The Peripatetic Albert, Round 3

[humanino]

Yes, thank you HallsofIvy. I was probably even less clear that you geometer. I was saying that, in the free fall referential coordinates, there is apparently no acceleration, and in any case, even non-uniform acceleration, the proper referential always make the acceleration disappear. That was really confusing in the context of this discussion, because of course this is not an inertial frame.

In the case of non-uniform acceleration, one can even find an inertial frame in which the acceleration disappear at a single point in spacetime. That was linked to an earlier discussion I had with Pete on GR, curvature and the (speculative but interesting) possibility that curvature does not encompass all gravitational effects, especially when topological defect play in the game. I should have been more careful in keeping my preoccupations out of the current discussion.

Thanks again every one.

 

[ostren]

... If one system is accelerating, even uniformly, with respect to another system it is no longer an inertial system, and you should be able to detect acclerations and thereby determine who is really moving. ...

I think not. Even though a thrusting event produces a sensation in the seat of the pilot's pants, it cannot be differentiated from a gravitational thing and so you are still technically unable to conclude "who is really moving". I find relativity easiest to understand when the observatory is always considered at rest. What power made the Earth and cosmos twist under your car? maybe just some fictitious force, especially considering that GR is full of such forces.

 

[humanino]

Welcome in PF ostren !

It depends ! I agree with you, but we had several misunderstanding during the discussion !
I think geometer was answering the initial question in the context of SR, without gravitational phenomena.

 

[russ_watters]

I think not. Even though a thrusting event produces a sensation in the seat of the pilot's pants, it cannot be differentiated from a gravitational thing and so you are still technically unable to conclude "who is really moving".

Since you can correlate the sensation in your pants (eh...I'll let that one go) with the turning of the steering wheel and the front wheels of the car, you can certainly conclude you are the one accelerating. Same goes for a rocket - you push the "fire" button, hear the engine start, see the flame, and feel the acceleration, you know you are the one accelerating.

 

[ostren]

Well, you might know it's you who is accelerating, or you might equally presume you have encountered a gravitational field; but in either case, you will not be able to make the unequivocal determination that you are "truly in motion". That last was the only conclusion that I was countering.

 

[geometer]

Well, you might know it's you who is accelerating, or you might equally presume you have encountered a gravitational field; but in either case, you will not be able to make the unequivocal determination that you are "truly in motion". That last was the only conclusion that I was countering.

That seems like a contradiction in terms - if you are accelerating, you must be moving. Can you give me an example of where you might be accelerating and not be moving?

 

[ostren]

That seems like a contradiction in terms - if you are accelerating, you must be moving. Can you give me an example of where you might be accelerating and not be moving?

Well, there is of course the obvious case of you being in deep intergalactic space and you feel a tug of acceleration, but it could just as readily be DEceleration that you are feeling... which leaves you stopped. But that doesn't answer the challenge well enough. The truth is that no object can categorically be deemed to be in absolute motion; only relative motion makes sense, for absolute motion implies an absolute preferred frame of reference, which has been shown to be pure myth. The relativistic explanation holds: any tug of acceleration felt could just as easily be a passing gravitational field, for no astute study of the behavior of light can betray otherwise.

 

[geometer]

Well, there is of course the obvious case of you being in deep intergalactic space and you feel a tug of acceleration, but it could just as readily be DEceleration that you are feeling... which leaves you stopped. But that doesn't answer the challenge well enough. The truth is that no object can categorically be deemed to be in absolute motion; only relative motion makes sense, for absolute motion implies an absolute preferred frame of reference, which has been shown to be pure myth. The relativistic explanation holds: any tug of acceleration felt could just as easily be a passing gravitational field, for no astute study of the behavior of light can betray otherwise.

But, when I feel the tug of acceleration, I can now say I know I am moving. I am either speeding up or slowing down, but I am moving.

 

[ostren]

But, when I feel the tug of acceleration, I can now say I know I am moving. I am either speeding up or slowing down, but I am moving.

