Implications of the statement Acceleration is not relative

[stevendaryl]

It seems to me you guys are just playing with words - proper, real, coordinate.
Try defining them before hitting one another on the head with them!

I always thought position was x,y,z - whatever they are, they are relative.

The modern way of thinking about it is that a position, such as a location on Earth, is absolute. The top of the Eiffel Tower is a definite spot; there is no ambiguity, or relativism involved. But there are infinitely many coordinate systems that can be used to specify a position.

In relativity, the primary thing is not a position, but an event, a point in space and time. So "the top of the Eiffel tower when Michelle Obama went up it" is an event, and it's absolute. But if I try to describe it using 4 numbers, for example, (latitude, longitude, altitude in meters, time in seconds since 1900), its description is relative to a coordinate system.

A spacetime path, giving the events that a traveler passes through, as a function of the time on his watch, is an absolute thing, because each event is absolute. But to describe the path as a set of 4 functions $x(\tau), y(\tau), z(\tau), t(\tau)$ is relative to a choice of a coordinate system.

The proper velocity of a path is again an absolute thing, while the components of the proper velocity are relative to a coordinate system. Proper acceleration is an absolute thing, while its components are relative to a coordinate system.

Yes you can invent a special acceleration and use the word "proper" for it.
But how can you MEASURE it in an experiment?

Yes, with the notion of "proper acceleration" used in General Relativity, one can measure its magnitude with an accelerometer. A simple accelerometer can be constructed by just taking a cubic box, putting a metal ball in the center, and then connecting the ball to the sides of the box using 6 identical springs. If the ball is exactly in the center, then the box has no proper acceleration. If the ball is closer to one wall, then the box is accelerating in the direction of the opposite wall.

 

[TrickyDicky]

The modern way of thinking about it is that a position, such as a location on Earth, is absolute. The top of the Eiffel Tower is a definite spot; there is no ambiguity, or relativism involved. But there are infinitely many coordinate systems that can be used to specify a position.

That is not the modern way, that is Newton's way, unless you consider Newton's the modern way of thinking (but we're not in the eighteenth century anymore, remember?). Anyway the rest of your post gets it right that the more modern relativist thinking considers events in space time rather that position in space as absolute, so I don't know what you meant by this introduction.

 

[Nugatory]

That is not the modern way, that is Newton's way, unless you consider Newton's the modern way of thinking (but we're not in the eighteenth century anymore, remember?). Anyway the rest of your post gets it right that the more modern relativist thinking considers events in space time rather that position in space as absolute, so I don't know what you meant by this introduction.

I must confess that I'm not reading Stevendaryl's point the way you are. The top of the Eiffel tower absolutely and unambiguously identifies a particular absolute coordinate-independent timelike worldline - and you'll notice that Stevendaryl carefully avoided identifying a "position" with a point in classical three-space.

 

[stevendaryl]

That is not the modern way, that is Newton's way, unless you consider Newton's the modern way of thinking (but we're not in the eighteenth century anymore, remember?). Anyway the rest of your post gets it right that the more modern relativist thinking considers events in space time rather that position in space as absolute, so I don't know what you meant by this introduction.

A position on the earth is absolute. A position in space isn't.

The Earth is a 3D object, while space is 4D in the modern way of looking at it.

 

[PeterDonis]

When I throw a ball, its acceleration (with respect to the only coordinate system that matters) is determined by its mass and the magnitude of the applied force.

Really? When you, standing on the surface of the Earth, throw a ball upward, its motion is determined purely by its mass and the force you apply? Then why does it come back down?

When the Earth and the stars move, the same law should apply. {Edit: Not exactly the same law. I realize that gravity will cause coordinate acceleration without applied force. But the moving Earth and stars have acquired kinetic energy with respect to the rocket. That energy must have come from somewhere.}

The ball changes its kinetic energy with respect to you even though you didn't exert any additional force on it; at some point in its trajectory, it is momentarily motionless with respect to you (up in the air at the instant it stops rising and starts falling back). Where did the kinetic energy you gave the ball go?

You give away the problem with the position you are trying to take when you say "not exactly the same law". That's just the point: if you want "the laws of physics" to be "the same" in all reference frames, so that you can always view yourself "at rest", then the laws of physics have to include counterintuitive things like the Earth and the stars changing direction and speed just because you fired your rocket engine. If you want the laws of physics to always look simple, then you have to restrict yourself to frames in which they look simple (because all the counterintuitive stuff cancels out in those frames). You can't have it both ways; you can't have both simple-looking laws *and* a free choice of frames; your choice of frames determines how simple the laws look in the frames you choose.

 

[GregAshmore]

The modern way of thinking about it is that a position, such as a location on Earth, is absolute. The top of the Eiffel Tower is a definite spot; there is no ambiguity, or relativism involved. But there are infinitely many coordinate systems that can be used to specify a position.

In relativity, the primary thing is not a position, but an event, a point in space and time. So "the top of the Eiffel tower when Michelle Obama went up it" is an event, and it's absolute. But if I try to describe it using 4 numbers, for example, (latitude, longitude, altitude in meters, time in seconds since 1900), its description is relative to a coordinate system.

