The Peripatetic Albert, Round 3

[geometer]

As others have pointed out, an observer can tell whether or not f=ma "works" for them just by doing local experiments, this can be done without thinking about the forces between the two bodies.

This is my point pervect. Since I can detect acclerations in my frame, I can therefore apply f=ma in my frame and conclude that I am moving. It may be difficult for me to tell if I am speeding up, or slowing down or turning depending on what other data I have available, but I can tell I am moving.

Depending on what data I have about the acceleration it may also be impossible for me to tell if I have suddenly encountered a gravitational field or the acceleration came from another source

 

[ostren]

If I fire my engine and I start to feel an acceleration, and I check an accelerometer on the marker I just dropped off my ship and see that it is not accelerating, I most certainly can deduce several things:

-There is no gravity field affecting these results (I consider it unreasonable to assume a gravity field coincidentally appeared at the instant I fired my rocket).

YOU consider it "unreasonable", yet relativists consider it not only reasonable, but necessary.

-I am accelerating, the marker is not.
-I am moving with respect to the marker, not the other way around.

No, the motion is perfectly mutual. You are drifting back into pre-relativity recidivism.

How many times do I have to say this before you accept it? All motion is relative. The words "real" and "imaginary" have nothing to do with anything.

Right... so you shouldn't go around saying things like in your post #50

"who is really moving" is talking about which of two objects is moving with respect to the other

There's no "which" to it; the motion is mutual!

And you shouldn't go around saying things like in your post #54

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

There's no "really" to it; whaddya mean by that silly word?

Is your explanation the same as Einstein's?

Refering to my website Addendum IV -- yes, it's the same as Einstein's.

 

[ostren]

You say there is no way to tell who is on a special frame. If this is true I could do a physics experiment in the accelerating car and on the Earth and the results would be identical. This is not true.

Lets try another thought experiment.
Two cars are sitting at a red light.
Granny in her 57 Chevy and some punk kid in a hotrod.
Each car has a passenger that is tossing a quarter in the air then catching it.
When the light turns green the kid in the hodrod hits the throttle but granny fell asleep.
The passenger in granny's car notices no change in his physics experiment (tossing and catching the quarter) but the passenger in the hotrod must now make a correction to catch his quarter.

In the hotrod, the quarter is tugged toward the rear. But the driver feels a tug toward the rear as well. And if you carefully examined a ray of light moving transverse to the road, it would appear to the hotrod's occupants to be tugged in that very same direction. So YES, there is asymmetry, but it is perfectly consistent with the presence of a gravitational field. Under relativity, you must ascribe the asymmetry to gravity, rather than claim someone is "really" in motion. I would have to think much too hard to tell you why... perhaps it's because a claim of unequivocal "motion" entails too too many confusing ramifications.

 

[kawikdx225]

In the hotrod, the quarter is tugged toward the rear. But the driver feels a tug toward the rear as well. And if you carefully examined a ray of light moving transverse to the road, it would appear to the hotrod's occupants to be tugged in that very same direction. So YES, there is asymmetry, but it is perfectly consistent with the presence of a gravitational field. Under relativity, you must ascribe the asymmetry to gravity, rather than claim someone is "really" in motion. I would have to think much too hard to tell you why... perhaps it's because a claim of unequivocal "motion" entails too too many confusing ramifications.

hmmmm... brain hurting!
Well if what you say is true then (as Russ said) the twin paradox would not be a paradox and both twins would be the same age after the trip. This disagrees with Einstein. Or am I missing something.

Here's what I think I know.
"I am really moving" is a meaningless statement
"I am really moving with respect to" is valid
"I am really accelerating" is valid

motion is relative
acceleration is absolute

geometer:
lol, no. just tryin to keep it colorful.

 

[ostren]

... Well if what you say is true then (as Russ said) the twin paradox would not be a paradox and both twins would be the same age after the trip. This disagrees with Einstein. Or am I missing something.

Thanks for keeping it colorful!

What you're probably missing about the Twin paradox is that the Lorentz tranform mandates more than just length contraction and time dilation (each of which is direction independent), but also a third element, that of time dissynchronicity, which is direction dependent. Actually there are a handful of adequate resolutions of the Twin Paradox, but of course none of them could be based on the astronaut twin being really*truly the one in motion. Didn't you check out my version yet?

 

[ostren]

... Minor nitpick, reilly - the force is equal and opposite, the acceleration is not. f=ma, and the Earth is a lot more "m" than that '57 chevy. Its good to bring us back to that example though: in the rocket example, the force of the engine acts on the rocket alone and a gravitational pull would act on the rocket and marker proportionally. ..