Are you? I think not. With respect to what reference are you moving? Unanswerable! The only answer is the relativistic one: there's no such thing as unequivocal motion. This is verified by light's behavior.

 

[russ_watters]

Are you? I think not. With respect to what reference are you moving? Unanswerable!

Wrong. If you feel acceleration that you know you caused (by hitting your engine fire button or turning your steering wheel), you can measure that acceleration and use it to calculate a change in velocity based on the assumption that you were at rest prior to the acceleration. Arbitary rest frame? Certainly, but that's Relativity!

 

[humanino]

Wrong. If you feel acceleration that you know you caused (by hitting your engine fire button or turning your steering wheel), you can measure that acceleration and use it to calculate a change in velocity based on the assumption that you were at rest prior to the acceleration. Arbitary rest frame? Certainly, but that's Relativity!

Devil advocate :
Someone fastidious could say : how do you know this is not also due to a gravitational wave ? Not solely due to your engine ? Imagine you have been fooled all your life by a deamon sending the right gravitational wave at the right instant you turn the engine, so that you never noticed anything wrong in the calculations of fuel consumption...

 

[ostren]

geometer said, "If one system is accelerating, even uniformly, with respect to another system it is no longer an inertial system, and you should be able to detect acclerations and thereby determine who is really moving."

The word "really" in the above citation is all that I was countering. There's never much dispute about relative motion.

 

[geometer]

Devil advocate :
Someone fastidious could say : how do you know this is not also due to a gravitational wave ? Not solely due to your engine ? Imagine you have been fooled all your life by a deamon sending the right gravitational wave at the right instant you turn the engine, so that you never noticed anything wrong in the calculations of fuel consumption...

If the demon's acceleration is in the same direction as the engine, you just go faster, but if you burn the engine, you consume fuel. If the demon's acceleration is exactly equal to, but opposite that of the engine, you don't accelerate so the question is moot (but you still burn the fuel).

But, looking at the bigger picture, who cares what causes the acceleration? In fact, the whole point behind general relativity is that you can't differentiate between the accleration caused by a gravitational field and that created by any other mechanism.

If you are accelerating with respect to any other system you are no longer an inertial system and therefore you can in principle determine that you are moving.

 

[ostren]

..If you are accelerating with respect to any other system you are no longer an inertial system and therefore you can in principle determine that you are moving.

Moving with respect to what? You don't have to be a non-inertial system to determine that you are moving relative to another frame. So the felt tug of acceleration provides no qualitative enhancement to one's assumptions.

 

[russ_watters]

Moving with respect to what? You don't have to be a non-inertial system to determine that you are moving relative to another frame. So the felt tug of acceleration provides no qualitative enhancement to one's assumptions.

Haven't you ever heard of inertial navigation? A submarine keeps an accurate fix on its position by measuring acceleration alone. All that is needed is a starting point. For a submarine, the starting point is provided by GPS right before the sub surfaces, but it doesn't need to be. In open water maneuvering, it can simply be "where I was 5 minutes ago relative to where I am now." On the spaceship, you can throw an obect overboard right before you start accelerating to mark your position, but you don't need to - and even if you do throw an obect out, you don't ever need to look at it to come back to it later - you don't have to have a physical marker for your position to use it as a reference point.

Also, when doing collision avoidance, maneuvering, doing dead reckoning, etc. ships use a "where I started from" reference frame that isn't necessarily fixed to a position on earth. Its simply not necessary.

 

[ostren]

I read all of the last post and am unimpressed. Acceleration is not an unequivocal guarantee of motion, and certainly not absolute motion (ie. "through space") because there's no such thing. The topic is relativity, and how one navigates through a body of water is hardly on point.

So you toss something out of a spacecraft and then accelerate away and return to find your "landmark". NO WAY is that any indication that you actually and unequivocally moved "through space". It's a moot point. Maybe you and the landmark were already in motion, and the engine thrust merely decelerated your craft to a standstill while the landmark continued on its merry way by momentum. But it's all made academic by Relativity anyway, because there's no such thing as unequivocal motion "through space", because "space" is not a medium -- space is the absence of any medium.

