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Author | Topic: Relative Motion (A Thought Experiment) | |||||||||||||||||||||||
AdminNosy Administrator Posts: 4755 From: Vancouver, BC, Canada Joined: |
Yes but relative to what? This is what I can't figure out. In the case of the earth moon system they are both moving relative to the center of mass. But i may come back to Mach's Principle.
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Tony650 Member (Idle past 4293 days) Posts: 450 From: Australia Joined: |
Hangdawg, your reply was linked to sidelined's post but you quoted me so I assume you were replying to me.
Hangdawg13 writes: Ohhh Now I get your question! Why don't the two fall into each other?!? Are the two orbiting each other about their barycenter or is the universe orbiting the two about their barycenter. Actually, that's not quite it. I'm not particularly interested in whether it's the bodies or the universe doing the moving. In a sense, it doesn't really matter. As far as I can see, there would be no distinction between the two. What I'm having a hard time with is the concept of non-relative motion. To me, this seems a contradiction in terms. What I'm trying to wrap my layman's head around is whether a tidally locked system actually can orbit, if it is isolated from any relative reference frame. The bodies are motionless, relative to each other. So if no external reference frame exists, what exactly is their orbit relative to?
Hangdawg13 writes: The only difference between the two orbiting each other and the universe spinning around them is that in the former they stay apart, and in the latter they fall together. Hmm...Are you sure about that? It may depend on what you mean by "the universe spinning around them." If you mean that space itself has some tangible quality that can move, rotate, etc then wouldn't it be indeterminable, from within, whether it was the universe (space) or the system doing the rotating? At the risk of phrasing this very badly, would "space moving against the bodies" not have exactly the same effect as "the bodies moving against space"? Also, it seems to me that if we allow for a "universal" rotation, we are again faced with the same problem. Namely, what is it rotating relative to? I should point out that we're getting into a pretty hazy area here, and though I've given it much thought over the years, I've come to no conclusions and certainly claim no expertise. The last few paragraphs have moved well and truly out of my knowledge base and into the field of my own personal speculations. So if anybody reading this sees gaping errors in what I've said, please do correct me.
Hangdawg13 writes: So motion MUST be in relation to space itself or else the two WOULD fall into each other. This would actually be an elegant solution to my problem but I don't know if relativity would allow it. Someone better versed in the theory can correct me if I'm wrong, but doesn't relativity forbid an "absolute" frame of reference? On the other hand, it is also my understanding that space is particulate at the quantum level the same way that matter is particulate at the atomic level. If this is correct then what you suggest may just be possible. I'm afraid, though, that this is still pure speculation on my part. Don't take anything I've said here as absolute (pardon the pun). I'm sure that all of the real scientists here will have a feast on my "amateur speculations".
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Hangdawg13 Member (Idle past 1012 days) Posts: 1189 From: Texas Joined: |
Isn't that exactly what we sent up that satellite to test? I THINK we sent the satellite up to detect space being dragged by the earth's gravitational field as it moves in it's orbit around the sun. What I am saying is that if this satelite does detect this, then a massive spinning planet or star would spin space slightly with it. If space is spinning with the planet and motion is relative to space, then the planet's rotational effects like centripetal acceleration would be slightly less than the theoretical. ...Kinda like those fancy new hubcaps that spin freely around the tires... If motion is not relative to space, then 'space-drag' should not effect these things. right? Maybe I'm completely off. If this could be detected, then this would mean space is an absolute reference frame I think.
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Tony650 Member (Idle past 4293 days) Posts: 450 From: Australia Joined: |
NosyNed writes: If MP is true then the merry go round would not see correolis forces alone in the universe, I think. But it would see them if MP isn't true. Hmm...So if Mach's principle is indeed accurate, motion may truly have no meaning in the hypothetical universe?
NosyNed writes: Got a spare Merry-go-round? Got a spare empty universe? Sure, I have a whole pile of empty universes sitting in the shed, out back. Want one? Heh, if only it were that simple, huh?
