Don’t have much time at present, but the claim that an object can be actually stationary to start with. And maybe that’s all. I’m too rushed to think clearly right now.
Relativity - Oh dear, here we go again!
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Paulhoff
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Nobody said that, but please point out to me something that isn't accelerating and/or decelerating in some direction.
Paul
Fredrik
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Any object whose world line is straight is stationary in an infinite number of inertial frames. (Any inertial frame in which the object's world line is parallel to the time axis will do). "Stationary" just means that the spatial coordinates are constant.
And if an object is stationary in one frame, there's of course an infinite number of inertial frames in which it's not stationary. (Any inertial frame in which the object's world line isn't parallel to the time axis will do).
And if an object is stationary in one frame, there's of course an infinite number of inertial frames in which it's not stationary. (Any inertial frame in which the object's world line isn't parallel to the time axis will do).
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Fredrik
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Please define what you mean by "truly, actually stationary". (And the definition must be abstract, otherwise it's not really a definition).
Fredrik
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Sure, the line defined by x=y=z=0 is both straight and parallel to the t axis. (Actually it is the t axis).
Can you give an example?
I'm sure that's not what you meant though. Do you mean an actual object, in our universe? Then we have a problem. First of all, everything in our universe is affected by gravity. We're discussing special relativity in this thread. SR is a theory that describes a universe without gravity, and also without quantum effects.
So how can I give you an example of an "actual" object that has a straight world line in flat space-time, when the actual space-time isn't flat? Should I think of an example in a fictional universe that's described by SR + Newtonian gravity + quantum mechanics? Would that be more satisfying than the mathematical answer I gave you above?
Even if we had lived in this fictional universe, the question of whether there exists an object with a straight world line would be irrelevant. What matters is that such world lines are possible in principle.
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Well, in a homogeneous expanding universe a typical galaxy is stationary by your definiton, because the coordinates are comoving. More explicitly, the straight worldine t = t, x = constant is a geodesic and describes the movement of a typical galaxy.
Fredrik
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True, but as I said, we're talking about special relativity here, and SR describes a universe without gravity. So I don't think we need to complicate things by bringing FRW solutions of GR into the picture.
Good point! I can’t think of any state where there wouldn’t be some form of gravity, solar wind, etc that would case some amount of acceleration. But this only provides further proves that everything is constantly being accelerated, and therefore is constantly moving - Thanks.Nobody said that, but please point out to me something that isn't accelerating and/or decelerating in some direction.
Paul
Are you serious? Using abstract mathematics as real proof! You do get out of the house occasionally, I hope.
<<<<<<noteHaven’t got one yet (actual, factual, real world that is). As Randi might say “The million dollars is safe”
Can you give an example?
Fredrik
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This is of course correct, but it has nothing to do with relativity, so it's irrelevant to what we've been discussing in this thread.
As proof?! I have no idea what you're talking about. We were talking about what a stationary object is! To explain that you have define it, and definitions are by necessity abstract.Are you serious? Using abstract mathematics as real proof! You do get out of the house occasionally, I hope.
I can't see why you guys think it makes sense to ask for one. (See my answer to Robinson above (#206) for more information).Haven’t got one yet (actual, factual, real world that is). As Randi might say “The million dollars is safe”
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Can you give an example?
Well, I thought you were talking about reality, so yes, I was asking about a real object.
I think I understand now, you are talking about made up stuff, not reality. This explains a lot.
Well, maybe to you. I thought the conversation was about relativity, not a fictional world.
True, but as I said, we're talking about special relativity here, and SR describes a universe without gravity. So I don't think we need to complicate things by bringing FRW solutions of GR into the picture.
It is all starting to make sense, I think. The discussion is about fictional stuff, stuff that can't exist. This might explain why it is so hard to follow. Like a world without matter, or a Universe without energy. It helps explain something, somehow, but it doesn't matter in reality.
No wonder it seems woo.
Please continue. It is still interesting, even if it isn't real.
Good - So we agree that in the actual, factual, real universe everything is moving relative to everything else?
Relative motions have nothing to do with Relativity!?
I think it has everything to with what we’re discussing.
Exactly! Is a stationary object a reality or an abstract concept?
Please don’t turn this debate into a pedantic/semantic one.
Ditto robinson’s reply.
Yes, it all relative. You cannot say an object is stationary, period. An object can only be stationary relative to some other object. In other words, if there is no movement between A and B, A is stationary relative to B, and B is stationary relative to A.
It's really as simple as that.
That's right. It's nothing new. And it's not an assumption. It's true by definition or logical deduction. If there is no movement between A and B, A is stationary relative to B, and B is stationary relative to B. What is not completely simple and clear about that?
No, there is no point in space that is absolutely at rest (meaning at rest relative to all other reference frames). On the other hand, any reference frame can be at rest relative to some other reference frame. For example, if there is no movement between A and B, A is stationary relative to B, and B is stationary relative to B.
Same answer as above....
There is no point in space that is absolutely at rest (meaning at rest relative to all other reference frames). On the other hand, any reference frame can be at rest relative to some other reference frame. If there is no movement between A and B, A is stationary relative to B, and B is stationary relative to B.
That is all that is being said.
Fredrik
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OK, I'm going to have to get philosophical here. There are some theories that can be said to be correct, i.e. they describe some aspect of our actual universe, rather than a fictional one. A good example is the statement "The Earth is round". (This is a theory. It didn't cease to be a theory the moment we became certain that it's correct. A theory is just a logically consistent statement about a possible universe. So the statement "The Earth is flat" is also a theory).