A spacetime path, giving the events that a traveler passes through, as a function of the time on his watch, is an absolute thing, because each event is absolute. But to describe the path as a set of 4 functions $x(\tau), y(\tau), z(\tau), t(\tau)$ is relative to a choice of a coordinate system.

I agree in principle with what you've said, but I question its practical utility. I read it this way: "A spacetime event (position and time) has a real existence apart from any coordinate system, yet can only be described in terms of some coordinate system." It would seem that the absoluteness of a spacetime event is metaphysical, because it cannot be verified empirically.

Furthermore, I believe that the rocket twin will deny what you say about "path", and all that follows from it. See below.

The proper velocity of a path is again an absolute thing, while the components of the proper velocity are relative to a coordinate system. Proper acceleration is an absolute thing, while its components are relative to a coordinate system.

I assume that you develop the absolute path of a particle in this way. An arbitrary coordinate system whose origin is at the object under scrutiny is chosen. Using myself as an example, X is to my right, Y is straight ahead, Z is out the "top" of my head. My path through spacetime is marked by placing a monument in space at regular time intervals (by my clock).

At each iteration of my clock, I place a monument. I inscribe on the monument the time as read from my clock. I also consult my accelerometer to determine (some calculation is necessary) the change in my orientation since the previous iteration. I inscribe the differential change in orientation on the monument that was placed at the previous iteration. The change in orientation is necessarily expressed as rotations about the axes of the arbitrarily chosen coordinate system. Finally, I take my measuring rod and place its end against the previously placed monument; I then read directly the distance traveled since the previous iteration. I write that distance on the previous monument. Thus, my friend can follow my path without a map (coordinate system) if he starts at the first monument, adjusts his orientation as directed, and travels the distance indicated. At each monument, he repeats the process.

All of that is well and good, if one accepts the premise that I am moving through space. But, if you will recall, I am that very obstinate occupant of the rocket who insists that he is not moving at all. In my world, there is only one monument, and my orientation does not change.

 

[stevendaryl]

I agree in principle with what you've said, but I question its practical utility. I read it this way: "A spacetime event (position and time) has a real existence apart from any coordinate system, yet can only be described in terms of some coordinate system." It would seem that the absoluteness of a spacetime event is metaphysical, because it cannot be verified empirically.

There's nothing metaphysical about it--it's very concrete. A meteor crashes to the Earth. That marks a unique event. You don't need to have coordinates for it. George Washington is born. That marks a unique event. A star goes supernova. That's a unique event.

On a piece of paper, you draw a dot. That dot is a unique location on the piece of paper. You don't need coordinates to know that it's unique. You don't need coordinates to know whether the dot is at the same location as the X that someone else drew on the paper.

I assume that you develop the absolute path of a particle in this way. An arbitrary coordinate system whose origin is at the object under scrutiny is chosen. Using myself as an example, X is to my right, Y is straight ahead, Z is out the "top" of my head. My path through spacetime is marked by placing a monument in space at regular time intervals (by my clock).

At each iteration of my clock, I place a monument.

Specifying the initial location of the monument isn't good enough. You have to also specify it's initial velocity.

A path through spacetime is a 4D analogue of a curve drawn a piece of paper. An event in spacetime corresponds to a point on the paper. A velocity of a path corresponds to the slope of the tangent line drawn through a curve.

I inscribe on the monument the time as read from my clock. I also consult my accelerometer to determine (some calculation is necessary) the change in my orientation since the previous iteration. I inscribe the differential change in orientation on the monument that was placed at the previous iteration. The change in orientation is necessarily expressed as rotations about the axes of the arbitrarily chosen coordinate system. Finally, I take my measuring rod and place its end against the previously placed monument;

Once again, an event is a single moment. You can't place a monument at a single moment, and you can't return to an earlier moment. The monument is going to follow its own path through spacetime, and when and if you get back to the same monument, it's not the same point in spacetime. Both you and the monument have moved since then.

All of that is well and good, if one accepts the premise that I am moving through space. But, if you will recall, I am that very obstinate occupant of the rocket who insists that he is not moving at all.

EVERYONE moves at all times. If you look at your watch, then wait a while and look at your watch again, the second look is a different event from the first event. You've traveled from one event to another. You've "moved" through spacetime.

Now, you can certainly choose a coordinate system so that the spatial coordinates of the second event are the same as the spatial coordinates of the first event. But there is no way to choose coordinates so that all coordinates are the same. There is no way to avoid having motion in spacetime.

 

[GregAshmore]

Really? When you, standing on the surface of the Earth, throw a ball upward, its motion is determined purely by its mass and the force you apply? Then why does it come back down?

I perhaps should have been even more careful in my wording. The "gravity" under discussion here is not the gravity of Earth or any massive body. For the purposes of the twin paradox problem, the gravitational field due to the mass of the Earth is ignored. The gravity under discussion is the gravity of unspecified origin that Einstein posits to explain the motion of the Earth when the rocket engine is fired. This gravity is purely the result of the choice of coordinate system, as I understand DaleSpam.

When I throw a ball in SR, its motion is indeed determined purely by its mass and the force I apply. It does not return. It continues to move forever at some constant speed.

The ball changes its kinetic energy with respect to you even though you didn't exert any additional force on it; at some point in its trajectory, it is momentarily motionless with respect to you (up in the air at the instant it stops rising and starts falling back). Where did the kinetic energy you gave the ball go?