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Tony650 Member (Idle past 4293 days) Posts: 450 From: Australia Joined: |
AdminNosy writes: In the case of the earth moon system they are both moving relative to the center of mass. Hmm...Well, in the end I may just have to take your word for that. I honestly don't see it. The centre of mass doesn't have any size, does it? It's not a volume, it's just a point, a location. I could understand if you were to say that their distance to the centre of mass changed. But I'm afraid I don't see how they can orbit it, in its frame of reference. If their distance to the centre of mass doesn't change then all that's left are their orbital paths around it. But a point, having zero size, is the same from all directions. If it had any size I could understand. But I don't see how the positions of the bodies along their orbital paths, produce any relative difference between themselves and their centre of mass, from one moment to the next. How can a dimensionless point be a relative frame of reference for two gravitating bodies whose distances from it never change but which just go "around" it? Again, I'm not disagreeing with you, I'm just asking questions. I'm sure you know this stuff better than I do. But try as I might, I sincerely can't see how this works. I'm not trying to be an annoyance or anything.
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NosyNed Member Posts: 9012 From: Canada Joined: |
You know we need Eta on this one. It's beyound the amateur guessing I'm doing.
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Tony650 Member (Idle past 4293 days) Posts: 450 From: Australia Joined: |
Yeah, me too.
I was hoping, right from the start, that he might chime in. Who knows? He still may. *crosses fingers* I apologize for pressing you on these points, Ned. I really hope you don't think I'm trying to be a pain. It's just that I've given this quite a lot of thought over the years and I've already considered most of what has been said. I'm not suggesting that what has been said is wrong, it's just that I've already found "problems" with much of it. Now, these "problems" may be nothing of the sort. I may just have some fundamental misunderstandings. If so, I really want to find out what they are. This one has been bugging me for a long time and every possible solution I try seems to lead to a paradoxical or unsatisfactory result. Anyway, I don't mean to push anyone. I'm just trying to figure out where I'm going wrong. Clearly something is amiss in my scenario and I'm just hoping to finally understand what it is.
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Dr Jack Member (Idle past 136 days) Posts: 3514 From: Immigrant in the land of Deutsch Joined: |
An orbit involves acceleration. Although relativity ensures you can never know whether you are moving or not (or, more profoundly, all fixed points are equal) you can still know about acceleration. On either the 'earth' or 'moon' you could know the other must be moving because it does not fall into you due to gravitational effects.
And what are they moving relative to? Wherever you choose as a fixed point. In a very real sense, this is the same situation we have in the universe. Nothing is staying still, so we simply choose our fixed point and work from there - if we're dealing with the solar system we tend to pick the sun as the fixed point. If we're dealing with the motion of stars in the Milky Way, we pick it's centre as our fixed point.
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sidelined Member (Idle past 6169 days) Posts: 3435 From: Edmonton Alberta Canada Joined: |
Tony650
What I'm trying to wrap my layman's head around is whether a tidally locked system actually can orbit, if it is isolated from any relative reference frame This was my mistake and you are of course correct.I erred with this statement.
One would orbit the other in reference to the other one It would not matter if they are tidally locked or not since without a background reference you could not say they orbit one another. You may be able to say that the body other than the one you are on was rotating as the surface view you had of it changed.
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Dr Jack Member (Idle past 136 days) Posts: 3514 From: Immigrant in the land of Deutsch Joined: |
It would not matter if they are tidally locked or not since without a background reference you could not say they orbit one another. You may be able to say that the body other than the one you are on was rotating as the surface view you had of it changed. Yes, you can. Acceleration can be deduced without a frame of reference, orbits are accelerational in nature.
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sidelined Member (Idle past 6169 days) Posts: 3435 From: Edmonton Alberta Canada Joined: |
MrJack
If there are no background stars to reference the accelerated motion by then how do you make a measurement?
