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Implications of the statement Acceleration is not relative

by GregAshmore
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GregAshmore
#289
Feb27-13, 07:31 PM
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Quote Quote by PeterDonis View Post
[]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.
We began by discussing the absoluteness of proper acceleration in the context of the twin paradox. We proceeded to discuss why certain objectors (the one in Taylor & Wheeler, and myself) believe that their objection has not been dealt with. I ended by saying that I am satisfied that the twin paradox is resolved with respect to kinematics, but I am not able to form an opinion with respect to dynamics. So, no, I do not assume that relativity is valid. Specifically, I do not assume that the non-inertial observer in the rocket can legitimately claim that he is at rest throughout. That must be proven.

So what makes a claim to be at rest "legitimate"? If the only requirement is the development of one set of equations that can be used by all at-rest observers to correctly predict the behavior of physical systems, then I don't think I have any quarrel with relativity. (I'm past worrying about whether time can vary with velocity.) But if the requirement is that the observed universe be "real" to every at-rest observer, then I'm not so sure.

As I read the posts in this thread regarding what the observer in the resting rocket observes, I saw the word "fictitious" many times. I'm fine with fictitious quantities if they allow me to correctly calculate what is going to happen. (I'm an engineer, not a theoretician.) But if the proceedings seen by the observer in the resting rocket are fictitious, I am not comfortable affirming the statement that all coordinate systems are equally valid. It seems to me that a coordinate system that has the stars violating a law of physics--traveling faster than light speed--has a lesser validity than a coordinate system that has the stars behaving within the laws of physics. The two coordinate systems may be equally useful, depending on circumstances, but they are not equally valid: one of them is telling a lie about the stars.

Quote Quote by PeterDonis View Post
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.
I don't know how proper acceleration is calculated. If the calculation has the rocket moving, then the observer in the at-rest rocket will dispute the definition. His claim is that he is absolutely at rest, as explained below.


Quote Quote by PeterDonis View Post
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.
It is claimed that motions are not absolute. That must be proven. You know it is true. I do not. I am not required to accept the premise until it is proven. Indeed, I ought not to accept it until it is proven. I am attempting to prove the premise.

The premise is that every observer may legitimately consider himself to be at rest. For the premise to have any meaning at all, "at rest" must mean "absolutely at rest". (Einstein says, "permanently at rest".) Every observer develops the laws of physics on the assumption that he is at rest in absolute space.

If all such observers are able to agree on one set of physical laws ("of the same form"), then the premise that motion is not absolute is proven to be true. If all observers are absolutely at rest, none are. But if just one observer can show that in his circumstances the laws of physics are unique, then the premise is falsified.

Quote Quote by PeterDonis View Post
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"
Your definition of physical assumes the truth of the premise that is to be proven.

Quote Quote by PeterDonis View Post
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.
I disagree. The task is for every resting observer to explain what he sees, expressed as laws of physics. The observer in the resting rocket sees the Earth moving by itself; that is an effect which must be explained.

Quote Quote by PeterDonis View Post
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.
Not so. The principle of relativity requires the rocket observer to explain how it is that the Earth moves while he remains at rest.

Quote Quote by PeterDonis View Post
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.
No again. The principle of relativity requires that the rocket be at rest, which implies that the Earth is moving by itself. The laws of physics must explain the motion of the Earth by itself, and the rocket by itself, if the principle of relativity is true.

If all we need to do is explain the relative motion of the rocket and Earth, I'm good with the force applied to the rocket. There is no reason at all to deal with the headache of induced gravitational fields in flat spacetime. But if the rocket must be allowed to rest, then the Earth must be moving by itself, and the headache must be endured.


Quote Quote by PeterDonis View Post
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.
Einstein says that "a gravitational field appears" when the rocket is fired. Then, "when the clock U1 has reached velocity v the gravitational field disappears."
PeterDonis
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Feb27-13, 08:20 PM
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Quote Quote by GregAshmore View Post
I do not assume that the non-inertial observer in the rocket can legitimately claim that he is at rest throughout. That must be proven.
Do you assume that an *inertial* observer can legitimately claim that he is at rest? If so, what's the difference? What makes an inertial observer special?

