Speed of the light and dilation of time

[Austin0]

In Michael Fontenot's CADO paper, he first discusses the "standard twin paradox":

I couldn't find where he explained why the velocity takes on a negative value at the turn-around point but GrammawSally's explanation of the dot product makes it automatic because the velocity vector points in the opposite direction so it isn't merely "a simple product of values".

the velocity takes on a negative value because of direction.
Relative clock desynchronization is direction dependent. The clocks ahead in the other frame are running ahead,
and vise versa. So as the traveler is moving away from the inertial position the clock at that position is running behind , so the vL is subtracted . After turnaround the inertial position is now ahead so the vL is added -(-vL)
As that is standard convention I didn't mention it .Other than the change of sign it is just a simple product.

Yes, she made that very clear. They're all from the inertial frame. If Michael ever says the same thing, it is not clear at all, partly because he is doing the same calculation for the "standard twin paradox" in the conventional way from the frame of the home twin where he introduces gamma but as fall as I can tell, it is never used in his CADO equation. I could never tell whether he was discussing the home twin's frame just for the conventional explanation or also for the CADO explanation.

In the linear case you can simply take measurements from the traveler frame and transform them to inertial values.

In the circular situation, using your approach, it might not be so simple. It seems like contraction of the circle would complicate the geometry.if done from the traveler frame

Yes. You then have to take the difference between the two vector angles to use in the dot product which simply means you multiply the magnitudes of the two vectors together and then multiply by the cosine of the delta angle.

I am starting to get this. Did you use a constant velocity magnitude or did that also vary with angle relative to the inertial observer?

I don't know why you think this is a different approach. She is simply providing some background to the setup of the scenario and then pointing out why you cannot just multiply L and v together (as Michael Fontenot implies, if not directly states, that you can) but rather why you need to treat L and v as vectors and use the dot product.

I said two approaches because in her initial post she clearly said calculated from the traveling frame. As I said I am quite sure Mike is right for linear motion.
It is just the circular path and this approach that prevents the simple product from applying
Haven't you ever calculated explicit value for a line of simultaneity intersecting some point?

Hopefully this has helped but if you need more clarification, just ask.

yes it has helped ,, thank you

 

[Austin0]

Austin0:

In Euclidean geometry, dot product of two vectors is simply product of their lengths with cosine of angle between them. If two vectors are parallel, it is thus the product of their lengths; if perpendicular, it is zero. More generally (in non-Euclidean geometry, e.g. for 4-vectors in SR spacetime), it is contraction of both vectors with the metric tensor. The CADO website specifies that L, v, and CADO_H are all as measured by the distant inertial observer. The CADO website requires only the Euclidean definition.

It is starting to sink in ,,, Thanks

Yes they can be measured by the inertial observer but CADO_T=gamma dt (traveler) -gamma (v times dx(traveler)) works just the same in the linear case.

 

[ghwellsjr]

yes it has helped ,, thank you

Your method of replying makes it impossible for me to automatically grab your quotes so I'll just respond to your post by copying and pasting:

I'm rapidly coming to the conclusion that GrammawSally's method using the dot product is not Michael Fontenot's scheme. It just happens to work in this case (and in the linear "standard twin paradox" case) because the father is traveling at a constant speed in the son's frame so there's always a linear relationship between the son's age (or time) and the father's age (or time) which is the factor gamma.

the velocity takes on a negative value because of direction.
Relative clock desynchronization is direction dependent. The clocks ahead in the other frame are running ahead,
and vise versa. So as the traveler is moving away from the inertial position the clock at that position is running behind , so the vL is subtracted . After turnaround the inertial position is now ahead so the vL is added -(-vL)
As that is standard convention I didn't mention it .Other than the change of sign it is just a simple product.

This "explanation" works fine for the "standard twin paradox" case since the traveler is always moving directly away from or towards the inertial observer but what does Michael expect us to do when the direction is at an angle?

In the linear case you can simply take measurements from the traveler frame and transform them to inertial values.

In the circular situation, using your approach, it might not be so simple. It seems like contraction of the circle would complicate the geometry.if done from the traveler frame

Yes, this is why I now think the dot product scheme is not the CADO scheme.

I am starting to get this. Did you use a constant velocity magnitude or did that also vary with angle relative to the inertial observer?

Yes, because in the son's frame, the father is traveling at a constant speed just under the speed of light. I didn't bother to use the actual speed of 0.9999999999c, I just used 1. It will make no noticeable difference in a plot and I could not calculate to the required degree of precision to show a difference anyway.

I said two approaches because in her initial post she clearly said calculated from the traveling frame. As I said I am quite sure Mike is right for linear motion.
It is just the circular path and this approach that prevents the simple product from applying

I'm not sure that she ever implied that she was using the traveling frame but maybe we'll have to await her response. However, as I mentioned earlier, I don't think her approach is the same as the CADO scheme except in those cases where the speed of the traveler is constant.

