Relative Motion (A Thought Experiment)

Ok, I thought I'd finally bite the bullet and open a topic of my own. This is not intended to be a controversial or debate topic, mind you. I don't want to start any fights. I'm just hoping to get some closure on one of the many questions I've had floating around in the back of my mind, unanswered, for years. It regards a thought experiment I read once which (as best as I can recall) goes like this.Visualize the universe as a black, empty void. Within it exists one sole body. It doesn't matter what it is but for familiarity's sake let's say it's the Earth. So only the Earth exists within the confines of four dimensional space-time, and nothing else.Now the question is...Can the Earth move? I don't mean can it be moved...I mean, can the Earth, in any sense of the word, be said to be "moving"?The reason I'm having trouble getting my mind around this one is because I can't see how "motion" can exist as an independent quality on its own. Isn't the very concept defined by the relative proximity and position of multiple bodies? It seems to me that for "The Earth is moving" to make any sense it must have something to be moving relative to.At the same time, I also have trouble accepting the idea that motion is some kind of indecisive quality that can (at least in principle) be switched on and off, dependent on the number of objects in the universe.Let's say that the one object is now a spaceship and I fire the engines. Am I moving? Is there any way that I could know if I was? There's nothing else out there for me to compare myself to but does this really mean that I'm not moving, or simply that there's no way for me to know that I'm moving?I find it hard to believe that "motion" would cease to exist in this way because it seems to me that there would then be other problems. For example, could a tidally locked system like Pluto and Charon exist on their own, without falling into each other?Let's say that our hypothetical universe now has two objects; a planet and its moon, in synchronous orbit. There is no "third-person perspective" where one can see the two bodies orbiting each other. The only observers are on the bodies themselves. As far as any of them can tell, there are two bodies in the universe, which sit, motionless, at a fixed distance from each other.Now do the bodies stay separated from each other or do they fall together? My thinking is that they would stay apart, just as they do in this universe but the question is why? How can they be said to be orbiting each other, in a universe where there is no other point of reference?Does the universe have some kind of characteristic whereby there are "absolute" locations in space? In other words, can things move relative to space itself? Or is this regressing too much back to the idea of the "ether"?I'm not sure how well I've explained this but I hope some of the physicists out there can help me understand it a little better.To be honest, I'm not sure if this is strictly a science question anyway. Personally, I find it to be more like those philosophical questions that drive you mad if you think about them too much.None the less, it's one that has had me scratching my head for a long time so here's hoping that someone out there can cure my ignorance (Eta perhaps?).Thanks for suffering through this, everyone. I know I prattle and I appreciate your patience.

Thanks Ned.And thanks to everyone for their replies. I apologize if I don't get to everybody's but I appreciate them all.

Ah, of course! I hadn't considered that. *slaps self on the wrist for missing something so blatantly obvious* Thank you, ::! I truly didn't intend it to be a trick question. The problem that I'm having is not simply that the Earth and moon don't appear to be orbiting each other, from either's perspective. What I'm having trouble understanding is how the concept of them orbiting each other has any meaning if there is nothing for them to be orbiting relative to.In principle, I agree with you. As I said previously, I simply can't imagine that the reality of motion is dependent on the number of extant bodies so I would concede that they should indeed be able to orbit each other. But I still can't shake this voice in my head that counters with, "Orbiting each other relative to what?"I guess what I'm trying to get clear in my mind is what motion actually is. If all motion is relative then how can the tidally locked bodies in our hypothetical universe "move" around each other? Since they are the only points of reference that exist, and neither of them moves relative to the other, can the concept of them orbiting each other have any meaning?

As a matter of fact, that's a thought that already occurred to me and it actually brought up another problem that I've been contorting my mind, trying to figure out.In a universe with one sole body (in this case the Earth) wouldn't the gravitational field have to emanate from the Earth itself? If so, can the Earth move relative to its own gravitational field?My "understanding" (and I use the word loosely) is that the geometry of space-time is a consequence of gravity. So if the Earth in this hypothetical universe were to "move" wouldn't its gravitational field warp space-time around it such that it was always positioned the same way within its universe? In other words, if there were only one extant gravitating body, would the universe not "follow" it, wherever it tried to go?Ugh, that last sentence was an absolute monstrosity! I hope my meaning was clear enough. My point is, if the universe will always be warped around its one gravitating body then can that body ever be said to move relative to the geometry of its universe, even if there are absolute locations in space?Oh brother! I know what I want to say but the words are extremely hard to find. I really hope what I've said isn't too unclear.

It's interesting that the more deeply I think about this, the more I find myself thinking along those lines. It does seem like a relatively neat solution to my problems but wasn't the ether hypothesis abandoned long ago?

