Could there be an absolute 'state of reference?'

[WannabeNewton]

Referencing Mach does not count as proper reference material I'm afraid. Mach's formulations of physical principles were non-rigorous, to say the least, and largely incompatible with SR and GR. Furthermore, this isn't a philosophy forum.

 

[PeterDonis]

Mach said, as you say (more or less), that the Copernican and ptolemic theories of motion were "equally valid."

I have said no such thing. Equivalence of different reference frames in relativity is not at all the same things as equivalence of two outdated "theories of motion". For one thing, both of those theories came with a lot of other baggage that is not there in relativity.

(I have another comment on this as well, but I'll put it in a separate post since it's a bit off the main thread of this discussion.)

And yet, he felt compelled to immediately add "But the world is only given once." His point was that the Earth could not be BOTH motionless and in motion, even if either view would allow accurate predictions. So, even the king of positivism did NOT claim that "equally valid" (for computational purposes) meant equally likely or equally "true" for objective, physical purposes.

Do you have a reference here? You appear to be giving a direct quote. Without seeing the quote in context I can't tell whether the interpretation of it that you are giving here is reasonable.

But in any case, what Mach said or didn't say is irrelevant here. We are talking about relativity, the scientific theory as it is practiced today, not about anyone's philosophical statements, whether it's Mach or Einstein or anybody else. The forum guidelines, which several people have already referred to, say that this is a physics forum, not a philosophy forum. Physics is an experimental science: that means that "equally valid for computational purposes" *is* "equally true for objective, physical purposes", because the only way physics has of distinguishing different theories or models or frames of reference is by comparing predictions with experimental results. If two frames of reference make the same predictions for all experimental results, they're equivalent as far as physics is concerned.

 

[Layman]

I would like to see less in the way of personal arguments by Layman, and more quotes from textbooks and/or the literature that would support the notion that his idea

Pervect, if there is any particular claim I make that someone disagrees with, I could take the time to do some online research, I guess. I've done it before, and know from experience that it is very time-consuming.

No one has yet given me a citation for any assertions they make, that I can recall. But I'm not looking for "scientific" explanations, really. There is a difference between a philosophical/theoretical claim (i.e. unproven postulates) and a scientific claim. My questions have been directed primarily towards a "thought experiments" (not an actual, scientific experiment) and/or definitions propounded by Einstein in his explication on the "relativity of simultaneity."

This is not a question of experiment, or fact. It is a matter of postulation, theorizing. To me, that's what the theory of special relativity boils down to: a theory.

I personally find some of the premises to be questionable, and am curious to see what explanations are forthcoming. Perhaps this is the wrong place to discuss such topics.

 

[WannabeNewton]

Perhaps this is the wrong place to discuss such topics.

It is. As far as the SR/GR forum is concerned we only discuss physics, not philosophy. As far as physics is concerned, SR is on as sound a footing as you can get for a physical theory.

 

[jtbell]

1. Banned from the forum? Does that mean it is an inutterable subject that cannot even be mentioned? I certainly didn't read it that way, but maybe I read it wrong.

It is OK to discuss it in a purely historical context, just as it is OK to discuss Ptolemy's cosmology as part of the history of cosmology in general. It is not OK to persistently argue in favor of it or present it as an alternative, against the current generally-accepted understanding of relativity, in the absence of evidence that there is serious current discussion of it in professional circles.

 

[PAllen]

Well, Peter, let me respond with this. As I'm sure you know, Ernst Mach was a hard-core positivist who Einstein greatly admired in his early years. Later, he thoroughly rejected Mach's positivism, long before that whole branch of theoretical philosophy was completely discredited. For many decades, the philosophical precepts of positivism absolutely ruled in the world of theoretical physics.

But, that's not my point. Mach said, as you say (more or less), that the Copernican and ptolemic theories of motion were "equally valid." And yet, he felt compelled to immediately add "But the world is only given once." His point was that the Earth could not be BOTH motionless and in motion, even if either view would allow accurate predictions. So, even the king of positivism did NOT claim that "equally valid" (for computational purposes) meant equally likely or equally "true" for objective, physical purposes.