Haven't you ever calculated explicit value for a line of simultaneity intersecting some point?

No. I don't bother with spacetime diagrams if that is what you are referring to. I just specify a scenario in terms of its co-ordinates. If the t co-ordinates for two events are the same, then they are simultaneous in that frame. If I want to see the what the co-ordinates look like in a different frame, I use the Lorentz Transformation.

 

[GrammawSally]

I'm rapidly coming to the conclusion that GrammawSally's method using the dot product is not Michael Fontenot's scheme.
[...]
this is why I now think the dot product scheme is not the CADO scheme.

They are the same. The Physics Essays paper gives the generalization of the CADO equation to two or three spatial dimensions. For the more familiar one dimensional twin paradox scenarios, the generalized CADO equation (with the dot product, and vectors for v and L) still works, but the paper and the webpage simplify the wording of the CADO equation in that case, probably just to make it simpler for non-physicists to understand and to use, and also to eliminate the need to specify the details of the spatial axis definitions for the typical simple twin paradox one dimensional problems.

(I've been spending a lot of time lately slowly working my way through that paper, so I'm more familiar with it than with the webpage. When I earlier stated the generalization of the CADO equation to two (or three) dimensions, I was assuming that it was also described on the webpage, but maybe it's not included there.)

I'm not sure that she ever implied that she was using the traveling frame but maybe we'll have to await her response.

All of the quantities on the right-hand-side of the CADO equation are according to the inertial person (because they are relatively easy to determine, and are widely understood). Once those quantities are known (for any given age of the traveler), the CADO equation then allows CADO_T (the current age of the inertial person, according to the traveler) to be easily calculated.

 

[ghwellsjr]

All of the quantities on the right-hand-side of the CADO equation are according to the inertial person (because they are relatively easy to determine, and are widely understood). Once those quantities are known (for any given age of the traveler), the CADO equation then allows CADO_T (the current age of the inertial person, according to the traveler) to be easily calculated.

Yes, but unless there is a constant time relationship between them, such as in this case, I don't understand how it works.

By the way, I'm still if the CADO scheme as depicted in my plot is what you meant when you said:

A person who is accelerating is still entitled to his own "point of view" about how other people are aging.

 

[Russell E]

They are the same. The Physics Essays paper gives the generalization of the CADO equation to two or three spatial dimensions.

It's a bit silly to call this the "CADO equation", and to attribute it to Mr. Fontenot. It is very standard and well known. See for example the discussion toward the end of this web page:

http://mathpages.com/rr/s2-09/2-09.htm

 

[ghwellsjr]

It's a bit silly to call this the "CADO equation", and to attribute it to Mr. Fontenot. It is very standard and well known. See for example the discussion toward the end of this web page:

http://mathpages.com/rr/s2-09/2-09.htm

I didn't see anything on that webpage that looked like the CADO equation:

CADO_T = CADO_H - v * L

Can you point out exactly where that is?

 

[Russell E]

I didn't see anything on that webpage that looked like the CADO equation:
CADO_T = CADO_H - v * L
Can you point out exactly where that is?

Sure. The formula following the words "so the above formula reduces to".

 

[ghwellsjr]

I didn't see anything on that webpage that looked like the CADO equation:
CADO_T = CADO_H - v * L
Can you point out exactly where that is?

Sure. The formula following the words "so the above formula reduces to".

The formula from your paper is:

τ₂ = t₁ - r • v

It superficially looks similar to GrammawSally's interpretation of the CADO formula (r replacing L and taking the dot product) but it doesn't look the same beyond that.

The commentary associated with the formula expresses to me that t₁ is the time of the traveler (the father) while τ₂ is the time of the inertial observer (the son). Did I get that wrong?

But in the CADO formula, the two variables that are multiplied have the same meaning as your formula but CADO_H is the inertial observer's time and "CADO_T denotes the traveler's conclusion about the home-twin's age" (or the time for the inertial observer). So in CADO, both times are for the inertial observer, it's just that they are the two different observer's opinion of that time, whereas in your paper, each time is for a different observer. If this isn't right, please point out where the errors are.

 

[Russell E]

The commentary associated with the formula expresses to me that t₁ is the time of the traveler (the father) while τ₂ is the time of the inertial observer (the son). Did I get that wrong?

You got it incompletely. The value of t₁ is indeed the coordinate time of an event on the traveler's worldline that we are trying to correlate with a time on the "home's" worldline, which is the spatial origin of the inertial coordinates. So, according to the home reference frame, the home time that corresponds to the traveler's event at t₁ is simply t₁. This is what you and Sally call "CADO_H". The value of t₂ is the home time that corresponds to the traveler's event at t₁ according to the traveler's instantaneous inertial frame of reference. This is what you and Sally call "CADO_T".

