space-time (remember that objects are always moving through space-time). When there is no energy or matter in the surrounding space-time then objects follow straight lines since space-time is flat. Now if you have some matter or energy to curve space-time then the shortest path isn't a straight line. The shortest path between any two points on a curved space is called a geodesic. All the falling body is doing is following the geodesic.

$\endgroup$
1
$\begingroup$

You might want to be inside a windowless falling capsule , for the context of a GR discussion. Maybe you are in orbit around the earth, or actually falling down. You would almost not feel gravity. However, you would see the tidal effects on a puff of smoke that would change shape over time. The smoke particles are following geodesics. Analysis of these geodesics will reveal the presence of earth and possibly its motion. You may want to set the origin of your space-time coordinates in the center of your capsule now. You may want to open a window to see if the earth is rotating below you or moving toward you to give you a good smack on the future branch of your time axis.

As for throwing a rock out the window, it will follow a space-time geodesic, that you can express in the coordinate system centered in your capsule. If you draw a graphic of its space-time trajectory, you will need extreme scaling on the time axis. Light goes really fast.

Instead of answering your question, I have asked you to change your context. Think of tidal forces rather than throwing rocks.

$\endgroup$

Not the answer you're looking for? Browse other questions tagged or ask your own question.