Quote Quote by GregAshmore View Post
So what makes a claim to be at rest "legitimate"?
To me the answer is "mu": the question itself presupposes that "at rest" has some absolute meaning. It doesn't; "at rest" is relative. That means the only requirement is indeed this:

Quote Quote by GregAshmore View Post
If the only requirement is the development of one set of equations that can be used by all at-rest observers to correctly predict the behavior of physical systems, then I don't think I have any quarrel with relativity.
Quote Quote by GregAshmore View Post
if the requirement is that the observed universe be "real" to every at-rest observer, then I'm not so sure.
"Real" is too vague a term to be useful here, IMO. Perhaps the term "fictitious" has given a wrong impression:

Quote Quote by GregAshmore View Post
I'm fine with fictitious quantities if they allow me to correctly calculate what is going to happen. (I'm an engineer, not a theoretician.) But if the proceedings seen by the observer in the resting rocket are fictitious, I am not comfortable affirming the statement that all coordinate systems are equally valid.
Events are real to all observers. Does that help? For example, if two twins meet up and find that one's clock has less elapsed time than the other's, that's not a "fictitious proceeding". But if one twin says "well, your clock had more elapsed time because you were higher up than me in a gravitational field while my rocket was firing", the gravitational field could be termed "fictitious". But that's more a matter of terminology or interpretation than physics; the traveling twin wants to interpret everything in his "rest frame", but that frame is non-inertial, so physics doesn't look as simple. "Gravitational field" is just a label he puts on the lack of simplicity; but the lack of simplicity is there because of the coordinates he chose. Nothing forces him to use coordinates in which he is always at rest.

Quote Quote by GregAshmore View Post
It seems to me that a coordinate system that has the stars violating a law of physics--traveling faster than light speed--has a lesser validity than a coordinate system that has the stars behaving within the laws of physics.
The stars aren't violating any laws of physics. The actual law is not "things can't travel faster than light"; it is "things can't move outside the local light cones". All of the stars' worldlines are within their local light cones.

Once again, you appear to want to have it both ways; you want the "laws of physics" to look simple, but you want to be able to choose any coordinates you like. You can't have both of those things.

Quote Quote by GregAshmore View Post
I don't know how proper acceleration is calculated.
You calculate the path curvature of the rocket's worldline. That can be done in any coordinates, including ones in which the rocket is at rest.

Quote Quote by GregAshmore View Post
It is claimed that motions are not absolute. That must be proven.
How do you want it proven? It has already been shown that "motion" depends on the coordinates you choose. What more do you need?

Quote Quote by GregAshmore View Post
The premise is that every observer may legitimately consider himself to be at rest. For the premise to have any meaning at all, "at rest" must mean "absolutely at rest".
I disagree; you can't call anything "absolute" if it depends on the coordinates you adopt.

Quote Quote by GregAshmore View Post
Every observer develops the laws of physics on the assumption that he is at rest in absolute space.
Really? When you're driving your car, do you intuit its physics based on the assumption that you are at rest in absolute space and everything else is moving? If you do, you're pretty unusual; most people talk about "going somewhere" in their car (or walking or bicycling or any other way, for that matter), not "making the grocery store come to me using my car".

Quote Quote by GregAshmore View Post
If all such observers are able to agree on one set of physical laws ("of the same form"), then the premise that motion is not absolute is proven to be true.
Then it's proven; GR provides just such a set of laws. But in some coordinates, a bunch of the terms in the equations become zero, so the laws look simpler in those coordinates.

Quote Quote by GregAshmore View Post
Your definition of physical assumes the truth of the premise that is to be proven.
Then give an alternative definition that doesn't make any assumptions relevant to the argument.

Quote Quote by GregAshmore View Post
The task is for every resting observer to explain what he sees, expressed as laws of physics.
I agree with this. But that doesn't imply this:

Quote Quote by GregAshmore View Post
The observer in the resting rocket sees the Earth moving by itself; that is an effect which must be explained.
No, the observer in the rocket sees the Earth moving *relative to him*. That's how the laws work. You don't get to declare by fiat that the laws *must* take a certain form, or *must* deal with "absolute rest" or "absolute motion". You have to find out whether they do by finding out what the laws are. It turns out that the actual laws--the laws of GR--do *not* talk about absolute rest or absolute motion; they talk only about relative rest and relative motion. If you want laws that talk about absolute rest and absolute motion, you're going to be disappointed, because there aren't any.

Quote Quote by GregAshmore View Post
The laws of physics must explain the motion of the Earth by itself, and the rocket by itself, if the principle of relativity is true.
No, the principle of relativity says that *relative* motion is what matters; it says that there is no such thing as absolute motion or absolute rest.