Mach argued that rotation had meaning only relative to distant stars (that is, that if you eliminate distant stars, rotation becomes undefinable). This is what Einstein was led to reject after finding that general relativity did not support this. In a universe empty except for two identical objects, each visually seeing the other circling them, if one felt no centrifugal force, the other would. In short, GR validated Newton more than Mach.

However, Galileo, Newton, Mach, and Einstein all agreed (and never changed their mind) that there is no absolute notion of 'who is moving'; the absolute [per Galileo, Newton, and Einstein] is who is accelerating or who is rotating. Note that the absolute notion of acceleration in no resolves who is moving. If feeling no acceleration, you accelerate, the stop accelerating, there is no objective way to say whether your starting state or your ending state (or neither) is the 'moving' state.

 

[PAllen]

Let me ask another series of questions, which you may dodge as you have so many:

Suppose you have several rockets with no windows. One is orbiting the earth, one orbiting the sun, one motionless relative to the sun and far away from it such that it is following a similar orbit to the sun around the galactic center, etc. Inside each rocket, please tell me a way to determine which is moving? All current knowledge says you cannot.

On the other hand, if one is spinning, this is easy to determine. If one fires its thrusters, this is easy to notice - but whether to consider that the starting state or the ending state (or both) is 'moving' is completely arbitrary.

 

[Layman]

Physics is an experimental science: that means that "equally valid for computational purposes" *is* "equally true for objective, physical purposes", because the only way physics has of distinguishing different theories or models or frames of reference is by comparing predictions with experimental results. If two frames of reference make the same predictions for all experimental results, they're equivalent as far as physics is concerned.

Then, Peter, I guess you would have to agree that non-SR theories which posit absolute simultaneity are the equivalent of SR. But everyone seems to be treating me like some kind of crank because I have noted that there is no known way to experimentally differentiate between the two types of theories.

Many papers have been written on such test theories of SR, and these are naturally the best source for studying those findings. But here is an excerpt from, and a citation to, a wiki summary on the topic:

"An experiment to test the theory of relativity cannot assume the theory is true, and therefore needs some other framework of assumptions that are wider than those of relativity. For example, a test theory may have a different postulate about light concerning one-way speed of light vs. two-way speed of light, it may have a preferred frame of reference, and may violate Lorentz invariance in many different ways...

Howard Percy Robertson (1949) extended the Lorentz transformation by adding additional parameters.[1] He assumed a preferred frame of reference, in which the two-way speed of light, i.e. the average speed from source to observer and back, is isotropic, while it is anisotropic in relatively moving frames due to the parameters employed. In addition, Robertson used the Poincaré–Einstein synchronization in all frames, making the one-way speed of light isotropic in all of them.[3][6] A very similar model was introduced by Reza Mansouri and Roman Ulrich Sexl (1977)...

By giving the effects of time dilation and length contraction the exact relativistic value, this test theory is experimentally equivalent to special relativity, independent of the chosen synchronization. So Mansouri and Sexl spoke about the "remarkable result that a theory maintaining absolute simultaneity is equivalent to special relativity." They also noticed the similarity between this test theory and Lorentz ether theory of Hendrik Lorentz, Joseph Larmor and Henri Poincaré. Though Mansouri, Sexl, and the overwhelming majority of physicists prefer special relativity over such an aether theory, because the latter "destroys the internal symmetry of a physical theory"."

http://en.wikipedia.org/wiki/Test_theories_of_special_relativity

 

[PeterDonis]

Mach said, as you say (more or less), that the Copernican and ptolemic theories of motion were "equally valid."

I want to comment separately on one aspect of this. What are we actually saying when we say that all frames of reference are "equally valid" in relativity?

Here's how I would put it: *physical laws* must take the same form in all frames of reference. (In special relativity, this is strictly true only for inertial frames; there can be extra terms in the equations in non-inertial frames. In general relativity, it's true without qualification.) Mathematically, physical laws are expressed as partial differential equations involving tensor quantities. So mathematically, the *same* equations for the physical laws must be valid in all reference frames.