Yes, I've considered this too. But again, I'm curious about the question of relativity.I know that they orbit around their centre of mass but do they actually move relative to their centre of mass? The way I visualize it (and I may be completely wrong about this), the two gravitating bodies and their centre of mass, relative to each other, stay exactly where they are. From all three points of reference wouldn't you see the other two as stationary (in a tidally locked system)?It seems to me that it would be necessary to have a point of reference outside the system. Anything that is a part of the system (planet, moon, and centre of mass) will see it as motionless. If we allow that they do orbit each other, we still won't see that from within because there is no relative motion within the system. The system "moves" as a whole, but only relative to an external point of reference.The point is that, in this scenario, there is no external point of reference. I think what I'm having trouble with is whether or not there is a difference between a system (any system) that has an "overall" motion with no external, relative point of reference and a system that has no "overall" motion at all.Just to be clear about this, I'm not arguing against the idea of motion. I for one believe that these hypothetical bodies could orbit each other, just as they do in this universe, and I have no doubt that you could detect the effects of their orbits.What I'm really trying to wrap my head around is how this can be when there is nothing relative to their orbit. My mind has trouble dealing with this one because "motion" would seem to lose all meaning if it's not relative to something. As far as I can tell, motion is defined as changes in relative proximity and orientation. If there is nothing to relate to, how can these changes occur?The same thing with the original question. Could a sole planet, alone in an otherwise barren universe, rotate for example? My mind seems to experience a jarring contradiction here. Almost simultaneously, I don't see why not...and...I don't see how it possibly could.Am I making any sense? Because I certainly don't feel like I am.

Seriously? You mean it's still considered a legitimate theory (or at least hypothesis)? I always thought it was one of those long dead ideas like the steady state theory. Well I must admit I'm surprised. I honestly had no idea that the concept was still given serious consideration. I always thought that the particle/wave duality of light showed how the ether was not necessary. As I understood it, light being able to travel as particles and waves, is capable of travelling through empty space, whereas a mere wave requires a medium through which to propagate.It seems that I misunderstood. That's what I get for being an "amateur scientist".Please feel free to set me straight on this.

Oh, whew! *wipes sweat off forehead* Don't do that to me, Lam! For a minute there, I thought I was going to have to learn that stuff all over again!So what I thought originally was correct then? Ok, cool. Thanks for clearing that up. I'd be lying if I said I completely understand it but I do have some idea. I understand that light is observed to have both particle-like and wave-like properties and I'm familiar with Young's two slit experiment.However I am not familiar with the underlying mathematics. As is the case with most of my scientific "understanding", my knowledge of this is limited to the concepts themselves and not the mathematical theory behind them. I just hope that this won't hold me back from fully appreciating the concepts. Thanks again for clearing that up. For a minute there, I really did think I was going to have some serious "unlearning" to do. Heh, never miss a trick, do you?What's funny is that as soon as I posted that I realized the irony of using that expression in a reply to you. I thought you'd pick up on it. Well done. *chuckle*Oh and to answer your question...I used to be, but I'm not quite as cheerful these days. You see, I've been under a lot of stress lately and...Oh! Gay! Right...Gotcha!No, I'm not. Don't worry, though. For what it's worth, I can assure you that you'll receive absolutely no judgement or hatred from me for being gay, Lam.

Heh, it's ok. Little Mr. Sensitive here takes things the wrong way sometimes.I understand that on forums like this, you kind of need to be on "troll alert" and I just wanted it to be clear that my questions are always sincere. I don't handle heated arguments well at all, which is why I tend to avoid participation in the "controversial" topics.But I digress. Yes, and this, I think, is what's causing my dilemma. Motion has real-world consequences which can be tested for and detected. How, then, can something with objective consequences be defined by completely subjective frames of reference?If the people on our hypothetical Earth test for, and find, signs that the Earth and moon are in orbit around each other, then what does this mean? In orbit relative to what? Not to each other. From either's perspective, the other doesn't move at all (if they are tidally locked). And if there is nothing else, what meaning can their "orbiting" possibly have?Hence my earlier suggestion that they may in fact be orbiting each other relative to space. But is this possible? Or would it violate relativity's principle that there can be no truly "absolute" frame of reference? Hmm...True to a point, but then, doesn't it have real-world consequences which can be objectively detected? This is one of my points of confusion: Does our lone system of tidally locked planet and moon suddenly lose its ability to be in orbit because of the lack of relative reference frames? I find this hard to believe.But then, if this isn't the case, doesn't that imply that motion is some kind of absolute quality which can exist without anything to be relative to? I find this equally hard to believe.So it seems that I'm stuck between those two frustrating places which are, as the saying goes, "hard". Yes, I thought that might have confused things a bit.How about this? Let's say that the "Earth" is inhabited by a society which, as yet, has no advanced knowledge of astrophysics. How could they tell whether their universe was one in which massive bodies orbit each other, or one in which they are "fixed" in the heavens, not affecting each other in any way?Now, I assume they could perform tests which would determine this (Coriolis effects on the Earth for instance) and if they found no indicators of motion then problem solved; there is no motion. But if they did then I am forced back to my original question; moving relative to what? At this point, so would I. But if the rules of gravitation work the same as they do in our universe does this mean that the gravitating bodies will inevitably fall together?If their "orbiting" has no meaning without at least one external frame of reference, then there is no distinction between a tidally locked orbiting system with nothing to relate its orbit to, and a system which is simply fixed in its place, not moving at all.This is my other major dilemma: Does motion simply "turn off" without a relative reference frame? Or "turn on" with a relative reference frame?If someone gets in a ship, flies away from our hypothetical Earth, and looks back when they are in a position such that they can see the Earth and moon orbiting, they have effectively "created" a relative frame of reference.So do all the laws of motion suddenly "kick in" now that the ship has broken uniform motion with the Earth/moon system? And do they "switch off" again when he returns to the Earth, reintegrating that relative frame of reference back into the Earth/moon system's uniform motion?I find this thought quite ludicrous, myself. But I don't see how it can be avoided unless their orbiting motion is permitted without any relative frame of reference. But if this is permitted then I am again forced back into a corner regarding my original question: Orbiting relative to what?I apologize for being so repetitive. I realize that I'm kind of arguing in circles here but I'm afraid this one really has me flamboozled. I seem to be faced with a paradox no matter which way I go.Also, I hope what I've written is intelligible enough. I'm having almost as much trouble putting my thoughts into words as I am with the problem itself. I hope my posts are at least partially comprehensible.