Quote Quote by GregAshmore View Post
Einstein says that "a gravitational field appears" when the rocket is fired. Then, "when the clock U1 has reached velocity v the gravitational field disappears."
Yes, but he's using the term "gravitational field" in different ways (possibly without realizing it). When he talks about the field being produced by "induction", there *has* to be a time delay involved; but that means there has to be *something* propagating even when the rocket is not firing. That something is the "gravitational field" that is produced by "induction", with the distant stars as the source. When he talks about the field appearing and disappearing, he's using "gravitational field" to mean the force that is felt only when the rocket is firing; but the "field" (the underlying whatever-it-is that produces the induction effect) is there whether or not the rocket is firing.
DaleSpam
#291
Feb27-13, 08:22 PM
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Quote Quote by GregAshmore View Post
Does this mean that we can put an engine-less pod in space (without gravity due to mass, per the scenario) and then select the appropriate mapping, at will and as needed, to accelerate the Earth and stars until the star of our choice meets up with the pod?
The meeting of worldlines is a frame-invariant geometric fact which cannot be changed through a choice of coordinates.
PAllen
#292
Feb27-13, 09:22 PM
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Quote Quote by GregAshmore View Post
Specifically, I do not assume that the non-inertial observer in the rocket can legitimately claim that he is at rest throughout. That must be proven.
What do you think of the following:
- The rocket cannot consider themselves inertial. This means that the simplest form of laws of physics cannot be used. A more complex way of expressing laws, that is also true (by natural vanishing of extra terms) for inertial motion, can be used. Thus, if you choose the more complex expression, laws are the same for all motion; however, this in no way changes that inertial and non-inertial motions are in-equivalent.
- The rocket is clearly at rest relative relative to itself. There is no escaping this, so it is clearly a legitimate thing to recognize.

Quote Quote by GregAshmore View Post
....
But if the requirement is that the observed universe be "real" to every at-rest observer, then I'm not so sure.
What can you possibly mean by this? How can the universe become less real because you are in a rocket firing thrust? I assume this isn't really what you mean, but I am at a loss for what you possibly could mean.
Quote Quote by GregAshmore View Post
As I read the posts in this thread regarding what the observer in the resting rocket observes, I saw the word "fictitious" many times. I'm fine with fictitious quantities if they allow me to correctly calculate what is going to happen. (I'm an engineer, not a theoretician.) But if the proceedings seen by the observer in the resting rocket are fictitious, I am not comfortable affirming the statement that all coordinate systems are equally valid. It seems to me that a coordinate system that has the stars violating a law of physics--traveling faster than light speed--has a lesser validity than a coordinate system that has the stars behaving within the laws of physics. The two coordinate systems may be equally useful, depending on circumstances, but they are not equally valid: one of them is telling a lie about the stars.
I can sympathize with this. Normally, you do not picture that distant mountains move rapidly when you turn your head. However, what would lead to a real problem is trying to apply the simplest form of laws to the 'turning head' frame. The simplest form of laws (only valid in inertial frames) says no matter can travel faster than the speed c. In the turning head frame, there are no limits on coordinate speed; but this law remains in a different way: no matter catches a pulse of light.
Quote Quote by GregAshmore View Post

I don't know how proper acceleration is calculated. If the calculation has the rocket moving, then the observer in the at-rest rocket will dispute the definition. His claim is that he is absolutely at rest, as explained below.
Proper acceleration calculation has been explained a few times in this thread. I will not repeat. However, I stress that proper acceleration can easily be non-zero for an observer at rest in non-inertial coordinates. As I tried to express it above, the rocket is indisputably at rest relative to itself. However, it is also indisputably non-inertial, which allows proper acceleration to be nonzero for an observer at coordinate rest.

I am not going to address the rest of your post because I am curious to your reaction to the above, first.
stevendaryl
#293
Feb27-13, 09:48 PM
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Quote Quote by GregAshmore View Post
As I read the posts in this thread regarding what the observer in the resting rocket observes, I saw the word "fictitious" many times. I'm fine with fictitious quantities if they allow me to correctly calculate what is going to happen. (I'm an engineer, not a theoretician.) But if the proceedings seen by the observer in the resting rocket are fictitious, I am not comfortable affirming the statement that all coordinate systems are equally valid. It seems to me that a coordinate system that has the stars violating a law of physics--traveling faster than light speed--has a lesser validity than a coordinate system that has the stars behaving within the laws of physics. The two coordinate systems may be equally useful, depending on circumstances, but they are not equally valid: one of them is telling a lie about the stars.
The laws of physics are not violated by noninertial coordinates, it's just that they have a different form when they are expressed in noninertial coordinates.