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Dr Jack Member (Idle past 136 days) Posts: 3514 From: Immigrant in the land of Deutsch Joined: |
Imagine you're in a car, cruising along the motorway (freeway in the US, I think?) at 85 mph. Although you're thumping along, you don't feel any motion. Now imagine you have to brake suddenly (some twat in a Passat cuts you up), you'll be thrown forward in your seat by the deceleration, and then as you get up to speed again when the idiot in the Passat gets out of your way you'll be pressed back in your seat.
Now, imagine doing the same thing but being blind-folded. Suppose for a second the car's so good you can't hear any wind or road noise, or fell any vibrations or bumping through the suspension. There's now no way you can tell the difference between travelling at 85mph, 40mph and being stationary. However as soon as your speed changes you can feel the acceleration acting on your body. Einstein's theories extend this principle. There is no experiment you can perform that can distinguish between being stationary, and travelling at speed but you can tell the difference between constant speed and changing speed. The same applies for your planets, although the forces produced will be tiny - they are still (theoretically) measurable. You'd also be able to tell you're in orbit because it's the only way two gravitationally attracted bodies could be arranged such that the could appear not be approaching one another.
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Tony650 Member (Idle past 4293 days) Posts: 450 From: Australia Joined: |
Note: Mr Jack, this reply turned out much longer than I intended. I apologize. You needn't reply to all of it (or even any of it) if you don't want to. Much of what I've written here is not directed specifically at you, I'm just trying to get my thoughts together, in general.
If you can help me with any of my questions then by all means, feel free! Just don't feel that you have to. I think I managed to get somewhat to the heart of my confusion this time (albeit in a rather roundabout way) so this is very much open for anyone to reply to. All are welcome to help if they wish.
Mr Jack writes: An orbit involves acceleration. Although relativity ensures you can never know whether you are moving or not (or, more profoundly, all fixed points are equal) you can still know about acceleration. On either the 'earth' or 'moon' you could know the other must be moving because it does not fall into you due to gravitational effects. Yes, I understand this. As I said, my concern is not really whether or not you could detect motion, it's simply that I don't see what this actually means if it has no relative frame of reference.
Mr Jack writes: And what are they moving relative to? Wherever you choose as a fixed point. In a very real sense, this is the same situation we have in the universe. Nothing is staying still, so we simply choose our fixed point and work from there - if we're dealing with the solar system we tend to pick the sun as the fixed point. If we're dealing with the motion of stars in the Milky Way, we pick it's centre as our fixed point. Yes but in the Earth/moon scenario, there's nothing to pick as our fixed point. All that exists is the system itself and empty space. I understand that you can pick any arbitrary point in space as your frame of reference but does this actually affect the "motion" of the system? I'm still having trouble communicating exactly where my confusion lies, I think. One of the main things that I'm trying to comprehend is the nature of motion itself. In other words, what is actually responsible for the effects of motion that we can objectively observe. Consider the following two versions of our thought experiment... Scenario 1: In our hypothetical universe, an inhabitant of the "Earth" performs a test to detect evidence of motion and they find it. They therefore conclude that the Earth/moon system is in synchronous orbit. Scenario 2: In our hypothetical universe, an inhabitant of the "Earth" performs a test to detect evidence of motion and they find none. They therefore conclude that the Earth/moon system is motionless; it sits fixed in its place. Now, here is my dilemma: Why does "Scenario 1" show evidence of motion while "Scenario 2" does not? In relativistic terms, what is the difference between the two? One possible answer is gravity; the Earth/moon system in "Scenario 1" has a gravitational field while the Earth/moon system in "Scenario 2" does not. However, do gravitating bodies actually move relative to their gravitational field? Or does it remain in uniform motion with them? If it moves with them then it is inadmissible as a relative frame of reference. I would intuitively tend to think that a gravitational field would maintain uniform motion with its gravitating body, but I don't actually know if this is correct. If I am mistaken on this point please correct me. Assuming I am correct (for now), I can't see any difference between these two systems, as far as relative frames of