... in the CADO formula... CADO_H is the inertial observer's time and "CADO_T denotes the traveler's conclusion about the home-twin's age" (or the time for the inertial observer). So in CADO, both times are for the inertial observer, it's just that they are the two different observer's opinion of that time.

Right, it's exactly the same as on the page I cited.

...in your paper, each time is for a different observer. If this isn't right, please point out where the errors are.

See above. Since the home worldline is the spatial origin of the inertial coordinates, the value of t₁ represents the coordinate time of the event on the traveler's worldline, and this is also (by definition) the time on the home worldline that corresponds to that event according to the home reference frame.

 

[Austin0]

The formula from your paper is:

τ₂ = t₁ - r • v

It superficially looks similar to GrammawSally's interpretation of the CADO formula (r replacing L and taking the dot product) but it doesn't look the same beyond that.

The commentary associated with the formula expresses to me that t₁ is the time of the traveler (the father) while τ₂ is the time of the inertial observer (the son). Did I get that wrong?

But in the CADO formula, the two variables that are multiplied have the same meaning as your formula but CADO_H is the inertial observer's time and "CADO_T denotes the traveler's conclusion about the home-twin's age" (or the time for the inertial observer). So in CADO, both times are for the inertial observer, it's just that they are the two different observer's opinion of that time, whereas in your paper, each time is for a different observer. If this isn't right, please point out where the errors are.

CADO_T = CADO_H - v * L

If the traveler is unprimed then:

CADO_T = $\gamma$(t-vdx)= CADO_H - v * L

where dx is traveler's distance from home
does this now look familiar?

 

[GrammawSally]

[...]
Yes, but unless there is a constant time relationship between them, such as in this case, I don't understand how it works.

By the way, I'm still if the CADO scheme as depicted in my plot is what you meant when you said:
[...]

I haven't been able to understand what you're asking in either of the above questions ... could you elaborate some?

Just guessing, but maybe the extra tip you might need (for the first question) is that all of the quantities in the CADO equation are functions of the traveler's age t. That dependence isn't explicitly shown in the equation, but it does need to be kept in mind.

And, just guessing for the second question, your plot looks reasonable, but a more useful plot (called the "age-correspondence graph" on the webpage) is CADO_T versus t, which is the home person's age, according to the traveler, plotted versus the traveler's age. That is the information that the traveler wants to know, at each instant t of his life. And, for comparison, it is interesting to also plot CADO_H versus t, which is exactly the same type of graph, except it's according to the home person. The two plots generally have quite different shapes, although for the usual twin paradox scenario (when the twins are co-located at the beginning and end of the trip), the beginning and end points of the two plots will always agree.

 

[Simplyh]

What are we talking about? Time dilatation as an effect of speed, is it not?

What we are talking about is how 2 inertial frames, with a certain relative movement, see each other's time. Then we can add all other effects we want.

Imagine 2 travellers parting from Earth in opposite directions at the same speed. They where both accelerated similarly and then left to an "inertial" movement. At certain places in space, at the same distance to Earth's original position, they must communicate to each other their respective time. Their relative speed is sufficiently high for the dilation time effect to be noticed but sufficiently slow for the communication at the speed of light to be practical. When the information arrives both observers compare their readings. Which reading should be expected to have a higher value? On Earth's frame (discounting Earth's movement) they should show the same time (though different from Earth time) because they had traveled the same distance at the same speed. But each observer should have received a reading showing that the other clock was working slower than his. How can this be possible?

Now going back to this thread's example. Most of you say: circular movement seems not to have a very important effect on ageing (though there's disagreement on the calculations) but it carries a strange unexplained effect: it makes the frame with the accelerated movement as The moving frame. Which principle allows you to do so?

For both observers the other should be ageing at very slow rate (as explained above) and, on top of that, the father should experiment an additional time distortion effect due to acceleration (even without changing speed, I disagree with you George, though this is not the topic).Your assumption (and based calculations) that the non inertial frame (in reality none of them is inertial) is The Sole and Only moving frame, thus applying SR assumptions Sole and Only to that frame is based on something that totally escapes my understanding. Can any of you please explain what makes it possible?

Divirtam-se

 

[ghwellsjr]

I haven't been able to understand what you're asking in either of the above questions ... could you elaborate some?

Just guessing, but maybe the extra tip you might need (for the first question) is that all of the quantities in the CADO equation are functions of the traveler's age t. That dependence isn't explicitly shown in the equation, but it does need to be kept in mind.

Yes, I'm aware of that. My only point is that since the speed of the traveling father is constant in the son's inertial (earth) frame, gamma is constant and we can determine the shape of the graph for the father's time purely from the son's time, which is what I did.