For example, Newton's laws of motion, when described using inertial Cartesian coordinates x and y, look like this:

[itex]\dfrac{d^2 x}{dt^2} = \dfrac{1}{m} F^x[/itex]
[itex]\dfrac{d^2 y}{dt^2} = \dfrac{1}{m} F^y[/itex]

If we change to a new coordinate system
[itex]r = \sqrt{x^2 + y^2}[/itex]
[itex]\theta = arctan(\dfrac{y}{x})[/itex]

then the same equations of motion look like this:

[itex]\dfrac{d^2 r}{dt^2} - r (\dfrac{d\theta}{dt})^2 = \dfrac{1}{m} F^{r}[/itex]
[itex]\dfrac{d^2 \theta}{dt^2} + \dfrac{2}{r} \dfrac{dr}{dt} \dfrac{d \theta}{dt} = \dfrac{1}{m} F^{\theta}[/itex]

They're the same laws of motion, except written in different coordinates. The form of the laws change in different coordinates, but the physical content does not.

Similarly, the rule of light-speed is, in differential form: If an object travels a distance [itex]\delta x[/itex] in time [itex]\delta t[/itex], then

[itex](c \delta t)^2 - (\delta x)^2 \geq 0[/itex]

That's what the law looks like in Cartesian coordinates. In general coordinates, the same law looks like this:

[itex]g_{\mu \nu}\ \delta x^\mu\ \delta x^\nu \geq 0[/itex] (summed over all indices [itex]\mu[/itex] and [itex]\nu[/itex])

where [itex]g_{\mu \nu}[/itex] are the components of the metric tensor in the new coordinate system. In an inertial Cartesian coordinate system, the metric tensor has the simple form

[itex]g_{tt} = c^2[/itex]
[itex]g_{xx} = g_{yy} = g_{zz} = -1[/itex]
(with all other components zero).

The laws of physics look different in noninertial or curvilinear coordinates, but they have the same physical content.
harrylin
#294
Feb28-13, 12:55 AM
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Quote Quote by nitsuj View Post
lol that's funny.

It's little tough for me to accept Dalespam's wording. But imo it's accurate.

From which perspective is physics described? I would say it is from the perspective of modeling. [..]
Somewhat yes; but a physicist never looses sight of the things that he is modelling - in the context of the topic here Einstein also didn't. Never confound physical entities with their mathematical representation.
DaleSpam
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Feb28-13, 06:43 AM
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Quote Quote by harrylin View Post
Somewhat yes; but a physicist never looses sight of the things that he is modelling - in the context of the topic here Einstein also didn't. Never confound physical entities with their mathematical representation.
Nobody here is doing that either.
A.T.
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Feb28-13, 07:35 AM
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Quote Quote by GregAshmore View Post
The two coordinate systems may be equally useful, depending on circumstances, but they are not equally valid: one of them is telling a lie about the stars.
You've got it backwards. Frames are preferred based on usefulness, not because some are more valid. For example, if you find it useful to use conservation of momentum, you prefer to use inertial frames, where conservation of momentum applies. But if you don't need conservation of momentum for your calculations, you might prefer some non-inertial frame. The fact that total momentum is not conserved in the non-inertial frame doesn’t make it less valid, just less practical in some cases. Nobody is lying here. Velocity is simply frame dependent.
GregAshmore
#297
Feb28-13, 05:43 PM
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Quote Quote by DaleSpam View Post
The meeting of worldlines is a frame-invariant geometric fact which cannot be changed through a choice of coordinates.
I'd like a yes or no answer.

After the resting rocket twin fires his engine and sees the Earth accelerate away, he eventually sees his target star approach. He can measure the distance to the star at intervals and verify that it is indeed getting closer. You have said repeatedly that the motion of the Earth is caused by the choice of coordinates, independent of the firing of the rocket. I give you the very same scenario, except without an engine in the rocket. Can you make the very same events happen? Can you cause the Earth and stars to move by selecting a certain coordinate system?
PeterDonis
#298
Feb28-13, 07:44 PM
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Quote Quote by GregAshmore View Post
I'd like a yes or no answer.
But you've asked two different questions. This actually illustrates well the difference between "motion" and relative motion.

