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.

Thread moved here from the Proposed New Topics forum.

At first I'd say no. All motion is relative, and if there is nothing to relate the earth to, then it can't be said to be moving.HOWEVER (there's always a 'however,' isn't there? ) it's not clear that there really is such a thing as a "single thing" in reality. Even elementary particles don't have well-defined existences in space-time. They're more like little "blurs" of probabilities which interfere with eachother and in that sense can be said to have relation or relativity. In actuality they are described as waves, and the peaks and valleys of a wave move in relation to eachother. Yes. The act of firing the engines actually propels billions of little particles AWAY from you out of your engines, and you are therefore moving relative to them. I think this is really a trick question. If the earth is spinning and the orbit of the moon is in perfect sync with rotation of the earth so that the moon sits in one place over the earth the whole time, the fact still remains that the moon is circling the earth even though it doesn't look like it from the earth or the moon. That's enough to maintain an orbit like it has presently.

Sounds like you're on your way to special relativity.

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The Laminator

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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"?

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It can be moving relative to the gravitational field, ie. your empty spacetime.

so the singularity of the big bang then had no movement, as to imply movement implies distance and time which were not yet created ???that is what I have trouble dealing with. The absolute perfection of nothing to the ordered chaos of something without a conscious trigger

I think this might be one of the current problems with the BB theory. This theory was made with the mindset that space is just that, space. But quantum physics shows us that space is more like a material. So if space itself should expand from nothing, what would that look like? It wouldn't be expanding in our 3 or 4 dimensions only like a literal bomb explosion. It would expand in every dimension at once. Sort of like increasing density rather than volume I guess?More to the point of the OP, I have read that a photon as it travels through space encounters an antiphoton causing an anhialation which then releases another photon, so a light wave would be more like a sound wave in this respect? So could not also the particles that make up the larger particles do the same? Isn't it Zero Point Energy that sustains larger particles?If space is made up of Zero Point Energy from a sea of particles with neither exact position or velocity, then I guess perhaps the lone earth could either be said to be in an exact position in space (though I have no idea what kind of coordinates to use) or have an exact velocity relative to the ZPE particle sea... or do two sets of two relativities equal no position or velocity? Gah...!This IS a mind bending thought.

This is not a problem with the BB. It is a problem with your understanding of it. In the BB it is exactly space itself that is expanding. It is, as you say, not expanding in our spacetime. There is no antiphoton or, rather, it is it's own antiparticle. If they do anihilate there is nothing left.I don't know what the rest is about. Nor am I likely to know the answer if it was clearer.Again, I offer some advice: going on about these kinds of things when we know pretty much zip about them is a waste of time. You may ask a few questions and hope someone who can talk sensibly about them comes by. Just making up stuff using some of the words you've picked up here and there is just gibberish.

It is possible, hypothetically, if we ever find a way to detect the ether.

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The Laminator

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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.

They are orbiting relative to the center of mass. I think that you could detect the motion because there would be "extra" coreolis forces. The center of the orbits isn't the center of the earth. It is under the earth's surface though. The earth then is being "flung around" a point under it's surface. I'm pretty sure that would be detectable.

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?

It wasn't abandoned. It was just given up.We know that light is both particle and wave.

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