# B Gravity and relative motion

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1. Mar 31, 2016

### Stephanus

Dear PF Forum,
I have a confusion about gravity. And frankly I don't know if this question belongs to this sub forum (cosmology, general physic?).
Gravity attracts object - Newton
Gravity curves space time - Einstein.

Why we revolve around a massive object?
Because that massive object curves space and we try to travel straight? So we revolve that massive object because that object curve space around it and we think that the curve is straight line?

But what if we don't travel. What if we just sit still and there's a massive object coming toward us. Would we still revolve that object once we are in its gravitational field? I think we are ALWAYS in its gravitational field no matter how far that object but its force (if we can call graviti, force) would be very weak.

A: So from our point of view.
Whoaa, there's a big object coming toward us, but why would we revolve around it. We just sit still right?

B: From that massive object point of view
Hey, there's a little thing coming toward me, well once it is in my gravitational field, it will revolve around me.

So no matter how the small object always revolves the bigger object right? And as I read somewhere the bigger object also revolves that small object but in a less curve.

=================================
So my conclusion is this.
In space time
A: Do we always move in time? I think so, we are getting older aren't we?
B: Do we ALWAYS MOVE in space? Does everything in this universe always move in space?
C: If B is true, is that why every object always revolves other object either bigger or smaller, because everything ALWAYS MOVE in space?

Thank you very much.

2. Mar 31, 2016

### Orodruin

Staff Emeritus
This is not correct. The correct statement is "because that massive object curves space-time".

It is not a straight line in space, it is a straight line in space-time. You are changing your time coordinate and thus traveling in space-time. There is no such thing as "not traveling", it would correspond to something existing only at one event.

3. Mar 31, 2016

### Stephanus

Okay, that we travel in time I can understand that. I'm gettting older 5 minutes from my first post. But does EVERYTHING ALWAYS MOVE in space also?

4. Mar 31, 2016

### Orodruin

Staff Emeritus
There is no way of defining "move in space" without referring to a particular coordinate system. You can always find a coordinate system in which an object moves, just as you can always find one where it stands still.

5. Mar 31, 2016

### Staff: Mentor

We still "travel" through time.

Edit: i am way too slow today!

6. Mar 31, 2016

### PeroK

I'm baffled by this post. We wouldn't revolve around the big object: it would smash into us, surely?

7. Mar 31, 2016

### teo del fuego

Einstein noted that as we sit still on Earth in its frame of reference, we are, nevertheless, traveling through time at the speed of light. Source: Brian Greene's Fabric of the Cosmos.

Is it correct to infer that Movement through space slows our movement through time?

[Moderator's note: edited to delete off topic content.]

Last edited by a moderator: Mar 31, 2016
8. Mar 31, 2016

### Staff: Mentor

Be cautious about Brian Greene's popularizations. He sometimes says "This is how it works" when it would be more accurate to say "This is NOT how it works, but it's easy to understand and won't be too misleading as long as you don't take it too seriously". There's nothing wrong with this as long as you understand what you're getting and the limitations of the explanation - but people are often confused when they try to carry the explanation too far.

Writers like Greene are one of the reasons why Physics Forums has a rule about acceptable sources.

9. Mar 31, 2016

### Stephanus

Why should it. If, say, there is a massive object coming toward us, but pass by say 1 million KM, would we "follow" it and eventually revolve around it?

10. Mar 31, 2016

### Orodruin

Staff Emeritus
No, not unless you by some other means accelerate to enter orbit around the object. Otherwise it would simply slingshot you.
https://en.m.wikipedia.org/wiki/Gravity_assist

11. Mar 31, 2016

### Ibix

What would moving slowly through time mean? That when I get to Monday you are still only on Sunday? That seems rather silly. I'm afraid that Nugatory has a point - Greene often over-simplifies things to the point of being wrong, and that's what's going on here.

What Greene has done is pick a particular coordinate system (conceptually, filled space with a regularly spaced array of clocks that he has synchronised by some method). He can always do this so that the four velocity of a chosen object is parallel to the direction he's chosen to call time and all his clocks are stationary with respect to it. Any other four velocity has a smaller component in the direction he's chosen to call time, which means that it's clocks will advance slower compared to the array of clocks Greene previously synchronised. But since Greene has a lot of freedom to choose what he calls "synchronised", that's just an artifact of his (entirely reasonable) choice.

12. Mar 31, 2016

### Stephanus

So you're saying that if we don't accelerate it doesn't slingshot us, it could just pass us not necesserily hits us?
This is my respons to
It might just passes us right.

13. Mar 31, 2016

### Stephanus

Movement through space [wrt, with respect to other body] slows our clock [wrt other body]
The only thing that you would feel is acceleration or gravity if you're bound on solid object such as your chair (which is bound to your floor which is bound to your house foundation which is bound to earth).
And you can never realize that your clock runs slow,
How can you?
Your clock runs slow, but your brain runs slow, too. You can't realize it, and everything around you, your heart beat, your digesting system, your chemical reaction, your music player, everything runs slow. You can't realize it can you. But if you see other thing that travels wrt you, you'll see its clock runs slow, too

14. Mar 31, 2016

### Ibix

More precisely, "slows our clock with respect to an array of clocks synchronised by Einstein's method (or many others but not all) and stationary with respect to the other body".

15. Mar 31, 2016

### teo del fuego

Thanks so much for your reply. As much as I like Greene's books, I find anything he does on TV unwatchable. Even to my layperson's limited knowledge, I see him consistently over-dramatizing things like quantum weirdness and special relativity. I will approach his books with more skepticism even though I don't have the knowledge base to criticize him.

[Mentor's note: Some text discussing another thread has been removed from this post because it is off-topic here]

Last edited by a moderator: Mar 31, 2016
16. Mar 31, 2016

### Orodruin

Staff Emeritus
No, this is not what I said. I said if you do not have other means of acceleration than the gravity, you will not enter orbit.

17. Mar 31, 2016

### Staff: Mentor

@Stephanus

Consider the Newtonian limit. If you are only under the influence of gravity then you will either travel in an elliptical path if you are in orbit or a hyperbolic path if you are not. There simply is not a gravity-only solution which goes from moving in a straight line wrt each other to revolving around each other.

When you get strong gravity in GR that can change, like you will not depart on a hyperbolic path if you cross the event horizon of a bkack hole.

18. Mar 31, 2016

### bahamagreen

In other words one needs to apply an acceleration to enter an orbit... this may be easier to grasp by considering the opposite - that one must apply an acceleration to exit an orbit.

19. Apr 1, 2016

### Stephanus

Okay, since we are in the heat of it, although this might be off topic. I'd like to ask anyway.

At the picture above,
1: Is there a chance that Red and Blue will collide?
2: Red and Blue must collide?
3: Is there a chance that Red and Blue will revolve each other?
4: Red and Blue must revolve?

Below
5: Is there a chance that Red and Blue will collide?
6: Red and Blue must collide?
7: Is there a chance that Red and Blue will revolve each other?
8: Red and Blue must revolve?