This paragraph does not apply; the ball does not reverse in SR.

You give away the problem with the position you are trying to take when you say "not exactly the same law". That's just the point: if you want "the laws of physics" to be "the same" in all reference frames, so that you can always view yourself "at rest", then the laws of physics have to include counterintuitive things like the Earth and the stars changing direction and speed just because you fired your rocket engine. If you want the laws of physics to always look simple, then you have to restrict yourself to frames in which they look simple (because all the counterintuitive stuff cancels out in those frames). You can't have it both ways; you can't have both simple-looking laws *and* a free choice of frames; your choice of frames determines how simple the laws look in the frames you choose.

The problem is not that the law of physics proposed by DaleSpam to explain the sudden movement of the Earth and stars at the firing of the rocket is not simple, or is not intuitive. The problem is that he has not proposed any law at all. Or at least, I do not recognize the statement "the movement of the Earth and stars was not caused by the firing of the rocket; it was caused by my choice of a certain set of coordinates" as a law of physics; certainly no other law of physics that I have learned looks like that. Furthermore, the statement borders on the delusional (I tried to find a neutral word; I could not; sorry) in that it denies the obvious causal connection between the firing of the rocket and the movement of the Earth and stars (which I made bold in the quote of your post).

That is how it seems to me. I don't really have the right to speak on the matter because I do not know anything about Christoffel symbols, and therefore cannot understand the line of reasoning taken by DaleSpam. It is much better for me to leave this alone for the time being. I only mentioned it in my summary because it is an outstanding issue that must eventually be addressed.

 

[PeterDonis]

The gravity under discussion is the gravity of unspecified origin that Einstein posits to explain the motion of the Earth when the rocket engine is fired.

Ah, ok, so the rocket is floating in flat spacetime. That clarifies things. But my comments still apply. See below.

This gravity is purely the result of the choice of coordinate system, as I understand DaleSpam.

If spacetime is flat, yes.

The problem is not that the law of physics proposed by DaleSpam to explain the sudden movement of the Earth and stars at the firing of the rocket is not simple, or is not intuitive. The problem is that he has not proposed any law at all.

No, the "problem" is that you have picked a scenario with a particular kind of simplicity, but then you want to choose a frame that doesn't match up with that simplicity. You have set your scenario in flat spacetime; in flat spacetime the laws of physics looks simplest in a global inertial frame. If you pick a non-inertial frame, like your "rest frame" when you fire your rocket, the laws of physics won't look as simple; they will have counterintuitive stuff in them like the Earth and the stars moving just because you fired your rocket engine. Once again, you can't have it both ways.

Or at least, I do not recognize the statement "the movement of the Earth and stars was not caused by the firing of the rocket; it was caused by my choice of a certain set of coordinates" as a law of physics; certainly no other law of physics that I have learned looks like that.

How about "you picked a reference frame that doesn't match up with the special properties of the spacetime you are in". Does that help?

The laws in question are the simple laws of flat spacetime. You already know them in an inertial frame. The talk about a "gravitational field" that appears when you choose non-inertial coordinates, or about the movement of the Earth and stars being caused by the choice of coordinates, is just a way of describing the fact that non-inertial coordinates make the laws look more complicated.

Furthermore, the statement borders on the delusional (I tried to find a neutral word; I could not; sorry) in that it denies the obvious causal connection between the firing of the rocket and the movement of the Earth and stars (which I made bold in the quote of your post).

What is this causal connection? How does the firing of your rocket make stars that are light years away suddenly move? It doesn't. It can't. Causal influences can only propagate at the speed of light; there's no way your firing your rocket engine here and now can make a star that is light years away move "right now".

This is one way that trying to choose a frame in which you are always at rest, when your motion is non-inertial, makes the laws of physics look more complicated: the laws of physics now have to include the possibility of "motions" that violate the usual rules of causality. The example Einstein used was rotation: if I consider myself, sitting here on the surface of the Earth, to be "at rest", then the stars must be moving around me faster than the speed of light.

But nothing can move faster than light! you say. Correct: but the "motion" of the stars due to my rotation is not a "real motion" that is subject to that law. The complete laws of physics in my "rest frame" now have to include the possibility of "fictitious motions" like the motion of the stars around me, or the motion of the stars in response to you firing your rocket engine, which can be faster than the speed of light and which can stop and start "instantly" if I change my state of motion, even though that "violates" causality.

Once more, you can't have it both ways. If you want simple, intuitive laws of physics, where there are no "fictitious motions" or "fictitious forces", you have to pick a reference frame that allows the laws to look that simple. If you insist on picking a frame where you are always at rest, even when you move non-inertially, the laws will not look simple in that frame. You can't avoid that trade-off.

 

[Dale]

Fair enough. But this may also lead to problems. For example, if proper time is measured by a clock, what is the proper time for the life of an individual particle? What clock do we read to measure its proper life span? This is of particular importance with regard to SR, as experiments with high speed particles are offered as evidence in support of the theory. We do not send a clock to accompany the particle on its journey in the accelerator. It seems to me that one is reduced to claiming that the particle is itself the clock. But if the particle is itself the clock, then there is no independent measure of the proper time that the particle existed, and thus no verification of the theory. There is no question that high speed particles live longer, as measured from our perspective. The question would be whether time in the rest frame of the particle is the same regardless of the speed of the particle measured in some other inertial frame, as the theory of SR requires. (I need to think about this some more; perhaps my logic is not entirely sound.)

