Relative Motion (A Thought Experiment)

If Einstein comes back from the dead, he'd bitch slap you for saying that

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The Laminator
For goodness's sake, please vote Democrat this November!

Mr.Jack But we are talking here about a void where we have no background by which to judge two bodies as gravitationally in orbit about one another.

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If Einstein comes back from the dead, he'd bitch slap you for saying that

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It's statements like this that sometimes makes me think of you as "Rocket Lite". Please stop cluttering up the board with such.Please make any response to this to the appropiate link, below.Adminnemooseus

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{Off-topic material deleted - Stop it! - AM}This message has been edited by Adminnemooseus, 07-29-2004 11:39 PM

No! You have done me a diservice and an injustice. I had NOTHING to do with this. I won't tolerate a ditatorship!

Doesn't matter. If the other body appears stationary compared to yours, and there is nothing holding it there, you must be in orbit. You don't have to see the motion to deduce that. If the two bodies were actually stationary they would accelerate towards one another and collide.

Believe it or not, I actually realized this after I posted. However I'd just finished a long ramble and didn't really feel like deleting it. So I thought I'd wait to see if this was in fact my error. Thanks for confirming it for me.So in essence, I was confusing motion and acceleration, correct? Is it correct then to say that "motion" actually produces no physical effects, only acceleration? If I understand correctly, only a change in motion will produce detectable results. But then, a "change in motion" is an acceleration, yes?Excellent! I think some of my confusion has finally been cleared up! Cool, I realize that now. Thanks!I actually did know this before but for some reason I wasn't applying it to this scenario. I think I was so hung up on the details that I was forgetting some of relativity's most basic concepts.Unfortunately I'm still having trouble understanding how the Earth can rotate in an empty void. I know this is an acceleration rather than a linear motion but doesn't acceleration also require a frame of reference? All points on the "Earth" remain a constant distance and orientation to all other points on the "Earth" (for the sake of this example), so in relative terms isn't it always in the same state?Maybe I'm just having trouble with this because of my preconceived ideas about motion (or in this case, changes in motion). It's possible that it doesn't require any relative frame of reference. The trouble is that I can't see how it is meaningful without it.Perhaps it comes down to how we arrive at our models of the physical world.For example, if we lived on this "Earth", we may hypothesize that the Coriolis Effect is caused by an unseen force (like gravity, for example). If this model accurately explains the observed phenomena, would it be any less correct than hypothesizing that it is caused by the "Earth" rotating? Would there actually be any way to test one against the other? Would there even be any qualitative difference between them?I think what I'm trying to work out (and at this point, I'm honestly not sure) is whether the Earth's "rotation" (in this thought experiment) is simply our subjective means of encoding the data into a coherent explanation, or is an actual reality, in its own right. If it is a true reality and not just our means of explaining/understanding our observations, then it must relate to either space or the "Earth" itself, as there is nothing else. And I can't see how a (mostly) solid mass can rotate (or move in any way) relative to itself.Clearly something is causing the Coriolis Effect on the "Earth" but can it actually be said to be its rotation? Does this have any meaning if there is nothing to relate its rotation to?I know that we are talking about a curved motion (an acceleration) but ultimately I keep running up against the same conceptual wall. If the "Earth" is rotating, its surface "pulls away" as it turns, so it's following a curved path. But again, what is it pulling away from? Heh, quantum theory is far more confusing than relativity! Hell, I actually understand relativity! Well, as far as laymen's understandings go anyway. In any case, relativity doesn't go entirely against common sense.But quantum physics...Ack! Particles are waves, waves are particles, the cat is dead, the cat is alive, jumps, tunnelling, the EPR Effect, 1/2 spin particles...Argh! Give me "simple" concepts like relativity any day.I seem to recall a quote (I don't remember who from, though) which was something to the effect of, "If quantum theory doesn't confuse you, you don't understand it." I loved the irony in that!One final note before signing off...I hope I don't seem like I'm just beating a dead horse with this topic. I honestly don't get it. I'm trying, though, and I really want to understand. If anyone can recommend any sites or URL's that may clear this up for me, I'll be quite happy to check them out.In the meantime, thank you to all the people who have helped me, thus far. I really appreciate it!

Mr.Jack How do you determine that you are not both stationary since in this thought experiment it is implied {I hope} that we are confined to one of the planets? Also what experiment do you perform on your planet to determine that the gravity on your planet is also on the other?

What shape is the other planet?

NosyNedTouche' Ned. Now how do we determine that this gravity acts between planets in such a way as to cause them to orbit one another?