That's directed to Reilly but I wish to venture this contribution. The gravitational potential gradient that mysteriously arises is precisely of such steepness as to stall your rocket -- ie. counter the engine thrust -- and accelerate the marker just so, as it is thence perceived to travel. It may be helpful to recall that the deflections/trajectories caused by gravity acting on an object do not depend on that object's MASS. Tower of Pisa, y'know?

 

[geometer]

We seem to be really hung up on the word really. What I really meant to really say was that in OneEye's original example he could conclude that he was moving with respect to the Earth and not vice-versa.

Ok Ostren - let's assume we are in a totally isolated laboratory in deep space, initially in an inertial condition. This laboratory has no visual communication with the space outside, and is empty except for you, me and an acclerometer of some kind. We now experience an acceleration (let's assume it's due to a preprogrammed rocket firing just for definiteness). What can we now conclude about our state of motion?

 

[ostren]

*** Nada ***

 

[geometer]

*** Nada ***

Why can I not apply f = ma and conclude that I am moving?

 

[pervect]

We seem to be really hung up on the word really.

Yep. And the word "truly" too.

What I really meant to really say was that in OneEye's original example he could conclude that he was moving with respect to the Earth and not vice-versa.

Really and truly? :-)

It seems to me that this thread has been about 50% arguing over semantics, and the other 50% over philosophy. I think everyone knows what happens next in all of the examples given, it's just a big argument about how to describe it.

 

[ostren]

Why can I not apply f = ma and conclude that I am moving?

I think that it may be because then relativity would not be such an elegant and consummate theory of physics. Even if you apply f=ma, as I said earlier that might only indicate that you are DEcelerating (to a stop?). There is too much ambiguity and yet if you stick to the application of relativity's Equivalence Principle, then nothing is left to be ambiguous. This is my best quick reply although perhaps a bit wanting :-)

 

[HallsofIvy]

Why can I not apply f = ma and conclude that I am moving?

You could apply f= ma and conclude (if f is not 0) that you are accelerating. The point is (and goes back to "Galilean Relativity") that force (which what you "feel") is proportional to acceleration, not velocity so you couldn't use that to determine your velocity- or even whether it is 0 or not.

It was the discovery that eletro-magnetic field DO depend on velocity rather than acceleration that led to the idea that we could use some sort of electro-magnetic experiments (i.e. light) to determine and "absolute" velocity. Relativity developed out of the fact that those experiments still didn't find anything!

 

[kawikdx225]

I don't think anyone was saying that you could determine your absolute velocity by studying your acceleration.
The argument is that if you know for a fact that you are accelerating (by use of an accelerometer) then doesn't that imply you are moving. Since acceleration is by defination a "change in velocity with respect to time" meter/second² ?

In other words, how can you accelerate if you are not moving? Granted you don't know how fast your velocity is or if you are speeding up or slowing down or just turning in a circle but you are moving.

 

[selfAdjoint]

You stomp on the gas and feel that push in your back. Must be moving! You look out the window and the telephone poles are standing still. Whaaaat?

You're driving on an endless belt that your spinning wheels drive backwards. Or, make up your own explanation. Acceleration does not always produce motion.

 

[geometer]

You stomp on the gas and feel that push in your back. Must be moving! You look out the window and the telephone poles are standing still. Whaaaat?

You're driving on an endless belt that your spinning wheels drive backwards. Or, make up your own explanation. Acceleration does not always produce motion.

Wait a minute! In this case you haven't accelerated yourself. You've acclerated the belt so you wouldn't detect the accleration. And, this acceleration did produce motion - it increased the rate at which the belt moves underneath you.

 

[selfAdjoint]

But the belt is running friction free on a planetoid that JUST HAPPENS to be rotating in the opposite direction.

The point, as I indicated, isn't the particular mechanism, but the general principle that the push you feel or don't feel has nothing to do, in and of itself alone, with how you are or are not moving. You have to look at the larger picture, or make reasonable assumptions like the designers of inertial guidance systems.

 

[kawikdx225]

But the belt is running friction free on a planetoid that JUST HAPPENS to be rotating in the opposite direction.

The point, as I indicated, isn't the particular mechanism, but the general principle that the push you feel or don't feel has nothing to do, in and of itself alone, with how you are or are not moving. You have to look at the larger picture, or make reasonable assumptions like the designers of inertial guidance systems.

geometer is right.
In your thought experiment an accelerometer in the car would measure 0 meters/second² therefore you are not accelerating.

Anytime your accelerometer reads 0 m/s²you cannot say"I am in motion" you must say "I am in motion relative to".

Anytime your accelerometer reads anything other than 0 m/s² you can say "I am in motion but I have no idea how fast or in which direction"

 

[ostren]

Kudos to geometer & kawikdx225 for finding selfAdjoint's error. The driver would NOT feel any push in his back in that scenario.

 

[geometer]

Seems like we have kind of run out of steam here, but it's been a great discussion and I think, has sharpened my thinking on this. Thanks!