However, the equations that express the physical laws have many different particular solutions. Solutions are expressed as numbers, or more generally functions or sets of functions, that describe particular physical entities that obey the physical laws. For example, in the train and embankment thought experiment, the coordinates I gave for each of the events of interest were part of the particular solution (to the physical laws) that describes that experiment. More generally, we could write down functions that describe the entire motion of the train, the embankment, the light emitted by the lightning flashes, etc. And it is perfectly possible for the solution that describes a particular experiment to *change* when we change reference frames. For example, the coordinates of the events of interest will, in general, change when we change frames. That doesn't change the physical laws: those equations are still the same. But the particular solution to those equations that describes the train-embankment experiment is different depending on whether we do it in the train frame or the embankment frame.

(Obviously, changing the "experiment" itself will also change the solution that describes the experiment. The CMBR was mentioned earlier in this thread: there is a particular solution to the Einstein Field Equation, which is the general equation expressing the physical law governing gravity in general relativity, that describes the CMBR as we observe it. But it is perfectly possible, physically, that the CMBR could have had different properties, in which case a different solution would describe it.)

So different solutions will be used to describe the same physical situation in different frames. And there is *no* requirement that those different solutions will describe the situation with equal simplicity, or be equally convenient for calculation. In the Copernican vs. Ptolemaic case, relativity would say that you can, of course, use a frame centered on the Earth to describe the motion of the planets, or you can use a frame centered on the Sun. Both are equally valid. But that doesn't mean that both descriptions are equally simple; the Sun-centered one is much simpler.

There is, actually, a physical fact that this greater simplicity is a reflection of: the solar system possesses an approximate "time translation" symmetry, which means that, to a certain approximation, its physical properties do not change with time--obviously things move relative to one another, but the motions are, to a certain approximation, periodic, so they can be described by a set of parameters that do not change with time. And the Sun-centered description of the solar system is much more closely "aligned" with the time translation symmetry of the system than the Earth-centered description is, meaning that the Sun-centered description comes much closer to being a description that does not change with time (because the motions with respect to the Sun-centered frame come much closer to being exactly periodic and therefore being expressible by a simple set of unchanging parameters). I say "much more closely aligned" because a description that was centered on the solar system's center of mass, which is not the same as the Sun's center, would be even closer to being "aligned" with the time translation symmetry. (I believe such a description is actually used for certain purposes in astronomy.)

So there can actually be physical features of a particular situation that make that situation look simpler in a particular reference frame. (Another example would be the fact that the description of the universe as a whole, including the CMBR but also including many other features such as its expansion, looks much simpler in a reference frame in which the CMBR is isotropic.) That can make such a frame much easier to use, practically speaking; but it doesn't make that frame any more "valid", physically, than any other frame, because the physical laws still look the same in every frame; the fact that some particular solutions to the laws have physical features that make them look simpler in a particular frame is a property of those particular solutions, not of the laws themselves.

 

[PAllen]

Then, Peter, I guess you would have to agree that non-SR theories which posit absolute simultaneity are the equivalent of SR. But everyone seems to be treating me like some kind of crank because I have noted that there is no known way to experimentally differentiate between the two types of theories.

Many papers have been written on such test theories of SR, and these are naturally the best source for studying those findings. But here is an excerpt from, and a citation to, a wiki summary on the topic:

"An experiment to test the theory of relativity cannot assume the theory is true, and therefore needs some other framework of assumptions that are wider than those of relativity. For example, a test theory may have a different postulate about light concerning one-way speed of light vs. two-way speed of light, it may have a preferred frame of reference, and may violate Lorentz invariance in many different ways...

Howard Percy Robertson (1949) extended the Lorentz transformation by adding additional parameters.[1] He assumed a preferred frame of reference, in which the two-way speed of light, i.e. the average speed from source to observer and back, is isotropic, while it is anisotropic in relatively moving frames due to the parameters employed. In addition, Robertson used the Poincaré–Einstein synchronization in all frames, making the one-way speed of light isotropic in all of them.[3][6] A very similar model was introduced by Reza Mansouri and Roman Ulrich Sexl (1977)...