This is merely a current technological limitation, not an in-principle limitation. In principle you could accelerate a regular clock up to .9999c and use it to measure the lifetime of particles that we shoot alongside it as it passes by. There will always be experiments that we would like to do but cannot currently accomplish.

However, what we can do with current tecnhology is to take modern clocks and make them so incredibly stable and accurate that we can measure relativistic effects with ordinary velocities. I.e. whether or not a velocity is "relativistic" or not depends on your sensitivity, and modern clocks are so exquisitely sensitive that we can measure relativistic effects at walking speeds.

Nonsense. I'm sitting at rest in my rocket the whole time. Don't tell me about choosing coordinate frames--there is only one coordinate frame that matters: mine.

This is the nonsense statement. It is nonsense for two reasons. First, because it uses an undefined concept. "Your frame" is non-inertial and there is no standard definition of a non-inertial object's frame. Second, because it is false. All coordinate systems have equal validity and yours is not particularly important and doesn't "matter" any more than any other coordinates.

When I throw a ball, its acceleration (with respect to the only coordinate system that matters) is determined by its mass and the magnitude of the applied force.

This statement is wrong. The acceleration in that frame is not only determined by the mass and magnitude of the applied force, but also by the fictitious force (gravity) acting on it in that frame.

When the Earth and the stars move, the same law should apply.

The same laws do apply to both, you just made a mistake in the case of the ball.

{Edit: Not exactly the same law. I realize that gravity will cause coordinate acceleration without applied force. But the moving Earth and stars have acquired kinetic energy with respect to the rocket. That energy must have come from somewhere.}

Actually, according to Noether's theorem, energy is NOT conserved in a non-inertial frame like that of the rocket. Energy is also frame variant.

A secondary (and less emotional) reaction is to ask the original question in a more precise way. What causes the spatial displacement between the rocket and the Earth to change?

What do you mean by "spacial displacement"? Do you just mean the coordinate displacement or are you thinking of some physical measure of displacement? If the latter, then exactly what measure are you thinking of?

 

[Dale]

The problem is that he has not proposed any law at all. Or at least ... no other law of physics that I have learned looks like that.

I can make it look more like a standard law of physics quite easily:$$\frac{d p^{\mu}}{d\tau} = f^{\mu} - {\Gamma^{\mu}}_{\nu\lambda} u^{\nu} p^{\lambda}$$Where f is the sum of the real four-forces acting on the particle, p is the four-momentum, u is the four-velocity, τ is the proper time along the particle's worldline, and $\Gamma$ is the Christoffel symbols in the coordinate system in question.

Furthermore, the statement borders on the delusional (I tried to find a neutral word; I could not; sorry) in that it denies the obvious causal connection between the firing of the rocket and the movement of the Earth and stars (which I made bold in the quote of your post).

It may be an obvious connection, but it is not a causal connection, as I clearly demonstrated earlier. If you would like to actually address the points that I made instead of making a blatantly fallacious rebuttal then I would be glad to discuss it.

 

[stevendaryl]

The problem is not that the law of physics proposed by DaleSpam to explain the sudden movement of the Earth and stars at the firing of the rocket is not simple, or is not intuitive. The problem is that he has not proposed any law at all. Or at least, I do not recognize the statement "the movement of the Earth and stars was not caused by the firing of the rocket; it was caused by my choice of a certain set of coordinates" as a law of physics; certainly no other law of physics that I have learned looks like that.

That's because you probably have used inertial Cartesian coordinates in physics. With inertial Cartesian coordinates, the relationship between applied force and coordinate acceleration is, as Newton wrote:

$m \dfrac{dV^\mu}{d \tau} = F^\mu$

where $V^\mu$ is the 4-velocity.

When you use noninertial or curvilinear coordinates, the relationship between applied force and coordinate acceleration is more complicated:

$m \dfrac{dV^\mu}{d \tau} +$fictitious force terms $= F^\mu$

So even when the applied force $F^\mu$ is zero, the coordinate acceleration $\dfrac{dV^\mu}{d \tau}$ can be nonzero due to "fictitious force" terms. Examples of such fictitious forces are the "g forces" due to acceleration, the "centrifugal force" and the "coriolis force". These "forces" are not due to any kind of physical interaction, but are artifacts of your choice of coordinate systems.

 

[harrylin]

That's done just to simplify the example. It's not a fundamental assumption of the explanation.

With instantaneous turnarounds, the proper distance along each leg is just algebra: $\sqrt{\Delta t^2-\Delta x^2}$. If we don't assume instantaneous turnarounds, we have to evaluate some sort of line integral. It's fairly easy to prove that in the limit as the turnaround time approaches zero, the line integral reduces to the simple algebraic calculation, so we use the latter when the details of the turnaround aren't important to the problem at hand.

In order to get a point through, simplification is fundamental to explanations...

 

[harrylin]

As you make your $\delta \tau$ small the SR predicted accelerometer reading becomes large while the actual accelerometer reading remains 0. [..]