And, just guessing for the second question, your plot looks reasonable, but a more useful plot (called the "age-correspondence graph" on the webpage) is CADO_T versus t, which is the home person's age, according to the traveler, plotted versus the traveler's age. That is the information that the traveler wants to know, at each instant t of his life.

Then just relabel the horizontal axis of my graph to read "t (ns)" for a gamma of one million.

And, for comparison, it is interesting to also plot CADO_H versus t, which is exactly the same type of graph, except it's according to the home person. The two plots generally have quite different shapes, although for the usual twin paradox scenario (when the twins are co-located at the beginning and end of the trip), the beginning and end points of the two plots will always agree.

Then just flip the plot along the diagonal.

Now since you have apparently agreed that my relabeled graph fulfills your desire for the father's own "point of view" about how his son is aging, I'd like to ask you what additional information the father obtains by our doing this exercise.

 

[Russell E]

CADO is essentially nothing but applying the fundamental transformations from a momentarily co-moving inertial frame.

Exactly, and it's ridiculous to assign it an acronym.

 

[GrammawSally]

[...]

I think I finally understand what you are doing: I think you must be stating results for the extreme case where gamma is very large and v is negligibly different from one, and apparently in that case, you've concluded (perhaps correctly) that you don't need any analysis beyond the inertial person's perspective to get the traveler's perspective with adequate accuracy.

That case isn't of any interest to me, because it doesn't bring out the most interesting and the more general aspects of special relativity for circular motion. Much more illuminating is a case like gamma = 2 and v = 0.866. If you try to determine the CADO_T vs t graph for that case, you will find that you do need to use the CADO equation: just analyzing it from the home person's perspective won't get you the traveler's perspective. You will find that the CADO_T vs t graph is quite different from the CADO_H vs t graph, and one can't be determined from the other in any trivially simple way.

Better yet, for this circular motion problem, why not derive an expression for the CADO_T vs t graph, with v and r (radius of circle) as parameters. For simplicity, you can let both the traveler and the inertial person be zero years old when they are initially co-located at the train station (theta = 0). Then get the CADO_T vs t graph for one circuit, 0 <= theta <= 2pi.

 

[Russell E]

If you try to determine the CADO_T vs t graph...

It isn't very useful to make up obscure non-standard terminology for trivial standard concepts and formulas. What you keep calling "CADO this" and "CADO that" is nothing but evaluating times in terms of the momentarily co-moving inertial reference frames of various objects.

 

[ghwellsjr]

What are we talking about? Time dilatation as an effect of speed, is it not?

Time dilation is an effect of the speed of an object in a single specified inertial Frame of Reference.

What we are talking about is how 2 inertial frames, with a certain relative movement, see each other's time. Then we can add all other effects we want.

No, that's not right. You are confusing how Coordinate Times differ between two inertial frames with a relative motion. Time dilation refers to the Proper Time of a moving object or clock in a single inertial frame.

Imagine 2 travellers parting from Earth in opposite directions at the same speed. They where both accelerated similarly and then left to an "inertial" movement. At certain places in space, at the same distance to Earth's original position, they must communicate to each other their respective time. Their relative speed is sufficiently high for the dilation time effect to be noticed but sufficiently slow for the communication at the speed of light to be practical. When the information arrives both observers compare their readings. Which reading should be expected to have a higher value? On Earth's frame (discounting Earth's movement) they should show the same time (though different from Earth time) because they had traveled the same distance at the same speed. But each observer should have received a reading showing that the other clock was working slower than his. How can this be possible?

Is this another rhetorical question? Are you questioning the validity of what you have just described?

Now going back to this thread's example. Most of you say: circular movement seems not to have a very important effect on ageing (though there's disagreement on the calculations) but it carries a strange unexplained effect: it makes the frame with the accelerated movement as The moving frame. Which principle allows you to do so?

The frame with the accelerated movement (the father's) is non-inertial which means two things: First, it is not easy to work with and second, there is no standard way to define non-inertial frames (like there is for inertial frames). For those two reasons, I have simply never been motivated to study or learn anything about them (until this thread, but I'm getting over that real fast). Relativity states that you can analyze a situation from any frame and it is just as valid and will provide all the significant answers that any other frame will. So why pick a difficult non-inertial frame when the very simple inertial frame will work just fine?

For both observers the other should be ageing at very slow rate (as explained above)

You explained correctly what happens in the single Earth frame for two observers with their own histories of motion. That's exactly what I did as I also used only the single Earth frame for two observers that have their own histories of motion. The difference between your example above and the example in this thread is that in your example the two observers never rejoined and so we never get to determine a frame independent aging difference between them whereas in the example in this thread, they rejoined 7.4 times per second (in the Earth frame) and so we can determine a frame independent continually growing age difference between the father and the son.

and, on top of that, the father should experiment an additional time distortion effect due to acceleration (even without changing speed, I disagree with you George, though this is not the topic).