By giving the effects of time dilation and length contraction the exact relativistic value, this test theory is experimentally equivalent to special relativity, independent of the chosen synchronization. So Mansouri and Sexl spoke about the "remarkable result that a theory maintaining absolute simultaneity is equivalent to special relativity." They also noticed the similarity between this test theory and Lorentz ether theory of Hendrik Lorentz, Joseph Larmor and Henri Poincaré. Though Mansouri, Sexl, and the overwhelming majority of physicists prefer special relativity over such an aether theory, because the latter "destroys the internal symmetry of a physical theory"."

http://en.wikipedia.org/wiki/Test_theories_of_special_relativity

As per your history here, you distort the source fundamentally (e.g. misattributing the views of Einstein, Mach, Galileo, etc.). You neglect this:

"Since the two-way speed of light in moving frames is anisotropic in both models, and only this speed is measurable without synchronization scheme in experimental tests, both models are experimentally equivalent and summarized as the "Robertson–Mansouri–Sexl test theory" (RMS).[3][6] On the other hand, in special relativity the two-way speed of light is isotropic, therefore RMS gives different experimental predictions as special relativity. By evaluating the RMS parameters, this theory serves as a framework for assessing possible violations of Lorentz invariance."

That is, to the extent that RMS is distinguishable from relativity it is disproven. To the extent that its parameter space include relativity, only those parameters that match relativity are confirmed.

 

[Layman]

However, Galileo, Newton, Mach, and Einstein all agreed (and never changed their mind) that there is no absolute notion of 'who is moving'; the absolute is who is accelerating or who is rotating. Note that the absolute notion of acceleration in no resolves who is moving. If feeling no acceleration, you accelerate, the stop accelerating, there is no objective way to say whether your starting state or your ending state (or neither) is the 'moving' state.

Yes, PA, I agree with this completely. Newton expounded at length on the topic. But he did not let quibbles over esoteric philosophical disputes impede his practical progress. For the solar system, he proposed using the barycenter (center of mass) as the point to be treated a "motionless" for purposes of assessing planetary motion, since with respect to it, everything else in the solar system was moving. He did this while freely acknowledging that the solar system, as a whole, could also have "shared" motions of many types and in many directions. That was irrelevant to his investigations of the relationships between the planets (and sun) themselves.

Likewise, for investigations of more cosmic proportions, he simply used the "fixed stars" as an "approximate" frame of rest. He freely acknowledged that this was not "absolute." He suggested that the "true" rest point would be the center of mass for the entire universe would be the best candidate for a truly "cosmic" rest point, but acknowledged that it could never be detected.

It's kind of strange how many people seem to think that Einstein somehow "corrected" Newton on these concepts. As you say, Newton was never under the misimpression that some "absolute rest point" could be detected.

 

[PeterDonis]

Then, Peter, I guess you would have to agree that non-SR theories which posit absolute simultaneity are the equivalent of SR.

As long as they make exactly the same physical predictions, yes, they are. I personally would discard such theories because of Occam's Razor: the concept of "absolute simultaneity" does no actual work in generating predictions (it can't, because SR can generate the exact same predictions without that concept and with nothing else changed), so it can just be thrown away without affecting the physical predictions at all. But Occam's Razor is more a matter of philosophy than physics, IMO. The real reason we don't discuss such theories on PF is that there's not much point. See below.

But everyone seems to be treating me like some kind of crank because I have noted that there is no known way to experimentally differentiate between the two types of theories.

I don't think that's what people were objecting to. See below.

Many papers have been written on such test theories of SR, and these are naturally the best source for studying those findings. But here is an excerpt from, and a citation to, a wiki summary on the topic

All this shows is that, as I said above, the concept of "absolute simultaneity" does no actual work in generating physical predictions. The test theory adds some parameters, then fixes their values so that the predictions are the same as SR's. (They have to do that, of course, because otherwise the test theory would disagree with experiment.) In other words, the added parameters are useless; you could throw them away and still have the exact same physical predictions. The whole test theory exercise just confirms that.