[..]SR predicts a very large accelerometer reading during the turnaround, and real free falling accelerometers read 0.

SR doesn't predict that an accelerometer in free fall will indicate a large acceleration. If you insist in this thread instead of starting it as a topic, I'll start that topic for you.

 

[harrylin]

[..] But this much I believe to be undeniably true of a purely SR treatment of a scenario in which two bodies, one inertial and the other non-inertial, separate from each other and then approach to reunion: the non-inertial body must experience unbalanced force at the transition from separation to approach. [..]

SR uses the inertial frames of classical mechanics. If you know classical mechanics, then you certainly understand that if you accelerate freely in a gravitational field, your accelerometer will read approximately zero. If you don't know that, we can discuss this in the classical forum.

 

[harrylin]

[..] The problem is not that the law of physics proposed by DaleSpam to explain the sudden movement of the Earth and stars at the firing of the rocket is not simple, or is not intuitive. The problem is that he has not proposed any law at all. Or at least, I do not recognize the statement "the movement of the Earth and stars was not caused by the firing of the rocket; it was caused by my choice of a certain set of coordinates" as a law of physics; certainly no other law of physics that I have learned looks like that. Furthermore, the statement borders on the delusional (I tried to find a neutral word; I could not; sorry) in that it denies the obvious causal connection between the firing of the rocket and the movement of the Earth and stars (which I made bold in the quote of your post). [..]

You evidently understand the question that Einstein attempted to address in 1918. Regretfully, few people who try to answer you understand the question. But in any case, nobody here gave support for the answer that Einstein gave, and neither does the physics FAQ.

 

[Dale]

SR doesn't predict that an accelerometer in free fall will indicate a large acceleration. If you insist in this thread instead of starting it as a topic, I'll start that topic for you.

The entire Langevin scenario is off-topic, but at this point it would take too much effort to split off and it doesn't make sense to do so, IMO.

Yes, SR does predict that. According to SR the proper acceleration is:$$a^{\mu}=\frac{d^2x^{\mu}}{d\tau^2}$$Where x is the worldline in an inertial frame and τ is the proper time along that worldline. That quantity is non-zero.

 

[Dale]

You evidently understand the question that Einstein attempted to address in 1918. Regretfully, few people who try to answer you understand the question. But in any case, nobody here gave support for the answer that Einstein gave, and neither does the physics FAQ.

It is hard to see how you can believe that there was any definite answer since you don't even know what he meant by the term "gravitational field".

 

[harrylin]

It is hard to see how you can believe that there was any definite answer since you don't even know what he meant by the term "gravitational field".

I even cited his answer several times.

 

[Dale]

I even cited his answer several times.

Yes, you did. But you never were able to identify what you thought he meant. Seems strange to claim that a quote is an answer when you don't claim to know what the quote is even referring to.

 

[harrylin]

[..] at this point it would take too much effort to split off [..]

A misunderstanding of something so basic and simple surely requires discussing - much more than the topic of this thread.
Promised thread started here: https://www.physicsforums.com/showthread.php?p=4284966

 

[harrylin]

Yes, you did. But you never were able to identify what you thought he meant. Seems strange to claim that a quote is an answer when you don't claim to know what the quote is even referring to.

Instead I claimed to know what he was referring to; however I don't try hard anymore to explain other people's explanations - that is usually futile.

 

[Dale]

I claimed to know what he was referring to

So, according to you, what exactly was he referring to with the term "gravitational field"? I believe it was the Christoffel symbols. You believe he was referring to _______.?

 

[Alain2.7183]

See this topic

https://www.physicsforums.com/showthread.php?t=490163

Thanks for that link. Very interesting. Lots of good stuff.

 

[harrylin]

So, according to you, what exactly was he referring to with the term "gravitational field"? I believe it was the Christoffel symbols. You believe he was referring to _______.?

Einstein definitely referred to a field of force that possesses the property of imparting the same acceleration to all bodies; according to his theory, the gravitation-field generates the accelerated motion.
- http://en.wikisource.org/wiki/The_F...in_the_extension_of_the_relativity-postulate.

 

[stevendaryl]

Einstein definitely referred to a field of force that possesses the property of imparting the same acceleration to all bodies; according to his theory, the gravitation-field generates the accelerated motion.
- http://en.wikisource.org/wiki/The_F...in_the_extension_of_the_relativity-postulate.

Yes, in Einstein's original discussion of the twin paradox, with elevators and all that, the way he put it was something like this: (paraphrased)

If an elevator in outer space accelerates downward, the people in the elevator will feel an apparent upward force lifting them toward the ceiling. This is the inertial force due to being at rest in an accelerated frame.

If you have the same elevator falling in a gravitational field, it's accelerating downward, but the people feel no forces, because the upward inertial force is exactly canceled by the downward gravitational force.
​

I can't find an online reference to the original argument, but I remember reading it once, and it seemed that Einstein talked about freefall as not the absence of any forces, but as an exact balance between gravitational forces and inertial forces so that they cancel. From the standpoint of today, that seems like a convoluted way of describing it.