But this issue was covered by yuiop in post #16 on the first page:

In the FAQ on the experimental evidence for relativity and the clock postulate section http://www.edu-observatory.org/physics-faq/Relativity/SR/experiments.html#Clock_Hypothesis it states:

"The clock hypothesis states that the tick rate of a clock when measured in an inertial frame depends only upon its velocity relative to that frame, and is independent of its acceleration or higher derivatives. The experiment of Bailey et al. referenced above stored muons in a magnetic storage ring and measured their lifetime. While being stored in the ring they were subject to a proper acceleration of approximately 10¹⁸ g (1 g = 9.8 m/s2). The observed agreement between the lifetime of the stored muons with that of muons with the same energy moving inertially confirms the clock hypothesis for accelerations of that magnitude."

Acceleration contributes nothing to time dilation.

Your assumption (and based calculations) that the non inertial frame (in reality none of them is inertial) is The Sole and Only moving frame, thus applying SR assumptions Sole and Only to that frame is based on something that totally escapes my understanding. Can any of you please explain what makes it possible?

I never said anything about The Sole and Only moving frame--where'd you get that idea from? I've always said that there is no Sole and Only frame, moving or otherwise. Let me repeat: any frame is just as good as any other frame, none is preferred, and none is disallowed. So why beat your head against the wall and pick a complicated frame when the Earth frame will do everything that any other frame will do?

If you would just follow your example of the two observers moving away from the Earth and defining and analyzing everything according to the single inertial Earth frame, there would be nothing more to explain and you should be able to understand everything.

Divirtam-se

Only English is allowed on these forums. Why do you keep doing this?

 

[ghwellsjr]

I think I finally understand what you are doing: I think you must be stating results for the extreme case where gamma is very large and v is negligibly different from one, and apparently in that case, you've concluded (perhaps correctly) that you don't need any analysis beyond the inertial person's perspective to get the traveler's perspective with adequate accuracy.

It's not the inertial person's perspective (whatever that means) that is important, it's the single Earth inertial frame that is sufficient to analyze what each observer perceives. If you want to do the analysis from another inertial frame, you need to use the Lorentz Transformation to establish the same scenario and after you do this you will find that this new frame produces the same analysis of what each observer perceives. If you know how to transform the scenario as defined in the initial inertial frame into a non-inertial frame, why do think any analysis will produce any additional insight into what any observer perceives?

That case isn't of any interest to me, because it doesn't bring out the most interesting and the more general aspects of special relativity for circular motion. Much more illuminating is a case like gamma = 2 and v = 0.866. If you try to determine the CADO_T vs t graph for that case, you will find that you do need to use the CADO equation: just analyzing it from the home person's perspective won't get you the traveler's perspective. You will find that the CADO_T vs t graph is quite different from the CADO_H vs t graph, and one can't be determined from the other in any trivially simple way.

And if you use some other definition of a non-inertial frame, you'll get a completely different graph. None is preferred, just like for inertial frames where each one can assign different time dilations to the observers.

By contrast, the Doppler analysis, which you can do from any frame, yields exactly one result and shows you what each observer actually perceives.

Better yet, for this circular motion problem, why not derive an expression for the CADO_T vs t graph, with v and r (radius of circle) as parameters. For simplicity, you can let both the traveler and the inertial person be zero years old when they are initially co-located at the train station (theta = 0). Then get the CADO_T vs t graph for one circuit, 0 <= theta <= 2pi.

Left as an exercise for the reader, correct?

Why should I do something that will not lead me one step closer to what any observer perceives? Why do you find this sort of exercise interesting?

 

[GrammawSally]

Why should I do something that will not lead me one step closer to what any observer perceives? Why do you find this sort of exercise interesting?

Because if I were actually traveling at high velocities in a spacecraft , far from Earth, I would want to know the current age of my twin brother back home. And I would know that any image I saw of him (either through a telescope, or on a TV screen) is old and out-of-date. I would know that image does not show me my twin's actual current age. I would want to correct for the transit time of the image. The CADO equation allows me to easily and quickly do that.

 

[ghwellsjr]

Because if I were actually traveling at high velocities in a spacecraft , far from Earth, I would want to know the current age of my twin brother back home. And I would know that any image I saw of him (either through a telescope, or on a TV screen) is old and out-of-date. I would know that image does not show me my twin's actual current age. I would want to correct for the transit time of the image. The CADO equation allows me to easily and quickly do that.

As I said to Underwood when he expressed the same opinion:

CADO has given you a false sense of security. You have bought into Michael Fontenot's dogmatism.