 

[humanino]

Russ-waters' argument does not really need any medium : as long as the gravitational field around is uniform, that would do.

 

[russ_watters]

Nor do I say anything about "absolute motion." I agree there is no such thing. So what is the problem? It almost seems like you are saying that since motion is absolute, motion doesn't exist. But that's a contradiction. It does exist, and it is relative.

So you toss something out of a spacecraft and then accelerate away and return to find your "landmark". NO WAY is that any indication that you actually and unequivocally moved "through space".

You moved relative to your arbitrary landmark. No one is saying anything here about the implications for absolute motion - because there are none. All motion is relative - relative to whatever you choose. If I choose an arbitrary landmark, choose to call it "stationary" and then observe my distance to it to be increasing, then I am moving.

 

[ostren]

Nor do I say anything about "absolute motion." ... No one is saying anything here about the implications for absolute motion..

Please have a look at my post #42, this thread, wherein that is answered. YOU may not have been trying to assert anything about absolute motion, but in geometer's original post, there was that clear implication.

You may need to review more of the thread to get a handle on this: one can discern relative motion without dispute, but I refuted geometer's claim that acceleration leads one to conclude "who is really in motion".

 

[russ_watters]

Please have a look at my post #42, this thread, wherein that is answered. YOU may not have been trying to assert anything about absolute motion, but in geometer's original post, there was that clear implication.

This quote:

geometer said, "If one system is accelerating, even uniformly, with respect to another system it is no longer an inertial system, and you should be able to detect acclerations and thereby determine who is really moving."

This does not imply anything about absolute motion. "who is really moving" is talking about which of two objects is moving with respect to the other, not whether either is "moving" absolutely. I think you misunderstood.

Ie, if a spaceship tosses a marker over the side, then fires its engines, you can't say its the marker that's "really moving" (with respect to the spaceship) because its the spaceship, not the marker, that's undergoing acceleration.

You can't even say both were moving and the ship decelerated and is now stopped because you then need a 3rd reference point from which to declare the ship moving at the beginning.

 

[ostren]

... I think you misunderstood. ...you can't say its the marker that's "really moving" (with respect to the spaceship) because its the spaceship, not the marker, that's undergoing acceleration..

I misunderstood nothing. When geometer said, "If one system is accelerating, even uniformly, with respect to another system it is no longer an inertial system, and you should be able to detect acclerations and thereby determine who is really moving." But acceleration does nothing to differentiate "who is really moving". There's no such determination. There's no preferred frame! If one pilot fires his thrusters and thence feels a G-force, that's no indication what*so*ever as to which of two arbitrary frames "is really moving". They are each moving relative to one another and acceleration cannot qualify that any better.

 

[geometer]

I misunderstood nothing. When geometer said, "If one system is accelerating, even uniformly, with respect to another system it is no longer an inertial system, and you should be able to detect acclerations and thereby determine who is really moving." But acceleration does nothing to differentiate "who is really moving". There's no such determination. There's no preferred frame! If one pilot fires his thrusters and thence feels a G-force, that's no indication what*so*ever as to which of two arbitrary frames "is really moving". They are each moving relative to one another and acceleration cannot qualify that any better.

I beg to differ ostren. You did misunderstand. Russ is right on. My point is that given two inertial coordinate systems, there is nothing you can do to tell which is moving relative to the other. However, as soon as one coordinate system experiences an acceleration, from whatever source, now you can tell who is moving with respect to whom. Accleration does differentiate which of two systems is moving with respect the other.

 

[geometer]

You can locally make acceleration disappear by changing the referential. Only locally. The free-fall observer not noticing the Earth gravitational field has a limited spave around him, otherwise he would notice the spherical shape.

Jumping back a few posts, I'd like to make a comment on this post also. Acceleration is actually a tensor of order 1. One of the main properties of tensors is that if they are non-zero in any coordinate system, they are non-zero in all coordinate systems. So, you can't make acceleration disappear by changing the reference frame.