Note that the word "round" is ambigously defined. It means "approximately spherical", and that's pretty ambiguous. When you see an object that you consider round, you can't always be sure that someone else would consider it round. This would be a serious problem if we were talking about a modern physical theory. Physicists have to know that they are talking about the same things, and the only way to make sure is to use exact definitions. The only way to do an exact definition is within the framework of mathematics. That's why definitions are always abstract. Concepts like "round" are hardly ever used in modern theories.
So what can you use instead? You can define the word "spherical" unambigiously, so you can use that. This will however have a funny side effect. If your theory says that "the Earth is spherical" everyone will agree about what the theory says, but now the theory can't possibly be correct. It would be an exact description of a fictional Earth, and at the same time an approximate description of the real Earth.
So, the ambigious theory "the Earth is round" can be said to be "correct" and to describe the actual Earth, because any experiment you perform will be consistent with the theory. (You may have to slightly adjust your opinion of what "round" means, if you e.g. discover that the tallest mountain is a bit taller than you thought, but experiments will never prove the theory wrong). The unambigous theory will however not be "correct" because a very simple experiment (just look out the window) will prove the theory wrong.
Does this mean that the ambigous theory is "better", or more scientific? Absolutely not. In physics it's far more important to make theories unambiguous than to make them "correct". The theory that describes the fictional ("spherical") Earth is more scientific than the theory that describes the actual ("round") Earth, because
a) you can't adjust your interpretation of the theory to make it stay "correct" as you perform better and better experiments, and
b) you can measure how much and in what way the theory deviates from being correct.
It's funny that only theories that are ambiguously defined have any chance of being "correct". Any unambigous theory that has been invented so far can be proven "wrong" by sufficiently accurate experiments. This means that all the best theories in physics, including general relativity and the standard model of particle physics, describe fictional universes!
This also means that theories can't be classified as "right" or "wrong". They are all "wrong", but some are a lot less wrong than others. Those are the "good" theories. Newtonian mechanics is a very good theory, special relativity is a better theory and general relativity is an even better theory than that. Only experiments can determine how good a theory is. Every experiment tries to "disprove" a theory, by finding a difference between the actual universe and the fictional universe described by the theory.
Special relativity is the theory that says that space and time can be represented by Minkowski space. But that's not all. If it was, the fictional universe described by the the theory would be completely empty. It wouldn't even contain a single particle of matter. So we have to add matter to the theory somehow. We do this by postulating that curves in Minkowski space that are consistent with a relativistic version of Newton's second law represent massive matter particles, and that Newton's law of gravity holds.
That's the complete theory. There are still no quantum mechanical effects in the universe described by SR, so it's certainly not wrong to call this universe "fictional".
Since it isn't possible to talk about reality without being ambiguous, I try to never talk about reality, unless the discussion is about how to test a theory experimentally.
My point is that it isn't possible to talk directly about reality without being ambigous. In a way, I'm talking about reality by talking about some "made up stuff" (a physical theory) that's a very good approximate description of reality. No one can do better than that. (Because of this, yours and ynot's request for a "real" example was pretty strange from my point of view).
The rest of your comments are very similar to the ones above, so my answer to them is the same.
Fredrik
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That's not what I said is correct, but I agree with this too. (What I said you were right about was that everything in the real world is accelerating).
Just about every time you quote me, you answer as if the text you quoted was about something different than it actually was. It's a little frustrating. What I said was that the fact that everything is accelerating in the real world has nothing to do with relativity.
What exactly has everything to do with what we're discussing, and why? (It's hard to know since you're constantly changing the subject and acting as if you don't).
I would have to say that it's an abstract concept, but I would say the same about an electron and the number 3, so I don't see the point of the question. We can only define these things exactly within the framework of a theory, so if we are talking about well-defined things we are always talking about abstract concepts. If the theory is a good approximate description of the universe, there might however be something in the real universe that's approximately described by the abstract concept. So I would also say that since SR is a good theory, it's a fact that something very much like a stationary object can exist in the real universe. The question of whether something that's very much like a stationary object in SR does exist in our universe is completely irrelevant, but if you really want an answer to that, the answer is "no".
Do you feel that we've made any progress at all in this discussion? Is there anything that you feel that you understand now that you didn't from the start?
When discussing something, common definitions are essential. The word "theory" has many meanings, so even in a philosophical conversation, it can be used ambiguously. A theory can become a scientific law, or it can be disproved, or it can be argued over on a message board.
- A good example is the statement "The Earth is round". That is a terrible example. From the earliest time it was viewed as spherical, not round.
All those remarks have the same error.
I think you are going off on a tangent here. The history of the theories about the Earths shape, as well as the current measurements of the true shape, are easy to find. And it isn't theory so much as a matter of practical application. A measurement of a distance is not a theory. We know exactly what the dimensions are, and can measure changes in these dimensions, taking the issue out of the theory category. Which is why your example is so terrible. We don't talk about theory when discussing a known measurement, that is beyond any question.
That makes little to no sense.
That is simply not true. I don't know where you are getting this from, but it may be philosophical, but it has little to do with reality.
Many many people talk directly about reality, and do things with reality, in reality, all the time, every day. That you think it can't be done is absurd. Think about your example of the shape of the planet. Right now many people and many devices are doing real things concerned and effected by the exact shape and dimensions of the planet. There is little or no error in the measurements. There is no doubt about when and where and how far.
I understand you are waxing philosophic, so consider this response in the same light. But with some reality thrown in for good measure.
It isn't hard to provide real examples of reality. The GPS information posted here was a good example of real science, real data based on reality.