No one said we didn't have Newton here. Do apples fall? Do we have GR?But you are right in that it would be much, much harder for Newton to figure out his rules and to generalize. Much!(added by edit)I think we could (Mach aside) detect the acceleration of the Earth due to the moon.This message has been edited by NosyNed, 07-31-2004 01:54 AM

Hey Ned!You know, I just realized that in all the time I've been reading on EvC (faaaar longer than I've been posting on EvC), I don't think I've ever read exactly what your field is. From what I can tell, you seem to be quite at home in the Dates and Dating forum; does your expertise tend towards Geology/Paleontology, perhaps?I hope you don't mind me asking, I'm just curious. It's actually quite hard to tell what your field is because you seem well rounded in just about everything.But I'm off topic.Just a general note to everyone in this thread:First of all, thank you all for your continued assistance. I appreciate everybody's time and patience.Also, I want to clarify my questions a little if that's at all possible. I feel like I've been anything but clear so far and I want to make sure that everyone actually knows what I'm having trouble with.I'm not particularly concerned with whether or not we can detect motion (or as Mr Jack pointed out, acceleration). What I'm having a hard time wrapping my head around is its non relative nature.In the rotating "Earth" scenario, for example, I'm willing to concede for the sake of argument that we could detect evidence of its "rotation". My problem is that I simply can't see what "rotation" means, in this context.This is causing me a conceptual mind-jolt like you wouldn't believe. If you can detect the applicable forces on the "Earth" then something must be causing them. The question is, "What?" The planet's rotation? Rotation relative to what? And yet, there those forces are! How else do you explain them?Damn! This is so frustrating! I feel like I'm just repeating the same things over and over again and never getting any clearer. I'm trying so hard to elaborate but I simply can't find the words. How about this?What is the "Earth's" rotation? Is it an actual reality that exists independent of our perception of it? Or is it perhaps something even more fundamental which we codify into our "rotating" model. Is it possible that there is something "deeper" (wrong word, I know) which causes the forces and our "rotating planet" model is simply our best explanation of the data?Ack! That was terrible!I'm not actually suggesting that this model is wrong, it is as correct as any other scientific model. What I mean is that while it is an accurate descriptive model, the concept of its rotation is still our abstraction and without a relative frame of reference, it doesn't actually exist, independently of our ability to assign random relative points to the space it occupies.Oh good grief! Did that make any sense, at all? It seems like the more I try to elucidate, the more incoherent I become. Somebody please tell me they can see where I'm coming from here.In any case, I'm having such a hard time making sense of this right now that what I've written here is almost certainly wrong. I'm afraid that, at the moment, it's simply the best my mind can do to maintain a consistent and (kind of) logical picture of the whole thing.I'd better leave it at that for the time being. I've been awake for way too long (about 40 hours) to be thinking about this now anyway. Sorry for taking up so much bandwidth on it.Sleepy time for me.

I don't actually know anything at all. Once upon a time before more than half of this forum was born I got a bachelor's in physics. Nothing I say here is a reflection on the department that granted that degree; I have forgotten everything!I spent my working life in software developement and DB administration and as a consultant and with IBM.The only thing I have going for me is that I like to read and learn. As for any apparent expertise that is a fake based on reading what others have written. And, I like to think, the ability to think moderately coherently and logically.

Heh, I wish I knew as "little" as you. Don't worry about that...I respect age and experience. Well even if you have, you still had to learn it in the first place. Give yourself some credit; a bachelor's in physics isn't something that just anyone could walk in and get. I doubt that I could. Software development? Were you a programmer, too? Just curious.Also, don't take this the wrong way, but how long ago did you start your work in computers? Did you work on the first desktop computers? Before that? After? I don't even know what the first desktop computers were, I'm just curious. You think very logically, I can vouch for that.As for reading and learning, I'm the same. And I think calling your expertise "fake" is a little harsh. Ultimately, most of what we know about these things will come from reading what others have written. For those of us that don't actually work in the necessary fields, it's the best we can do. But that's nothing to be ashamed of; it doesn't make our knowledge any less real or valuable.In fact, I'm always pleased to come across a fellow "self-taught scientist", especially when they are as knowledgeable as you; it gives me hope.Anyway, thanks for the info, Ned.

LOL, I was born within a month of Eniac. The first machine I programmed was an IBM 7044 which was the scientific machine superceded by the more general purpose 360 series.I was a bit late in the "PC" revolution. Friends owned various microprocessors in the '70's (a fortune slipped by again). I didn't touch one very seriously till using a Mac in 1985. It wasn't until 1993 that I started with a small software firm and ended up as a software development manager after the people who worked for me taught me C and C++ while I was their so-called "senior" engineer.It is nice to have experience respected. Just remember to question. One challenge for me working with more junior teams was to remind them that I can be wrong too, horribly wrong. It seems I can exude (I think it was the consulting -- at $200 an hour you have to be able to sound like you know what you are talking about) confidence and sound right even when I'm blowing pretty colored smoke out of one orifice or another. When time is short you can trust a track record to possible give a better quick decision but when there is time thinking is a good idea.