Silly classical + relativity question.

[h4x]

Hello, this is my first post on the forums and I would like to say this site is very interesting - I have read many physics books and am very interested in the subject. I suppose due to my poor reading comprehension or just general misunderstanding of the subject, I have a question about relatavistic phsycis and Newtons Classical methods:

I understand that planets gravitate towards each other; gravity is "the tendency of objects with mass to accelerate toward each other", G=m1m2/D^2. This is in accordance with Newtons law. Then, in Einstein's theory of relativity, this is wrong. There is no actual "force" between the two objects. Gravitation arises out of spacetime being curved by the presence of mass.

Therefore; Newtons law is wrong. Yet, in the definition of gravitation it cites: "The gravitational attraction of the Earth endows objects with weight and causes them to fall to the ground when dropped..".

Maybe I am uncomfortable with the word "dropped" and "fall" after all of this. I will try and describe a few situations I have envisioned:

If two tennis balls were out in deep space, and slowly "gravitating" towards each other, what would really be happening is that they are both bending space-time, and actually traveling straight in their own reference frame. Correct?

If that is correct, then if someone throws a tennis ball on earth, the same thing is happening again? The Earth is bending the space-time and the ball is not really "dropping", but more moving straight in its own reference frame as the Earth bends the space-time so the ball appears to fall/curve to the ground; but it really isn't falling.

I believe I am really hitting that mental block of jumping from common-sense Classical Mechanics into Relativity. I am fairly well read when it comes to Relativity; but I have had no formal learning and not many diagrams to aid me (im pretty visual).

Can anyone help me out or clear things up for me? Sorry for the long post. I actually feel better about how I feel now that I have typed out some thoughts. Thanks.

 

[LURCH]

It is a bit like having bees live inside your head, isn't it? GR puzzles most people, and that's perfectly normal.

I think that understanding GR will change your view of what gravity is (indeed, I can see that it already has), but the phenominon we call "falling" is still the same as it always was. Only the explanation for why things fall is changed. Gravity causes things to move toward each other in an accelerating fashion. We now know with a reasonable degree of certainty that this accelerating movement (I say "accelerating" because the two objects are accelerating relative to one another) is caused by the geometry of the space around them (like a curved surface), rather than by an attractive "force" (like a magnet).

But the actuall falling itself is still the same.

A lot of autors even contend that GR does not prove Newton wrong, because as you have already quoted, Newton was carefull to define gravity as "the tendency of objects with mass to acceerate towards one another". This "tendency" definitely exists. If Newton ever personally reffered to it as a "force", I do not know. I've even read quotes from him where he specifically sated that the cause of this motion was unknown, and said "I leave it up to the reader" to imagine why this happens.

Einstein was just a reader to took Newton up on that offer, and GR was the result.

 

[Entropia]

Hello! Thank you for sharing your question and thoughts with us. It's great to see your interest in physics and I'm glad you find this site interesting.

Firstly, it's important to understand that Newton's laws of motion are still applicable in our everyday lives and for most practical purposes. However, when it comes to very high speeds or extremely massive objects, Einstein's theory of relativity becomes necessary to accurately describe their behavior.

In the case of gravitation, Newton's law is a good approximation for most situations, but it breaks down at very high speeds or in the presence of extremely massive objects. This is where Einstein's theory of general relativity comes in. It offers a more complete and accurate understanding of gravity, by describing it as the curvature of space-time caused by the presence of mass.

To address your specific examples, in the case of two tennis balls in deep space, they would indeed be bending space-time and moving in their own reference frames. However, the effect of this curvature is very small and would not be noticeable in this scenario. On the other hand, in the case of a tennis ball being thrown on Earth, the effect of Earth's mass is much greater and therefore the curvature of space-time is more significant. This is why we observe the ball "falling" towards the ground. In reality, it is moving in its own reference frame, but the curvature of space-time makes it appear as if it is falling.

It's completely normal to have difficulty grasping the concepts of relativity, as they go against our common-sense understanding of the world. I would suggest reading more about the topic and looking for visual aids to help you visualize the concepts. There are also many great online resources and videos that can explain these concepts in a more accessible way.

I hope this helps clarify things for you. Keep exploring and learning about physics, it's a fascinating subject!