That's not to say the exercise was useless; it could have turned out that you could add parameters that *did* make a difference. In fact, one could argue that general relativity is just such a theory: it adds "parameters" in the form of letting the metric of spacetime be a variable (actually ten variables, since the metric is a symmetric 2nd-rank tensor in four dimensions), instead of a constant as it is in SR, and thereby generates a whole new set of predictions about gravity, while still maintaining all of the predictions of SR for situations where gravity is negligible.

This comparison illustrates, I think, what people are objecting to when someone keeps insisting that "absolute simultaneity" or some such concept is worth discussing. Why waste time discussing a concept that can be thrown away without affecting any physical predictions at all, when we could be discussing a concept like letting spacetime be curved, that *does* make all sorts of new physical predictions? SR is our standard theory for flat spacetime (i.e., when gravity is negligible); any other theory that makes the same predictions will just be SR plus some extra concept like "absolute simultaneity" that does no actual work. We don't see the point of spending too much time discussing such concepts here on PF, which is why the forum rules are the way they are.

 

[Layman]

As per your history here, you distort the source fundamentally (e.g. misattributing the views of Einstein, Mach, Galileo, etc.). You neglect this:

"Since the two-way speed of light in moving frames is anisotropic in both models, and only this speed is measurable without synchronization scheme in experimental tests, both models are experimentally equivalent and summarized as the "Robertson–Mansouri–Sexl test theory" (RMS).[3][6] On the other hand, in special relativity the two-way speed of light is isotropic, therefore RMS gives different experimental predictions as special relativity. By evaluating the RMS parameters, this theory serves as a framework for assessing possible violations of Lorentz invariance."

That is, to the extent that RMS is distinguishable from relativity it is disproven. To the extent that its parameter space include relativity, only those parameters that match relativity are confirmed.

PA, did you read this, which I already quoted?: "By giving the effects of time dilation and length contraction the exact relativistic value, this test theory is experimentally equivalent to special relativity, independent of the chosen synchronization. So Mansouri and Sexl spoke about the "remarkable result that a theory maintaining absolute simultaneity is equivalent to special relativity."

Again, this wiki summary is rather brief and there are many papers which elaborate in depth if you truly would like to learn more about these test theories and their comparability to SR.

 

[PAllen]

PA, did you read this, which I already quoted?: "By giving the effects of time dilation and length contraction the exact relativistic value, this test theory is experimentally equivalent to special relativity, independent of the chosen synchronization. So Mansouri and Sexl spoke about the "remarkable result that a theory maintaining absolute simultaneity is equivalent to special relativity."

Again, this wiki summary is rather brief and there are many papers which elaborate in depth if you truly would like to learn more about these test theories and their comparability to SR.

They only agree with SR predictions for parameter choices in which two way light speed is isotropic (while one way may not be). That makes the claimed absolute simultaneity undetectable in principle (as distinct from, say, critiques of string theory which might be unverifiable in practice, but is clearly verifiable in principle - near the Plank energy). Thus, adhering to them is exactly equivalent to adhering to Ptolemaic solar system models, and equally not part of science. Note that none of the creators of them as test frameworks adhered to them as physical theories.

 

[Layman]

That can make such a frame much easier to use, practically speaking; but it doesn't make that frame any more "valid", physically, than any other frame, because the physical laws still look the same in every frame; the fact that some particular solutions to the laws have physical features that make them look simpler in a particular frame is a property of those particular solutions, not of the laws themselves.

Peter, I don't quite see where the "laws looking the same" is a test of "physical validity." Let me use a simple example. I pick up a baseball, and bash it with a bat, knocking it over the fence. Who would say that the ball didn't move, just the outfield wall, and everything other stationary thing the Earth's frame. Would ANYONE claim that? I don't think so. All physics would go straight out the window. Any notion of F=MA, equal and opposite reaction, etc. would be invalidated by such a viewpoint.

It is one thing to say two viewpoints are equivalent for calculational purposes, and that they are equivalent for physical purposes, isn't it? That's my view, anyway. You can, I suppose, render every statement about physics "meaningless" by standing on philosophical postulates. Bishop Berkeley did this with his ultra-positivism. He made everything totally subjective, with no objective substance. Trees, for example, only existed if you looked at them. The second you looked away, they ceased to exist. "To be is to be perceived," was his motto.