 

[PeterDonis]

Yes, in Einstein's original discussion of the twin paradox, with elevators and all that, the way he put it was something like this: (paraphrased)

I don't think this is right. The argument as he puts it in the popular book Relativity: A Clear Explanation That Anyone Can Understand goes like this: suppose we have an "elevator" in empty space, and some kind of "being" attaches a rope to one end (there happens to be a hook on that end) and starts pulling on it. A man inside the elevator will be able to stand on its "floor" (the end opposite the hook) just as if the elevator were at rest in a gravitational field, and if he drops a rock, it will appear to him to accelerate downward just as if he were at rest in a gravitational field. Finally, the man wonders how the elevator can be at rest in a gravitational field when it's in the middle of empty space, but then he discovers the hook in the roof with the rope attached to it; the elevator is hanging at rest in the field.

There's no argument about "forces" or "balance of forces" at all; the argument is purely about the man's observations and how they can be accounted for equally well by the "being" pulling on the rope in free space or by the rope suspending the elevator at rest in a gravitational field.

This wasn't a discussion of the twin paradox, it was a discussion of the equivalence principle, so I don't know if you were thinking of some other discussion of his; but what you paraphrased doesn't seem like a discussion of the twin paradox either.

 

[ghwellsjr]

I can't find an online reference to the original argument, but I remember reading it once, and it seemed that Einstein talked about freefall as not the absence of any forces, but as an exact balance between gravitational forces and inertial forces so that they cancel.

The argument as he puts it in the popular book Relativity: A Clear Explanation That Anyone Can Understand goes like this...

I don't know if Einstein's 1920 book, Relativity: The Special and General Theory is the same one you just mentioned but it has an identical explanation as you can read in this online reference.

 

[stevendaryl]

I don't think this is right. The argument as he puts it in the popular book Relativity: A Clear Explanation That Anyone Can Understand goes like this: suppose we have an "elevator" in empty space, and some kind of "being" attaches a rope to one end (there happens to be a hook on that end) and starts pulling on it. A man inside the elevator will be able to stand on its "floor" (the end opposite the hook) just as if the elevator were at rest in a gravitational field, and if he drops a rock, it will appear to him to accelerate downward just as if he were at rest in a gravitational field. Finally, the man wonders how the elevator can be at rest in a gravitational field when it's in the middle of empty space, but then he discovers the hook in the roof with the rope attached to it; the elevator is hanging at rest in the field.

There's no argument about "forces" or "balance of forces" at all; the argument is purely about the man's observations and how they can be accounted for equally well by the "being" pulling on the rope in free space or by the rope suspending the elevator at rest in a gravitational field.

I certainly prefer that way of looking at the equivalence principle, but years ago, someone pointed me to something written by Einstein that took the (in my opinion, convoluted) approach of saying that for an elevator in freefall, the gravitational force canceled the "fictitious force due to acceleration". I will continue to search for this passage from Einstein.

 

[Samshorn]

In Langevin's "twin" example the accelerator reading is zero during turn-around; in early SR there was no "twin paradox". http://en.wikisource.org/wiki/The_Evolution_of_Space_and_Time

Could you point to (or quote) the specific passage in that reference where Langevin describes using gravity to give a turn-around with zero accelerometer reading?

 

[PeterDonis]

I don't know if Einstein's 1920 book, Relativity: The Special and General Theory is the same one you just mentioned

Looks like it, yes. I think the subtitle "A Clear Explanation That Anyone Can Understand" was added in a later edition.

 

[Dale]

Einstein definitely referred to a field of force that possesses the property of imparting the same acceleration to all bodies; according to his theory, the gravitation-field generates the accelerated motion.

The Christoffel symbols do that.

 

[GregAshmore]

I can make it look more like a standard law of physics quite easily:$$\frac{d p^{\mu}}{d\tau} = f^{\mu} - {\Gamma^{\mu}}_{\nu\lambda} u^{\nu} p^{\lambda}$$Where f is the sum of the real four-forces acting on the particle, p is the four-momentum, u is the four-velocity, τ is the proper time along the particle's worldline, and $\Gamma$ is the Christoffel symbols in the coordinate system in question.

I see it.

It may be an obvious connection, but it is not a causal connection, as I clearly demonstrated earlier. If you would like to actually address the points that I made instead of making a blatantly fallacious rebuttal then I would be glad to discuss it.

I went back to read the points you made. Here they are:

As I explained to harrylin, it doesn't. If you say "A causes B" then that means that the presence of A implies B. So, if we say that "a force applied to the rocket causes the Earth and all the stars to move" that means that a force applied to the rocket implies that the Earth and all the stars must move. In an inertial frame, there may be a force on the rocket without movement of the Earth, so the force on the rocket does not imply movement of the Earth. Therefore the force on the rocket does not cause the Earth to move.

Agreed that the force on the rocket does not cause the Earth to move.

So what does cause the Earth to move? The answer is that specific choice of non-inertial coordinates. That choice of coordinates implies that the Earth moves, regardless of the presence or absence of any rockets with any forces. Every time you use that choice of coordinates the Earth moves. So the choice of coordinates causes the Earth to move, not the rocket.

The text in bold is false. Prior to the firing of the rocket, selection and use of the rocket coordinates does not cause the Earth and stars to move. By the very same logic you used to prove that the force on the rocket does not cause the Earth to move, it is shown that the selection of coordinates does not cause the Earth to move. There may be a selection of the rocket coordinates without movement of the Earth; therefore the selection of rocket coordinates does not cause movement of the Earth.