Quote Quote by GregAshmore View Post
I give you the very same scenario, except without an engine in the rocket. Can you make the very same events happen?
This is question 1. The answer is no: you can't make the very same events happen because you can't change the relative motion of the Earth and the rocket (or the target star and the rocket) without a rocket engine. (We're assuming no other possibilities, i.e., no gravity, no aliens with tractor beams, etc. )

Quote Quote by GregAshmore View Post
Can you cause the Earth and stars to move by selecting a certain coordinate system?
This is question 2, and is *not* the same as question 1. The answer is yes, of course; just take the coordinate system in which the Earth, the rocket, and the target star are all at rest (since there's no rocket engine, they will always be at rest in this coordinate system), and boost it in some random direction. You now have a coordinate system in which all three are moving, at the same velocity.

What you can't do is get them moving at *different* velocities just by changing coordinate systems. But nobody was claiming that you could; that is, nobody was claiming that you could change their *relative* velocities just by changing coordinates. That's what takes the rocket engine; which is why relative motion is "physical" in a way that "motion" by itself is not.
Mentz114
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Feb28-13, 07:44 PM
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Can you cause the Earth and stars to move by selecting a certain coordinate system?
Yes. Any observer moving relative to the earth and stars will see them moving relative to himself. Velocity is relative.
GregAshmore
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Feb28-13, 07:46 PM
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Quote Quote by PeterDonis View Post
Do you assume that an *inertial* observer can legitimately claim that he is at rest? If so, what's the difference? What makes an inertial observer special?
I don't assume anything with regard to which kind of observer can consider himself at rest. The concept was never of more than passing interest until I read Einstein's book Relativity. I didn't have any pre-conceived ideas going in.

Quote Quote by PeterDonis View Post
To me the answer is "mu": the question itself presupposes that "at rest" has some absolute meaning. It doesn't; "at rest" is relative. That means the only requirement is indeed this:
I don't think that is true, as a matter of logic. "At rest" must have an absolute meaning for the observer who claims it. Einstein's stated goal was to show that acceleration does not have any absolute quality. Absolute acceleration (as Einstein used the term acceleration in the book referenced above) implies absolute motion. The charge is, "You are accelerating; therefore I am certain that you are in absolute motion." Einstein counters the charge with, "No. I am permanently at rest." For that statement to have any effectiveness against the charge of absolute motion, the rest spoken of must be absolute. If the rest is not absolute, then the observer must admit that he is moving, or at least might be moving. The observer must believe that he is absolutely at rest; he must evaluate all the evidence on the presumption that he is absolutely at rest. You will note that I have not said that any observer is absolutely at rest in actual fact, only that he evaluates what he observes on that basis. So for the resting rocket twin, the Earth moves by itself; the Earth moves absolutely.

I'm going to skip replying to some of your responses. I need more time, and probably more study, to give a good answer.


Quote Quote by PeterDonis View Post
Events are real to all observers. Does that help? For example, if two twins meet up and find that one's clock has less elapsed time than the other's, that's not a "fictitious proceeding". But if one twin says "well, your clock had more elapsed time because you were higher up than me in a gravitational field while my rocket was firing", the gravitational field could be termed "fictitious". But that's more a matter of terminology or interpretation than physics; the traveling twin wants to interpret everything in his "rest frame", but that frame is non-inertial, so physics doesn't look as simple. "Gravitational field" is just a label he puts on the lack of simplicity; but the lack of simplicity is there because of the coordinates he chose. Nothing forces him to use coordinates in which he is always at rest.
The difference in elapsed clock times is not the issue--not any longer. The issue is whether the rocket twin is moving in some absolute sense, or can legitimately claim to be at rest.

As to why the rocket twin insists on claiming that he is at rest, I guess you'ld have to ask Einstein. I certainly never thought to make an issue of it until I read his book.


Quote Quote by PeterDonis View Post
Once again, you appear to want to have it both ways; you want the "laws of physics" to look simple, but you want to be able to choose any coordinates you like. You can't have both of those things.
Again, the issue for me is not the simplicity or complexity of the equations. The issue is whether the claim to be at rest makes physical sense. See the end of this post.