Interesting philosophy, but hardly a practical basis for useful physics.

 

[PAllen]

Peter, I don't quite see where the "laws looking the same" is a test of "physical validity." Let me use a simple example. I pick up a baseball, and bash it with a bat, knocking it over the fence. Who would say that the ball didn't move, just the outfield wall, and everything other stationary thing the Earth's frame. Would ANYONE claim that? I don't think so. All physics would go straight out the window. Any notion of F=MA, equal and opposite reaction, etc. would be invalidated by such a viewpoint.

It is one thing to say two viewpoints are equivalent for calculational purposes, and that they are equivalent for physical purposes, isn't it? That's my view, anyway. You can, I suppose, render every statement about physics "meaningless" by standing on philosophical postulates. Bishop Berkeley did this with his ultra-positivism. He made everything totally subjective, with no objective substance. Trees, for example, only existed if you looked at them. The second you looked away, they ceased to exist. "To be is to be perceived," was his model.

Interesting philosophy, but hardly a practical basis for useful physics.

On the contrary, suppose you were in glider moving in tandem with the post hit baseball. All notions of F=MA and equal and opposite reaction would follow: the ball was initially moving rapidly towards you, the moving bat and batter hit it so it is now comoving with you.

At one point you agree with Newton, yet immediately you turn around and espouse principles completely at odds with Newton.

[edit: you would only need more complex equations if you wanted to use coordinates in which the baseball was stationary both before, after, and during the hit. Then you would need to introduce inertial forces (also called fictitious forces).]

 

[Layman]

On the contrary, suppose you were in glider moving in tandem with the post hit baseball. All notions of F=MA and equal and opposite reaction would follow: the ball was initially moving rapidly towards you, the moving bat and batter hit it so it is now comoving with you.

At one point you agree with Newton, yet immediately you turn around and espouse principles completely at odds with Newton.

PA, I don't understand what you're saying here. Not saying you're wrong, just saying I don't understand it. How would being in a glider lead one to believe that the Earth is moving while the glider stands absolutely still?

 

[PeterDonis]

Any notion of F=MA, equal and opposite reaction, etc. would be invalidated by such a viewpoint.

You should spend more time looking at how relativity would model this scenario using a frame in which the ball was at rest before making such claims. Such a model can capture F= MA, conservation of momentum (which is what "equal and opposite reaction" means), etc. perfectly well. You haven't really grasped what "the physical laws are the same in all reference frame" means; F = MA, conservation of momentum, etc. are physical laws, and they hold in all reference frames, so they hold in a frame in which the ball is at rest just as much as they hold in a frame in which the batter is at rest.

The rest of your post is just repeating what you've said before, without any new supporting argument and without addressing the responses I and others have previously given.

 

[PAllen]

PA, I don't understand what you're saying here. Not saying you're wrong, just saying I don't understand it. How would being in a glider lead one to believe that the Earth is moving while the glider stands absolutely still?

The glider, being inertial, would see all laws in their simplest form - all exactly as for earth. Then, the only reason for considering the glider moving (when in the glider) would be that the Earth is bigger. I have three times asked you to answer a set of questions about the fallacy of 'bigger wins', up to galactic scales where this is no center of mass point, in principle. You have completely dodged all these questions, while repeating fallacies like F=MA, and action reaction, would not hold in a so called moving frame [they would only not hold in a non-inertial frame].

 

[Layman]

Thus, adhering to them is exactly equivalent to adhering to Ptolemaic solar system models, and equally not part of science. Note that none of the creators of them as test frameworks adhered to them as physical theories.

You would seem to be in disagreement with Peter on this point, PA. You say the ptolemic system is "not part of science." As I understood him, he says if two theories produce the same predictions, then they are both equally valid, and hence presumably equally a "part of science."

 

[PeterDonis]

[they would only not hold in a non-inertial frame].