In the following I elaborate on why the selection of coordinates cannot cause the Earth and stars to move. In the process, I will have something to say about the premise at which you started your chain of logic.

We are discussing one and the same incident viewed by two observers, at rest on two bodies, Earth and rocket. (I need not repeat that all mass-induced gravity is ignored.) The two bodies have been separated for some time by the constant distance X. The rocket engine is fired. Coincidentally, the distance between the two bodies begins to increase. What is the cause of the change in distance between the two bodies?

First, an axiom and a postulate:

Axiom: Whatever the cause, it must be physical. This is self-evident in a discussion of physics.

Postulate: Whatever the physical cause, it must be the same for both observers. I think that this follows from the premise that there is one physical reality for all observers. However, I am not entirely confident that it is self-evident, as an axiom must be...as I understand the meaning of the terms axiom and postulate.


The cause according to the observer on Earth.
The observer on the Earth begins by claiming that the change in distance between the Earth and rocket is in fact the movement of the rocket. This follows from the claim of the observer on Earth that he is "anchored in place".

[Note: In my summary I said that, according to the principle of relativity, every observer may legitimately claim that he is at rest; anchored in place, as it were. It is the anchoring in place, the reckoning of the observer that his position is absolute, that makes the observer's coordinate system the only one that matters. His coordinate system is the only one that matters because it is the only one that is traceable to an absolute position. All other coordinate systems are derived in some way from his coordinate system. At this point in the discussion, it is a mistake to say that all coordinate systems are equally valid, for that is precisely the issue that is in question when discussing the validity of the principle of relativity.]

The observer on Earth goes on to claim that the movement of the rocket was caused by the force that was applied to the rocket. In support of the claim, he submits this evidence: the force is a physical phenomenon that was applied to the rocket only; only the rocket moved; the form of the motion correlates in a definite way with the force on the rocket and the mass of the rocket.

This is also the premise at which you started your chain of logic: "if we say that 'a force applied to the rocket causes the Earth and all the stars to move' that means..." (I'm sure that I have also said many times that the cause of the rocket's movement is the force on the rocket.)

Though the weight of evidence is great, the claim cannot be valid. It has been shown (by you) that the force on the rocket cannot be the cause of the movement of the Earth, as seen in the rocket frame. Therefore, expressed in relative terms, the force on the rocket cannot be the cause of the increasing distance between Earth and the rocket.

What can be the cause is the firing of the rocket. The firing of the rocket is a physical phenomenon. The force is the direct result of the firing of the rocket, so the rest of the case made by the observer on the Earth is valid.


The cause according to the observer on the rocket.
The observer on the rocket begins by claiming that the change in distance between the Earth and rocket is in fact the movement of the Earth and all the stars. This follows from the claim of the observer on the rocket that he is "anchored in place".

It might be questioned whether the stars move with the Earth, given that the distances between the rocket and the various stars have not been measured to verify that they are changing. However, it must be assumed that the stars move with the Earth, unless some cause for them to change their positions in relation to the Earth can be adduced.

What will the observer on the rocket consider as candidates for the cause of the movement of the Earth and stars?

It has been suggested that the choice of coordinates is the cause. This suggestion must be rejected, for the following reasons.

1. As noted above, the rocket coordinates may be chosen without resulting in the movement of the Earth and stars. Selection of the rocket coordinate system does not imply movement of the Earth and stars; therefore the selection of the rocket frame cannot be the cause of the movement of the Earth and stars.

2. A coordinate system is an abstraction, a mere convention used to identify a position in space. A coordinate system is not a physical entity, and therefore cannot be the cause of any physical phenomenon. (By the axiom.)

3. If in the rocket frame the selection of the coordinate system is the cause of the increasing distance between Earth and rocket, it must also be the cause in the Earth frame. (By the postulate.) No one has suggested that the choice of coordinates is the cause in the Earth frame.

4. If the selection of the coordinate system is the cause, then the firing of the rocket cannot be the cause in the Earth frame. (By the postulate.) But there is strong evidence that the firing of the rocket is the cause in the Earth frame.

The observer on the rocket will look for a physical cause. The only physical candidate is the firing of the rocket. There is strong evidence to support the candidate, and it satisfies both the axiom and the postulate.


It is now left to the rocket observer to derive the law of physics for the motion of the Earth and stars at the firing of the rocket...

Here I note that Einstein proposes a gravitational field which comes into existence when the rocket is fired. He suggests induction from the distant stars (and in the process rules out the coordinate systems as the cause):

To be sure, the accelerated coordinate systems cannot be called upon as real causes for the field, an opinion that a jocular critic saw fit to attribute to me on one occasion. But all the stars that are in the universe, can be conceived as taking part in bringing forth the gravitational field; because during the accelerated phases of the coordinate system K' they are accelerated relative to the latter and thereby can induce a gravitational field, similar to how electric charges in accelerated motion can induce an electric field.

http://en.wikisource.org/wiki/Dialog_about_objections_against_the_theory_of_relativity

In my view, a strong case can be made that such an induction cannot bring into being the required gravitational field because, as was pointed out by another, no causal signal can move faster than the speed of light.

 

[Dale]

The text in bold is false.