You cannot correct for the transit time of the image except by assuming that it is some particular arbitrary value and that assumption can give you any age you want (within limits). You don't know that any image you see of your brother is old and out-of-date. You are merely assuming that it is. You could just as reasonably assume that the image gets to you almost instantaneously and that it is no more out-of-date than an image you would see of him if he were just a few feet away from you.

What if your twin decides a year after you leave to get on his own spaceship and follow after you at a higher speed so that he will eventually catch up to you? How will CADO or any other method of calculating current age deal with that?

 

[Russell E]

Because if I were actually traveling at high velocities in a spacecraft , far from Earth, I would want to know the current age of my twin brother back home...

That doesn't make any sense, because using the co-moving inertial reference frame you can change the "current age" of your brother at will, simply by slight changes in your own state of motion. If you are far enough away, your brother's "current age" alternately increases and decreases by years as you pace back and forth in a room. As you pace in one direction, your brother is having his 20th birthday party right "now", and as you pace in the other direction he is long since dead and buried, and this alternates back and forth, now he's blowing out 20 candles, now he's dead, now he's blowing out 20 candles, now he's dead... Does this really signify "the current age of your brother", alternating between alive and dead in any meaningful sense? Do you really think that whether your brother is alive or dead right "now" can fluctuate depending on just slight differences in YOUR state of motion?

 

[ghwellsjr]

That doesn't make any sense, because using the co-moving inertial reference frame you can change the "current age" of your brother at will, simply by slight changes in your own state of motion. If you are far enough away, your brother's "current age" alternately increases and decreases by years as you pace back and forth in a room. As you pace in one direction, your brother is having his 20th birthday party right "now", and as you pace in the other direction he is long since dead and buried, and this alternates back and forth, now he's blowing out 20 candles, now he's dead, now he's blowing out 20 candles, now he's dead... Does this really signify "the current age of your brother", alternating between alive and dead in any meaningful sense? Do you really think that whether your brother is alive or dead right "now" can fluctuate depending on just slight differences in YOUR state of motion?

Although this is true, it misses the point. You can change the Current Age of a Distant Object simply by adapting a different simultaneity convention without changing your own state of motion. You can do it all in your mind without moving a muscle. And this has nothing to do with accelerating frames, the same thing is true for any two separated clocks even if they are stationary with respect to each other.

 

[PAllen]

Although this is true, it misses the point. You can change the Current Age of a Distant Object simply by adapting a different simultaneity convention without changing your own state of motion. You can do it all in your mind without moving a muscle. And this has nothing to do with accelerating frames, the same thing is true for any two separated clocks even if they are stationary with respect to each other.

Exactly! Distant simultaneity in all cases is just a convention, never something that can be observed. Only limit is you can't call something in your causal past or future simultaneous. Only thing you can say about 'Einstein simultaneity convention' for inertial frames is that it is simple and useful, never that it can be experimentally preferred over a different convention.

 

[Russell E]

Although this is true, it misses the point. You can change the Current Age of a Distant Object simply by adapting a different simultaneity convention without changing your own state of motion.

No, you missed the point. Sally espouses a specific definition for "current age", based on the momentarily co-moving inertial frames (which are perfectly well defined), and my comment addressed that specific definition, which does indeed require changing your state of motion in order to change your planes of simultaneity. You're making a different comment, namely, that Sally could, if she chose, define "now" differently. That's obviously true (for example, she could adopt the usual definition, i.e., that "now" consists of the entire region outside our past and future light cones), but it doesn't help to clarify for Sally why she should be dis-satisfied with the definition she is espousing.

 

[ghwellsjr]

No, you missed the point. Sally espouses a specific definition for "current age", based on the momentarily co-moving inertial frames (which are perfectly well defined), and my comment addressed that specific definition, which does indeed require changing your state of motion in order to change your planes of simultaneity. You're making a different comment, namely, that Sally could, if she chose, define "now" differently. That's obviously true (for example, she could adopt the usual definition, i.e., that "now" consists of the entire region outside our past and future light cones), but it doesn't help to clarify for Sally why she should be dis-satisfied with the definition she is espousing.

But look at the reason she adopted CADO in the first place:

Because if I were actually traveling at high velocities in a spacecraft , far from Earth, I would want to know the current age of my twin brother back home. And I would know that any image I saw of him (either through a telescope, or on a TV screen) is old and out-of-date. I would know that image does not show me my twin's actual current age. I would want to correct for the transit time of the image. The CADO equation allows me to easily and quickly do that.

She thinks there is an "actual current age" that she can know, just like Underwood believes, and it's a false notion. She claims to know that an image of her twin is old and out-of-date and it's simply not true that she can have that knowledge. She and Underwood need to understand that if they want truth, they have to settle only for what they can actually see, anything else is derived from whatever convention they adopt. In other words, if they want their sibling to be younger, they can adopt a convention that permits that. If they want their sibling to be older, they can adopt a convention that permits that.