 

[russ_watters]

ostren, if what you are saying were correct, there would be no possible resolution to Einstein's "twins paradox." You could accelerate a spaceship away from earth, claim Earth is the one accelerating while the spaceship is stationary, have the spaceship turn around and come back to earth, and calculate that the clock on Earth should have less elapsed time than the clock on the spaceship. But then you'd compare the clocks and find that its the clock on the spaceship that has less elapsed time.

And again, nothing in any of this has anything to do with absolute motion. You keep bringing it up, but it isn't what geometer meant and nothing I said implies it exists. Again, answering the question "who is really moving?" does not require an absolute frame of reference.

 

[ostren]

I have carefully read geometer's post #52 as well as Russ Watters' post #54 and I beg to differ with both. To geometer's claim that

..as soon as one coordinate system experiences an acceleration, from whatever source, now you can tell who is moving with respect to whom. Accleration does differentiate which of two systems is moving with respect the other.

I say you are way wrong and I stand by my post #51 unaltered. And your latter sentence above makes no sense: because each is obviously moving with respect to the other -- it's not lop-sided.

And to Russ's claim that

answering the question "who is really moving?" does not require an absolute frame of reference.

I have to ask what in tarnation you mean by the qualifier "really"?? Ah! some motion is "real" and other motion "imaginary".. is that your contention?

As for violating the twin paradox, Russ, you've got it all wrong. The turnaround twin's acceleration doesn't mean that time dilation is his alone because he is the one who is 'really' moving. Ooh, there's that funny word again.

No, the twin paradox can be resolved even assuming that the astronaut twin is STOCK STILL in space the entire time! For example, at my website, Addendum IV.

 

[kawikdx225]

I have carefully read geometer's post #52 as well as Russ Watters' post #54 and I beg to differ with both. To geometer's claim that I say you are way wrong and I stand by my post #51 unaltered. And your latter sentence above makes no sense: because each is obviously moving with respect to the other -- it's not lop-sided.

If you put an accelerometer on the spaceship and another on Earth you will find that it is "lop-sided" as the spaceship records acceleration and the Earth does not.

 

[ostren]

kawikdx225: This is about "who is really moving", a phrase employed by geometer and echoed by russ_watters. This is not about who is really experiencing a G-force.

 

[geometer]

And to Russ's claim that I have to ask what in tarnation you mean by the qualifier "really"?? Ah! some motion is "real" and other motion "imaginary".. is that your contention?

I don't mean to speak for you Russ, but what I meant by "really" refers back to the post that started this thread. In that post, OneEye commented that it was equally valid to say his turning of his car's steering wheel caused the Earth to turn as it was to say it caused his car to turn. He cited the equivalence prinicple from special relativity for this statement.

My comment was that by his turning of the steering wheel, he had introduced an acceleration into the picture, so the frame centered on and moving with his car was no longer an inertial frame and that the special relativity equivalence principle didn't apply any more. Further, since he felt the acceleration, it was possible for him to determine that it was actually he that was moving with respect to a frame stationary on the earth, and it was not the Earth that was moving with respect to a frame centered on his car.

 

[ostren]

Thank you for the compromise, geometer, but I was only partly right, I think. The car's brakes or gas pedal would produce acceleration that can NOT be differentiated from gravitation by means of the local study of light. But any rotational motion, any spinning, that component I do believe is confirmed by light's behavior to be unambiguous.

 

[geometer]

I think we still have a basic disagreement here ostren. Given a similar situation: If I am in my brand new Ferrari (hey, if we're going to pretend...) traveling down the freeway in a perfectly unaccelerated condition there is no way for me to tell whether I am moving with respect to the Earth outside my window or whether the Earth is moving and I am stationary. However, as soon as I step on the gas to pass Granny in her 57 Chevy I have introduced an acceleration into the picture and I can determine who is stationary with respect to whom.

It doesn't matter that I can't tell if the acceleration is due to gravity or to my pushing on the gas pedal; all accelerations produce equivalent effects. But, it does destroy my inertial status and enable me to differentiate the frame moving with me from any other inertial frame.