In SR, that's true; but as I said in a previous post, GR fixes that too. The GR versions of F = MA, conservation of momentum, etc. hold in all frames, inertial and non-inertial.

 

[PeterDonis]

You would seem to be in disagreement with Peter on this point, PA.

You missed the part where I said the "Ptolemaic system" is *not* the same as "a relativistic frame of reference in which the Earth is at rest." As I said, the Ptolemaic system includes lots of extra baggage that the relativistic description in a frame in which the Earth is at rest does not.

As I understood him, he says if two theories produce the same predictions, then they are both equally valid

And this doesn't apply to the Ptolemaic system anyway, since it did *not* make the same predictions as the Copernican system, and it certainly did not make the same predictions as our current theories do.

As for the relativistic frame in which the Earth is at rest, that's not a "different theory"; it's just SR applied using a particular frame. So all this stuff about different theories producing the same predictions is irrelevant if you're just talking about using different reference frames.

 

[Layman]

The glider, being inertial, would see all laws in their simplest form - all exactly as for earth. Then, the only reason for considering the glider moving (when in the glider) would be that the Earth is bigger. I have three times asked you to answer a set of questions about the fallacy of 'bigger wins', up to galactic scales where this is no center of mass point, in principle. You have completely dodged all these questions, while repeating fallacies like F=MA, and action reaction, would not hold in a so called moving frame [they would only not hold in a non-inertial frame].

PA, I'm not saying that "bigger wins," you are. That said, I won't say size is irrelevant, either.

Suppose you fill rocket with rocket fuel, light the torch, and it starts separating from the earth. How much "force" would it take for an object the size of the Earth to "suddenly" start moving away from the rocket (rather than vice versa)?

I have no idea, but I'm sure it's a hell of a lot more than is potentially available from the rocket fuel. The only practical view is that the rocket is NOT motionless and is in fact going "away from" earth.

 

[PAllen]

You would seem to be in disagreement with Peter on this point, PA. You say the ptolemic system is "not part of science." As I understood him, he says if two theories produce the same predictions, then they are both equally valid, and hence presumably equally a "part of science."

That's not how I read what he said. First, he never commented on Ptolemaic theories at least in any recent posts (I didn't search to the beginning). Second, he agreed that physicists, given two models with identical predictions, but one has additional undetectable elements and excess complexity, choose the former. The combination of a feature being both undetectable and superfluous renders it philosophy, not science (and arguably bad philosophy as well).

 

[PeterDonis]

That's not how I read what he said.

Me neither . See post #92. Your point is well taken as well.

 

[PeterDonis]

Suppose you fill rocket with rocket fuel, light the torch, and it starts separating from the earth...The only practical view is that the rocket is NOT motionless and is in fact going "away from" earth.

And then you, inside the rocket and feeling the acceleration produced by its engine, release a small object, say a marble, and watch it fall towards the stern of the rocket. Which is moving, the marble or you? The only practical view is that the marble is NOT motionless and is in fact falling away from you.

See the problem?

 

[PAllen]

PA, I'm not saying that "bigger wins," you are. That said, I won't say size is irrelevant, either.

Suppose you fill rocket with rocket fuel, light the torch, and it starts separating from the earth. How much "force" would it take for an object the size of the Earth to "suddenly" start moving away from the rocket (rather than vice versa)?

I have no idea, but I'm sure it's a hell of a lot more than is potentially available from the rocket fuel. The only practical view is that the rocket is NOT motionless and is in fact going "away from" earth.

Again, you are mixing a complex, non-inertial frame with inertial frames. In a frame moving uniformly relative to earth, the rocket and Earth are moving together, then rocket fires and decelerates to be at rest in this moving frame. Perhaps this is the frame of an asteroid, with the rocket ending up at rest relative to the asteroid.

Again, it seems you fundamentally don't understand Newtonian physics. You can't get to relativity without understanding Newtonian physics.

 

[PeterDonis]

Again, you are mixing a complex, non-inertial frame with inertial frames.

That's a problem if you do it unthinkingly, yes, but it's actually not quite the problem I was thinking of when I posed the question in my last post. I won't give the answer away yet since I want to see if Layman can see it for himself.