 

[Russell E]

But look at the reason she adopted CADO in the first place...

I quoted her reason in my message, and specifically addressed it, explaining why her "reason" is misguided, even on its own terms.

She thinks there is an "actual current age" that she can know...

Well, it is certainly possible to know standard inertial space-time coordinate systems, and to empirically distinguish them from other coordinate systems. And we can also determine the sequence of such coordinate systems in terms of which an arbitrary object is momentarily at rest. This isn't controversial. The problem with Sally's beliefs is not that her chosen concept of simultaneity is unknowable or meaningless. (It is neither.) And the problem isn't that her chosen concept is not the only possible concept of simultaneity. (Of course it's not.) The problem is that even Sally herself, if she grasped how her definition of "now" really works, would not consider it to be sensible or appealing.

 

[ghwellsjr]

The problem is that even Sally herself, if she grasped how her definition of "now" really works, would not consider it to be sensible or appealing.

And that is also true for all the other definitions.

 

[Russell E]

And that is also true for all the other definitions.

Well, as I said a few posts ago, the standard definition of "now" is the entire region outside the past and future light cones. Whether Sally or Mike would consider this to be a satisfactory definition is hard to say. (Maybe if Mike gave it an acronym and made a web page claiming to have discovered it...) Other people prefer a cosmological definition, e.g., the proper time along worldlines emanating isotropically from the CMBR, or some such. It isn't hard to think of definitions that are more satisfactory than the one based on momentarily co-moving inertial coordinates.

 

[ghwellsjr]

...the usual definition, i.e., that "now" consists of the entire region outside our past and future light cones...

Well, as I said a few posts ago, the standard definition of "now" is the entire region outside the past and future light cones.

I never heard of this "usual", "standard" definition of "now". Can you please provide some on-line references to it? I couldn't find any.

Whether Sally or Mike would consider this to be a satisfactory definition is hard to say. (Maybe if Mike gave it an acronym and made a web page claiming to have discovered it...)

We don't need sarcasm when discussing physics. There is nothing wrong with Mike's definition and there is nothing wrong with Sally adopting it. What's wrong is the idea that it reveals something about nature rather than that it is putting something arbitrary into nature and that it is superior (or inferior) to any other consistent arbitrary definition.

Other people prefer a cosmological definition, e.g., the proper time along worldlines emanating isotropically from the CMBR, or some such.

The issue of "now" is the issue of defining remote simultaneity which is all about defining a coordinate system with a coordinate time at each coordinate location. When the coordinate times at two coordinate locations are the same, they are the same "now". Those two events are simultaneous according to the defined coordinate system. Use a different coordinate system and the same two events may not be at the same time and no longer share the same "now".

It isn't hard to think of definitions that are more satisfactory than the one based on momentarily co-moving inertial coordinates.

It took an Einstein to think of the very "simple and useful" definition (as PAllen called it in post #269) but your claim of a "usual" and "standard" definition doesn't even permit deciding which events are simultaneous.

 

[Austin0]

Exactly! Distant simultaneity in all cases is just a convention, never something that can be observed. Only limit is you can't call something in your causal past or future simultaneous. Only thing you can say about 'Einstein simultaneity convention' for inertial frames is that it is simple and useful, never that it can be experimentally preferred over a different convention.

Do you think that a different synchronization could possibly produce isotropic invariance of light speed measurements?
I am not sure what you mean by simultaneity convention. To me it appears that the gamma function implemented through the Lorentz transformation for times at different locations in another frame is not so much a convention as an inevitable description of the physics of the real world. It describes the relative desynchronization of the clocks to be found at those locations and as such is assumed to be verified by actual experimentation.
How could a different synchronization produce a rational transformation between frames irrespective of the angle of their paths or relative velocities?
Of course clocks at distant locations in the same frame having the same proper time reading does not mean in any way that they are simultaneous.
Distant simultaneity is simply completely indeterminable by any means.

 

[PAllen]

Do you think that a different synchronization could possibly produce isotropic invariance of light speed measurements?
I am not sure what you mean by simultaneity convention. To me it appears that the gamma function implemented through the Lorentz transformation for times at different locations in another frame is not so much a convention as an inevitable description of the physics of the real world. It describes the relative desynchronization of the clocks to be found at those locations and as such is assumed to be verified by actual experimentation.
How could a different synchronization produce a rational transformation between frames irrespective of the angle of their paths or relative velocities?
Of course clocks at distant locations in the same frame having the same proper time reading does not mean in any way that they are simultaneous.
Distant simultaneity is simply completely indeterminable by any means.

A different convention would have isotropic two way speed of light, but anisotropic one way speed of light. There is no way to experimentally distinguish such a convention from the Einstein convention. Please look up some of the other long threads on this and not convert this thread into another thread on this well known fact.