Quote Quote by PeterDonis View Post
You calculate the path curvature of the rocket's worldline. That can be done in any coordinates, including ones in which the rocket is at rest.
Ok. I don't recall seeing this definition before. If it was in this thread, I missed it.


Quote Quote by PeterDonis View Post
Really? When you're driving your car, do you intuit its physics based on the assumption that you are at rest in absolute space and everything else is moving? If you do, you're pretty unusual; most people talk about "going somewhere" in their car (or walking or bicycling or any other way, for that matter), not "making the grocery store come to me using my car".
In the context, I thought it was clear that I meant every resting observer.


Quote Quote by PeterDonis View Post
Yes, but he's using the term "gravitational field" in different ways (possibly without realizing it). When he talks about the field being produced by "induction", there *has* to be a time delay involved; but that means there has to be *something* propagating even when the rocket is not firing. That something is the "gravitational field" that is produced by "induction", with the distant stars as the source. When he talks about the field appearing and disappearing, he's using "gravitational field" to mean the force that is felt only when the rocket is firing; but the "field" (the underlying whatever-it-is that produces the induction effect) is there whether or not the rocket is firing.
Prompted by the above, I read this again:
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.
This is consistent with the idea that the field is present all the time. But there seems to be a causal conundrum here. The effect of the field is not felt until the Earth accelerates. But the Earth only accelerates due to the effect of the field.

If no plausible explanation for the workings of the gravitational field can be given, the field must be considered a pure fiction; ad-hoc hand-waving. In that case, the absoluteness of acceleration is not removed, at least with respect to SR.

Personally, I have no stake in the argument. I don't care if there is an absoluteness to acceleration. But the issue was important to Einstein, and having come this far, I'd like to be able to evaluate whether he succeeded in eliminating the problem.
Mentz114
#301
Feb28-13, 07:59 PM
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Quote Quote by GregAshmore
If no plausible explanation for the workings of the gravitational field can be given, the field must be considered a pure fiction; ad-hoc hand-waving. In that case, the absoluteness of acceleration is not removed, at least with respect to SR.
Gravity produces coordinate acceleration but rockets produce proper acceleration. The field that Einstein conjures up produces the first kind - which is relative. But, as you have been told, proper acceleration is not coordinate dependent and cannot be made to disappear or appear by a change of coordinates. Proper acceleration is absolute in this sense.

Personally, I have no stake in the argument. I don't care if there is an absoluteness to acceleration. But the issue was important to Einstein, and having come this far, I'd like to be able to evaluate whether he succeeded in eliminating the problem.
Again, it's been said before, Einstein does not make it clear which kind he refers to so he has not succeeded, in my opinion.
PeterDonis
#302
Feb28-13, 08:09 PM
Physics
Sci Advisor
PF Gold
P: 6,149
Quote Quote by GregAshmore View Post
I don't assume anything with regard to which kind of observer can consider himself at rest.
Then we're missing a big piece of the puzzle, because we haven't established, have we, that the principle of relativity applies to *inertial* observers? If we haven't even done that, how can we do it for non-inertial observers?

Quote Quote by GregAshmore View Post
I don't think that is true, as a matter of logic. "At rest" must have an absolute meaning for the observer who claims it.
How can it be absolute if it's only true for the observer who claims it? Doesn't that make it relative? Isn't that the whole *point* of making the distinction between "relative" and "absolute"?

Quote Quote by GregAshmore View Post
Einstein's stated goal was to show that acceleration does not have any absolute quality.
Just like "motion" and "rest". That was his whole point. He wasn't trying to argue that motion or rest were absolute. If you think he was arguing that, you have seriously misunderstood his point.

Quote Quote by GregAshmore View Post
The charge is, "You are accelerating; therefore I am certain that you are in absolute motion." Einstein counters the charge with, "No. I am permanently at rest."
No, you're misunderstanding what he said. A better statement of his counter is: "No. There is no such thing as absolute motion. Acceleration, like motion and rest, is relative." Then he investigates what it takes to consistently maintain such a position, and finds out that to do so, we must also accept that a "gravitational field" (in the sense of "acceleration due to gravity"--mathematically, as DaleSpam pointed out, this corresponds to Christoffel symbols, first derivatives of the metric) is relative; it can be present in some frames and absent in others.