The text in bold is correct. You are confusing the rocket with a set of coordinates where the rocket is at rest. Or perhaps you are confusing the broad class of coordinate systems where the rocket is at rest with a specific choice of one such coordinate system.

The coordinates simply map events in spacetime to points in R4. If you use a mapping where the Chistoffel symbols at Earth are non zero then Earth accelerates, regardless of whether or not the rocket is firing it's engines or even whether or not the rocket exists. The choice of coordinates determines the Christoffel symbols and therefore the acceleration, not the rocket.

You are correct that there are multiple different coordinate systems which have legitimate claim to be the rockets coordinates. So the term is ambiguous, which is the reason I said "that specific choice of non inertial coordinates". Once you define it by identifying a specific mapping, then that mapping gives a unique prediction for Earth's motion.

The rest of your post is too long to digest. Please be more concise.

 

[PeterDonis]

Postulate: Whatever the physical cause, it must be the same for both observers. I think that this follows from the premise that there is one physical reality for all observers.

But it also requires you to define a "physical cause" as something that meets this requirement. See further comments below.

All other coordinate systems are derived in some way from his coordinate system. At this point in the discussion, it is a mistake to say that all coordinate systems are equally valid, for that is precisely the issue that is in question when discussing the validity of the principle of relativity.

Are we discussing the validity of the principle of relativity? I thought we were discussing how the (assumed to be valid) principle of relativity is applied to non-inertial observers. That is, I thought you were looking for "laws of physics" that could be applied by *any* observer who assumes himself to be at rest always. I didn't think you were questioning that such laws can exist.

What can be the cause is the firing of the rocket. The firing of the rocket is a physical phenomenon. The force is the direct result of the firing of the rocket, so the rest of the case made by the observer on the Earth is valid.

I don't necessarily disagree with this, but the only reason I can see for drawing a distinction between the firing of the rocket as "cause" and the force as "result" is that the force is frame-dependent; more precisely, the coordinate acceleration that results from the force is frame-dependent. In other words, you are maintaining that the firing of the rocket is an event that all observers must agree on, but the force is not.

But if there is a frame-independent way of measuring force, or acceleration, this argument fails. And there *is* a frame-independent way of measuring *proper* acceleration (but not coordinate acceleration), as you know. If that's the case, then on what basis do we distinguish the firing of the rocket as "cause" from the proper acceleration as "result"?

Note, again, that I'm not necessarily disagreeing; I'm pointing out what I think is a gap in your argument that needs to be filled. But this question also comes up in relation to "motion"; see further comments below.

However, it must be assumed that the stars move with the Earth, unless some cause for them to change their positions in relation to the Earth can be adduced.

The way you are stating this, along with the way you are stating your argument as a whole, presupposes that "motion" is something definite and absolute. It's not; it's frame-dependent. What you really should say here is that the stars and the Earth are not moving *relative to each other* (to the approximation we are working with here, anyway).

Similarly, before the rocket fires, Earth and the rocket are not moving *relative to each other*; but after the rocket fires, they are. It is the *relative* motion that needs to be explained; that is the thing that isn't frame-dependent. But your argument tries to explain the "motion of the rocket" or the "motion of the Earth", as if they were absolute. They're not.

In short, if you are going to take as an axiom that the cause must be "physical", then you should also take as an axiom that the *effect* must be physical as well. And since "physical", from the above, basically means "frame-independent", neither the motion of the Earth by itself, nor the motion of the rocket by itself, qualify as "effects" that need to be explained. Only the *relative* motion of the Earth and the rocket qualifies. The whole thing just boils down to: the firing of the rocket causes relative motion of the rocket and the Earth. That's all that's needed.

It is now left to the rocket observer to derive the law of physics for the motion of the Earth and stars at the firing of the rocket...

Same comment here: the laws of physics don't talk about the motion of the Earth, or the stars, or the rocket by themselves; they only talk about the *relative* motion of these things with respect to each other.

In my view, a strong case can be made that such an induction cannot bring into being the required gravitational field because, as was pointed out by another, no causal signal can move faster than the speed of light.

I'm not saying I necessarily prefer the "gravitational field" explanation, but the fact that causal signals can't move faster than light (which I brought up before) does not rule out "induction" as a source for the "gravitational field" Einstein talks about. Such an argument, if it were valid, would prove too much: it would prove that ordinary magnetic induction can't exist either. Obviously that's not true.

Consider the analogy with ordinary magnetic induction further. If you try to push a magnet that's in the field of another magnet, the first magnet feels an instant reaction force pushing back; it doesn't have to wait until a light signal has made a round trip to the other magnet. Why not? Because what causes the instant reaction force is not the field produced by the second magnet "right now", but the field produced by the second magnet one light-travel time ago. (For example, if the second magnet is 1 meter away, then the reaction force comes from the field emitted by the second magnet 3.3 nanoseconds ago.)

Similarly, if I am floating in free space and I fire a rocket, I feel a force, normally said to be due to "inertia". But it could also be attributed to the fact that I am in a "gravitational field" produced by the distant stars, just with a time delay; the contribution to the field from Alpha Centauri, say, is from Alpha Centauri as it was 4.3 years ago. The distant stars don't immediately feel any effect from my rocket firing, but I feel an immediate effect because the field at my location has already had plenty of time to propagate from the distant stars.