 

[GrammawSally]

[...]
Better yet, for this circular motion problem, why not derive an expression for the CADO_T vs t graph, with v and r (radius of circle) as parameters. For simplicity, you can let both the traveler and the inertial person be zero years old when they are initially co-located at the train station (theta = 0). Then get the CADO_T vs t graph for one circuit, 0 <= theta <= 2pi.

Here's what I get for that problem (r = 1, gamma = 2.0, CADO_H(0) = 0):

For any given angular position 0 <= theta <= 2pi of the traveler, the traveler's age is t = r * theta / v.

CADO_H(t) is of course equal to gamma * t.

CADO_T(t) = gamma * t - r * v * sin(v * t / r).

d{CADO_T(t)}/dt = gamma - v * v * cos(v * t / r).

The above derivative gives the slope of the CADO_T vs t graph (the "age-correspondence" graph). It is the time dilation of the home twin, according to the traveler. Since it is greater than one, the traveler says that the home twin is aging more quickly than he himself is, so it would more reasonably be called "time contraction", rather than "time dilation", in this case.

Here are a few values from the above equations:

theta t CADO_H CADO_T slope

0 0 0 0 1.25
pi/2 1.81 3.63 2.76 2.0
pi 3.63 7.26 7.26 2.75
3pi/2 5.44 10.88 11.75 2.0
2pi 7.26 14.51 14.51 1.25

If the home twin were at the center of the circle, the traveler would always conclude that the home twin was aging twice as fast as he himself was aging ... i.e., he would always exactly agree with the home twin about their respective ages.

But if the home twin is located on the circle (as in the above scenario), the traveler would conclude that near theta = 0, the home twin is aging faster than he himself is aging, but less than twice as fast. The home twin would be aging exactly twice as fast at theta = pi/2 and at theta = 3pi/2. And the home twin would be aging more than twice as fast at theta = pi.

The fact that the home twin is aging less than twice as fast as the traveler (according to the traveler) at theta = 0 was a surprise for me ... I had expected that the home twin's "time contraction" would be very nearly equal to gamma whenever the twins were very close together, but that's not what happens.

 

[ghwellsjr]

CADO_T(t) = gamma * t - r * v * sin(v * t / r).

If this is correct, then the graph I produced in post #238 is not correct. I was following your instructions in post #236 to take the dot product of L and v which is the cosine of the angle between the two vectors. I think you also need a factor of π/2 in the argument of the trig function. I think I know why you are using the sine function but I don't think you have taken into account the varying length nor the direction of L. Maybe if you do, your surprises will disappear.

HINT: when you want to show numbers in a table, wrap the table in a CODE tag.

 

[Austin0]

If this is correct, then the graph I produced in post #238 is not correct. I was following your instructions in post #236 to take the dot product of L and v which is the cosine of the angle between the two vectors. I think you also need a factor of π/2 in the argument of the trig function. I think I know why you are using the sine function but I don't think you have taken into account the varying length nor the direction of L. Maybe if you do, your surprises will disappear.

I want to find out if the approach I used is valid or not and if it is equivalent to the dot product method. I am now thinking it is equivalent but want to make sure.

But using the previous parameters circle C=314 ls in station frame. v close to c then:

At 90 deg. Calculated time at station is 28.5 sec.

180 deg. 157sec.

270 deg. 285.5 sec.

CADO_H is simply the coordinate time at the location (in the station frame in this case)

SO at 90deg. the local time is 78.5
L is the distance in the station coordinates
so at 90deg.the velocity vector would orthogonally intersect the position of the station observer (the tangent at the station point) so the distance in the station frame would be equal to the radius 50 ls ...multiplied by v =approx 50 sec this is just the lorentz math for determining relative clock desynchronization.between frames.
subtracting this from 78.5 gives CADO_T=28.5 ..the calculated simultaneous time at the station.

L is the vector position of the traveler, relative to the inertial person, according to the inertial person. v is the vector velocity of the traveler, relative to the inertial person, according to the inertial person.

So if the inertial person's position on the circle is taken as zero degrees, and if the traveler's position is momentarily at 90 degrees (CCW), then the L and v vectors will be neither perpendicular nor parallel, and L will have a magnitude greater than the radius of the circle. The dot product of L and v will be nonzero, and will have a magnitude less than the product of the magnitudes of L and v.

Maybe I misunderstood you, but it didn't sound to me like that's what you were doing.

It appears to me that in the case at 90 deg. that I used,, the cos (45deg)=.707 times the direct distance (70.7 )does equal the radius. SO in that particular instance there is agreement.
But I am not sure if this is true all around the circumference
This a drawing of a further point: is my approach of orthogonal projection equivalent to the dot product?

Looking at your graph it looks like my result is very close to yours for 90 deg. estimating values for a quarter of the circuit from your chart.
How do you get attached images to show up full size??