Quote Quote by GregAshmore View Post
In the context, I thought it was clear that I meant every resting observer.
Huh? The position you have been taking is that *every* observer can consider himself to be at rest, so "resting observer" just means any observer whatsoever. *I* can draw distinctions like the one I drew in what you quoted--nobody in real life considers himself to be "at rest" and the grocery store to be moving--because I'm arguing that relative motion is what's important anyway. But how can *you* draw such distinctions without undermining your whole position?

Quote Quote by GregAshmore View Post
The effect of the field is not felt until the Earth accelerates. But the Earth only accelerates due to the effect of the field.
No, the effect of the field is always there. When the rocket engine is off, the rocket and the person inside are free-falling in the field along with the Earth. When the rocket engine fires, it holds the rocket and the person inside at rest in the field so they can't free-fall with the Earth. So the Earth appears to accelerate "downward". But the field is always there; firing the rocket engine just prevents the rocket from free-falling in it.
GregAshmore
#303
Feb28-13, 08:11 PM
P: 221
Quote Quote by PAllen View Post
What do you think of the following:
My reply will have to wait until tomorrow night.
DaleSpam
#304
Feb28-13, 08:48 PM
Mentor
P: 17,306
Quote Quote by GregAshmore View Post
I'd like a yes or no answer.
OK

Quote Quote by GregAshmore View Post
Does this mean that we can put an engine-less pod in space (without gravity due to mass, per the scenario) and then select the appropriate mapping, at will and as needed, to accelerate the Earth and stars until the star of our choice meets up with the pod?
No.

Quote Quote by GregAshmore View Post
After the resting rocket twin fires his engine and sees the Earth accelerate away, he eventually sees his target star approach. He can measure the distance to the star at intervals and verify that it is indeed getting closer. You have said repeatedly that the motion of the Earth is caused by the choice of coordinates, independent of the firing of the rocket. I give you the very same scenario, except without an engine in the rocket. Can you make the very same events happen?
No.

Quote Quote by GregAshmore View Post
Can you cause the Earth and stars to move by selecting a certain coordinate system?
Yes.
A.T.
#305
Mar1-13, 04:04 AM
P: 4,053
Quote Quote by GregAshmore View Post
If no plausible explanation for the workings of the gravitational field can be given, the field must be considered a pure fiction
All of physics is that kind of "fiction". But it is "fiction" that describes nature quantitatively and allows predictions. Newton didn't explain the workings of his gravitational field either. He just quantified it, and this "fiction" still works great for most purposes.
GregAshmore
#306
Mar1-13, 05:29 PM
P: 221
Quote Quote by stevendaryl View Post
The laws of physics are not violated by noninertial coordinates, it's just that they have a different form when they are expressed in noninertial coordinates.

For example, Newton's laws of motion, when described using inertial Cartesian coordinates x and y, look like this:

[itex]\dfrac{d^2 x}{dt^2} = \dfrac{1}{m} F^x[/itex]
[itex]\dfrac{d^2 y}{dt^2} = \dfrac{1}{m} F^y[/itex]

If we change to a new coordinate system
[itex]r = \sqrt{x^2 + y^2}[/itex]
[itex]\theta = arctan(\dfrac{y}{x})[/itex]

then the same equations of motion look like this:

[itex]\dfrac{d^2 r}{dt^2} - r (\dfrac{d\theta}{dt})^2 = \dfrac{1}{m} F^{r}[/itex]
[itex]\dfrac{d^2 \theta}{dt^2} + \dfrac{2}{r} \dfrac{dr}{dt} \dfrac{d \theta}{dt} = \dfrac{1}{m} F^{\theta}[/itex]

They're the same laws of motion, except written in different coordinates. The form of the laws change in different coordinates, but the physical content does not.
True, the physical content does not change in this example.

Quote Quote by stevendaryl View Post
The laws of physics look different in noninertial or curvilinear coordinates, but they have the same physical content.
This is not true in the case of the resting rocket. In that coordinate system, no force acts on the Earth, yet it accelerates. And, an unbalanced force acts on the rocket, yet it does not accelerate. Both of these phenomena violate Newton's law as quoted above. Additional physical content is proposed as an explanation: a gravitational field. Additional term(s) are needed in the equation to express the behavior of the additional physical content. I believe (but can't say for sure) that the equation presented by DaleSpam in #278 corresponds to the law of Newton quoted above, with the addition of Christoffel symbols to account for the gravitational field.


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