reference go. Why, then, does one show evidence of motion while the other does not? What is the arbitrator of which system feels the effects of motion and which system does not? At present, this is what has me the most confused. I understand that we can speak of motion in terms of any arbitrary point we define as our relative frame of reference, but what is "real" motion; the kind that produces objective physical evidence? For example, let's return to the original thought experiment, for a moment; the Earth, alone in the universe. In relative terms, it can't be meaningfully said to be "rotating", correct? Ok, so I launch a probe into space, such that it orbits the Earth, at a constant altitude, at approximately 23° to the equator, completing one full orbit every 24 hours. Now, the probe is orbiting the Earth but the Earth itself is still motionless, so it shouldn't show any evidence of motion. But there are only two relative frames of reference in this universe; the Earth and the probe. How, then, is it any more accurate to describe the probe as being in orbit around a stationary Earth than it is to describe them as both sitting in fixed locations, relative to each other, while the Earth rotates on its axis, relative to the probe? Now, if all motion is relative (and this, I think, is the heart of the matter), would you expect to see Coriolis forces suddenly spring to life, on the Earth, if you arbitrarily assigned the frame of reference to the probe, instead of the Earth? As ridiculous as this seems to me, I can't see any way around it if motion is purely relative. Does our subjective determination as to "which one is the frame of reference" cause the Earth to display (or cease to display) evidence of motion? This seems completely nonsensical to me but that's pretty much where I am, right now. I feel kind of stuck on this point. In short, if the relative frame of reference doesn't produce the forces that we use to infer motion, what does? What is actually responsible for the effects we see?
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Tony650 Member (Idle past 4293 days) Posts: 450 From: Australia Joined: |
sidelined writes: This was my mistake and you are of course correct.I erred with this statement. That's cool, sidelined. Not a problem! In any case, my focus has changed gradually during the course of this thread. I seem to keep finding new aspects of this concept to be confused about. Actually, I've started to narrow it down and I think I've figured out what one of my major points of confusion is. Rather than rewrite it all here, though, check out my reply to Mr Jack (message 58). I prattled far more than I meant to in that post but I think I finally managed to organize my thoughts, at least to a point. Anyway, feel free to read it and reply if you wish.
sidelined writes: It would not matter if they are tidally locked or not since without a background reference you could not say they orbit one another. You may be able to say that the body other than the one you are on was rotating as the surface view you had of it changed. Yes, I realize that they needn't be tidally locked. Just to explain, I understand that aside from rotation relative to the other body, a non-tidally locked system would be essentially the same, for the purpose of this exercise, as a tidally locked one. By and large, we've been concerned with their orbits, not their rotation. However, I didn't want to make things any more confusing than they already were, so for the sake of simplicity, I opted for a system which would show no relative motion, at all. I figured there was no point in complicating things even further. Especially for me; I'm confused enough as it is!
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Dr Jack Member (Idle past 136 days) Posts: 3514 From: Immigrant in the land of Deutsch Joined: |
Scenario 1: In our hypothetical universe, an inhabitant of the "Earth" performs a test to detect evidence of motion and they find it. They therefore conclude that the Earth/moon system is in synchronous orbit. Scenario 2: In our hypothetical universe, an inhabitant of the "Earth" performs a test to detect evidence of motion and they find none. They therefore conclude that the Earth/moon system is motionless; it sits fixed in its place. It is impossible to find evidence of motion. You can only decide that you are moving next to a point you have decided to consider fixed. What the earth/moon inhabitants can find is acceleration.
In relative terms, it can't be meaningfully said to be "rotating", correct? Ok, so I launch a probe into space, such that it orbits the Earth, at a constant altitude, at approximately 23 to the equator, completing one full orbit every 24 hours. Wrong. It can be said to be rotating. It is only constant linear motion that you cannot talk about. Constant linear motion produces no forces. (And don't worry about it; relativity makes no sense, much like Quantum theory). This message has been edited by Mr Jack, 07-28-2004 09:44 AM
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