Thanks for the link, Ned!And yes, I would agree with you. But this isn't really my problem.In this example, the merry-go-round is spinning relative to the ground beneath it, the trees around it, the other rides, the people, etc. It has a clearly definable relative motion. The Earth/moon system (or the sole "Earth") in our thought experiment does not.I'm not saying that you couldn't detect such effects in our hypothetical universe. What I'm trying to figure out is: If you were to detect such things in a system totally isolated from any relative frame of reference, what would it mean?Would it mean it was moving? Again, relative to what? Does the concept of one body's motion actually mean anything without a second body moving relative to the first? Isn't that precisely what motion is? Yes, I am vaguely (very vaguely) familiar with this. Is Mach's principle generally believed to be true?For that matter, is there any way we can even test it? By its nature, it would seem to be a difficult idea to either confirm or disprove. As we are stuck in this universe, how does one go about demonstrating that a body's inertial properties are different in a universe with a different overall mass? Or perhaps I misunderstand the principle?If I understand it correctly, am I right in thinking that, in the original thought experiment (with only the Earth), motion would in fact not be possible, as there is no mass outside of the Earth itself?Or would it mean that the Earth could (possibly) move but it would simply feel no inertial effects of its movement? Perhaps, then, the Earth in this scenario could move but you could never know because it would show none of the physical characteristics of motion.Unfortunately this, in my opinion, makes things even worse. What meaning can the "motion" of the Earth have if it not only has no relative frame of reference, but also shows no physical evidence of motion?Of course I may be completely misunderstanding (or misapplying) Mach's principle. Please feel free to correct me (or point me in the right direction) if I am wrong on this.

Exactly! This is what I'm confused about; it's all relative. If the Earth is all that exists in the universe then there is nothing to relate it to. So in what sense can it "move"? Yes, that was a poor choice of words, on my part. I apologize. Your phrasing ("relative to the reference frame") cuts right to the point, I think. I've been fumbling my words a bit and this may help me make myself a little clearer. Thank you.In the original example there is one frame of reference (the Earth). Now, what this ultimately comes down to (I think) is whether or not one reference frame can move without another. If only one frame of reference exists and no other, can it be said to "move" or is this a meaningless concept? Yes but relative to what? This is what I can't figure out. In a tidally locked system wouldn't they be motionless, relative to each other? From the Earth, the moon would appear motionless, and from the moon, the Earth would appear motionless, correct? If there is no other frame of reference to relate them to then how can they meaningfully be said to have any motion, orbital or otherwise?I apologize for all these questions. I'm not trying to be confrontational, I'm just having trouble getting this. I hope I'm not coming off as a know-it-all.I'm well aware that most of the people here are far more educated in these subjects than I am and it's not my intention to disagree with anyone. I'm just having trouble reconciling some of this with my layman's understanding.Anyway, I'm sure that the problem lies in my own misunderstanding or lack of education so please don't think that I'm ever being argumentative. I genuinely want to understand this and I very much appreciate everyone's patience.

Hangdawg, your reply was linked to sidelined's post but you quoted me so I assume you were replying to me. 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? 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. 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".

Hmm...So if Mach's principle is indeed accurate, motion may truly have no meaning in the hypothetical 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?

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.

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.