 

[Layman]

And this doesn't apply to the Ptolemaic system anyway, since it did *not* make the same predictions as the Copernican system, and it certainly did not make the same predictions as our current theories do.

I'm not sure what different predictions you have in mind with respect to planetary motion, Peter. I could refer you to a lot of sources, but here's very abbreviated one from PBS, with an excerpt (it's the first one I came across).

" That Ptolemy could place Earth at the center of the universe and still predict the planets’ positions adequately was a testament to his ability as a mathematician. That he could do so while maintaining the Greek belief that the heavens were perfect—and thus that each planet moved along a circular orbit at a constant speed—is nothing short of remarkable."

http://www.pbs.org/wnet/hawking/universes/html/ptole.html

Of course I'm not talking about the method of arriving at predictive results, or about the assumptions and rationale of the theory. Just the result, such as: There will be a total eclipse of the sun as viewed from Paraguay at 7:00 P.M. on June 27, 2053.

 

[PAllen]

In SR, that's true; but as I said in a previous post, GR fixes that too. The GR versions of F = MA, conservation of momentum, etc. hold in all frames, inertial and non-inertial.

True, but I didn't want to go there. Also, of course, there are still locally inertial frames in which there is the major simplification of the connection vanishing (locally) and metric becoming Minkowski (locally). This is equivalent to inertial forces vanishing.

 

[PeterDonis]

" That Ptolemy could place Earth at the center of the universe and still predict the planets’ positions adequately was a testament to his ability as a mathematician.

"Adequately" is not the same as "with the same accuracy as Copernican theory", nor is it the same as "with the same accuracy as our current theories". Different accuracy means different predictions; measurements can distinguish less accurate from more accurate predictions. Which is exactly what they did as advances in astronomy made the Ptolemaic theory obsolete. (Which did *not*, btw, happen when Copernicus published his theory; it wasn't until Kepler that a Sun-centered system was developed that was actually more accurate in its predictions than the Ptolemaic system. It was Tycho Brahe's more accurate observations that forced Kepler to abandon Copernicus' circular orbits and consider ellipses, and that was what made the difference in predictive accuracy.)

 

[Layman]

"Adequately" is not the same as "with the same accuracy as Copernican theory", nor is it the same as "with the same accuracy as our current theories". Different accuracy means different predictions; measurements can distinguish less accurate from more accurate predictions. Which is exactly what they did as advances in astronomy made the Ptolemaic theory obsolete. (Which did *not*, btw, happen when Copernicus published his theory; it wasn't until Kepler that a Sun-centered system was developed that was actually more accurate in its predictions than the Ptolemaic system. It was Tycho Brahe's more accurate observations that forced Kepler to abandon Copernicus' circular orbits and consider ellipses, and that was what made the difference in predictive accuracy.)

Heh, Peter, I knew when I made that post that you would jump on the word "adequately." As I said then, I could find other sources, but didn't bother. But here's another, now.

"In 1543, the geocentric system met its first serious challenge with the publication of Copernicus' De revolutionibus orbium coelestium (On the Revolutions of the Heavenly Spheres), which posited that the Earth and the other planets instead revolved around the Sun. The geocentric system was still held for many years afterwards, as at the time the Copernican system did not offer better predictions than the geocentric system, and it posed problems for both natural philosophy and scripture. The Copernican system was no more accurate than Ptolemy's system, because it still used circular orbits."

http://en.wikipedia.org/wiki/Geocentric_model

P.S.: I see that your post already noted theoretical improvements made by kepler, etc. But do you doubt that ptolemic astronomy could not also be "tweaked" (as Copernican theory was) to accommodate newly observed (measured) phenomena? I don't.

 

[PeterDonis]

But here's another, now.

Which says the same thing I said in the post you quoted. What's your point?

 

[PeterDonis]

True, but I didn't want to go there.

We will end up going there if the answer I was thinking of to my "see the problem?" question gets revealed.

 

[Layman]

Which says the same thing I said in the post you quoted. What's your point?

See the P.S. I added to my last post.